Conversion of solar to electrical energy

Solar energy is collected by a concave mirror and directed onto a body located within a container which is lined with solar cells. The heated body radiates energy to the solar cells. The solar cells convert a portion of such radiated energy to electricity. Another portion is converted to heat which is removed by a heat exchanger. A third portion of the radiated energy which is not absorbed by the solar cells or their support structure is reflected back to the radiating body to help maintain its temperature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to sun powered electrical generators, and more 
particularly to a method and apparatus for changing the spectral 
characteristics of the sun rays before directing them onto the solar 
cells, to make them more closely match the response function of the solar 
cells. 
2. Description of the Prior Art 
As is well known, a solar battery is a dc power source made up of several 
solar cells connected in series or parallel or both, to deliver useful 
amounts of power when illuminated by sunlight. Heavy duty photo-electric 
batteries are used in space satellites, control devices, emergency 
telephone power supplies, portable radios, and other places. Generally 
speaking a solar cell is a heavy duty photovoltaic cell; that is, a 
self-generating cell that can produce usefully high voltage and current 
when exposed to sunlight. One known type of solar cell is the silicon 
cell. This type of photovoltaic cell delivers the highest output for a 
given light intensity. In a typical silicon cell an n-silicon layer is 
applied to a metal back plate which becomes the negative output electrode. 
A thin p-type layer then is formed on, or diffused into, the exposed face 
of the n-type layer. Finally, for ohmic contact, a sprayed-on (or plated) 
strip is applied to the p-type layer to serve as the positive output 
electrode. 
A typical solar battery comprises one or more planar panels of photovoltaic 
cells which in use are oriented to face towards the sun. Not all of the 
solar energy received by the solar battery is converted to electricity. 
Some of it merely heats the solar battery and other portions of it are 
either reflected back towards the sun or passes through the solar battery. 
In accordance with the present invention, the solar spectrum is converted 
to a new spectrum which more closely matches the response function of the 
solar cells, so that less heat is generated and more electricity produced 
by the photon energy which reaches the solar cells. Also, according to an 
aspect of the invention, the long wave length photon energy which is 
allowed to pass through conventional solar batteries is collected and 
utilized for the purpose of further increasing the efficiency of the 
system. 
The present invention involves the use of reflectors or mirrors for 
concentrating solar energy at the focus of the reflector. This practice is 
per se old and has been utilized in solar heating systems wherein an 
object to be heated (i.e. a fluid conduit) is located at the focus of a 
curved reflector. It is also known per se to reflect infrared photon 
energy back towards a source of radiant energy, as is disclosed in U.S. 
Pat. No. 3,331,707, granted July 18, 1967 to John J. Werth. However, it is 
not known to increase the efficiency of a solar battery system by changing 
the spectral characteristics of the photon energy received from the sun 
before directing it onto solar cells for generating useful electricity. 
SUMMARY OF THE INVENTION 
According to the present invention, one or more curved reflectors are used 
for collecting sun rays and concentrating them on a black body which is 
spaced from a group of photovoltaic cells which form a solar battery. The 
sun rays heat the black body to make it a radiating body. The spectrum of 
photon energy radiated from such body to the solar battery more closely 
matches the response function of the photovoltaic cells than do the direct 
sun rays. The spectral peak of the converted rays very closely match the 
electron production threshold of the photovoltaic cells. This results in a 
greater amount of the energy which is absorbed by the photovoltaic cells 
being converted to electrical energy and a smaller amount being converted 
to heat which must be carried away. 
According to one aspect of the invention, the photon energy which initially 
passes through the photovoltaic cells is reflected back to the black body 
to help maintain its temperature and in that way can be utilized and help 
increase the efficiency of the system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 show a first embodiment of sun powered electrical generator 
incorporating the concepts of the present invention. It comprises a 
parabolic or similar concave reflector 10 which in use is positioned 
towards the sun to collect and concentrate the sun's rays onto a black 
body or radiator 12 within a spherical cavity 14 lined by photovoltaic 
(viz. solar) cells 16. 
As shown by FIG. 2, the spherical cavity may be formed in a housing 18 
which includes a window 20 for receiving the concentrated sun rays. A 
coolant jacket 22 immediately surround a reflector 24 which immediately 
surrounds the photovoltaic cells 16. A coolant fluid is introduced into 
space 22 via an inlet passageway 26 and is removed therefrom via an outlet 
passageway 28. 
Another possible configuration is a Cassegrainian system which offers the 
possibility of using a back wall as a radiator for some of the heat that 
must be rejected. Two embodiments of a Cassegrainian system are 
illustrated by FIGS. 3-5. FIG. 5 is an axial sectional view of the 
circular embodiment shown by FIG. 3, but it could just as well be a 
transverse sectional view of the cylindrical configuration shown by FIG. 
