Batteries: thermoelectric and photoelectric – Photoelectric – Panel or array
Reexamination Certificate
2001-06-21
2002-10-22
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Panel or array
C136S259000, C126S683000, C126S685000, C126S686000, C126S687000, C126S698000, C359S742000, C359S743000, C359S838000
Reexamination Certificate
active
06469241
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of solar power collection. More particularly, the present inventor relates solar collectors, solar concentrators, and spectrum splitters for improved collection solar power collection efficiency.
BACKGROUND OF THE INVENTION
Solar cells are photovoltaic devices that convert sun light into electricity. In order for photovoltaic devices to significantly contribute to the nationwide energy supply, both the cost per installed kilowatt of photovoltaic generating capacity and the price charged for a kilowatt-hour of electricity generated from such devices, must be reduced. Current research extends in two directions, including solar concentration and spectrum splitting. Efficiencies in solar power generation result from concentration and/or spectrum splitting.
Prior solar light concentrators have used either one axis or two axis standard Fresnel lenses, curved prismatic lenses, curved mirrors or other reflective and or refractive optical devices to concentrate solar energy for conversion to electricity using the photovoltaic cells. These photovoltaic cells are commonly referred to as solar cells. The prior optical concentrator devices enable a significant reduction in the solar cell area to be achieved for a given level of power output and thus greatly reduce the costs associated with the otherwise very expensive solar cells. At a specified operating temperature, higher sunlight to electricity conversion efficiencies are achieved by employing solar cells designed for operation at the higher concentration ratios.
Due to the finite size of the sun, practical limits of solar energy concentration are approximately one hundred times per axis of concentration with both reflecting and refractive lenses, such as a Fresnel lens. Concentrators designed for a single axis of concentration can achieve a one hundred suns concentration ratio whereas two axis concentrators can achieve up to a 10K suns concentration ratio. Practical solar cells that are designed for high concentration conversion operate in the range of 0.1K to 1K suns concentration ratio. In general, the higher the concentration ratio to be achieved, the more stringent are the optics accuracy and the pointing requirements with respect to the direction of received sun light.
A Fresnel lens is characterized as a sawtooth refractive optical lens. A Fresnel lens is disclosed in U.S. Pat. No. 4,069,812. The Fresnel lens has been applied to solar energy collection. The Fresnel lens may be a curved prismatic type lens that performs that function of spectrum splitting the received sunlight. When designed for spectrum splitting the practical concentration ratio is reduced from approximately one hundred suns to approximately twenty suns. This reduction in concentration ratio is because spectrum splitting the sunlight spreads the incoming sunlight into its component frequencies.
Although many concentrator devices are capable of achieving high concentration ratios, the ultimate efficiency and related economics are limited by the conversion performance of the associated solar cells. Solar cells are very efficient at converting light to electrical energy at a single bandgap optimized frequency. However, sunlight is composed of a wide spectrum of different frequencies. The overall efficiency of the solar cells diminishes rapidly at frequencies that are above or below the bandgap frequency of the selected solar cells. Many proposals have been made to increase solar array efficiency by using two or more solar cells with appropriately spaced bandgaps to span a greater portion of the incident solar spectrum. Each bandgap is selected to best match the input spectral portion and thus obtain maximum efficiency. Traditional design practices address this problem by developing solar cells that can be vertically stacked. Ideally, each cell in the stack is optimized for a specific frequency and associated solar cell bandgap. However, this bandgap stacking approach presents numerous design challenges that are primarily associated with solar cell substrate mismatches, frequency response and transmission characteristics of the cell materials, and electrical compatibility of the various cell characteristics. Designs directed to meet a number of challenging design factors associated with vertically stacked cells, are limited by the number of bandgap junctions that can be stacked using conventional semiconductor thin film processes. Ideally, each bandgap is optimized for a different frequency response that accumulates into a spectral range. Hence, the spectral range is limited when using a limited number of bandgap junctions, for example, between two to four bandgap junctions that can be achieved in a vertical stack using conventional semiconductor fabrication processes. In order to satisfy substrate requirements, additional compromises are made in the design and frequency characteristics of the stack. Furthermore, the bandgap stacking design approach disadvantageously have complex multilayer and multistep manufacturing processes that result in expensive solar cells with significantly lower than theoretically achievable efficiencies. Due to the complexity and manufacturing difficulty of multijunction vertically stacked solar cells, current devices operate with only two or three bandgaps. Technology limits may soon reach a viable four bandgap solar cell configuration. Ultimately, the vertically stacked approach is limited in the number of bandgaps and associated conversion efficiencies that can be achieved. Due to the complex multilayer multistep manufacturing processes involved, the vertical junction design also increases the production costs.
Solar cell designers recognized that when the solar cells are separated spatially, that is horizontally rather than vertically overlaid, each cell can be separately designed and manufactured on respective unique and optimized substrates. This respective substrate processing technique would eliminate the complex manufacturing processes and inefficiencies associated with vertically stacked solar cells. Each solar cell could be individually optimized for a specific bandgap optimized for a small portion of the solar spectrum without concern as to substrate mismatch and process design impediments of juxtaposed solar cells within an array of frequency sensitive solar cells. In addition, the horizontal spatial arrangement would allow for the integration of much larger number of different bandgaps, for example, six to ten. With proper design optimization, 60% to 70% of sunlight to electricity conversion efficiency is achievable. The horizontal spatial arrangement also allows design of employing spatially separated cells of different bandgaps to share a common substrate by selection of different cell materials and optimizing the design for each desired bandgap of response. For the horizontal spatial arrangement approach to suitably function, an additional step occurs in the optical process prior to reaching the solar cells. The solar spectrum must be split into respective frequencies and then optically redirected to each respective recipient solar cell. The optical redirection can be accomplished with a prism based optics system. The prism based optic system can be manufactured in the form of a Fresnel lens. In such a prism optical system, the prism device provides spectrum splitting with limited concentration in a single axis. The employment of a prism to split the solar spectrum decreases the ultimate concentration ratio that could otherwise be achieved. As a consequence, the limited achievable concentration of the prism optics with the spectrum splitting approach offsets the advantages over higher concentration.
The combination of both high concentration and spectrum splitting would allow for the development of a low cost solar array capable of leveraging the benefits of both features for minimizing solar cell area with high conversion efficiency. Prior collector systems with either high concentration ratio or high efficiency spectrum splitting conversion have been used but n
Diamond Alan
Reid Derrick Michael
The Aerospace Corporation
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