The increasing trend for global energy demand and environmental concerns have enforced us to seek for renewable energy, particularly from the most abundant sunlight. Hence, development of selective solar absorbers (SSAs) with a high absorptance in the solar wavelengths (0.3 to ≈2.5 μm) and low emittance in the infrared thermal radiation wavelengths (≈2.5–40 μm) are being explored. Researchers at Department of Applied Physics and Applied Mathematics, Columbia University, Department of Chemistry, Columbia University Department of Materials Science and Engineering, Stanford University, USA, have demonstrated a simple, “dip and dry” technique based on galvanic displacement reaction to fabricate solar absorbing plasmonic nanoparticle coated foils (PNFs) for SSAs at room temperature.
The fabrication process involves immersion of Zn foil in aqueous CuSO4 solution for 30–60 s, in which the Cu2+ ions are reduced to metallic Cu nanoparticles by Zn on its surface (Figs. 1(a) and 1(b)). The galvanically deposited Cu nanoparticles on Zn appears as a black layer (Fig. 1(c)), with a strong solar absorptance (Fig. 1(d)) and an excellent optical selectivity (α = 0.96 and ε = 0.08) (Fig. 1(e)).
Fig. 1 (a) Schematic of the deposition process – formation of Cu nanoparticles on Zn by galvanic displacement reaction; (b) SEM image of the Cu nanoparticle layer on the PNF; c) Photograph of PNF; (d) Schematic depicting the high solar absorptance and low thermal emittance of PNF (Thickness of the arrows indicates their intensity); and (e) Spectral reflectance of PNF (α = 0.96, ε= 0.08) and the ideal SSA at 100 °C.
The effect of immersion time, concentration of CuSO4 and temperature on the galvanic deposition of Cu nanoparticles on Zn is reflected in the spectral reflectance at normal incidence in the wavelength range of 400 nm to 14 μm (Figs. 2(a)-2(c)). It is evident that longer immersion time, higher concentrations of CuSO4, and higher temperatures employed for deposition of Cu nanoparticles have lead to a lower reflectance across the wavelength. An increase in thickness as well as the diameter of the Cu nanoparticle layer lead to a lower reflectance. An increase in surface roughness of the Cu nanoparticle layer also causes a lower reflectance. The morphology of the Cu nanoparticles could also be altered by varying the type of anions as well as with the addition of surface active agents in the solution.
Fig. 2 Variation in spectral reflectance across the wavelength as a function of (a) immersion time; (b) concentration of CuSO4; and (c) temperature.
The extent of change in solar absorptance, emittance and efficiency as a function of immersion time, concentration of CuSO4 and temperature are shown in Figs. 3(a)-3(c). Only a small variation in the α (≈0.83) and ε (0.03 to 0.06) are observed with an increase in immersion time from 15 s to 45 s (Fig. 3(a)). Both α (0.43 to 0.94) and ε (0.02 to 0.24) are increased with an increase in concentration of CuSO4 from 2.5 mM to 50 mM (Fig. 3(b)). Similarly, a reasonable increase in α (0.86 to 0.93) and ε (0.02 to 0.17) are observed with an increase in temperature up to 0 °C to 75 °C (Fig. 3(c)). For efficient harvesting of solar energy, an SSA must have possess a high α(θ) and a low ε at all incidence angles. The SSAs developed in this work exhibit an excellent wide-angle solar absorptance, with α(θ) ranging from 0.96 at 15°, to a peak of 0.97 at ≈35°, to 0.79 at 80°.
The adhesion between the Cu nanoparticle layer and the Zn substrate is very strong. Reflectance measurements fails to indicate any significant change in the solar absorptance, emittance and efficiency before and after the adhesion testing. Accelerated thermal aging at 200 °C in air up to 96 h indicates only a small decrease in α/ε from 0.94/0.13 to 0.90/0.09, suggesting its better thermal stability. The observed variation in and with experimental parameters employed for the deposition of Cu nanoparticles indicate that it would be possible to maximize the efficiency of PNFs’ by suitably tuning the experimental parameters. Thus the “dip and dry” technique proposed in this study offers many avenues to the optical selectivity of the SSAs.
T.S.N. Sankara Narayanan
Fig. 3 Extent of change in solar absorptance, emittance and efficiency, as a function of (a) immersion time; (b) concentration of CuSO4; and (c) temperature.
For more information, the reader may kindly refer: Jyotirmoy Mandal et al., Scalable, “Dip-and-Dry” Fabrication of a Wide-Angle Plasmonic Selective Absorber for High-Efficiency Solar–Thermal Energy Conversion, Adv. Mater. 2017, 1702156, DOI: 10.1002/adma.201702156
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