Modifying existing solar cells for temperature contro

Scott, R. R.

English

NASA Technical Reports Server

ATIONAL 
IN ISTRATION 
SOLAR CELL INCL ' JDING SECOND SURFACE MIRROR" 
Filed Jan. 30. 1968 
https://ntrs.nasa.gov/search.jsp?R=19710001574 2020-03-17T02:34:01+00:00Z
3,532,558 
8 mc 8EC49N 
ACE s 
Administrator of the National Aero- 
ace Administration, with respect to an 
invention of Ralph R. Bcott, Levittown, Pa. 
Piled Jan. 30,1968, Ser. No. 701,654 
U.9. C1. 136-89 9 Claims 
A CT OF 
A fused silica cover plate is bonded over a solar cell 
with portions of the plate overhanging the edges of the 
cell and a reflective coating is formed on the second or 
lower surface of the overhanging portions to  reduce the 
amount of heat absorbed by the cell and hence reduce the 
overall temperature. 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is subject 
to the provisions of Section 305 of the National Aeronau- 
tics and Space Act of 1958, Public Law 85-568 (72 Stat. 
435; 42 USC 2457). 
BACKGROUND OF THE INVENTION 
This application relates to solar cell arrays for convert- 
ing solar radiant energy to electrical energy, and more 
particularly to a method for modifying an existing array 
to  reduce the overall temperature of the array without 
any major redesign of the structure. 
Energy converting solar cell arrays used to  provide elec- 
trical power for a spacecraft are naturally designed to fit 
the power requiremens of&he paricular spacecrafg The 
effiicency of the silicon cells usually used for such arrays 
decreases as temperature increases and hence there are 
mximum operating temperature limits. Since some space- 
crafts travel much closer to  the sun than others, it be- 
comes necessary to design a solar cell array so that it8 
maximum operating temperature fits the flight path of the 
particular spacecraft However, it is desirable&% mod- 
ifications be made to  existing arrays without changing the 
basic size or shape of the array, even though this may in 
general involve accepting a reduced power output. Also, 
the design change should be one that permits relatively 
easy adjustment to fit the prescribed requirements. 
SUMMARY OF THE INVENTION 
Briefly stated, a solar cell having a cover plate made 
of light transmitting material and including portions over- 
hanging the edges of the cell is formed with a reflective 
coating on the underside of the overhanging portions. 
Thus, the effective absorptivity to emissivity ratio for the 
surface surrounding the cell is kept as low as possible. 
Averaging this ratio with the higher ratio for the active 
solar cell surface results in a reduced effective value for 
the overall surface and hence a reduced overall tempera- 
ture. Advantageously, the amount of the temperature re- 
duction may be controlled by suitably choosing the size of 
a 
10 
15 
20 
25 
30 
35 
the overhanging area with respect to the active solar cell 
surface. In a preferred arrangement, the reflective surface 
may be conveniently produced by evaporating a metallic 
coating directly onto the undersides of the overhanging 
portions of the fused silica cover plate. 
DETAILED DESCRIPTION OF TKE SPECIFICATION 
AND DRAWINGS 
For a more thorough understanding of the invention 
refer now to the following detailed description and draw- 
ings in which: 
FIG. 1 is a plan, partially schematic view of a solar 
cell module shown on a supporting envelope; 
FIG. 2 is an enlarged cross-sectional view of the module 
of FIG. 1 taken along line 2-% 
FIG. 3 is a greatly enlarged cross-sectional view of a 
single cell assembly on line 3-3 of FIG. 1 .  
Referring now to the drawings, there is shown a solar 
cell module with a plurality of rectangular solar cell as- 
semblies 10, including photovoltaic cells 11, and arranged 
in two spaced parallel rows 12 and 14. The assemblies 10 
are bonded by a suitable layer 15 of adhesive to a sub- 
strate or envelope 16. The substrate serves as a strong but 
lightweight support for the cell assemblies 10 and is typ- 
ically an aluminum honeycomb structure 17 insulated from 
the cell 11 by a layer 18 of resin impregnated glass fiber 
cloth. An elongated electrical conductor 19 extends paral- 
lel to the cell rows 12 and 14 in contact with the upper 
surface of each cell along its inner edge I l a  which forms 
one terminal for each cell. A similar elongated electrical 
conductor 20 is attached to  the other terminal formed on 
the inner edge 1Pb of the lower surface of each cell. 
The remainder of the upper surface of each cell has 
bonded thereto, by a suitable transparent layer of cement 
21, a thin fused silica cover plate 22 which includes por- 
tions 24,25, and 26 that overhang the edges of the cell 11. 
The edges of the plate 22 are slightly spaced from adja- 
cent plates. 
The center portion 22a of the silica cover d a t e  22 which 
40 extends over ihe active surface of cell 11 ;s coated with 
a thin blue multilayer interference filter 28 on the surface 
next to the cell. The outer surface of the silica cover plate 
