Circuit for converting solar energy into AC power

A circuit for converting solar energy into ac power for supplementary household power has a number of solar photovoltaic cells connected in parallel in groups, with the various groups connected in series (30), and a bridge arrangement of four switching devices (31, 32, 33, 34) each operated to pass current in one direction, with the series-connected groups of cells (30) connected between positive and negative bridge terminals (1, 2), and means for connecting the bridge arms (3, 4) across the primary coils of a transformer (312), with the secondary coils thereof connected to the switching devices to control the phase of passed current, the output at the bridge arms (3, 4) being connectable to the household ac.

FIELD OF THE INVENTION 
The present invention relates to an adaptor for converting solar energy 
into ac to supplement household ac. 
DESCRIPTION OF THE PRIOR ART 
The conversion of solar energy into electricity via available technologies 
so far has not been economically feasible except in exceptional, limited, 
applications. The solid-state photovoltaic cells or solar cells, which are 
user-convenient, are used 1) more conventionally in conjunction with 
storage batteries and thus as a dc power supply at hand or for later use, 
and 2) in conjunction with an inverter circuit to provide ac power into 
the (household) power line. These and other applications have been well 
known to those with solar cell concerns. 
Beside the still rather high cost of the solar cells themselves the costs 
of the batteries and/or circuits required do make the system cost of the 
solar-to-electricity applications out of the consumers' reach. Typical 
costs for solar cells are still in $10 per kilowatt range. The batteries 
and circuits' costs generally are more than that of the solar cells. There 
have been many attempts to produce cells with higher efficiency and less 
expensive, and projection of cost around $5/KW has been made. 
The inverter generally is designed to operate starting with a dc voltage 
source. Thus the solar cell output is stored/regulated by a battery or at 
least a very large capacitor. An electronic chopper then provides the 
chopped dc to be stepped up to the desired voltage via a transformer or a 
series of voltage doublers. A pulse-shaping circuit then shapes the 
amplified chopped voltage into a desired sine wave, say a 220 volt ac. 
(The circuit portion, from the batteries to this ac output, is now 
commonly used as part of an Un-interrupted Power Supply or UPS. The latter 
includes an additional rectifying or converter section which converts 
household ac power into dc for charging the batteries). To provide power 
into the ac municipal power lines the sinusoidal wave then must be 
phase-synchronized with the power line such that the latter absorbs the ac 
power. The said sequence is depicted in FIG. 1. 
It is to be noted that the electrical energy from the solar cells must 
undergo many steps of transformation, and each step cannot have 100% 
efficiency. Thus the many steps involved not only incur higher cost but 
also result in poor efficiency. Some of the losses are in providing the 
energy necessary to effect the functioning of the chopper, amplifier, 
shaper and synchronizer circuits. 
Reference 1, a Canadian patent issued in 1977 with a "Japanese priority 
date" 1973, represents an earlier version of the UPS using SCR as chopping 
switches, time-controlled by a central logic circuit. Pulse shaping is 
accomplished using inductor-capacitor tuned loads, and amplification is 
via a transformer. To reduce harmonics and afford better phase control 
more sophisticated chopping and pulse-shaping techniques have been 
introduced, such as in References 2 and 3. Some circuits are very 
complicated but the essential functions as described in FIG. 1 remain the 
same or with only slight modifications. Our new invention to be described 
below is not intended to serve as a UPS, but rather as a supplemental ac 
power controlled by and fed into a "live" ac power line, employing solar 
power in the most efficient and least expensive way. 
SUMMARY OF THE INVENTION 
The present invention provides A circuit for converting solar energy into 
ac power supplementary to household ac power, comprising the combination 
of a plurality of solar photovoltaic cells chosen to be as nearly 
identical as possible means for connecting said cells in parallel in one 
or more groups of a certain number of cells each group having a group 
short-circuit current, such that when illuminated that group's 
short-circuit current exceeds a value of application interest; means for 
connecting said groups of parallel-connected cells in series of forming an 
ensemble such that the open-circuit voltage of the ensemble is 5-20% 
greater than the absolute value of a peak voltage of the household ac 
power; four units of switching devices each of which can be operated to 
pass a current in one directing and having a positive and negative 
electrode, the units being connected in a bridge configuration in which 
the positive electrodes of the two units are electrically connected at a 
second terminal, and in which one unit of said two units is connected to 
one unit of said other two units, through a third terminal, the other unit 
of said two units being connected to the other unit of said other two 
units through a fourth terminal; means for connecting the ensemble 
comprising the series connected groups of parallel solar cells to said 
bridge with the positive voltage terminal of the ensemble connected to the 
first bridge terminal and the negative voltage terminal of the ensemble to 
the second bridge terminal; an isolation transformer having a primary coil 
and a pair of input terminals and at least four low voltage secondary 
output coils which are insolated from one another and from the primary 
coil each having a pair of output terminals; means for connecting the 
third and fourth terminals of said bridge to the terminals of the primary 
coils of the transformer; means for connecting the same terminals of the 
primary coil of said isolation transformer to a plug which can be plugged 
into the ac power line; and means for connecting the output terminals of 
each secondary coil across a pair of control electrodes of the switching 
devices such that in use the voltages on the control electrodes of devices 
on the opposite arms of the bridge have the same ac phase as each other 
and a phase which is 180.degree. different from the ac phase on the 
control electrodes of the other two switching devices on adjacent arms of 
the bridge.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
Turning to the Figures, FIG. 1 represents the qualitative description of 
the state-of-the-art technique for using solar photovoltaic power as a 
power supplement to the ac power system and should be self-explanatory. 
