Comparison of molten salt heat recovery options in the Cu-Cl cycle of hydrogen production

This paper presents a comparative review of different options for recovering heat from molten CuCl in the Cu-Cl cycle of hydrogen production. Various technologies exist for recovering heat from the molten CuCl in the cycle, but each has its advantages and challenges. In this paper, different parameters such as heat transfer rate, additional materials in the cycle, energy efficiency, temperature retention, economics, material issues, and design feasibility are considered in the evaluation of methods. It is shown that casting/extrusion, atomization methods, with a separate vessel using water as a coolant and rotary/spinning atomization using air as a coolant, can be considered as the most efficient and reliable methods for heat recovery from molten CuCl.     

Experimental study of effect of anolyte concentration and electrical potential on electrolyzer performance in thermochemical hydrogen production using the Cu-Cl cycle

An important process in the copper-chlorine water splitting cycle for hydrogen production is electrolysis which occurs after a series of cycle steps that produce the constituents for the anolyte of the electrochemical cell. In this investigation, an anolyte mixture of HCl/CuCl/H₂O of varying concentrations is circulated through the electrolyzer to assist in optimizing its performance. It is observed that the concentration and temperature of the anolyte directly affect the process. The efficiency of the electrolyzer is adversely affected, after running a series of experiments, due to copper deposition on the membrane. An important implication of the results is that, to determine the optimal electrolyzer performance, one needs to vary the flow rate and the concentration of anolyte, for a given constant voltage source. In addition, this work demonstrates that aqueous CuCl₂ can be recovered from the waste solution exiting the electrolyzer and recycled to the hydrolysis reactor.     

Design of systems for hydrogen production based on the Cu-Cl thermochemical water decomposition cycle: Configurations and performance

In this study, we analyze several Cu-Cl cycles by examining various design schemes for an overall system and its components, in order to identify potential performance improvements. The factors that determine the number and effective grouping of steps for new design schemes are analyzed. A thermodynamic analysis and several parametric studies are presented for various configurations. The energy efficiency is found to be 44% for the five-step thermochemical process, 43% for the four-step process and 41% for the three-step process, based on the lower heating value of hydrogen. Also, conclusions regarding implementation of these new configurations are discussed and the potential benefits ascertained.     

Clean hydrogen production with the Cu-Cl cycle - Progress of international consortium, II: Simulations, thermochemical data and materials

This second of two companion papers presents the latest advances of an international team on the thermochemical copper-chlorine (Cu-Cl) cycle of hydrogen production. It specifically focuses on simulations, thermochemical data, advanced materials, safety, reliability and economics of the Cu-Cl cycle. Aspen Plus simulations of various system configurations are performed to improve the cycle efficiency. In addition, simulations based on exergo-economic and exergy-cost-energy-mass (EXCEM) methods for system design are presented. Modeling of the linkage between nuclear and hydrogen plants demonstrates how the Cu-Cl cycle would be integrated with an SCWR (Super Critical Water Reactor; Canada’s Generation IV reactor). Chemical potentials, solubilities, formation of Cu(I) and Cu(II) complexes and properties of Cu₂OCl₂, Cu(I) and Cu(II) chloride species are reported. In addition, the development of new advanced materials with improved corrosion resistance is presented. In particular, the performance of new anode electrode structures and thermal spray coatings is presented. This companion set of two papers presents new advances in a range of key enabling technologies for the thermochemical copper-chlorine cycle.     

Development and test of a solar reactor for decomposition of sulphuric acid in thermochemical hydrogen production

