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Hydrogen energy in electrical power systems: a review and future outlook, 1. introduction, 2. current status of hydrogen production and storage, 2.1. review of production of hydrogen from electrolytic water, 2.2. developments in hydrogen storage technologies, 2.3. application of integration of hydrogen storage with renewable energy sources, 3. key technologies for hydrogen electrification, 3.1. hydrogen electrification technology, 3.2. hydrogen ancillary service technologies, 3.3. hydrogen–electric power systems, 4. key technologies for hydrogen–electric coupling, 4.1. load-side electric–hydrogen coupling, 4.2. power-side electric–hydrogen coupling, 5. outlook for the development of hydrogen energy applications, 5.1. reducing electrolysis costs and improving preparation yields, 5.2. development of new materials to enhance hydrogen storage capacity, 5.3. optimizing control methods to improve power generation efficiency, author contributions, data availability statement, conflicts of interest.

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Dai, S.; Shen, P.; Deng, W.; Yu, Q. Hydrogen Energy in Electrical Power Systems: A Review and Future Outlook. Electronics 2024 , 13 , 3370. https://doi.org/10.3390/electronics13173370

Dai S, Shen P, Deng W, Yu Q. Hydrogen Energy in Electrical Power Systems: A Review and Future Outlook. Electronics . 2024; 13(17):3370. https://doi.org/10.3390/electronics13173370

Dai, Siting, Pin Shen, Wenyang Deng, and Qing Yu. 2024. "Hydrogen Energy in Electrical Power Systems: A Review and Future Outlook" Electronics 13, no. 17: 3370. https://doi.org/10.3390/electronics13173370

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Designing a hydrogen generation system through PEM water electrolysis with the capability to adjust fast fluctuations in photovoltaic power

• Kumar, Raj Kapur
• Samuel, Paulson

—Hydrogen can be produced through water electrolysis using photovoltaic (PV) power in a way that is both pollution-free and friendly to the environment, rendering hydrogen a clean energy source. Proton Exchange Membrane (PEM) water electrolysis is an efficient hydrogen generation process because of its compact design, fast response, high efficiency and high current density. The fast fluctuations in solar power, however, are difficult for PEM water electrolysers to control effectively. To address the problems caused by intermittent power, a system for producing hydrogen consisting of a solar panel, battery, and commercial PEM electrolyser, alongside an electrical control strategy, is proposed. The Maximum Power Point Tracking (MPPT) Perturb and Observe (P&O) technique is employed for PV systems, while a bidirectional charge controller manages the charging and discharging of the battery. A Proportional-Integral (PI) controller is also utilised for the PEM electrolyser to ensure efficient operation. This setup provides a practical solution for hydrogen production. It enhances efficiency and reduces the risk of damage to the electrolyser, thereby making the process more sustainable and reliable. The proposed approach has been validated by simulation under different conditions. A comparative analysis has also demonstrated that the proposed system can perform well despite having less PV power. When comparing systems with and without batteries for energy storage, the system with the battery for energy storage has a 10% higher energy efficiency under the same operating conditions. This suggests that adding energy storage can increase energy utilization, maintain the DC link voltage constant, and achieve stable hydrogen production.

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How the U.S. Is Building a Sustainable Hydrogen Economy

The u.s. department of energy's regional clean hydrogen hubs program uses economic clusters to drive innovation, cut costs and accelerate green hydrogen adoption across the country..

The Bipartisan Infrastructure Law, passed by Congress in 2021, includes up to $7 billion to fund a Regional Clean Hydrogen Hubs Program. The U.S. Department of Energy, the program’s administrator, states that its goal is to facilitate the creation of “ large-scale, commercially viable hydrogen ecosystems .” In its main publications on the program, the Department of Energy does not detail how taxpayer dollars for hydrogen ecosystems will translate into the holy grail of “commercial viability” for green hydrogen. Yet there is reason to believe that the Hubs concept can help bring about this outcome. The key idea is that of the “cluster.”

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The term “cluster” as an economic construct was explained by Harvard Business School Professor Michael Porter in 1998 as “geographic concentrations of interconnected companies and institutions in a particular field.” The sprawling set of activities around the production of wine in California is one of many examples Porter advances. The great benefit of clusters, Porter says, is that they encourage interactions across cluster members that foster a variety of productivity enhancements. He describes how the bases of competition at play in the pre-globalized twentieth century have lost their force in the globalized twenty-first century: “The enduring competitive advantages in a global economy lie increasingly in local things – knowledge, relationships, motivation – that distant rivals cannot match.”

Without the cluster framework, the parties sponsoring each of the seven hydrogen hub projects selected for funding by the Department of Energy would appear to have a somewhat random aspect. To take the Pacific Northwest Hydrogen Hub (PNWH2) as an example, one notes representation on the Board of Directors from energy engineering and equipment company Mitsubishi Heavy Industries and hydrogen production companies Fortescue Future Industries and NovoHydrogen . This certainly makes sense. If the goal is to encourage green hydrogen production, a hub needs to include technologists and producers. And if this were a twentieth-century undertaking, hub membership might stop there.

However, one also notes Board representation from ammonia producer Atlas Agro , natural gas utility AltaGas , and public transit provider Lewis County Transit . The heterogeneous nature of these parties notwithstanding, it can quickly be seen that each is a potential consumer of green hydrogen. For Atlas Agro, green hydrogen is the main feedstock for the production of green ammonia. For AltaGas, green hydrogen could be blended into its pipeline natural gas to reduce its carbon footprint. For Lewis County Transit, green hydrogen will fuel its small but growing fleet of fuel-cell-powered transit buses. Porter argues that the co-existence of suppliers and customers in a cluster can create a virtuous cycle in which heightened awareness of customer needs drives improvement of product/service bundles, and heightened awareness of producer capabilities leads to articulation of evolving customer needs—all without breaking down the arm’s length relationships at the heart of competition-based economies.

Further perusal of the Board reveals representation from the Washington State Labor Council and the Cowlitz Indian Tribe. From a twentieth-century perspective, this is actively surprising. Why include organized labor and other community stakeholders in Hub leadership? True, workers will be needed to staff plants, but producers typically meet that need through established hiring processes. Porter places a great deal of emphasis on the workforce dimension of clusters, pointing out how they can create environments in which regional workers are motivated to acquire knowledge and skills relevant within a cluster and local institutions are motivated to offer programs that will allow them to do so.

A final look at the Board makeup reveals representation from the Oregon Department of Energy. This is perhaps the most unexpected finding of all. Twentieth-century thinking recognizes that government actors perform essential functions in society but that their modes of encouraging economic development are normally at a disinterested remove (and that when they are not encouraging economic development, their actions often have adverse effects on businesses). Porter has little patience with this view. On the contrary, he says that “Governments – both national and local – have new roles to play. They must ensure the supply of high-quality inputs such as educated citizens and physical infrastructure . . .” By sitting at the table with other Hub developers, the Oregon Department of Energy will gain the perspectives it needs to be able to coordinate with other agencies within and beyond the Oregon state government to help them discharge their duties in a way that is as supportive as possible of Hub development.

The embrace of the cluster concept by the sponsors of the PNWH2 was no accident. On the contrary, clusters have been an explicit focus of policymakers in Washington State since at least the 2022 launch of the Department of Commerce’s Innovative Cluster Accelerator Program (ICAP) . The Consortium for Hydrogen And Renewably Generated E-Fuels (CHARGE) was among the first recipients of funding under ICAP. CHARGE was launched by Washington State University in 2021 with the mission of making Washington State “ a global hub for commercializing alternative fuels .” Its cluster orientation was apparent in the relationships it built with the Pacific Northwest National Laboratory, selected utilities and public utility districts, and a variety of private-sector players. CHARGE was subsequently instrumental in convening the proponents of the PNWH2 when the DOE announced the Hydrogen Hubs program in September 2022 and is represented on the PNWH2 Board.

The economics of green hydrogen are currently challenging, with production costs at a significant premium over those of gray hydrogen. Technoeconomic analyses suggest that this is not a permanent reality but is rather an artifact of the early stage of green hydrogen development. A 2023 analysis suggests that green hydrogen will be able to close the cost gap as we enter the next decade and that “The dramatic drop in the price of green hydrogen is due to two key factors: economies of scale, and supportive policy.” Porter would argue that clusters create a context in which both factors can be applied highly effectively (as long as scale is understood in his revised terms: “A cluster allows each member to benefit as if it had greater scale”). It would appear that the creators of the Hydrogen Hubs program agree with him.

• SEE ALSO : Japan’s Mitsubishi Electric Sees Surging Demand for a Special Type of A.I. Hardware

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Essay on Hydrogen Energy | Types | Renewable Energy | Energy Management

Here is an essay on ‘Hydrogen Energy’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Hydrogen Energy’ especially written for school and college students.

Essay on Hydrogen Energy

Essay Contents:

• Essay on the Limitations of Hydrogen Energy

Essay # 1. Introduction to Hydrogen Energy:

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Electrical energy is the most convenient form of energy because it can be easily controlled, transported and converted into heat and work at very high efficien­cies. The only shortcoming of electrical energy is that it cannot be stored in large quantities. Alternative energy of future is hydrogen energy which can also be easily stored in addition to other qualities of electrical energy.

But is highly inflammable and special handling precautions are needed during its production, transportation, storage and utilization. Hydrogen is a secondary fuel that is produced by utilizing energy from a primary source. Water and solar energy are freely and abundantly available in nature on earth. Hydrogen can be produced from water by using solar energy. All plants and hydrocarbons (fossil fuels) are sources of hydrogen.

Hydrogen can be stored as gas underground or in high pressure cylinders. Also it can be stored in liquid form at low temperatures. There are a number of metals and alloys which form solid hydrides with hydrogen. The hydrogen can be easily recovered by heating metal hydrides. Thus metal hydrides provide a possible means for hydrogen storage in solid form. Hydrogen can be similarly transported as gas through pipes or storage cylinders or as liquid hydrogen or as metal hydrides.

Hydrogen has the highest energy content per unit mass. Its burning process is non-polluting and it can be used in fuel cells to produce both electricity and useful heat. It can be used as a fuel directly in gas turbines or spark-ignition engines. It can be used as motor vehicle fuel in urban transportation where air pollution problems are critical.

Its specific energy content is almost three times that of hydrocarbon fuels. Therefore it can be directly used as aircraft fuel for air transport. Hydrogen has been used as a fuel for space craft’s. A H 2 – O 2 fuel cell liberates energy and also water as sole material product for the use of space craft passengers.

The simplest practical way to obtain hydrogen from water is its electrolysis using electricity. The latter can be generated from removable energy sources like solar energy, wind energy, geothermal energy.

Hydrogen has huge market. However, enormous capital investment is re­quired for its production, distribution and storage. Special design precautions are needed for the safe operation of equipment and systems. This is presenting the essential obstacle in the quick introduction of hydrogen.

Essay # 2. Utilization of Hydrogen Energy:

Hydrogen has the following properties which make it an attractive alternative energy source:

1. At room temperature and pressure, hydrogen is a light gas. Its density is only 1/14th of that of air and 1/9th that of natural gas.

2. At atmospheric pressure, hydrogen can be liquefied at – 253°C. The liquid hydrogen has a specific gravity of 0.07 which is 1/10th that of gasoline.

3. The standard heating value of hydrogen gas is 12.1 MJ/m 3 compared with 38.3 MJ/m 3 for natural gas.

4. The heating value of liquid hydrogen is 120 MJ/kg or 8400 MJ/m 3 as compared to 44MJ/kg or 32000 MJ/m 3 of aviation petrol. The specific energy of hydrogen liquid is superior to gasoline on mass basis but inferior on volume basis.

5. The flame speed of hydrogen when burning in air is much greater than for natural gas.

6. The ignition energy to initiate combustion is less for hydrogen than for natural gas.

7. Detonation can occur between hydrogen-air mixture between 18 and 59 percent. The internal combustion engine on hydrogen fuel can work from very rich (excess fuel) to very lean (excess air) mixture. The adjustment of air fuel ratio is less critical than for gasoline engine.

8. Mixture of hydrogen and air are combustible over a wide range of com­position. The flammability limits are from 4 to 74 percent by volume of hydrogen in air at ordinary temperatures.

9. The combustion of hydrogen with oxygen from air results in release of energy and water as by-product.

10. The burning process of hydrogen is pollution free.

The possible areas of use of hydrogen in the near future are as follows:

1. Production of Useful Heat:

(i) In a high-temperature combustion of hydrogen with oxygen or air.

(ii) In a low-temperature flameless catalytic combustion with extremely low NO x emission.

2. Power Generation:

(i) In reactors with direct steam generation.

(ii) In high-temperature and membrane fuel cells.

3. Cogeneration of Heat and Electricity:

(i) In internal combustion engine based cogeneration plants.

(ii) In combined-cycle power plants.

4. Automotive and Aircraft Fuel:

(i) Environmentally friendly fuel for motor vehicles.

(ii) Aircraft fuel.

5. Energy Storage:

(a) Compressed hydrogen gas storage under high pressure.

(ii) Metal hydrides.

6. Synthesis of Fuels:

(i) Raw material to produce methanol, ammonia or hydrocarbon using carbon dioxide or nitrogen from air.

(ii) Raw material for manufacture of gaseous fuels.

Essay # 3. Hydrogen Energy for Air and Surface Transport:

1. Jet Fuel:

The high energy density 33.3 kWh/kg of liquid hydrogen against 12.7 kWh/kg of conventional jet fuel is the main advantage in air transportation where hy­drogen energy can be used. Although volume of liquid hydrogen would be greater than regular fuel but this could be accommodated on a large aircraft.

The cold liquid hydrogen can also be used directly or indirectly for cooling engine and airframe surfaces of high speed aircrafts. Liquid hydrogen may be the only practical fuel for hypersonic aircraft when developed. Because of smaller total weight, it may be possible to achieve shorter take-off runs, steeper climbing path, smaller engine thrust and less noise production. `

The favorable diffusion properties and high thermal conductivity of hydrogen help to use shorter combustion chambers. Wide range of ignition for H 2 – O 2 mixtures (5% to 75%) by volume of hydrogen helps better control of engine operation especially under part load conditions and reduction in NO x emission.

The heat required to vaporize hydrogen for the engines can be obtained from certain outer skin of wings and fuselage. This helps to cool the boundary layer. The laminar boundary layer so developed helps to reduce drag and fuel con­sumption.

The main problems are economic production of hydrogen, infrastructure for fueling of aircrafts and sitting of bulky hydrogen tanks in the aircraft body.

2. Road Vehicles:

The use of hydrogen fuel in engines of automobiles, buses, trucks and farm machinery can help conserve petroleum products and reduce atmospheric pol­lution. A mixture of hydrogen gas and air of constant ratio is introduced into the manifold. The engine speed and power are controlled by varying the quality of mixture entering the cylinder with the help of a throttle valve.

In another design, hydrogen gas under pressure can be directly injected into the engine cylinder through a valve and air is admitted through another intake valve. The spark advance has to be retarded because of higher speed of flame of hydrogen in air. The engine emission will not contain carbon monoxide and hydrocarbon because the only product of combustion is H 2 O. In order to control NO x emis­sion, the cylinder exhaust containing H 2 O is injected to reduce combustion temperature.

Storage of hydrogen as compressed gas or metal hydrides for the vehicles is a dis-advance against hydrogen fuel because of lesser energy density per unit volume and the weight of metal hydrides is also excessive. The best way to use hydrogen as vehicle fuel is the use of fuel cells. The electricity generated in the fuel cells operating with hydrogen fuel can be uti­lized to operate electric motors for propelling the vehicle.

Essay # 4. Power Generation Using Hydro Energy:

1. Central Power Plants:

The plants working on natural gas can be changed to hydrogen-fired plants without significant technological changes. These plants may be combined-cycle power plants with gas and steam turbines or cogeneration plants with gas turbine and internal combustion engines. Hydrogen-fired plants compared to gas-fired plants have lower capital cost and higher efficiency.

Hydrogen fuel has higher combustion velocity and flame temperature as compared to natural gas. Higher combustion velocity results in unstable com­bustion and higher flame temperature leads to higher No x emissions.

2. Autonomous Power Plants:

Hydrogen fuelled fuel cells can be used for domestic power generation as well as industrial power generation. Alkaline fuel cells can be used for produc­ing electricity from pure hydrogen and oxygen. Fuel cells with acid electrolytes can be operated with impure hydrogen and hydrocarbons. High temperature fuel cells with molten carbonate electrolytes operating at 600°C are advanced generation fuel cells for electricity and heat production in cogeneration plants.

Essay # 5. Miscellaneous Applications of Hydro Energy:

Hydrogen can be used for domestic cooking replacing LPG. The burner design has to be changed with bigger holes and air supply system to take care of greater flame speed and low specific energy per unit volume when hydrogen is used as fuel.

Hydrogen can be usefully used in radiant space heating with flameless combustion on a catalytic surface. In this case combustion temperature is low with negligible NO x formation.

Hydrogen has many advantages over industrial gases for production of heat and other uses.

Essay # 6. Hydrogen Storage and Distribution:

There are five principle methods that have been considered for hydrogen storage:

1. Compressed gas storage. Hydrogen is conveniently stored for many applications in higher pres­sure cylinders. The method is rather expensive and bulky.

2. Liquid storage as cryogenic storage in vacuum insulated or super in­sulted storage tanks. The liquid hydrogen fuel used as rocket propellant in the space programme is stored in large tanks.

3. Line packs system where it is allowed to vary the pressure in the trans­mission and distribution system.

4. Underground storage of hydrogen gas in depleted oil and gas fields or in aquifer systems. This is the cheapest way to store large amounts of hydrogen for subsequent distribution.

