A Numerical Evaluation of Thermotropic Materials for Polymer Solar Thermal Collectors

Abstract

A series of numerical studies of thermotropic materials are conducted to develop a theoretical basis for the selection and design of phase change thermotropic materials for polymer collectors. The optical requirements for a thermotropic material to provide overheat protection are identified. In the clear state, high solar-weighted transmittance (≥80%, preferably ≥85%). To protect an absorber of polypropylene, material temperature limit of 115 �C, the solar-weighted reflectance of the thermotropic material must be ≥50%. To determine how to achieve these optical requirements, the radiative transfer within thermotropic materials is modeled with a Monte Carlo ray tracing algorithm. A parametric study of the radiative transfer in thermotropic materials is conducted. The results are presented as dimensionless plots of transmittance and reflectance as a function of the overall optical thickness τL, the scattering albedo ω, and the particle size parameter x. The study demonstrates that to achieve ≥85% transmittance in the clear state, the optical thickness must be low. To achieve ≥50% reflectance in the translucent state necessitates a significant increase in optical thickness with temperature. Additionally, the material must have small particles (x ≤ 2.5) and be weakly absorbing (ω ≥ 0.990). For example, for a size parameter of 2, the optical thickness in the clear state must be ≤0.35. The optical thickness in the translucent state must be ≥10 for a scattering albedo of 0.995. The data contained in these dimensionless plots can be used to identify and optimize thermotropic materials. A method for indentifying potential thermotropic material combinations is presented. Using encapsulated particles in a thermotropic material is also investigated. The transmittance and reflectance of hydroxystearic acid in poly(methyl methacrylate) are predicted as a function of shell refractive index, shell thickness, and particle volume fraction. The study demonstrates that a thermotropic material with encapsulated particles can achieve the optical requirements for use in a solar collector. Additionally, the study reveals that a wide range of shell relative refractive indices, from 0.95 to 1.0, and thicknesses, up to 35 nm, provide acceptable optical performance.