Decomposition of sulphuric acid is a key step of sulphur based thermochemical cycles for hydrogen production by thermal splitting of water. The Hybrid Sulphur Cycle (HyS) consisting of two reaction steps is considered as one of the most promising cycles: firstly, sulphuric acid is decomposed by high temperature heat of 800–1200 °C forming sulphur dioxide, which in a second step is used to electrochemically split water. Compared to conventional water electrolysis only about a tenth of the theoretical voltage is required making the HyS one of the most efficient processes to produce hydrogen by concentrated solar radiation. As a result, this thermochemical cycle has the potential to significantly reduce the amount of energy required for water splitting and to efficiently generate hydrogen free of carbon dioxide emissions. The European research project HycycleS aims at a technical realisation of the HyS. One objective of the project is to develop and qualify a solar interface, meaning a device to couple concentrated solar radiation into the endothermal steps of the chemical process. Therefore, a test reactor for decomposition of sulphuric acid by concentrated solar radiation was developed and tested in the solar furnace of DLR in Cologne. Tests in concentrated solar radiation were carried out for temperatures of the honeycomb up to 950 °C decomposing sulphuric acid of 50 and 96 weight-percent. Mass and energy flow of the process were calculated in order to determine energy efficiency and chemical conversion. The influence of process parameters like temperature, flow rates and space velocity on chemical conversion and reactor efficiency was analysed in detail. If catalysts like iron oxide (Fe₂O₃) and mixed oxides (i.e. CuFe₂O₄) were used a conversion of SO₃ to SO₂ of more than 80% at a thermal efficiency of over 25% could be reached.     

Thermodynamic viability of a new three step high temperature Cu-Cl cycle for hydrogen production

A new three step high temperature Cu-Cl thermochemical cycle for hydrogen production is presented. The performance of the proposed cycle is investigated through energy and exergy approaches. Furthermore, the effects of various parameters, such as the temperatures of the steps of the cycle and power plant efficiency, on various energy and exergy efficiencies are assessed with parametric studies. The results show that the exergy and energy efficiencies of the proposed cycle are 68.3% and 32.0%, respectively. In addition, the exergy analysis results reveal that the hydrogen production step has the maximum specific exergy destruction with a value of 150.9 kJ/mol. The results suggest that proposed cycle may provide enhanced options for high temperature thermochemical cycles by improving thermal management without causing a sudden temperature jump/fall between the hydrogen production step and other steps.     

Thermodynamic performance assessment of boron based thermochemical water splitting cycle for renewable hydrogen production

The present study is related with the thermodynamic performance assessment of renewable hydrogen production through Boron thermochemical water splitting cycle. Therefore, all step efficiencies and overall cycle efficiency are calculated based on complete reaction. Additionally, a parametric study is conducted to determine the effect of the reference environment temperature on the overall cycle efficiency. In this regard, exergy efficiencies, exergy destruction rates and also inlet and outlet exergy rates of the cycle are calculated and presented for various reference temperatures. The exergy efficiency of the cycle is calculated as 0.4393 based on complete reaction and occurs at 298 K. This study has shown that Boron thermochemical water splitting cycle has a great potential due to cycle performance. As a result, Boron based thermochemical water splitting cycle can help achieve better environment and sustainability due to high exergetic efficiency. By the way, economic and technical issues of the storage and transportation of the hydrogen can find a proper solution if the hydrogen production reaction of the Boron thermochemical water splitting cycle takes place on-board of a vehicle.     

Thermodynamic analysis and optimization of an integrated solar thermochemical hydrogen production system

In this study, thermodynamic analysis of solar-based hydrogen production via copper-chlorine (Cu–Cl) thermochemical water splitting cycle is presented. The integrated system utilizes air as the heat transfer fluid of a cavity-pressurized solar power tower to supply heat to the Cu–Cl cycle reactors and heat exchangers. To achieve continuous operation of the system, phase change material based on eutectic fluoride salt is used as the thermal energy storage medium. A heat recovery system is also proposed to use the potential waste heat of the Cu–Cl cycle to produce electricity and steam. The system components are investigated thoroughly and system hotspots, exergy destructions and overall system performance are evaluated. The effects of varying major input parameters on the overall system performance are also investigated. For the baseline, the integrated system produces 343.01 kg/h of hydrogen, 41.68 MW of electricity and 11.39 kg/s of steam. Overall system energy and exergy efficiencies are 45.07% and 49.04%, respectively. Using Genetic Algorithm (GA), an optimization is performed to evaluate the maximum amount of produced hydrogen. The optimization results show that by selecting appropriate input parameters, hydrogen production rate of 491.26 kg/h is achieved.     