4. 
Referring to FIG. 5, the parallel sun rays 30 are concentrated by a 
parabolic or similarly curved primary reflector 32 onto the face of a 
convex (e.g. hyperboloidal) secondary reflector 34 which is located 
generally at the focus of the reflector 32. Reflector 34 directs the sun 
rays through a window 36 formed at the apex of reflector 32 and 
concentrates them on a black body or radiator 38 which is mounted within a 
housing 40. Housing 40 includes a planar rear wall portion 42 and a 
parabolic or similarly shaped front wall 44. As shown by FIG. 6, the black 
body 38 is positioned generally at the focus of a concave mirror 46 formed 
on the inner surface of wall 44. 
The concentrated sunlight which enters through the window 36 serves to heat 
the black body 38 to make it a radiating body. Some of the energy which 
radiates from it is radiated directly towards the photovoltaic cells 48 
which are mounted on the inner side of wall 42. The photons which are 
radiated against the concave mirror surface 46 are reflected by such 
surface to the photovoltaic cells 48. 
The photovoltaic cells 48 are mounted on a reflector 50 which serves to 
reflect the photon energy which is not absorbed by the photovoltaic cells 
48 or wall 42 back to the black body 38, to help maintain its temperature, 
as will hereinafter be discussed in some detail. 
Wall 42 is constructed to radiate some of its heat but it also includes a 
coolant jacket 52 through which a coolant is circulated for removing heat. 
To understand how the present invention increases the conversion efficiency 
of solar cells, one needs to recognize that a photon above the electron 
hole pair production threshold does not use all its energy in the 
production of an electron hole pair. For example, a 0.2 .mu.m photon only 
requires 20% of its energy for the production of electricity, as the graph 
of FIG. 7 indicates. The remaining energy (or power) which amounts to 80% 
for a 0.2 .mu.m photon, essentially appears as kinetic energy which 
ultimately must be removed as sensible heat. This fractional use of photon 
energy means that a large fraction of the power contained in the solar 
spectrum above the threshold is not available for conversion to 
electricity, as the graph of FIG. 8 shows. All the power below the 
threshold is also lost, of course. Thus, only the white area under the 
curve is available for conversion even before solid state conversion 
efficiencies are considered. This situation is not satisfactory. Two 
possibilities occur which offer improvement. First, one can shift the 
spectral peak to more closely match the response function of the solar 
cells themselves. The spectrum plotted in FIG. 9 is a 3,000.degree. K. 
black body; it can be seen that the fractional use above the threshold is 
greatly improved as compared to the previous solar spectrum (FIG. 8). 
The second possibility is to use the power available below the threshold by 
reflecting the photons of these wave lengths back to the radiating body to 
help maintain its temperature. In practice, of course, some loss must be 
expected for this reflection, but the net effect of these two 
possibilities increases the maximum theoretical efficiency of spectral 
conversion from 46% in the solar spectrum to about 75% for a 3,000.degree. 
K. spectrum. This is a significant improvement and in actuality represents 
a different design aim for the construction of solar cells. 
Conventional solar cell usage attempts to maximize spectral absorption and 
accept whatever conversion can be obtained. In accordance with the present 
invention, however, one attempts by cell design to minimize absorption at 
wave lengths where the conversion of efficiency is not high. It is 
desirable for the cell to be transparent to wave lengths that cannot be 
used efficiently. This then, is a different cell design aim than for 
conventional usage and as such represents a large potential for 
improvement. 
The efficiency of a cell for conventional usage is power-out over power-in 
where the latter is the total spectral power, viz. 
##EQU1## 
For the concept of the present invention, efficiency is still defined as 
power-out over power-in, but the net power-in is only that fraction of the 
total spectrum that is actually absorbed. This is, of course, less than 
the total spectral power and the efficiency is thus larger than for 
conventional usuage with the same output power. 
##EQU2## 
As an example of the effect of a spectral shift on the power utilization, 
defined as the fraction of absorbed power converted to electricity, which 
is the limiting efficiency of this concept, consider a 280 .mu.m thick 
N/P-P+ cell exposed to the solar spectrum. The power utilization is 16% 
and the efficiency to the total spectrum is 13%. This relationship is 
shown by the graph of FIG. 10. Consider the same cell exposed to a 
3,000.degree. K. black body spectrum. The power utilization is increased 
to 21.8%, a 36% increase over the solar spectrum. This is a sufficient 
increase in the upper limit of efficiency obtainable and is available 
without optimizing the cell designed for this use. In fact, this 
particular cell design is fairly well optimized for conventional usage, 
which is not a particular advantage for the concept of the present 
invention. 
