Abdulrahman bin Jumah
Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia. E-mail: abinjumah@ksu.edu.sa
First published on 22nd August 2024
Green hydrogen, a versatile and sustainable energy carrier, has garnered increasing attention as a critical element in the global transition to a low-carbon economy. This review article comprehensively examines the production, applications, and potential of green hydrogen, accompanied by the challenges and future prospects associated with its widespread adoption. The production of green hydrogen is a central focus, due to its environmental benefits and distinctive characteristics. The article delves into the various techniques and technologies employed in green hydrogen production, emphasizing the need for cost reduction and increased scale for economic viability. Focusing particularly on applications, the review discusses the diverse sectors where green hydrogen demonstrates immense promise. Challenges and limitations are explored, including the intermittent nature of renewable energy sources, high production costs, and the need for extensive hydrogen infrastructure. The article also highlights the pressing need for innovation in electrolysis technology and materials, emphasizing the potential for cost reduction and increased efficiency. As industries gradually transition to green hydrogen as a cleaner feedstock, its demand and cost-competitiveness are projected to increase. This review article thoroughly evaluates the current status of green hydrogen and provides valuable insights into its potential role in the transition to a sustainable energy system.
Type | Name | Country | Power capacity (MW) | Year | Ref. |
---|---|---|---|---|---|
Biomass power plant | Alholmens Kraft power plant | Finland | 240 | 2002 | 11 |
Hydroelectric power | Three Gorges dam | China | 22![]() |
2003 | 12 |
On-shore wind farm | Gansu wind farm | China | 7965 | 2009 | 13 |
Biomass power plant | Ironbridge power plant | United Kingdom | 740 | 2012 | 14 |
Biomass power plant | Polaniec biomass power plant | Poland | 220 | 2012 | 15 |
Photovoltaics & hydroelectric power | Longyangxia dam solar park | China | 2130 | 2015 | 16 |
Parabolic trough and solar power tower (CSP) | Ouarzazate solar power station | Morocco | 580 | 2016 | 17 |
Photovoltaics | Bhadla solar park | India | 2245 | 2018 | 18 |
Photovoltaics | Huanghe hydropower hainan solar park | China | 2200 | 2020 | 19 |
Taking proactive measures against climate change, as indicated by the International Energy Agency (IEA) study, could bring about $26 trillion in economic benefits and 65 million new jobs by 2030.20 Hence, renewable energy sources are increasingly gaining traction to achieve environmentally friendly and sustainable energy systems. This is due to their non-carbon, widespread availability, and high-energy-density characteristics.21–23 Broadly speaking, the utilization of renewable energy stands as the most appealing approach, holding the potential to substantially reduce or even eliminate the reliance on fossil fuels. Renewable energy sources have experienced a significant increase in generation and adoption over the past decade. Some of these sources are even harnessed for large-scale electricity production, as seen in the case of solar energy,24,25 wind energy,26–28 biomass energy,29,30 and ocean energy.31,32
Hydrogen (H2), a gas that is both colorless and odorless, possesses remarkable flammability. Several sources, such as biomass, natural gas, and water, can be used to obtain hydrogen, which is the lightest and most abundant element in the universe.33–35 Utilizing H2 as a fuel source involves the transformation of this gas into electricity within a hydrogen fuel cell (FC). These cells are distinguished by their exceptional efficiency, boasting rates of up to 60%. Moreover, they demonstrate an environmentally friendly character, as they generate no detrimental emissions, yielding only water and heat as by-products.36,37 H2 stands as an eco-friendly fuel, emitting no environmentally harmful molecules during combustion or oxidation at lower temperatures.38,39 H2 shows significant potential for reducing carbon emissions in the energy sector and achieving net-zero production by 2050. Driven by these compelling attributes of hydrogen, several nations have just unveiled plans and initiatives aimed at establishing sustainable, renewable (green) hydrogen ecosystems.40–42
H2 can be generated from both sustainable and non-sustainable origins, resulting in the categorizations of green, blue, and gray hydrogen.43 These types and their origins are shown in brief in (Fig. 1).
The production of H2 through the utilization of fossil fuels is classified as gray H2, denoting its association with environmental consequences and carbon emissions resulting from the combustion of these finite resources.44 The majority of present-day H2 production stems from fossil fuels incorporating no carbon dioxide capture. While these stands as the most direct approach to hydrogen generation, its sustainability is questionable.45 Gray H2 is acquired through processes that yield greenhouse gas emissions exceeding 36.4 grams of carbon dioxide per megajoule (MJ), regardless of whether these processes rely on renewable or non-renewable resources.46,47 In the current landscape, the foremost origins of the H2 supply can be attributed to the exploitation of coal and natural gas. The industrial utilization of H2 spans across the globe; however, the act of producing H2 presents a notable concern due to the substantial carbon dioxide emissions it contributes annually. This interplay underscores the delicate balance between the practical applications of H2 and the environmental ramifications inherent in its generation.48,49 In summary, the production of gray H2 from fossil fuels carries significant environmental implications and carbon emissions. While fossil fuels dominate current H2 production, their sustainability remains uncertain.
The outcome of this process is blue H2, which emerges from the utilization of fossil fuels in combination with methods involving carbon utilization, storage, and absorption.50,51 Blue H2 is commonly synthesized from natural gas, frequently employing steam reforming techniques coupled with carbon capture and storage. While certain approaches to blue H2 production involve carbon absorption, it's important to note that this method doesn't inherently eliminate carbon emissions.52 Producing a substantial quantity of blue H2 could play a vital role in supporting the expanding worldwide and local H2 supply chains and their associated fuels. The highest projected efficiency for carbon dioxide absorption stands between 85% to 95%, resulting in a leakage of around 5% to 15% of the total carbon.53
H2 derived from sustainable and renewable resources is classified as green H2, signifying its origin through environmentally friendly methods that harness sources like solar, wind, or hydropower.54–56 An increasingly prominent technique within this sector, garnering noteworthy focus in recent times, is the electrolytic generation of H2.57 The production of green H2 using renewable energy sources is expected to increase rapidly in the near future. Multiple ongoing and forthcoming initiatives are aligned with this trajectory.58,59 Nonetheless, achieving substantial cost reduction necessitates increased mass production, dedicated research, and comprehensive development efforts. In accordance with this pattern, the scope of projects has experienced exponential expansion in recent times. H2 generated from renewable sources has the hypothetical to greatly enhance renewable energy output. It is currently technically viable and has the imminent potential to become a prominent global economic contender.60 Anticipations from experts indicate that, by 2050, the cost of green H2 will probably fall to a level below $1 per kilogram, thus rendering green H2 a more competitive option. This underscores the pressing requirement for persistent research and development efforts in the realm of H2 energy. Such investments are essential because H2 is forecasted to emerge as the preferred choice for fuel in the forthcoming years, primarily due to its substantial energy content and environmentally advantageous attributes.61
Some of the already used methods, such as pyrolysis, have several advantages, including the production of H2-rich fuel, rapid and efficient decomposition of feedstock, and great flexibility. However, there are also certain disadvantages, such as the high energy requirements and the potential for tar formation. Gasification offers several advantages, including the capability to convert a wide range of feedstocks, extraordinary productivity, and the potential to generate value-added products alongside H2 production. Nevertheless, there are also some disadvantages, such as electrode deactivation, substantial energy requirements, and the necessity for exceptionally durable equipment. Reforming offers several advantages, including a high capacity to reform diverse materials, cost-effective construction, rapid response capabilities, and compactness. However, there are also some drawbacks, such as high energy requirements, a significant reduction in electrode lifetime, and the need for catalyst regeneration. The data included in Table 2.
