Spinel Cu–Mn–Cr Oxide Nanoparticle-Pigmented Solar Selective Coatings Maintaining >94% Efficiency at 750 °C

Abstract

High-temperature concentrating solar power (CSP) system is capable of harvesting and storing solar energy as heat toward cost-effective dispatchable solar electricity. Solar selective coating is a critical component to boost its efficiency by maximizing solar absorptance and minimizing thermal emittance losses. However, maintaining a high solar-thermal conversion efficiency >90% for long-term operation at ≥750 °C remains a significant challenge. Herein, we report spray-coated spinel Cu–Mn–Cr oxide nanoparticle-pigmented solar selective coatings on Inconel tube sections maintaining ≥94% efficiency at 750 °C and ≥92.5% at 800 °C under 1000× solar concentration after 60 simulated day-night thermal cycles in air, each cycle comprising 12 h at 750 °C/800 °C and 12 h cooling to 25 °C. The solar spectral selectivity is intrinsic to the band-to-band and d-d transitions of nonstoichiometric spinel Cu–Mn–Cr oxide nanoparticles. This feature offers a large fabrication tolerance in nanoparticle volume fraction and coating thickness, facilitating low-cost and scalable spray-coated high-efficiency solar selective absorbers for high-temperature CSP systems.

Extracting Optical Parameters of Cu–Mn–Fe Spinel Oxide Nanoparticles for Optimizing Air-Stable, High-Efficiency Solar Selective Coatings

“…Therefore, the most reasonable explanation for the solar absorptance degradation of Cu–Mn–Fe oxide spinels is the oxidation of Fe 2+ ions in the octahedral sites upon thermal cycles, thus increasing the Fe 3+ (oh)/Fe 3+ (th) ratio and suppressing the d–d optical transitions between Fe 2+ ions and Fe 3+ ions in the octahedral sites to reduce the optical absorption. This is also consistent with the fact that iron-free CuMn 2 O 4 and Cu–Mn–Cr oxide NP (Syn42) coatings are both thermally stable upon 750 °C/room temperature thermal cycling in air (ref ).…”

“…X-ray photoelectron spectroscopy (XPS) analysis reveals that Cu 1+ , Cu 2+ , Mn 3+ , Mn 4+ , and Fe 3+ ions are simultaneously present in CuFeMnO 4 in our previous work in ref . In the case of Cu–Mn–Fe spinel oxides, some Fe 2+ ions may exist due to partial reduction of Cu 2+ to Cu 1+ and oxygen deficiency .…”

“…For the calculations of thermal emittance ε and overall efficiency η therm , the absorption and scattering cross sections are obtained by extrapolating the refractive index data into the mid-IR regime (Figure S9), and the silicone matrix absorption in the IR range is also taken into account using reference silicon coating samples without NPs. As shown previously in refs and , the latter is the dominant factor in thermal emittance. Since Syn24 and Syn42 offer the highest solar absorptance with the same volume concentration and coating thickness, we only focus on Syn24 and Syn42 here with the corresponding optical data in Figure d.…”

“…The best performances are Syn24 and Syn42 NP coatings with a solar absorptance of 96.5% and 96.9%, respectively. Further considering the data we reported recently in ref for Cu–Mn–Cr spinel oxide NPs, we can summarize the following results on modifying the optical properties of CuMn 2 O 4 spinel NPs by Cr and Fe alloying: As shown in Figure a, in the case of Cu–Mn–Fe spinel oxide NPS, the indirect bandgap at ∼1.9 eV is almost independent of the Mn/(Mn + Fe) ratio, while the direct bandgap shows a trend of decreasing with increasing Mn composition. The faster shrinkage of the direct bandgap leads to the more directness of the fundamental bandgap; i.e., CuMn 2 O 4 (Mn/(Mn + Fe) = 1) becomes a direct band gap material.…”

A Superhydrophilic, Light/Microwave-Absorbing Coating with Remarkable Antibacterial Efficacy

“…15 Spinel metal oxides are a series of attractive compounds with special chemical and physical properties that have potential for many important applications, including solar energy transformation, catalysis, and magnetic recording media. 16,17 More recently these materials have, in particular, drawn attention as solar absorber coatings. Kim et al 18 prepared a two-layered solar absorbing coating, which consisted of an outer CuFeMnO 4 layer and an inner CuCr 2 O 4 layer.…”