To see how cell design could be altered to increase power utilization, a 
calculation was made for the 280 .mu.m thick N/P-P+ cell and compared with 
measured data available in literature. The agreement is good. The 
calculation was then performed for thinner cells and the power utilization 
was found to increase as expected. 
It should be emphasized that this alteration in cell design, that is, 
simply decreasing the thickness, is rather crude. The point to be made is 
that the potential exists for substantially increasing the power 
utilization of cells by design alteration. 
FIGS. 12-14 show a test facility which was successfully used to demonstrate 
the advantages of the present invention. It comprises a frame 52 mounted 
for pivotal movement about a horizontal axis 58 between two upright 
standards 54, 56. The standards 54, 56 are firmly secured at their lower 
ends to a support base 60. 
A large concave reflector 62 (a search light reflector) is mounted onto the 
frame 52. This was done by cutting a circular opening 64 in a front wall 
portion 66 of frame 52 and beveling the edge of the opening 54 so that it 
closely matches the angle of the rear side of the reflector 62. A 
plurality of retainers, some of which are designated 68, are secured to 
the front face of wall 66. They each include a portion which overlaps the 
edge of the reflector 62, so that such edge is trapped between such member 
68 and the beveled edge of the opening 64. 
A secondary convex reflector 70 is supported generally at the focuss of the 
primary concave reflector 62. Reflector 70 includes a convex face 72 and 
an axial shaft 74 which threads into a cylindrical block 76. Cylindrical 
block 76 is located inside a support ring 78 and is secured thereto by a 
plurality of radial screws 80. The screws 80 are threaded in and out as 
necessary for the purpose of positioning the centerline axis of block 76, 
shaft 74 and the reflector 70 on the centerline axis of the reflector 62. 
The reflector 70 is rotated like a bolt for the purpose of threading shaft 
74 into and out from the block 76 until the mirror surface 72 is properly 
located relative to the focus of reflector 62. Support ring 78 may be 
supported by a plurality of support arms 82 which extend between it and 
securement points on the front wall 66 of frame 52. 
A housing for a black body, a third reflector and an array of photovoltaic 
cells is mounted in the space between the two reflectors 62, 72. This was 
done so that it would not be necessary to cut an opening at the apex of 
reflector 62. Housing 84, may include a plurality of mounting arms which 
extend forwardly from a peripheral portion thereof and extend into sockets 
88 which are carried by a pair of members 90 which span across the face of 
reflector 62. The members 86 are moved endwise through the sockets until 
the housing 84 is properly positioned and then they are secured in place. 
The members 90 are movable both endwise and transversely for the purpose 
of making the axis of housing 84 coincide with the axes of reflectors 62 
and 72. Clamps 92 are used for holding the ends of members 90 in proper 
position. 
Referring to FIG. 14, the housing 84 includes a planar forward wall 94 in 
which a central window 96 has been formed. Ten photovoltaic cells 98 are 
mounted on the inner surface of wall 94, about the window. The portion of 
the wall 94 not occupied by the photovoltaic cells 98 is covered by a 
polished aluminum plate 100 constituting a reflector for that portion of 
the non-absorbed photon energy which does not fall on the cells. 
Additional reflector means, for reflecting back the non-absorbed wave 
length which passes through the cells, is carried by the wall means at a 
location behind the cells. By way of non-limitative example, the 
additional reflector means may be a reflective coating applied to the back 
sides of the cells. A concave reflector 102, constituting the reflector 
portion of a sealed beam headlight, is secured to the wall 94, with its 
mirrored surface directed toward the solar cells 98 and the polished 
aluminum reflector 100. Wall 94 is formed to include a coolant jacket 
through which a coolant is circulated, for removing heat. 
The black body 104 in this test facility is a three-quarter inch diameter, 
one-sixteenth inch thick carbon disc. It is located generally at the focus 
of the reflector 102. 
In operation, the test installation is faced towards the sun and the frame 
52 is tilted until the front wall 66 is substantially perpendicular to the 
sun's rays. The sun's rays which fall on reflector 62 are concentrated 
thereby onto the convex reflector 72 which in turn redirects such rays 
through the window 96 and concentrates them onto the black body 104. The 
sun's rays heat the black body 104 and it becomes a radiating body. A 
portion of the radiated energy falls directly on the photovoltaic cells. A 
second portion is directed towards the near surface of the reflector and 
is reflected by it to the photovoltaic cells. A portion of the photon 
energy absorbed by the photovoltaic cells is converted to electrical 
energy and the remaining absorbed energy heats the cells and the wall 94 
and is removed by both radiation and a coolant fluid that is circulated 
through the coolant jacket. The unabsorbed photon energy is reflected by 
the mirror surface 102 around the cells and by the mirror surfaces behind 
the cells back to the black body, either directly or indirectly via a 
reflection from the mirrored surface.