The Performance of Small-Pore Microporous Aluminophosphates in Low-Temperature Solar Energy Storage: The Structure-Property Relationship

The utilization of the reversible chemical and physical sorption of water on solids provides a new thermal energy storage concept with a great potential for lossless long‐term storage. The performance of microporous aluminophosphates in heat storage applications is highlighted by a comparative thermogravimetric and calorimetric study of three known materials (SAPO‐34, AlPO4‐18, APO‐Tric) and is correlated with their structural features. The maximum water sorption capacity is similar for all three samples and results in a stored energy density of 240 kWh m−3 in the 40–140 °C range. The elemental composition influences the gradual (silicoaluminophosphate SAPO‐34) or sudden (aluminophosphates AlPO4‐18, APO‐Tric) water uptake, with the latter being favourable in storage systems. The driving force for the determined sorption process is the formation of highly ordered water clusters in the pores, which is enabled by rapid and reversible changes in the Al coordination and optimal pore diameters. The ease with which changes in the Al coordination can occur in APO‐Tric is related to the use of the fluoride route in the synthesis. The understanding of these fundamental structure/sorption relationships forms an excellent basis for predicting the storage potential of numerous known or new microporous aluminophosphates and other porous materials from their crystal structures. A model that predicts the heat storage potential of numerous known or new microporous aluminophosphates is proposed based on a comparative thermogravimetric and calorimetric study of three known structures. The formation of highly ordered water clusters in the pores is determined to be a driving force for sudden water uptake in a narrow relative pressure range.