Type | Drawbacks | Benefits | Ref. |
---|---|---|---|
Pyrolysis | High required energy | High flexibility | 62–64 |
Fast and efficient feedstock decomposition | |||
Possibility of tar formation | H2-rich fuel production | ||
Gasification | Need for high resistant equipment | High productivity | 65–67 |
High required energy | Ability to convert a variety of feedstocks | ||
Deactivation of electrodes | High potential to produce value-added products along with H2 | ||
Reforming | Considerable reduction in electrode lifetime | High-speed responding quality | 68–70 |
Need for catalyst regeneration | Cheap construction costs | ||
High energy requirement | High capacity to reform varying materials | ||
Compactness |
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Fig. 2 Representation of the SMR process.71 |
Plasma technologies work by energizing gas streams through electrical discharge (Fig. 3). This process results in the generation of various components, including positively charged ions, negatively charged electrons, neutrals, reactive and excited species, an electromagnetic field, and photons. In conditions that deviate from normal atmospheric pressure and temperature, these phenomena facilitate the efficient conversion of biomass into H2 through oxidation.76,77 Conversion methods utilizing plasma technology hold promise for the production of valuable chemicals in addition to H2, and their generation of harmful pollutants is virtually negligible.78,79 Furthermore, by integrating with complementary processes, these methods can yield a H2 product of exceptionally high purity.80
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Fig. 3 Representation of the production of green H2 with plasma route.75 |
Kuo and colleagues75 conducted a comprehensive analysis by a DC plasma torch reactor to evaluate the suitability of various biomass feedstocks for H2 production. Their investigation encompassed a diverse range of sources, including pine wood chips, grape marc, forest residues, rice straw, and macroalgae. The chief aim of their research was to discern how the choice of biomass influenced not only H2 production but also the formation of harmful compounds and the overall gasification yield achieved through plasma technology. Notably, the outcomes revealed a consistent H2 concentration of 68 mol% in the syngas generated from all the studied biomass sources. Non-woody biomass sources showed a higher presence of sulfur compounds compared to woody sources, which can be explained by the inherent traits of non-woody materials. Among the tested options, pine wood emerged as the most favorable choice due to its exceptional efficiency in plasma gasification and the minimal presence of impurities in the resulting syngas, underlining its potential for sustainable H2 production.
Wu and colleagues81 conducted a comprehensive investigation into the presentation of methanol decomposition through a unique liquid-phase discharge setup, designed for enhanced visualization. The use of a high-speed camera in their study gave a detailed understanding of methanol decomposition and liquid-phase discharge processes. By changing the electrode spacing, the researchers were able to create two different plasma discharge modes: discharge (GD) and gliding arc discharge (GAD) glow. GD's current and voltage curves closely resemble the sinusoidal waveform of AC power supplies, with a discharge power range of 130.4 to 460.2. In contrast, GAD exhibited a unique feature of bipolar pulses, characterized by high transient peak currents (ranging from 420.6 to 690.9 mA), resulting in a lower discharge power of 30.7–110.3 W. Initial analyses revealed that GAD's energy consumption was notably lower than that of GD, primarily because of disparities in their discharge characteristics. By optimizing the process, we achieved an energy consumption rate of 1.63 kW h per cubic meter of H2 for hydrogen production. The result of this approach was a gaseous product with a maximum hydrogen proportion of 63.21%, and carbon monoxide as the primary byproduct at 26.38%. These findings shed light on an efficient and sustainable method for hydrogen production.
Tabu and their team82 accomplished the development of low-temperature, atmospheric pressure plasma reactors utilizing the principles of gliding arc (glidarc) discharges and transferred arc (transarc). These reactors were meticulously designed, constructed, and meticulously characterized to facilitate the conversion of low-density polyethylene (LDPE), serving as a representative model for plastic waste, into H2. Their experimental findings revealed a clear relationship between voltage levels and H2 production rates and efficiency in both reactors. As voltage levels increased, H2 production exhibited a steady rise. The transarc reactor achieved a maximum H2 manufacture of 0.33 mmol g−1 LDPE, while the glidarc reactor surpassed this with a peak hydrogen production of 0.42 mmol g−1 LDPE. The transarc reactor showed increased hydrogen production with a narrower electrode-feedstock spacing. However, the glidarc reactor exhibited greater hydrogen generation when flow rates were moderate. Remarkably, despite their significantly different operational modes, both reactors delivered comparable H2 production results. These findings represent a substantial step forward in the utilization of plastic waste for H2 generation, offering valuable insights into the effectiveness of the transarc and glidarc technologies.
Conventional approaches to H2 production encompass processes like water electrolysis, biomass gasification, coal gasification, and steam methane reforming. However, many of these methods, particularly those relying on fossil fuels, are associated with substantial carbon emissions, which run counter to the aim of achieving carbon neutrality. Future H2 production should prioritize renewable resources and minimizing carbon emissions.83–85
Water electrolysis, despite being naturally endothermic, needs a higher voltage than the theoretical electrolysis voltage because of ohmic and overpotential loss.
In a study conducted by Gandia et al.87 they conducted, simulations to explore the production of H2 through wind energy. Their findings revealed significant temperature fluctuations during transient operation, with notable temperature spikes observed under high-power generation conditions and, conversely, temperature decreases during low-power generation scenarios. Notably, the study identified a safety concern related to gas crossover, specifically the presence of O2 in the H2 stream and H2 in the O2 stream. The crossover gases reached high concentrations, especially when the gas volume decreased due to low power generation. This was mainly because the current determined the total of hydrogen gas manufactured. This underscores the importance of addressing safety considerations in H2 production processes. Hence, the variability in the power supply altered the condition of the electrolyzer, impacting the purity of the gas produced.88 Furthermore, The research by Ursúa et al. (Fig. 4),86 they found that during the operation of an alkaline water electrolyzer without additional devices, a requirement was established to maintain a minimum power load of 40%. The objective is to maximize the employment of renewable energy resources by evaluating if it's feasible to operate alkaline water electrolyzers below the minimum power load.86 They aimed to improve the utilization of renewable energy sources. The outcomes of their study revealed that the system could sustain operation for up to 20 minutes under these conditions. Moreover, by implementing these adjustments, they were able to reduce the frequency of operational halts in a water electrolyzer powered by photovoltaic energy by half. Consequently, this approach led to an enhancement in energy efficiency by an additional 6.3%.
