Device for converting solar energy

A device is described for converting solar energy into electric power for a load, which device is provided with photocells and electric accumulators. Switching means are provided which at least during starting of the load connect this load to the output terminals of the array of photocells so that during starting the current through the load is determined by the current supplied by the photocells. The device makes efficient use of the available solar radiation, is of simple design and is reliable.

The invention relates to a device for converting solar energy into electric 
energy for supplying a load which exhibits an in-rush effect. The device 
is provided with photocells for converting the solar energy into electric 
energy and with electric accumulators which can store the energy supplied 
by the photocells. 
Such a device is known from French Patent Specification No. 2,041,243. This 
Patent Specification describes an automatic system for adapting a d.c. 
generator with variable characteristics (photocells) to a load. 
In a device of the type mentioned in the preamble the accumulators are 
charged by the solar cells during the day. The energy thus stored is 
utilized for driving an electric motor, for example the motor of a water 
pump in very sunny and dry regions. The stored energy may be employed for 
numerous other purposes such as for lighting by means of incandescent 
lamps. 
In devices which employ solar energy an energy accumulator is necessary 
owing to the irregular character of the energy received from the sun. This 
radiation energy exhibits a daily cyle and furthermore depends on 
climatological conditions. In the case of solar cell arrays which are 
stationarily disposed or which have different orientations for different 
seasons, the radiation energy which is received depends on the inclination 
factor of the sun relative to the vertical of the arrays. The electrical 
energy supplied by a photoelectric generator thus exhibits variations as a 
function of time, which makes certain applications difficult or even 
impossible if no storage unit is provided for the energy which is 
produced. 
The electrical resistance of a d.c. motor which is used, for example, in a 
device in which solar energy is utilized for pumping water is kept as low 
as possible so as to minimize losses owing to the Joule effect and to 
achieve maximum efficiency. The accumulator will then supply a large 
in-rush current which has an adverse effect on the lifetime of certain 
elements of the device. 
In order for the motor to start smoothly known devices are equipped with 
current limiting circuits. The use of such circuits may present problems. 
Moreover, such circuits also consume power. 
It is an object of the present invention to provide a device of the type 
mentioned in the preamble which does not have the above-mentioned 
drawbacks, which is of simple design and is reliable, has a high 
efficiency and operates the load smoothly. 
The device according to the invention is therefore characterized in that 
the number of photocells and their arrangement is sufficient to provide a 
peak power and a peak photocell current which under optimum insolation is 
equal to or is slightly greater than the nominal power and the nominal 
current, respectively, of the load, and that the device comprises 
detection and switching means for connecting the load during the load 
starting period to the output of the array of photocells only. As a 
result, during starting the current through the load is determined only by 
the current supplied by the photocells. 
According to the invention use is made of the fact that the photocells can 
be employed as current sources. As long as the voltage across the 
photocells remains below a specific value, these cells supply a 
substantially constant current which is independent of the voltage across 
the cells. When the load, for example a motor, is switched on the 
photocells limit the current supplied to the load and when the motor 
reaches its nominal speed it is connected to the accumulators.

In the known device of FIG. 1a the load, which consists of a motor M, is 
connected directly to the terminals of the accumulators A.sub.1 and 
A.sub.2. Starting of the motor, which in the case where the device is 
utilized for pumping water out of a well, coincides with the start of a 
pumping cycle, is controlled by the state of charge of the accumulators, 
i.e. by the accumulator voltage Ua. The motor/pump assembly is started 
before the accumulator is overcharged (the upper value of Ua Uass). 
A pumping cycle is terminated when the voltage across the accumulators 
drops below a lower threshold voltage Uasi. The accumulator voltage is 
detected by the system S.sub.11, which system ensures that the limiting 
circuit L.sub.11 is rendered operative upon starting and is 
short-circuited after the in-rush starting period. 
The current limiting circuit L.sub.11 may consist of resistors with 
progressively decreasing values, an automatic rheostat, or a circuit with 
several parallel-connected power transistors, as shown in FIG. 1b. 
FIG. 1c represents the current Im through the motor, the accumulator charge 
Q = .intg. Ia dt (where Ia is the current through the accumulators), and 
the voltage Ua across the accumulators, all as a function of time. When 
the accumulator charge reaches the value Qs, i.e. when its voltage Ua 
reaches the value Uass, the motor is started. When the accumulator voltage 
has decreased to the lower level Us (i.e. Uasi in FIG. 1a) a pumping cycle 
is terminated. The circuit S11 detects when the charge Q.sub.s is reached 
and activates the current limiter L11 and with the switch contact open. 
