For a large part of the United States, August means bright sunshine, sweltering heat, and high power bills. Scientists and engineers have worked for decades to harness all that light and heat efficiently, and one of the most exciting new developments in the world of solar power comes from the smart combination of two different sun-catching technologies.

Many of ARPA-E’s technology programs seek to break down silos and build new technological communities around a specific energy challenge. The Full-Spectrum Optimized Conversion and Utilization of Sunlight (FOCUS) program is bringing together the photovoltaic (PV) and concentrated solar power (CSP) communities to develop hybrid and cost-competitive solar energy systems. Previously, CSP and PV communities did not have much experience working together on solar technology development. ARPA-E’s FOCUS program has enabled experts from both disciplines to develop a hybrid option that could capture maximum energy out of the full solar spectrum, generating both electricity and usable heat within the same system.

The solar cells in these new, hybrid technologies convert some of the sunlight directly to electricity, which can be used immediately, while the heat can be stored and used to generate electricity later. Unfortunately, CSP mirror systems use only direct sunlight, or the light that comes in a straight beam from the sun, whereas PV has the capacity to convert both direct and diffuse light to electricity. Diffuse sunlight has been scattered by the atmosphere; when the scattering is weak, the sky looks blue, but when it’s strong, the sky looks hazy.

Photo: Schematic of the PVMirror concept, in which PV and CSP are hybridized via spectrum splitting

Arizona State University is developing a PVMirror technology that modifies the CSP mirror design, replacing the usual curved concentrating mirror with a curved structure incorporating silicon solar cells and a dichroic optical film. Dichroic films allow some colors of light to pass through while reflecting the rest, replacing the ordinary mirrored coating. The PV cells absorb only the colors of sunlight that can be used to efficiently generate electricity, while the other colors (blue and infrared) are reflected. Because the optical film conforms to the curved PVMirror, this light is concentrated at the PVMirror’s focus where a black tube, called a thermal absorber, is placed. The reflected portions of sunlight heat this tube; the heat is carried away by a flowing oil, and used later to power a steam turbine and generate electricity.

Photo: Project lead Dr. Zachary Holman from Arizona State University shows a section of the PVMirror to World Bank Group President Dr. Jim Yong Kim

Photo: Project lead Dr. Zachary Holman (center) from Arizona State University and his researchers conducting test measurements of the PVMirror system

In the below video, ARPA-E Deputy Director for Technology Dr. Eric Rohlfing discusses the FOCUS program. The video also features interviews with innovators from the Arizona State University and University of Arizona project team and showcases how the FOCUS program is combining the best elements of two types of solar to get the most out of the full solar spectrum.