HEAT TRANSFER STUDIES ON A PACKED BED HEAT EXCHANGER CONTAINING CAPSULES HAVING PHASE CHANGE MATERIAL

Abstract

Energy storage device is one of the key technologies for energy conservation. Presently in world, conventional energy resources (e.g. coal, crude oil, natural gas and nuclear energy) are used to supply the energy for almost all purposes. Because of the limitations of conventional energy resources (they would not last long) and the pollution and environmental deterioration caused by their use, alternative energy resources are being developed and encouraged for future use. Solar energy is the most promising and abundant one in nature. Energy storage is required essentially for using solar energy and its systems because of its intermittent nature and because of the mismatch between time and rate of supply and demand. Thermal energy storage in phase change material is one of the viable options. In the present work, the heat transfer characteristics and energy storage capacity involved in packed bed heat exchanger containing capsules having phase change material are studied experimentally. In this present work cylindrical bottles were used as storage capsules, and paraffin wax was chosen as PCM. The objective of the present study. is to investigate the thermal characteristics of phase change material (paraffin wax) inside vertical cylindrical capsules contained in a packed bed of latent heat energy storage unit. The important operating parameters are the Reynolds number (mass flow rate) and Stefan number (heat transfer fluid inlet temperature) for a packed bed energy storage system. It has been observed that as the mass flow rate (Reynolds number) increases for the same capsules resulting in higher heat transfer and consequently the phase change period is reduced. It has been seen that for the higher Reynolds number or Stefan number, the time required for storing energy is less. For higher inlet temperature the heat transfer rate increases and hence the complete melting time (charging time) decreases. Also it has been found that the average heat transfer coefficient increases with Reynolds number. A correlation of charging time with Reynolds number and Stefan number has been developed. Also a correlation of average heat transfer coefficient with Reynolds number and Stefan number has been developed.