Investigating performance improvement of solar collectors by using nanofluids

The present review is an extensive perspective of the research progress arisen in the performance of solar collector using nanofluids. The increase in the price of fossil fuels and rapid depletion of conventional energy sources are among the major energy concerns. Solar collector, as a kind of green and renewable energy device, can help us stay out of these energy concerns. Low efficiency and high cost of solar collectors compared with the conventional devices persuade scientists and engineers to make effort to increase performance of solar collectors. Nanofluid - the suspension of nanoparticles into a basefluid - has predominant characteristics because of nanoparticles' small size and high surface area. Many researchers evaluated these special properties of nanofluids, using several methods and techniques. Mathematical and numerical methods are practiced and experimental methods come to validate the results. Using nanofluid instead of conventional fluid improves heat transfer as well as optical and thermal properties, efficiency, transmittance and extinction coefficient of solar collector. Based on comprehensive studies, it has been also realized that the thermal properties of nanofluid such as thermal conductivity have significant effect on improving the efficiency of direct solar absorption collectors. On the other hand, using nanofluid is a big challenge in terms of economical aspects. Moreover, there is a lack of study on the effect of nanofluid's optical properties such as transmittance and extinction coefficient on the performance of solar collector. Similarly, effort should be made to perform two-phase analysis of nanofluid and study properties of nanofluid with more than one type of nanoparticle. (C) 2013 Elsevier Ltd. All rights reserved.