Dixit, Ambesh

Ni-Co/Nanoporous Al2O3/Aluminum spectrally selective structures are grown using electrochemical anodization of alumina, followed by electrochemical pigmentation of Ni and Co metallic particles simultaneously. The pigmentation of Ni and Co particles into nanoporous anodized alumina (AA) is achieved using the reduction of metal ions in the aqueous electrolyte. The measured absorptivity and emittance are ∼ 0.90 ± 0.05 and 0.14 ± 0.04, respectively. These structures are subjected to the ten identical thermal cycling upto 300°C in inert N2 and air ambient conditions. The optical performance suggests that absorptivity is nearly same with slight enhancement in the emittance values as compared to the structure without any heat treatment. The accelerated corrosion studies suggest that Ni-Co co-pigmented AA structures are relatively corrosion robust as compared to that of the AA structure.

Ni–Co pigmented anodized alumina as spectrally selective coating is realized using electrodeposition. Ni and Co metals are pigmented into nanoporous alumina by the reduction of metallic ion from aqueous solution. The measured spectral absorptance and thermal emittance are ∼0.90 ± 0.05 and 0.14 ± 0.04, in 0.2–2.5 μm and 2.5–25 μm wavelength window, respectively for the optimized Ni–Co co-pigmented alumina spectrally selective coatings. The optimized solar selective coating samples are heat treated in open environmental condition at 300 °C for 100 h to evaluate their thermal stability. The absorptance is nearly unaffected after heat treatment, whereas slight enhancement in thermal emittance is observed as compared to the unheated sample. After thermal evaluation, corrosion characteristics are also evaluated and observed that Ni–Co co-pigmented coatings are more corrosion resistant than anodized alumina structures, suggesting that these co-pigmented solar selective coating may be suitable for mid temperature absorbers in ambient conditions.

Solar selective coatings (SSC) are integral part of the receiver in any concentrated solar thermal system for efficient conversion of incident solar energy into thermal energy. The most common solar thermal systems are based on parabolic trough collectors, which are operating at 350 °C. Thus, the absorber coating should be robust and efficient to operate at such temperatures without any degradation. There are limited such solar selective coatings and very few are commercially available. These are cermet based structure and are stable at high temperature in vacuum. These structures are prone to chemical reaction in open-air condition, which results in degraded optical performance of SSC structures. Thus, there are issues and challenges in SSCs, especially for higher operating temperatures, which is essential to realize the higher efficiency of solar thermal systems. The work is focusing on such constraints towards realizing high temperature solar selective coatings.

SiO2/(ZnO/Sn-In2O3)n=4/SS multilayer structure as a spectrally selective coating (SSC) is developed using a low-cost sol-gel assisted dip-coating process on stainless steel based planer and tube samples. The X-ray diffraction and atomic force microscopic measurements are carried out to understand the development of structural and microstructural features on the developed coating structures. The optical measurements are carried out using Ultraviolet–visible spectroscopy (UV–Vis) and Fourier transform infrared (FTIR) spectrometers, and the observed absorptance and emittance values are ∼0.85 and 0.14 for the developed coating structures. These coatings showed enhanced thermal stability up to 350 °C in open-air conditions and enhanced corrosion resistance against saline conditions, i.e., 3.5% by weight in water solution. The coating process is extended on a foot-long stainless steel (SS) tube having a one-inch diameter to demonstrate the feasibility of the developed technique. Thus, the developed process showed potential for large-scale, low-cost coating with desired optical properties for solar thermal applications.