Selection of thermotropic materials for overheat protection of polymer absorbers

• A parametric study of radiative transfer in thermotropic materials is conducted.
• The study provides a basis for the selection of thermotropic material combinations.
• The key predictor of optical behavior is the relative refractive index.
• A potential thermotropic combination is short chain alkanes in polycarbonate.

Thermotropic materials offer the potential to provide passive overheat protection for polymer solar absorbers. These materials are comprised of a matrix in which a second material, referred to as the scattering domain, is dispersed as small particles. Overheat protection is provided by a change in transmittance and reflectance at elevated temperature. The magnitude of this change depends on the change in the relative refractive index between the matrix and the scattering domain, the volume fraction and size of the dispersed particles, and the thickness of the material. To predict the effect of these parameters on the normal-hemispherical transmittance and reflectance, thermotropic materials are modeled as a non-absorbing slab comprised of discrete, anisotropic scattering, spherical particles embedded in a matrix material. A Monte Carlo ray tracing algorithm predicts the transmittance and reflectance of the slab. The model predictions are compared with: the analytical solution for a slab of non-absorbing, non-scattering media, and the measured transmittance of 0.3 mm thick polymer samples containing 400 nm particles. A parametric study of the effects of the design parameters on the transmittance is presented to identify potential material combinations which will produce a thermotropic composite capable of providing overheat protection for flat plate solar collectors. Relatively short chain alkanes or low molecular mass polyethylene in a matrix of polycarbonate are identified as promising materials.