INVESTIGATION OF HEAT TRANSFER CHARACTERISTICS OF ENERGY STORAGE SYSTEM USING PCM CAPSULES

Abstract

Thermal energy storage has recently attracted increasing interest related to thermal applications such as space and water heating, waste heat utilization, cooling and air-conditioning. Energy storage is essential whenever there is a mismatch between the supply and consumption of energy. Use of phase change material (PCM) capsules assembled as a packed bed is one of the important methods that has been proposed to achieve the objective of high storage density with higher efficiency. A review of the heat transfer studies in packed beds indicates that sufficient research work has been carried out on packed beds used in applications like chemical reactors, compact heat exchangers and heat storage equipments. In these studies various types of packing materials and geometries have been used and each packing material has different heat transfer characteristics. Packed bed latent heat thermal energy storage with PCM capsules as packing elements has been proposed to be an efficient and simple method of handling phase change material in storage beds. During charging mode, the hot heat transfer fluid carrying energy from the source is circulated through the tank where the PCM inside the capsules absorbs latent heat and melts. During discharging mode, cold fluid returning from the load is circulated through the tank causing solidification of the encapsulated PCM. The thermal behavior of the systems which undergo phase change has been found to have different characteristics during charging and discharging modes. During charging (melting), the heat exchange surface is in contact with the liquid (melted PCM) and therefore convection is possible. During discharging (solidification), the liquid solidifies on the heat transfer surface and an immobile layer of solid material (solid PCM) continuously grows as it gives up heat of fusion. Since this solid layer generally has low thermal conductivity as it grows, heat transfer is impeded, and in turn it increases time required for the energy release. It is observed that there is a substantial difference in heat transfer phenomena in melting and solidification. Each of these processes (melting and solidification) undergoes various stages (called `modes'); each stage having different heat transfer mechanism and rates. The heat transfer behavior of the packed bed system using PCM capsules can be understood first by studying heat transfer in a single individual capsule and then by employing that knowledge, the heat transfer behavior of packed bed containing such capsules can be investigated.