EXPERIMENTAL INVESTIGATION OF PACKED BED SOLAR THERMAL STORAGE HAVING LARGE SIZE PACKING MATERIALS

Abstract

Solar energy is the most important energy resource among all renewable energy resources due to its quantitative abundance. It is easy to convert solar energy into thermal energy and this method is most widely accepted for the utilization of this abundant renewable energy resource. However, due to intermittent nature of solar energy availability, an energy storage system is required to be attached with solar collectors to store energy for use when solar energy is not available. A storage system therefore constitutes an important component of the solar energy utilization system. The heat energy may be stored as sensible heat or latent heat. The energy storage taking place without the phase change is the sensible heat storage whereas in case of latent heat storage phase change of material take place. In sensible heat storage when rocks and pebbles are packed in container then it is known as packed or rock bed and they are most commonly used storage system due to its low cost. An extensive literature review was carried out on packed bed storage system.Literature review reveals that most of the experimental work done on packed bed storage system use air as heat transfer fluid. Keeping this in view it was proposed to investigate the behavior of storage tank with water and oil (therminol – vp1) as heat transfer fluids. In order to investigate the storage tank with different HTFs, an experimental set up comprising of sensible heat storage tank containing bed elements, a heating set up, an electric pump, temperature and fluid flow measuring devices has been designed, fabricated and commissioned. It was planned to collect temperature data at different locations of packed bed. Based on the experimental study it has been found that the increment in temperature is faster when water was used as HTF as compared to oil as HTF. The temperature variations were also observed with respect to different volume flow rate of HTFs. It was found out that increment at low flow rate was higher due to more contact time of HTF with the storage materials. The temperature is found to be decreased first along the axis but at the bottom part it again start increasing due to storage of HTF in the lower plenum. Beside this, different properties like Nusselt number, Reynolds number, Friction factor, Pressure drop across bed, Amount of Energy stored and the efficiency of storage tank has been computed.