A reality check on long-term thermochemical heat storage for household applications

Long-term thermochemical and sorption heat storage processes for household applications are struggling to fulfill promises, probably because of an initial misunderstanding about the nature of these processes and the definition of their performance criteria. This perspective paper focuses on long-term thermochemical heat storage for space heating and domestic hot water production, which are the main applications targeted by the vast majority of studies. It provides a documented view on the technology, while questioning the relevance of the idea of long-term thermochemical heat storage for household applications and the performance criteria used to evaluate the corresponding systems. The authors present a fundamental analysis of this storage technology, examining the nature of the processes and the most realistic potential availabilities on the application side. Additionally, the criteria used to assess the performance of the materials, processes or systems are discussed. Finally, based on data collected on many prototypes available in the literature, some of the limitations of these processes have been highlighted. It appears that seasonal thermochemical heat storage is subjected to significant thermal losses. The volumetric energy storage density exhibited by the processes based on solid hydrates or aqueous solutions is prohibitive for long-term thermal energy storage for domestic hot water and, in most cases, for house heating. So far, the performance or advantages of thermochemical energy storage have been probably overestimated in the literature. The authors of this paper suggest that, before undertaking new research activities on thermochemical heat storage, researchers should carefully consider the challenges pointed out here.