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Fig. 4 Configurations for the integration of electrolysers with renewable energies in stand-alone systems.86 |
In their extensive research, Stansberry et al.89 embarked on a series of experiments employing a 60 kW proton exchange membrane (PEM) water electrolyzer driven by a combination of wind power and photovoltaic sources. Within the complex system, the most significant energy loss, marked by the inadvertent release of hydrogen gas, was observed in the pressure swing adsorption dehumidification unit, closely followed by energy losses within auxiliary equipment and the power consumption associated with alternating/direct current (AC/DC) conversion units. The overall efficiency of water electrolysis was greatly affected by the accumulation of these losses, especially at lower electric power levels, resulting in a rated current drop below 50%. These fluctuations in power delivery resulted in similar adverse scenarios for the electrolyzers. As such, it becomes paramount to improve a comprehensive sympathetic of the mechanisms underlying the varying capabilities to support such fluctuating operations. This understanding can be attained by shedding light on the factors that dictate these abilities, which encompass factors like cell structures and the integration of auxiliary equipment. These insights are essential for improving the competence of water electrolysis in renewable energy systems.
Photovoltaic and wind energy production operate in diverse time cycles and exhibit varying power output fluctuations. These traits give rise to several challenges in the operation of electrolyzers. Notably, fluctuating power can lead to electrode degradation owing to abrupt shifts in electrode potential. A significant factor in this degradation is the reverse current generated during operational halts, resulting in a substantial deterioration of electrode performance.76
h+ + □ + H2O → H+ + □ + OH | (1) |
O2 + 2e− + 2H+ → H2O2 | (2) |
O2 + e− → O2− | (3) |
H2O2 + O2− → OH + OH− + O2 | (4) |
h+ + □ + OH− → OH | (5) |
H+ + e− → H | (6) |
H + H → H2 | (7) |
The foundation of photocatalytic H2 production lies in the semiconductor photocatalyst, which utilizes solar energy to split water. When light of a specific wavelength (a photons with a particular energy) strikes the photocatalyst, it energizes electrons from the valence band to the conduction band (CB). This event results in the creation of electron–hole (e−–h+) pairs, which play a pivotal role in driving the redox reactions occurring on the surface of the photocatalyst. Basic water splitting illustration is found in (Fig. 5).
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Fig. 5 Illustration of the photocatalyst water splitting process, (1) the absorption of light radiation from a light source, (2) the separation of electron–hole pairs, and (3) redox reaction. |
Yan and his team,92 achieved a remarkable milestone by developing a novel Ni2P/NiS@polymeric carbon–oxygen semiconductor (PCOS). Their work resulted in a groundbreaking achievement, with a notable production rate of 70.2 μmol h−1 of O2 and 150.7 μmol h−1 of H2 produced for every 100 mg of photocatalyst. Interestingly, the reaction solution also exhibited the presence of H2 peroxide, initially at a rate of approximately 100 μmol h−1 over the first 2 hours. This hydrogen peroxide had a detrimental impact on the photocatalyst's performance. However, the introduction of MnO2 effectively mitigated this negative effect, resulting in excellent and stable rates of photocatalytic H2 and O2 production.
Ruan and colleagues (Fig. 6)93 introduced a groundbreaking method that marks the inaugural attempt to leverage a straightforward ethylenediaminetetraacetate (EDTA) etching process. Their goal was to enhance the number of active surface sites on photocatalysts and reduce particle size, all while preserving high crystallinity. Among the tested materials, STO-2 demonstrated remarkable performance, achieving the highest activity levels. Specifically, it facilitated H2 production at an impressive rate of up to 310 μmol g−1 h−1 and O2 evolution at 155 μmol g−1 h−1. What makes their work even more intriguing is that the EDTA etching technique holds substantial promise for broader applications. Since EDTA can interact with a wide array of metals, this uncomplicated method has the potential to be further refined for the modification of various photocatalysts, enhancing their performance in a myriad of applications.
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Fig. 6 Schematic diagram of photocatalytic water splitting of samples.93 |
Saleh and the research team94 delved into an exploration of various TiO2 nanocomposites enriched with two co-catalysts: Cu or Pt nanocrystals in the 3–4 nm range. These nanocomposites were synthesized through different methods, including photo-deposition, hydrothermal, and incipient wet impregnation. The results yielded a noteworthy discovery: the optimal H2 generation occurred with a mass filling of 0.3 wt% for both co-catalysts. What's particularly remarkable is that, even in the absence of a precious metal like Pt, the Cu/TiO2 nanocomposites, produced through the photo-deposition method, demonstrated a preliminary degree of 24 mmol h−1 g−1. This rate was 3.5 times greater than those synthesized using the hydrothermal method and 1.4 times greater than those produced with the impregnation method. Conversely, for Pt co-catalysts, the highest rate was observed in the impregnation-synthesized composites, clocking in at 58 mmol h−1 g−1, surpassing the rates from the photo-deposition and hydrothermal synthesis methods by 1.6 and 1.1 times, respectively.
From (Table 3). The current analysis underscores that non-green methods of hydrogen production, including SMR and coal gasification, demonstrate superior efficiency and cost-effectiveness when compared to their green counterparts, such as electrolysis powered by renewable energy. Despite their environmental drawbacks, these conventional methods offer a more mature and economically viable pathway for large-scale hydrogen production in the immediate term. However, the urgent need to mitigate climate change and reduce greenhouse gas emissions necessitates a dual focus in future research endeavors.
Production method | Energy efficiency% | Production cost, € per kg H2 | Characters |
---|---|---|---|
SMR | 70–85 | 0.56–1.12 | High efficiency, low cost, mature technology, large emissions |
Partial oxidation of methane | 60–78 | 0.78–1.68 | |
Coal gasification | 50–70 | 0.56–1.12 | |
Electrolysis of water (fossil energy) | 62–82 | 1.79–3.36 | High power consumption, high cost, high H2 purity |
Wind electrolysis of water | — | — | Zero emissions, high cost, low conversion rate |
Solar electrolysis of water | — | — |
It is imperative to enhance the cost-competitiveness and efficiency of green hydrogen production technologies. Significant advancements are required in areas such as electrolyzer technology, renewable energy integration, and novel catalyst development to bridge the gap between green and non-green hydrogen production. Furthermore, comprehensive cost-benefit analyses and life cycle assessments should be prioritized to ensure that the environmental benefits of green hydrogen are realized without compromising economic feasibility. Therefore, the next frontier in hydrogen research should aim to lower the production costs of green hydrogen while simultaneously improving its efficiency. This dual approach will not only facilitate a more sustainable hydrogen economy but also align with global environmental and economic goals. Only through such concerted efforts can we transition to a truly sustainable and scalable hydrogen infrastructure.
