Selective Solar Absorber Materials: Nanostructured Surfaces

Current solar to thermal energy conversion technologies, including concentrated solar power (CSP) and solar water heaters (SWH) utilize absorber surfaces that collect incident solar radiation. However, these absorber surfaces emit thermal energy (at their temperature) in the infrared (IR) spectrum, resulting in decreased overall efficiency for solar-to-thermal conversion. Selective absorber surfaces are highly absorptive in the solar spectrum, yet highly reflective in the infrared spectrum and therefore have the potential to minimize thermal energy loss. Copper Oxide (CuO) nanostructures are a candidate selective absorber material due to high absorptivity in the solar spectrum (about 95%), relatively high reflectance in the IR spectrum, scalability, and ease of fabrication. The aim of this study was to analyze optical properties and thermal stability of CuO surfaces in order to assess its feasibility as a selective absorber material. CuO nanostructures were synthesized on copper via chemical wet processing. Samples were thermally cycled to simulate day/night cycles in a typical SWH application. A cycle consisted of 12 hours of heating at 200*C and 12 hours of cooling to ambient temperature. Samples were cycled 1, 2, 3, 8, and 10 times. Surface optical properties were characterized using Ultraviolet-Visible Spectroscopy (UV-Vis) and Fourier Transform Infrared Spectroscopy (FTIR) and compared to optical properties of Pyromark*, the industry standard. Reflectance in the IR spectrum of CuO samples was found to increase after initial heating, whereas the absorptivity decreased. This tradeoff in optical performance resulted in an overall efficiency that remained relatively stable between 0 and 10 cycles (69.5 1.6%, 70.2 1.6%, respectively). CuO samples were found to be roughly 10% more efficient (optical conversion) than Pyromark* (rlpyromark,3x = 59.5 0.7%), indicating that CuO samples have the potential to be an efficient selective absorber material. Thesis Supervisor: Evelyn N. Wang Title: Associate Professor of Mechanical Engineering