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BACKGROUND OF THE INVENTION
1. Field of the Invention:
This invention relates to generation of electrical energy from solar cells
and more particularly to a method and apparatus for enhancing the
efficiency of energy production to render solar cells competitive with
more conventional sources of power.
2. Description of the Prior Art:
In an article titled "Design Considerations for a 50-watt Photovoltaic
Power System Using Concentrated Solar Energy" by Beckman et al., published
in Solar Energy, Volume 10, No. 3, 1966 at Page 132 et seq., there is
disclosed a solar cell array on which solar energy is concentrated and
wherein the cells are supported on a cooling unit which is part of a
forced flow water system that has a heat exchanger remote from the solar
cells. The device disclosed in the article operates by transferring heat
out of the system simultaneously with the heat input from the solar energy
collector and thus requires substantial pumping and heat exchanging
capacity.
An article titled "Performance of Silicon Solar Cells at High Levels of
Solar Radiation" by Pfeiffer et al., Transactions of the ASME, January,
1962, Page 33 et seq., describes an experiment in which solar cells are
placed in a waterproof capsule that has a mylar window through which solar
energy is introduced. The water is circulated through the system at a
substantial rate to maintain a relatively low cell temperature.
An article titled "Use of Concentrated Sunlight with Solar Cells for
Terrestrial Applications" by Ralph, Solar Energy, Volume 10, No. 2, 1966
Page 67 et seq., discloses a solar concentrator for solar cells operating
without auxilliary cooling equipment to offset temperature rise in the
solar cells. He states that "Forced-air or water-cooling equipment could
be used to offset this temperature rise; however, such equipment is
costly, complicated, and would consume power, . . ."
The following U.S. Pat. together with other Patents in Class 136-89
disclose solar energy concentrators: Nos. 588,177; 3,232,795; 3,279,457;
3,350,234; 3,376,165; 3,419,434; and 3,427,200.
SUMMARY OF THE INVENTION
Contrary to known prior art procedures for enhancing the efficiency or
power output of solar cells, the present invention employs a substantial
quantity of heat absorbing material, such as water, to absorb heat energy
during the 8 hours or so of intense solar energy in order to limit the
temperature rise of the solar cells. The stored heat energy is then
transferred from the heat energy absorbing medium during the balance of
the diurnal period. Accordingly, the first cost and recurring operating
cost of the equipment are minimized so that improved efficiencies are
achieveable. Efficiencies can be improved to a degree that solar cells
used in the apparatus and method of the present invention can compete with
other sources of electrical power.
An object of the invention is to provide an improved solar photovoltaic
power supply which concentrates sunlight on a solar cell array to increase
the power output of the array while maintaining the cells at a relatively
low temperature for high conversion efficiency. This object is achieved by
operating the cells in a quantity of heat absorbing medium, such as water,
which quantity is large enough to absorb sufficient heat energy to limit
the temperature rise during daylight hours. During the balance of the
diurnal cycle when there is no sunlight, the heat energy is transferred to
the relatively colder environment so as to operate the cells at high
efficiency without expending power for pumping the medium through a
conventional heat exchange system.
Another object is to provide a very economical source of electric power by
increasing the power that a solar cell array can produce by orders of
magnitude, thus reducing the cost of power so produced to a point where it
becomes competitive with other sources of energy. This object is achieved
in part because the heat energy absorbing medium, water, is universally
available and is exploited in a relatively simple inexpensive structure
provided according to the invention.
A futher object is to provide improved efficiency of power generation from
solar cells by providing a system that exploits temperature conditions
throughout the diurnal cycle so that heat is absorbed in the medium during
daylight hours and is released more efficiently during cooler periods in
the diurnal cycle.
Still another object of the invention is to provide an improved solar
photovoltaic power supply which can operate without attention and which
requires only occasional maintenance such as cleaning of the reflective
surface. The present invention achieves this object by providing apparatus
that has a minimum number of moving parts so that breakdowns due to wear
are minimized or eliminated.
