Can nanofluids improve solar collector efficiency

Abstract

Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.

Direct solar collectors are designed to absorb solar energy into a carrier fluid which then transports the heat away to some storage area. To increase the efficiency of the solar collectors the design is changed to that of a direct solar collector and nanoparticles are suspended in the heat transfer fluid. A two dimensional model of a static nanofluid in a solar collector is presented in this paper. Two equations are integral in describing the problem mathematically; the radiative transfer equation accounts for the attenuation of solar radiation through the depth of the collector and the heat equation describes the distribution of heat in the collector. The source term that accounts for the volumetric heat release is derived under the assumption of independent scattering. Taking advantage of the small aspect ratio the full model is approximated by the one-dimensional limit. Furthermore, a decaying exponential function is used to approximate the source term which allows for an exact analytical solution to be presented. In power generation applications high operating temperatures and maximum energy absorption are favourable. Therefore, the collector efficiency is measured in terms of the Carnot number and total solar energy absorption. For a fixed collector height, varying nanoparticle concentrations and exposure times are investigated to maximise collector efficiency