A review and comparative evaluation of thermochemical water splitting cycles for hydrogen production

Analysis and assessment of a new solar assisted sodium hydroxide thermochemical hydrogen production cycle

3.1 Hydrogen Production

Hydrogen is a commercially important element. Basically, there are several methods of hydrogen production that have been commercially used, such as Steam Methane Reforming (SMR), High Temperature Steam Electrolysis (HTSE), and thermochemical cycles, like Sulphur-Iodine (SI). Among these methods, SMR is the most widely used for large-scale hydrogen production, with conversion efficiency between 74–85% and it has commercially used in some fertilizer industries in Indonesia. Steam reforming is a method to convert alkane (natural gas) compounds to hydrogen and carbon dioxide (synthetic gas) by adding moisture at high pressure and temperature (35-40 bar; 800-900°C). These hydrogen production technologies can be coupled with different nuclear reactors based on the heat required in the process. The High Temperature Gas-cooled Reactor (HTGR) using helium as a coolant, has a high outlet temperature (900°C), so it can potentially be used to supply for process heat for hydrogen production, coal liquefaction/gasification or for other industrial processes requiring high temperature heat. Hydrogen production cost from SMR method is influenced by a range of technical and economic factors. The fuel component of natural gas needed in the SMR method can be replaced by nuclear heat from a nuclear power plant (NPP) operating in cogeneration mode (i.e. simultaneous producing electric power and heat), hence contributing to the reduction of carbon dioxide in the process.In the SMR method, fuel costs are the largest cost component, accounting for between 45% and 75% of production costs. Therefore, there is opportune to assess the economics of hydrogen production by using nuclear heat. The economic evaluation is done by using IAEA HEEP-4 Software.The results comprise cost break up for 2 cases, coupling SMR process for hydrogen production with: (1) 2 HTGRs of 170 MWth/unit; and (2) 1 HTGR of 600 MWth/unit. The cost of hydrogen production is highly depend on the scale of the NPP as energy source and results indicated that hydrogen production cost of the 1 HTGR Unit600 MWth (Case 2) has a lower value (1.72 US$/kgH2), than the cost obtained when 2 HTGR units of 170 MWth each (case 1) are considered (2.72 US$/kgH2). For comparison, the hydrogen production cost by using SMR with carbon capture and storage (CCS) with natural gas as fuel is 2.27 US$/kgH2.

Assessment and multi-objective optimization of a vanadium-chlorine thermochemical cycle integrated with algal biomass gasification for hydrogen and power production

The technology to use nuclear heat to thermally split water into hydrogen and oxygen attracts more and more attentions at present. This paper discusses some challenges to couple nuclear heat with thermochemical hydrogen production cycles. The challenges include matching the maximum heat grade of thermal chemical cycles and nuclear reactors, and extracting heat from nuclear reactors. Sulfur-iodine and copper-chlorine cycles are taken as typical examples for analysis and discussion. The heat grade and quantity required by each step of the cycles are discussed. The maximum heat grade of sulfur-iodine cycle is higher than 800°C which cannot be easily coupled by GenIV nuclear reactor and other sources of heat must be provided. In comparison, the maximum heat grade of copper-chlorine cycle is 530°C which can be coupled by more nuclear reactors such as advance Gen-III and future Gen-IV nuclear reactor. It is concluded that thermochemical cycles with lower temperature requirement are easier to couple with present and future generations of reactors. Low temperature thermochemical cycles such as copper-chlorine cycles are recommended to match the heat grade of most nuclear reactors. Some methods are proposed to couple heat between a thermochemical cycle and nuclear power generating station. Several heat extraction methods such as using working fluid of nuclear reactor to provide heat to thermochemical cycles are proposed in this paper.