Yet another object of the invention is to provide a method for operating a
solar cell which method continues throughout the diurnal cycle.
Performance of the method permits elimination or reduction of power
consuming and wear producing parts and mechanisms.
The foregoing together with other objects, features and advantages will be
more apparent after referring to the following specification and the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plot of cell temperature versus power output for a typical
solar cell.
FIG. 2 is a cross sectional elevation view of an apparatus designed
according to the present invention.
FIG. 3 is a view at enlarged scale of the solar cell array in the apparatus
in FIG. 2.
FIG. 4 is a cross sectional view taken along line 4--4 of FIG. 3.
FIG. 5 is a plot of temperature versus time of day for illustrating an
important characteristic of apparatus according to the invention.
FIG. 6 is a cross sectional elevation view of an alternate embodiment of
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring more particularly to drawings reference numeral 12 indicates the
power output of a commercially available solar cell in the temperature
range of -20.degree. C to +80.degree. C, temperature being plotted on the
abscissa. For the purposes of illustration, 100% output is assumed to
occur at 30.degree. C, the output being plotted on the ordinate. As can be
seen from curve 12 in FIG. 1, the output of the solar cell decreases
substantially as the temperature of the cell increases, a phenomenon well
known to those skilled in the art. Concentrating solar energy on a solar
cell increases the output of the solar cell but at the same time raises
the temperature of the solar cell so that the efficiency of power output,
without providing for cooling of the cell, is substantially reduced. The
present invention achieves both solar concentration and limitation of
temperature rise so as to enhance substantially the output of the solar
cell.
Referring to FIG. 2, a solar cell array identified generally at 14 is
mounted in alignment with an opening 16 formed in a parabolic reflector
wall 18 which constitutes a part of a Cassegrain concentrating system. The
system also includes a secondary hyperbolic reflector 20 which is
supported in fixed space relation to parabolic reflector wall 18 by
radially extending struts 22. The Cassegrain concentrator, which is
disclosed in the U.S. Pat. No. 2,985,783, for example, is not per se a
part of the invention and exemplifies any suitable structure for
concentrating solar energy onto a relatively small area. Suffice it to
say, solar energy traversing a path S is reflected by the concave surface
of parabolic wall 18 to the convex surface of reflector 20 and thence
through opening 16 to the solar cell array 14.
Spanning opening 16 in parabolic wall 18 is a window 24 that can be glass,
Plexiglass or like material that is solar energy previous - liquid
impervious. At the periphery of parabolic wall 18 and rearward of the
reflective surface thereof is an impervious cylindric wall 26. The
cylindric wall terminates rearward of wall 18 where it is spanned by an
impervious rear wall 28 so as to form a watertight reservoir 30. The
reservoir is bounded by parabolic wall 18, window 24, cylindric wall 26
and rear wall 28. In the embodiment of FIG. 2, rear wall 28 is of
parabolic shape; such shape improves the balance or symmetry of the
structure but is not critical.
Solar array 14 is supported within chamber 30 by means of brackets 32; the
solar cell array is supported in alignment with window 24 and spaced
therefrom so that the image of the solar rays emanating from secondary
reflector 20 is focused on the array.
Solar cell array 14 can be formed, for example, of seven identical solar
cells 34 which as shown in FIG. 3 are hexagonal shaped so as to afford
closely spaced mounting thereof. The solar cells are preferably about 2
inches in maximum dimension to facilitate construction of the same from a
standard two inch boule of crystal material. The shape of the cells and
their number is not critical, but will be determined from cost and
convenience. The cells will have conducting grids on their active
surfaces, which is known to present art and practice, but which is
especially important in this device to minimize series resistance at high
currents which would reduce efficiency. The spacing between the solar cell
array and the window is approximately 1/8 inch to 1 inch to allow
circulation of the coolant medium but to avoid substantial absorption of
the solar radiation by the coolant medium. The solar cells are adhesively
secured to an insulative ring 36 which is supported at the inner ends of
brackets 32. The solar cells are connected in series, by conductors not
shown, and because each solar cell produces an output voltage of about 0.4
volts, the voltage output of the series combination in the example
considered is about 2.8 volts. The output voltage of the series
combination is connected via conductors 38 to a pair of output terminals
40 which extend through wall 28 in a watertight mount of conventional
form.
