A review on optical properties and application of nanofluids in direct absorption solar collectors (DASCs)

As an abundant, cheap and pollution-free renewable energy resource, solar energy offers a great alternative to conventional fossil resources and is expected to play an increasingly significant role in our global energy future. Solar thermal collectors are conventional devices for capturing solar radiation. Direct absorption solar collector (DASC) is a new generation of solar thermal collectors in which the transport medium is directly exposed to the incident solar radiation. To improve direct absorption collectors absorption and heat transfer capabilities, molecular-level additives are added to their working fluids. As a production of the emerging world of nanotechnology, nanofluids - which are engineered suspensions of nanoscale materials in host fluids - have shown remarkable improvement in thermo-physical and optical properties of conventional heat transfer fluids. This review presents a comprehensive survey of the recent studies on optical characterization, relevant equations and theoretical modeling approaches, and application of nanofluids in direct absorption solar collectors. Possible issues and challenges encountered in potential exploitation of nanofluids in DASCs are addressed as well.