Hybrid concentrated photovoltaic and thermal power conversion at different spectral bands

Abstract

The option to use the beam down optics of a solar tower system for large-scale and grid-connected concentrated photovoltaic (PV) cells is examined. The rationale is to use this system to split the solar spectrum. Part of the spectrum can be utilized for PV cells. For instance, but not limited to, mono-crystalline silicon cells can convert the 600–900 nm band to electricity at an efficiency of 55–60%. The rest of the spectrum remains concentrated and it can be used thermally to generate electricity in Rankine–Brayton cycles or to operate chemical processes. Two optical approaches for a large-scale system are described and analyzed. In the first concept, the hyperboloid-shaped tower reflector is used as the spectrum splitter. Its mirrors can be made of transparent fused silica glass, coated with a dielectric layer, functioning as a band-pass filter. The transmitted band reaches the upper focal zone, where an array of PV modules is placed. The location of these modules and their interconnections depend on the desirable concentration level and the uniformity of the flux distribution. The reflected band is directed to the second focal zone near the ground, where a compound parabolic concentrator is required to recover and enhance the concentration to a level depending on the operating temperature at this target. In the second approach, the total solar spectrum is reflected down by the tower reflector. Before reaching the lower focal plane, the spectrum is split and filtered. One band can be reflected and directed horizontally to a PV array and, in this case, the rest of the spectrum is transmitted to the lower focal plane. To illustrate the feasibility of these options, commercial silicon cells with antireflective coating, intended to operate under concentrated solar radiation in the range of 200–800 suns, were chosen. The results show that 6.5 MWe from the PV array and 11.1 MWe from a combined cycle can be generated starting from a solar heat input of 55.6 MW.