Standby efficiency and thermocline degradation of a packed bed thermal energy storage: An experimental study.

The waste heat potential from industrial processes is tremendous and if it can be utilized it may significantly contribute to the mitigation of climate change. A packed bed thermal energy storage system is a low-cost storage technology that can be employed to enable the utilization of waste heat from industrial processes. This system can be used to store excess heat and release this energy when it is needed at a later time. To ensure the efficient operation of a packed bed thermal energy storage, its characteristics in standby mode need to be studied in great detail. In the present study, the standby efficiency and thermocline degradation of a lab-scale packed bed thermal energy storage in standby mode is experimentally investigated for different flow directions of the heat transfer fluid during the preceding charging period. Results show that for long standby periods, the standby efficiency is significantly affected by the flow direction of the heat transfer fluid. The maximum entropy generation rate for a 22 h standby process with the flow direction of the heat transfer fluid from the bottom to the top in the preceding charging process is twice as high as for the same process with reversed flow direction. Energy efficiency is 5% higher whilst exergy efficiency is even 18% higher in the process with reversed flow direction. • Standby mode of a packed bed thermal energy storage is experimentally investigated. • Measurement data is processed by second law analysis. • Energy and exergy efficiencies of max. 92% and 83% are measured. • Standby efficiencies depend on the air flow direction in the charging period. • The impact of natural convection on long standby periods is significant. [ABSTRACT FROM AUTHOR]