Convective heat transfer performance of sand for thermal energy storage
Golob, Matthew Charles
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This thesis seeks to examine the effective convective heat exchange of sand as a heat exchange medium. The goal of this exploratory research is to quantify the heat transfer coefficient of sand in a proposed Thermal Energy Storage (TES) system which intends to complement solar thermal power generation. Standard concentrator solar thermal power plants typically employ a heat transfer fluid (HTF) that is heated in the collector field then routed to the power generators or TES unit. A fairly clear option for a TES system would be to utilize the existing HTF as the working storage medium. However, the use of conventional HTF systems may be too expensive. These fluids are quite costly as the quantity needed for storage is high and for some fluids their associated high vapor pressures require expensive highly reinforced containment vessels. The proposed storage system seeks to use sand as the storage medium; greatly reducing the expenses involved for both medium and storage costs. Most prior TES designs using sand or other solids employed them in a fixed bed for thermal exchange. The proposed TES system will instead move the sand to drive a counter flow thermal exchange. This counter flow design allows for a much closer temperature of approach when compared to a fixed bed. As cost and performance are the primary goals to tackle of the proposed system, the evaluation of the sandâ s thermal exchange effectiveness in a flowing state is necessary. Experiments will be conducted to measure the effective heat transfer coefficient between the sand and representative solid surfaces used as the heat transfer conduits. Additional experiments that will be looked at are wear caused by the sand as a consideration for long term design viability as well as angle of repose of the sand and its effect on scoop design for improved materials handling. Key investigational aspects of these experiments involve the sand grain size as well as shape of the heat exchanger surfaces. The thesis will evaluate the resulting convective heat transfer coefficient of the sand as related to these features. The data will then be compared and verified with available literature of previously studied characteristic thermal properties of sand. The measured and confirmed data will then be used to further aid in a design model for the proposed TES system.