Solar array output regulator using variable light transmission

A solar cell voltage regulating arrangement which is particularly advantageous for use in spacecraft includes a light valve such as a liquid crystal cell disposed before the solar cell, to thereby controllably vary the light transmission to the solar cell. A control arrangement varies the transmission of the liquid crystal to control the output voltage.

BACKGROUND OF THE INVENTION 
This invention relates to a control apparatus for the output voltage or 
current of a solar cell or array of solar cells, and particularly relates 
to those in which variable light transmissive devices, such as liquid 
crystal devices, are used as control regulators. 
Solar cells and solar arrays are in widespread use for generation of 
electrical power, especially in remote or inaccessible locations. In 
particular, solar cell arrays are used to provide electrical power on 
spacecraft. FIG. 1 illustrates, in highly simplified form, a prior art 
spacecraft 10 including a body 12 and a pair of solar panels or solar cell 
arrays 14a, 14b mounted on corresponding booms 16a, 1b. Booms 16a and 16b 
may be arranged on bearings, such as bearing 18b, to allow for rotation of 
the booms for positioning the solar cell arrays. 
FIG. 2 is a simplified block diagram of a prior art arrangement for 
controlling the output voltage of the solar arrays of a spacecraft. In 
FIG. 2, the solar panels 14a and 14b of FIG. 1 are illustrated together as 
a single block designated 14. When sunlight falls on solar panel 14 of 
FIG. 2, an electrical voltage is generated between terminals 20 and 22, 
which voltage can be used to energize a utilization apparatus, illustrated 
as a resistor 24. Electrical current flows under the impetus of the solar 
panel voltage from terminal 20, through a bus conductor 26 and slip rings 
28 to load 24. Slip rings 28 are provided to allow a continuous electrical 
path notwithstanding the rotation of booms 16 relative to body 12 of the 
spacecraft of FIG. 1. The return path for current from load 24 to solar 
panel 14 is by way of another slip ring 28b and, as is well known to those 
skilled in the art, another conductor, illustrated in FIG. 2 as the common 
ground on the spacecraft. 
In FIG. 2a, a resistor 30 and a zener or avalanche diode 32 form a voltage 
divider 33 which is coupled between bus 26 and ground, for generating 
reference voltage at node 34. A controller illustrated as a block 36 
includes an amplifier (not illustrated) which compares a sample of the 
voltage from bus 26 with the reference voltage at node 34, and produces an 
error signal which is applied by way of a conductor 90 to the control 
input terminal of a shunt load, illustrated as a block 38. Shunt load 38 
is coupled by a slip ring 28c across a portion of the solar array. Shunt 
load 38 responds to the error voltage by varying its conduction, to 
thereby interact with the current producing ability of solar panel 14 in a 
manner which reduces the voltage on bus 26 toward the value established by 
control block 36. Such schemes are notoriously well known and are but one 
form of a degenerative or negative feedback control system. 
FIG. 2b illustrates a portion of solar panel 14. In FIG. 2b, solar panel 14 
includes a plurality of arrayed solar cells 40a, 40b, 40c. . . , each of 
which has an upper surface 41 and a lower surface. Upper surface 41 
includes a light input port or aperture, which is merely a light-sensitive 
portion of surface 41. When light, illustrated by a photon symbol 46, 
"enters" the aperture, a voltage is generated between upper surface 41 and 
the lower surface of each solar cell 40. Conductors (not illustrated) 
associated with the upper and lower surfaces of each solar cell provide 
ohmic contact between interconnection conductors 42 and the solar cells. 
Each conductor 42 of FIG. 2b connects the top surface of one solar cell to 
the bottom surface of the adjoining solar cell, thereby forming an 
electrical series connection by which the individual voltages produced by 
each cell can be added to produce a larger output voltage. Parallel 
connections of a number of such series strings may be made to increase the 
current output capability of the solar panel. Upper surface 41 of each 
solar cell 40 is protected by a coverglass 44, as for example upper 
surface 41 of solar cell 40b is protected by a coverglass 44b. The 
coverglass may itself have a conductive coating on its outer surface (the 
surface remote from the solar cell) to reduce arc discharges. 
A disadvantage of the scheme of FIG. 2 lies in the thermal problems of 
spacecraft and the high power dissipation of shunt load 38 of FIG. 2 
during operation. In a particular spacecraft under design, a solar array 
is used in which 90 solar cells are connected in series, each of which 
produces about 0.5 volts at maximum power. A sufficient number of cells 
are parallelled to produce an output current of 40 amperes (A), and shunt 
regulation is used in conjunction with 58 of the 90 series-connected cells 
to maintain a desired bus voltage of 28 volts. The 32 series cells which 
are not shunt regulated produce a maximum of 16 volts, whereupon the 
voltage across the 58 cells being regulated must be reduced from as much 
as 29 volts to 12 volts to meet the desired bus voltage. Assuming that the 
maximum current being shunted is 30 A, the total shunt power is 30 A 
multiplied by 12 volts, which equals 360 watts. In the above-mentioned 
spacecraft design, dissipation of this amount of heat requires that the 
shunt circuits be mounted on heat sink brackets associated with the solar 
array. The shunt elements and the associated brackets in this particular 
instance weigh approximately 22 lbs. It is desirable to reduce the high 
thermal dissipation in a shunt load, and to eliminate the weight 
associated with such a control scheme. 