The circuit S11 also short circuits current limiter L11 after the in-rush 
period by closing the switch contact in parallel therewith. If the 
accumulator voltage drops below the value Uasi, the circuit S11 opens the 
switch contact and makes the current limiter inoperative so that no 
current can flow thru it. 
The operation of the device of FIG. 1a as a function of time is illustrated 
in FIG. 2. In this Figure: 
the curve C.sub.1 represents the variation of the total current Ip supplied 
by the photocells BPX; 
C.sub.2 represents the charging periods of the accumulators, and 
C.sub.3 represents the pumping cycles .gamma..sub.1, .gamma..sub.2, 
.gamma..sub.3 etc. 
FIG. 2 clearly shows the intermittent character of the operation of said 
device. The peak power which is supplied by the photocells is smaller than 
the nominal power demand of the load, specifically if the load consists of 
a motor pump. 
The effective daily duration of intermittent pumping (and thus the daily 
output) is proportional to the number of photocell arrays. The pump which 
is energized by the device of FIG. 1a generally operates for a 
comparatively short time. In the case of intensive solar radiation the 
motor is started many times a day. The current limiting system L.sub.11 in 
FIG. 1a will consume power each time that the motor is started. This power 
consumption constitutes a pure loss. Moreover, the current limiting system 
becomes more intricate when higher reliability is required. 
It might be considered advantageous to dispense with the current limiting 
system. However, in that case the switching circuits, the load and the 
accumulators will be subjected to large in-rush currents. Furthermore, 
certain motor elements will be heated owing to the large in-rush currents 
each flowing time that the motor is started, which has an adverse affect 
on these elements. 
In accordance with the invention the load is connected to the terminals of 
the photocells only during the in-rush starting period. In that case it is 
not necessary to employ a current limiting system. This is because a 
photocell may be regarded as a generator which supplies a constant current 
if the voltage remains below a specific threshold voltage, which voltage 
will be referred to as the "knee voltage" hereinafter (see Uc in FIG. 7). 
For a voltage higher than the knee voltage the current supplied by the 
photocell decreases rapidly. Furthermore, the current supplied by the 
photocells depends on the illumination. 
In the devices of FIGS. 3a, 3b and 4 the accumulators are connected to the 
photocells via diodes. 
The diodes between the photocells and the accumulators have different 
functions: 
The accumulator cannot discharge via the photocell; thus the stored energy 
cannot return to the photocells as a pure loss. 
During the in-rush starting period of the motor the diodes are cut off and 
the motor voltage, which is proportional to the motor speed, can initially 
be very low. The accumulators are then temporarily disconnected during 
this period. 
When changing from a series connection of the accumulators (for example 24 
V) to a parallel connection (12 V), the two diodes will prevent an 
excessive current surge between the two accumulator branches if these are 
not correctly balanced. 
Because the peak current supplied by the photocell arrays in the case of 
optimum insolation has been selected not lower than the nominal current 
consumed by the load (for example a water-pump motor), the operation is 
substantially continuous. In the case where the accumulators are fully 
charged and the photocell arrays supply a current which is smaller than 
the starting current of the load, which may happen in particular when the 
solar radiation is still weak (generally every morning), the accumulators 
cannot be overcharged because the relay contact S.sub.1 is closed. This 
contact is closed when the control system S.sub.1a (or S.sub.1b) detects 
an accumulator voltage which is higher than a threshold voltage Uass. The 
current supplied by the photocells can then be passed through the motor, 
so that overcharging of the accumulators is avoided. 
Normally, contact S.sub.1 closes when the photocurrent Ip reaches the upper 
threshold value Ipss. However, if the accumulator voltage U.sub.A is below 
the value Uasi, then contact S.sub.1 remains open even if the photocell 
current Ip exceeds the value Ipss. This allows the battery to regain its 
normal state of charge more rapidly. 
In this case it may often be desirable to allow the load (for example the 
motor of a water-pump) to start until a nominal speed is reached. This can 
be realized with the arrangement of FIG. 4. In this arrangement the load 
can also operate if the solar radiation is weak, e.g. half the above 
mentioned optimum value. The arrangement of FIG. 4 can be obtained from 
that of FIG. 3b with the aid of conventional automatic switching means 
which can be made to respond to the insolation level. In FIG. 4 two 
photocell arrays are connected in parallel with each other, and so are the 
accumulators. The two groups of power elements are connected in series 
with the load, preferably via a semiconductor diode. 
