No abstract

Although renewable solar power plants are rapidly proliferating, high cost and the intermittent availability of solar power are still significant barriers for its penetration into the energy grid system. Concentrating solar power (CSP) offers an attractive alternative due to its integration with cost-effective thermal energy storage systems. To further reduce the cost of CSP, it is imperative to operate the plants at higher temperatures for enhanced efficiency. One of the key components for next-generation high-temperature CSP is the solar absorbing coating materials. In this work, we have developed tandem-structured solar absorbing layers with CuFeMnO 4 and CuCr 2 O 4 black oxide nanoparticles (NPs). These tandem structures exhibited a remarkably high solar-to-thermal conversion efficiency, or figure of merit (FOM), of 0.903, under the condition of 750 o C operating temperature and a solar concentration ratio of 1000. More importantly, the coating showed unprecedented durability, as demonstrated from long-term isothermal annealing

Concentrating solar power is becoming an increasingly important part of the renewable energy portfolio. However, further cost reduction is desired to make CSP competitive with traditional energy technologies. Higher operating temperature is considered an attractive avenue leading to higher power conversion efficiency and lower cost, but tremendous technical challenges exist with higher temperature operation of CSP, with one of the main issues being the lack of a high-performance solar absorbing material that is durable at 750 o C or above. In this work, a black oxide material, made of cobalt oxide nanoparticles, is synthesized and utilized as a high-temperature solar absorbing material. The nanoparticles are embedded in a dielectric matrix through a scalable spray coating process. The top layer of the coating is further improved with light-trapping structures using sacrificial fillers introduced from the same coating process. After the surface modification of cobalt oxide coating, we achieved a high thermal efficiency of 88.2%. More importantly, the coating shows no degradation after 1,000-hour annealing at 750 o C in air, while the existing commercial light absorbing coating was reported to degrade by long-term exposure at high temperature. Our findings © 2015. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 suggest that the materials and processes developed here are promising for solar absorbing coating for future high-temperature CSP systems.

Structural color arises from geometric diffraction; it has potential applications in optical materials because it is more resistant to environmental degradation than coloration mechanisms that are of chemical origin. Structural color can be produced from self-assembled films of colloidal size particles. While the relationship between the crystal structure and structural color reflection peak wavelength is well studied, the connection between assembly quality and the degree of reflective structural color is less understood. Here, we study this connection by investigating the structural color reflection peak intensity and width as a function of defect density and film thickness using a combined experimental and computational approach. Polystyrene microspheres are self-assembled into defective colloidal crystals via solvent evaporation. Colloidal crystal growth via sedimentation is simulated with molecular dynamics, and the reflection spectra of simulated structures are calculated by using the finite-difference time-domain algorithm. We examine the impact of commonly observed defect types (vacancies, stacking fault tetrahedra, planar faults, and microcracks) on structural color peak intensity. We find that the reduction in peak intensity scales with increased defect density. The reduction is less sensitive to the type of defect than to its volume. In addition, the reflectance of structural color increases as a function of the crystal thickness, until a plateau is reached at thicknesses greater than about 9.0 μm. The maximum reflection is 78.8 ± 0.9%; this value is significantly less than the 100% reflectivity predicted for a fully crystalline, defect-free material. Furthermore, we find that colloidal crystal films with small quantities of defects may be approximated as multilayer reflective materials. These findings can guide the design of optical materials with variable structural color intensity.

In contrast to strong plasmonic scattering from metal particles or structures in metal films, we show that patterns of arbitrary shape fabricated out of multilayer hyperbolic metamaterials become invisible within a chosen band of optical frequencies. This is due to anomalously weak scattering when the in-plane permittivity of the multilayer hyperbolic metamaterials is tuned to match with the surrounding medium. This new phenomenon is described theoretically and demonstrated experimentally by optical characterization of various patterns in Au-Si multilayer hyperbolic metamaterials. This anomalously weak scattering is insensitive to pattern sizes, shapes, and incident angles, and has potential applications in scattering crosssection engineering, optical encryption, low-observable conductive probes, and optoelectric devices.

No abstract

Effects of shape anisotropy on colloids reconfiguration kinetics: crystals of ellipsoids melt at rates 5.7 times faster than spheres.

Tandem structured spectrally selective coating layer of copper oxide nanowires combined with cobalt oxide nanoparticles, Nano Energy, http://dx.Abstract Increasing the light absorption across the wide solar spectrum has important implications for applications in solar-thermal and photovoltaic energy conversion. Here, we report novel tandem structures combing two different materials with complementary optical properties and microstructures: copper oxide (CuO) nanowires (NWs) and cobalt oxide (Co 3 O 4 ) nanoparticles (NPs). Copper oxide NWs of 100-200 nm in diameter and 5µm long are grown thermally on copper foil in air and cobalt oxide NPs of 100-200 nm in diameter are synthesized hydrothermally. Tandem structures of spectrally selective coating (SSC) layer are built with three different methods: spray-coating, dip-coating of cobalt oxide NPs into copper oxide NWs forest, and transferring of copper oxide NWs layer onto cobalt oxide NPs layer. The tandemstructured SSC layers fabricated from the spray-coating, dip-coating and transferring methods exhibit figure of merit (FOM) values of 0.875, 0.892 and 0.886, respectively, which are significantly higher than that of the starting copper oxide NWs (FOM = 0.858) and cobalt oxide NPs (FOM= 0.854). Our results demonstrate the efficacy of using novel tandem structures for enhanced light absorption of solar spectrum, which will find broad applications in solar energy conversion.