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Noyan, O.F.; Hasan, M.M.; Pala, N. Hydrogen Energy Eco-System. Encyclopedia. Available online: https://encyclopedia.pub/entry/43541 (accessed on 04 September 2024).
Noyan OF, Hasan MM, Pala N. Hydrogen Energy Eco-System. Encyclopedia. Available at: https://encyclopedia.pub/entry/43541. Accessed September 04, 2024.
Noyan, Omer Faruk, Muhammad Mahmudul Hasan, Nezih Pala. "Hydrogen Energy Eco-System" Encyclopedia, https://encyclopedia.pub/entry/43541 (accessed September 04, 2024).
Noyan, O.F., Hasan, M.M., & Pala, N. (2023, April 26). Hydrogen Energy Eco-System. In Encyclopedia. https://encyclopedia.pub/entry/43541
Noyan, Omer Faruk, et al. "Hydrogen Energy Eco-System." Encyclopedia. Web. 26 April, 2023.
Hydrogen Energy Eco-System
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This entry is adapted from the peer-reviewed paper 10.3390/en16031484

Climate change primarily caused by the greenhouse gases emitted as a result of the consumption of carbon-based fossil fuels is considered one of the biggest challenges that humanity has ever faced. Moreover, the Ukrainian crisis in 2022 has complicated the global energy and food status quo more than ever. The permanency of this multifaceted fragility implies the need for increased efforts to have energy independence and requires long-term solutions without fossil fuels through the use of clean, zero-carbon renewables energies. Hydrogen technologies have a strong potential to emerge as an energy eco-system in its production-storage-distribution-utilization stages, with its synergistic integration with solar-wind-hydraulic-nuclear and other zero-carbon, clean renewable energy resources, and with the existing energy infrastructure.

hydrogen renewables production

1. Introduction

Anthropogenic CO2 emissions that cause global warming have been increasing at a worldwide scale since the end of 19th century. With a combined 66.4% of worldwide fossil fuel use and 67.8% of global fossil CO2 emissions in 2021, China, the United States, the European Union, Japan, Russia, and India continued to lead the world in CO2 emissions. India and Russia had the biggest relative increases in fossil CO2 emissions among the six countries in 2021 in contrast with 2020 [1]. This situation has motivated the search for clean energy solutions. As one of the most important alternatives, hydrogen (H2) has a long history of being used as fuel, dating back to 1806 when the first internal combustion engine (ICE) was powered by a mix of hydrogen and oxygen. With the increasing global environmental concerns, hydrogen has recently emerged as a major secondary energy source to store and transport energy produced from other sources, since it can be used without direct emissions of air pollutants or greenhouse gases. Hydrogen, as a synthetic fuel, can be produced from a variety of low-carbon sources, such as wind and solar, and can store energy efficiently with high energy content per unit mass, which further contributes to reducing the environmental impact. Developing hydrogen technologies are expected to provide a pathway to a resilient and sustainable energy eco-system by: (1) using hydrogen as an efficient energy storage and transport medium to replace carbon-based fuels; (2) storing the electricity to meet weekly or monthly imbalances in supply and demand in renewable energy (RE) systems; and (3) replacing the carbon-intensive hydrogen production methods with green ones to supply for the demand of all industries [2].

