Modern trends in the development of solar energy dictate the need to search for new materials and designs to improve the efficiency of converting solar radiation into electrical energy. In this context, quasi-crystalline films, due to their unique optical properties, are of considerable interest as selective absorbers of solar radiation.
A feature of quasicrystals is the absence of a periodic structure typical of traditional crystals, which leads to the emergence of photonic forbidden zones and the formation of complex spectral characteristics of reflection and transmission of light. By varying the composition and parameters of the quasi-crystalline film, it is possible to achieve high absorption capacity in the visible and near infrared ranges, where the main energy of the solar spectrum is concentrated, and low radiation in the thermal range, minimizing energy losses due to re-radiation.
To efficiently utilize solar thermal energy, selective absorbing surfaces must efficiently absorb solar radiation (αs) and emit minimal heat (ϵh). Important characteristics include resistance to oxidation and diffusion, especially at elevated temperatures. Quasi-crystalline materials are a promising class of compounds that exhibit the necessary resistance.
Using a genetic algorithm, the structure of a thin-film selective absorber consisting of dielectric and quasi-crystalline layers was optimized. The sandwich structure consisting of a dielectric, a quasi-crystal, and a dielectric deposited on a copper substrate demonstrates high selectivity values: αs = 0.86 and ϵh (at 400 °C) = 0.051. Theoretically, the use of metal-dielectric materials can provide even higher characteristics. The optical properties of metal-dielectrics, in which a quasi-crystalline material is used as a metal component, were calculated using the Bruggeman theory. The system including a metal-ceramic film and an additional anti-reflection coating on a copper substrate demonstrates αs = 0.92 and ϵh (at 400 °C) = 0.048.
Numerical optimization is a key tool in the design of selective absorbers based on quasi-crystalline films. Computational electrodynamics methods, such as the finite element method or the finite difference method in the time domain, allow modeling the interaction of electromagnetic radiation with the quasi-crystalline structure and determining the optimal film parameters that provide maximum absorption efficiency.
The optimization process takes into account such parameters as film thickness, alloy composition, degree of structure ordering, and the presence of additional layers that improve optical characteristics. The goal is to maximize the integral absorption coefficient in the solar spectrum while simultaneously reducing thermal radiation. The structures obtained as a result of numerical optimization can be used to create highly efficient solar collectors and thermophotovoltaic energy converters.
Author: T. Eisenhammer
Institute: Ludwig Maximilian University of Munich, Department of Physics, Amalienstrasse 54, 80799 Munich, Germany