4E evaluations of salt hydrate-based solar thermochemical heat transformer system used for domestic hot water production

A critical step toward the widespread use of renewables is the development of effective energy storage technology. An impressive solution for energy storage and heat upgrade is the salt hydrate-based solar thermochemical heat transformer (THT). The article aims at the temperature lift effects of pressurization-assisted THT systems employing different salts to fulfill the heat requirements of domestic hot water (DHW) generation. To grasp the sustainability of the GJ-level THT systems, energy, exergy, economic, and environmental (4E) assessments are performed under various working conditions. Results manifest that the majority of THT systems enable discharging temperatures (Tdis) to surpass 65 °C, matching regular DHW production. Tdis can be further boosted by the two-stage pressurization whereas at the expense of lowering thermodynamic properties. The SrBr2-based system almost exhibits the best 4E performances with a Tdis of 74.3 °C, although its levelized energy cost (LEC) of 0.1162 $/kWh is slightly higher than that of the LiOH-based system (0.1147 $/kWh). Both systems yield great useable heat, up to 11,667 MJ and 11,140 MJ, respectively, with maximum exergy efficiency of 89.16 % and 63.41 %. Albeit capable of generating higher temperature DHW (≥90 °C), the useable heat and thermodynamic performances of the FeCl2 and CaCl2 based systems are unsatisfactory. By contrast, the K2CO3 and LiOH based systems render higher temperature DHW while ensuring acceptable thermodynamic properties and useable heat. Targeting the regular and higher temperature DHW productions, the lowest CO2 emissions are separately achieved by the SrBr2 and LiCl based systems, i.e., 15 kg/MWh and 56.2 kg/MWh; and the former shows the slowest growth rate in carbon emission with increased Tdis. Augmenting solar irradiation and duration contributes to reducing LEC, and the ideal operating conditions in thermo-economic performance may differ from system to system.