SUMMARY OF THE INVENTION 
A light-operated electric generator includes a light admitting aperture and 
terminals at which a direct voltage is generated when light enters the 
aperture. A suitable generator is a silicon solar cell or an array of 
interconnected solar cells. A source of reference voltage is coupled to a 
first terminal of an amplifier, and the other terminal of the amplifier is 
coupled to a sample of the voltage produced by the generator. The 
amplifier generates an error signal representing the difference between 
the direct voltage generated by the generator and the desired voltage. A 
controllable light transmission arrangement is located before the light 
admitting aperture or apertures of the generator, to thereby control all 
or a portion of the light entering the aperture. The controllable light 
transmitting arrangement includes electrical energization terminals, and 
is adapted for allowing light to enter the light admitting aperture of the 
generator in a first electrical energization state, and for preventing 
light from entering the light admitting aperture of the generator in a 
second electrical energization state. A feedback coupling arrangement is 
connected between the amplifier output and the electrical energization 
terminals of the controllable light transmitting arrangement, for 
translating the error signal to the variable light transmitting device in 
a degenerative or negative feedback manner, whereby a closed feedback loop 
is formed which tends to maintain the direct voltage near the desired 
voltage. In a particular embodiment of the invention, the controllable 
light transmitting arrangement is a plurality of liquid crystal cells 
regulating a like plurality of solar cells. In a particular embodiment, a 
spacecraft includes a plurality of such solar cells and a liquid crystal 
array associated with a solar panel, and the error signal is converted 
into an alternating square wave of variable amplitude for energizing the 
liquid crystal cells.

DESCRIPTION OF THE INVENTION 
FIG. 3a is a simplified block diagram of a control system in accordance 
with the invention. In FIG. 3a, a solar panel 314 is made up of 
interconnected solar cells 340 as illustrated in FIG. 3b. In FIG. 3b, a 
plurality of solar cells 340a, 340b, 340c . . . are arrayed and 
electrically connected in series by electrical conductors 342b, 342c . . . 
in a manner similar to that described in conjunction with FIG. 2b. In FIG. 
3b, each solar cell 340 has a liquid crystal cell 348 located before or in 
front of its light aperture 341, instead of a coverglass as in FIG. 2b. 
For example, a liquid crystal cell 348c is located above surface 341 of 
solar cell 340c. The physical mounting arrangements are not illustrated. 
Ideally, the solar cell apertures and the light-controlling paths of the 
liquid crystal cells are in registry, between light 346 and light aperture 
341, and each light-controlling path substantially covers the associated 
light aperture. Each liquid crystal cell includes a pair 350x, 352x of 
electrodes, where x is an index letter representing the particular cell. 
Control voltages may be applied to electrodes 350, 352 of each solar cell 
for controlling the light transmission therethrough. A pair of bus 
connectors 350, 352 are connected to electrodes 350x, 352x, respectively, 
of each liquid crystal cell, so that all the liquid crystal cells are 
electrically driven in parallel, or energized by the same voltage at the 
same time. 
In FIG. 3a, voltage produced by solar panel 314 is applied by way of a 
conductor 370, slip ring 372, the ground connection and bus 326, slip 
rings 328 and connector 327 to a load or utilization apparatus, 
illustrated as a resistor 324. A resistor 330 in conjunction with a zener 
or avalanche diode 332 forms a voltage divider 333 coupled between bus 326 
and ground, which produces a reference voltage at a node 334. The 
reference voltage is applied to a control circuit illustrated as a block 
336, which compares the reference voltage with a sample of the voltage on 
bus 326, and generates an error signal representative of the difference 
therebetween on a conductor 337. The error signal varies in amplitude in 
response to deviation of the bus voltage away from the desired value. The 
error signal is applied to a DC-to-AC square wave inverter illustrated as 
a block 338. Inverter 338 "chops" the error signal to produce a variable 
amplitude symmetrical alternating square wave on a conductor 390. A light 
valve illustrated as 360 is illustrated as being juxtaposed with solar 
panel 314, in a position between the solar panel and light illustrated by 
a photon symbol 362. Light valve 360 includes an array of liquid crystal 
cells, each located before or in front of the light admitting aperture of 
the corresponding solar cell, as described in conjunction with FIG. 3b. 
Many types of liquid crystal cells are known. Basic liquid crystal cells, 
however, can be said to have two conditions, depending upon their 
electrical energization. In a first state, they are transparent, thereby 
allowing light to pass therethrough. This is a transmissive condition. In 
a second state, they do not allow light to pass, but instead either absorb 
or reflect the light. This is a non-transmissive or light-blocking 
condition. The transition between the transmissive and non-transmissive 
states is gradual, so a continuous variation of light transmission can be 
obtained in response to the control signal. 
