Test and analysis of a flat plate latent heat storage design

Thermal storage technologies are a key component for increasing energy efficiency and assisting in the integration of regenerative energy sources in the energy market. In latent heat energy storage, the storage material changes phase as energy is charged into the storage. This makes use of the large amount of enthalpy that is absorbed or released during phase change of a material. Since a storage unit is a link in the chain between supply and demand of heat, it has to be adapted for each particular application. The aspect that is most difficult to adapt is the required power level. The inherent heat transfer in phase change materials (PCMs) is typically low and a limiting factor. Therefore, heat transfer structures have been developed that increase the surface area between the PCM and the heat transfer fluid. In proven design concepts for latent heat storage, a tube bundle is immersed in PCM as a heat exchanger between the storage medium and the heat transfer fluid. In order to increase the power level, the heat transfer surface is increased with exterior fins on the tubes. A new concept that was designed, tested and analyzed at DLR is a storage adaptation of a flat plate heat exchanger. The design tested replaces the secondary heat exchanger medium with isolated chambers of storage material. The primary heat transfer fluid flows through the one side of the flat plate thermal storage, transferring heat across the steel encasements to the storage medium. A flat plate lab storage unit was built to prove this design concept. The testing of this storage unit, including various temperature gradients, flow rates and the insertion of heat transfer structures, is reported in this paper