5. Storage as metal hydrides in chemically bound form. A number of metals and alloys form solid compounds by direct reaction with hydrogen gas. The metal hydrides can be transported in solid form. When the hydride is heated, hydrogen is released for use.

Hydrogen Transportation:

1. Long distance hydrogen gas transmission pipelines of lengths greater than 90 km must be supplied with booster compressors. Therefore, the cost of transmitting hydrogen by pipelines must include the cost of pip­ing, compressor and power consumption by compressors. Another prob­lem of hydrogen transmission is hydrogen embrittlement of the pipeline materials.

2. Hydrogen in bulk can be transported and distributed as the liquid in double-walled insulated tanks. Distribution of liquid hydrogen by pipe­lines, jacketed with liquid nitrogen can also be considered.

3. Hydrogen can also be transported as a solid metal hydride. The main drawback is the heavy weight of hydride relative to its hydrogen yield.

Safety Precautions:

Hydrogen is highly inflammable and explosive and can lead to fire and serious accidents. The production, storage and distribution of hydrogen require special precautions.

1. The system should be designed to withstand the explosion pressures.

2. The system should be designed to withstand pressure surges.

3. Proper exposition-relief system must be provided.

4. Flame traps, flame suppressors, explosion-relief devices and rapid-closing devices must be used.

5. The design, manufacture, storage should follow Petroleum Act.

Essay # 7. Hydrogen Production:

1. From Fossil Fuels:

The conventional hydrogen production processes are shown in Fig. 17.1.

A. Natural Gas/Naptha:

1. The natural gas or naptha is reformed with steam at 900°C to produce a mixture of gases by the following reaction:

Hydrogen is produced by electrolytic dissociation of water.

Hydrogen as a Form of Energy

• Categories: Hydrogen

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Article Contents

Introduction, 1 overview of green hydrogen production, 2 energy transition with green hydrogen, 3 the perspective of green hydrogen energy, 4 conclusions, acknowledgements, conflict of interest statement, data availability, green hydrogen energy production: current status and potential.

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Ali O M Maka, Mubbashar Mehmood, Green hydrogen energy production: current status and potential, Clean Energy , Volume 8, Issue 2, April 2024, Pages 1–7, https://doi.org/10.1093/ce/zkae012

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The technique of producing hydrogen by utilizing green and renewable energy sources is called green hydrogen production. Therefore, by implementing this technique, hydrogen will become a sustainable and clean energy source by lowering greenhouse gas emissions and reducing our reliance on fossil fuels. The key benefit of producing green hydrogen by utilizing green energy is that no harmful pollutants or greenhouse gases are directly released throughout the process. Hence, to guarantee all of the environmental advantages, it is crucial to consider the entire hydrogen supply chain, involving storage, transportation and end users. Hydrogen is a promising clean energy source and targets plan pathways towards decarbonization and net-zero emissions by 2050. This paper has highlighted the techniques for generating green hydrogen that are needed for a clean environment and sustainable energy solutions. Moreover, it summarizes an overview, outlook and energy transient of green hydrogen production. Consequently, its perspective provides new insights and research directions in order to accelerate the development and identify the potential of green hydrogen production.

Nowadays, the technology of renewable-energy-powered green hydrogen production is one method that is increasingly being regarded as an approach to lower emissions of greenhouse gases (GHGs) and environmental pollution in the transition towards worldwide decarbonization [ 1 , 2 ]. However, there is a societal realization that fossil fuels are not zero-carbon, which leads to significant thinking about alternative solutions.

The global energy system ought to drastically change from one mostly reliant on fossil fuels to one that is effective and sustainable with low carbon emissions to meet the goals of the Paris Agreement. Accordingly, >90% is the required global CO 2 emission decrease and the projected direct contribution of renewable energy to the necessary emission decrease is 41% [ 3 , 4 ]. Hydrogen (H 2 ) is a cost-effective, environmentally friendly alternative for energy consumption/storage [ 5 , 6 ]. In addition, it can contribute to making a low-carbon society a reality and largely boost the share of hydrogen [ 7 ].

Hydrogen technologies have been considered an approach to strengthening various economic sectors since the COVID-19 pandemic. The potential of hydrogen is currently the subject of an important consensus, partly due to an increased ambitious climate policy [ 8 , 9 ]. In addition, hydrogen can be used in fuel cell technology in the power generation sector and many other sectors, such as industry, transport and residential applications, which reflects its potential for decarbonization [ 10–12 ].

Several initiatives and projects worldwide are rapidly rising, reflecting the outstanding political and commercial momentum that the development of hydrogen as a zero-carbon fuel is undergoing. The growing boost is caused by the decreasing cost of hydrogen produced by renewable energy sources, or ‘green hydrogen’, and the urgent need to reduce GHG emissions [ 3 , 13 ]. However, green hydrogen is expected to increase in prominence over the next few decades and attain high commercial viability [ 13 , 14 ]. Producing hydrogen can be done using coal, methane, bioenergy and even solar energy; however, green hydrogen production is one of the pathways [ 15 , 16 ].

Numerous countries consider hydrogen the next-generation energy management response, and they increasingly support adopting hydrogen technology intended to create a decarbonized economy. Therefore, many strategies and plans for developing and implementing hydrogen have been made [ 17 ].

By 2050, according to Anouti et al. [ 18 ], there could be 530 million tonnes (Mt) of demand globally for green hydrogen, or hydrogen produced with fewer carbon dioxide emissions. Consequently, it would displace ~10.4 billion barrels of oil, which is equivalent to ~37% of the pre-pandemic world oil production [ 18 , 19 ]. Based on its forecast, the worldwide market for green hydrogen exports may be worth $300 billion annually by 2050, creating ~400 000 jobs in the hydrogen and renewable-energy industries [ 18 ].

Based on the technique used to produce hydrogen, the energy source used and its effects on the environment, hydrogen is categorized into various colour shades, including blue, grey, brown, black and green [ 20 ]. Using the steam-reforming/auto-thermal reforming method, grey hydrogen is extracted from natural gas but CO 2 is emitted into the atmosphere as a by-product. When the steam-reforming method converts natural gas into hydrogen and the CO 2 emissions from the process are captured, this is known as blue hydrogen. The most prevalent type of hydrogen used today is brown hydrogen, mainly produced via the gasification of hydrocarbon-rich fuel, in which CO 2 is released into the atmosphere as a by-product. However, green hydrogen is produced by water electrolysis, which is powered by renewable energy resources [ 18 , 21 , 22 ].

Green hydrogen is already competitive in regions with all the appropriate conditions [ 15 ] and will play a significant role in achieving sustainable development goals (SDGs) for the UN 2030, based on the agenda for sustainable development adopted wholly by UN Member States. The specified section of SDG 7 depends on ‘Affordable and Clean Energy’ [ 23 , 24 ]. For this reason, many efforts have been made to attain this goal globally in recent years.

Therefore, continuing on from those issues mentioned above in the introduction, in this paper, we analyse green hydrogen production technologies and investigate several aspects of the significance of the growth of the green hydrogen economy (GEE). The key objective of this study is to highlight the potential and progress of green hydrogen production and its significance in meeting energy needs. The paper is organized as follows. Section 1 summarizes the introduction, Section 2 presents an analysis of the energy transition with green hydrogen, Section 3 details a general overview of green hydrogen production, Section 4 specifics the perspective of green hydrogen energy production and Section 5 summarizes the conclusions and recommendations for future work.

There are several uses for hydrogen, including energy storage, power generation, industrial production and fuel for fuel cell vehicles. Hence, hydrogen production from green energy sources is essential to meet sustainable energy targets (SETs) as the globe attempts to move to a low-carbon economy.

Green hydrogen production requires large amounts of renewable energy and water resources. Thus, areas with an abundance of renewable energy resources, as well as accessibility to water sources, have been determined to be optimal for producing huge amounts of green hydrogen. However, to allow green hydrogen to be more economically viable than fossil fuels, advances in technology and cost reductions must be made.

In order to achieve the target for the expansion of green hydrogen production and utilization, details ought to be established at the level of the authorities. They can facilitate adoption, on the one hand, by increasing manufacturing capacity and guaranteeing an ongoing renewable energy source and, on the other, by increasing the need for green hydrogen alongside its derivatives and developing a system for storing and transporting hydrogen [ 25 ].

This paper performed a literature review to screen >100 papers related to Google Scholar/Web of Science to consider precisely green energy production by filtering the information in a large number of literature papers in science databases. Figs 1 and 2 illustrate the visualized literature network diagrams; hence, searching for keywords in science databases maps the intensity of relations/strengths among items. The analysis, which determined the research relationships of networks for visualization and exploration, utilized the VOSviewer. The categorical evaluation relies on the occurrence and frequency of keywords in related publications. The red cluster (lower left) represents initial development words trend links, the blue cluster (upper center) represents the second stage of development and the green cluster (lower right) links the green hydrogen words. Fig. 1 displays and signifies the mapping of the intensity of relations among words. In recent years, more research has focused on developing green hydrogen production from 2016 to 2023. Fig. 2 elucidates the keywords of scientific mapping and field trends. The blue cluster (lower left) represents the trend of research development from 2016 to 2019 and the bright maroon cluster (upper right) represents the trend of research development from 2020 to 2023.

Characterizes scientific mapping and relations between words

Characterizes keywords of scientific mapping and developing field trends from 2016 to 2023

The technology of green hydrogen can play a vital role in energy storage. Electrolysis can be utilized for producing hydrogen by using a surplus of renewable energy produced when demand is low. Whenever required, hydrogen can be used directly in various applications or stored and subsequently turned back into power using fuel cells. Hydrogen can be stored in different ways, either in the form of liquid, gaseous fuel or solid state; thus, the storage method is determined based on the consumption approach or export. In addition to resources such as solar and wind, this makes it possible to integrate renewable energy into the grid. This may lower the overall cost of the hydrogen yield.

Long-haul transportation, chemicals, and iron and steel are only a few industries that can benefit from the decarbonization of clean hydrogen produced using renewables, fossil fuels, nuclear energy or carbon capture. These industries have had difficulty in reducing their emissions. Vehicles fuelled by hydrogen would enhance the security of energy and the quality of air. Although it is one of the few alternative energy sources that can store energy for days, weeks or months, hydrogen can facilitate the incorporation of various renewable energies into the electrical grid.

Hydrogen storage technology, either underground or surface storage, gives more effectiveness and is more reliable to utilize; also, storage on a large scale has advantages in terms of energy demand and flexibility of the energy system [ 26 ]. The important consideration of storing hydrogen efficiently and safely is vital for many applications, such as industrial processes and transportation.

The transition towards green hydrogen will create new job opportunities in several sectors, including manufacturing, fuel cells, infrastructure, and operation and maintenance of electrolysers. Moreover, the development of the green hydrogen sector has the potential to promote economic growth, produce income through exports, bring in investments and drive scientific breakthroughs in the field.

Green hydrogen technological progress is the focus of ongoing studies and developments. Hence, this encompasses enhancing the effectiveness of electrolysis procedures, making affordable fuel cells, investigating cutting-edge materials for hydrogen storage and raising the overall efficacy of hydrogen systems. The range of applications for green hydrogen will grow due to technological improvements that will lower costs, boost effectiveness and expand their usage. State-of-the-art electrolyser devices and their development are based on decreasing the cost of manufacturing, enhancing efficiency and increasing the role played by electrolysis in the global hydrogen economy.

However, before worldwide commerce in hydrogen becomes a feasible, affordable option on a large scale, numerous milestones must be accomplished. The key is a techno–economic analysis used to investigate the circumstances required for such a trade to be profitable. The scenarios are for predicting the hydrogen trade outlook towards 2050 in which hydrogen production and costs of transportation are accessible. The trade of hydrogen is expected to develop in local markets to a great extent.

Based on a global plan through a ‘pathway toward decarbonization and net-zero emissions via 2050’ in the 1.5°C scenario, ~55% of the hydrogen traded globally by 2050 will be transported through a pipeline. The vast majority of the hydrogen network would rely on already-built natural gas pipelines that can be converted to transport pure hydrogen, greatly lowering the cost of transportation [ 27 , 28 ]. Hence, if we examine the economic and technological production capability of green hydrogen globally over various scenarios, we can evaluate the prognosis for the global hydrogen trade in 2030 and 2050 [ 27 ].

Progress and optimization of the hydrogen supply chain are important for comprehending the potential of hydrogen as a sustainable and clean energy carrier. Moreover, socio-economic aspects through providing a labour market can extend to the supply chain by deploying/installing renewable-energy devices. Thus, as technology and infrastructure continue to develop, the hydrogen supply chain is anticipated to play a substantial role in the shift to a low-carbon energy system.

Further outlook of green hydrogen to extend knowledge to include outreach approaches incorporating hydrogen-related topics into the curriculum might include online sources, community workshops and collaborations with educational institutions.

Accordingly, many factors have led numerous countries to endorse adopting green hydrogen technology projects. These aim to create a decarbonized economy and reduce GHG emissions, considering hydrogen as an alternative for sustainable energy management. Table 1 summarizes the breakdown of recently announced ongoing investment projects in green hydrogen production.

List of large green hydrogen planned/ongoing projects

Achieving the 1.5°C scenario includes a commercially viable form of large-scale production of hydrogen and commerce. The electricity needed for the production of hydrogen should be adequate and not take away from the electricity needed for other vital and more productive purposes. Thus, this leads to increased scale and acceleration of renewable-energy development at the core of the transition to green hydrogen.

Green hydrogen has the potential to play a crucial role in the development of a cleaner and more sustainable energy future as costs decrease, technology improves and supportive policies are put in place [ 34 ]. Fig. 3 depicts a potential pathway for producing hydrogen from green energy resources. An environmentally friendly renewable-energy supply, so-called biogas, is produced whenever organic matter, including food scraps and animal waste, breaks down. The biomass gasification of organic materials or agricultural waste can be gasified in a controlled environment to harvest a mixture of hydrogen. The biogas produced may be used to generate energy, heat houses and fuel motor vehicles.

Potential pathway for producing hydrogen from green energy

Electrolysis is a procedure that uses electrolysers to separate water into hydrogen and oxygen, utilizing electricity produced by renewable sources such as solar technology, including photovoltaic (PV) and concentrating solar power (CSP), wind or hydropower. The hydrogen produced can then be used for numerous purposes, such as fuel cells or industrial processes, or it can be stored. The basic production of hydrogen via electrolysis using electricity to split molecules in water into hydrogen and oxygen is given by:

It is important to mention that another method—the so-called photoelectrochemical (PEC) hydrogen production technique—depends on the use of solar radiation to drive the water-splitting process directly; PEC cells transform solar energy into hydrogen [ 35 , 36 ]. Although this technology is still in its infancy, it indicates promise for producing hydrogen sustainably and effectively [ 35 ].

Owing to their capability for photosynthetic oxygen production, algae have been recommended as a potential resource for the production of green hydrogen. Some types of algae can also produce ‘hydrogen gas as a by-product of their metabolism’ under certain conditions. Green hydrogen production from algae is based on the biohydrogen production technique, which is a subject of interest and ongoing study [ 37 , 38 ]; however, it is not commonly used in industrial practice yet [ 39–41 ].

Electrolysers ought to function at a higher usage rate to reduce the expenses of producing hydrogen, although this is incompatible with the curtailed supply of restricted energy [ 42 ]. Several research publications suggested the idea of using direct seawater electrolysis to produce hydrogen and oxygen [ 43–45 ].

The shift towards clean energy using green hydrogen necessitates collaboration among industries, governments, communities and research institutions. It offers a chance to increase sustainable growth, diversify sources of energy and decrease emissions of GHGs [ 14 ]. Table 2 details the world’s green hydrogen production capacity (in EJ) and potential by region distributed on continents. The top high potential was in sub-Saharan Africa, at ~28.6%, followed by the Middle East and North Africa, at ~21.3%. Then, the following other regions across the continent are listed.

Breakdown of the potential of global green hydrogen production by region [ 46 ]

Green hydrogen, from an economic perspective, represents a large economic opportunity. It includes the potential to promote the growth of new industries, the creation of employment opportunities and economic expansion. Thus, countries with abundant renewable energy resources can use green hydrogen generation to export energy, diversify their economy and lower their dependency on fossil fuels.

The production of hydrogen can assist in reducing curtailed systems that use a significant amount of variable energy from renewable sources [ 42 ]. Herein, green hydrogen is considered a technological development catalyst from a technical development perspective. Technology advances in the field are anticipated to result from research and development initiatives to increase electrolysis efficiency, lower costs and create improved materials and methods. This perspective highlights the innovative potential and development of green hydrogen technology.

Moreover, green hydrogen is considered an essential catalyst of the energy shift from the perspective of that transition. Subsequently, clean energy sources such as wind and solar power provide a method of integrating and balancing energy from renewable sources. Green hydrogen may increase the shares of clean energy sources in the energy system by offering grid flexibility and long-term energy storage.

It is clear that the movement towards the global transition is accelerating based on the energy transition policies and carbon-neutrality targets of different nations [ 47 ]. The investments in green hydrogen projects are progressing and taking place globally, including the USA, Germany, Austria, Saudi Arabia and China, to name a few. These countries have taken a step forward towards implementing large-scale projects of green hydrogen [ 15 , 42 ].

Energy from hydrogen can be utilized in numerous fields encompassing industry, electricity, construction, transportation, etc. [ 47 ]. Fig. 4 elucidates the schematic flow of perspectives on green hydrogen production. The demand for green hydrogen has recently evolved since more recent sources have become the latest insights on its current status and projections. The need for green hydrogen is anticipated to increase over the coming years as green technologies develop and the urgency to battle climate change grows. The demand is also needed for environmental aspects of climate change mitigation, decarbonization, technological developments and policy support.