2. Overview of the Global Energy Landscape

After the world economy had been affected by COVID-19 pandemics, the impacts of the Russia–Ukraine conflict in 2022 on international energy supplies are causing further vital consequences. Russia is one of the largest fossil fuel producers in the world, being third in the production and exporting of crude oil; second in the production and the largest in the export of natural gas (NG); and the third in the export of coal. It is the producer of 12% of the world’s oil demand, 17% of the NG demand, and 5% of the coal demand and deeply integrated into Europe’s distribution networks. The European Union (EU) and United Kingdom, combined, import about 40% of the NG, about 35% of the crude oil and more than 45% of the coal from Russia [3][4]. The Russia–Ukraine conflict and sanctions have increased the interest in H2 as a potential cost-efficient, clean, renewable, sustainable fuel for the future.
In addition to its impact on fossil fuels, the Russia–Ukraine conflict also affected the biofuel ecosystem. Russia is the world’s largest exporter of wheat and the second largest exporter of sunflower oil, while Ukraine is the world’s largest exporter of sunflower oil, the fourth largest exporter of maize, and the fifth largest exporter of wheat. Russia and Ukraine produce nearly 25% of the world’s wheat and barley, 20% of the world’s corn, and over 60% of its sunflower oil [5][6]. Rising food prices due to the conflict raised the concerns for potential famine risk and rekindled the “food versus biofuel” argument. In the US, the leading biofuels producer, 36% of total corn production was used as biofuels (blended into petroleum) in 2021. In the EU, 12 million tons of grain, including wheat and maize, is turned into ethanol—around 7% of the bloc’s production [6]. It was estimated that the calories diverted to biofuel production under the current policies and future commitments are equivalent to the annual needs of 1.9 billion people [7].
The EU has developed a strategy concentrating on emission-free renewable H2 and plans to install 40 gigawatts of green H2 electrolyzer capacity by 2030. Germany recently started a collaboration effort with Australia to decrease the production cost of renewable H2 and accelerate the innovation process in both countries and to minimize reliance on Russian natural gas [8]. China identified H2 as one of the six industries of the future in its 5-year economic plan [9][10]. Japan aims at having 800,000 H2 fuel cell vehicles (FCVs) and 900 H2 refueling stations by 2030 [11]. South Korea has a target to supply 10% of its energy demands with H2 by 2030, increase this rate to 30% before 2040 and make it the country’s largest energy carrier by midcentury [12]. India sees H2 as the opportunity for “quantum leap” towards energy independence by 2047 and shapes its energy policy accordingly [12]. The US Department of Energy (DoE) plans to invest $9.5B for the commercialization of innovative green H2 production technologies with the goal of reducing the production cost by 80% within the next decade. Australia, Saudi Arabia, Morocco, United Arab Emirates and Oman have announced strategies to develop clean H2 to diversify their energy portfolios [13].
Important developments in H2 production, storage, distribution, and utilization have been demonstrated in recent years. As a fuel, H2 has considerably higher energy per unit weight than other fuels such as gas, diesel, and methanol (see Figure 1) [14]. Therefore, it has an immense market potential for the transportation sector, and industry. H2 storage is one of the key factors in the H2 economy. RE electricity is widely stored in chemical batteries but the limited capacity, lifetime, and high cost of batteries have limited their applications. H2 could be employed as an energy carrier, like a battery, for RE electricity. The stored H2 can be used for electrical power production in a gas turbine or fuel cell, or mechanical power production in an ICE. Solid-state materials in standard temperature and pressure conditions (STP) are seen as a promising method of storing H2, especially for mobile systems. In terms of H2 utilization, the deployment of fuel cell (FC) systems opened a new window to H2 fuel success in the transportation sector. Currently, H2-powered engines based on proton exchange membrane (PEM) FCs are deployed worldwide.
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Figure 1. Comparison of specific energy (energy per mass or gravimetric density) and energy density (energy per volume or volumetric density) for several fuels based on lower heating values.
H2 is defined as a clean energy source but is mostly produced via the combustion of fossil fuels (grey or black-brown category). For commercial production of H2, four methods come forward, three of which use fossil fuels: steam methane reformation, oxidation, and gasification. The fourth one is electrolysis [15]. When the electricity for H2 production is supplied by clean electricity, (solar photovoltaics -PV-, concentrating solar power, wind, wave, hydropower, and geothermal), the produced H2 is regarded as zero-carbon green H2. This means cleanliness, inexhaustibility, and independence from geopolitical effects. Today, there are highly efficient electrolyzers to produce H2 as an environmentally friendly replacement for fossil fuels in all sectors. The integrated RE H2 system and economy is a supply chain from production to storage, transportation/distribution and utilization (Figure 2). In an established large-scale H2 infrastructure the most featured difficulties are cost reduction and the scaling up of this chain.
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Figure 2. Integrated RE H2 system and economy.

