Hydrogen from solar energy,a clean energy carrier from a sustainable source of energy

Solar energy is going to play a crucial role in the future energy scenario of the world that conducts interests to solar-to-hydrogen as a means of achieving a clean energy carrier. Hydrogen is a sustainable energy carrier, capable of substituting fossil fuels and decreasing carbon dioxide (CO₂) emission to save the world from global warming. Hydrogen production from ubiquitous sustainable solar energy and an abundantly available water is an environmentally friendly solution for globally increasing energy demands and ensures long-term energy security. Among various solar hydrogen production routes, this study concentrates on solar thermolysis, solar thermal hydrogen via electrolysis, thermochemical water splitting, fossil fuels decarbonization, and photovoltaic-based hydrogen production with special focus on the concentrated photovoltaic (CPV) system. Energy management and thermodynamic analysis of CPV-based hydrogen production as the near-term sustainable option are developed. The capability of three electrolysis systems including alkaline water electrolysis (AWE), polymer electrolyte membrane electrolysis, and solid oxide electrolysis for coupling to solar systems for H₂ production is discussed. Since the cost of solar hydrogen has a very large range because of the various employed technologies, the challenges, pros and cons of the different methods, and the commercialization processes are also noticed. Among three electrolysis technologies considered for postulated solar hydrogen economy, AWE is found the most mature to integrate with the CPV system. Although substantial progresses have been made in solar hydrogen production technologies, the review indicates that these systems require further maturation to emulate the produced grid-based hydrogen.     

Comparative assessment of hydrogen production methods from renewable and non-renewable sources

In this study, we present a comparative environmental impact assessment of possible hydrogen production methods from renewable and non-renewable sources with a special emphasis on their application in Turkey. It is aimed to study and compare the performances of hydrogen production methods and assess their economic, social and environmental impacts, The methods considered in this study are natural gas steam reforming, coal gasification, water electrolysis via wind and solar energies, biomass gasification, thermochemical water splitting with a Cu–Cl and S–I cycles, and high temperature electrolysis. Environmental impacts (global warming potential, GWP and acidification potential, AP), production costs, energy and exergy efficiencies of these eight methods are compared. Furthermore, the relationship between plant capacity and hydrogen production capital cost is studied. The social cost of carbon concept is used to present the relations between environmental impacts and economic factors. The results indicate that thermochemical water splitting with the Cu–Cl and S–I cycles become more environmentally benign than the other traditional methods in terms of emissions. The options with wind, solar and high temperature electrolysis also provide environmentally attractive results. Electrolysis methods are found to be least attractive when production costs are considered. Therefore, increasing the efficiencies and hence decreasing the costs of hydrogen production from solar and wind electrolysis bring them forefront as potential options. The energy and exergy efficiency comparison study indicates the advantages of biomass gasification over other methods. Overall rankings show that thermochemical Cu–Cl and S–I cycles are primarily promising candidates to produce hydrogen in an environmentally benign and cost-effective way.     

A review on integrated thermochemical hydrogen production from water

Hydrogen production from water splitting is considered one of the most environmentally friendly processes for replacing fossil fuels. Among the various technologies to produce hydrogen from water splitting, thermochemical cycles using chemical reagents have the advantage of scale up compared to other specific facilities or geological conditions required. According to thermochemical processes using chemical redox reactions, 2-, 3-, 4-step thermochemical water splitting cycles can generate hydrogen more efficiently due to reducing temperatures. Increasing the number of cycles or steps of thermochemical hydrogen production could reduce the required maximum temperature of the facility. In addition, recently developed hybrid thermochemical processes combined with electricity or solar energy have been studied on a large scale because of the reduced cost of hydrogen production. Additionally, hybrid thermochemical water splitting combined with renewable energy can result in not only reducing the cost, but also increasing hydrogen production efficiency in terms of energy. As for a green energy, hydrogen production from water splitting using sustainable and renewable energy is significant to protect biological environment and human health. Additionally, hybrid thermochemical water splitting is conducive to large scale hydrogen production. This paper reviews the multi-step and highly developed hybrid thermochemical technologies to produce hydrogen from water splitting based on recently published literature to understand current research achievements.     

