Nanofluid-Based Absorbers for High Temperature Direct Solar Collectors

Solar collectors with surface receivers have low overall energy conversion efficiencies due to large emissive losses at high temperatures. Alternatively, volumetric receivers promise increased performance because solar radiation can be transferred into a fluid medium, which subsequently reduces the concentrated heat at the surface. Nanofluid-based direct solar receivers, where nanoparticles in a liquid medium can scatter and absorb solar radiation, have recently received interest to efficiently distribute and store the thermal energy. We present a combined modeling and experimental study to investigate the efficiency of fluid-based solar receivers seeded with carbon-coated absorbing nanoparticles. A simple one-dimensional transient heat transfer model was developed to compare idealized surface receivers to idealized volumetric receivers. In the limit of idealized behavior, volumetric receivers were shown to be more efficient than selective-surfaces for high concentration levels (C > 100) and/or tall receiver designs (H > 10 cm). Furthermore, volumetric receivers allowed the working range of the fluid to be extended to higher temperatures while maintaining relatively high receiver efficiency. We also designed and custom built a cylindrical receiver to experimentally demonstrate the concept of nanofluid-based receivers and validate the modeling efforts. We explored the effect of particle characteristics such as size, distribution, and material properties, as well as collector parameters such as absorbing depth and level of solar concentration. The work offers design guidelines for the development of efficient volumetric receivers for future solar thermal energy conversion systems.