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Half of today's global energy consumption is in the form of heating and cooling. Solar collectors are the most promising sustainable alternative to fossil fuels in this sector. The most important component in a solar collector is the receiver, which by use of a selective surface absorbs and converts solar irradiance to thermal energy. Herein, a novel selective surface for low-to mid-temperature solar collectors is developed, studied and presented. The surface is produced by electroplating a cobalt-chromium coating on a stainless steel substrate using an electrolyte based on a deep eutectic solvent. Our method makes use of trivalent instead of traditionally used hexavalent chromium, which significantly reduces health-related issues and makes it more environmentally benign. We obtain a coating of chromium doped cobalt where the surface exhibits an absorptance and emittance of 0.96 and 0.14, respectively, giving it a solar-to-thermal efficiency of 0.95. An observed loss in optical efficiency, is shown to correlate to an oxidation of the metallic cobalt to Co₃O₄ at elevated temperatures. We further show that this oxidation can be mitigated by dip-coating a protective silica top coating, which concurrently improves the optical selectivity of the surface. The present selective surface is efficient, cheap, scalable, and easy to produce sustainably, making it competitive to industry standards. We foresee that our method will have impact on the advancement of improved low-to mid-temperature solar collectors, assisting a faster transition towards a sustainable society.

Solar energy will be a crucial part of the sustainable, fossil free energy production of the future. A majority of this will be produced by solar collectors and photovoltaics. Important for the efficient utilization of the incident solar energy for both technologies are a cover glass with antireflective coatings giving it a high solar transmittance. In the current paper we describe the development of antireflective mesoporous silica coatings on low-iron float glass using the aerosol-based nFOG™ deposition technique. The coatings exhibit a hexagonal and closed pore structure, high smoothness, superhydrophilic properties (contact angle <5°) and consistent thicknesses of approximately 110 nm. This is in line with optimal thickness determined from simulations of the antireflective behavior. Low-iron float glass coated on both sides show a highly reproducible solar weighted transmittance of 95% in the wavelength range 300–2500 nm and an antireflective effect increasing with incident angle. The smoothness, closed pores and low contact angle indicate a high cleanability, which in combination with the high transmittance render a competitive broadband antireflective coating well adapted for solar glass applications.

The use of antireflective coatings to increase the transmittance of the cover glass is a central aspect of achieving high efficiencies for solar collectors and photovoltaics alike. Considering an expected lifetime of 20–30 years for solar energy installations, the durability of the antireflective surfaces is essential. Here, a novel antireflective SiO₂ coating with a hexagonally ordered closed pore structure, produced with an aerosol-based sol-gel method is benchmarked against two commercial coatings; produced with acid etching and sol-gel roll coating. The optical and mechanical properties together with contact angle characteristics were evaluated before and after various durability tests, including climate chamber tests, outdoor exposure, and abrasion. Compared to the commercial antireflective coatings with open pore structures, the novel coating performed in parity, or better, in all tests. Based on the results of humidity freeze and industrial climate chamber tests, it appears that the coating with closed pore structure has a better ability to prevent water adsorption. Additionally, the closed pore structure of the coating seems to minimize the accumulation of dirt and deposits. The abrasion and cleanability test further confirm the advantages of a closed pore structure, showcasing the coating's mechanical durability. While the coatings exhibit similar hardness and reduced elastic modulus, the closed pore coating proves to be even harder after undergoing the industrial climate chamber test, but also slightly more brittle, as indicated by the probability of crack initiation. In summary the closed pore structure is well suited for tempered and arid climates, making it a truly competitive alternative to existing antireflective coatings.

Solar collectors have the potential for significant climate change mitigation by substituting heat produced with fossil fuels. To achieve this, collectors with highly efficient solar absorbers are essential. Carbon nanotubes are highly absorbing, sustainable, cheap, and thermally stable, making them a promising material for solar absorbers. However, achieving a high solar absorptance and low thermal emittance (solar selectivity), while maintaining good thermal stability and scalability is challenging. Here, we present a selective coating based on multi-walled carbon nanotubes and silica (SiO₂). A water-based dispersion enabled by carboxyl functionalization of the carbon nanotubes (CNT_F) is spray coated on a stainless steel (SS) substrate and fixated with sol-gel dip coated silica. The SS/CNT_F/SiO₂ surface exhibits an optical selectivity dependent on CNTF area load and with 0.83 gCNT m⁻² a solar absorptance and thermal emittance of 0.94 and 0.40, respectively, is achieved. The coating also demonstrates excellent thermal stability, with an estimated lifetime of >25 years at working temperatures ≤222°C. All together, we show that by using scalable and cheap technology, concurrent with sustainable materials and a simple structural design, we can manufacture a coating that exhibits properties suitable for low-to-mid-temperature applications. Our study highlights the potential of carbon-based solar absorbers.

Realizing the huge potential of solar thermal collectors depends on the reduction in levelized cost of energy, intimately related to production techniques, supply chains, industrial incorporation, and component cost. A key component in solar thermal collectors is the receiver with its solar selective coating, leveraging a high solar weighted absorptance, αS, and low thermal emittance, εT, to maximize the solar-to-thermal conversion efficiency. Herein, a solar selective multi-walled carbon nanotube (MWCNT) silica composite is deposited on a thermally induced oxide undercoating using highly scalable, cheap and sustainable methods and materials. The undercoating is optically tuned through manipulation of destructive interference to reduce reflectance in the visible wavelength region, to compliment the absorptance of the CNT composite dominated by π-plasmon excitation centered in the UV-region. Optimization of the CNT composite composition and layer stack configuration is achieved by the successful development of models to simulate the optical properties of both the oxide undercoating and the MWCNT silica composite top-coating. The tools and methods developed here take us closer to achieving sustainable and cost competitive coatings needed to realize the potential of solar thermal.