Performance of Graphite-Dispersed Li₂CO₃-K₂CO₃ Molten Salt Nanofluid for a Direct Absorption Solar Collector System

Considered to be the next generation of heat transfer fluids (HTFs), nanofluids have been receiving a growing interest over the past decade. Molten salt nanofluids have been shown to have great potential as an HTF for use in high temperature applications such as direct absorption solar collector (DAC) system. Very few studies using molten salt nanofluids as the HTF in a DAC receiver can be found in the open literature. This study aimed to develop a 3D computational fluid dynamics model of the receiver of a DAC using graphite-nanoparticle-dispersed Li₂CO₃-K₂CO₃ molten salt nanofluid to investigate the effects of design and operation parameters on receiver performance. Receiver total efficiency using Li₂CO₃-K₂CO₃ salt was compared with that using solar salt nanofluid. Spectral properties of the base fluid and nanoparticles were modeled as wavelength-dependent and the absorption of the solar radiation was modeled as a volumetric heat release in the flowing heat transfer fluid. Initial results show that the receiver efficiency increases with increasing solar concentration, decreasing nanoparticle volume fraction, and decreasing receiver length. It was also found that the Carnot efficiency increases with increasing receiver length and nanoparticle volume fraction, and decreasing solar concentration and inlet velocity. Comparative study shows that solar salt HTF could provide higher total efficiency. However, a higher operating temperature of Li₂CO₃-K₂CO₃ will allow for a greater amount of thermal energy storage for a smaller volume of liquid.