Spectrally selective tandem absorbers for photothermal conversion in high temperature solar thermal systems
Solar energy is the inexhaustible and abundant energy resources on the earth which can be a best substitute for the fossil fuels. Over the past couple of decades, all the solar technologies are rising very steadily in two main branches including photovoltaics and solar thermal. One of the major components of solar thermal system is receiver, which plays an important role to enhance the photo-thermal efficiency by absorbing maximum amount of solar radiation with a minimum heat loss. In this regard, our objective was to fabricate a spectrally selective absorber coating for receiver which should have a high absorptance of ≥ 0.95 in the solar spectrum (0.25-2.5 μm) and a low thermal emittance of ≤ 0.05 in the infrared region (2.5-25 μm). In addition, while considering the real field applications, these coatings should exhibit exceptional thermal (> 450 °C) and environmental stability in different operational conditions. In spite of the outstanding thermo-chemical and thermo-physical stability of the ultra-high temperature ceramics (UHTCs), there are only a few reports on the spectral selectivity of these materials. Therefore, in the first part, we have utilized DC and RF magnetron sputtering system to prepare TiB2/TiB(N)/Si3N4 -based multilayer absorber coating. A systematic investigation was carried out to understand the influence of various deposition parameters including target power, deposition time and reactive gas flow on the spectral selectivity of the coating. The optimal process parameters lead to a high absorptance of 0.964 and an emittance of 0.18. However, the film is inadequate in terms of environmental stability. We have subsequently developed W/WAlN/WAlON/Al2O3 -based multifunctional novel coating using magnetron sputtering. The rationale behind this specific coating design was based on good optical properties and high diffusion block ability of transition metal -based oxides and oxynitrides. The optimally fabricated coating has a superior spectral selectivity with a maximum absorptance of 0.958 and a low emittance of 0.08. Based on the extensive analysis using the transmission electron microscopy (TEM), phase modulated spectroscopic ellipsometry along with computational analysis, we have manifested that the optical constants of each layer decrease from substrate to surface of coating, leading to enhance the photo-thermal efficiency. A prolonged thermal annealing established that the spectral properties of the coating could be retained at 500 °C in air for 150 hrs, indicating a service durability of ~ 25 years. Also, the directional and hemispherical emissivity is not compromised during annealing at 500 °C in air and in vacuum for 12 hrs. It is noteworthy to mention that the recently conducted high temperature testing (30 cycles at 450°C) in simulated solar field environment at Sandia National Laboratory establishes excellent thermal shock resistance property of the coating. In summary, a broad spectrum of ceramic combinations as tandem absorber coatings was investigated, and attempts were made to demonstrate the governing physical phenomena to explain the origin of spectral selectivity of ceramic absorbers. This dissertation will also provide guidelines to develop multilayer ceramic absorber coating for high temperature photo-thermal conversion systems.