Moreover, the kinetics of hydrogen absorption and desorption is a critical focus area, with researchers aiming to enhance these rates to facilitate rapid and efficient hydrogen storage cycles. This involves exploring nanostructured materials, which can offer increased surface areas and improved kinetics, and composite materials, which can tailor properties through synergistic interactions. Stability, both thermal and chemical, is also crucial to ensure durability over many cycles and to maintain performance without degradation. Safety and cost-effectiveness further underpin the practical deployment of these materials, necessitating that they be non-toxic, non-explosive, and economically viable for large-scale use. As such, the development of hydrogen storage materials is a multifaceted challenge, requiring a comprehensive approach to optimize capacity, kinetics, stability, and safety while maintaining economic feasibility.
The effectiveness of materials in storing H2 is closely tied to their physical and chemical characteristics, with a particular emphasis on their thermodynamic and kinetic properties.95 Up to now, the predominant technological challenge in establishing a sustainable H2 economy has been the creation of effective H2 storage systems. When evaluating methods and materials for H2 storage, it's essential to consider various factors, including the design of high-pressure tanks, the densities of H2 in terms of weight and volume, refueling speed, energy efficiency, cost, durability, adherence to standards, technical readiness, and comprehensive assessments of both life cycle and efficiency.96 To make H2 suitable for transportation, it's essential to enhance its energy density. Several methods have been suggested to achieve this, including liquefaction, compression, the formation of metal hydrides, and the utilization of liquid organic transporters such as conversion into energy carriers like methanol and ammonia.97,98
Furthermore, NH3 can be effectively broken down into a blend of N2 and H2 gases, resulting in the generation of larger quantities of high-purity H2 while leaving no carbon footprint. This distinguishes it from hydrocarbon-based organic carriers like methane and methanol, which inevitably produce carbon dioxide. NH3 also offers the advantage of being easier to maintain in a liquid state due to its lower boiling point compared with H2 or methane. At room temperature, NH3 can be liquefied with a moderate pressure of 1.0 MPa.110,111
Economic assessments have demonstrated that NH3 holds greater promise in comparison to conventional fuels as methanol, liquefied petroleum gas (LPG), natural gas, gasoline, and hydrogen, primarily due to its absence of CO2 emissions.112 Furthermore, when compared to liquid H2 (8.49 MJ L−1) and compressed H2 (5.0 MJ L−1 at 70.0 MPa and 25 °C), liquid NH3 boasts a greater volumetric energy density, measuring at 10.5 MJ L−1.113 NH3 further distinguishes itself with a superior heat of combustion compared to liquid H2 at 8.58 MJ L−1 and nearly doubles the value of compressed H2 at 5.0 MJ L−1. Additionally, NH3 is less dense than air (0.769 versus 1.225 kg m−3 at standard temperature and pressure). Under atmospheric conditions, gaseous NH3 can disperse relatively quickly into the atmosphere, mitigating the potential explosion and fire hazards in case of accidental release. The fire risk is lower for NH3 than H2, mainly because NH3 has a higher auto-ignition temperature.114
Selecting suitable metal hydride materials for high efficiency in reversible hydrogen storage and release involves considering several critical factors. First, the hydrogen storage capacity of the material is paramount; it should possess a high gravimetric (wt%) and volumetric capacity to store sufficient hydrogen. For instance, magnesium hydride (MgH2) offers a high hydrogen storage capacity of about 7.6 wt%. Secondly, the thermodynamics of the material, specifically the operating temperatures and pressures for hydrogen absorption (hydrogenation) and desorption (dehydrogenation), must be suitable for practical applications. A moderate enthalpy of formation is essential to balance storage capacity and ease of hydrogen release. Sodium alanate (NaAlH4), which operates at around 150 °C and 5 MPa, exemplifies favorable thermodynamics.
Kinetics is another crucial factor; the material should exhibit fast kinetics for both hydrogen absorption and desorption to minimize energy losses and reduce the time required for charging and discharging. For example, adding Ti-based catalysts to sodium alanate significantly improves its kinetics. Stability over multiple hydrogenation/dehydrogenation cycles is also vital, as the material must resist degradation and maintain its structural integrity and hydrogen storage capacity. LaNi5H6 is noted for its good cycling stability and moderate operating conditions.
Safety and environmental impact are also key considerations. The material should be safe to handle, with minimal risk of toxicity or flammability, and should have a low environmental impact during production, use, and disposal. Magnesium hydride, for instance, is relatively safe and environmentally benign compared to more reactive or toxic materials like lithium hydride. Cost and availability are important practical concerns; the material should be cost-effective and readily available for large-scale applications. MgH2, being abundant and inexpensive, is a popular choice for many applications.
Enhancements through additives and composites can significantly improve the performance of metal hydrides, enhancing their kinetics and thermodynamic properties. For example, mixing MgH2 with transition metal catalysts like Ti or Ni can dramatically enhance its hydrogen absorption and desorption rates. Finally, the suitability of a material depends on the specific application, whether for stationary storage, mobile applications, or portable devices. LaNi5H6, with its moderate pressure and temperature requirements, is suitable for various applications, while MgH2, due to its higher operational temperatures, is more apt for stationary storage.
Gao and colleagues124 introduced a solid-solution MAX phase TiVAlC catalyst directly into the MgH2 system, without the need for etching treatment, to enhance H2 storage performance. At 300 °C, the optimized MgH2-10 wt% TiVAlC composite can absorb about 4.82 wt% of H2 at 175 °C in 900 seconds and release around 6.00 wt% of H2 in 378 seconds. Impressively, even after undergoing 50 isothermal H2 absorption/desorption cycles, the composite exhibits exceptional cyclic stability and retains 99.6% of its capacity, which is 6.4 wt%. The abundant electron transfer at the external interfaces with MgH2/Mg is what gives the TiVAlC catalyst its remarkable catalytic activity. Abundant electron transfer occurs at internal interfaces (Ti3AlC2/TiVAlC) due to the presence of an impurity phase, Ti3AlC2, enhancing electron transfer and showing strong H2 affinity. This study is the first to explore the impact of impurity phases, which are commonly found in MAX phases, on all catalyst activity. It provides a distinct method for designing composite catalysts that enhance the hydrogen storage capabilities of MgH2.