There is an inlet opening to reservoir 30 such as a valved opening 42 in
cylindric wall 26. Valved opening 42 permits reservoir 30 to be filled
with a suitable non-conductive heat absorbing medium such as water. As
will be discussed in more detail hereinbelow, the volume of reservoir 30
is sufficient that the temperature rise of the heat absorbing medium and
of solar cells 34 is confined within a range of efficient operation of the
cells. For more efficiently transferring the heat energy stored in the
medium in chamber 30 there is a plurality of radially extending cooling
fins 44 mounted on the exterior of cylindric wall 26.
A support structure 46 is secured to rear wall 28. Support structure 46 has
a lower arcuate surface 48 which forms one race of a bearing having
rollers, schematically shown at 50, and a lower race 52. Lower race 52 is
rigid with an azimuth ring 54, the lower surface of which forms a race for
a bearing that includes balls 56 and a rigid base ring 58. Conventional
means schematically shown at 59E and 59A are provided for positioning the
elevation and azimuth of the structure so that it is pointed toward the
sun during daylight periods in order that solar energy is maximally
concentrated through window 24 onto solar cell array 14.
In order to achieve the high efficiency afforded by the present invention,
reservoir 30 is of a size to contain a quantity of heat absorbing medium
sufficient to absorb heat energy produced during the solar energy
producing portion of the diurnal period and to limit the temperature rise
of solar cells 34. The heat energy absorbed by the medium is transferred
to the atmosphere at a more advantageous time i.e. during a portion of the
diurnal period when no sunlight is present and consequently when the
temperature of the environment is lower. Referring to FIG. 5, wherein the
time during a diurnal period is plotted on the abscissa and the ambient
temperature in degrees centigrade is plotted on the ordinate, curve 60
indicates the air temperature variation in a typical environment. In the
example plotted in FIG. 5, the temperature varies from a high of about
30.degree. C to a low of about 11.degree. C. The temperature of the medium
in reservoir 30 is indicated by curve 62 in FIG. 5 and is assumed to be
T.degree. above the air temperature. The difference between the maximum
heat energy absorbing medium temperature and the minimum temperature is
identified in FIG. 5 as .DELTA.t. Employing known principles of
thermodynamics the following formula is derived:
##EQU1##
wherein E is the electrical energy produced by the cell (kilowatt hours),
.DELTA.t is the increment of temperature rise of the water coolant and the
cells above the starting temperature (.degree. C), n.sub.p is the
electrical conversion efficiency of the solar cell, n.sub.c is the optical
conversion efficiency of the concentrator system which focuses sunlight on
the cell, C is the heat capacity of the water (in kilocalories per
kilogram .degree. C= calories per gram .degree. C), J is the mechanical
equivalent of heat (kilowatt hours per kilocalorie), and W is the weight
of coolant in kilograms. Solving the above equation for W produces the
following:
##EQU2##
W indicating the weight of water that will absorb sufficient heat energy
to limit .DELTA.t to a prescribed amount. In one structure designed to
practice the present invention, the parameters of the above equation have
magnitudes listed below:
E=(0.14 kw) (8 hrs) = 1.12 kwhrs
n.sub.p = 10% = .1
n.sub.c = 50% = .5
C = 1 kcal/kgm for water
.DELTA.t = 20.degree. C
j = 4.186 joules/cal
= 1.163 .times. 10.sup.-.sup.3 kwhrs/kcal
Solving the above equation for W produces W=963 kilograms or 2,119 pounds
of water. With such quantity of water in the example represented in FIGS.