Green hydrogen production perspectives

A study reported that hydrogen has a significant potential role in supporting the globe in meeting decarbonization goals/net-zero emissions by 2050 and limiting the global warming phenomenon to 1.5°C because it can reduce ~80 GT (gigatonnes) of CO 2 emissions by 2050 [ 48 ].

The potential of green hydrogen relies on geographic location and abundant natural resources. Hence, water, solar energy, wind and hydro-energy and organic materials are available. The development in infrastructure enables the widespread implementation of green hydrogen and important infrastructure progress is required. It comprises establishing hydrogen refuelling and building electrolysis plants, storage systems, etc.

Furthermore, investment projects would be viable in desert areas, where large projects might be constructed using solar PV and CSP to generate electricity. Subsequently, electricity can be used to produce enough hydrogen for the local market and export the surplus. Hence, these will help economic development in countries with great potential for solar radiation intensity over the years.

The economies of scale enabled via a developing global market for clean energy sources and green hydrogen will continue to drive down overall expenses [ 29 ]. However, the most economical way to use green financing will be to focus on helping the initial phases of the expansion of green hydrogen generation during a period when the investment takes place [ 49 ]. The investment cost is the main aspect to be considered while designing a hydrogen plant. Therefore, a core desired feature is low-levelized energy costs from renewable energy resources and electrolysers. These will make the project more feasible, efficient and cheap for the production of green hydrogen. The environmental impact of green hydrogen production is a key tool for attaining global climate goals—the potential to guarantee a more sustainable and environmentally friendly future for our planet.

This paper summarizes the outline of green hydrogen, its contribution and its potential towards net-zero emissions. Hence, its viewpoint provides new insights to accelerate the expansion of green hydrogen production projects. In order to accelerate the implementation of green hydrogen, scholars, industries and governments worldwide will contribute to the research and development of the technology. It is considered a feasible option for lowering emissions of GHGs, encouraging energy independence and helping in shifting to a low-carbon, environmentally friendly energy system.

There has been development of hydrogen technology that has significantly progressed to meet energy needs. Therefore, green hydrogen yield, which depends on renewable energy resources, has recently become a more attractive option due to decreased expenditure. Thus, it has the potential to mitigate environmental issues, promote economic expansion and contribute to the transition of the entire world to sustainable and clean energy systems. To adequately realize the potential of green hydrogen, challenges, including lower expenses, development of infrastructure and industrial scale, remain important factors.

A worldwide market for green hydrogen could emerge, enabling assignees with abundant renewable resources to export surplus electricity in the form of hydrogen. Therefore, this could assist countries in switching to a more sustainable energy mix and decrease their dependence on fossil fuel imports. Future work includes developing/deep analysis of a cost-effective, high-efficiency electrolyser device that will decrease the overall cost of green hydrogen yield.

Many grateful thanks go to the Libyan Authority for Research Science and Technology, and many thanks go to the staff in the Libyan Centre for Research and Development of Saharian Communities. Also, thanks to the anonymous reviewers for their constructive comments in improving this paper.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data sharing does not apply to this perspective paper, as no new data sets were created during this research.

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Hydrogen is a clean fuel that, when consumed in a fuel cell, produces only water. Hydrogen can be produced from a variety of domestic resources, such as natural gas, nuclear power, biomass, and renewable power like solar and wind. These qualities make it an attractive fuel option for transportation and electricity generation applications. It can be used in cars, in houses, for portable power, and in many more applications.

Hydrogen is an energy carrier that can be used to store, move, and deliver energy produced from other sources.

Today, hydrogen fuel can be produced through several methods. The most common methods today are natural gas reforming (a thermal process), and electrolysis. Other methods include solar-driven and biological processes.

Thermal Processes

Thermal processes for hydrogen production typically involve steam reforming, a high-temperature process in which steam reacts with a hydrocarbon fuel to produce hydrogen. Many hydrocarbon fuels can be reformed to produce hydrogen, including natural gas, diesel, renewable liquid fuels, gasified coal, or gasified biomass. Today, about 95% of all hydrogen is produced from steam reforming of natural gas.

Learn more about:

• Natural gas reforming
• Coal gasification
• Biomass gasification
• Reforming of renewable liquid fuels .

Electrolytic Processes

Water can be separated into oxygen and hydrogen through a process called electrolysis. Electrolytic processes take place in an electrolyzer, which functions much like a  fuel cell  in reverse—instead of using the energy of a hydrogen molecule, like a fuel cell does, an electrolyzer creates hydrogen from water molecules.

Learn more about  electrolytic hydrogen production .

Solar-Driven Processes

Solar-driven processes use light as the agent for hydrogen production. There are a few solar-driven processes, including photobiological, photoelectrochemical, and solar thermochemical. Photobiological processes use the natural photosynthetic activity of bacteria and green algae to produce hydrogen. Photoelectrochemical processes use specialized semiconductors to separate water into hydrogen and oxygen. Solar thermochemical hydrogen production uses concentrated solar power to drive water splitting reactions often along with other species such as metal oxides.

Learn more about  photobiological processes ,  solar thermochemical processes , and  photoelectrochemical processes .

Biological Processes

Biological processes use microbes such as bacteria and microalgae and can produce hydrogen through biological reactions. In microbial biomass conversion, the microbes break down organic matter like biomass or wastewater to produce hydrogen, while in photobiological processes the microbes use sunlight as the energy source.

Learn more about biological hydrogen production from microbial biomass conversion and  photobiological processes .

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Hydrogen: A renewable energy perspective

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Hydrogen has emerged as an important part of the clean energy mix needed to ensure a sustainable future. Falling costs for hydrogen produced with renewable energy, combined with the urgency of cutting greenhouse-gas emissions, has given clean hydrogen unprecedented political and business momentum.

This report is also available in Japanese     (日本語) . 

This paper from the International Renewable Energy Agency (IRENA) examines the potential of hydrogen fuel for hard-to-decarbonise energy uses, including energy-intensive industries, trucks, aviation, shipping and heating applications. But the decarbonisation impact depends on how hydrogen is produced. Current and future sourcing options can be divided into grey (fossil fuel-based), blue (fossil fuel-based production with carbon capture, utilisation and storage) and green (renewables-based) hydrogen. Green hydrogen produced through renewable-powered electrolysis is projected to grow rapidly in the coming years.

Among other findings:

• Important synergies exist between hydrogen and renewable energy. Hydrogen can boost renewable electricity market growth and broaden the reach of renewable solutions.
• Electrolysers can add demand-side flexibility. In advanced European energy markets, electrolysers are growing from megawatt to gigawatt scale.
• Blue hydrogen is not inherently carbon free. This type of production requires carbon-dioxide (CO 2 ) monitoring, verification and certification.
• Synergies may exist between green and blue hydrogen deployment, given the chance for economies of scale in hydrogen use or logistics.
• A hydrogen-based energy transition will not happen overnight. Hydrogen use is likely to catch on for specific target applications. The need for new supply infrastructure could limit hydrogen use to countries adopting this strategy.
• Dedicated hydrogen pipelines have existed for decades and could be refurbished along with existing gas pipelines. The implications of replacing gas abruptly or changing mixtures gradually should be further explored.
• Trade of energy-intensive commodities produced with hydrogen, including “e-fuels” could spur faster uptake or renewables and bring wider economic benefits.

Additional analyses

Green hydrogen strategy: a guide to design, sub-saharan africa: policies and finance for renewable energy deployment, international co-operation to accelerate green hydrogen deployment, 100% renewable energy scenarios: supporting ambitious policy targets, green hydrogen for sustainable industrial development: a policy toolkit for developing countries, related content.

Key Enablers to Triple Renewables by 2030: Policy and Regulations

The Role of Sustainable Bioenergy in Supporting Climate and Development Goals

Record Growth in Renewables, but Progress Needs to be Equitable

Innovative Policymaking is Crucial to Drive Green Hydrogen Market and Ensure its Sustainable Production

IRENA-WTO report highlights role of trade in developing green hydrogen markets

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For Many, Hydrogen Is the Fuel of the Future. New Research Raises Doubts.

Industry has been promoting hydrogen as a reliable, next-generation fuel to power cars, heat homes and generate electricity. It may, in fact, be worse for the climate than previously thought.

By Hiroko Tabuchi

It is seen by many as the clean energy of the future. Billions of dollars from the bipartisan infrastructure bill have been teed up to fund it.

But a new peer-reviewed study on the climate effects of hydrogen, the most abundant substance in the universe, casts doubt on its role in tackling the greenhouse gas emissions that are the driver of catastrophic global warming .

The main stumbling block: Most hydrogen used today is extracted from natural gas in a process that requires a lot of energy and emits vast amounts of carbon dioxide. Producing natural gas also releases methane, a particularly potent greenhouse gas.

And while the natural gas industry has proposed capturing that carbon dioxide — creating what it promotes as emissions-free, “blue” hydrogen — even that fuel still emits more across its entire supply chain than simply burning natural gas, according to the paper, published Thursday in the Energy Science & Engineering journal by researchers from Cornell and Stanford Universities.

“To call it a zero-emissions fuel is totally wrong,” said Robert W. Howarth, a biogeochemist and ecosystem scientist at Cornell and the study’s lead author. “What we found is that it’s not even a low-emissions fuel, either.”

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Hydrogen production, storage, utilisation and environmental impacts: a review

• Open access
• Published: 06 October 2021
• Volume 20 , pages 153–188, ( 2022 )

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• Ahmed I. Osman   ORCID: orcid.org/0000-0003-2788-7839 1 ,
• Neha Mehta 1 , 2 ,
• Ahmed M. Elgarahy 3 , 4 ,
• Mahmoud Hefny 5 , 6 ,
• Amer Al-Hinai 7 ,
• Ala’a H. Al-Muhtaseb 8 &
• David W. Rooney 1  

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A Correction to this article was published on 31 March 2022

This article has been updated

Dihydrogen (H 2 ), commonly named ‘hydrogen’, is increasingly recognised as a clean and reliable energy vector for decarbonisation and defossilisation by various sectors. The global hydrogen demand is projected to increase from 70 million tonnes in 2019 to 120 million tonnes by 2024. Hydrogen development should also meet the seventh goal of ‘affordable and clean energy’ of the United Nations. Here we review hydrogen production and life cycle analysis, hydrogen geological storage and hydrogen utilisation. Hydrogen is produced by water electrolysis, steam methane reforming, methane pyrolysis and coal gasification. We compare the environmental impact of hydrogen production routes by life cycle analysis. Hydrogen is used in power systems, transportation, hydrocarbon and ammonia production, and metallugical industries. Overall, combining electrolysis-generated hydrogen with hydrogen storage in underground porous media such as geological reservoirs and salt caverns is well suited for shifting excess off-peak energy to meet dispatchable on-peak demand.

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• Environmental Chemistry

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Introduction

The continual growth and rapid urbanisation of the world population and economy have resulted in an enormous increase in energy need, urging the switch from fossil-based fuels into alternative clean renewables (Dawood et al. 2020 ). Consequently, global decarbonisation in the transportation, industry and electricity generation sectors is crucially needed to mitigate anthropogenic climate change (Fawzy et al. 2020 ; Osman et al. 2021a ). In this context, there has been a growing interest from scholars and industries with versatile production routes. There is abundant availability of renewable sources used in hydrogen production; however, the variable and intermittent nature of these resources is the major challenge in the transition towards a hydrogen economy. Hence, this calls for technical accommodation, especially for balancing variable renewable supply, i.e. solar, wind and others, and varying energy demand. Furthermore, cost-effective production methods, policies, research and development and hydrogen infrastructure development are areas that need more investigation when transitioning towards the hydrogen economy.

More than 100 current and planned hydrogen production technologies are reported to date, with over 80% of those technologies are focused on the steam conversion of fossil fuels and 70% of them are based on natural gas steam reforming. However, in order to minimise carbon footprint emissions, a wider range of hydrogen extraction processes, such as methane pyrolysis and seawater electrolysis using alternative energy sources, must be addressed. All hydrogen production routes are highlighted in Fig.  1 .

Hydrogen production routes, including renewables, fossil fuels and nuclear, with hydrogen being produced in power plants, pharmaceutical applications, synthetic fuels or their upgrades in transportation, ammonia synthesis, metal production or chemical industry applications

Hydrogen is the most abundant element in the universe, and due to its reactivity, it only exists on earth in compounds such as water and organic materials. It is an odourless, flammable and colourless gas, which is leading to its safety concern, especially if a leak is not detected and gas collects in a confined area; it can ultimately ignite and causes explosions. Furthermore, metal hydrogen embrittlement is an issue as it could damage pipelines and containers due to its small molecular size; thus, it escapes through materials. The higher heating value (HHV) of hydrogen is 141.8 MJ/kg at 298 K, and the lower heating value is 120 MJ/kg at the same temperature. This is significantly higher than that of most fuels such as gasoline with a value of 44 MJ/kg at 298 K. However, liquid hydrogen has a lower energy density by volume than hydrocarbon fuels such as gasoline by a factor of four with a density of 8 MJ/l versus density of 32 MJ/l. While hydrogen gas has a high energy density by weight but a low energy density by volume compared to hydrocarbons, it requires a larger tank to store. For example, as opposed to liquified natural gas, liquified hydrogen contains 2.4 times the energy but takes 2.8 times the volume to store. At the same time, the low temperature for liquified hydrogen storage at ambient pressure and a temperature of −253 °C raises quite a few risks. When exposed, it can cause cold burns; furthermore, leakage can result in a combination of liquefied air and hydrogen, resulting in an explosive mixture or the formation of flammable or explosive conduits (Atilhan et al. 2021 ; El-Halwagi et al. 2020 ).

Like electricity, hydrogen is an energy carrier and not an energy source; using it to store renewable energies instead of being wasted when not in use is crucial since it is storable, utilisable and transportable (Parra et al. 2019 ; Abe et al. 2019 ).

Hydrogen cleanness and colour coding

Dawood et al. (Dawood et al. 2020 ) reported the four main stages in hydrogen economy: production, storage, safety and utilisation, where hydrogen purification and compression (subsystems) need to be considered along with the life cycle assessment (LCA) when selecting the production method for hydrogen. Hydrogen cleanness level is described in the literature with many colour coding: mainly green, blue and grey, which relies only on the production route, i.e. hydrogen origin, and fails to assess the deep cleanness of the produced hydrogen (Merzian and Bridges 2019 ), for instance: (1) Grey hydrogen is produced using fossil fuels such as natural gas, one tonne of hydrogen produced in this way is responsible for 10 tonnes of carbon dioxide (Dvoynikov et al. 2021 ), as shown in Fig.  2 ; (2) blue hydrogen is produced from fossil fuels like grey hydrogen but with combination of carbon capture and storage to mitigate emissions; (3) green hydrogen is typically produced from 100% renewable sources such as wind or solar energies with lower carbon footprint; (4) brown hydrogen is produced from gasification of coal-based fuel; and (5) turquoise hydrogen is produced from the thermal decomposition of natural gas, i.e. methane pyrolysis or cracking by spitting methane into hydrogen and carbon at a temperature range from 600 to 1200–1400 °C (Dvoynikov et al. 2021 ). This process produces black carbon (soot) as a by-product instead of carbon oxide emissions in the grey hydrogen, allowing for the sequestration of carbon emissions in the form of solid carbon. However, carbon stability in this black soot is critical for long-term carbon sequestration, along with the utilisation of renewable energy sources in the high-temperature process to achieve carbon neutrality. Interestingly, hydrogen could be produced with a negative carbon footprint via biogas pyrolysis.

Hydrogen colour coding for various manufacturing processes. Green hydrogen is produced using renewable energy sources such as solar or wind energy, followed by water electrolysis. Grey and brown hydrogen are produced by methane steam reforming and coal gasification, respectively, and when combined with carbon capture and storage, blue hydrogen is produced. Turquoise hydrogen is produced through the pyrolysis of methane, with solid carbon as a by-product

However, this colour coding is not precise as it assumes that green hydrogen always has low-carbon emission than blue or grey hydrogen, which is not applicable in all cases. Blue hydrogen, for example, is regarded as less safe than green hydrogen, even though it releases no carbon at the point of use or during the entire process, while green hydrogen may do. For instance, bioenergy feedstocks such as biomass emit greenhouse gas emissions such as CH 4 , SO x , NO x and CO 2 during their growth or thermochemical conversions. Furthermore, the carbon capture and storage technique used in the blue hydrogen reduces toxic emissions significantly. The manufacture of photovoltaic panels as renewable energy technology also has a significant carbon footprint and generates various types of waste, liquid and gaseous by-products that are hazardous to the environment. Starting from the extraction of quartz and other materials used to manufacture solar panels, this is coupled with the carbon and sulphur emission in the energy-intensive process when producing metallurgical silicon. Moreover, the solar panel has a 30-year lifespan, and then, it must be handled as a particular waste at its end of life.

A recent LCA study compared environmental impacts for steam methane reforming with water electrolysis using wind, solar photovoltaic, hydropower, solar thermal and biomass gasification as energy sources (Al-Qahtani et al. 2021 ). It was concluded that among all the technologies evaluated, solar photovoltaic electrolysis had the most damaging environmental implications because of the significant acidification potential in the photovoltaic panel production phase and the relatively poor efficiency of photovoltaic systems.

Thus, measuring the emitted greenhouse gas emissions accurately in the entire production process along with the life cycle of the equipment used is crucial. This is required to determine how green is the green hydrogen and how blue is the blue hydrogen. A recently proposed model for improved hydrogen colour coding consisted of a hydrogen cleanness index followed by the number of depth levels (Han et al. 2021 ). For instance, 80 green-4 means hydrogen is produced via renewable resources; however, it is not a zero-emission process, only 80% green, due to emissions related to the process. The number after the colour, which in this case is 4, indicates that greenhouse gas emissions (CO 2-e ) linked with the purification during the production route have been considered. This model still requires much more analysis to decide the start and end of the continuum thresholds for each colour, as well as the evaluation depth levels and related weight for each level.