3. Renewable Energies

RE globally and technically promises more zero-carbon action. Hydropower and three intermittent energies, solar, wind, and tidal are resources that can forever be used to produce and store H2 via the electrolysis of water, which is also a medium of energy storage. The global RE generation capacity has reached 2.8 TW as of 2021. Within this capacity, hydropower accounts for 1.3 TW while solar and wind powers account for 126 GW and 110 GW, respectively. The share of RE in the global electricity generation increased to 29% in 2020, up from 27% in 2019 [15]. The costs for RE continued to fall in 2021 with a 15% and 13% reduction in the cost of electricity from onshore wind and offshore wind, respectively, and a 13% reduction in the cost of solar PV generation compared to 2020, despite the rising materials and equipment costs. In 2021, about 60% of newly installed RE power had lower costs than the world’s cheapest coal-fired option in the G20 countries. The benefit from RE in 2022 has been unparalleled, given the fossil-fuel price crisis: (i) in Europe, new solar and wind power added in 2021 had lifetime costs per kWh that, on average, were four to six times lower than the 2022 minimal producing expenses of fossil fuels; (ii) the additional RE capacity developed in 2021 may globally lower the cost of producing electric power in 2022 with a minimum of 55 billion USD; and (iii) solar and wind power alone prevented the import of fossil fuels, mostly NG, in the amount of 50 billion USD in Europe during January to May 2022 [16]. As a historical turning point, the 2022 crisis showed that without RE, conditions, particularly in the developed countries, would be much more challenging for consumers, economies, and the environment. Utility-scale solar PV and wind turbines have become effectively competitive over fuel and CO2 cost in Europe.
Solar and wind energy, with their short project lead times and relatively lower initial investment, present affordable pathways for countries to quickly reduce and eventually phase out fossil fuels while limiting the economic impact. The recent report “Renewable Power Generation Costs in 2021” published by the International Renewable Energy Agency (IRENA) states that “RE is by far the cheapest form of power today. 2022 is a stark example of just how economically viable new RE power generation has become. It frees economies from volatile fossil-fuel prices and imports, curbs energy costs and enhances market resilience. Today’s situation is a devastating reminder that RE and energy saving are the future” [16].

4. Clean RE for an Integrated H2 Energy System

H2 is the most abundant and lightest element (2.016 × 10−3 kg/mol) in the universe, and energy-efficient fuel on the Earth (calorific or heating value). It is the richest in energy per-unit mass (140 MJ/kg), and almost three times higher than solid fuels (50 MJ/kg) (see Figure 1). It exists mainly in compounds such as water, methane, H2S, living organisms, organic waste, etc. It is environmentally benign, since water is the only emission product at conversion to energy, and unlike electricity, it can be stored. RE H2 produced either at a central production plant or onsite at a H2 refueling station (HRS) could help achieve carbon-free energy economy. Despite the fact that onsite H2 production by electrolysis typically has higher costs than production at central plants due to limited production capacity, it was shown that the at-the-pump prices end up being similar due to the additional H2 transport cost for the case of central production. H2 production cost constitutes the largest portion of the levelized H2 price at the pump due to the high capital investment in both RE electricity generation and water electrolysis. As the relevant technologies advance, the at-the-pump H2 price is estimated to fall substantially. In a 2022 study, the future at-the-pump prices of RE H2 produced onsite at an HRS in the 26 EU countries are estimated as 11 euros/kg in 2020, decreasing to 5 euros in 2050 while the European Commission (EC) target is 1.8 euros/kg H2 by 2030. With a 3 euro/kg H2 subsidy for RE H2 production, it is expected that the price targets can be reached earlier. Moreover, the subsidy can enable RE H2 to reach an energy-based cost parity with diesel fuel (0.034 euros/MJ for H2 compared to 0.038 euros/MJ for diesel) by 2030. Furthermore, since H2 FCVs being more efficient, they can travel 1.3 times further compared to diesel trucks using the same amount of energy. Therefore, considering both increasing diesel prices and vehicle efficiency, the 3 euros/kg subsidy can pave the way to make H2 cost-competitive against diesel well before 2030. Although subsidies could be a promising way to lower prices, to achieve true climate benefits, robust regulations on sourcing electric power are necessary in addition to direct financial support for RE H2 production [17][18][19][20][21][22].
It has been shown that integrated PV-battery- H2 systems can sustain an affordable electricity cost over the system’s lifetime owing to the energy storage components. Similarly, a multi-optimization problem has been studied for integrated wind-PV energy systems for RE H2 production to assess its exergo-economic performance and to determine the optimal operation conditions [23]. However, when the H2 is integrated with other RE systems the overall complexity of the combined system increases the uncertainties in the power load which requires careful design to address the nonlinear characteristics and added variables [24][25].