Li and his team125 developed nanosheets of a medium-entropy alloy called CrCoNi. The addition of these nanosheets greatly boosted MgH2's capacity for storing hydrogen at low temperatures. The dehydrogenation temperature of 9 wt% CrCoNi modified MgH2 decreased by 130 °C from 325 °C to 195 °C, surprisingly. Additionally, the composite of MgH2–CrCoNi discharged 4.84 wt% of hydrogen in only 5 minutes at 300 °C and absorbed 3.19 wt% of H2 in just 30 minutes at 100 °C (at 3.2 MPa). There was a decrease in activation energy by 45 kJ mol−1 for dehydrogenation, and a decrease by 55 kJ mol−1 for rehydrogenation. Through extensive cyclic kinetics analysis, it was discovered that the 9 wt% CrCoNi-doped MgH2 showed exceptional strength even subsequently 20 cycles, with a mere 0.36 wt% decrease in H2 capacity. The stability of CrCoNi was confirmed by XRD patterns during the cyclic reaction process. Additionally, there was a uniform dispersion of CrCoNi nanosheets on the surface of MgH2, resulting in numerous catalytic active sites and facile diffusion pathways with low energy barriers. Exceptional kinetic performance was achieved due to the synergistic catalysis that facilitated the rapid absorption and release of hydrogen atoms across the Mg/MgH2 interface.
A novel technique was developed by Zhang and the research team126 to boost the dehydrogenation and rehydrogenation capabilities of MgH2. The introduction of carbon-wrapped Ti and Co bimetallic oxide nanocages (Ti–CoO@C) made this possible. Through a precise hydrothermal method, the nanocages were synthesized and then mixed with MgH2 using mechanical ball milling. The hydrogen desorption was notably influenced, as MgH2 with 5 wt% Ti–CoO@C began desorbing hydrogen at 185.6 °C, a considerable 160.2 °C decrease compared to pure MgH2. Within a short span of 5 minutes, the composite released an astonishing 6.3 wt% H2 at 275 °C. The MgH2 + 5 wt% Ti–CoO@C composite exhibited a significant reduction in activation energy for H2 desorption/absorption, dropping from 169.19 kJ mol−1 and 83.61 kJ mol−1 for MgH2 to 137.76 kJ mol−1 and 35.17 kJ mol−1, respectively. Furthermore, the composite displayed exceptional stability, with no significant decline in performance observed even after 20 cycles. The catalyst's even distribution and the in situ formation of titanium and MgO are responsible for the remarkable hydrogen storage performance. In addition, the promoting effect of Mg2Co/Mg2CoH5 functioned as a H2 pump, thereby contributing to the improved performance. Furthermore, carbon played a vital part in catalyst nanosizing and in reducing the strength of the Mg–H bond in MgH2. As a result, the 5 wt% Ti–CoO@C + MgH2 composite exhibits outstanding hydrogen storage capabilities.
Ali and the research team129 successfully developed CoTiO3 through the solid-state method, and this novel material proved highly operative in ornamental the desorption behavior of NaAlH4 for H2 storage. The introduction of dissimilar weight percentages of CoTiO3 (ranging from 5 wt% to 20 wt%) had a profound impact. NaAlH4's initial desorption temperature significantly decreased due to the inclusion of CoTiO3 catalysts. In the first desorption stage, the temperature decreased to about 130–160 °C, and in the second stage, it decreased to around 182–198 °C. These temperatures are much lower compared to untreated milled NaAlH4. The composite samples showed significantly faster desorption kinetics at 150 °C. A range of 3.0–3.7 was observed during the release of the NaAlH4–CoTiO3 composite. The activation energies for the two stages of NaAlH4 desorption were greatly decreased. They were lowered to 85.5 and 91.6 kJ mol−1, which is a reduction of 30.7 and 35.5 kJ mol−1 compared to untreated milled NaAlH4, respectively. The formation of Al–Co and Al–Ti alloys during the desorption of NaAlH4–CoTiO3 is responsible for the remarkable catalytic effect of CoTiO3. These discoveries create new possibilities for the advancement of efficient catalysts for NaAlH4, showing its potential for H2 storage purposes.
In a theoretical simulation by Mekky,130 the research explored the characteristics of pure Na12Al12H48, and their variations with an interstitial doping of C, H, and Ti atoms. These clusters are being considered as a talented system for H2 storage. The study found that, when compared to the interstitial space-doped clusters, the pure Na12Al12H48 clusters exhibited greater stability. The introduction of interstitial space-doped C, Ti, and H atoms into Na12Al12H48 did not significantly alter the lattice structure, and, notably, these atoms acted more than catalysts rather than traditional “interstitial space doping” elements. Additionally, the study found that the Na12Al12H48 cluster displayed greater stability, but less chemical reactivity compared to the interstitial-doped clusters. When interstitial space-doped C, H, and Ti atoms were added to Na12Al12H48, the lattice structure remained largely unchanged. This confirms that Ti, C, and H atoms play a catalytic role rather than simply being interstitially doped into space.
Urunkar and their team131 conducted a numerical analysis of a hydride reactor occupied with sodium alanate, specifically examining the absorption process within multiple tubes. They developed a mathematical model for the hydride reactor based on various governing equations and validated it using ANSYS Fluent. In general, water or oil is used in the hydride reactor to transfer heat while absorbing H2. The study replaced traditional heat transfer fluid with nanofluid for its better heat exchange properties. The research yielded results across several parameters, including the choice of nanoparticle material, nanoparticle concentration, H2 supply pressure, and the inlet temperature of the heat exchange fluid. The absorption rate of the CuO/HTF nanofluid showed significant improvement, specifically at a 5 vol% concentration, surpassing other concentrations and selected nanofluids. This improvement translated to a 14% reduction in H2 absorption time under specific conditions. Moreover, the CuO/HTF nanofluid with a 5 vol% concentration exhibited superior thermodynamic performance in comparison to other nanofluids, resulting in a 10% increase in heat exchange rate for the hydride reactor. The study found that the CuO/HTF nanofluid with a 5 vol% concentration performed better than the other nanofluids in the hydride reactor. This highlights the benefits of using nanofluids in this application.
The evaluation of various storage methods for green hydrogen reveals a diverse array of options, each with distinct advantages and challenges. Compressed hydrogen and liquid hydrogen offer straightforward and mature technologies but are hindered by high energy requirements and safety concerns related to pressurization and cryogenic temperatures. Chemical manufacturing of hydrogen carriers such as ammonia, methanol, and formic acid presents a promising alternative, providing a more stable and potentially safer means of storage and transportation. However, these methods require further optimization to improve the efficiency of hydrogen release and to reduce associated carbon emissions. On the other hand, metal hydrides, including sodium alanate and magnesium hydride, demonstrate significant potential due to their high hydrogen storage densities and relatively moderate operating conditions. Nevertheless, the commercialization of metal hydride storage is currently limited by issues related to material cost, kinetics, and cyclic stability.
