Salt hydrate based thermochemical systems cascaded with high temperature mechanical heat pumps for waste heat recovery

Abstract

The efficient use of waste heat is essential for addressing global energy needs and achieving carbon neutrality targets. High-Temperature Heat Pumps (HTHPs) play an important role in this endeavor. However, for their efficient operation, these devices need a continuous supply of heat at temperatures higher than their evaporators’ temperatures. As a result, heat at temperatures below HTHP’s evaporator temperature is wasted. This study aims to design and thermodynamically analyze a cascaded system where, next to the HTHP, a thermochemical system based on salt hydrates is integrated, which is able to upgrade the low-grade waste heat of the HTHP. The system incorporates a dual-reactor salt hydrate configuration in which the HTHP’s waste heat is utilized in two steps: as a driving force to generate water vapour that reacts in one reactor, producing higher temperature heat, and to charge the other reactor. The present study focuses mainly on the heat recovery performance of the thermochemical heat transformer or heat pump coupled with the HTHP. The study emphasizes the need for open reactor operation and presents a procedure for screening appropriate salt hydrates. A thermodynamic analysis is presented to evaluate the merits of the optimal system. Notably, using K2CO3 salt hydrate, thermodynamic analysis shows promising performance, delivering heat of 62.28 kW per kg/s of air at 57.81 % efficiency at 145 °C. The cascaded system has an overall efficiency of 85.33 % at 145 °C, with an estimated heat upgrading efficiency of 49.33 %.

Keywords

• High temperature heat pump
• Waste heat recovery
• Thermochemical energy
• Thermochemical heat transformer
• Salt hydrates
• Cascaded heat pump