Batteries: thermoelectric and photoelectric – Photoelectric – Panel or array
Reexamination Certificate
2001-09-11
2003-02-04
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Panel or array
C136S249000, C136S251000, C136S261000, C136S262000, C136S252000, C136S244000, C257S433000, C257S434000, C257S437000, C257S443000
Reexamination Certificate
active
06515217
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to solar collectors, and more particularly to a solar collector having a three-dimensional array of different types of monolithic photovoltaic cells enclosed within an enclosure having reflective surfaces to provide improved efficiency, extended operating life and reduced manufacturing cost.
BACKGROUND
Solar or photovoltaic cells (PVCs) are semiconductor devices having P-N junctions which directly convert radiant energy of sunlight into electrical energy. Conversion of sunlight into electrical energy involves three major processes: absorption of sunlight into the semiconductor material; generation and separation of positive and negative charges creating a voltage in the PVC; and collection and transfer of the electrical charges through terminal connected to the semiconductor material. PVCs are widely known and commonly used in a variety applications, including providing electrical energy for satellites and other space craft, marine vessels, installations in areas not served by a grid of an electric utility company, and portable consumer electronics devices such as radios, tape/compact disc players and calculators.
Heretofore PVCs have not been widely used as a main or even auxiliary source of power for residences and businesses having access to conventional power sources, for example, through a power grid of an electric utility company. There are several reasons for this, the most important of which is cost. Electricity produced from solar cells tends to be relatively expensive compared to that available from conventional power sources such as hydroelectric, oil-fired, coal fired and nuclear power plants.
Although the cost of installing, maintaining and repairing solar electric generation arrays or systems is not insignificant, the greatest cost associated with solar energy is the cost of the manufacturing the PVCs. Referring to
FIG. 1
, prior art PVCs
20
are typically formed on an ultra-pure silicon wafer or substrate
22
, which in itself can cost from about 300 hundred to about 5 thousand dollars apiece depending on size. For example, an 8 inch diameter silicon commonly used in manufacturing PVCs typically costs about 2.5 thousand dollars. Furthermore, traditionally a large number of individual PVCs
20
were fabricated on a single substrate
22
by (i) depositing or growing a doped layer of semiconductor material, such as silicon, over the substrate
22
including a dopant of an opposite type; (ii) patterning and etching the substrate
22
with the doped layer thereon to form individual PVCs
20
; (iii) depositing a metal layer over the etched substrate
22
; (iv) patterning and etching the metal layer to form vias, contacts and lines interconnecting the individual PVCs
20
; and (v) inspecting and testing the finished PVCs
20
to remove from an output circuit defective PVCs. The time, equipment and skilled operators required to perform each of the above steps makes the cost of solar electricity extremely expensive, and impractical for just about any use for which an alternative conventional energy source is available.
In an effort to reduce costs, some of the latest generations of PVCs have been monolithic PVCs in which substantially the entire surface of a substrate is taken up a by single large PVC, thereby eliminating much of the time and costs associated with patterning and etching the doped layer and the metal layer. However, this approach has not been wholly successful, since unlike with a substrate having numerous individual PVCs which can be individually removed from the output circuit, a single defect at any point in the monolithic PVC would render the entire substrate useless. In practice, this has resulted in yields well below 40%, offsetting or completely negating any cost savings realized with this approach.
Yet another problem with prior art PVCs is their efficiency in converting available light into electrical energy. This is particularly a problem for solar electric systems having limited power generating capability. That is, because usable solar energy is available for only a fraction of a day, when it is available the PVCs must generate energy to meet current demands and generate sufficient energy to be stored for use when usable solar energy is unavailable. Thus, conventional solar electric systems must either have relatively large numbers of PVCs, which as explained above are costly, or have a high degree of efficiency. Unfortunately, prior art PVCs are typically only from about 10 to 14% efficient.
Referring to
FIG. 2
it is seen that a major reason for this poor efficiency is that a significant or even a large proportion of the light incident on a surface
24
of the PVC
20
is simply reflected away again. There have been several attempts in the prior art to remedy this including anti-reflective coatings on the surface of the PVC, and the use of a concentrator or lens
26
to enhance collection of incident light, as shown in FIG.
3
. However, these solutions have not been wholly satisfactory for a number of reasons. One reason is that the addition of anti-reflective coatings or lens further increases the costs of fabricating the PVCs. More fundamentally, due to band-gap energy, which is a characteristic of every particular type of PVC, the PVC is capable of utilizing or converting into electricity only a narrow range of light wavelengths incident thereon, no matter how much light is concentrated on or prevented from being reflected from the surface of the PVC. For example, although solar radiation includes wavelengths from 2×10
−7
to 4×10
−6
meters, silicon based PVCs having a band gap energy of about 1.1 electron volts (eV) are capable of utilizing only wavelengths from about 0.3×10
−6
to about 3.0×10
−6
meters. Similarly, gallium-arsenide (GaAs) based PVCs, aluminum-gallium-arsenide (AlGaAs) based PVCs, and germanium (Ge) based PVCs have band gap energies of 1.43, 1.7 and 0.5 eV respectively, and are therefore sensitive to other wavelengths.
One possible approach to overcoming this inherit limitation of prior art PVCs
20
is shown in FIG.
4
.
FIG. 4
is a sectional side view of a PVC
20
having several separate layers
28
,
30
,
32
, of semiconducting material to form several different p-n junctions, each having a different band gap energy and each sensitive to a different wavelength of light to enhance the utilization of incident light. Unfortunately, the additional processing steps required to fabricate this multilayer PVC makes the approach prohibitively expensive for all but those applications, such as satellites and spacecraft, for which no alternative exists. Moreover, certain types of PVCs, such as GaAs, AlGaAs or Ge based PVCs, are easily damaged by exposure to high levels of short wavelength or ultraviolet radiation.
Accordingly, there is a need for a solar collector that is inexpensive to fabricate, highly efficient in its utilization of available solar radiation, and which has an extended operational life.
The present invention provides a solution to these and other problems, and offers other advantages over the prior art.
SUMMARY
It is an object of the present invention to provide a solar collector having an array of photovoltaic cells with improved efficiency, extended operating life and reduced manufacturing cost.
According to one aspect of the present invention, the solar collector includes a number of substrates arranged in a two-dimensional array of, each substrate having a monolithic photovoltaic cell (PVC) formed on a surface thereof for converting light incident thereon into electrical energy. The PVCs include at least two different types of PVCs receptive to different wavelengths of light and having different band gap energies. The array of substrates are enclosed within an enclosure having a top-wall with an anti-reflective coating through which light is passed to the PVCs, and bottom and sidewalls having reflective coatings to reflect at least a portion of light incident thereon onto the PVCs. Preferably,
Diamond Alan
Dorsey & Whitney LLP
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