Thermophysical properties of copper compounds in copper–chlorine thermochemical water splitting cycles

This paper examines the relevant thermophysical properties of compounds of copper that are used in thermochemical water splitting cycles. There are four variants of such Cu–Cl cycles that use heat and electricity to split the water molecule and produce H₂ and O₂. Since the energy input is mainly in the form of thermal energy, the Cu–Cl water splitting cycle is more efficient than water electrolysis, if the electricity generation efficiency for electrolysis is taken into account. Various chemicals are recycled within the plant, while the overall effect is splitting of the water molecule. The system includes several reactors, heat exchangers, a spray dryer, and an electrochemical cell. This paper identifies the available experimental data for properties of copper compounds relevant to the Cu–Cl cycle analysis and design (Cu₂OCl₂, CuO, CuCl₂, CuCl). It also develops new regression formulae to correlate the properties, which include: specific heat, enthalpy, entropy, Gibbs free energy, density, formation enthalpy and free energy. No past literature data are available for the viscosity and thermal conductivity of molten CuCl, so estimates are provided. The properties are evaluated at 1 bar and a range of temperatures from ambient to 675–1000 K, which are consistent with the operating conditions of the cycle. Updated calculations of chemical exergies are provided as follows: 21.08, 6.268, 82.474, and 75.0 kJ/mol for Cu₂OCl₂, CuO, CuCl₂ and CuCl, respectively. For molten CuCl, the estimated viscosity varies from 1.7 to 2.6 mPa s for the envisaged range of temperatures. A Riedel-like equation is proposed to correlate the vapor pressures with the temperature for molten CuCl.     

Heat recovery from molten CuCl in the Cu–Cl cycle of hydrogen production

The copper–chlorine (Cu–Cl) cycle of thermochemical hydrogen production requires heat recovery from molten CuCl at various points within the cycle. This paper examines the convective heat transfer between molten CuCl droplets and air in a counter-current spray flow heat exchanger. This direct contact heat exchanger is analyzed as a proposed new method of recovering heat from the solidified molten CuCl. Effective thermal management within the Cu–Cl cycle is crucial for achieving high thermal efficiency. The cycle’s efficiency is improved drastically when all heat released by the products of reactions is recycled internally. Recovering heat from molten CuCl is very challenging due to the phase transformations of molten CuCl, as it cools from liquid to different solid states. In this paper, a spray column direct contact heat exchanger is analyzed for the heat recovery process. A predictive model of heat transfer and droplet flow is developed and then solved numerically. The results indicate that full heat recovery is achieved with a heat exchanger diameter of 0.13 m, and heights of 0.6 and 0.8 m, for a 1 and 0.5 mm droplet diameter, respectively. Additional results are presented and discussed for heat recovery from molten CuCl in the thermochemical Cu–Cl cycle.     

Liquid sodium versus Hitec as a heat transfer fluid in solar thermal central receiver systems

Selection of an appropriate HTF is important for minimising the cost of the solar receiver, thermal storage and heat exchangers, and for achieving high receiver and cycle efficiencies. Current molten salt HTFs have high melting points (142–240 °C) and degrade above 600 °C. Sodium’s low melting point (97.7 °C) and high boiling point (873 °C) allow for a much larger range of operational temperatures. Most importantly, the high temperatures of sodium allow the use of advanced cycles (e.g. combined Brayton/Rankine cycles). In this study, a comparison between the thermophysical properties of two heat transfer fluids (HTFs), Hitec (a ternary molten salt 53% KNO₃ + 40% NaNO₂ + 7% NaNO₃) and liquid sodium (Na), has been carried out to determine their suitability for use in high-temperature concentrated solar thermal central-receiver systems for power generation. To do this, a simple receiver model was developed to determine the influences of the fluids’ characteristics on receiver design and efficiency. While liquid sodium shows potential for solar thermal power systems due to its wide range of operation temperatures, it also has two other important differences – a high heat transfer coefficient (～an order of magnitude greater than Hitec) and a low heat capacity (30–50% lower than Hitec salt). These issues are studied in depth in this model. Overall, we found that liquid sodium is potentially a very attractive alternative to molten salts in next generation solar thermal power generation if its limitations can be overcome.     