Energy and exergy analyses of magnesium-chlorine (Mg-Cl) thermochemical cycle

Increasing demand for hydrogen are driving the need for the development of more efficient methods to produce hydrogen on a large scale. The main methods for hydrogen production include reforming of hydrocarbons such as natural gas, coal gasification, biological processes, water electrolysis, and thermo-chemical cycles. The processes involving hydrocarbons contribute to CO2 emissions, and biological processes may not be cost effective on a large scale. Electrolysis is commercially viable but may be too inefficient for large-scale applications. Hence the interest in using thermochemical cycles for large-scale hydrogen production. Thermochemical cycles produce hydrogen through a series of chemical reactions that result in the splitting of water at much lower temperatures (~500-1000ºC) than direct thermal dissociation (>2500oC). All other chemical species in these reactions are recycled resulting in the consumption of only heat and water to produce hydrogen and oxygen. Since water rather than hydrocarbons are used as the source of hydrogen in these thermochemical cycles, no carbon dioxide emissions are produced and the hydrogen is highly pure.Although there are hundreds of possible thermochemical cycles, the hybrid-sulfur (HyS) process is the only practical, all-fluid, two-step thermochemical cycle. If the energy input into the process is solar energy, solar radiation is used in a solar receiver/reactor to provide the energy needed to vaporize and decompose sulfuric acid. The resulting sulfur dioxide (SO2) is used in the second step consisting of an SO2-depolarized electrolyzer (SDE) that electrochemically oxidizes SO2with water to form sulfuric acid at the anode and hydrogen at the cathode. All sulfur species are recycled, and the overall reaction is the splitting of water to form hydrogen and oxygen. Excess SO2 is stored during daylight operation and used at night to permit continuous electrolyzer operation and hydrogen production. Electricity is supplied from a companion solar-electric plant or obtained from the grid. Approximately 80% of the energy input to the process is solar-thermal energy, and 20% is electricity for the electrolyzer and auxiliaries. We have developed and patented a gas-fed SDE that we tested over a range of operating conditions (e.g., current, temperature, SO2 flow rate) and design variations (e.g., catalyst type and loading, membrane type and thickness). Results of our research will be presented and the challenges that remain in making this an economically viable process for large-scale hydrogen production will be presented.

In order for hydrogen to play a key role in the move towards renewable energy, CO2-neutral hydrogen must be generated efficiently, inexpensively, and at a large scale. This article focuses on thermochemical water-splitting cycles, especially when coupled with solar energy. There are hundreds of possible thermochemical cycles, but only a few have been considered commercially viable. Commonalities among thermochemical cycles are a series of reactions that split water at lower temperatures (∼500–1000°C) than thermal dissociation (>2500°C), with other species recycled in the system. Thermochemical cycles can be direct (all chemical steps) or hybrid (i.e., a combination of chemical and electrochemical steps) processes. If concentrated solar energy is used for the thermal dissociation step, all thermochemical cycles can be classed as solar thermochemical hydrogen (STCH) processes. This article discusses two cycles: 1) a direct thermochemical cycle (zinc oxide cycle), and 2) a hybrid cycle (hybrid sulfur cycle). The latter combines a chemical thermal dissociation step with an electrochemical step. As two-step cycles, their appeal is in their relative simplicity. Although probably several years away, the outlook for solar thermochemical hydrogen generation is strong, especially with continued advances in materials, and system design and optimization.

Assessing the potential of thermo-chemical water splitting cycles: A bridge towards clean and sustainable hydrogen generation

The objective of this study is to evaluate the feasibility of cogeneration of electricity and hydrogen using a high temperature, concentrated solar power central receiver with supercritical carbon dioxide as the working fluid. Carbon dioxide is heated directly in a high efficiency receiver to a temperature of 720°C, which then provides thermal input in series to a supercritical carbon dioxide (sCO2) Brayton cycle for producing electricity and then to a thermochemical hydrogen production cycle. Three different methods of producing hydrogen are examined: the 4-step copper-chlorine (Cu-Cl) thermochemical cycle, the hybrid sulfur (HyS) thermochemical cycle, and two different electrolyzers: the alkaline electrolyzer and the polymer exchange membrane electrolyzer. Simulations developed in Aspen Plus were used to estimate the thermochemical hydrogen production efficiency, while the solar receiver and sCO2 Brayton cycle performance were based on results of prior work. The initial results show that the overall solar-to-electricity and H2 efficiencies for the Cu-Cl cycle, hybrid sulfur cycle, alkaline electrolyzer, and polymer exchange membrane electrolyzer are 39.6%, 26.1%, 29.6%, and 22.8%, respectively. These systems’ hydrogen capacities are 864, 715, 524, and 403 kg/h, respectively, for an equivalent sCO2 Brayton cycle.

Comparison of sulfur–iodine and copper–chlorine thermochemical hydrogen production cycles

A critical review on integrated system design of solar thermochemical water-splitting cycle for hydrogen production

A review on hydrogen production thermochemical water-splitting cycles

A study on comparative environmental impact assessment of thermochemical cycles and steam methane reforming processes for hydrogen production processes

Investigation of a new integrated energy system with thermochemical hydrogen production cycle and desalination