22 is formed with a thin antireflection coating 38 to en- 
hance the absorptivity of the cover plate 22 and hence 
The overhanging portions 24, 25, and 26 of the silica 
cover plates have formed on their second or lower surface 
a thin reflective metallic coating 32 effectively making 
these portions second surface mirrors. This coating 32 is 
50 preferably silver or aluminum and may be vacuum depos- 
ited. If desired the lower surface of the reflective coating 
32 may be protected by a suitable layer, not shown, of 
materid such as Inconel, a trade name for a composition 
of approximately, 80% nickel, 5% iron, and 15% chro- 
The solar cells 11 are semiconductors wafers prefer- 
ably made of silicon but including a dissimilar semicon- 
ductor materia1 so that a p-n junction is created. The func- 
tion of a photcrvoltaic cell is of course to convert light 
60 energy, falling on its surface to useful electric energy. 
Most solar cells utilize silicon semiconductors in that they 
have good response to sunlight. The spectral response of 
45 the cell 11. 
55 mium. 
3,532,551 
3 
silicon begins in the short infrared region, extends through .82. However, due to the increased area with the over- 
the visible region, and stops short of the ultraviolet region. hang, a greater quantity of heat producing radiation is 
The radiant energy from the sun which is outside of the emitted from the combined assembly PO than without 
spectral response range of a silicon solar cell merely the overhang portions. Thus the addition of the plate 
increases the temperature of a cell which it strikes. Also, overhanging portions results in a reduction of the solar 
since the efficiency of a silicon photovoltaic cell is rela- absorptivity as well as an increase in the effective infrared 
tively low, a considerable portion of the absorbed radia- emissivity of the cell assembly. The net result is that 
tion is converted to heat rather than electricity. with the reflective overhang portions the ratio of the 
The power output of the silicon photovoltaic cell de- absorptivity to emissivity values for the cell assemblies 40 
creases as temperature increases, at a rate of approxi- is decreased and the overall temperature of the array 
mately 0.6% per degree centigrade as the temperature is 1s decreased with a minimum reduction of the array power 
raised above 10" centigrade. At colder temperatures this output due to lost area (for energy conversion. 
coefficient becomes progressively smaller and may exhibit While only a single embodiment of the invention has 
small positive values. Hence, as the distance from the sun been illustrated, it will be understood that other variations 
of a particular solar cell array increases, the efficiency will 15 may now be apparent to one skilled in the art. Accord- 
increase because of the lower temperature; however, the ingly, it is intended that all such variations that fall within 
available energy per unit area naturally decreases as the the true spirit and scope of the invention be included in 
distance from the sun increases. As a result, it is of course. the appended claims. 
necessary to consider the varying distances from the sun What is claimed is: 
that a spacecraft will be, in determining the design of its 20 1. A solar cell assembly comprising: 
solar cell array along with the power requirements of a solar cell; 
the craft. As mentioned above, it is desirable that a con- a cover plate made of material that is a transmitter of 
venient arrangement be provided for modifying an array solar energy to which the cell is responsive for photo- 
to fit a given need without changing the basic size or voltaic action, the plate including portions overhang- 
shape of the array. If a spacecraft is to travel closer to 25 ing the cell; and 
the sun than that for which a particular array was origi- means on the underside of said cover plate overhanging 
iially designed it is necessary to accommodate the result- portions which reflects a greater quantity of solar 
ing temperature increase which would occur. energy than is transmitted therethrough to thereby 
The fused silica cover plates 22 are transparent to the decrease the overall temperature of the assembly 
radiant energy which produces the photovoltaic action 30 with a minimum reduction of the assembly power 
in silicon solar cells and hence the plates do not inhibit output due to lost area for energy conversion. 
the absorptivity of this energy. On the other hand, the 2. A solar cell assembly as defined in claim 1 wherein 
plates cause an increase in the infrared emissivity of the said reflective means comprises a metallic coating on the 
cell assemblies, and a high value of infrared emissivity underside of said cover plate. 
allows the cell assemblies to reject more heat to cold 35 3. A solar cell assembly as defined in claim 2 wherein 
space and thus operate at a lower temperature. The said metallic coating is silver. 
coating 21 of the cement is also transparent to essentially 4. A solar cell assembly as defined in claim 2 wherein 
