Innovative approaches to waste heat recovery: Coupling high temperature vapour compression heat pumps with salt hydrate thermochemical systems

Efficient utilization of waste heat is a crucial method to meet global energy demands and carbon neutrality. High Temperature mechanical Heat Pumps (HTHPs) are vital in this context but are limited by evaporator temperature constraints. This study proposes an innovative approach to upgrading and reintegrating waste heat from HTHPs (105–140 °C) by coupling them with a dual reactor salt hydrate based Thermo-Chemical Energy Storage (TCES) system. Operating in a quasi-continuous mode, the system utilizes the waste heat from the HTHP to drive hydration (discharge) in one reactor and dehydration (charge) in another. The key to heat upgrading lies in the evaporator of the TCES system, which governs system performance. Therefore, an empirical relation has been developed to determine its optimum temperature as a function of waste heat temperature, heat upgrade temperature, and the thermal properties of the salt hydrate. Subsequently, the system performance with K2CO3-H2O was assessed by applying the first law of thermodynamics, with the evaporator temperature of the TCES varied from 100 °C to 90 °C. Lowering the evaporator temperature of the TCES enhances thermal output but is constrained by the HTHP's temperature requirements. The system delivers 55.4 kW per kg/s of air with a heat upgrade efficiency of 45.7 %, using waste heat at 140 °C and the evaporator of the TCES at 100 °C. This study attempts to establish a framework for designing efficient thermally driven cascaded heat pumps.