1-5 the maximum temperature of the solar cells 34 is 40.degree., a
temperature at which the efficiency of the cells is in excess of 90%.
At the termination of the solar energy producing portion of the diurnal
period, assumed to occur around 4:00 p.m. in the example of FIG. 5, the
air temperature begins to decrease. At a rate depending on the exterior
surface area of the walls defining reservoir 30, the temperature of the
medium correspondingly decreases so that by the time solar energy
commences again, the temperature of the heat energy absorbing medium is
lowered whereby another cycle of efficient operation can be acheived. Of
course it will be obvious from the foregoing that the cooling of the
medium occurs without the expenditure of power since the cooling is
deferred until the non-solar energy producing portion of the diurnal
period.
The area of the walls of the chamber together with the area of fins 44 to
achieve the mode of operation shown in FIG. 5 can be derived from the
following equation and the following parameters.
##EQU3##
In one structure of the form shown in FIG. 2 and designed according to the
present invention, the diameter of cylindric wall 26 is about 61/2 feet,
the depth about 1 foot, with 12 fins 3 inches wide around the
circumference, a size sufficient to afford a 17.6 square meter area for
transferring heat. The value of k used in the example is an average figure
appropriate to this example, but varies somewhat for different surface
orientations.
The operation of the apparatus of FIG. 2 will now be summarized in
conjunction with graphs of FIGS. 1 and 5. After the structure shown in
FIG. 2 is installed at a suitable site, reservoir 30 is filled with water
through valved opening 42. Thereafter the structure is caused to follow
the sun during the portion of the diurnal period when the sun is visible.
Movement on the bearing structure together with the reflector composed of
parabolic surface 18 and parabolic surface 20 assure impingement of the
maximum amount of solar energy through window 24 onto solar cell array 14.
The power required for positioning means 59A and 59E is taken from the
solar cells, but is only about one-thousandth horsepowers because the
structure is well balanced and the bearings are designed to afford minimal
friction. As the temperature of the solar cells 34 in array 14 rises,
there is a corresponding rise in temperature of the water in reservoir 30.
Because the solar cell array is spaced from window 24, water entirely
surrounds the array. The local high temperature region in reservoir 30
induces circulation within the reservoir so that the medium, i.e. water,
within the reservoir is uniformly heated. Because of the large quantity of
the medium the temperature rise is limited to about 20.degree. C so that
the efficiency of the solar cells is maintained. The electric power output
on terminals 40 can be connected to load and/or to storage batteries as is
desired. Because of the substantial quantity of water within reservoir 30
and the fact that the water circulates without energy consumption,
maximization of the power output is achieved.
At the end of the solar energy producing portion of the diurnal period, the
ambient temperature begins to decrease as shown in FIG. 5. Because of the
substantial surface area of the walls of reservoir 30 and fins 44 the heat
energy stored in the medium within the reservoir is transferred to the
surrounding air so that when the next solar energy producing portion of
the diurnal cycle occurs, the solar cells are at a relatively low
temperature for efficient energy generation. Because of the great quantity
of water in the reservoir, the normal cooling during dark portions of the
diurnal cycle is exploited to achieve maximum utilization of the energy
produced by the solar cells.
An alternate form of the apparatus of the invention is shown in FIG. 6.
Because the embodiment in FIG. 6 has many elements corresponding in
structure and function to the elements of the embodiment in FIG. 2, the
same reference numerals with the addition of a prime are in part employed
in FIG. 6. That is to say, there is a main parabolic wall 18' and a
secondary parabolic reflector 20' supported in spaced relation to the main
wall by struts 22'. Centrally of parabolic wall 18' is an opening 16' in
which is mounted a solar energy pervious window 24'. Supported behind
window 24' by brackets 32' is a solar cell array 14' which is
substantially identical to array 14 described in more detail in reference
to FIGS. 3 and 4. Behind parabolic wall 18' and surrounding photo cell
array 14' is a reservoir defined by a cylindric wall 26' and a rear wall
28'. Conductors from the photo cell array extend through rear wall 28' at
a waterproof joint to output terminals 40' to afford electrical connection
with external circuitry.