FIG. 2 is a qualitative depiction of a photovoltaic cell consisting of a 
semiconductor cell body layer 103 designed according to the solar-cell 
state-of-the-art, with a patterned top metal electrode 101 and a back 
metal electrode 102. 
FIG. 3 is a schematic representation of several photovoltaic cells, having 
the top and bottom electrode labelled 1011 and 1021 for cell number 1, 
1012 and 1022 for cell number 2, . . . etc., connected in parallel to form 
a compound cell with electrodes 201, 202. 
FIG. 4 is a schematic representation of several compound cells of FIG. 3, 
having the top and bottom electrodes 2011 and 2021 for the first compound 
cell, 2012 and 2022 for the second compound cell, . . . etc., connected in 
series to form an ensemble with the electrodes 301, 302. 
FIG. 5 shows the current-voltage (I-V) characteristic of the ensemble 
described in FIG. 4 with I along the y-axis 24 and V along the x-axis 23 
and showing a characteristic short-circuit current at 22 and the 
open-circuit voltage at 21. 
FIG. 6 shows the schematic diagram of an embodiment in accordance with the 
invention, showing the "bridge" configuration 1-2-3-4 having four 
directional switching units 31, 32, 33 and 34 and a transformer 312 
connected such that the ac line power terminals 310, 311 can energize the 
transformer at 38 and 39, and energize the bridge at 3 and 4 while the 
solar cell ensemble described in FIGS. 2-4 is represented as a battery 30 
connected in series with a diode 314 at 35 such that the negative terminal 
of the battery is connected to the bridge terminal 2 and the negative 
terminal of the diode is connected to the bridge terminal 1. The battery 
and the diode may exchange places in the series connection between 
terminals 1 and 2 with no operational difference. 
FIG. 7 shows a schematic details of a directional switching unit comprising 
a silicon control rectifier (SCR) 315 and a coil 328 which is one of the 
secondary coils of the transformer 312 of FIG. 6, and said coil 328 is 
connected in parallel with the series combination of the diode 326 and 
resistor 327 at 318 and 322 while the negative terminal of 326 is 
connected to 327 at 320, to form a network connections across the gate 317 
and cathode 316 of the SCR with a resistor between 317 and 318, finally 
forming the whole switching unit having positive electrode 313 at the 
anode of the SCR and the negative terminal 314 at the tie point of 316, 
321 and 322. 
It is understood that there are 3 more such switching units similarly 
constructed as shown appropriately connected in FIG. 6, with the coils 
similar to 328 as secondary coils of 312 but their phases arranged such 
that 31 and 34 are operated "ON" when 32 and 33 are "OFF", and vice versa 
Alternatively, the switching devices may comprise field effect transistors 
(FET), bipolar transistors, insolated-gate bipolar transistors (IGBT), 
gate-turn-off thyristors (GTO) or MOS-controlled thyristors (MCT). 
FIG. 8 shows a schematic diagram of a possible alternate scheme of 
energizing the gate-cathode pair in lieu of the less complex 
diode-resistor network scheme for the same function depicted in FIG. 7, 
and it should be understood by those skilled in the art the additional 
merits therein. 
FIG. 9 shows the ac line voltage 410 as dashed curve, the voltage between 1 
and 2 of FIG. 6 as the piece-wise continuous curve 49, and the current 
waveform 41-42-43-44-45-46-47-48 between 3 and 4 of FIG. 6. 
Referring now to FIGS. 2 and 3 it is evident that in principle any 
requirement on the magnitude of current could be met by paralleling a 
sufficient number of cells into a compound cell. From FIGS. 4 and 5 it is 
also evident that in principle any open-circuit voltage 21 can be obtained 
using sufficient number of compound cells in series to form the ensemble, 
which can also supply the desired short-circuit current 22. It is arranged 
that the sum total of the open-circuit voltage is 5-20% greater than the 
absolute value of the peak voltage of the ac. power line. 
Furthermore, it is readily apparent to those skilled in the art that there 
are other series parallel combinations of the cells which can produce the 
same set of open circuit voltage and short circuit current. 
