Batteries: thermoelectric and photoelectric – Photoelectric – Panel or array
Reexamination Certificate
2000-04-19
2001-06-26
Chapman, Mark (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Panel or array
C359S853000
Reexamination Certificate
active
06252155
ABSTRACT:
TECHNICAL FIELD
The present invention is directed to concentrators, each associated with a plurality of solar cells, for concentrating solar radiation by employing either trough-, trapezoidal-, and cone-shaped near-ideal, non-imaging concentrator elements, with or without a graded refractive index (GRIN) or a reflective second stage that replaces the refractive optics. The 2-dimensional version of the second stage may use a biaxial graded index of refraction profile, whereas the 3-dimensional version employing a cone-shaped first stage may use a radial index profile.
BACKGROUND ART
Solar cells find extensive application in space-related vehicles for providing power from the sun. Obviously, high efficiency solar cells, such as multijunction (MJ) cells, are preferably utilized; however, such high efficiency solar cells tend to be expensive. Due to the high overall expense of satellite launch, deployment, and operation in space, it is desirable to extend the lifetime of the solar cells, whether in earth orbit, at various altitudes, or on an interplanetary mission. It is further desirable to provide a concentrator for solar cells in space environments that yields higher concentration ratios (on the order of 50× or more), while at the same time providing shielding against charged particles (mostly electrons and protons) within the energy range of a few KeV to 100 MeV. Further, such a concentrator must be light-weight and compact and must not require excessive solar tracking.
Solar cells employed in space applications require protection from damaging effects of solar radiation. Radiation damage-induced degradation of the power output of silicon solar arrays is well-known. One approach to reduce this problem is through the use of gallium arsenide (or other III-V compound semiconductor) solar cells. Such III-V devices evidence a superior radiation hardness as compared to silicon solar cells. However, such III-V devices are also more expensive, as compared with silicon solar cells.
Cover glasses are commonly used to shield the solar cells from part of the damaging radiation. However, the effectiveness of such cover glass protection is limited, and leads either to solar cell life limitations because the cover glasses have to be thin or to weight penalties if the cover glasses are made too thick. Further, such cover glasses fail to provide any concentration of the solar radiation. The concentration ratio is thus 1× and no savings in cell cost is evidenced.
Concentrators have been developed for space applications. One such example is discussed in U.S. Pat. No. 4,494,302, which uses a Cassegrain mirror concentrator. A solar cell is exposed to concentrated sunlight in the Cassegrain mirror concentrator in which the light is focused by a secondary mirror on the solar cell attached to the front of a primary mirror. The back of the primary minor is made black, and thus serves as a radiator, dissipating the heat from the solar cell. However, such concentrators suffer from a tracking problem, in that with concentrators of large ratio, the image easily moves out of the focal point as the sun's position moves away from the normal to the aperture plane of the module. For example, for a 2-D ideal concentrator trough providing a concentration ratio of 500×, a shift in position of the sun by only 0.1° from normal is all that is necessary for the image to be out of the focal point. Thus, acceptance angle of any concentrator must be optimized so as to minimize the output power losses caused by tracking errors.
Trough-shaped, non-imaging, ideal concentrator elements are known for concentrating solar radiation onto earth-based solar cells and for increasing acceptance angle; see, e.g., U.S. Pat. No. 4,964,713 to Goetzberger, and U.S. Pat. Nos. 3,923,381 and 4,114,592, both to Winston. Such concentrator elements may comprise two stages. The Winston patents disclose a first stage comprising a trough with parabolic sides which are mirrored and a second stage comprising a dielectric material using total internal reflectance (TIR). The Goetzberger patent discloses a first stage comprising a trough with parabolic sides in which the trough comprises glass or plastic and a second, smaller stage comprising either the same or a different material, also having parabolic sides.
Goetzberger's trough of glass or plastic adds undesirable weight for space applications, while for practical purposes, Winston's combination only achieves a concentration of about 5 times (5×) and requires an undesirably long distance from the entrance of the first stage to the surface of the solar cell.
Thus, a concentrator that provides excellent off-normal performance, reduced focal distance (hence, reduced mass), high concentration, and light weight is desired.
DISCLOSURE OF INVENTION
In accordance with the present invention, a solar concentrator that provides the above desirable features, while avoiding most, if not all, of the prior art limitations is provided. The solar concentrator comprises two stages. The first stage comprises a trapezoid-shaped (2-D/3-D) concentrator cusp unit having two major opposed sides joined by two ends. The inner surfaces of the first stage concentrator are mirrored. Further, the ends have two flat, angled surfaces, while the two sides have optimized Bezier surfaces that approximate the paraboloid surfaces used in the prior art. In one embodiment, the second stage comprises a bi-axial gradient refractive index element, in which two gradient refractive index materials, each having a high index surface and a low index surface, are joined together along their high index surfaces. The two ends of the bi-axial element are flat, while the two sides have optimized Bezier surfaces that approximate paraboloid surfaces. The top surface of the bi-axial element is provided with a cylindrical surface, while the bottom, or exit, surface is ground flat and polished. The high index boundary is parallel to the side surfaces of the first stage unit.
A solar cell that is mounted on a thin, high thermal conductivity, ceramic substrate is bonded to the flat exit surface of the second stage of the concentrator of the present invention. An array of such concentrators and solar cells, in which the solar cells are electrically interconnected, may then be deployed for converting solar energy into useful electrical energy.
The concentrator of the present invention evidences much lower mass than prior art concentrators. Further, as the normal onto the array, or panel, of solar cells is out of alignment with the sun as the spacecraft wobbles or changes attitude in space, the concentrator will continue to operate, at relatively high efficiencies, due to the large off-normal performance capability. Concentration ratios in the order of 50× are realized with the present concentrator. A three-dimensional (3-D) version of the same concept is suitable to evidence concentration ratios in excess of 300× with very good off-normal performance.
In a second embodiment, the GRIN lens of the second stage is replaced by a reflector second stage that also has Bezier optimized sides.
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 numeral designations represent like features throughout the FIGURES.
REFERENCES:
patent: 3923381 (1975-12-01), Winston
patent: 4114592 (1978-09-01), Winston
patent: 4494302 (1985-01-01), Knechtli et al.
patent: 4883522 (1989-11-01), Hagerty et al.
patent: 4964713 (1990-10-01), Goetzberger
patent: 5405453 (1995-04-01), Ho et al.
patent: 6057505 (2000-05-01), Ortabasi
W.T. Welford et al, “The Optics of Nonimaging Concentrators”, Academic Press, Inc., pp. 77-79 (1978).
Chapman Mark
Collins David W.
LandOfFree
Space concentrator for advanced solar cells does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Space concentrator for advanced solar cells, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Space concentrator for advanced solar cells will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2452904