Design of Volumetric Solar Flow Receivers

The development of efficient solar thermal receivers has received significant interest for thermal energy to electrical power conversion and heating applications. Volumetric receivers, where the incoming solar radiation is absorbed in a fluid volume, have advantages over state-of-the-art surface absorbers owing to the reduced heat losses at the surface. To efficiently distribute and store the thermal energy in the volume, nanoparticles can be suspended in the liquid medium to scatter and absorb the incoming radiation. In such systems, however, compact models are needed to design and optimize the performance. In this paper, we present an analytical model that can be used to perform parametric studies to investigate the effect of heat loss, particle distribution, and flow rate on receiver efficiency. The analytical model was formulated by modeling the suspended nanoparticles as embedded heat sources. The heat equation was solved with the surface heat losses modeled using convective losses based on Newton’s law of cooling. The analytical solution provides a convenient tool to predict two-dimensional temperature profiles for a variety of heat loss and inlet fluid temperature conditions. The efficiency of the receiver is defined as the ratio of the amount of thermal energy transported by the fluid to the total incident solar energy. For very large lengths the thermal energy carried by the fluid reaches a maximum steady value as the amount of heat loss equals the incident solar energy. The model can be used to estimate the approximate receiver lengths required to achieve near peak bulk fluid temperature. The results from this study will help guide experimental design, as well as practical flow receivers for solar thermal systems. Predictions made on a channel of 1mm depth with a solar concentration of 1 show that there exists a maximum system efficiency of 0.3373 for a dimensionless receiver length of 1.66.