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The efficiency of the conversion of solar energy into thermal energy is determined by the optical and thermal properties of the selective coating, in particular, the solar absorptance and thermal emittance at the desired temperature of the specific application. Photothermal techniques are the most appropriate methods to explore these properties, however, a quantitative determination using photothermal radiometry, which is based on the measurement of emitted radiation caused by the heating generated by a modulated light source, has proven to be elusive. In this work, we present experimental results for selective coatings based on electrodeposited black nickel-nickel on both stainless steel and copper substrates, as well as for commercial TiNOX coatings on aluminum, illustrating that the radiation emitted by the surface depends on the optical absorption, thermal emissivity and on the light-into-heat energy conversion efficiency (quantum efficiency). We show that a combination of photothermal radiometry and photoacoustic spectroscopy can successfully account for these parameters, and provide values for the emissivity in agreement with values obtained by Fourier-transform infrared spectroscopy. [ABSTRACT FROM AUTHOR]

• Selective black cobalt coatings (BC) are viable for solar thermal collectors. • Electrodeposition is a robust and scalable BC fabrication method. • BC films on Pt are stable to 750 °C; on stainless steel – nickel to 300 °C. • The BC selective film is hydrophobic and stable under weathering. • The BC coating has been demonstrated in solar thermal collectors. We report on the electrodeposition of nickel/black cobalt selective coatings, and evaluate the performance stability as a function of the substrate material, the temperature of heat treatments, and under accelerated weathering conditions. The black cobalt absorber material is shown to be stable up to 750 °C, and the performance stability for a stainless steel substrate with a nickel/black cobalt selective coating after 200 h at 300 °C is demonstrated, with a solar absorptance of 95% and thermal emittance of 7% (at 100 °C). The galvanostatic electrodeposition process has been scaled up to collector size, and is demonstrated both for a flat plate and a tubular substrate. A copper flat plate of 193 cm × 12 cm connected to a riser tube was coated, and a single-fin solar thermal collector was evaluated under realistic outdoor conditions. The collector produced essentially the same amount of heat as an identical reference collector with a commercial TiNOX selective coating. In addition, the coating was applied to a copper tube for a 3 m parabolic trough collector. The selective coating is hydrophobic and withstands an accelerated weathering test, illustrating the viability of black cobalt electrodeposition for the fabrication of solar collectors for low and high temperature applications. [ABSTRACT FROM AUTHOR]

• Electrodeposition of black nickel selective coatings has been scaled up. • Passivation of the backside of the copper substrates improves the stability. • Solar absorptance and thermal emittance affect collector performance. • An electrodeposited 1.74 m2 solar collector has been fabricated. • Electrodeposited collector performance compares well with commercial collectors. The selective coating is the heart of a solar heating system, where the coating needs to efficiently absorb sunlight and heat up the substrate, which subsequently transfers the generated heat to a fluid. Selective coatings generally have an IR reflecting interlayer that maintains low thermal emittance from the substrate in order to optimize the thermal efficiency of the collector. We report on the electrodeposition of a bright nickel/black nickel selective coating onto a copper solar collector substrate, and compare the optical properties and thermal stability with commercially available options, including copper oxide and sputter-deposited nitrogen-doped titanium dioxide. We have fabricated identical solar collectors of an aperture area of 1.74 m2 in order to compare the performance of the three selective coatings, and we have determined the thermal efficiency versus temperature difference in a realistic outdoor setting under sunlight. The collector with the electrodeposited selective coating shows overall better performance than the copper oxide-based collector, and at temperatures above about 50 °C, it achieves equal performance as the titanium dioxide-based collector. One of the reasons for the lower thermal losses for the electrodeposited collector appears to be the protection of the backside of the copper flat plates with a nickel coating. Taking the low investment and materials cost into account, the electrodeposited black nickel/bright nickel/copper flat-plate collector may be attractive for commercial implementation in solar water heaters. [ABSTRACT FROM AUTHOR]

Black nickel is an interesting, economical, and abundant solar absorber material for low temperature solar-to-thermal energy conversion systems, and thin films can be prepared by electrodeposition. Critical parameters include the energy conversion efficiency and thermal stability, which depend on the design of the selective coating and the substrate material. In this work, we show that a metallic nickel interlayer is required in order to achieve thermal stability of black nickel coatings on copper, although the solar-to-thermal conversion efficiency for these multilayer coatings is slightly lower than for coatings of black nickel directly electrodeposited onto copper. The black nickel films were electrodeposited from a nickel chloride bath at pH 6.5 in order to avoid thermal stability issues related to the presence of sulfur. XPS and Raman spectroscopy illustrate that the composition of the black nickel film is complex, containing Ni(OH) 2 , NiO, NiOOH and metallic Ni. We show that the electrodeposition parameters strongly affect the performance of the selective coating. [ABSTRACT FROM AUTHOR]

Solar absorber coatings are the heart of solar-thermal power systems and their desired properties intimately depend on the operational temperature. Specifically for medium to high temperature applications, the systems require a thermally-stable selective coating with optimal optical properties. Black metal mixed oxides with the spinel crystal structure are interesting prospects as a new family of selective absorber materials. In this work we report on the indirect electrodeposition of a spinel black oxide of the type Mn x Co 3-x O 4 , using a nitrate solution; the electrochemical reduction of nitrate results in the formation of surface hydroxide leading to the co-precipitation of Co and Mn hydroxides, and the resulting mainly amorphous film is subsequently converted to the spinel material by sintering. The optimal precursor chemistry and electrodeposition conditions were obtained using a combinatorial method employing the Hull cell. After optimization and thermal annealing, spinel Mn x Co 3-x O 4 films (x ≈ 0.7) with solar absorptance of α = 89.5% and thermal emittance of ε = 14.9% (at 100 °C) were obtained, illustrating their promising optical properties. The thermal stability of the coatings at medium and high temperatures (250 °C and 560 °C), as well as under harsh accelerated weathering conditions, was found to be excellent. The scalability of the electrodeposition method was confirmed by deposition onto a stainless steel tube. [ABSTRACT FROM AUTHOR]