Life cycle assessment of three types of hydrogen production methods using solar energy

A comprehensive life cycle assessment (LCA) is carried out for three methods of hydrogen production by solar energy: hydrogen production by PEM water electrolysis coupling photothermal power generation, hydrogen production by PEM water electrolysis coupling photovoltaic power generation, and hydrogen production by thermochemical water splitting method using S–I cycle coupling solar photothermal technology. The assessment also contains an evaluation of four environmental factors which are global warming potential, acidification potential, ozone depletion potential, and nutrient enrichment potential. After conducting a quantitative analysis of all three methods with environmental factors being considered, a conclusion has been drawn: The global warming potential and the acidification potential of the thermochemical water splitting by S–I cycle coupling solar photothermal technology are 1.02 kg CO₂-eq and 6.56E-3 kg SO₂-eq. And this method has significant advantages in the environmental impact of the whole ecosystem.     

Life cycle assessment of various hydrogen production methods

A comprehensive life cycle assessment (LCA) is reported for five methods of hydrogen production, namely steam reforming of natural gas, coal gasification, water electrolysis via wind and solar electrolysis, and thermochemical water splitting with a Cu–Cl cycle. Carbon dioxide equivalent emissions and energy equivalents of each method are quantified and compared. A case study is presented for a hydrogen fueling station in Toronto, Canada, and nearby hydrogen resources close to the fueling station. In terms of carbon dioxide equivalent emissions, thermochemical water splitting with the Cu–Cl cycle is found to be advantageous over the other methods, followed by wind and solar electrolysis. In terms of hydrogen production capacities, natural gas steam reforming, coal gasification and thermochemical water splitting with the Cu–Cl cycle methods are found to be advantageous over the renewable energy methods.     

The reactor design for photoelectrochemical hydrogen production

The heat transfer and flow characteristics of a photoelectrochemical (PEC) hydrogen generation reactor are investigated numerically. Four different reactor designs are considered in this study. The solar irradiation is separated into short and long wavelength parts depending on the energy band gap of the photoelectrode used. While short wavelength part is used to generate electron and hole pairs, the long wavelength part is used to heat the system. Because the energy required for splitting water decreases as temperature is increased, heating the reactor by using the long wave energy increases the system efficiency. Thus, how the long wavelength energy is absorbed by the reactor is very important.The results show that more long wavelength energy kept inside the reactor can increase the solar-to-hydrogen efficiency, η_SH. For Fe₂O₃ photoelectrode, careful reactor design can increase η_SH by 11.0%. For design D under 4000 W/m² irradiation and a quantum efficiency of 30%, η_SH is found to be 14.1% and the hydrogen volume production rate is 166 L/m² h for Fe₂O₃. Effects of several parameters on the PEC hydrogen reactor are also discussed.     

