EFFECT OF RADIAL HEAT TRANSFER IN THE PEBBLE BED THERMAL ENERGY STORAGE TANK COUPLED TO A SOLAR ORGANIC RANKINE CYCLE

asme-orc2015 Tracking Number 152

Organic Rankine cycle is an efficient and a scalable technology to convert low temperature heat into electricity available from a number of heat sources like geothermal or waste heat from industrial processes or low concentration solar collector systems. The nature of the above mentioned heat sources can be different from one another, wherein some of these can be expected to yield continuous and constant power supply while, others such as concentrated solar systems are bound to have fluctuations. In such cases, a thermal energy storage (TES) system can be used to suppress the effect of fluctuations and also if needed to generate power during non-solar hours. Presently, there are a number of storage options available, for example one tank or two tank technology with heat storage media as pure heat transfer fluid (HTF). One tank technology definitely has the cost advantage factor; but to reduce the cost further, pebble bed systems are used, wherein a fraction of heat is stored in the pebbles depending upon the void fraction of the bed. This paper analyzes the heat transfer mechanisms in the pebble-bed storage medium and reports the issues associated with it. A case study is performed considering the storage requirements for an R-134a ORC operating between 150 °C and 45 °C with a cycle efficiency of 10 %. A100 kWe system is analyzed to mitigate the diurnal fluctuations in direct normal insolation (DNI) using propylene glycol as the HTF. DNI is calculated for a typical day of vernal equinox on the tropic of Cancer, which approximately represents the mean of Indian latitude. Variation in DNI is recorded using ASHRAE clear sky model for single axis tracking in case of parabolic collectors. A cylindrical geometry is considered for the TES tank in the present analysis. TESs reported in the literature use a central inlet and outlet configurations which are prone to have thermally isolated regions in the extreme corners. The theoretical models and procedures developed for these system do not capture radial effects of heat transfer. Though, ideally it is desirable to have no radial effects on heat transfer in order to maximize the energy storage capacity, may often not be practically realizable. In this regard, governing equations considering the 3-dimensional geometry need to be solved to account for radial effects. This is achieved by modeling the fluid flow in TES using a CFD tools. Three geometrical configurations are modeled using a commercial solver, Fluent 6.0, ANSYS for various cases of inlet and outlet port positions a) central inlet and outlet, b) uniform inlet and uniform outlet and c) diametrically opposite inlet and outlet ports on the lateral surface of the cylindrical TES tank. In case of central inlet and outlet, formation of thermally isolated corners is observed in the extreme corners of the geometry. Theoretically, temperature at the outlet of the TES is supposed to be steady; however, CFD results reveal that high temperature fluid is discharged from the TES system after a 20% of the charging cycle. These issues are found to be absent for the case of uniform inlet and outlet configuration, which often is difficult to practically realize. Therefore, a new configuration is proposed by placing the inlet and outlet ports at extreme ends to mitigate the above issues, the details of which are provided in the paper.