Determining parameters of heat exchangers for heat recovery in a Cu–Cl thermochemical hydrogen production cycle

A heat exchanger is a device built for efficient heat transfer from one medium to another. Shell and tube heat exchangers are separated wall heat exchangers and are commonly used in the nuclear and process industry. The CuCl cycle is used to thermally crack water in to H₂ and O₂. The present study presents the heat exchanger thermal design using analysis of variance for heat recovery from oxygen at 500 °C, coming from the molten salt reactor. Polynomial regressions in terms of the amount of chlorine in the oxygen, the mass flow rate on the tube side, and the shell's outlet temperature are estimated for various exchanger parameters and the results are compared with the bell Delaware method. Based on energy and exergy analysis, this study also discusses the best possible path for the recovered heat from oxygen. Optimal heat exchanger parameters are estimated by Design-Expert^® Stat-Ease for most effective heat recovery.     

Preparation and thermal stability of ternary sulfate molten salt

以硫酸钠、硫酸钾和硫酸镁为原料,采用在硫酸钠-硫酸钾二元共晶盐中加入硫酸镁的方法制备三元硫酸熔盐。应用TG-DSC联用分析仪、热常数分析仪、X射线衍射仪以及热循环法对复合熔盐的熔点、相变潜热、热导率、比热容、分解点以及热稳定性进行表征。结果表明：所制备的三元硫酸熔盐熔点分布在667.5～669.7 ℃之间,较二元熔盐熔点降低了160 ℃左右,硫酸镁含量为30%（质量分数）的三元硫酸熔盐相变潜热值最大为94.3 J/g,比热容最大为1.13 J/(g·K)（720℃≤T≤800℃）,导热系数为0.41 W/(m·K),分解温度为1070 ℃,经50次热循环后,相变潜热值降低约4.34%,熔点和物相保持基本恒定,具有良好的热稳定性。该研究为硫酸盐作为高温传热蓄热介质提供了依据。     

氯化物熔盐单质及其混合物热物性子动力学模拟研究

采用分子动力学模拟的方法对3种常用的氯化物熔盐单质（KCl、NaCl和LiCl）、二元混合氯化物熔盐（LiCl-KCl和 LiCl-NaCl）和三元混合氯化物熔盐（KCl-LiCl-NaCl）的比热容、粘度和热导率进行系统研究。研究中粘度采用平衡分子动力学方法进行计算,而热导率采用非平衡分子动力学方法进行计算。结果表明,3种氯化物熔盐单质的比热容模拟值与实验值相比误差均在6.8%以内;NaCl、KCL、LiCl熔盐单质的粘度平均误差分别为5.3%、10.9%以及11.7%;热导率除了LiCl外,其余两种氯化物熔盐的热导率误差均在9%以下。与文献不同混合比例的NaCl-KCl的热导率平衡分子动力学方法模拟值比较,偏差均在12.5%以内。综合结果表明模拟结果均与实验值表现出较好的一致性。为了更好地从微观结构上理解氯化物熔盐的热物性,通过计算径向分布函数对熔融盐体系及局部结构进行分析。     

Coupled multiphase heat and mass transfer of a solid particle decomposition reaction with phase change

In this paper, a multiphase reacting system with thermal decomposition of solid reactant particles and phase transition of a molten product at a gas–liquid–solid interface are analyzed. A new predictive model is developed for the mass and heat transfer, as well as the reaction kinetics. The predicted results fit well with experimental data. The analysis is applied to the decomposition of solid particles of CuO–CuCl₂ (copper oxychloride) in a molten salt environment of the thermo-chemical copper–chlorine (Cu–Cl) cycle of hydrogen generation. Oxygen gas is released from the thermal decomposition of copper oxychloride particles. This reaction establishes the high temperature limit of the thermo-chemical cycle at 530 °C. A Stefan boundary condition is used to track the position of the moving solid–liquid interface as the solid particle decomposes to produce molten product under the input of a heat flux at the surface. The results of conversion of CuO–CuCl₂ from both a thermo-gravimetric (TGA) microbalance and large laboratory scale batch reactor experiments are analyzed and the rate of endothermic reaction is measured to determine the heat transfer rate. The resulting reaction rate is incorporated into the model, and the controlling resistances are analyzed. The predicted results are successfully compared and validated against experimental data.     

