Numerical evaluation of a Carnot battery system comprising a chemical heat storage/pump and a Brayton cycle

In this work, a novel Carnot battery system comprising a chemical heat storage/pump (CHS/P) and a Brayton cycle is presented. The reversible chemical reactions selected for heat storage/pump are the dehydration of calcium hydroxide and the hydration of calcium oxide. The re-conversion of heat of reaction into electricity is provided by a Brayton cycle, in which the working fluid is supercritical carbon dioxide (sCO2). This Carnot Battery system is compared to the heat storage system comprising a two-tank molten salt sensible heat storage (SHS) in which the maximum temperature of salt storage is selected as 560°C. It is demonstrated that the round-trip efficiency of the CHS/P can range from 37.4% to 41.7% when evaporation temperature of water for the heat output reaction of the chemical heat storage system is selected, respectively, as 80°C or 160°C, while the system comprising the SHS can reach a round-trip efficiency up to 40.3%. It is also estimated that the total volume of storage required for the CHS/P is 66% smaller than in SHS system. The exergy method was applied on both systems. The analysis of relative irreversibility of the systems shows that under the assumptions given in this work the major loss of exergy occurs when converting electricity into heat rather than transferring heat from the storage to the sCO2. Mapping charts of rational efficiency for the dehydration and hydration reactions have been determined for the main combinations of reaction temperatures and condensation/evaporation temperatures: those can be a guide for future efficient design of Carnot batteries including CHS/P. Finally, a comparison with other energy storage technologies highlights that the proposed CHS/P Carnot battery has an energy density of 280 Wh/l, which is one of the highest values in comparison with other energy storage technologies reported in Literature.