Transition metal boride based multilayer solar selective coatings for concentrated solar power application

Abstract

Renewable power technologies with zero CO2 emissions are the key to regulate the current global warming scenario. Solar energy is the most affordable and sustainable green energy source. One hour of solar radiation has energy equivalent to the world’s annual energy consumption. But due to its low energy density, it requires efficient technology like concentrated solar power (CSP) to harvest and convert to useful energy. Coated with spectrally selective absorber materials, the receiver tubes of the CSP plant play the most pivotal role by converting the incident solar energy to thermal energy. The cost of heat/power generation can be reduced by increasing the photo-thermal conversion efficiency of the CSP plant. This objective can be accomplished by increasing the solar selectivity of the absorbers or increasing the operating temperature of the plant. Therefore, in recent years, significant research is being carried out to develop absorbers that can operate at high temperatures for a longer duration without undergoing degradation. Solar selective absorbers based on carbides, nitrides, borides, and oxides of transition metals, known as ultra-high temperature ceramics, have gained more attention in recent years due to their high thermal and chemical stability, good thermal conductivity, and mechanical properties. This work demonstrates our efforts to develop transition metal boride-based spectrally selective absorber coatings for CSP application. The first part of the work deals with enhancing the solar selectivity and thermal stability of TiB2-based multilayer absorber coating by employing double-layer anti-reflection coating. In the second part of the work, I will present the results obtained with a new NbB2-based multilayer absorber coating. The processing challenges in our attempt to fabricate TiB2 and NbB2-based multilayer absorber coatings with high absorptance in solar irradiation range and low emissivity in the infrared region will be discussed. The developed coatings exhibit good thermal stability at 500°C for 250 hrs in vacuum. Extensive experiments have been carried out to investigate the microstructure and optical properties of the optimised coatings. Based on the obtained results, we have attempted to understand the governing physical phenomena to explain the origin of spectral selectivity and the thermal stability of the developed ceramic coatings.