Sorption based long-term thermal energy storage – Process classification and analysis of performance limitations: A review

Abstract In sorption heat storage, one of the sources of discrepancy between theoretical material based energy storage potential and resulting system performance is the choice of process type. In this paper, in order to understand this performance deviation, a sorption heat storage process categorisation is proposed. This is followed by a review of reported sorption systems categorised according to the proposed process classification. An analysis of the reported systems is then undertaken, focusing on the ratio of resulting temperature gain in sorption (ad- or absorption), compared to required temperature lift in desorption. This measure is termed temperature effectiveness and enables a form of system performance evaluation in the broad landscape of sorption thermal energy storage demonstrators. It is argued that other performance parameters such as volumetric energy storage density and volumetric charge and discharge power density are not adequate for comparison due to the highly varying testing conditions applied. From the system evaluation, it is seen that best temperature effectiveness is generally found in a closed, transported process with the ability of single sorbent pass and true counter flow heat exchange.