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

Abstract

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 10kg 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°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 2h storage time. While the energy recovery ratio decreases slightly with charging temperature due to heat losses, energy storage density increases strongly with temperature from 9.5kWhm−3 at 175°C to 20.4kWhm−3 at 250°C. The measured energy storage density during continuous operation corresponds to 93% of the theoretical maximum for lossless operation and to 76% for 2h 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.