Lightweight parabolic mirrors for solar concentrators have been fabricated using carbon fiber reinforced polymer (CFRP) and a nanometer scale optical surface smoothing technique. The smoothing technique improved the surface roughness of the CFRP surface from ~3 micron root mean square (RMS) for as-cast to ~5 nm RMS after smoothing. The surfaces were then coated with metal, which retained the sub-wavelength surface roughness, to produce a high-quality specular reflector. The mirrors were tested in an 11x geometrical concentrator configuration and achieved an optical efficiency of 78% under an AM0 solar simulator. With further development, lightweight CFRP mirrors will enable dramatic improvements in the specific power, power per unit mass, achievable for concentrated photovoltaics in space.
A proof-of-concept 11x concentrator made with lightweight carbon fiber parabolic mirrors achieved up to 77.5% optical efficiency. The primary loss of efficiency was due to shape deviation from the nominal parabola, as the smoothed surface of the mirrors provided excellent specular reflectance over the visible and near-infrared wavelengths. Further optimization of this system will include improving the shape, reducing the thickness of the polymer smoothing layer, and making thinner CFRP composite for overall mass reduction. In addition, we will investigate the stability of the system in vacuum and under elevated temperatures and thermal cycling consistent with operation in space. Overall, this UV curable nano-meter scale smoothing process for making CFRP mirrors offers promising performance for an ultra-light concentrated photovoltaic system intended for space applications.