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.     

Clean hydrogen production with the Cu-Cl cycle - Progress of international consortium, I: Experimental unit operations

Advancement of the thermochemical copper-chlorine (Cu-Cl) cycle for hydrogen production is reviewed and discussed in this paper. Individual unit operations and their linkage into an integrated cycle are being developed by a Canadian consortium, as part of the Generation IV International Forum (GIF) for hydrogen production with the next generation of nuclear reactors. This paper focuses on the consortium’s latest advances on the Cu-Cl cycle, particularly with respect to hydrogen production with Canada’s Generation IV reactor, called SCWR (Super-Critical Water Reactor). Other heat sources may also be utilized for the Cu-Cl cycle, such as solar energy or industrial waste heat. In this first of two companion papers, recent developments in Canada’s nuclear hydrogen program are reported, specifically unit operation experiments of the Cu-Cl cycle and system integration. The following second companion paper will present system modeling with Aspen Plus, corrosion resistant materials, thermochemistry, safety, and reliability aspects of the Cu-Cl cycle.     

Performance analysis of a novel hydrogen production system incorporated with hybrid solar collector and phase change material

In this technical article, a novel experimental setup is designed and proposed to produce a hydrogen by using solar energy. This system comprises a hybrid or photovoltaic Thermal (PVT) solar collector, Hoffman's voltameter, heat exchanger unit and Phase Change Material (PCM). The effect of PCM and mass flow rate of water on the hybrid solar collector efficiency and hydrogen yield rate is studied. This experimental results clearly showed that by adding the thermal collector with water, decreases PV module temperature by 20.5% compared with conventional PV module. Based on the measured values, at 12.00 and 0.011 kg/s mass flow rate, about 33.8% of thermal efficiency is obtained for water based hybrid solar collector. Similarly, by adding Paraffin PCM to the water based thermal collector, the maximum electrical efficiency of 9.1% is achieved. From this study, the average value of 17.12% and 18.61% hydrogen yield rate is attained for PVT/water and PVT/water with PCM systems respectively.     

Life cycle assessment of alternatives for hydrogen production from renewable and fossil sources

New processes under development for producing hydrogen have been assessed using a life cycle methodology and compared to conventional ones. The aim of this paper is to determine the main obstacles to be beaten or the critical aspects to be addressed to ensure the feasibility of these processes. Water photosplitting, solar two-step thermochemical cycles and automaintained methane decomposition with different lay-outs were studied. They have been compared to methane steam reforming with CCS and electrolysis with different electricity sources.     

Comparative study of the activity of nickel ferrites for solar hydrogen production by two-step thermochemical cycles

In this work, we compare the activity of unsupported and monoclinic zirconia – supported nickel ferrites, calcined at two different temperatures, for solar hydrogen production by two-step water-splitting thermochemical cycles at low thermal reduction temperature. Commercial nickel ferrite, both as-received and calcined in the laboratory, as well as laboratory made supported NiFe₂O₄, are employed for this purpose. The samples leading to higher hydrogen yields, averaged over three cycles, are those calcined at 700 °C in each group (supported and unsupported) of materials. The comparison of the two groups shows that higher chemical yields are obtained with the supported ferrites due to better utilisation of the active material. Therefore, the highest activity is obtained with ZrO₂-supported NiFe₂O₄ calcined at 700 °C.     

Numerical investigation of heat and mass transfer during hydrogen desorption in a large-scale metal hydride reactor coupled to a phase change material with nano-oxide additives

