Numerical evaluation of the optical properties of encapsulated phase change particles for thermotropic materials

Phase change thermotropic materials have been proposed as a low cost method to provide passive overheat protection for polymer solar thermal absorbers. One challenge to their development is control of the size of the phase change particles dispersed within the matrix. Here we explore encapsulation as a means to resolve this challenge with a focus on the selection of materials, including the encapsulating shell, to achieve desirable optical behavior. Hydroxystearic acid (HSA) particles in a matrix of poly(methyl methacrylate) (PMMA) is down selected from candidate materials based on its optical properties and the melt temperature of the dispersed phase. The optical properties (normal-hemispherical transmittance, reflectance, and absorptance) as a function of the properties of the encapsulation shell and the particle volume fraction are predicted at a wavelength of 589 nm using a Monte Carlo ray tracing model. A range of shell relative refractive indices, from 0.95 to 1, and thicknesses, up to 35 nm, can be employed to achieve greater than 80% transmittance in the clear state and greater than 50% reflectance in the translucent state.