Carbon black nanofluid synthesis for use in concentrated solar power applications

Abstract

Direct absorption solar collectors offer possible improvement in efficiency over traditional surface absorbing collectors, because they have fewer heat transfer steps and has the ability to utilise higher radiation fluxes. Carbon black based nanofluids, in a base fluid of salt water, were synthesised by a two-step method where the carbon black nanoparticles were treated with a surfactant, TWEEN-20, in a 1:2 mass ratio and sonicated for 60 minutes to break up agglomerates. The synthesised nanofluids showed stability for over 31 days. The different carbon black concentration nanofluids' solar irradiation absorption properties were compared with each other and with the base fluid of salt water in a concentrating, as well as non-concentration scenario. It was found that the carbon black nanofluids showed excellent absorption properties over the entire solar radiation spectrum. A 1 m2 concentrating unit using a two-axis tracking system, with two mirrors and a 1 m diameter circular Fresnel lens, was used to concentrate solar radiation on a direct absorption solar collector flow cell with a 10 cm2 collection area. An optimum concentration of 0.001 volume % carbon black was found to show a 42 % increase in heating rate, compared to that of salt water. The collector was, however, hampered by high energy losses and the maximum collector efficiency achieved was only 46 %, 23 % higher than that of salt water. The overall system efficiency was only 22 %. This low efficiency can be attributed to the high optical concentration losses (50 % - 70 %) present in the concentrating unit.