Abstract
Flat-plate solar collector systems are devices used to convert solar energy to thermal energy. These devices require a selective coating that absorbs all sunlight but does not emit energy in the near IR; hence, a multilayer system of metal/IR-reflector/metal oxide is often used. Many substrates are used, including metals such as copper, stainless steel, and aluminum. Aluminum is an interesting substrate for its lower cost and good optical properties. Electrodeposited materials have gained importance in the industry for the easy handling and good results in scale-up. However, in general it is difficult to electrodeposit onto aluminum due to the ever-present thin insulating oxide film. In this work, we propose an adherent interlayer film based on copper, which is electrodeposited onto aluminum, and serves as a basis for the subsequent electrodeposition of a selective coating consisting of bright nickel and the absorber film of electrodeposited black nickel. Solar collectors based on aluminum substrates and this electrodeposited selective coating are specifically useful for thermo-solar applications at low to medium temperature.
Electrochemistry studies including cyclic voltammetry, rotating disk cyclic voltammetry, and galvanostatic curves are carried out for all the different layers. The properties of the coatings are subsequently characterized by reflectance spectra using UV-Vis, NIR and IR spectrophotometers equipped with integrating spheres. The solar absorptance is calculated by weighting the reflectance spectrum against the solar radiation spectrum ASTM G173-03. The thermal emittance is calculated weighting the reflectance spectra against the black body radiation function at 100 °C. The morphology and homogeneity of the film are evaluated with scanning electron microscopy, and energy dispersive spectroscopy. X-ray photoelectron spectrometry is used to determine the film composition. The crystalline structure of the films is studied by X-ray diffraction and Raman measurements. The second and third layer, bright nickel, and black nickel, respectively, are electrodeposited on top of the Cu layer using a previously reported method. We characterize the selective coating electrodeposited onto aluminum/Cu/bright nickel and show that this is a viable technology for the fabrication of low-cost and high efficiency flat plate solar collectors. In addition, we demonstrate the scale-up of the electrodeposition process, and characterize the performance of a complete flat-plate solar collector device of 1.8 m2 gross area.
Acknowledgements
The authors gratefully acknowledge CONACYT, Módulo Solar S.A. de C.V. and IER-UNAM for funding through the Mexican Center for Innovation in Solar Energy (CeMIE-Sol), Project P-81.