Hydrogen production routes

According to the International Energy Agency (IEA), green hydrogen could help reduce our carbon footprint if major challenges such as infrastructure, logistics, cost-effective manufacturing methods and safety are overcome. Globally, hydrogen is responsible for about 843 metric tonnes of CO 2 emissions per annum, equal to the combined total emissions of the UK and Indonesia (IEA 2019 ). The global hydrogen demand is projected to increase from 70 million tonnes in 2019 to 120 million tonnes by 2024 (Global hydrogen market insights 2020 ; Atilhan et al. 2021 ; Safari and Dincer 2020 ). In 2025, the largest global green hydrogen plant will be built, with a capacity of 237,250 tonnes per annum, i.e. 650 tonnes/day hydrogen output through electrolysis and 4 gigawatts of renewable energy from wind, solar and storage.

A wide range of resources is available for hydrogen production, mainly fossil-based and renewable fuels (Dawood et al. 2020 ; Saithong et al. 2019 ; Osman et al. 2020 a). The former is the more mature and most common used industrially as it is a cost-effective method that deploys cracking or reforming fossil-based fuels. In 2016, hydrogen production globally was about 85 million tonnes used in petroleum, metal industry, fertiliser, food processing, semiconductor production, power plants and generations (Chen and Hsu 2019 ; El-Emam and Özcan 2019 ; Acar and Dincer 2019 ).

There are many ways to extract hydrogen from hydrogen-containing materials, either hydrocarbon or non-hydrocarbon, such as photonic, electric, chemical, bioenergy, heat and a combination of those methods together (Abe et al. 2019 ; El-Emam and Özcan, 2019 ; Osman et al. 2020 b). Table 1 shows different hydrogen production routes with different energy sources, technology readiness level (TRL) and their % energy efficiency.

Advances and challenges in water electrolysis

Water is typically purified and then sent to an electrolyser, which produces hydrogen and oxygen. The hydrogen is then dried, purified and compressed from a 10.3 to 413.7 bar pressure, and then stored in a tank. Although the electrolysis pathway offers a 100% renewable route for hydrogen production, it represents less than 5% of worldwide hydrogen production (Han et al. 2021 ). Despite this low percentage contribution, water electrolysis is gaining momentum for various reasons such as zero-carbon emissions, the absence of unwanted by-products such as sulphates, carbon oxides and nitrogen oxides, and high hydrogen purity. The cost of producing hydrogen through electrolysis would be reduced by approximately 70% over the next decade, allowing for the widespread adoption of a green hydrogen production approach.

By 2040, the worldwide market for hydrogen electrolysers is expected to have grown by 1000-fold. Aurora Energy Research predicted that about 213.5 gigawatts of projects will be completed over the next 19 years; this compares to an estimated 200 megawatt that is currently in service. They reported that 85 per cent of anticipated projects are in Europe, with Germany accounting for 23 per cent of expected global electrolyser capacity. The European Union has already set a goal of 40 gigawatts of electrolyser capability by 2030 (Research, 2021). If all this power is available, it will supply up to 32 million tons of hydrogen per year, which is already half of the currently demanded hydrogen. In a 1.5-degree climate change mitigation scenario, meeting 24% of energy demand with hydrogen will necessitate massive amounts of additional renewable electricity generation. To power electrolysers in this scenario, approximately 31,320 terawatt-hours of electricity would be required, i.e. more than is currently produced globally from all sources combined (BNEF 2020 ). Besides, an investment of more than $11 trillion in manufacturing, storage and transportation infrastructure would be required.

Proton exchange membrane (PEM) along with alkaline anion exchange membrane (AEM) and concentrated potassium hydroxide solution KOH are the most common techniques used in low-temperature water electrolysis. The key benefit of alkaline anion exchange membrane electrolysis over other methods is lower cost since no platinum group metals are used as catalysts herein. The main challenge, however, is the low rate of hydrogen production and the instability of the alkaline method owing to its susceptibility to pressure drop (Dvoynikov et al. 2021 ; Yu et al. 2019 ). A typical electrolysis system consists of two metal electrodes, an anode and a cathode, separated by a membrane and immersed in an electrolyte solution (Zhu et al. 2019 ). As an electric current flows through the solution, oxygen and hydrogen bubbles rise above the anode and cathode, respectively. Both electrodes are typically coated with a catalyst to reduce the amount of energy needed to liberate hydrogen from water.

However, large amounts of freshwater would be needed to generate hydrogen, and these supplies are already depleted worldwide; thus, the utilisation of seawater will be an option to overcome this issue. However, seawater utilisation in hydrogen production is associated with challenges such as the corrosion of chloride ions in seawater to the anode metal. Hung et al. reported a solution to this issue by designing the anode material as a porous nickel foam pan collector coated with an active and inexpensive nickel and iron catalyst, which showed strong conductivity and corrosion resistance. It is worth noting that, while using freshwater is more expensive than using seawater, the cost of water usually accounts for less than 2% of the total cost of hydrogen production via electrolysis (Milani et al. 2020 ). The affordability and accessibility of freshwater is one side of the coin, while inexpensive and sustainable green energy alternatives are the other, and the proximity of these two supplies, i.e. renewable energy and freshwater, does not always coincide. The main areas that need further investigation in water electrolysis are reducing the capital cost of electrolysis technology, finding water resources and increasing efficiency.

According to the recent literature summarised in Table 1 , membrane reactor technology is increasingly being recognised as an encouraging route to expand clean hydrogen production paths from hydrocarbons and hydrogen purification. At least 99.8% can be achieved without any gas purification using a proton exchange membrane analyser (Jorschick et al. 2021 ).

Recently, it was reported for Australia that the levelised cost of hydrogen (LCOH) for steam methane reforming could reach a cost of $(1.88–2.30)/kg H 2 and $(2.02–2.47)/kg H 2 for coal gasification production routes. In comparison, the LCOH via electrolysis technologies costs between $4.78 and $5.84/kg H 2 for alkaline electrolysis and $6.08–7.43/kgH 2 for proton exchange membrane technologies (Milani et al. 2020 ).

When using partial methane oxidation for hydrogen production via synthesis gas, the average cost is 1.33 euros/kg H 2 , while the cost of large-scale H 2 processing ranges between 1 and 1.5 euro/kg H 2 (Dvoynikov et al. 2021 ). It is important to note that the economic viability of using natural gas or related petroleum gas for hydrogen production should be seen in the light of transportation systems or the direct use of hydrogen on-site of the gas or oil plant.

In terms of blue hydrogen, carbon capture and utilisation lower greenhouse gas emissions but raise the overall production cost. Chemical looping reforming, for instance, has a comparatively short life cycle, global warming potential and low fossil fuel intake. Nevertheless, adding carbon capture and liquefaction process units raises the expense of the steam methane reforming by 18% and autothermal reforming processes by 2% (Atilhan et al. 2021 ). The process of liquefying hydrogen absorbs approximately 30% of the energy content of hydrogen. Additionally, keeping liquified hydrogen under one atmospheric pressure and at a low temperature of −253 °C is difficult. Furthermore, evaporation and leakage can occur even with robust insulation, losing typically 1 per cent of the stored volume per day (Atilhan et al. 2021 ).

Biomass gasification

Biomass gasification is seen as one of the most feasible, sustainable and potentially carbon-neutral alternatives to generate hydrogen (Saidi et al. 2020 ). Since biomass is a renewable feedstock that absorbs atmospheric carbon dioxide during growth, it has a much lower net CO 2 footprint than fossil-based fuels. However, the economic feasibility of hydrogen output from biomass must be closely related to the availability and affordability of raw materials in the local area. The biomass physicochemical properties, distribution and hydrogen rate are the main attributes of the supply materials. Since biomass feedstocks vary widely in structural composition and shape, all of these characteristics must be taken into account when combining the feedstock with the appropriate conversion technology (Srivastava et al. 2020 ).

Consequently, moisture, energy and ash contents are the core criteria for evaluating biomass utilisation in this route. The hydrogen yield from biomass is comparatively poor since the hydrogen content of biomass is roughly 5.9 wt% compared to 25 wt% for methane (natural gas), and the energy content is also low due to high oxygen content within the biomass of 40%. Thus, techno-economic studies backed by adequate life cycle assessment evaluation are crucial in this matter. Since biomass has a lower density, transportation and storage costs for either biomass feedstock or the produced hydrogen should be well justified in terms of economies of scale. In certain ways, these characteristics would make it impossible for biomass-based hydrogen production to compete with common natural gas such as steam methane reforming method unless new regulatory frameworks such as carbon tax favour competitively sustainable hydrogen production routes.

Biomass gasification, like coal, is the most practical process for biomass feedstocks because it produces the best yield at high temperatures, generally, 500–1400 °C, where the overall reaction is presented in Eq.  1 . Interestingly, the integration between biomass gasification and carbon capture and storage can potentially lead to an overall negative carbon footprint.

Advances and challenges in fossil-based hydrogen production route

The breakdown of the long-chain hydrocarbon via gasification, reforming or pyrolysis reaction routes is required for hydrogen production from fossil-based feedstocks. The primary product in the reforming reaction is the synthesis gas (a mixture of H 2 and CO), followed by H 2 separation via autothermal reforming, steam methane reforming, partial oxidation or membrane reforming. Another well-known method that is commonly used in hydrogen production is the gasification of fossil fuels, such as coal gasification (Milani et al. 2020 ).

Al-Qahtani et al. evaluated and compared the most common hydrogen generation routes on a monetary basis, such as steam methane reforming, coal or biomass gasification, methane pyrolysis with or without carbon capture and storage technology. Besides, the hydrogen production from the water via electrolysis derived from solar or nuclear energy were also assessed. They reported that, at the moment, steam methane reforming with carbon capture and storage appeared to be the most viable alternative (Al-Qahtani et al. 2021 ).

Steam methane reforming and methane pyrolysis

The primary feedstock for steam methane reforming is natural gas, predominantly methane mixed with other hydrocarbons and carbon dioxide (Osman 2020 ) Natural gas and steam reaction occur in a two-step reaction, as shown in Eq.  2 at high temperatures, followed by an interaction between the carbon monoxide and the produced hydrogen along with the unreacted natural gas. Following that, more steam is supplied to react with carbon monoxide in a water–gas shift reaction (WGSR), as shown in Eq.  3 , to recover further hydrogen and convert carbon monoxide into carbon dioxide. The entire process efficiency is around 76% (Al-Qahtani et al. 2021 ). The entire process releases a significant amount of carbon dioxide emissions, which may be decreased by installing carbon capture and storage technology, removing and separating the flue gases from the product stream. Following that, an amine solvent such as monoethanolamine absorbs about 90% of the carbon dioxide emission, and then, the processed flue gas stream is released into the environment. Afterwards, carbon dioxide is thermally desorbed and compressed to 110 bars for storage. The integration between steam methane reforming and carbon capture and storage (SMR + CCS) technologies has an energy efficiency of 68 per cent, owing mostly to the energy necessary to regenerate the monoethanolamine and the power required for compression. After the WGSR, hydrogen is further purified to 99.99 per cent in both situations, with or without carbon capture and storage, in a pressure swing adsorption unit, which is also utilised in the gasification technology such as coal or biomass gasification routes.

Regarding methane pyrolysis at high temperatures, thermally or catalytically, the processes degrade hydrocarbons into hydrogen and solid carbon, as shown in Eq.  4 . Because there is no oxygen in the process, no carbon oxides are generated, possibly removing the requirement for subsequent processing stages such as the WGSR and lowering the capital and operating expenditures compared to steam methane reforming (Al-Qahtani et al. 2021 ). The greater H 2 content in the product gas stream has the potential to reduce downstream clean-up operations significantly. The cost of methane pyrolysis is heavily influenced by the natural gas prices, processing method and solid carbon by-product.

Coal gasification

During the coal gasification process at high temperatures ranging from 800 to 1300 °C and 30–70 bar pressures, coal is partially oxidised in oxygen or air atmosphere into synthesis gas, as shown in Eq.  5 . The synthesis gas is typically composed of carbon monoxide and dioxide, hydrogen and unreacted methane, where the WGSR process (Eq.  3 ) enriches the syngas further to recover additional hydrogen. Thus, combining Eqs.  3 and 5 will lead to the overall reaction as in Eq.  6 . Coal gasification is less efficient than steam methane reforming with 55%, although it has a larger single-train capacity.

Bibliometric analysis

Key research studies were identified to summarise state of the art and discover knowledge gaps in the hydrogen production and LCA research arenas. The advanced search tool for publications from the Web of Science was used for this study, using the terms ‘Hydrogen production’ AND ‘ Life cycle assessment ’ as inputs. The results were manually scanned, and 24 most complete and relevant studies published from 2019 to 2021 were selected for review in the present study.

• Life cycle assessment

Life cycle assessment (LCA) is recognised as a comprehensive tool to evaluate environmental impacts associated with products and processes. There are many hydrogen production methods, such as steam methane reforming, electrochemical routes through water electrolysis using renewable power sources, thermochemical pathways involving renewable feedstock as the hydrogen carrier and biological processes (Valente et al. 2021 ; Owgi et al. 2021 ). However, environmental sustainability based on LCA remains one of the key requirements for selecting these processes for hydrogen production (Falcone et al. 2021 ). This is because policymakers need to adopt transformative solutions based on robust data and evidence-based research to identify processes that go beyond a one-fits-all approach.

To this end, we reviewed 24 LCA studies published from 2019 to 2021 on hydrogen production and life cycle assessment (Table 2 ). The four main stages defined by ISO 14040 and IS0 14,044 for conducting LCA are: (1) goal and scope definition, (2) life cycle inventory analysis, (3) environmental impacts assessment and (4) life cycle interpretation (Al-Muhtaseb et al. 2021 ).

Goal and scope of the life cycle assessment

The first stage of LCA consists of defining a goal and the scope of the study. This stage determines whether a study would be attributional or consequential, what functional unit will be considered to evaluate environmental impacts and the extent of the system boundary. This is an important initial step as the questions to be answered determine the results and associated policy implications.

Types of life cycle assessment: attributional and consequential

Life cycle assessment studies can be broadly classified into two categories: (1) Attributional LCA incorporates immediate physical flows such as raw materials, energy and emissions involved across the life cycle of a product (Jeswani et al. 2020 ), and (2) consequential LCA accounts for how physical flows can change as a consequence of an increase or decrease in demand for the product system under study (Earles and Halog 2011 ). It includes unit processes inside and outside the product's immediate system boundaries; therefore, consequential LCA studies are more suited for policy decisions. However, as LCA for hydrogen production remains at an embryonic stage, attributional studies are more commonly found. Nevertheless, both attributional and consequential approaches were considered for the purpose of this study.

Functional unit

In LCA, the functional unit is a measure of the purpose of the studied system, and it provides a reference by which the inputs and outputs can be related. This enables the comparison of two essentially different systems. The definition of the functional unit is intricately linked to the goal of an LCA study. It was observed that ~ 42% of the reviewed studies used ‘kg of hydrogen produced’ as the functional unit (Fig.  3 ). While some studies provided results considering hydrogen as an energy carrier and therefore recorded functional unit as ‘energy produced in MJ or kWh’. Very few studies reported ‘distance travelled in km’ as a functional unit when hydrogen was utilised as fuel for vehicles. The choice of different functional units for the same product, i.e. hydrogen, shows the challenges associated with comparing LCA models.

Types of functional units used in the life cycle assessment studies reviewed in the present work (N = 24)

System boundary

In LCA, the system boundary definition profoundly impacts the materials, processes and emissions considered for evaluation. As such, system boundary limits can also considerably influence the calculation of environmental impacts (Collotta et al. 2019 ). The two commonly studied kinds of system boundary for hydrogen production are ‘cradle-to-gate’ or ‘well-to-pump’ that includes processes only until production and ‘cradle-to-grave’ or ‘well-to-wheel’, which incorporates emissions during end use as well.

The generalised system boundary used for conducting the LCA of hydrogen production and consumption includes: (1) raw materials and primary energy sources such as natural gas, coal, biomass, nuclear energy and water; (2) the hydrogen production processes, for instance, water electrolysis and thermochemical processes. Some processes may also consider hydrogen purification as a subsystem to the production; (3) storage of hydrogen in underground caves or compressed tanks; (4) transportation of hydrogen in liquified or compressed gaseous form using trucks and tube trailers or pipelines; (5) emissions during end use such as by hydrogen trains or generation of power using hydrogen; and (6) finally, waste treatment processes from these systems such as emissions to land, air and water (Fig.  4 ).

Generalised system boundary used for conducting life cycle assessment of hydrogen production and consumption. This includes various raw materials such as solar, wind, biomass, coal, water and natural gas

During the review, we observed that studies employed an array of processes and limits in system boundary for conducting LCA of hydrogen production and consumption (Table 3 ). There were only a handful of studies that considered emissions during the use phase. However, given the increasing interest in using hydrogen as a clean energy carrier, it is important to consider the emissions during the use phase and conduct LCAs that present ‘well-to-wheel’ estimates.

Allocation approaches

The allocation approach refers to both ‘partitioning’ and system expansion/substitution method. The allocation approach has been identified to significantly control the values obtained for environmental impacts (Finnveden et al. 2009 ). Allocation approaches are required because the life cycle of a product can consist of many multifunctional processes. Therefore, it is imperative to allocate the environmental impacts between the different coproducts generated by the same process in a justified manner.