5. H2 Production

Though hydrogen is the most common element constituting ~70% of the universe, free hydrogen is not readily available on earth. It is typically bound into molecules such as water and hydrocarbons, particularly methane (CH4). It can be separated from these molecules by using energy in various forms. H2 is identified with a spectrum of colors depending on its source or production method: (1) black-brown (from coal), (2) gray (hydrocarbons without carbon capture and storage—CCS), (3) blue (hydrocarbons with CCS), (4) turquoise (pyrolysis), (5) pink (nuclear powered electricity), (6) yellow (solar energy), (7) red (high-temperature catalytic process), (8) white (naturally-occurring geological hydrogen) and (9) green (clean RE electricity) (Figure 3). Natural gas (NG), crude oil, coal, and water electrolysis processes are the most commonly used sources for H2 production, with shares of 49, 29, 18, and 4%, respectively [26].
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Figure 3. The spectrum of H2 production processes.

5.1. Black-Brown and Gray H2 Production

By far the most widely used H2 production method is the steam reforming of natural gas, also known as steam methane reformation (SMR). This is one of the cheapest H2 production technologies in terms of operation and production costs [27], and is also renowned for its high efficiency operation [28]. Steam and natural gas are heated together up to ~900 °C at high pressure in the presence of a nickel-based catalyst. The process yields a mixture of carbon monoxide (CO) and H2 known as syngas which is further treated in the water–gas shift reaction to produce more hydrogen and carbon dioxide. The H2 produced by this method is named gray H2. However, it has serious downsides, for instance: (1) as an endothermic reaction it requires huge amount of heat supply at extremely high temperature; (2) generating heat to produce extra steam (exceeds the stoichiometric requirement by two to three times) which is required for the curving of deactivation events like coking and moving the equilibrium position of the reaction more in the direction of the H2 generation, and (3) complex extraction of H2 from a combination of other outcomes (CO, CO2, CH4, and H2O) by employing pressure swing adsorption (PSA, separates gases in a mixture of gases to obtain a high degree of purity) and water–gas shift reactors [29]. In a similar process called gasification, steam and oxygen are used to break molecular bonds in black coal or lignite (brown coal) to produce syngas consisting of CO, H2, CO2, CH4, and water vapor. These processes also result in substantial CO2, which must be captured and stored (CCS). To address these limitations, a carbon-efficient process concept was developed that converts captured CO2 to CO2 in two steps. It prevents direct contact between O2 and CH4, lowering safety risks. The external energy demand is reduced by coupling endothermic and exothermic reactions, and the operating temperature to produce H2 is reduced by more than 150 K when compared to conventional CH4 reforming. Iron, nickel, and calcium oxides are used in this process as solid intermediates, which operates between 923 and 998 K. By simultaneously delivering steam and methane to the materials in a fixed bed, an H2-rich stream is created in the first phase, and a CO-rich stream is created in the second step by renewing the materials with a combination that is like air. H2 concentrations of over 65 mol% and CO2 conversion rates of over 80% were reached in proof-of-concept tests. According to cyclical experiments, carbon deposition is reduced, and the ratio of generated H2 to CO is around 2. Experimental findings demonstrate that the process idea enables conventional methane reforming for the generation of syngas to avoid the limitations of thermodynamic equilibrium [29].
Biomass is another source of feedstock for H2 production. As gasification is a well-established and commercially available technology, it is commonly applied to biomass like coal. The oxidation eventually results in a combination of CO, H2, CO2, CH4, N2, and higher hydrocarbons gases [30]. After the creation of syngas H2/CO2 a WGS reactor is used for H2 production with high temperature [30]. It is really hard to obtain pure H2 as biomass produces large amounts of tar even with high temperature operation due to higher hydrocarbons. The efficiency of the biomass ranges from 35–50% [31]. The moisture contents in biomass limit the efficiency of this process as it needs to be vaporized. Gasification of biomass for H2 production are not yet commercially established due to high operation costs and the complicated process for cleaning the H2 from tar.
Approximately 6% of the globally extracted NG and 2% of the globally extracted coal are used for H2 production. Over 95% of current H2 production (black-brown and grey) leading over 2% of global annual CO2 emissions by 2020 is by steam reforming of NG, its partial oxidation, and coal gasification; very little of it is green. Producing H2 in pure form for industry is energy intensive. The global demand for H2 is for its utilization in chemical production (40%), oil refineries (33%), metallurgical industries (3%), and the rest is used in synthetic fuel and petrochemical production, semiconductor and glass manufacturing, welding processes, powering FCVs, and in the food industries [32][33].