It is clear that while several methods show promise, no single storage technology currently meets all the criteria for widespread adoption. Therefore, ongoing research is essential to address the technical and economic barriers associated with each storage method. Future studies should focus on enhancing the efficiency of hydrogen release, reducing material costs, and improving the safety and feasibility of large-scale deployment. By advancing these areas, the development of an optimal hydrogen storage solution can be accelerated, thereby facilitating the broader adoption of green hydrogen as a key component of the global energy transition.
Hydrogen has 2.4 times more energy per unit mass than methane. However, due to its low density, its lower heating value (LHV) per unit volume is three times lower than methane. This results in a reduction of the energy content in the gas blend as the hydrogen concentration increases. From a safety perspective, higher hydrogen concentrations raise the risk of fire and explosion. Hydrogen has a much broader flammability range (5.3 times) and detonation limit range (7.1 times) compared to methane. Additionally, it has a significantly lower ignition energy (14.5 times lower), making it more easily ignitable and increasing the fire risk.
Utilizing current pipeline systems to blend hydrogen with natural gas (Table 4) offers the most affordable means of transporting significant quantities of hydrogen over long distances without requiring new infrastructure. Nonetheless, because hydrogen molecules are smaller and have unique physical characteristics, including lower density and viscosity, the mixture exhibits behavior distinct from that of pure natural gas.139,140 This introduces potential safety hazards for pipelines designed specifically for natural gas. To keep the energy output consistent, the mixture with hydrogen may require higher flow rates, leading to increased operating pressures that could surpass the design limits of the compressors and pipelines originally meant for natural gas. Hence, it is essential to consider redesigning these systems to safely transport the hydrogen blend and to identify any risks and operational challenges associated with varying hydrogen concentrations. It is crucial to maintain a uniform mixture of the blended gas along the entire pipeline. Significant density differences between the gases can cause them to separate, leading to varied flow behaviors and leak issues. This separation can result in inconsistent energy distribution and operational challenges in the pipeline.134
Project | Country | Network | Electrolyser capacity | Hydrogen blend % |
---|---|---|---|---|
HyP SA | Australia | Distribution | 1.2 MW | 5% |
ATCO-CEIH | Australia | Distribution | 0.15 MW | 5–25% |
HyDeploy | UK | Distribution | 0.5 MW | 20% |
Jupiter 1000 | France | Transmission | 1.0 MW | 6% |
M. Ozturk et al.,142 designed and studied an integrated system to produce renewable hydrogen and blend it with natural gas from the Black Sea for widespread use in Turkey. They focused on a case study for the city of Zonguldak, aiming to use the natural gas reserves more efficiently and environmentally. The study primarily investigates blending natural gas with 20% hydrogen by volume. Renewable energy sources such as wind, solar, and wave were evaluated for their hydrogen production capacities, yielding 1432 kg, 174210 kg, and 1257 kg, respectively. The study examined the impact of this hydrogen addition on gas consumption and the lifespan of natural gas reserves. With 20% hydrogen, annual gas consumption increased from 46.55 billion cubic meters (bm3) to 54.11 bm3, while natural gas consumption decreased from 46.55 bm3 to 43.29 bm3, extending the reserve lifespan from 11.6 years to 12.5 years. Emissions of CO and CO2 dropped significantly, from 0.05 g day−1 and 32% to 0.02 g day−1 and 28%, respectively, as the hydrogen content increased from 5% to 20%. However, NOx emissions rose from 4.08 g day−1 to 7.54 g day−1 with the same hydrogen increase.
M. Ozturk et al. (Fig. 7),141 conducted an experimental investigation to analyze the impact of adding hydrogen to natural gas on emissions and combustion performance. They burned natural gas and various natural gas-hydrogen blends (with 10%, 20%, and 30% hydrogen by volume) in identical gas stoves and measured emissions of CO, CO2, and NOx. The results showed that increasing the hydrogen content improved combustion efficiency from 39.32% to 44.4%. Higher hydrogen ratios reduced CO2 and CO emissions, but NOx emissions varied. A life cycle analysis assessed the environmental impact of the different blending scenarios. With a blend containing 30% hydrogen, the global warming potential decreased from 6.233 to 6.123 kgCO2 equivalents per kgblend, and the acidification potential dropped from 0.0507 to 0.04928 kgSO2 equivalents per kg blend compared to pure natural gas. However, there were slight increases in human toxicity, abiotic depletion, and ozone depletion potentials per kg blend, rising from 5.30 to 5.52 kg 1,4-dichlorobenzene (DCB) equivalents, 0.0000107 to 0.00005921 kg Sb equivalents, and 3.17 × 10−8 to 5.38 × 10−8 kg CFC-11 equivalents, respectively.
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Fig. 7 Illustration of the benefit from blending green H2 with natural gas.141 |
In the steel industry, hydrogen serves two main purposes: (1) reducing iron oxide content in the blast furnace (BF) production process and the gas-based direct reduction iron (DRI) process, (2) it functions as a fuel for various heating applications, such as assisting in sintering production, the pelletizing process, and heating ladle furnaces, among others.147,148 Introducing hydrogen-enriched gases into the BF results in a reduction in the viscosity and density of the gas mixture. This reduction in density and viscosity leads to a lower pressure drop and faster heat exchange between the gas mixture and the materials being processed in the BF. As a result, this contributes to enhancing the efficiency of heat utilization in the BF.147 Simultaneously, during the reduction in iron oxides by hydrogen eqn (8), the diffusion capacity of hydrogen is 3.74 times greater than that of carbon monoxide. The internal micro and macro pores of the iron ores enable H2 to efficiently reach the reaction interface through diffusion.149 Consequently, if the H2/CO ratio is higher, the reduction rate will be faster with the same volume fraction of reduction agents. Numerous scholars have substantiated this through studies on the dynamic aspects of iron oxide reduction.147,150–155
Fe2O3 + 3H2 → 2Fe + 3H2O | (8) |
It was clear now that hydrogen metallurgy offers several advantages. Firstly, it produces H2O as a reduction product, reducing reliance on fossil fuels like coal and coke and decreasing CO2 emissions. Additionally, H2 serves as a superior reductant compared to CO, thanks to its higher calorific value, lower density, enhanced penetration, and faster reduction rate. The availability of abundant raw materials for H2 production ensures a readily available supply. Moreover, H2 metallurgy can stimulate the rapid growth of DRI processes by substituting natural gas with H2, which is valuable in localities with limited natural gas resources, such as China. In general, H2 metallurgy plays a role in the sustainable development of iron and steel enterprises.