Life cycle inventory analysis

Life cycle inventory analysis includes data collation for all the inputs and outputs for processes within the system boundary. In general, the more the processes included in the system boundary, the more complex, challenging and cumbersome is the inventory analysis. This also explains the fact that many studies did not include all the processes ranging from raw material acquisition to end-of-life management (summarised in Table 3 ). The two different kinds of data to be collected for an LCA study are: (1) foreground data for foreground systems which includes primary data that can be easily modified or improved and (2) background data for background systems typically comes from Life Cycle Inventory databases (Silva et al. 2020 ). Background systems support the foreground systems. Table 2 details the databases/data sources incorporated in LCA studies on hydrogen production such as Ecoinvent, expert communications, Greenhouse gases, Regulated Emissions and Energy use in Transportation.

Environmental impacts assessment

Midpoint and endpoint indicators.

Global warming potential due to emissions of greenhouse gases and depletion of fossil fuels was the centre of the attention in the environmental indicators for hydrogen production, with 100% of the studies accounting for either of these two categories (Table 4 ). More than half (54%) of the reviewed studies computed environmental impacts in categories that go beyond global warming potential and net energy use/performance. These environmental impacts included but were not limited to acidification, eutrophication, abiotic depletion, marine, freshwater and terrestrial ecotoxicity, and human toxicity.

Global warming potential expressed as kg CO 2 equivalent relates to greenhouse gas emissions; abiotic depletion recorded in kg Sb equivalent is linked to depletion of minerals, peat and clay; acidification reported in kg SO 2 equivalent is due to the emission of acidifying substances; eutrophication measured as kg PO 4 3− equivalent is due to release of nutrients; particulate matter formation calculated as PM 2.5/PM 10 equivalent relates to the emission of PM 2.5 (particulate matter with ≤ 2.5 µm in diameter) and/or PM10 (particulate matter with ≤ 10 µm in diameter). Photochemical oxidation (commonly called as ‘summer smog’) occurs in stagnant air, in the presence of pollutants such as NO x , non-methane VOCs and others. Ozone layer depletion evaluates the global loss of ozone gas caused by trichlorofluoromethane (CFC-11) of the same mass. Land use calculated in m 2 is categorised as the transformation of urban, agricultural and natural land. Damage to terrestrial, freshwater and marine ecosystems is measured by ecotoxicity potential. Finally, human toxicity is caused due to the potential human health impacts of carcinogenic and non-carcinogenic pollutants.

The midpoint categories are aggregated to present results as endpoint categories such as human health, damage to ecosystem quality in the form of loss of species and resources depletion (Osman et al. 2021b ). It is argued that the environmental impacts should be presented as midpoint categories to prevent oversimplification or misinterpretation of environmental impacts (Kalbar et al. 2017 ). This is because endpoint indicators entail weighting of impacts. Evidently, only one study was identified that presented environmental impacts for both midpoint and endpoint indicators (Ozturk and Dincer 2019 ).

Uncertainty and sensitivity analysis

Uncertainty arises in LCA studies due to sparse and imprecise nature of the available data and model assumptions (Cherubini et al. 2018 ). It is, therefore, imperative to consider and compute these uncertainties quantitatively to reach transparent, robust and trustworthy decisions.

There has been a vast development on the methods to imbibe these uncertainties in LCA models such as parameter variation and scenario analysis, classical statistical theory (e.g. probability distributions and tests of hypothesis); Monte Carlo simulations, bootstrapping and other sampling approaches; nonparametric statistics, Bayesian analysis, fuzzy theory; and the use of qualitative uncertainty methods (Finnveden et al. 2009 ).

This review recorded that 67% of the studies used scenario analysis to account for parameter uncertainty (Fig.  5 ). Together with comparative studies mentioned in (Table 2 ) and scenario analysis in Fig.  5 , this value reaches 96%, i.e. all but one study performed comparative and/or scenario analysis (Cvetković et al. 2021 ). This can be attributed to the dearth of the data and the serious effort required to conduct an LCA of biohydrogen production via anaerobic digestion (Cvetković et al. 2021 ). Furthermore, it was noted that 8% of the studies employed Monte Carlo simulations to propagate parameter uncertainties in the model.

Details of the scenario, sensitivity and Monte Carlo simulations (to propagate uncertainty) conducted in the reviewed studies (N = 24). Scenario analysis was conducted in 67% of the reviewed studies

Sensitivity analysis is conducted to distinguish processes in the hydrogen production chain that contribute to the burdensome environmental footprints. Relatedly, if environmental impacts are to be minimised, these will be the processes where future research should focus on (Al-Muhtaseb et al. 2021 ). 42% of the studies reviewed here conducted sensitivity analysis.

Interpretation of results

This stage of the LCA includes making interpretations, drawing conclusions and distinguishing the processes that can be improved to increase the environmental feasibility of the system. This stage could also involve presenting and communicating results to stakeholders. Table 5 summarises key findings from the reviewed studies.

Key findings and recommendations for future life cycle assessment studies

Life cycle assessment is a complex tool that sits at the interface between science, engineering and policy. Despite this inherent complexity, it is recognised as a comprehensive tool to evaluate environmental impacts associated with products and processes. We reviewed LCA studies published from 2019 to 2021. This section draws recommendations for policymakers to create a sustainable hydrogen economy and LCA practitioners to conduct future studies.

During the review, no two LCA studies were identified to be similar. Differences in the geographical and temporal span, functional units and system boundaries considered, and environmental impact categories were reported. Therefore, it is recommended that the policymakers pay heed to the modelled processes and extent of the system boundary for making decisions for creating a sustainable hydrogen economy.

Most of the studies did not encompass processes, inputs and outputs for ‘cradle-to-grave’ LCA analysis. Thus, future studies should conduct ‘cradle-to-grave’ evaluation for robust decision-making.

About 54% of the reviewed studies computed environmental impacts in categories that go beyond global warming potential and depletion of fossil fuels. It is crucial to assess environmental impacts in more categories. Otherwise, there can be the issue of burden shifting, where hydrogen production processes are developed to mitigate climate change and energy security, however, leading to severe environmental and human health impacts such as acidification, eutrophication and human toxicity.

Finally, focusing on production pathways, only eight studies were identified that computed environmental impacts for biohydrogen, showing that there is a considerable knowledge gap in production processes utilising bio-based feedstocks.

Hydrogen underground storage

There are ambitious goals of the Paris agreement for climate change to be met than ever by 2050. However, the continuous increase in carbon dioxide (CO 2 ) emission generated by the use and storage of fossil fuels has created a clear demand for alternative sources of clean and renewable energy (Ochedi et al. 2021 ). Solar and wind energy, however, provide intermittent and volatile power sources (as shown in Fig.  6 ) that are requiring backup solutions and/or energy storage at scales comparable to their power generation capacity (i.e. longer-term TWh storage solutions). In particular, some industrial sectors are hard to be decarbonised. To help balance the energy supply and demand, a capability of various energy storage technologies, with a dynamic combination of daily, weekly and seasonal storage, can reduce CO 2 emissions per unit of energy provided.

[A] Diurnal time series shows the matching of load, wind and solar of a typical day during the winter season for Europe with 15th and 85th percentiles for each average day time series. [B] Annual time series of weekly averages illustrate the seasonal correlation (i.e. excess/shortage) of load, wind and solar. Electricity generation and demand normalised over the corresponding average value. [C] Schematic round-trip efficiency for a short-term (e.g. battery, brown line) and long-term (e.g. power to hydrogen, black line) storage technology. The figures were adapted from (2017) and (Gabrielli et al., 2020 )

To date, the technical feasibility and economic attractiveness for developing large-scale, lithium-ion-based and seasonal energy storage batteries can be challenging to be implemented and provide an energy supply during high demand times. Such shortfall can be eliminated by storing the excess renewable energy chemically—in the form of hydrogen—in the subsurface aquifers, salt caverns and/or exhausted hydrocarbon reservoirs in the so-called Underground Seasonal Hydrogen Storage (USHS). The usage of hydrogen as an energy carrier can be a promising solution for clean energy because of its non-toxicity, high specific energy and non-CO 2 emission after combustion. The challenge is to find hydrogen storage materials with high capacity. USHS, therefore, can be one of the most promising solutions for offsetting seasonal mismatch between energy generation and demand (Fig.  6 ), firstly for medium- and long-term storage while increasing contribution to low-carbon energy supply. Despite the vast opportunity provided by USHS, maturity still is considered low, with several uncertainties and challenges (Heinemann et al. 2021 ).

Hydrogen-based economy requires a large gas transport infrastructure. It has been suggested that existing natural gas pipe networks could be used to transport hydrogen (Melaina et al. 2013 ; Panfilov 2016 ). The gases would be transported as a mixture and separated afterwards. Some methods for separating mixtures of methane and hydrogen, particularly gas membrane separation, appear promising (Ockwig and Nenoff 2007 ).

Geologically, underground formations are suitable for storing hydrogen, which may then be used as a carrier of chemical energy produced in times of surplus energy production, stored for several months and ultimately retrieved for re-electrification when it is needed most (Bauer et al. 2013 ; Bauer et al. 2017 ). As an illustration of the possible storage potential, a system volumetric capacity (i.e. the Net Energy Density) of hydrogen-based flow battery stores approximately 2.7 kWh/L (NREL) of electrolyte, and hence, an exhausted million-barrel oil field would hold > 3 TWh of electricity. This is equivalent to 30 weeks’ output from a large offshore wind farm which is far more than is needed to eliminate the intermittency issues associated with such a facility. Hence, it was proved that only a few offshore gas fields are required to store enough energy as hydrogen to balance the entire seasonal demand for UK domestic heating (Mouli-Castillo et al. 2021 ).

Thermophysical properties of hydrogen

After hydrogen is produced at the surface from one of the technologies, it must be transported to a seasonal storage facility in a liquid or gas phase. Moreover, hydrogen can also be stored on the surfaces of solids (i.e. by adsorption) or within solids (i.e. by absorption) (El-Eskandarany 2020 ). During the loading cycle, where the power demand is at a peak, hydrogen can be easily re-converted for electrical generation.

Hydrogen can be considered as an ideal gas that may occur in various states over a wide temperature range and even at high pressures. Here, the thermophysical properties of hydrogen at the conditions relevant to the underground hydrogen storage were provided. One of its most important thermophysical characteristics is its low density, making it necessary for any practical application to compress the hydrogen or liquefy it. At intended storage depths, the density and dynamic viscosity of hydrogen are iteratively calculated using equation of state (EOS) and following (Span et al. 2020 ). Primarily, the hydrogen density (kg/m 3 ) mainly increases with increasing pressure while dynamic viscosities (μPa.s) significantly increase with increasing temperature, as shown in Fig.  7 . At low temperatures of − 262 °C, hydrogen is solid with a density of 70.6 kg/m 3 . At higher temperatures, hydrogen is a gas with a small density of 0.089 kg/m 3 at 0 °C and at a pressure of 1 bar. The extent of hydrogen's liquid state can be presented as a narrow zone between the triple and critical points, with a density of 70.8 kg/m 3 at − 253 °C.

(left) Density [kg/m 3 ], (right) dynamic viscosity [µPa.s] of hydrogen at representative P–T conditions which are typical for Underground Hydrogen Storage system. The calculations were carried out by the authors, using the fundamental properties of Hydrogen as an ideal gas. By the time pressure of > 35 MPa is reached, a deviation of 15% from the real values is expected

Three potential technologies for hydrogen storage, therefore, can be considered according to combinations of pressure and temperature relevant to the storage conditions (Table 6 ):

Cryo-compressed hydrogen storage (CcH 2 ) and liquid hydrogen (LH 2 ) storage: storage of hydrogen as a liquid requires cryogenic temperatures because the boiling point of hydrogen at one-atmosphere pressure is − 253 °C with a density of close to 71 kg/m 3 . These properties make storing hydrogen under standard atmospheric pressure and temperature extremely difficult due to the high cost and safety issues. Whereas other gases can be liquefied around the standard temperature of 20 ºC, this is unfortunately practically impossible for hydrogen. Therefore, hydrogen needs compression into cryogenic vessels that can be pressurised to 25–35 MPa. Accordingly, the size of liquid hydrogen requires larger tanks reaching about three times larger than the currently used gasoline tank (El-Eskandarany, 2020 ).

For pressure ranges between 5 and 30 MPa and temperature between 25 and 130 °C, hydrogen can safely be stored as a gas in underground geological formations. For USHS, hydrogen must be transported to a wellhead for underground storage. The hydrogen must then be compressed to be injected at sufficient pressure to enter the geological formation at the in situ pressure and temperature. Different potential geological storage sites for USHS are shown in Fig.  10 and will be discussed in more detail in the following sections.

Additionally, pressurised hydrogen gas takes a great deal of volume compared with, for example, gasoline with equal energy content—about 30 times bigger volume at 10 MPa gas pressure (El-Eskandarany 2020 ). USHS basically implies the reduction of the enormous volume of hydrogen gas due to the reservoir pressure gradient (Fig.  8 ). One kilogram of hydrogen in ambient temperature and at atmospheric pressure occupies a volume of 11 m 3 .

Normalised volume of hydrogen at the pressure–temperature (over the range of geothermal gradients) conditions plotted as a function of depth. Grey horizontal line at 800 m marks the minimum depth recommended for hydrogen injection, where it can be found as a supercritical phase at pressure and temperature conditions relevant for USHS (above 1.3 MPa)

Fluid dynamics of hydrogen in a brine-saturated porous medium

In the context of the USHS system, the cyclic injection of hydrogen into (and possible retrieval from) a brine-filled permeable formation is part of multi-phase flow problems that have been studied extensively (Hashemi et al. 2021 ; Liebscher et al. 2016 ). In this case, a two-phase hydrogen–brine system is immiscible—the fluids are separated by a capillary interface. Likewise, the CO 2 geological storage, an important first approximation to the behaviour of the hydrogen–brine system, is found via applying a group of dimensionless ratios and solubility (and hence its mobility) that analyse the dynamics of two-phase immiscible flow systems (Ringrose et al. 2021 ). Viscous/capillary ( N vc ) and gravity/viscous ( N gv ) ratios are, respectively, the characteristic time ratios for fluid to flow in the transverse direction due to capillary and gravity forces to that in the horizontal direction due to viscous forces using the assumption of (Zhou et al. 1997 ). The two fluids here are assumed to be vertically segregate due to the gravity and density difference. Both ratios can be formulated in Eqs.  7 and 8 as follows:

where u x is the total flow velocity in the horizontal (x) direction, ∆x and ∆z are the system dimensions, μ nw is the viscosity of the non-wetting phase (hydrogen), k av is the average permeability, ∆ρ is fluid density difference, g is the acceleration due to gravity and (dP c /dS w ) is the capillary pressure gradient as a function of wetting-phase saturation.

Around the injection/production wellbore, viscous-dominated conditions are expected to occur due to the high-pressure gradient (Ringrose et al. 2021 ). However, within the reservoir and away from the injection/production wellbore region, gravity-dominated conditions are expected to occur. Such ratios, therefore, can be used to expect the fluid dynamic behaviour of the hydrogen-brine flow system and determine which factors are likely to be most critical, particularly when assessing large-scale macroscopic fluid flow, where the capillary and gravity forces become important enough to be not neglected.

Another important factor for USHS is the solubility of hydrogen in the resident formation fluid (water/brine). Therefore, forecasting the phase equilibria (solubility of hydrogen in brine and water content in the hydrogen-rich phase) under the geological storage conditions (i.e. at different temperatures, pressure and molality) is necessary for the study of hydrogen mobility and reactivity, as well as the control, monitoring and optimisation of the storage. Based on new experimental datasets, Chabab et al. developed predictive models to estimate the water content in the hydrogen-rich phase and precisely capture the salting-out effect on hydrogen solubility (Fig.  9 ) (Chabab et al. 2020 ).

Solubility of hydrogen in pure water as well as the brine of different molalities (up to 5 M), as a function of pressure (up to 25 MPa), and at the temperature of 50ºC [a] and 100ºC [b]. The symbols represent experimental results from the literature (Chabab et al., 2020 ). The solid, dotted and dashed lines represent the hydrogen solubilities calculated by the e-PR-CPA, SW and geochemical models, respectively. The figure is modified from Chabab et al., ( 2020 )

Large-scale hydrogen geological storage

A promising solution to help balances the energy supply from renewable intermittent sources and demand is hydrogen as an energy carrier for clean energy and must be accompanied by energy storage systems. The benefits of using hydrogen are because of its non-toxicity, high specific energy and non-CO 2 emission after combustion. However, the challenge is to find hydrogen storage materials with high capacity. Large-scale underground storage of natural gas has been practised successfully for many decades, with a global total of 413 billion standard cubic metres (BSCM) of natural gas storage accommodated in depleted gas fields (80%), underground aquifers (12%), and engineered salt caverns (8%) (Perry 2005 ), as shown in Fig.  10 . Here, these types of underground hydrogen storage systems have been considered (Lord et al. 2014 ; Panfilov 2010 ).

Schematic diagram of different processes which are associated with hydrogen production using electrolysis, seasonal storage in geological formations and/or salt caverns, utilisation for ammonia production and re-electrification of hydrogen using fuel cells. The figure shows different potential storage mediums for the hydrogen in the underground geological formations: reservoir/aquifer and salt caverns. The dimensions are not to scale

Depleted hydrocarbon reservoirs

More often than not, depleted hydrocarbon reservoirs are appealing targets for USHS because of their storage capacity, proven seal, previous knowledge of reservoirs characterisation and existing infrastructure (i.e. natural gas pipeline network). Nevertheless, various physical, chemical and microbial processes are associated with USHS in hydrocarbon reservoirs (Heinemann et al. 2021 ) (summarised in Fig.  10 ).