Thermolysis reactor scale-up for pilot scale CuCl hybrid hydrogen production

Alternative hydrogen production methods are being explored with the goal of finding efficient and economical process. The copper-chlorine (CuCl) cycle for hydrogen production has been the focus of the Clean Energy Research Laboratory (CERL) at the University of Ontario Institute of Technology (UOIT). The CuCl cycle has lower thermal energy requirements compared to other methods and utilizes waste heat from power plants and/or industrial processes. The cycle is comprised of the electrolysis, hydrolysis and thermolysis reaction steps. Decomposition of copper oxychloride (CuOCuCl₂) occurs in the thermolysis reactor at 500–530 °C. A novel thermolysis reactor design is presented here with the purpose of scaling it up to a pilot at an industrial site. Using a dual heater configuration, simulations showed decomposition temperatures were achieved between 3.5 and 4 h with 2.0 kg of CuCl as the feedstock. With 10 kg of CuCl, the reactor would reach decomposition temperatures after 10 h of heat up. Improved start-up thermal performance was observed during experiments with solidified CuCl and a reduction of startup time.     

Properties of some salt hydrates for latent heat storage

For the utilization of low-grade heat the latent storage of thermal energy is of great advantage because the heat can be preserved at a constant temperature perfectly matched to the special purpose of application. Investigations on the heat capacities, enthalpies of fusion, densities, crystallization behaviour and other chemical and physical properties have shown that the following salt hydrates are especially suitable media for storing low-grade heat. The eutectic mixture of water and 3.92% by weight of sodium fluoride, melting point (MP) = - 3.5°C, is extremely convenient and cheap for refrigerating or other cooling purposes. Lithium chlorate trihydrate, LiClO₃. 3H₂O, MP = +8.1°C has an extremely high storage capacity and other advantageous properties as a storage medium in cooling systems, but a very high price will limit its application. Calcium chloride hexahydrate, CaCl₂. 6H₂O, MP = + 29.2°C, is a suitable and cheap storage medium for heating purposes. For the same application disodium hydrogen phosphate dodecahydrate, Na₂HPO₄. 12H₂O, MP = + 35.2°C, is even better because of the larger storage capacity per unit volume and other advantages which largely compensate the higher material cost. the unique properties of potassium fluoride tetrahydrate, KF. 4H₂O, MP = +18.5°C, make it especially suitable for storing low-grade heat. It can directly function as an energy sink and as an energy reservoir in heat collecting and consuming systems. Examples of the practical applicability for residential heating, temperature levelling and cooling are described.     

Thermal cycle testing of calcium chloride hexahydrate as a possible PCM for latent heat storage

In order to study the changes in latent heat of fusion and melting temperature of calcium chloride hexahydrate (CaCl₂·6H₂O) inorganic salt as a latent heat storage material, a thousand accelerated thermal cycle tests have been conducted. The effect of thermal cycling and the reliability in terms of the changing of the melting temperature using a differential scanning calorimeter (DSC) is determined. It has been noticed that the CaCl₂·6H₂O melts between a stable range of temperature and has shown small variations in the latent heat of fusion during the thermal cycling process. Thus, it can be a promising phase change material (PCM) for heating and cooling applications for various building/storage systems.     