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Fig. 8 Illustration of the manufacturing of different organic materials using carbon capture and green hydrogen production. |
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Fig. 10 Visual representation of the chemical structure of (a) methanol, (b) ammonia, (c) methane, and (d) formic acid. |
Dongliang and colleagues165 introduced an innovative approach for H2 production coupled with CO2 application in the coal-to-methanol (CTM) process. They termed this new approach the GH-CTM process, designed to enhance material integration, carbon efficiency, and methanol yield. Through comprehensive process modeling, parameter optimization, and simulations, the results demonstrated remarkable improvements compared to the conventional CTM process. The GH-CTM process exhibited a 10.52% higher energy efficiency, an 85.64% reduction in CO2 emissions, and a remarkable 124.67% increase in methanol production. In addition, the proposed process had significantly slowed production costs, 23.95% less than the traditional CTM process. Notably, the payback period for investment in the GH-CTM process was substantially shorter, at 2.8 years, compared to the CTM process's 7.2 years. Moreover, the GH-CTM process experienced a 47.37% increase in internal rate of return compared to the traditional CTM process. This new approach shows potential for introducing green H2, utilizing CO2, and transforming coal into valuable chemicals sustainably.
A preliminary assessment by Sollai and their team166 looked into a power-to-fuel plant setup for generating 500 kg h−1 of renewable methanol using green H2 and captured CO2. They developed a comprehensive process model employing the Aspen Plus tool, which simulated all aspects of the plant and the system as a whole. Once the process was optimized, a comprehensive economic analysis was performed, considering operating and capital costs derived from real-world experience at a commercial scale, with a projected operational lifetime of 20 years. Through the analysis, it was determined that the LCoM is 960 € per t, which translates to around 175 € per MW h. While the study showed that, as of the present, the technology isn't yet economically competitive, with the LCoM exceedingly double the prevailing international methanol price of 450 € per t, it does indicate a potential shift towards competitiveness in the medium-term future, largely driven by evolving European policies. Additionally, the research revealed that LCoM is particularly influenced by factors such as electricity prices, electrolyzer capital costs, and the plant's capacity factor.
![]() | (9) |
As outlined by MacFarlane et al.,168 various approaches for green ammonia production can be categorized: First-generation green ammonia involves capturing carbon emissions post-ammonia production and storing it, resulting in what is referred to as “blue ammonia”. Second-generation green ammonia focuses on producing ammonia from environmentally friendly feedstocks, namely N2 and H2. This approach aims to transform the traditional Haber–Bosch process into a sustainable source. Third-generation green ammonia entails departing from the conservative Haber–Bosch process and adopting alternative methods that prioritize high stability, sustainability, and the use of renewable sources for ammonia production.
Currently, various methods exist for the indirect generation of environmentally friendly ammonia, such as microbial electrolysis,169 photosynthesis,170 dark fermentation,171 and electrolysis.172 Electrochemical techniques have garnered significant attention in numerous nations.173
An enhanced optimization-based simulation model was introduced by Zhao and their team174 to simulate the long-term sustainability of green manufacturing. They investigated the effect of significant institutional incentives and the collaborative effects of O2 on investments. According to the study, the estimated levelized cost of ammonia is about 820 USD per t, which is nearly twice the current market price. Several factors were identified as pivotal in green ammonia investments, including the operational rate, the electrical efficiency of electrolyzers, electricity costs, and ammonia pricing. China's energy transition was greatly influenced by carbon pricing and VAT exemptions. To bridge the gap, a subsidy of about 450 USD per t would be needed based on the current pricing, but this could be lowered by 100 USD per t through the implementation of O2 synergy. Comparatively, green NH3 production exhibited both environmental and economic advantages when contrasted with inter-regional power transmission. The study thus advocates a balanced approach, leveraging both options to address integrating O2 manufacturing into H2 production and renewable power curtailment processes. By mitigating renewable power curtailment, this research aims to encouragement the increase of the H2 economy in China.
Ishaq and colleagues175 conducted a study to determine if offshore wind energy can be used to produce green ammonia and green H2. They used water and air as the main inputs. The green ammonia would then be transferred to onshore demand points using ships or pipelines. They performed a comprehensive year-long transient analysis to ensure the technology's reliability and adherence to quality standards before field deployment. Their approach involved integrating offshore wind energy with a water electrolysis unit and a seawater desalination system to generate renewable H2 for subsequent green ammonia synthesis. In terms of costs, the economic analysis revealed that the offshore wind farm and electrolyzer made up the majority, with 45% and 29% allocations, respectively. In order to meet the constant demand for green ammonia at a rate of 554 kg h−1, they concluded that an 80000 kg H2 storage system was necessary, which could also provide ammonia to external customers. The study further illustrated the variations in ammonia production capacity over the course of the year, both with and without the H2 storage system. Importantly, it demonstrated that integrating a H2 storage system could ensure a steady supply of green ammonia throughout the year.
Bouaboula and the research team176 developed a new Techno-Economic (TE) modeling method. Their goal was to optimize the operation and design of a pilot-scale Green Ammonia plant. The intermittent nature of renewable energy sources is taken into account in this novel TE model. In order to deal with this, we examined multiple site locations that had consistent meteorological data each year. Furthermore, the model includes a unique Energy Management Strategy (EMS) to ensure a continuous power supply for the Haber–Bosch (HB) reactor. The EMS ensures the smooth distribution of power from renewable sources to charge and discharge Energy Storage Systems (ESS). Two main Key Performance Indicators (KPIs) were used to evaluating the plant's performance: Levelized Cost of Ammonia (LCOA) and HB Load Factor (LF). The findings indicated that the implemented EMS effectively reduced the fluctuations in RE sources by optimally distributing power across different time slots. Consequently, the HB LF rose by 56% to 65%, based on the particular RE setup. The increase in LF resulted in lower plant costs due to higher production yield outweighing investment and operational expenses. The PV/Battery scenario, consisting of 6 MW of PV and 11 MW h of battery capacity, was identified as the most efficient plant configuration, with a LCOA of $774 per t NH3. By 2050, the estimated cost of NH3 could decrease to $250 per ton according to a forecast. Furthermore, it suggests that green ammonia is expected to be economically competitive with conventional fossil fuel approaches by 2030.
