Examination of the heat transfer potential of an active latent heat storage concept

Efficient storage of thermal energy is a key element for the transition of the energy system to renewable energy sources. To reach a high exergetic efficiency, latent heat thermal energy storages offer the opportunity to charge and discharge thermal energy at a constant temperature level. As most Phase Change Materials (PCM) have a low thermal conductivity, the layer of solidified PCM material growing on the heat exchanger surface during discharging typically limits the specific heat transfer to very low values. The state of the art in overcoming this effect is the increase of the heat transfer area by heat transfer structures. Another possibility to increase the heat flux is by means of active latent heat storage concepts. With active concepts, the thickness of the solidified PCM layer can be minimized and controlled by an active motion of the PCM or the heat transfer surfaces. In the presented rotating drum concept, a partially submerged drum is rotating in liquid PCM. A cooled Heat Transfer Fluid (HTF) passes through the drum resulting in a heat flux from the PCM to the HTF. The resulting solidified PCM layer at the outside of the rotating drum is constantly removed by a scraper. The thickness of the PCM layer and therefor the thermal resistance is a function of the rotation speed. Subsequently it is possible to control the heat flux by varying the rotation speed of the drum. An analytical model of the heat transfer has been developed. A detailed examination of the different heat transfer mechanisms shows that the heat transfer is no longer limited by the heat conductivity of the PCM. Instead the heat transfer from the drum wall to the HTF becomes the limiting factor. In the presentation, an improved test rig, aiming for a maximized heat flux density by minimizing the PCM layer thickness and maximizing the heat transfer to the HTF is introduced and experimental results are presented