While one can transfer know-how and technology from underground natural gas storage and underground carbon storage, some of the challenges USHS faces are peculiar. In both compressed gas and liquid forms, the low density of hydrogen makes the seasonal storage of hydrogen in porous media (and possible retrieval) problematic. With a mass–density ratio of less than 0.01 compared to water for most relevant subsurface storage conditions, H 2 is very light. Consequently, an H 2 plume would experience strong buoyancy forces (i.e. the stronger the buoyancy forces, the higher the potential for hydrogen leakage), and water upconing towards the extraction borehole may occur (Heinemann et al. 2021 ; Sainz-Garcia et al. 2017 ).

This limitation is felt most strongly during the hydrogen retrieval from the subsurface. The gas saturation around the production well required to keep a gas well flowing is of major concern since it will impact and reduce the production and ultimately will kill the well. The thinner the hydrogen plume will be, the lower gas saturation and the higher accumulation of resident formation brine in the downhole. Therefore, the dynamics of the USHS system require a wellbore model capable of describing/predicting the conditions (pressure and temperature) in the extraction borehole as the fluid(s) flow up (or the liquid accumulation at the bottom of) the borehole.

Water upconing is the change in the hydrogen–water contact profile due to drawdown pressures. This phenomenon can be seen as the name implies: a cone of water formed below the perforations. One way to avoid upconing during H 2 production is the use of a cushion gas (Kim et al. 2015 ; Oldenburg 2003 ), usually a cheaper and denser gas like nitrogen (N 2 ), which helps prevent water flooding of the gas plume when H 2 is being produced. This concept is well known in underground natural gas storage and has previously been proposed for USHS (Cao et al. 2020 ).

Additionally, it is important to note that USHS involves cyclic hydrogen injection (i.e. during power surplus) into and withdrawal (i.e. during energy demand) from the geological formations, where changes in the reservoir pressure may induce fatigue in the caprock and lowering the fracturing pressure at which hydrogen commences to leak through a seal rock. Therefore, assessing the sealing capacity to hydrogen (or hydrogen column height) will be crucial to keeping the risk of the potential upward leakage of hydrogen through the sealing caprock at a minimum. Seal rocks have fine pore and pore throat sizes that, in turn, generate hydraulically tight low-permeability caprocks with high capillary threshold pressures. High threshold pressures, together with wettability and interfacial tension (IFT) properties, determine the final column height that a seal can hold, thereby affecting the ultimate reservoir storage volumes. Compared to the underground natural gas storage, higher capillary entry pressures are expected to occur for hydrogen due to its higher interfacial tension (Hassanpouryouzband et al. 2021 ; Naylor et al. 2011 ). Therefore, hydrogen can be stored at a higher pressure in the reservoir than methane, with a reduced risk of geomechanical failure.

On the hydrogen injection into a storage reservoir, a very small fraction of hydrogen will dissolve into the formation fluids (Chabab et al. 2020 ), and water vapour may contaminate the hydrogen phase due to chemical disequilibrium. Hydrogen losses through diffusion need to be considered, as the diffusion ability of hydrogen is several times more than that of CO 2 and methane, to such an extent that hydrogen can travel between the structures of ice-like crystals (Hassanpouryouzband et al. 2020 ).

In order to show the influence of the large density difference (Fig.  11 ) between the injected gas (hydrogen) and the resident formation fluid (brine) on the hydrogen plume migration during the seasonal storage period, we numerically simulate the injection of 10-ton kg of hydrogen over 10 days and its storage for 35 days. We used the numerical simulator PorousFlow Module, open-source software for solving parallel tightly coupled nonlinear THM processes in porous media (Wilkins et al. 2021 ; Wilkins et al. 2020 ). It is based on the MOOSE framework (Gaston et al. 2009 ) and its internal architecture relies on state-of-the-art libraries for finite element analysis (Kirk et al. 2006 ) and nonlinear iterative algebraic solvers (Balay et al. 2019 ). The simulation results are shown in Fig.  11 . It is shown from the simulation standpoint that the leakage rate of hydrogen is going to be the biggest challenge due to the very high mobility of hydrogen, the small molecule size, the high dispersion rate and the large density difference between the hydrogen and brine. Therefore, a proper tightness assessment of the caprock above the reservoir is required to prove its effectiveness for any possible hydrogen leakage. In addition, we propose expressly storing H 2 /CH 4 gas mixtures to improve the density contrast with the water. The mixed gas can, upon demand, then be extracted and transported in the same natural gas pipelines.

Hydrogen–brine displacement in an idealised 2D horizontal cross section (i.e. geological storage formation). The injection wellbore is located at the left-hand side of the simulated domain. The subfigures are showing only the first 50 m horizontal distance from the injection well with 10 × horizontal exaggeration. The horizontal exaggeration is 10x. [A] the reservoir is fully saturated with brine (i.e. before the hydrogen injection start). The migration of the hydrogen phase after [B] 9 days, [C] 23 days, [D] 36 days and [E] 45 days

Subsurface microorganisms, including methanogens, sulphate reducers, homoacetogenic bacteria and iron(iii) reducers can make use of H 2 as an electron donor, which may lead to an unwanted accumulation of biomass in the vicinity of the injection borehole and/or loss of H 2 (Ganzer et al. 2013 ; Hagemann et al. 2015 a). The local rate of the biochemical reactions depends on the number of the particular microorganism (Hagemann et al. 2015 b). Hence, an important problem for the modelling of USHS is the description of microbial growth and decay functions. Microbial conversion of hydrogen can only occur if the hydrogen is in the aqueous phase. A mixture of hydrogen with another gas means it will have a lower partial pressure and hence lower solubility in water. It was stated that if the temperature of the formation is higher than 122ºC or the salinity is higher than 5 M NaCl, the hydrogenotrophic microbial activity becomes highly unlikely (Thaysen and Katriona 2020 ). Hence, if a storage reservoir is hot enough, one can combine hydrogen storage with CO 2 , since methanogenic microbial activity will be limited by the temperature constraint. Further, a high-pressure environment is toxic for some microorganisms.

Considering the deep depleted gas-condensate reservoirs, the risks are minimised here due to the presence of well-defined geological traps related to previously formed gas reservoirs. Unfortunately, the risk of migration from the target storage formation cannot be eliminated completely, particularly due to the re-pressurisation and change of the stresses and the long-term well integrity issues of the casing and cement.

Salt caverns

Another underground storage medium, which could be used under certain conditions and locations, is the usage of salts caverns as high-pressure gas storage facilities (Fig.  10 ) (Gabrielli et al. 2020 ; Hassanpouryouzband et al. 2021 ; Pudlo et al. 2013 ; Foh et al. 1979 ). Based on energy storage capacity (GWh) and discharge timescale, storing hydrogen in salt caverns can afford utility-scale, long-duration energy storage to meet the market need to shift excess off-peak energy to meet dispatchable on-peak demand. Salt caverns can hold substantial promise due to the self-sealing nature of the salt and the ability to customise the size and often shape of the caverns (Lord et al. 2014 ). However, the inaccessibility of the salt caverns in the area where hydrogen production is can be a limiting factor.

Salt caverns can be artificially constructed in the salt formation (or salt dome) by injecting water through an access wellbore, dissolving the salt and generating large volumes of brine in the so-called solution mining process. This process is associated with retrieving a large quantity of brine which requires disposal in an eco-environmental way. Finding suitable disposal repositories for brine disposal can be economically problematic due to higher costs for constructing longer pipelines which eventually may slow down or even hinder the permitting process. During the hydrogen withdrawing from the caverns under constant pressure, part of this saturated brine can be injected into the caverns to maintain the caverns' pressure and stability. Cushion gas, therefore, is not needed under these operating conditions (Foh et al. 1979 ; Taylor et al. 1986 ).

Compared to depleted oil and gas reservoirs, the key advantages for storing hydrogen in salt caverns are: (1) salt surrounding the caverns is highly impermeable and virtually leakproof where the only possibility for gas loss is escaped through leaky wells (Lord et al. 2014 ). (2) Salt does not react with hydrogen (Bünger et al. 2016 ). (3) Withdrawal of ‘discharge’ of hydrogen is highly flexible in rate, duration and volume with lower cushion gas requirements to avoid rock breakage. (4) Caverns are a mature, financeable storage technology that has been successfully used to store compressed gases for over 75 years with possible extensions for USHS.

The city of Kiel’s public utility, as an illustration, has been storing town gas with a hydrogen content of 60–65% in a gas cavern with a geometric volume of about 32,000 m 3 and a pressure of 8–16 MPa at a depth of 1330 m since 1971 (Kruck et al. 2013 ; Carpetis, 1988 ) estimated the hydrogen storage capacity for cavern volume of 500,000 m 3 and a casing shoe depth of 1000 m a pressure range of 180 to 60 bar is suitable of 4.0 Mio kg hydrogen (47 Mio m 3 (st)) and a cushion gas of 2.2 Mio kg (26 Mio m 3 (st)). For an economic prospect, the total installed costs, including wellbore drilling, compressors and gas treatment, were estimated to be about € 100 million (Michalski et al., 2017 ). Compared to energy storage in Li-ion batteries with a cost of 100 €/kWh, USHS in salt caverns offers a significant cost reduction potential in the total investment cost by a factor of 100.

Storage of hydrogen in the form of methane (natural gas) may be a preferable alternative for overcoming the storage problems associated with storing pure hydrogen in geological formations. When there is a surplus of renewable energy in the summer, hydrogen can be produced through water electrolysis. Furthermore, when this hydrogen and carbon dioxide combine in the methanation reaction, methane is produced, which can then be stored in a geological reservoir for winter use. This could be accomplished through a methane reforming reaction followed by using a fuel cell to generate electricity that can be fed into the power grid.

In short, hydrogen storage in a geological medium can offer a viable option for utility-scale, long-duration energy storage, allowing the hydrogen economy to grow to the size necessary to achieve net-zero emissions by 2050. While the operational experience of storing town gas in salt caverns provides considerable proof of its viability and operational best practice, full-scale deployment of USHS has yet to be evaluated for any associated risks and public acceptance of viewpoints, similar to the potential for induced seismicity.

• Hydrogen utilisation

Fuel and power systems

Globally, the heat generated from domestic as well as industrial activities contributes by 33 and 50% of the carbon dioxide emissions and universal energy consumption rate, respectively (Dodds et al. 2015 ). The majority of gaseous emitted by the conventional burning process of natural gas are implicated in numerous environmental contamination issues (i.e. greenhouse gaseous emissions). The primary source of carbon dioxide emissions was energy consumption, with a global emissions rate of 33.1 gigatonnes in 2018, mainly resulting from the burning of fossil fuels. Contrarily, applying hydrogen gas as an alternative fuel to natural gas has proved to be an efficient pathway to reduce greenhouse gaseous emissions. Once it is generated from renewable energy sources, as shown in Fig.  1 , it can directly participate in the decarbonisation process in the energy sector thanks to its reacting nature, whether combusted or utilised in the fuel cell. The hydrogen is currently produced by conventional (non-renewable sources) of 18%, 30% and 48% from coal, heavy oil/naphtha and natural gas, respectively, which was negatively responsible for releasing about million 560 tonnes of carbon dioxide per year (Lui et al. 2020 ).

Moreover, given the costly natural gas employed throughout the power-producing framework (i.e. requires a huge area to store), hydrogen appears to be a viable option as a fuel feeding to gas turbines (Bicer and Khalid 2020 ). The utilisation of hydrogen in the central heating system instead of natural gas offers numerous merits: comparable operational activity and an increased heat generation rate with minimal harmful emissions (Dodds et al. 2015 ). Several factors, such as the Wobbe index, should be considered before forwarding hydrogen to various appliances. Generally, Wobbe index values differ considering the chemical composition of the gas. The Wobbe index number of pure hydrogen is about 48 MJ/m 3 ; it falls within the permissible natural gas integrity extent for the vast majority of burners (Zachariah-Wolff et al. 2007 ). Supplying the operating system with a fuel beyond the Wobbe index band can negatively result in some operational problems (i.e. incomplete combustion and burner overheating). Clearly, attributing to the hydrogen's higher combustion velocity compared with the natural gas fuel, advanced burners with specialised technical specifications must be operated with hydrogen as a fuel feed stream.

Furthermore, the overabundant electricity generated from power facilities can be transformed into hydrogen, which can be either directed to the existing natural system (direct consumption) or chemically converted into chemicals used in different industrial aspects (Collet et al. 2017 ). Besides, hydrogen can be used individually in the aerospace industry or in combination with oxygen as propellants. The mentioned liquid mixture (oxygen and liquid) generates a large amount of energy and makes it more suitable for space applications. Because of releasing water during hydrogen combustion, in addition to its high efficacy compared with gasoline, these characters qualify it to be employed as an automotive fuel (Gurz et al. 2017 ).

Hydrogen employment in power systems

Hydrogen is enormously used to store and transport energy in a variety of power applications, typically illustrated in Fig.  1 and discussed as follows (Parra et al. 2019 ):

Storing of energy and auxiliary services

Given the hydrogen's high storing efficacy, hydrogen-based energy storage has gained traction for storing energy over a medium/long term and in auxiliary services in the last decades. It can meet energy storage requirements over a broad timescales to avoid any defect (shortage) that may occur between the product and the demand (required) of energy (Al Shaqsi et al. 2020 ). Recently, renewable energy production has grown rapidly; however, certain renewable energy supplies are sporadic and seasonally dependent. As a result, the produced renewable energy should be stored in a dependable form that is resistant to the fluctuation in those energy sources (Mehrjerdi et al. 2019 ). In particular, the most popular types of energy storage are: (1) power-to-power, (2) power-to-heat and (3) power-to-gas (Widera 2020 ). Hydrogen, in comparison, has a large energy storing capacity, a great storing time and flexibility. It has the ability to reduce energy volatility and absorb the surplus of energy production. Practically, it can deal with the economic and seasonal variations issues. Hydrogen can exceptionally balance between the resultant and required energies by storing the surplus energy when the production rate exceeds the required one as well as in times at which the electricity's price is minimal and reuse it in the opposite cases. Contrarily, hydrogen can be forwarded to generate electricity in the high energy demand.

Moreover, the storing capacity of hydrogen is higher than batteries, as it may range to weeks or months, unlike batteries that may extend (limited) for hours (Bocklisch 2016 ). Otherwise, hydrogen can be subjected to transform renewable resources to produce energy during different climatic conditions in different seasons. The storage capacity of hydrogen is estimated to reach up to megawatt-hours (1000 Kilowatts hours), even terawatts-hours, which is considered a high value by considering that of batteries (i.e. kilowatts hours). A slew of hydrogen power storage plants has been commenced worldwide, showing the technology's potency for the large scale. Examples of power plants established to produce and store hydrogen are Underground Sun Storage, Orsted and SoCalGas in Austria, Denmark and USA, respectively (Home | SoCalGas, https://www.socalgas.com ).

In the Underground Sun Storage, the energy derived from wind and solar renewable resources is stored beneath the earth's surface. Referring to the difficult storing of the produced energy from renewable resources, the rest released power in reprocessed into hydrogen via electrolysis process and conserved for the futuristic challenges. The findings of the plant outlines revealed that it has the efficiency to equilibrate the basic energy requirements in line with the various seasonal variations. Other projects were established to face the shortage between the system supply and demand. Orsted plant was designed to operate the electrolysers by subjecting the oversupply of energy generated from wind farms to them. Another project launched by SoCalGas on campus succeeded in directly converting the produced hydrogen from the solar electric system into methane inside a bioreactor.

Besides, hydrogen is hugely accounted as an assistant tool for providing the energy sector (grid) with the necessary services such as frequency maintenance and voltage strengthening via electrolysers and fuel cells (Bird et al. 2016 ). In the HAEOLUS facility (Haeolus. https://www.haeolus.eu/ ), the oversupply of wind generation is directly fed into an electrolyser to generate hydrogen, which is subsequently forwarded into fuel cells to be used later for various purposes (utilities, data transmittance, systems controlling and others) (Larscheid et al. 2018 ). Another form of energy storage can be achieved by regulating the grid frequency near its normal value (50–60 Hz) by injecting or consuming energy in a coordinated manner to maintain the gap between the product and the required power. Numerous regulation reserves have been installed in different European grid systems. Commonly, frequent containment and restoration reserves have been used to handle the frequencies through the distributed control systems. The first mentioned controlling scenario supplies a steady feed stream in case of occurring a sudden corruption in frequency in a very short period, whereas the latter can tolerate a longer corruption beyond the 30 s. The twice services can be attained via electrolysers and fuel cells by incrementing or decreasing their power setpoints related to frequency signals (Alshehri et al. 2019 ).

Besides, hydrogen-based equipment can contribute to voltage support by adjusting their power factor to meet the local voltage support requirements, which can be accomplished using inverter or rectifier monitoring systems (Alshehri et al. 2019 ). Some troubles such as blackout can occur in power plants, which was conventionally faced using a diesel Genset. The use of fuel cells may have the advantage to realise this scope given its no emissions and noiseless nature. These studies imply the profitability of hydrogen scaling up in the power sector.

Power-to-gas

Power-to-gas is a process in which electrical energy is used to generate a combustible gas. Since hydrogen is thought to be a combustible gas with a large power density, power-to-hydrogen technologies are increasing (Eveloy and Gebreegziabher 2018 ). Because of the combustibility nature of hydrogen, it has been inserted into gas applications. The hydrogen generated from the electrolyser can be converted into methane by the methanation process, which is either pumped to the natural gas grid operating system or stored to achieve the financial budget for the energy market (Gondal 2019 ). By the literature, numerous pilot projects have been commenced worldwide with the highest establishment rate of 85% in Europe, followed by the USA and Japan (Thema et al. 2019 ). Among different European countries, Germany constructed a power-to-gas plant with a maximum production capacity of (40–100 megawatts) to be directed for industrial purposes, and it will pump in the natural gas grid operating system from 2022 (Romeo et al. 2020 ).

