Combining Phase Change and Thermochemical Processes for Air Conditioning and Long-Distance Thermal Transport

This talk presents my group’s work on combining phase change and thermochemical processes to improve air conditioning, dehumidification, and long-distance thermal transport. I discuss our work on Smart ThermOREsponsive (STORE) desiccants, which undergo a hygroscopic-to-hydrophobic phase transition at a lower critical solution temperature (LCST). This LCST phase change facilitates thermochemical processes (water absorption/desorption) and can improve air conditioning/dehumidification performance. Next, I discuss our work on facilitating long-distance thermal transport by combining phase change separation processes (distillation) and reversible thermochemical reactions.<br/><br/>I first present thermodynamic models for air conditioning/dehumidification cycles that use STORE desiccant LCST phase transitions.[1, 2] I examine two classes of LCST materials: LCST polymers which undergo a solid-solid phase transition and LCST ionic liquids which undergo a liquid-liquid phase transition. The LCST regeneration mechanism leads to fundamental advantages relative to traditional desiccants. The performance of traditional desiccants is intrinsically tied to psychrometrics because they regenerate by desorbing <i><u>gaseous</u></i> water into a stream of hot dry air. In contrast, the performance of STORE desiccants is intrinsically tied to materials chemistry. STORE desiccants instead regenerate by releasing <i><u>liquid</u></i> water through their LCST phase transition. Our modeling work shows that these STORE desiccant cycles can have lower regeneration temperatures and higher coefficients of performance than traditional desiccant cycles.<br/><br/>I next present a thermodynamic model for a “thermochemical heat pipe” that uses reversible liquid-phase endothermic/exothermic reactions to efficiently transport waste heat over long distances. More specifically, the thermochemical heat pipe uses waste heat to both (i) drive the endothermic cleavage of an adduct molecule into two small reactant molecules and (ii) chemically separate the reactant molecules via distillation. The separated molecules are then pumped to their destination where they are combined in an exothermic reaction that supplies heat to users (and chemically reforms the adduct molecule). The adduct molecule is then pumped back to the waste heat source for re-use. Thermodynamic analysis of the system shows that heat transport efficiencies of ~30% are possible when only the thermochemical energy storage component is used. This efficiency can be boosted up to ~70% by incorporating heat exchangers that recover the sensible and latent energy components prior to long distant thermal transport. Pump work analysis shows that thermal transport over thousands of kilometers is feasible when transport is done in the liquid phase.<br/><br/>[1] Kocher, Yee, and Wang, Energy Conversion and Management, 253, 115158 (2022)<br/>[2] Rana and Wang, Energy Conversion and Management, 201, 118029 (2024)