5.2. Blue H2 Production

H2 produced from NG processing coupled with CCS is referred to as blue H2 being carbon neutral. Blue H2 production is predicted to reach 80 million metric tons by 2050 [34]. The climate change impact of H2 produced from fossil fuels coupled with CCS is still debated, especially in regard to the leakage risks of indefinite long-term storage of methane gas [35]. H2 produced using fossil-fuel feedstock causes greenhouse gas (GHG) emissions, even when CCS is used. These emissions could be substantial, and the cost of CCS is higher than frequently assumed. The percentages of GHG releases and those of CCS during blue H2 production process determine the impacts of blue H2 on the climate and the competitiveness of blue H2 regarding global environment seems to depend on high CCS and negligible GHG releases. However, according to a research analysis the positive effects of blue H2 production in terms of GHG emissions and climate change are insignificant and the use of blue H2 seems challenging to justify on climate grounds [36]. Establishing H2 supply chains on the basis of fossil fuels, as many national strategies forecast, may be incompatible with decarbonization goals [18]. Therefore, developing advanced supply chains in combination with high CCS capture rates is vital for blue H2 to be a sustainable option for an actual net zero hydrogen economy [37].
In a study of optimizing blue hydrogen production for future energy systems, a northern European perspective based on Germany was investigated [38]. Blue routes provide significant benefits in the hydrogen industry but limited gains in the electricity sector. Systems using CCS (CO2 Capture and Storage) become significantly less expensive than those depending just on renewables in an energy system of the future when vast amounts of carbon-free fuels are required for usage in sectors like long-distance transportation and industrial. Another conclusion is that paths that allow CCS would prioritize carbon-free fuels above electrification. The study found that when conventional CCS technology were used, system costs decreased by 29% and CO2 emissions decreased by 106% (biomass’s negative emission), with hydrogen generated for less than the cost of power. With the possibility for integrated power cycles that enable variable power and hydrogen generation, advanced blue hydrogen technologies might provide an extra 12% decrease in system costs with extra negative emissions because of extremely high CO2 capture metrics. Since handling the intermittent hydrogen fluxes resulting from such an operational strategy proved to be expensive, scenarios with lower variable renewable energy shares proved to be more cost-effective. Originally, these concepts were created to better integrate higher shares of wind and solar energy [38].

