Dense Granular Flows as a New Heat Transfer Fluid for Concentrated Solar Power

Abstract

The increasing interest in concentrated solar power as a new form of renewable energy necessitates an improvement in overall system efficiency. Current heat transfer fluids employed to capture the concentrated heat demonstrate limited working temperature ranges. This study sought to investigate the use of dense granular flows as a possible new heat transfer fluid, as ceramic particles present virtually no restriction on working temperature. A bench-scale system simulating a single tube of a concentrated solar power central receiver was constructed and used to evaluate the heat transfer properties of the flow at low temperatures. Ceramic particles, 270μm in diameter, were gravity-fed through a vertical tube, resulting in granular flows with particle packing fractions of approximately 60%. Radial temperature profiles were measured and used to calculate the mean temperature of the fluid at different axial tube locations. The heat transfer coefficient was then calculated based on the input heat flux and measured wall and mean temperatures. The effect of the mass flow rate on the heat transfer coefficient was examined by using different orifices at the tube exit. As expected, the heat transfer coefficient increased with increasing flow rate. Heat transfer coefficients ranging from 330 to 380 W/m2-K were obtained for bulk temperatures ranging from 40 to 70°C. Previous works demonstrated comparable heat transfer coefficients at higher bulk temperatures. Thus, our preliminary heat transfer coefficient results demonstrate the potential of dense flows of ceramic particles for obtaining beneficial heat transfer properties at extremely high operating temperatures.