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Waste heat recovery in the energy intensive industry is one of the most important measures for the mitigation of climate change. The utilization of just a fraction of the theoretically available waste heat potential would lead to a significant reduction of the primary energy consumption and hence a reduction of greenhouse gas emissions. The present study examines the integration of a packed bed thermal energy storage for waste heat recovery in the iron and steel industry. Along with the highly fluctuating availability of excess heat the main difficulty of waste heat recovery in industrial processes is the high amount of powder that is transported by the hot exhaust gases. Therefore, investigations focus on the pressure drop and powder hold-up in a packed bed thermal energy storage that is operated with a gas-powder two phase exhaust gas as heat transfer fluid with the ultimate goal to assess its suitability and robustness under such challenging operational conditions. The results indicate, that 98 % of the powder that is introduced into the system with the heat transfer fluid during charging accumulates in the packed bed. Remarkably, most of the powder hold-up in the packed bed is concentrated near the surface at which the heat transfer fluid enters the packed bed. When reversing the flow direction of the heat transfer fluid to discharge the storage with a clean single phase gas, this gas is not contaminated with the powder that has been accumulated in previous charging periods. Further, the radial distribution of the powder hold-up in the packed bed is observed to be even which indicates that there is no risk of random flow channel formation that could affect the thermal performance (storage capacity, thermal power rate) of the system. The results reinforce the great potential of packed bed thermal energy storage systems for waste heat recovery in the energy intensive industry., Comment: submitted to Journal of Energy Storage

The waste heat potential from industrial processes is huge and if it can be utilized it may contribute significantly 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. The maximum entropy generation rate for a 22-hour standby process with the flow direction of the heat transfer fluid from bottom to top in the preceding charging process is twice as high as for the same process with the reverse flow direction. Energy and exergy efficiencies are lower for the process with reverse flow direction by 5% and 18% respectively., Comment: submitted to Applied Energy