The volume of reservoir 30' is substantially less than reservoir 30 in FIG.
2 in consequence of which the structure of FIG. 6, even when filled with
water is lighter weight. Accordingly a more simplified base 64 is provided
for supporting the structure. Base 64 includes suitable means for
maintaining the concentrator pointed at the sun during solar energy
producing portions of the diurnal period. Communicating with reservoir 30'
is an outlet hose 66 and an inlet hose 68. The hoses communicate with a
storage chamber 70, which has fins 72 on the exterior thereof, and in
order to distribute the absorbed heat energy throughout the quantity of
water contained in reservoir 30' and chamber 70, there is a low power
circulator 73. The combined volume of reservoir 30' and chamber 70 is the
same as the volume of reservoir 30 described hereinabove. Consequently,
there is a sufficient quantity of heat energy absorbing medium to absorb
the heat generated during the solar energy producing portion of the
diurnal period to limit the temperature rise of solar cells. Circulator
73, because it only circulates the water to achieve temperature
equilibrium within reservoir 30' and chamber 70 consumes very little power
as compared with the power required to drive the high speed pump employed
in the Beckman publication cited hereinabove. Moreover, because the weight
of the water in reservoir 30' is less than the weight of the water in
reservoir 30, the positioning apparatus in base 64 of the embodiment of
FIG. 6 consumes less power so that the system of FIG. 6 affords
substantially the same efficiency as the system of FIG. 2.
The operation of the embodiment of FIG. 6 is substantially identical to
that referred to above. As the structure tracks the sun, energy is
produced by the solar cells and the temperature rise of the solar cells is
limited to about 20.degree. C because of the heat energy absorbed in the
water disposed in reservoir 30' and chamber 70. Circulator 73 slowly moves
the water through the system so that the heat energy absorbed in the
relatively warm water in reservoir 30' is distributed to the water in
chamber 70. After termination of the portion of the diurnal period during
which solar energy is produced, the absorbed heat is transferred to the
relatively cooler atmosphere, fins 72 contributing to such cooling. In
addition, the exterior surface areas of reservoir 30" contribute to heat
energy transfer.
Thus it will be seen that the present invention provides an apparatus for
producing electric power which is efficient and inexpensive in that it
employs a few solar cells with concentrated energy directed thereon and
which consumes little or no power for limiting the temperature rise of the
cells. Because the method is operative throughout the entire diurnal
period, the normal atmosphere cooling during nightime is exploited to
optimize energy generation. Moreover, because the apparatus is designed to
employ water as the heat energy absorbing medium, the apparatus is
relatively light in weight during transport and installation at a site.
When installation has been completed, water from a local source is
supplied to the system and operation proceeds as described above.
To underscore the important advance represented by the present invention it
is pointed out that, based on costs applicable at the time of filing the
application, for a plurality of systems that will supply 1 kilowatt
continuous is about $2,500 to $3,000; this amount is approximately equal
to the capitalized cost that public utilities require to deliver energy to
a home. As a basis of comparison with current art in photovoltaic power
supplies, the previously cited Ralph reference quotes $20 per watt or more
than $60,000 to supply 1 kilowatt continuous by the method proposed
therein. In addition to the favorable cost comparisons with conventional
sources of energy, the present invention produces no pollutants and avoids
consumption of limited natural resources. Although several embodiments
have been shown and described, it will be obvious that other adaptations
and modifications can be made without departing from the true spirit and
scope of the invention.
* * * * *
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