The present invention is related generally to solar power, employing photovoltaic cells, and, more particularly, to novel concentrating photovoltaic modules that achieve extremely high efficiencies.
State-of-the-art single junction solar arrays as well as concentrators using single junction solar cells utilize only a limited portion of the available solar spectrum, thereby wasting the remainder of available energy outside of their limited spectral response. The limitation is caused mainly by two basic xe2x80x9cphoton lossxe2x80x9d mechanisms within the cells, namely, (1) loss by longer wavelengths and (2) loss by excess energy of photons. In the former case, photons with energy smaller than the xe2x80x9cenergy bandgapxe2x80x9d or xe2x80x9cforbidden gapxe2x80x9d Eg (direct bandgap semiconductor) or Egxe2x88x92Ephonon (indirect bandgap semiconductors where Ephonon is the phonon quantum of energy) cannot contribute to the creation of electron-hole pairs. In the latter case, in the spectrum range of interest, one photon generates only one electron-hole pair. The rest of the energy larger than the bandgap is dissipated as heat. Photons with energy hv greater than Eg thus can only use a portion of Eg of their energy for generation of electron-hole pairs. The excess energy raises the temperature of the solar cell and degrades its performance. Thus, even high quality cells with excellent quantum efficiencies, such as GaAs, exhibit relatively modest conversion efficiencies since they cannot respond to more than a relatively small portion of the incident spectrum.
One way of circumventing this limitation is the use of two or more different bandgap cells that are stacked, or monolithically grown, in a vertical manner. Such a multi-junction (MJ) system with appropriately chosen bandgaps can span a significantly greater portion of the incident solar spectrum than achievable with single-junction cell systems. Such multi-junction solar cells are well-known. For example, three-junction cells have been devised that can control a relatively larger portion of the solar spectrum, and are further described below. Because of their potential for very high efficiencies, MJ cells have enjoyed increased interest over the last two decades.
At a recent NCPV (National Center for Photovoltaics) meeting in Denver, Colo. on Apr. 16-19, 2000, it was reported that triple-junction GaInP2/GaAs/Ge concentrator cells developed by NREL (National Research Energy Laboratory) and Spectrolab have achieved 32.3% at 47 suns and 29% at 300 suns (AM1.5, 25C), with an obvious drop of 3.3% (absolute) or 10.2% (relative), indicating one of the many limitations of MJ concentrator systems at higher concentrations. It should be kept in mind that the above-mentioned encouraging achievement with a pulsed solar simulator does not represent a real life situation. Under actual operating conditions, the MJ concentrator system performance can drop more than 12 to 15% (absolute) against the bare cell performance and defeat the use of high efficiency MJ cells. Some of the major concentration-related performance losses in MJ cells are caused by the following shortcomings: absorption of light in the top cells, chromatic aberrations caused by the concentrator optics, flux non-uniformity on the cells, limited heat removal from the top cells, current limitation in the cells, series resistance, shadowing losses due to finger contacts on the cells, and limited acceptance angle for photon incidence on the cells. Most of these limiting factors apply to all conventional concentrator types based on a variety of cells. MJ cells, however, are more vulnerable to most of these performance-limiting factors.
The relative deterioration of MJ cells becomes worse as the number of junctions increases. Several authors in the field have predicted that for vertically stacked or monolithically-grown systems, limited improvements are expected beyond triple-junction cells. A recent press release by Boeing (Spectrolab) on Aug. 15, 2001, confirmed that a triple junction cell developed by Spectrolab and NREL has reached a conversion efficiency of 34% (a world record at that time) at 400xc3x97. That appears to be very much the limit of three-junction cells. Four-junction cells are predicted to be able to reach upper 30% and lower 40% efficiencies. Theoretical studies have shown that to achieve this kind of efficiency level, a four-junction cell system requires a 1 eV bandgap III-V cell that meets all requirements including: optical, thermal, and electronic issues involved. In spite of extensive efforts, this material remains elusive.
Another shortcoming of the monolithic MJ cells lies in the limitation of complementary bandgap cell materials with matching lattices. In vertically-grown MJ cells, all the adjacent xe2x80x9csub-cellsxe2x80x9d must have matching or slightly mis-matching lattices for proper performance. Thus, even the best bandgap matched sub-cell cannot result in a multi-junction cell if their lattices mis-match. This requirement narrows down significantly the available set of sub-cells that could be used.