Influence of carbon nanotubes and nano-alumina on the thermal performance of nitrate phase change materials for thermal storage

硝酸熔融盐具有良好的热物性能,在聚光太阳能发电技术中得到广泛运用,通过提高熔盐的比热容,可以提高其蓄热能力。将硝酸锂、硝酸钾和硝酸钠以共晶比52：30：18制备硝酸盐储热材料,通过添加纳米氧化铝颗粒及碳纳米管研究其对熔融盐比热的影响。通过超声振动和蒸发在硝酸熔融盐中加入纳米氧化铝颗粒和碳纳米管。采用直接合成法制备含纳米氧化铝和碳纳米管分别为0.06%、0.5%、1%和2%的硝酸盐纳米流体。通过SEM观察了纳米氧化铝颗粒及碳纳米管在硝酸盐中的分散性,当纳米氧化铝颗粒浓度为1%时发生团聚,这种团聚会影响纳米流体的比热容。碳纳米管添加量小于2%时分散性好。通过差示扫描量热仪（DSC）测量了硝酸盐纳米复合材料的比热容,纳米氧化铝浓度为1%的硝酸盐纳米流体在固态（110℃）和液态（140℃）下分别是纯硝酸盐比热容的1.44倍和3.48倍。CNTs浓度为1%的硝酸盐纳米流体在固态（110℃）和液态（140℃）下分别是纯硝酸盐比热容的5.07倍和4.17倍。DSC结果表明当纳米颗粒浓度大于1%时,纳米流体的比热降低。本研究讨论了提高纳米流体比热容的机制,得到硝酸盐比热容的增加与纳米颗粒的高比表面积和高能量相关。     

Estimation of thermodynamic properties of the ternary molten salt system, LiF-NaF-BeF2, by the modified Peng-Robinson equation

The molten salt reactor (MSR), which is one of the generation IV reactors, can meet the demand of transmutation and breeding. The thermodynamic properties of the molten salt system like LiF-NaF-BeF₂ influence the design and construction of the fuel salt and coolant in the MSR for the new generation. In this paper, the equation of state of the ternary system 15%LiF-58%NaF-27%BeF₂, over the temperature range from 873.15 to 1 073.15 K at one atmosphere pressure, is described using a modified Peng-Robinson (PR) equation. The densities of the ternary system and its components are estimated by this equation directly, and compared with the experimental data. Based on the equation of state, the other thermodynamic properties such as the enthalpy, entropy and heat capacity at constant pressure are estimated by the residual function method and the fugacity coefficient method respectively. The densities calculated by PR equation are highly in agreement with the experimental data, and the enthalpy, entropy and heat capacity evaluated by the two different methods are consistent with each other. It can be concluded that the modified PR equation can be applied to evaluate the density of the molten salt system, and it is recommended that it be used as the basis to estimate the enthalpy, entropy and heat capacity of the molten salt system.     

Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis

A copper chloride (CuCl) electrolyzer that constitutes of composite proton exchange membrane (PEM) that functions at medium-temperature (>100 °C) is beneficial for rapid electrochemical kinetics, and better in handling fuel pollutants. A synthesized polybenzimidazole (PBI) composite membrane from the addition of ZrO₂ followed with phosphoric acid (PA) is suggested to overcome the main issues in CuCl electrolysis, including the copper diffusion and proton conductivity. PBI/ZrP properties improved significantly with enhanced proton conductivity (3 fold of pristine PBI, 50% of Nafion 117), superior thermal stability (>600 °C), good mechanical strength (85.17 MPa), reasonable Cu permeability (7.9 × 10⁻⁷) and high ionic exchange capacity (3.2 × 10⁻³ mol g⁻¹). Hydrogen produced at 0.5 A cm⁻² (115 °C) for PBI/ZrP and Nafion 117 was 3.27 cm³ min⁻¹ and 1.85 cm³ min⁻¹, respectively. The CuCl electrolyzer efficiency was ranging from 91 to 97%, thus proven that the hybrid PBI/ZrP membrane can be a promising and cheaper alternative to Nafion membrane.     

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.     