Furthermore, several power-to-gas infrastructures have been installed in the regions rich in solar and wind renewable resources. A realistic study is displayed in the HAEOLUS project (north of Norway). Chiefly, its core idea was based on using 2.5 Megawatts proton exchange membrane electrolyser to transform the produced wind power generated from wind farms into hydrogen, which can be consumed in various aspects. HyCAUNAIS project displays the viability of running a resilient power to gas facility in conjunction with the methanation approach by equipping a nominal 1 megawatts electrolysis area to produce hydrogen, which was methanated and inserted into natural gas grid operating system or combined with biomethane generation area from landfill biogas (HYCAUNAIS – Storengy – Europe en BFC. https://www.europe-bfc.eu/beneficiaire/hycaunais-storengy/ ).

Lately, fuel cells have gained worldwide attention as efficient and environmentally friendly energy generators. Practically, they are integrated electrochemical devices widely used to convert the delivered chemical energy into its electrical counterpart via redox reactions (Yuan et al. 2021 ). Regarding their efficacy for energy generation, they can be served as energy carriers. Fuel cells are composed of two electrodes (i.e. anode and cathode) separated by electrolytes responsible for the migration of ions between electrodes (Ogawa et al. 2018 ). There are numerous types of fuel cells such as alkaline fuel cell, direct carbon fuel cell, direct methanol fuel cell, microbial fuel cells, molten carbonate fuel cells, phosphoric acid fuel cell, proton exchange membrane fuel cells and solid acid fuel cells.

Table 7 displays different types of fuel cells with their operational conditions and efficiency%. During system operation, hydrogen is passed to the anode while oxygen is passed to the cathode. At the anode, the hydrogen molecules are split into protons and electrons by a catalyst. The positive hydrogen particles can pass through the membrane to the cathode side, but the negative cannot. However, electrons change their path by being forced to the circuit and generating electric current. At the cathode, the hydrogen protons, electrons and oxygen combine to produce a water molecule which is the end product of this reaction. Among different types of fuels (i.e. hydrocarbons and chemical hydrides), applying hydrogen in fuel cells is eco-friendly because it does not expel any pollutants (Psoma and Sattler 2002 ). It works within low temperatures ranges comparing with the internal combustion engine. As mentioned before, the end product of the hydrogen-based fuel cell is water, whereas the end products of diesel/natural gas-based fuel cells are carbon dioxide and greenhouse gases (Xu et al. 2021 ). The main differences between fuel cells and traditional batteries are presented as follow: (1) operational mode of fuel cells is mostly like the traditional batteries, but the latter requires an electrical powering to run, (2) batteries can store hydrogen, unlike fuel cells that can provide a continuous electricity supply wherever hydrogen (fuel) and oxygen (oxidising agent) are available from outside sources. In addition to the mentioned differences, the batteries electrodes are steadily consumed during their extended usage, which entirely differs (not found) in the fuel cells (Spingler et al. 2017 ; Aydın et al. 2018 ).

Co-generation and tri-generation distribution systems

Interestingly, fuel cells can be employed to optimise the efficiency of different power systems and reduce the overall production cost of these processes in several aspects, including co-generation systems (i.e. heat + power/cold + power) or tri-generation systems (i.e. cold + heat + power). Co-generation is the sequential generation of two different forms of beneficial energy from a primary single source (fuel cells). In that case, the electricity generated from fuel cells is used to meet the electrical demand, and the released heat is directed towards the heating activities. As a result, total efficiency will be about 95%. Systematically, co-generation fuel cell systems consist of different components, including fuel processors, power suppliers, heat recovery unit, energy (thermal/electrochemical) storage unit, control devices, additional apparatus (i.e. pumps) and stack. Commercially, a large number of facilities have been launched to improve the performance of co-generation systems. Different co-generation projects were erected around the world. In Japan, the plant installed by the ENE-FARM project (300,000 units/2018) simultaneously supplied the home with electricity and heat necessary for daily activities by using proton exchange membrane fuel cells ranged from 0.3 to 1 kilowatt. Initially, liquefied petroleum gas feedstock streams are fed into a reformer, where they are converted into hydrogen, which is further combined with oxygen inside the fuel cells to produce water, electricity and heat used later for various residential purposes (Yue et al. 2021 ). Recently, the manufacturing of micro-co-generation fuel cells has grown in Europe. Besides, more than 1000 micro-combined heat and power fuel cells were launched in 10 European countries between 2012 and 2017. The primary European plant for a micro-co-generation fuel cell was the ENE. Field project (ene.field. http://enefield.eu/ ). An LCA study was successfully performed for the mentioned project, and simply it revealed that co-generation fuel cell was environmentally in nature compared with other gas boilers and heat pumps strategies considering its less greenhouse gaseous emissions. PACE was another project, firstly started in 216, whereas about 2800 of combined heat and power fuel cells are fabricated. Briefly, the overall development in the electrical efficiency through the two inspected projects were 60 and 95%, respectively (Home - PACE Pathway to a competitive European fuel cell micro-cogeneration market. https://pace-energy.eu/ ).

Tri-generation strategy is an improved strategy of co-generation in which a single primary source achieves the required cooling by thermally driven equipment. The working principle of heat pumps mainly stands on producing cooling from a thermal source. Typically, this can be achieved by using condenser and evaporator types of equipment. The gas released from absorbent/adsorbent is cooled down in the condenser and converted into a liquid by releasing its heat (refrigeration process). Then, the cooled down fluid continues to an evaporator, whereas it is evaporated by losing its contained heat. Significantly, the tri-generation fuel cells simultaneously reduce carbon emissions and enhance energy efficacy (Yue et al. 2021 ). Fong and Lee ( 2014 ) reported that employing a 593 kilowatts solid oxide fuel cell and absorption chillers, the carbon emissions were notably decreased by about 50% with an increase in the energy efficacy up to 75% (Fong and Lee 2014 ). A simulated 339 kilowatts solid oxide fuel cell combined with a combustor and a heat recuperation system proficiently recovered about 267 kilowatts of heat with an efficacy of 84%. Besides, they announced that 339 kilowatts solid oxide fuel cells provided with an absorption chiller generated about 303.6 kilowatts of cold with an efficacy of 89% (Yu et al. 2011 ).

Transportation sector

Compared with conventional battery-powered powertrains, vehicles based on hydrogen fuel (hydrogen-fuelled vehicles) represent a promising solution to surpass them. Globally, the sales rate of hydrogen-fuelled vehicles is anticipated to be 3% and enhanced up to 36% in 2030 and 2050, respectively (Path to Hydrogen Competitiveness: A Cost Perspective - Hydrogen Council. https://hydrogencouncil.com/en/ ). Currently, innumerable vehicles companies are developing their operating system to be hydrogen-based, attributing to its dependability and quality. Toyota has evolved Mirai fuel cell vehicles by using proton exchange membrane fuel cells with a volume power density and maximum power productivity of 3.1 km/L and 144 kilowatts, respectively. The hydrogen-fuelled vehicles can be driven by different forms of hydrogen (i.e. liquid and compressed). The compressed (high pressurised) hydrogen is the most appropriate form in the vehicles storage system of Clarity and NEXO; hydrogen-based fuel cell vehicles developed by Honda and Hyundai companies, respectively. At the same time, liquid hydrogen operates Hydrogen 7 vehicle improved by BMW company (Yue et al. 2021 ). Moreover, regional multi-unit trains powered by hydrogen have been entered into service in Europe and are projected to gain more economic benefits. Approximately 30% of presently employed diesel fleets may be phased out in the future (Study on the use of Fuel Cells and Hydrogen in the Railway Environment - Shift2Rail. https://shift2rail.org/publications/study-on-the-use-of-fuel-cells-and-hydrogen-in-the-railway-environment/ ).

Among different modes of transportation, the aviation division is regarded as the fastest transportation mode with anticipated annual growth in air traffic. The most common aircraft fuel is kerosene. Various aviation fuels often display a set of specifications, such as resistance to corrosion and severe temperature changes (Tzanetis et al. 2017 ). It is worth noting that petroleum accounts for the majority of the fuel used in the aviation sector. To improve energy preservation and reduce the negative environmental effects of fossil fuels, alternative, less harmful fuels such as liquid hydrogen are developed and thought to be eco-friendly. Table 8 presents some variations in the physicochemical properties between hydrogen and kerosene fuels. Refrigerated hydrogen fuel can be potentially better than kerosene as aviation fuel. It emits fewer greenhouse gaseous emissions and is easily produced from a variety of sources. Aside from that, the operating hydrogen-fuelled aircraft is characterised by minimal maintenance costs, long lifetime engines, high energy content and better combustion.

Furthermore, some constraints may arise during hydrogen utilisation as aviation fuel, such as depressed ignition energy, high flammability and the possibility of unburned traces forming that promotes metal embrittlement. Furthermore, the hydrogen admission with the onboard technology instead of inserting into the grid commercially allows its manufacturing companies to resell it (Nanda et al. 2017 ). The National Renewable Energy Laboratory manifested that the hydrogen cost in the mentioned case ranges from 3 to 10 USD/Kg, while the most traded hydrogen cost is about 13.99 USD/Kg. To sum up, liquid hydrogen presents admirable efficacy as an aviation fuel for reducing greenhouse gaseous emissions, resulting in a significant improvement in air quality. Furthermore, by using hydrogen-based aviation fuels, over-reliance on traditional fuels could be decreased. The total cost of aircraft powered by liquid hydrogen is predominately associated with the cost of production and storage technologies (Eichman et al. 2012 ).

Recently, the global navigation movement in terms of maritime shipping has become increasingly important in the movement of different types of goods worldwide, which is in line with tremendous industrial progress in various fields. Unfortunately, this, in turn, led to an increase in the consumption of conventional fuels (i.e. diesel and heavy fuels). Regrettably, the pollution created by ships significantly implicates about 2.5% of the universal greenhouse gaseous emissions. Furthermore, bunkering activities broadly contribute to the leakage of heavy fuels in the aquatic environment, consequently posing a threat to the ecosystem. It was announced that carbon dioxide emissions associated with shipping activities release about 3.3% of the global emissions (Vogler and Sattler 2016 ). Other gaseous emissions such as nitrogen oxide and sulphur oxide are also associated with shipping activities. Accordingly, the maritime industry seeks more environmentally alternative fuels than conventional ones to overcome these obstacles (Prussi et al. 2021 ). Numerous suitable substitutes in different states, gas (i.e. hydrogen, propane) and liquid (i.e. bio-oil, methanol and ethanol) are used to compensate for the usage of traditional fuels (Al-Enazi et al. 2021 ; Abou Rjeily et al. 2021 ). Among them, hydrogen can be employed in maritime activities in two routes: (1) internal combustion engines and (2) fuel cells (Banawan et al. 2010 ). Relatively, fuel cells meet the energy requirements needed by ships sailing for long distances travelling and supply the ancillary energy requirements of larger ships in contrast to the other battery-powered ones. Numerous studies have been conducted to assess the feasibility of using hydrogen in maritime activities. Deniz and Zincir ( 2016 ) stated that hydrogen had a durable, safe and bunker capability criterion, qualifying as a favourable fuel for shipping. Although they reported that liquefied natural gas has the preference to be used as an alternative fuel, they recommended more research studies on utilising hydrogen as an effective alternative fuel (Deniz and Zincir 2016 ).

Production of hydrocarbon fuels

Production of hydrocarbon fuels via fischer–trospch pathway.

Syngas (synthesis gas), a mixture of carbon monoxide and hydrogen, is a product of different thermochemical conversion processes (i.e. pyrolysis, gasification and others) and can be utilised by two scenarios: (1) direct fuel or (2) transformed into transportation fuels via Fischer–Trospch synthesis process and syngas fermentation (Wainaina et al. 2018 ). The two strategies are categorised as gas-to-liquid transformation strategies that can generate hydrocarbon fuels and alcohols based on syngas feedstock stream (Gruber et al. 2019 ). Normally, the Fischer–Trospch strategy (exothermic) operates at 200–350 °C and 1.5–4 MPa for reaction temperature and pressure, respectively (Okolie et al. 2019 ). Majorly, it comprises three main stages: (1) syngas production, (2) syngas treatment and (3) transforming into hydrocarbon fuels associated with their upgrading. Besides the production process of transportation fuel, other valuable products (i.e. paraffin, naphtha and others) can be produced. Significantly, the as-produced green fuels based on the Fischer–Trospch process have numerous advantages over petroleum-based fuels. They have excellent burning characters, elevated smoking points and free of heavy contaminants. The physicochemical properties of resultant fuels depend heavily on reaction conditions (i.e. reactor type, heating rate, residence time and others) (Sun et al. 2017 ). The given equations from (Eqs. 9 , 10 , 11 , 12 , 13 , 14 and 15 ) explicates the synthesis of different products (i.e. alkanes, alkenes, oxygenated products, methanol, ethanol and dimethyl ether via the Fischer–Trospch process by participating in hydrogen. The hydrogen/carbon monoxide ratio is a critical controlling parameter in the Fischer–Trospch synthesis process (Bermudez and Fidalgo 2016 ). Different types of catalysts (i.e. copper-based catalysts) can be used to optimise the yield of the Fischer–Trospch process.

Synthesis of alkanes:

Synthesis of alkenes:

Synthesis of alcohols:

Synthesis of carbonyl:

Synthesis of ethanol:

Synthesis of methanol:

A ratio of H 2 / CO of 2:1 is preferable for the synthesis of hydrocarbon fuels via water—gas shift reaction as given in Eq.  15 :

Dimethyl ether is admirable commonly realised as an efficient alternate for diesel fuel (Kim and Park 2016 ). Distinctly, numerous physicochemical features characterise liquefied petroleum gas, such as anti-corrosive, anti-carcinogenic, less nitrogen oxide and carbon monoxide emissions during its burning, less engine noise and high cetane number (Dincer and Bicer, 2020 ). In general, dimethyl ether can be produced by (1) direct route (combined single step of methanol synthesis and dehydration) or (2) indirect route (separated methanol synthesis and dehydration steps) as shown in Eqs. ( 16 , 17 and 18 ) (Gogate, 2018 ):

Direct route (single step):

Indirect route (two steps):

Dehydration of methanol:

Production of hydrocarbon fuels via Syngas fermentation pathway

Syngas fermentation (biorefining) pathway is regarded as the interconnection between the biochemical and thermochemical scenarios (Thi et al. 2020 ). It produces value-added products (i.e. alcohols) from syngas by flexibly employing several groups of microorganisms at different reaction temperatures of 37–40 °C and 55–90 °C for mesophilic (i.e. Clostridium autoethanogenum ) and thermophilic (i.e. Moorella thermoacetica ), respectively. During the process, the feedstock of syngas can be simply converted into alcohols (i.e. ethanol) via two subsequent stages via (1) producing acetyl coenzyme A and then (2) its transformation into ethanol. Other alcohols and chemicals (i.e. acetate, butanol and formate) can be synthesised by acetogenic bacteria (Park et al. 2017 ). Regarding several operational advantages characterised to syngas fermentation such as (1) no necessity for using costly pretreatment step, (2) process' versatility with different biomass composition, (3) independent on the hydrogen/carbon monoxide ratio in the feedstock upstream, (4) high selectivity of as-used microorganisms and (5) moderate (ambient) working parameters with no necessity for catalysts usage or its poising trouble, they support it over the Fischer–Trospch process. However, there are some operational challenges such as (1) weak solubility of the gas in the liquid state, (2) complicated bioreactor design, (3) existence of impurities and (4) low yield of production. Briefly, integration between different thermochemical, biochemical and hydrothermal routes can effectively compensate for the shortage of individual techniques and maximise productivity (Rigueto et al. 2020 ).

Refining of crude oil and petroleum products

Commercially, hydrogen is conceived as an upgrading (improving) agent for crude oil products and petroleum distillates in terms of hydrocracking and hydroprocessing and processes. The hydrocracking process is defined as treating heavier hydrocarbons with hydrogen to simultaneously split them into lighter derivatives and enhance the hydrogen/carbon ratio (El-Sawy et al. 2020 ). In hydroprocessing, various heteroatoms such as nitrogen, sulphur, oxygen and heavy metals are majorly captured from petroleum products via different treatment processes named: hydrodenitrogenation (Dasgupta and Atta 2020 ), hydrodesulphurisation (Han et al. 2018 ), hydrodeoxygenation (Yfanti and Lemonidou 2020 ) and hydrodemetallisation (Rana et al. 2020 ), respectively, as displayed in Eqs. ( 19 – 21 ).

Hydrodenitrogenation:

Hydrodesulphurisation:

Hydrodeoxygenation:

This can be achieved by reacting the upstream feedstock (heavy oils and petroleum products) with hydrogen through catalytic reaction, resulting in removing these contaminants and saturating the aromatics (C–C) bonds. The elimination process of these contaminates from feedstocks directly contributes to fuel upgrading because they deactivate the as-used catalysts due to their adsorption on the surfaces of the catalyst (blocking of active catalyst sites). Recently, the appeal for inserting hydrogen in hydroprocessing has been increasingly growing (Al Obaidi et al. 2018 ). From the environmental point of view, the key cause of this pattern is the combination of strict environmental legislation governing gaseous greenhouse emissions and other particulate contaminants, as well as product quality specifications. Generally, numerous upgrading techniques are directed to improve the physicochemical properties of heavy oils by decreasing their viscosity and carbon/hydrogen ratio at the same time (Misra et al. 2017 ).

