Development of novel thermodynamic cycles of chemical heat pump for high-temperature heat lifting

The efficiency of Carnot Batteries (CB) for large-scale renewable energy storage is constrained by low coefficients of performance (COP) in electric-to-heat conversion. While mechanical heat pumps can theoretically elevate this efficiency, their deployment is limited by high capital costs associated with large complex turbomachinery. Chemical heat pumps offer a promising alternative, yet existing systems typically fail to simultaneously achieve both high-temperature heat and high efficiency. Here, three novel high-temperature chemical heat pump cycles based on Ca(OH)2 and Mg(OH)2 are proposed to convert renewable electricity into stable 400 - 560oC heat efficiently. A comprehensive thermodynamic analysis quantified the energy performance of each cycle. Expander integration in the Ca(OH)2/CaO single-stage cycle (Case 2) enables recovery of high-grade sensible heat from superheated vapor, generating 52.09 kW of electrical power while maintaining COP at 1.39. A novel two-stage cycle (Case 3) coupling Ca(OH)2/CaO and Mg(OH)2/MgO concentrate heat output in the high-temperature range. At a Ca-based hydration temperature of 560oC, the two-stage cycle achieves a COP of 1.64, a 17.14% improvement over traditional single-stage cycles. A compressor is further introduced in the two-stage cycle (Case 4) to lower the required Mg-based dehydration temperature, thereby expanding operational flexibility. For CB applications, the total heat output of the two-stage cycle can be utilized, whereas only partial heat output of the single-stage cycle is applicable. This distinct advantage enables the proposed two-stage cycle to achieve an efficiency η of 1.84 times that of the conventional cycle. These results demonstrate significant potential for efficiently utilizing renewable energy through advanced ‘electric-to-heat’ conversion.