Thermal design of a solar hydrogen plant with a copper-chlorine cycle and molten salt energy storage

In this paper, the operating temperature ranges of various solar thermal energy technologies are analyzed, with respect to their compatibility with solar hydrogen production via thermochemical cycles. It is found that the maximum temperature of 530 °C required by the oxygen production step in the Cu-Cl cycle can be supplied by current solar thermal technologies. The heat requirements are examined for the Cu-Cl cycle and it is found that the heat source must be sufficiently high and above the maximum temperature requirement of the Cu-Cl cycle, in order to match the heat requirements of the cycle. The quantity of molten salt and solar plant dimensions for capturing and storing solar heat for an industrial hydrogen production scale are also estimated for 24 h operation per day. The flow characteristics and heat losses of molten salt transport in pipelines are studied while considering the influences of pipeline diameter, heat load and weather conditions. The heat loss from a solar salt storage tank is also calculated based on various tank diameters and heights. The intermediate product of molten salt produced in the oxygen production step gives the Cu-Cl cycle a significant advantage of linkage with current high temperature solar thermal technologies. This allows flexibility for integration of the Cu-Cl cycle and solar thermal plant. Using a thermal network analysis of the Cu-Cl cycle, the layout options for the integration of a Cu-Cl cycle with various solar thermal technologies are presented and discussed in this paper.     

Multiphase reactor scale-up for Cu–Cl thermochemical hydrogen production

This paper examines selected design issues associated with reactor scale-up in the thermochemical copper–chlorine (Cu–Cl) cycle of hydrogen production. The thermochemical cycle decomposes water into oxygen and hydrogen, through intermediate copper and chlorine compounds. In this paper, emphasis is focused on the hydrogen, oxygen and hydrolysis reactors. A sedimentation cell for copper separation and HCl gas absorption tower are discussed for the thermochemical hydrogen reactor. A molten salt reactor is investigated for decomposition of an intermediate compound, copper oxychloride (CuO·Cl₂), into oxygen gas and molten cuprous chloride. Scale-up design issues are examined for handling three phases within the molten salt reactor, i.e., solid copper oxychloride particles, liquid (melting salt) and exiting gas (oxygen). Also, different variations of hydrolysis reactions are compared, including 5, 3 and 2-step Cu–Cl cycles that utilize reactive spray drying, instead of separate drying and hydrolysis processes. The spray drying involves evaporation of aqueous feed by mixing the spray and drying streams. Results are presented for the required capacities of feed materials for the multiphase reactors, steam and heat requirements, and other key design parameters for reactor scale-up to a pilot-scale capacity.     

Experimental investigation of a calcium chloride-water heat storage system for off-peak energy supply and solar heating

One of the main difficulties in applying heat storage systems utilising the latent heat of calcium chloride hexahydrate is the incongruent character of the melting of the hexahydrate. The heat storage capacity of the system is considerably diminished by the separation of α-CaCl₂.4H₂O, forming a new phase. In externally non-mixed systems the separation of the tetrahydrate is practically irreversible and this causes the rapid deterioration of the heat storage system. During the experiments described in this paper the effects of the change in composition of the CaCl₂, 6H₂O heat storage material, of the impurities (NaCl, KCl, MgCl₂) and of thermoconvective flow on the formation of α-CaCl₂.4H₂O were investigated in externally non-mixed systems. It has been established that with a heat storage packing of more dilute (CaCl₂.6·3H₂O) total composition than the peritectic composition, favourable storage properties can be attained. To achieve this, the solid impurities have to be removed from the molten phase and the electric cable providing the heat input must be arranged to provide for adequate concentration equalisation by the convective thermal flow.     