These apparent limitations represent a formidable bottleneck in the development of high and very high efficiency (and therefore cost-competitive) concentrator systems in the near future. According to analytical studies, ideal four bandgap cell systems utilizing a new 1 eV material can improve the solar to electricity conversion efficiency over 48% at 500 suns. Even at a cost of $250/Watt for such a system, the effective cell system cost for a 500xc3x97 flux concentrator can be as low as $0.50/Watt. At this cost level, the concentrators would be ahead of the long range goals of the Department of Energy for PV flat plate technology (installed system cost of $1.00/Watt to $1.50/Watt by the year 2030), if the balance of concentrator system could be built for $0.50/m2 to $1.00/m2. Thus, very high cell and system efficiencies are paramount to achieve the long term cost goals for photovoltaics in general.
In the late 1990s, NASA and JPL scientists proposed an alternative technique, called xe2x80x9cRainbowxe2x80x9d, to circumvent the problems of vertical MJ systems and improve the performance of multi bandgap cell systems. Their method is to split the solar spectrum into several frequency bands and focus each frequency band onto separate cells with corresponding energy bandgaps. The Rainbow multi-bandgap system represents a combination of solar cells, concentrators, and beam splitters. The use of separate discrete cells offers the widest possible scope of semiconductor choices. Based on data for xe2x80x9crealxe2x80x9d cells and optical components, Rainbow was expected in 1997 to convert over 40% of incident solar energy to electricity at the system level.
To the knowledge of the present inventor, this concept has never come to a closure, presumably due to extreme difficulties encountered with the associated optics. In addition, this space system would only have a concentration ratio of a maximum of 20xc3x97, i.e., much lower than the 500xc3x97 or more to reduce the effective cell cost dramatically. A thorough literature search has shown that in the past, the very promising method of spectral splitting and simultaneous use of discrete solar cells with different bandgaps has never reached its potential capacity and the technology was never exploited fully. The invention disclosed herein represents a straight-forward approach to achieve break-through performance levels and with it to rapidly lower the cost of solar energy to competitive levels.
In accordance with the present invention, a photovoltaic cavity converter (PVCC) module is provided which can operate at a concentration range of about 500 to 1,000 suns and a power output range of a few kiloWatts to 50 kiloWatts when combined with a primary dish concentrator and a secondary concentrator. The disclosed PVCC module is an enabling technology to reach very high solar-to-electricity conversion efficiencies. Connecting a plurality of such modules together in a power plant permits obtaining a power generating capacity on the order of several hundred megawatts.
The PVCC module comprises:
(a) a housing having a cavity of any optimized closed shape inside the housing, the cavity having an internal surface area As and including an opening for admitting pre-focused solar radiation into the cavity, the opening having an entrance aperture area Ai, where Ai is smaller than As;
(b) a plurality of single junction solar cells within the cavity, at least some of the solar cells each having different energy bandgaps so their composite spectral responses simultaneously and fully span the solar spectrum; and
(c) at least one wavelength filter associated with each solar cell, wherein the wavelength filter is selected from the group consisting of Rugate filters and a combination of Rugate filters and stack interference filters, thereby providing selective transmission and reflection of incident solar radiation to assist in maximizing the utilization of a region of the solar spectrum by solar cells having an appropriate bandgap.
The use of the cavity containing a plurality of single junction solar cells of different energy bandgaps and simultaneous spectral splitting of the solar spectrum provide the following improvements over the best state-of-the-art (see Table I, below), namely, high efficiency multi-junction (MJ) solar cells operating under high solar flux concentration. The improvements listed in Table I are due to the change from a vertical cell architecture of the MJ cells (where the cells that respond to different parts of the solar spectrum are stacked or monolithically grown on top of each other) to a lateral geometry within the spherical cavity (where the cell strings made of the single junction cells operate next to each other without mutual interference). The purpose of the cavity with a small aperture for the pre-focused solar radiation is to confine (trap) the photons to a high degree so that they can be recycled effectively and used by the proper cells provided with the proper xe2x80x9cpass/rejectxe2x80x9d spectral filters, thus the name Photovoltaic Cavity Converter (PVCC).
Other objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description and accompanying drawings, in which like reference designations represent like features throughout the FIGURES.