Exergy efficiency and thermocline degradation of a packed bed thermal energy storage in partial cycle operation: An experimental study

To enable the exploitation of the industry sectors' huge waste heat potential this study investigates the utilization of packed bed thermal energy storage systems for the waste heat recovery in the iron and steel industry. The main goal is to assess the partial cycle operation of a packed bed thermal energy storage with industrial exhaust gas that is contaminated with high amounts of metal powder. A continuously rising accumulation of powder particles inside the packed bed requires the storage to be charged from the bottom in order to facilitate the removal of powder hold-up. Therefore, investigations focus on the dependency of the thermal performance of the storage on the flow direction of heat transfer fluid during charging/discharging. Exergy efficiency and thermocline degradation are evaluated as key performance indicators for different flow directions of the heat transfer fluid using a lab-scale test rig. Results show that, compared to charging from the top, a slightly faster thermocline degradation occurs for a storage that is charged from the bottom. Still, energy and exergy efficiencies in partial cycle operation are well above 85% and 80% respectively regardless of the heat transfer fluid flow direction. The thermal power rate during discharging is stable between 74% and 93% with respect to the maximum input rate for both charging the storage from the top and from the bottom. All in all, packed bed thermal energy storage systems are found to be suitable for waste heat recovery in industrial processes. Only a marginal deterioration of the thermal performance of the storage has to be expected if the storage is required to be charged from the bottom.