Dual Focus Cassegrainian Module Can Achieve >45% Efficiency

Various types of multijunction solar cells have now been demonstrated with energy conversion efficiencies over 40%. Higher cell efficiencies are still possible. NREL, Fraunhoffer and others have announced >40% Inverted metamorphic triple-junction (IMMTJ) InGaP/GaAs/GaInAs cells. One key feature of these cells is that the lowest bandgap cell is no longer a Ge cell. The IMMTJ lowest cell bandgap now allows for a 4 junction cell either integrated or separate. Herein, it is observed that the dual focus Cassegrainian (DFC) module can be fitted with a InGaP/GaAsP/InGaAs triple junction cell at its primary focus and with a GaSb or other IR sensitive cell at its secondary focus. This can allow a straight forward rapid path to a combined cell efficiency of >44%. The potential advantages of this configuration are, first, a rapid path to a combined cell efficiency of 44%, and second, given the fact that the heat load is divided into two locations, both cells will run cooler giving a higher module efficiency. DUAL-FOCUS CASSEGRAIN (DFC) MODULE CONCEPT Maximizing conversion efficiency in solar photovoltaic systems is a current focus of leading cell manufacturers [1] and [2]. The reasons are obvious: a cost effective 50% efficient solar system would radically change the renewable energy production landscape in the United States and the world. While recent work on improving the cell efficiencies to over 40% in the lab (which can result in up to 30% at the system level in actual use) shows promise to improve overall concentrated solar PV systems, no group has proposed or defined a truly ground breaking system design that can shatter previous conversion records and holds the potential to be economically viable in the very near term. The path by which a DFC enables higher conversion efficiency is by the addition of a fourth solar cell at a separate location that then converts a previously unused portion of the solar spectrum into electricity. The fourth cell adds to the system conversion efficiency by converting long wave infra-red light that would otherwise not be converted by the triple junction (TJ) cell. There are two choices for the fourth IR sensitive cell. Either the simple GaSb diffused junction IR cell or the epitaxial InGaAs/InP IR cell can be used as the fourth cell. Figure 1 shows the spectral response of a current state of the art IMMTJ cell [1], with an additional InGaAs TPV cell [3], and figures 2 and 3 show how the addition of a TPV cell to a best in class National Renewable Energy Lab IMMTJ can result in up to 46% conversion efficiency. Figure 1: Spectral Response of an Emcore [1] Inverted Metamorphic Triple Junction Solar Cell and a 0.6 eV InGaAs TPV Cell.