Design Optimization of a Compact Standalone CaSO4 Solar Chemical Heat Pump Under Moderate Solar Irradiation

Standalone solar chemical heat pumps (SCHPs) have attracted increasing attention as promising technologies for utilizing renewable solar thermal energy for heating and cooling applications. However, residential deployment remains challenging because reducing collector area and installation space generally leads to a decline in thermochemical storage performance. In particular, practical design guidelines for minimizing system size while maintaining performance under moderate solar irradiation conditions such as in autumn have not been sufficiently established. In this study, a standalone CaSO4 solar chemical heat pump driven solely by solar thermal energy and a photovoltaic-powered heat exchange medium (HEM) circulation system was experimentally investigated. First, the influence of HEM flow rate on sensible and thermochemical heat storage was evaluated using a baseline configuration equipped with two solar collectors. Subsequently, the collector area was reduced by employing a single collector. Finally, the apparatus layout was optimized through adjustment of the collector tilt angle for autumn operation in Japan and reduction in piping length by vertically installing the CHP unit. The results showed that increasing the HEM flow rate enhanced heat transfer and promoted the dehydration reaction of CaSO4. Under the optimum condition, a conversion of approximately 93% and a chemical heat storage capacity of 537 kJ were achieved. Furthermore, the optimized layout significantly reduced thermal losses and improved solar energy utilization. Although the collector area was reduced by 50%, the optimized Type 3 configuration achieved a chemical heat storage capacity of 462 kJ, corresponding to 84% of that of the baseline system. In other words, a 50% reduction in collector area resulted in only a 16% reduction in heat storage capacity. These results demonstrate that system-level optimization can substantially mitigate the performance loss associated with collector downsizing. The proposed configuration provides a practical pathway toward compact residential SCHPs with reduced installation space requirements while maintaining high thermochemical heat storage performance.