A systematic multi-step screening of numerous salt hydrates for low temperature thermochemical energy storage

In this paper, the potential energy storage density and the storage efficiency of salt hydrates as thermochemical storage materials for the storage of heat generated by a micro-combined heat and power (micro-CHP) have been assessed. Because salt hydrates used in various thermochemical heat storage processes fail to meet the expectations, a systematic evaluation of the suitability of 125 salt hydrates has been performed in a three-step approach. In the first step general issues such as toxicity and risk of explosion have been considered. In the second and third steps, the authors implement a combined approach consisting of theoretical calculations and experimental measurements using Thermogravimetric Analysis (TGA). Thus, application-oriented comparison criteria, among which the net energy storage density of the material and the thermal efficiency, have been used to evaluate the potential of 45 preselected salt hydrates for a low temperature thermochemical heat storage application. For an application that requires a discharging temperature above 60°C, SrBr2·6H2O and LaCl3·7H2O appear to be the most promising, only from thermodynamic point of view. However, the maximum net energy storage density including the water in the water storage tank that they offer (respectively 133kWhm-3 and 89kWhm-3) for a classical thermochemical heat storage process are not attractive for the intended application. Furthermore, the thermal efficiency that would result from the storage process based on salt hydrates without condensation heat recovery appears also to be very low (lower than 40% and typically 25%). Even for application requiring lower discharging temperature like 35°C, the expectable efficiency and net energy storage density including the water storage remain low. Alternative processes are needed to implement for salt hydrates in low temperature thermochemical heat storage applications.