Sustainable hydrogen production options and the role of IAHE

In this paper, some potential sustainable hydrogen production options are identified and discussed. There are natural resources from which hydrogen can be extracted such as water, fossil hydrocarbons, biomass and hydrogen sulphide. In addition, hydrogen can be extracted from a large palette of anthropogenic wastes starting with biomass residuals, municipal wastes, plastics, sewage waters etc. In order to extract hydrogen from these resources one needs to use sustainable energy sources like renewables and nuclear. A total of 24 options for sustainable hydrogen production are then identified. Sustainable water splitting is the most important method of hydrogen production. Five sustainable options are discussed to split water, which include electrolysis, high temperature electrolysis, pure and hybrid thermochemical cycles, and photochemical/radiochemical methods. Other 19 methods refer to extraction of hydrogen from other materials than water or in conjunction with water (e.g., coal gasification with CO₂ capture and sequestration). For each case the achievable energy and exergy efficiency of the method were estimated based on state of the art literature screening for each involved process. In addition, a range of hydrogen production capacity is determined for each of the option. For a transition period to hydrogen economy nuclear or solar assisted coal gasification and fossil fuel reforming technologies – with efficiencies of 10–55% including CO₂ sequestration – should be considered as a viable option. Other “ready to be implemented” technology is hydro-power coupled to alkaline electrolysers which shows the highest hydrogen generation efficiency amongst all electrical driven options with 60–65%. Next generation nuclear reactors as to be coupled with thermochemical cycles have the potential to generate hydrogen with 40–43% energy efficiency (based on LHV of hydrogen) and 35–37% exergy efficiency (based on chemical exergy of hydrogen). Furthermore, recycling anthropogenic waste, including waste heat, waste plastic materials, waste biomass and sewage waters, shows also good potential as a sustainable option for hydrogen production. Biomass conversion to hydrogen is found as potentially the most efficient amongst all studied options in this paper with up to 70% energy efficiency and 65% exergy efficiency.     

Prospects of solar thermal hydrogen production processes

This article provides a critical discussion of prospects of solar thermal hydrogen production in terms of technological and economic potentials and their possible role for a future hydrogen supply. The study focuses on solar driven steam methane reforming, thermochemical cycles, high temperature water electrolysis and solar methane cracking. Development status and technological challenges of the processes and objectives of ongoing research are described. Estimated hydrogen production costs are shown in comparison to other options. A summary of current discussions and today's scenarios of future use of hydrogen as an energy carrier and a brief overview on the development status of end-use technologies characterise uncertainties whether hydrogen could emerge as important energy carrier until 2050. Another focus is on industrial hydrogen demand in areas with high direct solar radiation which may be the main driver for the further development of solar thermal hydrogen production processes in the coming decades.     

Two-step thermochemical electrolysis: An approach for green hydrogen production

Electrolysis and thermochemical water splitting are approaches to produce green hydrogen that use either an electrical potential (electrolysis) or a chemical potential (thermochemical water splitting) to split water. Electrolysis is technologically mature when applied at low temperatures, but it requires large quantities of electrical energy. In contrast to electrolysis, thermochemical water splitting uses thermal energy, as thermal energy can typically be supplied at a lower unit cost than electrical energy using concentrating solar power. Thermochemical water splitting, however, typically suffers from high thermal losses at the extremely high process temperatures required, substantially increasing the total energy required. We show how, by combining electrical and chemical potentials, a novel and cost-efficient water splitting process can be envisioned that overcomes some of the challenges faced by conventional electrolysis and thermochemical water splitting. It uses a mixed ionic and electronic conducting perovskite with temperature-dependent oxygen non-stoichiometry as an anode in an electrolyzer. If solar energy is used as the primary source of all energy required in the process, the cost of the energy required to produce hydrogen could be lower than in high-temperature electrolysis by up to 7%.     

Investigation of hydrogen production performance of a reactor assisted by a solar pond via photoelectrochemical process

In this study, the hydrogen production performance of a reactor assisted by a solar pond by photoelectrochemical method is examined conceptually. The main components of the new integrated system are a solar pond, a photovoltaic panel (PV) and a hybrid chlor-alkali reactor which consists of a semiconductor anot, photocathode and cation exchange membrane. The proposed system produces hydrogen via water splitting reaction and also yields the by products namely chlorine and sodium hydroxide while consumes saturated NaCl solution and pure water. In order to increase the efficiency of the reactor, the saturated hot NaCl solution at the heat storage zone (HSZ) of the solar pond is transferred to the anot section and the heated pure water by heat exchanger in the HSZ is transferred to cathode section. The photoelectrode releases electrons for hydrogen production with diminishing the power requirement from the PV panel that is used as a source of electrical energy for the electrolysis. The results confirm that the thermal performance of the solar pond plays a key role on the hydrogen production efficiency of the reactor.     

