PVMirror: A New Concept for Tandem Solar Cells and Hybrid Solar Converters

As the solar electricity market has matured, energy conversion efficiency and storage have joined installed system cost as significant market drivers. In response, manufacturers of flat-plate silicon photovoltaic (PV) cells have pushed cell efficiencies above 25%-nearing the 29.4% detailed-balance efficiency limit-and both solar thermal and battery storage technologies have been deployed at utility scale. This paper introduces a new tandem solar collector employing a "PVMirror" that has the potential to both increase energy conversion efficiency and provide thermal storage. A PVMirror is a concentrating mirror, spectrum splitter, and light-to-electricity converter all in one: It consists of a curved arrangement of PV cells that absorb part of the solar spectrum and reflect the remainder to their shared focus, at which a second solar converter is placed. A strength of the design is that the solar converter at the focus can be of a radically different technology than the PV cells in the PVMirror; another is that the PVMirror converts a portion of the diffuse light to electricity in addition to the direct light. We consider two case studies-a PV cell located at the focus of the PVMirror to form a four-terminal PV-PV tandem, and a thermal receiver located at the focus to form a PV-CSP (concentrating solar thermal power) tandem-and compare the outdoor energy outputs to those of competing technologies. PVMirrors can outperform (idealized) monolithic PV-PV tandems that are under concentration, and they can also generate nearly as much energy as silicon flat-plate PV while simultaneously providing the full energy storage benefit of CSP.