Design of diatomite‐based hydrated salt composites with low supercooling degree and enhanced heat transfer for thermal energy storage

In this work, diatomite (DM) was calcined at 400°C to obtain the pure surface and pores (DM‐1); then, the three kinds of shape‐stable composite phase change materials (ss‐CPCMs) of CaCl₂·6H₂O (CCH)/DM‐1, CH₃COONa·3H₂O (SAT)/DM‐1, and Na₂SO₄·10H₂O (SSD)/DM‐1 were prepared by impregnation method. The hydrated salts were uniformly adsorbed on the surfaces and into the pores of DM‐1 by capillary action and surface tension. The addition of nucleating agent effectively reduced the supercooling degrees of hydrated salts, and the heterogeneous nucleation mechanism was employed to explain the supercooling suppression. The results showed that the supercooling degrees of the three hydrated salt ss‐CPCMs with optimal nucleating agent were less than or equal to 0.6°C. Moreover, the thermal conductivities of ss‐CPCMs were significantly improved by adding graphite, and the addition of 10 wt% graphite could increase the thermal conductivity of hydrated salt ss‐CPCMs by at least 70%. The package capacity of CCH, SAT, and SSD in three ss‐CPCMs with appropriate contents of nucleating agent and graphite was 58.1, 56.1, and 56.3 wt%, respectively. The DSC results showed that the phase change temperatures of the three ss‐CPCMs were approximately 28.8 to 57.8°C, and the latent heat was approximately 108.7 to 149.4 J/g. The XRD and FT‐IR results exhibited that the three ss‐CPCMs indicated understanding chemical compatibility. In addition, the results of 200 melt‐solidification cycles demonstrated that the ss‐CPCMs had excellent thermal cycle reliability. Thus, the hydrated salt/DM‐1 ss‐CPCMs showed great potential as heat storage materials for various heat storage applications.     

Design and energy evaluation of a stand‐alone copper‐chlorine (Cu‐Cl) thermochemical cycle system for trigeneration of electricity,hydrogen, and oxygen

In this article, a new stand‐alone Cu‐Cl cycle system (SACuCl) for trigeneration of electricity, hydrogen, and oxygen using a combination of a specific combined heat and power (CHP) unit and a 2‐step Cu‐Cl cycle using a CuCl/HCl electrolyzer is presented. Based on the self‐heat recuperation technology for the CHP unit and the heat integration of the Cu‐Cl cycle unit, the power efficiency of the SACuCl for 5 prescribed scenarios (case studies) is predicted to achieve about 48% at least. The SACuCl uses the technologies of the dry reforming of methane and the oxy‐fuel combustion to achieve a relatively high CO₂ concentration in the flue gas, and CO₂ emissions for power generation could be almost restricted by 0.418 kg/kWh. From the aspect of the electricity required for hydrogen production, it is verified that the 2‐step Cu‐Cl cycle system is superior to the conventional water electrolyzer because the CHP process supplies the heat/electricity for Cu‐Cl thermochemical reactions and a thermoelectric generator is connected to the exhaust gas for recovering the power consumption from the compressor and the CuCl/HCl electrolyzer. Finally, the heat exchanger network and the pinch technology are employed to determine the optimum heat recovery of the Cu‐Cl cycle. In case 5 analyzed for the SACuCl, the electricity required for the heat‐integrated 2‐step Cu‐Cl cycle is predicted to dramatically decrease from 4.39 to 0.452 kWh/m³ H₂ and the cycle energy efficiency could be obviously increased from 23.77 to 31.97%.     

Advances in unit operations and materials for the CuCl cycle of hydrogen production

This paper presents recent advances by an international team of five countries – Canada, U.S., China, Slovenia and Romania – on the development and scale-up of the copper–chlorine (CuCl) cycle for thermochemical hydrogen production using nuclear or solar energy. Electrochemical cell analysis and membrane characterization for the CuCl/HCl electrolysis process are presented. Constituent solubility in the ternary CuCl/HCl/H₂O system and XRD measurements are reported in regards to the CuCl₂ crystallization process. Materials corrosion in high temperature copper chloride salts and performance of coatings of reactor surface alloys are examined. Finally, system integration is examined, with respect to scale-up of unit operations, cascaded heat pumps for heat upgrading, and linkage of heat exchangers with solar and nuclear plants.