A review on solar energy-based indirect water-splitting methods for hydrogen generation

The distinguish generation methods regarding hydrogen generation using solar energy as a triggering agent are discussed in this paper, specifically indirect techniques. Two broadly classified processes are direct and indirect. The Direct processes exhibit high thermal efficiency, but their low conversion efficiency, maximum heat dissipation, and the lack of readily available heat resistive materials in abundance put the indirect processes relatively on the higher rank. The indirect methods include bio photolysis, thermochemical, photolysis, and electrolysis. There are promising features of indirect ways. Bio-photolysis provides zero pollution; the photolysis method reduces the carbon footprint in the environment; Thermochemical is meritorious in low electricity consumption due to high heat generation in the process; Electrolysis proves its worth in negligible pollution and considerable efficiency. The energy and exergy efficiency for hydrogen yielding are compared, and it is found that electrolysis has the highest energy and exergy efficiency. In terms of raw material availability, thermochemical ranks very low as compared to photolysis (abundant solar energy), bio-photolysis (a readily available bio-agent), and electrolysis (electrolytic agents to carry out the process).     

Thermodynamic analysis of a photoelectrochemical hydrogen production system

The thermodynamic analysis of photoelectrochemical (PEC) hydrogen production is performed in this work for air mass 1.5 solar insolation. Because the energy required for splitting water decreases as temperature is increased, heating the system by using the long wavelength energy will increase the system efficiency.     

环境低负荷制氢技术及集成制氢系统

讨论了各种环境低负荷的制氢技术。SPE电解水制氢技术成熟,将成为未来主要制氢方法之一。生物化学制氢和半导体光解水制氢仅以太阳能为能源,前景广阔。生物质制氢清洁、节能,值得推广。环境低负荷集成制氢系统综合多种技术,是制氢技术发展的一个趋势。     

A comparative thermodynamic analysis of samarium and erbium oxide based solar thermochemical water splitting cycles

This paper reports a thermodynamic comparison between the samarium and erbium oxide based solar thermochemical water splitting cycles. These cycles are a two-step process in which the metal oxide is first thermally reduced into the pure metal, and the produced metal can be used to split water to produce H₂. The metal oxides can be reused for multiple cycles without consumption. The effect of water splitting temperature on various thermodynamic parameters which are essential to design the solar reactor system for the production of H₂ via water splitting reaction using the samarium and erbium oxides is studied in detail. The total amount of solar energy needed for the thermal reduction of samarium and erbium oxides is estimated. The amount of heat energy released by the water splitting reactor is calculated. Also, the cycle and solar-to-fuel energy conversion efficiency for both cycles are determined by employing heat recuperation. Obtained results indicate that the efficiencies associated with these cycles are comparable to the previously studies thermochemical cycles. It is observed that higher water splitting temperature favors towards higher efficiencies. At constant thermal reduction temperature = 2280 K, by employing 50% heat recuperation, the solar-to-fuel energy conversion efficiency for the samarium cycle (30.98%) is observed to be higher than erbium cycle (28.19%).     

High temperature electrolysis of hydrogen bromide gas for hydrogen production using solid oxide membrane electrolyzer

Using solid oxide membrane, this paper presents the theoretical modeling of the high temperature electrolysis of hydrogen bromide gas for hydrogen production. The electrolysis of hydrogen halides such as hydrogen bromide is an attractive process, which can be coupled to hybrid thermochemical cycles. The high temperature electrolyzer model developed in the present study includes concentration, ohmic, and activation losses. Exergy efficiency, as well as energy efficiency parameters, are used to express the thermodynamic performance of the electrolyzer. Moreover, a detailed parametric study is performed to observe the effects of various parameters such as current density and operating temperature on the overall system behavior. The results show that in order to produce 1 mol of hydrogen, 1.1 V of the applied potential is required, which is approximately 0.8 V less compared to high temperature steam electrolysis under same conditions (current density of 1000 A/m² and temperature of 1073 K). Furthermore, it is found that with the use of the presented electrolyzer, one can achieve energy and exergy efficiencies of about 56.7% and 53.8%, respectively. The results presented in this study suggest that, by employing the proposed electrolyzer, two-step thermochemical cycle for hydrogen production may become more attractive especially for nuclear- and concentrated solar-to-hydrogen conversion applications.     