For the object of reducing heat consumption in hydrogen metal hydride (MH) storage units during the discharging cycle, the nano-PCM (i.e. phase change material containing nano-oxides) strategy is adopted herein for accelerating the release of the latent heat (LH) stocked in the PCM to the MH. The process was assessed in a large-scale horizontal cylindrical reactor equipped with 4 PCM tubes distributed homogenously in the MH-bed. Mass and heat transfer were computationally analyzed in the diverse regions of the MH-nano-PCM system using a 2D numerical model developed with Fluent 15.0 CFD-software. Temporal temperature profiles (average and contours), MH-dehydrogenation efficiency, velocity contours and PCMs solidification rate were established in the presence (5% v/v) and absence of four types of nano-oxides (Al₂O₃, MgO, SnO₂ and SiO₂). Remarkable results were obtained. The nano-PCM system participated in the MH-discharging by providing latent heat (LH) and changing its physical phase. The MH was completely discharged within 700 s. Nano-oxides additions improved the solidification rate of the PCM (i.e. accelerating the release of the LH) by more 50%, with a strong dependency on the PCM-tubes position. The PCM-tube above the H₂-charging pipe solidifies more quickly than the other tubes, probably to the gravitational effect. The outcomes of this research provide insight into the use of nano-PCMs as a thermal supplier in MH storage systems during the discharging cycle.     

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

The development of clean hydrogen production methods is important for large-scale hydrogen production applications. The solar thermochemical water-splitting cycle is a promising method that uses the heat provided by solar collectors for clean, efficient, and large-scale hydrogen production. This review summarizes state-of-the-art concentrated solar thermal, thermal storage, and thermochemical water-splitting cycle technologies that can be used for system integration from the perspective of integrated design. Possible schemes for combining these three technologies are also presented. The key issues of the solar copper-chlorine (Cu–Cl) and sulfur-iodine (S–I) cycles, which are the most-studied cycles, have been summarized from system composition, operation strategy, thermal and economic performance, and multi-scenario applications. Moreover, existing design ideas, schemes, and performances of solar thermochemical water-splitting cycles are summarized. The energy efficiency of the solar thermochemical water-splitting cycle is 15–30%. The costs of the solar Cu–Cl and S–I hydrogen production systems are 1.63–9.47 $/kg H₂ and 5.41–10.40 $/kg H₂, respectively. This work also discusses the future challenges for system integration and offers an essential reference and guidance for building a clean, efficient, and large-scale hydrogen production system.     

Study of the first step of the Mn2O3/MnO thermochemical cycle for solar hydrogen production

In this work, a complete thermodynamic study of the first step of the Mn₂O₃/MnO thermochemical cycle for solar hydrogen production has been performed. The thermal reduction of Mn₂O₃ takes place through a sequential mechanism of two reaction steps. The first step (reduction of Mn₂O₃ to Mn₃O₄) takes place at teomperatures above 700 °C, whereas the second reaction step (reduction of Mn₃O₄ to MnO) requires temperatures above 1350 °C to achieve satisfactory reaction rates and conversions. Equilibrium can be displaced to lower temperatures by increasing the inert gas/Mn₂O₃ ratio or decreasing the pressure. The thermodynamic calculations have been validated by thermogravimetric experiments carried out in a high temperature tubular furnace. Experimental results corroborate the theoretical predictions although a dramatically influence of chemical kinetics and diffusion process has been also demonstrated, displacing the reactions to higher temperatures than those predicted by thermodynamics. Finally, this work demonstrates that the first step of the manganese oxide thermochemical cycle for hydrogen production can be carried out with total conversion at temperatures compatible with solar energy concentration devices. The range of required temperatures is lower than those commonly reported in literature for the manganese oxide cycle obtained from theoretical and thermodynamic studies.     

Immobilization of calcium oxide solid reactant on a yttria fabric and thermodynamic analysis of UT-3 thermochemical hydrogen production cycle

UT-3 cycle has been considered as one of the most promising thermochemical processes for hydrogen production. In order to make the cycle practical, however, the solid reactants in the cyclic reactions must have adequate lifespan and better kinetics. In this paper, hydrolysis reaction of calcium bromide, the slowest process in the cycle, was investigated theoretically and experimentally. A new type of calcium oxide reactant was fabricated by dispersing and fixing it on a yttria fabric via a comparatively straightforward and inexpensive immobilization process. The characteristics and cyclic performance of the prepared fabric samples were evaluated and compared with the conventional calcium oxide pellets. It was shown that the calcium oxide immobilized on the yttria fabric had continuous higher reactivity and comparable hydrolysis rate compared with the conventional calcium oxide pellets. A theoretical analysis of the thermodynamic cycle was also conducted. The effect of excess steam on the equilibrium conversion was significant; however, the reaction temperature was limited due to the melting point of calcium bromide. By continuously removing the product gas, the conversion in the hydrolysis reaction which is the slowest reaction in the cycle could be completed theoretically. This hypothesis was found to be true based on the experimental tests.     