Production of ammonia

Ammonia is deemed one of the essential chemicals largely employed in industrial fertiliser activities with huge global production rates. The biggest ammonia production plant has projected to achieve a daily capacity rate of 3300 metric tons (Brightling 2018 ). Broadly, ammonia can be introduced as fertiliser in the agriculture sector. Additionally, it is provided to various industries such as polymers processing, explosives, refrigerant, pharmaceuticals, gas sensors and fuel cells. The ammonia synthesis process is promoted by the catalytic reaction between hydrogen and nitrogen elements through the Haber process (Arora et al. 2018 ). It is performed in the as-designed reactor under operating conditions of 20–30 Mpa and 300–500 °C for pressure and temperature, respectively, using KOH-promoted finely divided iron catalysts with the required energy of 2.5 EJ (Tolod et al. 2019 ).

Moreover, the hydrogen addressed to the ammonia synthesis process is primarily derived from steam gas reforming, which is not regarded as environmentally friendly. Accordingly, there is an increasing interest in other green and sustainable ammonia synthesis pathways, such as electrochemical hydrogen manufacturing techniques and photocatalytic nitrogen fixation (artificial photocatalysis). The distinctiveness of the electrochemical ammonia synthesis pathways routes is controlled by the employed energy sources. Hydrogen can be generated from water employing an electrolysis process using renewable green sources (i.e. wind and solar energy) and, hence, reduce harmful greenhouse gaseous emissions (Bicer and Dincer 2017 ).

Metallurgical industries

Generally, hydrogen can produce oxy-hydrogen flames in industrial metallurgical activities and act as a reducing agent to obtain metals from their ores. During the oxy-hydrogen flames synthesis process (exothermic reaction), hydrogen is allowed to react with oxygen at very high temperatures (3000 °C) to produce oxy-hydrogen flames, used later for cutting and welding working on non-ferrous metals (Polverino et al. 2019 ). Otherwise, hydrogen is reliably characterised by its high ability to recover (reduce) metals from the aqueous solutions of their salts (hydrogen reduction). The metals may be powdered for later metallurgical usage or incorporated into a composite material. Chemically, hydrogen can interact with the elements of periodic tables in three ways: (1) ionic bond formation between the elements of Ia and IIa groups, (2) interstitial solid solution between the elements of VIa, VIIa and VIII groups and (3) metallic bond between elements of IIIa, Iva and Va groups. Additionally, the electrostatic shielding phenomenon can be generated by attributing the hydrogen's capability to capture free electrons and the self-trapping of metals. Likewise, the small particle size of hydrogen effectively facilitates the process of metal–hydrogen interaction (Agrawal et al. 2006 ).

To ensure the long-term production of clean and green hydrogen, it is crucial to conduct a critical assessment of various production routes and their environmental impacts, as well as seasonal storage and utilisation options. Hydrogen is produced from either fossil-based or renewable feedstocks; however, each route has advantages and disadvantages. The current hydrogen colour coding is imprecise, assuming that green hydrogen always has lower carbon emissions than blue or grey hydrogen, which is not always accurate.

Water electrolysis is gaining momentum; however, meeting 24% of energy demand with hydrogen in a 1.5-degree scenario of climate change mitigation will necessitate massive amounts of additional renewable electricity generation. In this scenario, approximately 31,320 terawatt-hours of electricity would be required to power electrolysers, which is more than is currently produced globally from all sources combined. Furthermore, > $11 trillion in manufacturing, storage and transportation infrastructure would be needed. The affordability and accessibility of freshwater is one side of the coin, and the proximity of these two supplies, i.e. renewable energy and freshwater, is the other. Water electrolysis research priorities include lowering the capital cost of electrolysis technology, locating water resources, find utilisation routes for the produced oxygen and increasing the efficiency of the process.

In terms of biomass gasification, the economic feasibility of producing hydrogen from biomass must be closely related to the availability and affordability of raw materials in the surrounding area. The main characteristics of the supply materials are the biomass physicochemical properties, distribution and hydrogen rate. Because biomass feedstocks differ greatly in structural composition and shape, all of these factors must be considered when combining the feedstock with the appropriate conversion technology. In conclusion, there are challenges associated with the most common hydrogen generation routes, such as steam methane reforming, water electrolysis, coal or biomass gasification, methane pyrolysis with or without carbon capture and storage technology.

To understand advances in evaluating environmental impacts due to hydrogen production, we performed an intensive critical review of 24 life cycle assessment studies published from 2019 to 2021, including methods and findings. The important methodological approaches and key findings observed were:

No two life cycle assessment studies were identified to be similar. There were differences in the geographical and temporal span, functional units and system boundaries considered, and environmental impact categories assessed. Therefore, it is recommended that the policymakers pay heed to the modelled processes and extent of the system boundary for making decisions for creating a sustainable hydrogen economy.

Many life cycle assessment studies did not encompass processes, inputs and outputs for ‘cradle-to-grave’ analysis. Thus, future research should pay more attention to ‘cradle-to-grave’ evaluation for robust decision-making.

In addition to the global warming potential and depletion of fossil fuels, environmental impacts in more categories for hydrogen production processes must be evaluated.

Furthermore, large-scale energy storage is key in securing the energy supply chain for the next energy transition using electrolysis-generated hydrogen. The Underground Seasonal Hydrogen Storage (USHS) holds great potential to overcome the natural temporal fluctuations inherent in renewable energy production at the scale required to achieve net-zero by 2050. The selection of geological porous media for USHS should be based on a comprehensive geological investigation that includes an assessment of their utility on both a basin and regional scale, fluid flow behaviour of hydrogen in brine-saturated subsurface reservoirs, an assessment of storage capacity, the safety of long-term storage, geochemical and biological reactions triggered by hydrogen injection, the geomechanical response of the subsurface to hydrogen storage and other measures. The discussed procedures can lead to informed decision-making in terms of forecasting best-operating strategies and ensuring safe and efficient hydrogen storage installation. Further research to integrate the theoretical studies with existing experimental USHS trials is required to minimise the uncertainty that might be associated with the feasibility of large-scale hydrogen storage. Finally, blending the need with the various utilisation routes such as fuel production, ammonia production, metallurgical industries and power systems is crucial in the hydrogen economy.

Change history

31 march 2022.

A Correction to this paper has been published: https://doi.org/10.1007/s10311-022-01432-x

Abbreviations

Carbon capture storage and utilisation

Levelised cost of hydrogen

Water–gas shift reaction

Underground Seasonal Hydrogen Storage

Proton exchange membrane fuel cells

Phosphoric acid fuel cells

Solid oxide fuel cells

Molten carbonate fuel cells

Direct methanol fuel cells

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Acknowledgements

The authors would like to thank OQ Oman for their generous financial support (project code: CR/DVC/SERC/19/01). The authors would also like to acknowledge the support of the Sustainable Energy Research Centre at Sultan Qaboos University. Ahmed Osman and David Rooney wish to acknowledge the support of The Bryden Centre project (Project ID VA5048). The Bryden Centre project is supported by the European Union’s INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB). Neha Mehta acknowledges funding from the Centre for Advanced Sustainable Energy (CASE). CASE is funded through Invest NI’s Competence Centre Programme and aims to transform the sustainable energy sector through business research.

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Environmental Science Department, Faculty of Science, Port Said University, Port Said, Egypt

Ahmed M. Elgarahy

Egyptian Propylene and Polypropylene Company (EPPC), Port-Said, Egypt

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Mahmoud Hefny

Geology Department, South Valley University, Qena, Egypt

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Osman, A.I., Mehta, N., Elgarahy, A.M. et al. Hydrogen production, storage, utilisation and environmental impacts: a review. Environ Chem Lett 20 , 153–188 (2022). https://doi.org/10.1007/s10311-021-01322-8

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Green hydrogen can be stored in a liquid form. Wolfgang Kumm/picture-alliance/dpa/AP Images

Green Hydrogen: Could It Be Key to a Carbon-Free Economy?

Green hydrogen, which uses renewable energy to produce hydrogen from water, is taking off around the globe. Its boosters say the fuel could play an important role in decarbonizing hard-to-electrify sectors of the economy, such as long-haul trucking, aviation, and heavy manufacturing.

By Jim Robbins • November 5, 2020

Saudi Arabia is constructing a futuristic city in the desert on the Red Sea called Neom. The $500 billion city — complete with flying taxis and robotic domestic help — is being built from scratch and will be home to a million people. And what energy product will be used both to power this city and sell to the world? Not oil. The Saudis are going big on something called green hydrogen — a carbon-free fuel made from water by using renewably produced electricity to split hydrogen molecules from oxygen molecules.

This summer, a large U.S. gas company, Air Products & Chemicals, announced that as part of Neom it has been building a green hydrogen plant in Saudi Arabia for the last four years. The plant is powered by 4 gigawatts from wind and solar projects that sprawl across the desert. It claims to be the world’s largest green hydrogen project — and more Saudi plants are on the drawing board.

Green hydrogen? The Saudis aren’t alone in believing it’s the next big thing in the energy future. While the fuel is barely on the radar in the United States, around the world a green hydrogen rush is underway, and many companies, investors, governments, and environmentalists believe it is an energy source that could help end the reign of fossil fuels and slow the world’s warming trajectory.

“It is very promising,” said Rachel Fakhry, an energy analyst for the Natural Resources Defense Council. Experts like Fakhry say that while wind and solar energy can provide the electricity to power homes and electric cars, green hydrogen could be an ideal power source for energy-intensive industries like concrete and steel manufacturing, as well as parts of the transportation sector that are more difficult to electrify. “The last 15 percent of the economy is hard to clean up — aviation, shipping, manufacturing, long-distance trucking,” Fakhry said in an interview. “Green hydrogen can do that.”

Germany has allocated the largest share of its clean energy stimulus funds to green hydrogen.

Europe, which has an economy that is saddled with high energy prices and is heavily dependent on Russian natural gas, is embracing green hydrogen by providing funding for construction of electrolysis plants and other hydrogen infrastructure. Germany has allocated the largest share of its clean energy stimulus funds to green hydrogen. “It is the missing part of the puzzle to a fully decarbonized economy,” the European Commission wrote in a July strategy document.

Hydrogen’s potential as a fuel source has been touted for decades, but the technology has never gotten off the ground on a sizeable scale — and with good reason, according to skeptics. They argue that widespread adoption of green hydrogen technologies has faced serious obstacles, most notably that hydrogen fuels need renewable energy to be green, which will require a massive expansion of renewable generation to power the electrolysis plants that split water into hydrogen and oxygen. Green hydrogen is also hard to store and transport without a pipeline. And right now in some places, such as the U.S., hydrogen is a lot more expensive than other fuels such as natural gas.

While it has advantages, says Michael Liebreich, a Bloomberg New Energy Finance analyst in the United Kingdom and a green hydrogen skeptic, “it displays an equally impressive list of disadvantages.”

“It does not occur in nature so it requires energy to separate,” Liebreich wrote in a pair of recent essays for BloombergNEF. “Its storage requires compression to 700 times atmospheric pressure, refrigeration to 253 degrees Celsius… It carries one quarter the energy per unit volume of natural gas… It can embrittle metal; it escapes through the tiniest leaks and yes, it really is explosive.”

In spite of these problems, Liebreich wrote, green hydrogen still “holds a vice-like grip over the imaginations of techno-optimists.”

Green hydrogen is produced using renewable energy, making it a CO2-free source of fuel. SGN

Ben Gallagher, an energy analyst at Wood McKenzie who studies green hydrogen, said the fuel is so new that its future remains unclear. “No one has any true idea what is going on here,” he said. “It’s speculation at this point. Right now it’s difficult to view this as the new oil. However, it could make up an important part of the overall fuel mix.”

Hydrogen is the most abundant chemical in the universe. Two atoms of hydrogen paired with an atom of oxygen creates water. Alone, though, hydrogen is an odorless and tasteless gas, and highly combustible. Hydrogen derived from methane — usually from natural gas, but also coal and biomass — was pioneered in World War II by Germany, which has no petroleum deposits. But CO2 is emitted in manufacturing hydrogen from methane and so it’s not climate friendly; hydrogen manufactured this way is known as gray hydrogen.

Green is the new kid on the hydrogen block, and because it’s manufactured with renewable energy, it’s CO2-free. Moreover, using renewable energy to create the fuel can help solve the problem of intermittency that plagues wind and solar power, and so it is essentially efficient storage. When demand for renewables is low, during the spring and fall, excess electricity can be used to power the electrolysis that is needed to split hydrogen and oxygen molecules. Then the hydrogen can be stored or sent down a pipeline.

Such advantages are fueling growing interest in global green hydrogen. Across Europe, the Middle East, and Asia, more countries and companies are embracing this high-quality fuel. The U.S. lags behind because other forms of energy, such as natural gas, are much cheaper, but several new projects are getting underway, including a green hydrogen power plant in Utah that will replace two aging coal-fired plants and produce electricity for southern California.

The Middle East, with the world’s cheapest wind and solar power, is angling to be a major player in green hydrogen.

In Japan, a new green hydrogen plant, one of the world’s largest, just opened near Fukishima — an intentionally symbolic location given the plant’s proximity to the site of the 2011 nuclear disaster. It will be used to power fuel cells, both in vehicles and at stationary sites. An energy consortium in Australia just announced plans to build a project called the Asian Renewable Energy Hub in Pilbara that would use 1,743 large wind turbines and 30 square miles of solar panels to run a 26-gigawatt electrolysis factory that would create green hydrogen to send to Singapore.

As Europe intensifies its decarbonization drive, it, too, is betting big on the fuel. The European Union just drafted a strategy for a large-scale green hydrogen expansion, though it hasn’t been officially adopted yet. But in its $550-billion clean energy plan, the EU is including funds for new green hydrogen electrolyzers and transport and storage technology for the fuel. “Large-scale deployment of clean hydrogen at a fast pace is key for the EU to achieve its high climate ambitions,” the European Commission wrote.

The Middle East, which has the world’s cheapest wind and solar power, is angling to be a major player in green hydrogen. “Saudi Arabia has ridiculously low-cost renewable power,” said Thomas Koch Blank, leader of the Rocky Mountain Institute’s Breakthrough Technology Program. “The sun is shining pretty reliably every day and the wind is blowing pretty reliably every night. It’s hard to beat.”

BloombergNEF estimates that to generate enough green hydrogen to meet a quarter of the world’s energy needs would take more electricity than the world generates now from all sources and an investment of $11 trillion in production and storage. That’s why the focus for now is on the 15 percent of the economy with energy needs not easily supplied by wind and solar power, such as heavy manufacturing, long-distance trucking, and fuel for cargo ships and aircraft.

The Fukushima Hydrogen Energy Research Field (FH2R), a green hydrogen facility that can generate as much as 1,200 normal meter cubed (Nm3) of hydrogen per hour, opened in Japan in March. Toshiba ESS

The energy density of green hydrogen is three times that of jet fuel, making it a promising zero-emissions technology for aircraft. But Airbus, the European airplane manufacturer, recently released a statement saying that significant problems need to be overcome, including safely storing hydrogen on aircraft, the lack of a hydrogen infrastructure at airports, and cost. Experts say that new technologies will be needed to solve these problems. Nevertheless, Airbus believes green hydrogen will play an important role in decarbonizing air transport.

“Cost-competitive green hydrogen and cross-industry partnerships will be mandatory to bring zero-emission flying to reality,” said Glen Llewellyn, vice president of Zero Emission Aircraft for Airbus. Hydrogen-powered aircraft could be flying by 2035, he said.

In the U.S., where energy prices are low, green hydrogen costs about three times as much as natural gas, though that price doesn’t factor in the environmental damage caused by fossil fuels. The price of green hydrogen is falling, however. In 10 years, green hydrogen is expected to be comparable in cost to natural gas in the United States.

A major driver of green hydrogen development in the U.S. is California’s aggressive push toward a carbon-neutral future. The Los Angeles Department of Water and Power, for example, is helping fund the construction of the green hydrogen-fueled power plant in Utah. It’s scheduled to go online in 2025.

A company called SGH2 recently announced it would build a large facility to produce green hydrogen in southern California. Instead of using electrolysis, though, it will use waste gasification, which heats many types of waste to high temperatures that reduce them to their molecular compounds. Those molecules then bind with hydrogen, and SGH2 claims it can make green hydrogen more cheaply than using electrolysis.

California officials see green hydrogen as an alternative to fossil fuels for diesel vehicles.

California officials also see green hydrogen as an alternative to fossil fuels for diesel vehicles. The state passed a Low Carbon Fuel Standard in 2009 to promote electric vehicles and hydrogen vehicles. Last month, a group of heavy-duty vehicle and energy industry officials formed the Western States Hydrogen Alliance to press for rapid deployment of hydrogen fuel cell technology and infrastructure to replace diesel trucks, buses, locomotives, and aircraft.

“Hydrogen fuel cells will power the future of zero-emission mobility in these heavy-duty, hard-to-electrify sectors,” said Roxana Bekemohammadi, executive director of the Western States Hydrogen Alliance. “That fact is indisputable. This new alliance exists to ensure government and industry can work efficiently together to accelerate the coming of this revolution.”

Earlier this year, the U.S. Department of Energy announced a $100 million investment to help develop large, affordable electrolyzers and to create new fuel cell technologies for long-haul trucks.

In Australia, the University of New South Wales, in partnership with a global engineering firm, GHD, has created a home-based system called LAVO that uses solar energy to generate and store green hydrogen in home systems. The hydrogen is converted back into electricity as needed.

All these developments, says Blank of the Rocky Mountain Instiute, are “really good news. Green hydrogen has high potential to address many of the things that keep people awake at night because the climate change problem seems unsolvable.”

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