Experimental investigation of a heat pump coupled to a thermal storage for charging a Carnot battery
Carnot batteries (CBs) are commonly charged using heat pumps, yet experimental data for model validation remain limited. We designed and commissioned a flexible, highly instrumented, laboratory-scale test bench and report initial CB charging experiments. The setup uses a 3.3 kW reciprocating compressor in the heat pump and incorporates two unpressurized water storage tanks for energy storage; the condenser heats water transferred from one tank to the other. Two plate heat exchangers ensure deep subcooling below 25 °C, which reduces throttling losses and enables CB start-up from ambient temperature. Tap water entering at ambient temperature served as heat source, limiting the minimum temperature. We compare a zeotropic working fluid mixture (propane/pentane) with two pure fluids (propane and butane), focusing on irreversibilities in the condenser and compressor. We assess the impact of operating conditions on cycle and component efficiencies by varying the upper storage temperature from 45 °C to approximately 90 °C and exploring compressor outlet temperatures ranging from 70 to 118 °C. The process-dependent overall compressor efficiency of the mixture ranges from 47 to 57%, which exceeds the efficiency of pure butane. The mixture reduces exergy destruction rates during heat rejection to the sensible storage tanks compared to the pure fluids, but increases exergy destruction rates during evaporation against a near-isothermal heat source, the latter reduces the second-law efficiency. Finally, a reciprocating-compressor model is compared with the measurements, indicating its potential for heat pump simulation.