Efficiency of hydrogen production systems using alternative nuclear energy technologies

Nuclear energy can be used as the primary energy source in centralized hydrogen production through high-temperature thermochemical processes, water electrolysis, or high-temperature steam electrolysis. Energy efficiency is important in providing hydrogen economically and in a climate friendly manner. High operating temperatures are needed for more efficient thermochemical and electrochemical hydrogen production using nuclear energy. Therefore, high-temperature reactors, such as the gas-cooled, molten-salt-cooled and liquid-metal-cooled reactor technologies, are the candidates for use in hydrogen production. Several candidate technologies that span the range from well developed to conceptual are compared in our analysis. Among these alternatives, high-temperature steam electrolysis (HTSE) coupled to an advanced gas reactor cooled by supercritical CO₂ (S-CO₂) and equipped with a supercritical CO₂ power conversion cycle has the potential to provide higher energy efficiency at a lower temperature range than the other alternatives.     

Prospects for solar only operation of the hybrid sulphur cycle for hydrogen production

The hybrid sulphur process is one of the most promising thermochemical water splitting cycles for large scale hydrogen production. While the process includes an electrolysis step, the use of sulphur dioxide in the electrolyser significantly reduces the electrical demand compared to conventional alkaline electrolysis. Solar operation of the cycle with zero emissions is possible if the electricity for the electrolyser and the high temperature thermal energy to complete the cycle are provided by solar technologies.This paper explores the possible use of photovoltaics (PV) to supply the electrical demand and examines a number of configurations. Production costs are determined for several scenarios and compared with base cases using conventional technologies. The hybrid sulphur cycle has promise in the medium term as a viable zero carbon production process if PV power is used to supply the electrolyser. However, the viability of this process is dependent on a market for hydrogen and a significant reduction in PV costs to around $1/W_p.     

碳中和愿景下绿色制氢技术发展趋势及应用前景分析

围绕目前主流的绿色制氢技术,综述国内外“绿氢”技术的最新研究进展,重点阐述电解水制氢技术（碱性电解水法、质子交换膜电解水法、固体氧化物电解水法）、太阳能分解水制氢技术（光催化法、光热分解法、光电化学法）以及生物质制氢技术（热化学转化法、微生物法）的产氢原理、技术难点和改进方法等,讨论比较各类“绿氢”技术的优缺点,分析未来绿色制氢技术的应用前景和发展方向。     

太阳能制氢研究现状及展望

综述了国内外制氢研究现状。对常用的太阳能制氢方法：直接热分解法、热化学循环法、光电化学分解法(PEC)以及光催化法进行了分析，指出了各种方法的研究难点和重点。并结合我国的现状提出目前我国应该把光电化学分解法和2步热化学循环法作为研究的重点。     

Potential methods for geothermal-based hydrogen production

In this study, four potential methods are identified for geothermal-based hydrogen production, namely, (i) directly from the geothermal steam, (ii) through conventional water electrolysis using the electricity generated from geothermal power plant, (iii) using both geothermal heat and electricity for high temperature steam electrolysis and/or hybrid processes, (iv) using the heat available from geothermal resource in thermochemical processes to disassociate water into hydrogen and oxygen. Here we focus on relatively low-temperature thermochemical and hybrid cycles, due to their greater application possibility, and examine them as a potential option for hydrogen production using geothermal heat. We also present a brief thermodynamic analysis to assess their performance through energy and exergy efficiencies for comparison purposes. The results show that these cycles have good potential and become attractive due to the overall system efficiencies over 50%. The copper–chlorine cycle is identified as a highly promising cycle for geothermal hydrogen production. Furthermore, three types of industrial electrolysis methods, which are generally considered for hydrogen production currently, are also discussed and compared with the above mentioned cycles.