Experimental investigation of a rotating drum heat exchanger for latent heat storage

Kurzfassung

With latent heat thermal energy storages, thermal energy can be stored at a constant temperature level with high storage density using the enthalpy of the solid-liquid phase change of a material. During the discharge process of a latent heat thermal energy storage, phase change material (PCM) solidifies at the heat transfer surface and increases the thermal resistance. This decreases the transferred thermal power with time and the state of charge. The rotating drum heat exchanger, experimentally investigated in detail for the first time in this paper, overcomes this limitation by removing the PCM layer. While a heat transfer fluid passes through the inside of the rotating drum, which is partially immersed in liquid PCM, the PCM solidifies at the outer side. The solidified layer is removed at each rotation by a fixed scraper. Thus, the layer thickness and the thermal power are kept constant over time. The solidified PCM can be stored separately from the liquid phase, resulting in a complete independence of thermal power and storage capacity. A commissioned experimental test rig using a low temperature PCM is used for the investigation of the heat transfer potential, the layer thicknesses, the mechanical energy needed for the removal of the solidified layer and as a proof of concept. The experimental data show a consistent heat transfer which is increasing for higher rotational speeds. With the presented test rig, the heat transfer density is up to 6.8 kW*m^-2 based on the total drums shell surface of the rotating drum at a temperature difference of 5 K between the melting point of the PCM and the temperature of the heat transfer. Adhering liquid PCM increases the total heat transfer by up to 60% as the liquid PCM solidifies also after the surface left the liquid PCM. While the measured solidified layer decreases to below 0.05 mm with higher rotational speeds, the adhering layer is slightly increasing. The results show the high potential of the rotating drum heat exchanger concept for the generation of steam out of a high temperature PCM.