Experimental assessment of the cyclability of the Mn2O3/MnO thermochemical cycle for solar hydrogen production

The cyclability of Mn₂O₃/MnO thermochemical cycle for solar hydrogen production has been experimentally evaluated. The results of three consecutive cycles show a stable hydrogen production per mass of initial solid that is in agreement with the maximum expected amount according to the stoichiometry of the process. The characterization of the material recovered after each cycle shows a mixture of different manganese oxide phases that are completely converted into MnO in the subsequent thermal reduction, maintaining the productivity of the cycles. Based on that, a modification of the thermochemical cycle scheme is proposed taking into account the differences observed between the first cycle and the following ones. MnO₂/MnO thermochemical cycle appears as a promising alternative, working in the same temperature range but with a theoretical hydrogen production per unit mass of solid manganese oxide almost twice than that obtained with the conventional Mn₂O₃/MnO cycle. Finally, the results of exergy efficiency of the complete cycle give new insights into the commercial possibilities of the cycle for hydrogen production, demonstrating the sustainable cyclability of the process regarding the manganese containing materials at lower temperatures than those theoretically reported in literature and consequently with higher exergy efficiencies that the common values associated to this cycle.     

Commercial activated carbon for the catalytic production of hydrogen via the sulfur-Iodine thermochemical water splitting cycle

Eight commercial activated carbon catalysts were examined for their catalytic activity to decompose hydroiodic acid (HI) to produce hydrogen; a key reaction in the sulfur-iodine (S-I) thermochemical water splitting cycle. Activity was examined under a temperature ramp from 473 to 773 K. No statistically significant correlation was found between the measured catalyst sample properties and catalytic activity. Four of the eight samples were examined for one week of continuous operation at 723 K. All samples appeared to be stable over the period of examination.     

Thermodynamics and kinetics analysis of hydrogen absorption in large-scale metal hydride reactor coupled to phase change material-metal foam-based latent heat storage system

Phase change materials (PCMs) have recently been coupled with metal hydride storage tanks (MHSTs) to store adsorption heat and subsequently deliver it for hydrogen desorption through melting and solidification cycles. This method might reduce process costs by eliminating the use of HTF (i.e. heat transfer fluid). However, thermodynamics and kinetic data are scarce for large-scale MH-PCM applications, particularly when PCM is loaded in metal foams (MFs) to promote heat transfer. The current work aimed to develop a 2D model for simulating H₂-absorption in a LaNi₅-metal bed integrating a PCM-MF unit in a large-scale tube-and-shell heat exchanger. The constructed model (via Fluent 15.0 CFD-platform) was first-validated using referenced experimental data. The resulting heat transfer was analyzed for different MFs [aluminum, copper, nickel and titanium] of different porosities (0.1–1.0). Excellent outcomes were retrieved. By trapping the H₂-absorption heat, the MF-PCM unit improved the LaNi₅ hydriding. The LaNi₅ charging was achieved after ～500 s, independently of the MF type and porosity. The PCM melting rate depends on tube position, porosity and the MF type. It increased with MFs incorporation (order of enhancement: Cu > Al > Ni > Ti) and MF-porosity decrease (from ε = 100% to 10%). Besides, the PCM tube above the H₂-feeding pipe melts more quickly than the other tubes, presumably to the gravitational-force effect. Longer times (i.e. 9 000 s to >16 000 s, depending on tube position) were recorded for complete melting of the PCM when ε_MF = 100%; however, when ε_MF is less than 80%, the required time for total melting was tremendously reduced to less than 500 s. Nevertheless, the MFs porosity could not be decreased considerably to avoid a huge loss of material storage (PCM), thereby diminishing the thermal storage performance of the PCM matrix.