Reference is now made to FIGS. 6 and 7. Evidently the points 310, 38 and 3 
are electrically the same point. The gate-cathode biases of the switching 
units 31 and 34, although electrically isolated from each other, are both 
of the same phase as the ac voltage at the 310-38-3 common point. When 
this common point is positive (with respect to the other common point 
311-39-4) the switching units 31 and 34 are conducting and 32, 33 are 
blocking. Under this condition, and because the open-circuit voltage 21 of 
the ensemble 30 is larger than the voltage at the 310-38-3 common point, 
there is a current of a value I given by the value on the curve in FIG. 5 
at the instantaneous voltage value V of the common point flowing 
positively from 3 to 38 to 310 and negatively from 4 to 39 to 311. 
Conversely when the common point 310-38-3 is negative the switching units 
31 and 34 become blocking, while the units 32 and 33 are conducting, and 
the similar current I flows positively from 4 to 39 to 311 and negatively 
from 3 to 38 to 310. 
The voltage and current described above appear graphically as 410 and 
41-42-43-44-45-46-47-48 in FIG. 9. The secondary coils are connected so 
that the switching devices 31 and 34 have the same ac. phase, and 
similarly devices 32 and 33 have the same phase as each other. Devices 31 
and 36 have phases which differ by 180.degree. from the phase of devices 
32. The voltage difference between 3 and 4, indicated by 49, corresponds 
to said current at any instant according to the I-V characteristic in FIG. 
5. 
FIG. 8 shows a schematic circuit diagram of a possible alternate scheme of 
energizing the gate-cathode pairs of the switching units. The circuit 
could be tailored to be more effective by one skilled in the art of 
controlling the SCR behaviours. 
Finally it should be evident that the diode 314 in FIG. 6 serves to block 
any current from other parts of the circuit to flow into the solar cell 
ensemble 30. The STAC circuit in FIG. 6 therefore would only add ac power 
to the ac line when the cells are illuminated but will not absorb power 
from the line under insufficient illumination. 
As a discrete package the invented circuit, to be called "Solar to AC" or 
STAC adaptor, comprises the transformer with four secondaries, the four 
SCR "units" (each with the appropriate gate drive network and driven by 
the transformer secondary voltage with appropriate phase) and the (safety) 
series diode. The package has one two-prong (35, 2) terminal to connect to 
the solar cell "battery" and one two-(or three-) prong (310, 311) plug to 
plug into the ac "house line". The STAC adaptor is thus a very simple, 
inexpensive, safe and self-operating, and highly efficient. 
Inasmuch as the STAC current waveform resembles a "square" ac wave, the 
amplitude of the fundamental frequency wave, from Fourier analysis, is 
approximately 4/.pi. times the "square height". And because modern loads, 
such as computers, often convert the ac house-power into dc power supplies 
needed for the electronics, the square-wave nature of STAC actually is 
more effective in terms of ac energy conversion into dc. 
The STAC package can also be applied to any pair of ac lines and hence are 
suitable supplementary to the 3-phase power line as well (one STAC per 
pair of one phase) The STAC's may also be arranged to provide different 
amounts of supplemental current into different one-phase pairs of the 
three-phase lines, to meet the unequal demands of unbalanced loads often 
occurring in three-phase applications. 
While it is true that the STAC adaptor will not absorb appreciable power 
from the ac line, small losses in the gate drive transformer and the 
possible danger of atmospheric electromagnetic pulse as a safety measure 
the STAC adaptor should be disconnected from the ac power line during 
night time. A separate automated control circuit to effect the 
disconnection could be designed into the STAC adaptor, but this is not 
included as part of the present invention. 
From the description above it should be evident that the circuit differs 
from those in the state of the art in that it makes use of the 
current-source nature of the solar cell itself as the means to simplify 
the circuit and thus lowering the cost, and achieving the maximum 
efficiency. 
In order to minimize costs and maximize the efficiency of solar energy 
utilization as supplementary AC power source a new "Solar to AC" or STAC 
adaptor makes use of the current-source nature of the solar cell, the 
highly efficient switching capability of the switching device such as the 
silicon-controlled rectifier (SCR), and the AC line phase itself. The 
newly invented STAC adaptor circuit is simple, inexpensive, 
self-regulated, safe and efficient. 
While the invention has been particularly shown and described with 
reference to several preferred embodiments thereof, it will be understood 
by those skilled in the art that various changes in form and details may 
be made therein without departing from the spirit and scope of the 
invention. 
References 
1. Tamil, et al., Canadian Patent No. 1016998, issued 77/09/06, "Power 
Conversion Systems". 
2. Butler, D. M., U.S. Pat. No. 4,075,034, Feb. 21, 1978, "Solar 
Converter". 
3. Bobier, J. A., Brown, G. E., U.S. Pat. No. 4,742,291, May 3, 1988, 
"Interface Control for Storage Battery based alternate energy systems".