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Experimental and numerical analysis of heat transfer in a particle-based concentrated solar power receiver
Ketchem, Tyler C.
Ketchem, Tyler C.
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2015
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A new concept has been developed by the National Renewable Energy Laboratory to realize the advantages of particle-based concentrated solar power with thermal energy storage. This concept uses an array of horizontal, hexagonal absorber tubes to capture sunlight and transfer heat to particles flowing over the heated tube surfaces. The granular flow heat transfer characteristics are critical to the design of the system but are not well understood. This research seeks to characterize heat transfer within this solar receiver concept by experimentally measuring heat transfer coefficients and developing a model capable of simulating a full-scale receiver by utilizing existing heat transfer correlations. Experiments conducted on a small-scale tube array showed the dynamic nature of the particle flow and the dependence of the heat transfer coefficients on particle contact. The top tube face has particle contact along its full length, the side face has intermittent contact and the bottom face has little to none. The top face heat transfer coefficient was found to be roughly 320 W/m^2-K, over ten times greater than the bottom face, while the average heat transfer coefficient for a single tube was roughly 175 W/m^2-K using 300 micron particles. A numerical model was then developed that includes both the solar and particle sides of the absorber tubes. Three-dimensional view factor relations capture incoming flux and reradiation effects while a unique heat transfer correlation from the literature was used for each face. The top face was treated as an inclined plate, the side face as a smooth-walled vertical channel, and the bottom face by considering heat transfer in a thin channel of the pure gas phase. Relevant flow parameters for these correlations were obtained through a one-dimensional model of the particle flow dynamics. Simulation of the full-scale receiver at intended operating conditions shows significant improvement in the convective and effective heat transfer coefficients due to increased bulk conductivity and thermal diffusivity of the granular flow at high temperatures and greatly increased contributions from radiation.
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