SECOND LAW OPTIMIZATION OF A PACKED BED SOLAR ENERGY SENSIBLE HEAT STORAGE SYSTEM

Abstract

Thermal energy storage is resorted to when there is a wide time gap between the availability of a resource and the demand. The energy is stored when it is available in amount more than the demand and withdrawn when the resource is not available and energy is required. Thermal energy storage technologies are grouped into three main classes: Sensible, Latent and Thermo-Chemical storage. Packed bed sensible heat energy storage consist of solid material of good heat capacity packed in a storage tank through which the heat transport fluid is circulated. Solar collector supplies hot fluid, which flows through the bed to transfer the thermal energy to the solid particles. The stored thermal energy can be recovered by making cold ambient fluid flow through the bed. The design of the packed bed energy storage system should be based on available energy and thermohydraulic performance rather than thermal performance alone. A thermohydraulic optimization process has been developed in order to determine the set of optimum values of the system parameters (sphericity and void fraction) to result in the best "thermohydraulic performance" of the packed bed solar energy storage system. Based on the above, optimization plots have been prepared in terms of optimum values of sphericity and void fraction as function of operating parameters of the solar energy utilization system, namely, the temperature rise parameter and insolation. These plots can be used by the designer to select suitable bed elements.