Pignataro et al.179 presented three management strategies (MSs) for controlling the H2 storage tank and methanation unit in the power-to-gas system. The most influential operational variables were determined through a systematic comparison of these MSs, and their impact on system performance was evaluated. The first strategy, denoted as MSA, stood out as the most straightforward of the three. When the produced H2 falls within the operational range, MSB closely resembled MSA in its behavior when operational constraints were breached. The control algorithm of MSC was similar to MSB, but the storage tank supplied different amounts of additional H2 during “in-range” methanation operations. While the methanation unit was running, we considered a scenario where the setpoint for methanation matched the flow rate from the electrolysis system (ES). The findings indicated that MSA and MSB exhibited similarities in the methanation unit and CH4 production utilization factor. Despite this, MSB demonstrated greater efficiency in handling methanation unit shutdowns, albeit with the drawback of needing a bigger storage system. On the other hand, MSC demonstrated the highest CH4 production but had more shutdowns and used a smaller storage system. Nonetheless, the results consistently showed a low average state of charge (SOC) for the storage in all MSs, suggesting that the system components may not have been sized optimally. Further investigation is needed to explore how resizing different subsystems impacts system performance and cost. Ultimately, the selection of the management strategy varies on the goal and feasibility of utilizing excess H2 in the power-to-methane system.
In a comprehensive study, Garcia-Luna et al.180 focused on integrating waste biomass oxycombustion with a power-to-methane system. Their approach primarily relies on using photovoltaic solar energy to drive PEM electrolysis and produce H2 and O2. The gases are utilized in a sub-critical steam power cycle for waste combustion. Depending on the operational strategy, an air separation process utilizing cryogenic distillation can generate an extra O2. Following purification and compression, the CO2 stream is directed towards the methanation reactor. The researchers created a quasi-stationary model to simulate the entire plant and assess integration efficiency under different operational conditions. According to their study, the entire plant integration shows high efficiency, with a CO2 reduction associated efficiency penalty of approximately 6% points on average per year. The system reduces emissions by using waste biomass as the primary fuel source, resulting in a −610 kgCO2 per MW h reduction compared to biomass plants without CO2 capture. Furthermore, a comprehensive annual techno-economic study shows an average levelized electricity cost of €56 per MW h and an average green CH4 production cost of €12 per MW h. The results support the implementation of this system in both new and retrofitted biomass power plants because the CO2 capture cost is around 65.66 € per ton of CO2.
Gong and his team184 developed an advanced integrated system that merges methanol selective oxidation reaction and the H2 evolution reaction. By incorporating a power management system, this system operates on a UDC RF-Pulsed-TENG. At the cathode, green H2 is produced in this setup, while simultaneously generating the high-value chemical product, FA, at the anode. Applying a constant voltage of 1.8 V to the electrochemical cell after power management resulted in 1.68 times increase in the green H2 production rate. The entire system produces green H2 at a rate of 14.69 μL min−1 with 100% Faraday efficiency. Additionally, it allows for the simultaneous and quick generation of pure green H2 and valuable FA using clean energy sources.
SOFCs | MCFCs | PAFCs | AFCs | PEMFCs | |
---|---|---|---|---|---|
Electrolyte | Ceramics | Molten carbonate | Phosphoric acid | Potassium hydroxide | Polymeric membrane |
Charge carriers | O2− | CO32− | H+ | OH− | H+ |
Operating temperature | 500 to 1000 °C | 600 to 700 °C | 150 to 220 °C | 50 to 200 °C | −40 to 120 °C (150 to180 °C in high temp. PEMFCs) |
Electrical efficiency | Up to 65% | Up to 60% | Up to 45% | Up to 70% | Up to 65–72% |
Primary fuel | H2, biogas, or methane | H2, biogas, or CH4 | H2 or reformed H2 | H2 or cracked ammonia | H2, reformed H2, methanol in direct methanol fuel cells |
Primary applications | Stationary | Stationary | Stationary | Portable and stationary | Portable, transportation, and small-scale stationary |
Power delivery (2019) | 78.1 MW | 10.2 MW | 106.7 MW | 0 MW | 934.2 MW |
The concept of the H2 economy is not far-fetched. Presently, gray and blue H2 are priced between $1.20 and $2.40 per kilogram, subject to the expense of carbon capture and storage. The cost of green hydrogen is approximately $4.85 per kilogram, taking into account an electricity cost of $53 per MW h and an efficiency of 65% at nominal capacity based on the lower heating value. However, it is anticipated that declining renewable electricity costs, enhanced electrolyzer efficiency, and reduced capital expenses will bring the cost of green H2 to below $2.00 per kilogram by 2030, making it competitive with gray H2 across various sectors, including industry.195 The refining sector is projected to see a rise in demand for H2 in the next decade, reaching approximately 41 million metric tons per year.196 Also, the demand for methanol and ammonia is projected to experience substantial growth in the foreseeable future, driven by their usage in agriculture and their role as efficient energy carriers.197
Several critical factors determine the economic viability of green H2 production plants. Firstly, it is essential to have a substantial H2 demand to justify the investment in such facilities. The industrial sector, particularly in applications such as steel production and chemicals, presents a significant opportunity for the utilization of green H2. Moreover, the economic feasibility is further enhanced when electricity prices are low, as the energy-intensive electrolysis process relies on affordable power sources. Additionally, the presence of high carbon taxes incentivizes industries to transition towards cleaner energy sources, making green H2 a cost-effective solution for reducing emissions.
However, there are numerous challenges and opportunities associated with green H2 production. One of the foremost challenges is the integration of these facilities with the existing energy grid. It can be challenging to balance intermittent renewable energy sources, such as solar and wind, with the steady demand for H2. Effective grid integration and energy storage solutions are critical to address this issue. Moreover, transportation and storage of H2, whether in gaseous or liquid form, pose logistical challenges. Infrastructure development is required to facilitate the efficient distribution and utilization of green hydrogen.
Lastly, policy support is paramount for the growth of the green H2 sector. Governments can incentivize investment through subsidies and regulations that promote cleaner energy sources. By addressing these challenges and capitalizing on the opportunities, green H2 production can become a transformative force in promoting sustainable energy solutions and reducing carbon emissions.
Despite these challenges, the outlook for green hydrogen integration into large-scale projects is promising, aligning with the vision for a sustainable future by 2050. Technological advancements and increased production scales are expected to lower the cost of green H2. As renewable energy becomes more competitive, the production costs of green H2 will benefit from reduced energy expenses. Research and development efforts are anticipated to yield more efficient and cost-effective electrolysis technologies, with breakthroughs in catalyst materials and cell designs significantly enhancing production efficiency. Establishing hydrogen infrastructure, such as pipelines and storage solutions, will facilitate the broad adoption of green H2 across various industries, including transportation and power generation. Supportive government policies, including subsidies and carbon pricing mechanisms, are increasingly recognizing the role of green H2 in decarbonizing industries, encouraging investment and growth in this sector. Regions rich in renewable energy are exploring the potential for exporting green H2, creating new economic opportunities and fostering international cooperation. Industries like chemicals and steel are progressively transitioning to green H2 as a cleaner feedstock, driving demand and further reducing costs through economies of scale.
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