Heat lost or stored: Experimental analysis of adsorption thermal energy storage

Adsorption thermal energy storage systems are currently intensively investigated due to their high energy storage density and their potential for long-term storage. Still, thermal storage systems suffer from heat losses which need to be quantified to apply storage in practice. In this paper, we determine the heat losses of a closed unit for adsorption thermal energy storage with 10 kg of zeolite 13X. We demonstrate that heat-loss coefficients can be determined accurately from steady-state measurements of the adsorber. We analyze the storage performance for charging temperatures of 175-250 degrees C. Our storage unit offers high thermal efficiencies: above 91% during charging and higher than 95% during discharging. For the investigated range of charging temperatures, the energy recovery ratio varies between 85-91% for continuous cyclic operation, and reduces to 69-74% for cyclic operation with 2 h storage time. While the energy recovery ratio decreases slightly with charging temperature due to heat losses, energy storage density increases strongly with temperature from 9.5 kW h m(-3) at 175 degrees C to 20.4 kW h m(-3) at 250 degrees C. The measured energy storage density during continuous operation corresponds to 93% of the theoretical maximum for lossless operation and to 76% for 2 h storage time. The results show that adsorption thermal energy storage can provide for efficient storage. For short-term storage, the energy storage density can be enhanced significantly by charging at high temperatures. For long-term storage, heat losses can strongly reduce the storage density. (C) 2016 Elsevier Ltd. All rights reserved.