the same type of energy as the plate. The blue optical said metallic coating is aluminum. 
filter layer 28 selectively reflects the ultraviolet portion of 5. A solar cell assembly as defined in claim 1 wherein 
the solar energy which would only contribute heat to the 40 the Portions of said cover plate directly over the solar 
cell assemblies 10. The filter 28 further serves to shield 
the transparent bonding cement 21 from harmful ultra- 6. A solar cell assembly as defined in claim 1: 
violet radiation which might degrade the quality of the including an electrical conductor in contact with one 
bond. 
wherein said cover plate overhangs the other edges of 
and the filter 28, additional measures are necessary to 
reduce temperature if the solar array is to travel close 7. A solar cell assembly as defined in claim 1 wherein 
to the sun. The addition of the plate overhanging portions 
cell has an interference filter thereon above said cell. 
edge of the cell; and 
the cell. 
In sptie of the beneficial aspects of the cover plates 22 45 
said cover plate is made of fused silica; and 
24, 25, and 26 d the cover plates 22 and the-reflective 
coating 32 on their second or lower surfaces solves this 50 
problem by substantially decreasing the temperature of the 
overall array. In one example of the invention, the absorp- 
tivity value of the cell assembly within the active cell area 
22a was .74 (with 1.0 being a perfect absorber) while 
the absorptivity value of the overhang portions 24, 25, 55 
and 26 of the cell assembly was .05 to .08. Consequently, 
by averaging the low absorptivity value of the overhang 
area with that of the active cell area, the overall absorp- 
tivity average value is decreased. The magnitude of the 
overall absorptivity decrease is controlled by the ratio 60 
of overhang area to active cell area. Since the overhang 
portions are in close proximity to the main portion of 
the cell and the assemblies are small, an efficient absorp- 
tivity averaging results, and the temperature gradient 
across the cell assembly is quite small even though the 65 
silica cwer  plate is not a very good conductor of heat. 
In this regard, if desired a potting compound 34, indicated 
by the broken lines in FIG. 2, may be positioned between 
the overhang portions 24-26 and the substrate 16' and in 
contact with the edges of the cells 1 to conduct heat and 70 
further reduce any temperature gradients. 
In the above example, the emissivity value of the plate 
overhang portions 24, 25, and 26 with the reflective coat- 
ing 32 remained about the same as the emissivity value 
for the active solar cell surface, which was approximately 7.j 
including an antireflective coating formed on the outer 
surface of said cover plate. 
8. A photovoltaic cell array comprising: 
a plurality of photovoltaic cells electrically connected 
to each other; 
a cover plate over the light responsive top surface of 
each of said cells, each plate including a portion over- 
hanging its respective cell, the cover plates being of 
material that is an effective transmitter of radiant 
energy within the responsive range that produces 
photovoltaic action in the cells and is an effective 
emitter of radiant heat which is outside the respon- 
sive range; and 
means on the underside of said cover plate overhanging 
portions which reflects a greater quantity of radiant 
energy than is transmitted therethrough to decrease 
the overall temperature of the array with a minimum 
reduction of the array power output due to the lost 
area for energy conversion. 
9. A cell array as defined in claim 8 wherein: 
said cells are primarily made of silicon, have a gen- 
erally rectangular shape, and are arranged in two 
closely spaced parallel rows with said conductor ex- 
tending parallel to said rows and overlapping an edge 
of each cell; and 
said cover plates are made of fused silicon and over- 
hang the other three edges of its respective cell, with 
3,532,551 
the adjacent edges of the cover plates being slightly 3,419,433 12/1968 Seifer _ _ _ _ _ _ _ _ _ _ _ _ _ _ _  136-89 
spaced from each other. 3,076,861 2/1963 Samulon et al. _ _ _ _ _ _ _ _  136-89 
2,567,331 9/1951 Gaiser et al. 
OTHER REFERENCES References Cited 
UNITED STATES PATENTS 0. S. Heavens, “Practical Applications of Thin F i b s  
2,688,564 9/1954 Forgue. in Optics,” in Butterworth Scientific Pub., 1955, pp, 207, 
2,873,303 211959 Rittner _------------ 136-89 215, 216. 
3,009,006 11/1961 Kostelec _ _ _ _ _ _ _ _ - _ _ _  136-89 F. C. Campbell, “Effects of Radiation on Transmittance 
3,038,952 611962 Ralph -------------- 136-89 of Glasses and Adhesives,” in Proc. 17th Annual Power 
3,049,622 AhlStrOm et al. ------- 136-89 lo Sources Conf., October 1963, PP. 19-22. 
3,350,234 10/1967 Ule _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _  136-89 
3,370,986 211968 Amsterdam et al. ----- 136-89 ALLEN B. CURTIS, Primary Examiner 
3,375,141 3/1968 Julius ____--_--_----_ 136-89 
8/1962 


Solar cell including second surface mirrors Patent

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