A Parametric Numerical Study of Optical Behavior of Thermotropic Materials for Solar Thermal Collectors

Abstract

A Monte Carlo model is applied to determinate the steady state, solarweighted optical properties of potential thermotropic composite materials for overheat protection of polymer solar absorbers. The key results are dimensionless plots of normalhemispherical transmittance, reflectance and absorptance as a function of particle size parameter, scattering albedo, and overall optical thickness. The optical behavior of thermotropic materials at different temperatures is represented by a change in the relative refractive index which affects the scattering albedo and optical thickness. At low temperatures where overheat protection is not required, referred to as the clear state, the overall optical thickness should be less than 0.3 to ensure high transmittance for the preferred particle size parameter of 2. At higher temperatures where overheat protection is required, referred to as the translucent state, the overall optical thickness should be greater than 10 and the scattering albedo should be greater than 0.995 to achieve 50% reflectance. A case study of low molecular weighted polyethylene in poly(methyl methacrylate) is presented to illustrate use of the results to guide the design of thermotropic materials.