5.3. Turquoise H2 Production

Like gray and blue H2, turquoise H2 also uses methane as a feedstock, but the process is driven by heat produced with electricity rather than through the combustion of fossil fuels. In the process called pyrolysis, hydrogen is separated from methane (i.e., NG) in one step via flow through a molten metal catalyst in a bubble column at higher temperatures (1065 °C). This pyrolysis process is an endothermic reaction where two molecules of H2 in gaseous form and one molecule of carbon in solid form is produced from one molecule of methane. This reaction is advantageous over methane reforming as it can be performed in near ambient pressure. Theoretically, this reaction produces a larger amount of solid carbon than H2 with the ratio of 1 to 3 in favor of carbon. However, there is no GHG-free methane reform process. Industrial quality solid carbon can be used as manufacturing feedstock, for example for carbon-fiber production or landfilled which are simpler than capturing and sequestering CO2. Since it is not released into the atmosphere and does not pollute groundwater in landfills, turquoise H2 may contribute to low-carbon H2 production in the future. Pyrolysis has not been fully commercialized yet for H2 production. There are variants of the process that are being developed including thermal, plasma, and catalytic decomposition. The required process temperature for thermal, plasma, and catalytic decomposition are over 1000 °C, over 2000 °C, and below 1000 °C. Plasma processes realized on industrial scale are still being further developed. The other processes are at an early stage of development [39].

5.4. Pink H2 Production

In a global zero-carbon energy system, nuclear-produced H2 has a potential place. In this alternative method, H2 can be produced by steam of which heat source is nuclear power plant. Nuclear energy can be used, as a source of electricity and heat, to produce H2 efficiently and with little to no CO2 emissions.
The nuclear alternative has had the challenge of high cost in the past, but that is now changing in the midst of a global energy crisis causing significant price hikes for fossil fuels as well as making global supplies less secure [40]. Nuclear power is the second largest low-carbon source of electricity after hydropower, supplying 10% of the world’s electricity. However, nuclear power generating capacity will need to more than double to achieve net zero by 2050 [41].
Light water reactor technology presents great potential for H2 production. The reactors reach an operating temperature of ~300 °C, while district heating and seawater desalination processes require ~150 °C. Numerous district heating systems have been in operation for decades. This heat could also be utilized for the production of fresh water, H2 or other products in cogeneration (simultaneous production of electricity, heat and heat-derivative product) [41][42]. Electricity and heat produced at temperature levels of 300 °C in small modular reactors can be used in solid-oxide electrolyzer cells (SOEC). Six small modular reactors with a total capacity of 300 MW can meet the annual H2 demand of a mid-sized ammonia plant (73,000 tons of H2/year) [43]. In the longer term, advanced nuclear reactors will be able to provide the higher temperatures required for industrial processes such as steel and cement production, as well as H2 production by thermochemical water splitting (with some reactor designs having coolant outlet temperatures of 800–1000 °C). A single 1000 MW nuclear power reactor could produce more than 200,000 tons of H2 each year to fuel more than 400,000 FCVs or more than 16,000 long haul FC trucks [44]. In the near future, small-to medium-sized reactors can be dedicated, on an industrial scale, to making H2, which also takes away the cost of storage and transportation. Moreover, using base-load surplus of nuclear or any RE power grid H2 can be produced for the local market by a mix of electrolysis and thermal processes. Nuclear power, by its potential for further development, can be a part of efforts to achieve global climate targets, and it is a sufficiently experimented alternative which can stimulate clean H2 production preferences.