In operation of the arrangement of FIG. 3a, control block 336 generates an 
error voltage representative of the difference between the desired voltage 
on bus 326 and the actual voltage. The error signal, if not within the 
correct voltage range for driving electrodes 350 and 352 of light valve 
360, can be amplified or attenuated to the correct value and applied 
directly to the liquid crystal, without conversion to a square wave. 
However, at the present state of the art, the flow of direct current 
through the liquid crystal material tends to degrade the material, and 
conversion to a "square" wave alternating voltage is desirable. The 
peak-to-peak amplitude of the square wave varies in response to the error 
signal. The variable amplitude square wave is applied from inverter 338 to 
light valve 360 poled to reduce the transmission of light therethrough 
with increases in the bus voltage, to thereby form a degenerative feedback 
loop by which the voltage is stabilized. The poling of the control signal 
causes the p-p amplitude of the square wave to either increase or decrease 
in response to increases in bus voltage, as required by the particular 
control characteristics of the light valve. 
FIG. 4 illustrates details of portions of the arrangement of FIG. 3a. In 
FIG. 4, elements corresponding to those of FIG. 3a are designated by the 
same reference numerals. In FIG. 4, control circuit 336 includes a voltage 
divider 410 with resistors 412 and 414 coupled between bus 326 and ground. 
A sample of the bus voltage appears at node 411. A first high gain 
amplifier 416 has its inverting (-) input connected to node 411 and its 
non-inverting (+) input connected to node 334 of voltage divider 333, and 
produces an error signal representative of the difference between the 
reference voltage and the sample of the bus voltage. A voltage-following 
differential power amplifier 420 connected for noninverting operation 
amplifies the error signal, and boosts the current capability of amplifier 
416, and a corresponding amplifier 422 connected for inverting operation 
amplifies the error signal with the same gain as the noninverting 
amplifier 420 and also boosts the output capability of amplifier 416, to 
produce at their outputs a pair of error signals of mutually opposite 
polarity and equal amplitude relative to ground. Mutually 
opposite-polarity portions of error voltage are applied over conductors 
337a and 337b to the emitters of bipolar switching transistors 424, 426 
respectively. The bases of transistors 424 and 426 are coupled to the 
collectors of bipolar transistors 430 and 432 whose emitters are connected 
to ground. This arrangement allows a ground referenced square wave source 
428 to interface with the bases of transistors 430 and 432 to effect 
proper switching. Square waves produced by source 428 alternately drive 
one pair of transistors 424; 430 and 426; 432 into conduction and 
non-conduction, respectively, and then vice versa. When switch transistors 
424 and 430 conduct and switch transistors 426 and 432 do not conduct, the 
output voltage from power amplifier 420 is coupled to conductor 390, and 
when switch 426 and 432 conduct and 424 and 430 do not conduct, the 
equal-and-opposite voltage output of power amplifier 422 is coupled to 
conductor 390. Thus, a variable amplitude, symmetrical square wave is 
generated for application over conductor 390 to the liquid crystal array. 
At the present state of liquid crystal technology, the amount of light 
transmission can be varied by about 15/1 to 20/1, and the voltage required 
to effect this change is about 0 to 15 volts AC peak-to-peak. 
The power dissipated by a liquid crystal cell, when energized by an AC 
voltage, varies from one to 300 microwatts per square centimeter, 
depending upon the type of cell and its construction. Taking 150 
microwatts per centimeter as an average, and taking the array area to be 
in the vicinity of 108,000 square centimeters, the total power required to 
activate the liquid crystal array is about 16 watts. This is much less 
than the power dissipated by a shunt regulator, thereby easing the thermal 
management problem. Sixteen watts is easily dissipated to space directly 
from the liquid crystal cells. Also, there is no additional weight 
involved in using the liquid crystal cells because they replace the 
existing solar cell cover glass. This results, in the abovementioned 
example, in a 221b weight saving because of the elimination of the shunt 
circuits. Also, weight and complexity can be saved by the elimination of 
high current carrying slip rings for the shunt regulator connection. These 
are replaced by fewer low current sliprings for the liquid crystal control 
voltage. 
At the present state of the art, liquid crystal material tends to increase 
in viscosity as temperature decreases, and the response time within which 
the liquid crystal modules can change their orientation increases. This 
may tend to slow down the control loop. Temperatures of the solar panel 
can drop to as much as -110.degree. C. in the Earth's shadow. The response 
time of the loop may be very slow, or the liquid crystal material may 
freeze. However, since no light falls on the solar cell under these 
conditions, no control is needed. Heaters may be used to prevent damage to 
the liquid crystals under these conditions, if desirable. 
At high temperatures, the molecules of the liquid crystal material become 
disoriented from their aligned positions, which tends t reduce the 
transmission of light. Thus, light transmission, and therefore maximum 
output voltage, tends to be reduced even in the absence of control signal 
at high temperatures, which is ordinarily the condition in which maximum 
sunlight reaches the solar panel. This provides "fail safe" type of 
operation. 
Other embodiments of the invention will be apparent to those skilled in the 
art. For example, the liquid crystal cells may be connected in 
series-parallel as well as in parallel, to change the energizing voltage 
range.