Novel sol–gel based selective coatings: From coil absorber coating to high power coating

Abstract

Single layer black selective coatings for solar thermal energy conversion were produced by low cost sol–gel dip-coating, spin-coating and coil-coating methods, the last made on the industrial line of the Alanod company (DE). For the deposition, pigment dispersion was prepared by milling a mixture of a black spinel (FeMnO4) pigment, whose surface was modified with trisilanol heptaisobutyl POSS dispersant, dried and re-milled with (3-glycidoxypropyl)trimethoxysilane (GLYMO) and Ti(i-OPr)4 acting as binders. The corresponding organic–inorganic hybrid consisted of silica clusters linked together with poly(ethyleneoxide) chains acting as a network former. The as-prepared nanocomposite dispersion served for the deposition of coil absorber coatings (~800m) on aluminium coil in an industrial plant in the Alanod factory. They gave solar absorptance of 0.905 and thermal emittance of 0.12 and excellent homogeneity. The stability of the coatings at 400°C was excellent but corrosion resistance was moderate (5 days in salt spray).The coil absorber coating was then used for making a High Power (HIPO) solar coating with solar absorptance of 0.92 and thermal emittance of 0.075, which was achieved by exposure of the as-prepared coil to 400°C. The isoconversion kinetics approach was used for assessing the thermal loading (time and temperature of heat treatment) required to provide a HIPO coating from the coil absorber. The improvement of selectivity was investigated by infrared spectroscopy and XPS measurements, revealing that HIPO coatings deposited on stainless steel (AISI 316) survived 1216h at 460°C and 400h at 500°C without any change of selectivity, while after exposure to 500°C for 1216h, thermal emittance increased to 5–7% but solar absorptance remained unchanged. Thermal emittance changes observed at thermal loading were explained with the help of infrared spectra analysis, revealing the formation of iron oxide on the surface of the HIPO coating. The deposited HIPO coating is an interesting option for temperature and corrosion stable coatings for applications in concentrated solar power (CSP) plants for solar processing heat generation.