5.5. Green H2 Production

H2 produced by water electrolysis using electricity from clean RE, namely, the green H2, promises to help meet global energy demand while contributing to climate action goals, and represents the most promising energy carrier for the zero-carbon economy. Falling RE energy prices coupled with the declining cost of electrolyzers and increased efficiency due to technology advancements have increased the commercial feasibility of green H2 production. Assuming these costs continue to fall, green H2 can be produced for $0.70–$1.60/kg by 2050, a price range competitive with NG [45].
In 2003, the European Commission (EC) suggested that the EU should attain a H2-based economy in 2050 and estimated that 35% of newly produced vehicles will be fueled by zero-carbon H2 in 2040 [46]. This strategy updated in 2022 aims to ensure low-carbon H2 technologies be able to be deployed at large scale to cover all hard-to-decarbonize sectors. Projects varying from 20 MW to above 100 MW are being developed with the current H2 costs of 5–8 €/kg. The EC launched a call for a 100 MW electrolyzer to scale up manufacturing facilities to multi-GW per annum, to attain the goal of 1.5 $/kg of green H2 by 2025 provided that low-cost RE power is ensured. In this way, it is believed that green H2 production cost parity (or even superiority) with fossil- fuels could be achieved as early as 2025. Deploying zero-emission H2 vehicles (heavy-duty FC H2 buses, trucks, and trains) is an essential part of EU’s H2 policy. They will require a very reliable, high-capacity HRS capable of delivering several tons each day. The regulation requires one HRS available every 150 km along the Trans-European Transport Network and in every urban node by 2030. In order to fully unleash the potential of H2 technologies and establish them as a mainstream means of decarbonization in all transportation modes, cost reduction, increased performance and lifetime, sustainability, recycling, and eco-design in transport FC system components and vehicles are needed [47]. Another H2 -hub plan approved by the EC in July 2022 allocates €5.4 billion to support 41 technology development projects. A small hub near Hamburg, Germany, is already under construction, and a larger hub is being built at the Port of Rotterdam in Netherlands. Portugal will produce green H2 using wind power and transport it into the Port of Rotterdam. Spain houses Europe’s largest green H2 facility with a 100 MW solar farm. In USA, the Intermountain Power Project will also have a 220 MW electrolysis system based on PV generators to produce H2 on site, along with the facilities to store up to 300 GW hours of the gas in underground salt domes [48]. Studies on China’s potential of green H2 production showed that the efficiency of the H2 production from wind power is significantly higher than that from solar power. These efficiencies and green H2 production in each province of China are expected to significantly increase by 2030 [49]. In a recent study in Sweden the feasibility of producing green H2 using electrolysis with RE electricity in HRS was investigated. With hourly solar radiation and wind speed data, and electricity price, simulations were used to assess the cost components of H2 production. The study revealed that integrating the electricity grid in green H2 production was important, wind speed was critical for cost reduction, whereas solar radiation had less influence. Further, a combination of solar and wind could provide better performance in an off-grid scenario. The most encouraging finding was the cost, which was competitive with reported costs in other EU countries [50].
In addition to the currently existing technologies, efficient and direct conversion of sunlight through a photocatalytic process to split ocean water without use of electricity can pave the way to a clean production route. A unique, CdS/ZnTHPP binary nanocomposite was recently created and employed as a reliable photocatalyst for the generation of H2 in the presence of artificial sunlight which is all solid state. It demonstrated a conversion rate of almost ten times greater than that of pure CdS nanorods, the data showed that this nanosystem had remarkable photocatalytic activity. This innovation may lead to further improvement toward high-efficient photocatalytic green H2 productions for real applications [51].
The inherent production intermittency of RE sources makes operation of energy systems with large RE components reliably and securely quite challenging. In an energy system with 100% RE the infrastructures for carbon-free H2 production by electrolysis, and storage and/or transportation can overcome the intermittency of RE. The commercial production of H2 by electrolysis of water can achieve efficiency up to 81%. Less than 0.1% of H2 production globally comes from water electrolysis today, and the H2 produced by this means is mostly used in markets where high-purity H2 is necessary (electronics and polysilicon), but electrolysis, as an energy-intensive process for H2 production, is still confronting challenges [52][53][54][55][56][57]. Electrolysis can be classified into three types: (i) alkaline (AWE), (ii) proton-exchange membrane (PEM), and iii) solid-oxide steam (SOSE). H2 production by AWE is well established technology up to the megawatt range for commercial level (Figure 4a).
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Figure 4. (a) H2 generation from alkaline electrolyzer. (b) schematics of PEM water electrolyzer (Solid polymer electrolyte membrane is the same as proton exchange membrane).

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Omer Faruk Noyan
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