(392b) Characterizing Thermochemical Energy Storage Materials for Heat Decarbonization in Buildings

The buildings sector is a significant consumer of primary energy, of which 60% is used for space conditioning. To decarbonize these thermal loads, promising solutions include solar heating and renewable electrification, but their intermittency necessitates the use of thermal energy storage (TES) to match demand and supply. Of the different TES material categories, thermochemical materials (TCMs) exhibit volumetric energy densities that are at least 3 higher than phase change materials (PCMs) and sensible heat storage, making it attractive for TES in buildings where space is limited. Furthermore, TCMs exhibit negligible heat loss (i.e., no self-discharge) as they store and release energy via reversible chemical reactions between a salt and water vapor (closed loop) or moist air (open loop). Selection of these salt hydrate TCMs requires careful consideration of multiple thermodynamic properties and material morphology that influences both heat and mass transport kinetics, leading to reaction irreversibilities and loss of storage capacity during cycling. To this end, we discuss two promising approaches to mitigate hygrothermal and structural stability challenges with TCMs: (i) encapsulation of the salt into a highly porous matrix, and (ii) formation of binary salt mixtures with complementary phase diagrams and hydration kinetics. We also show the importance of particle size on the hydration/dehydration kinetics. Overall, these composites can enhance water vapor diffusion and energy storage density, while maintaining structural stability under cycling to achieve high thermal power outputs. This approach also extends the operational window of individual salts, and it enables the addition of thermally conducting filler materials to improve the thermal conductivity of TCMs.