When the motor starts the voltage across the motor is very low because the 
back e.m.f. of the motor is very small due to its low initial speed. For 
example at an internal resistance of the motor, r = 0.1 .OMEGA., and a 
current I.sub.m = I.sub.p = 10 A, the motor voltage, Umotor = 1 V. In FIG. 
4 the voltage at point B relative to the voltage at point A is then for 
example 2 V. The potential difference across the accumulators which are 
charged and connected in parallel will be 12 V. The voltage across the 
photocells is then 10 V but negative. The operating point of the 
photocells is then located in the quadrant Q.sub.2 in FIG. 7. In the case 
of FIG. 3b, however, the operating point of the photocells during starting 
is located in the quadrant Q.sub.1 near zero voltage. 
The motor, whose torque depends on the current through the motor, starts 
and then gains speed. A back e.m.f. is produced which is roughly 
proportional to motor speed and increases to a value of for example 23 V, 
which voltage is attained when the motor operates at the nominal speed. At 
the same time the operating point of the photocells moves towards the 
quadrant Q.sub.1 until the intersection is reached of the curve which 
represents the current supplied by the photocells as a function of the 
insolation with the approximately straight load line (see the dashed 
horizontal line in FIG. 7). 
In the arrangement of FIG. 4 the load can also start and subsequently 
operate normally, i.e. rotate with the nominal motor speed, if the current 
Ipp which is supplied by each photocell array is small. In the case of two 
photocell arrays Ipp may be half the nominal current through the motor. 
Moreover, depending on the illumination the number of parallel connected 
accumulators and photocell arrays can be increased so that the current Ipp 
may be reduced even further. 
After the starting period has elapsed changeover is possible from the 
arrangement of FIG. 4 to the arrangement of FIG. 3a or 3b by means of 
conventional switching means. 
The switching systems of FIGS. 3a and 3b comprise threshold detectors 
S.sub.1a, S.sub.2a or S.sub.1b, S.sub.2b, S.sub.2b. These threshold 
detectors may be operational amplifiers or simple coils which switch the 
relays S.sub.1, S.sub.2. 
Such a switching system operates as follows: S.sub.1 is closed either in 
the case that the accumulators are fully charged and the photocell arrays 
supply a total current Ip which is smaller than the starting current of 
the load, or in the case that the current through the current detector B 
exceeds an upper threshold current Ipss, i.e. if the photocells are 
adequately exposed to solar radiation and supply a current comparable to 
the nominal motor current. S.sub.2 is closed when the motor speed is 
correct (stationary operation), the voltage across the motor (Um) then 
being approximately equal to the accumulator voltage (Ua). S.sub.1 opens 
if the current becomes smaller than a lower threshold current Ipsi. 
Subsequently, S.sub.2 can be opened once the accumulator voltage which is 
then approximately equal to the motor voltage, has attained the lower 
threshold Uasi, so that the accumulators are protected against too low a 
state of charge. In the parallel arrangement of FIG. 3b, the voltage 
across both the photocells and the accumulators is half the voltage across 
the series arranged photocells and accumulators of FIG. 3a. Consequently, 
the motor speed is then half the speed it was in the case of a 
series-connection. In the arrangement of FIG. 3b the pump can be started 
when the amount of solar radiation is half the amount of solar radiation 
necessary to enable the motor to start in the arrangement of FIG. 3a. The 
torque of the motor pump assembly is substantially proportional to the 
current. In the case of FIG. 3b the motor rotates at half the speed, while 
the efficiency is only slightly lower than in the case of FIG. 3a. 
FIG. 5 by way of example shows the apparatus by means of which switching is 
possible from one of the arrangements of FIGS. 3a, 3b and 4 (series 
arrangement, parallel arrangement, and series-parallel arrangement) to 
another of these arrangements. In the diagram of FIG. 5 the points A, B 
and C correspond to the points A, B and C of FIGS. 3a, 3b and 4. The 
arrangement of FIG. 3a corresponds to the positions of the switches 
I.sub.1 through I.sub.6 represented by the solid lines. The arrangement of 
FIG. 3b corresponds to the switch positions represented by the dashed 
lines. If the switches I.sub.1 through I.sub.6 are in the positians 
represented by the dotted lines the arrangement of FIG. 4 is obtained. It 
is obvious that all the switches should be operated simultaneously by one 
switching command signal. The switches of FIG. 5 can be controlled 
automatically, for example, by means of a solenoid operated in response to 
a system parameter such as the insolation level. 
The conditions for switching between the various arrangements will be 
described later on with reference to FIG. 7. 
With respect to the choice between the various arrangements as a function 
of the amount of solar radiation which is received, for example if two 
photocell arrays are available, the following is to be noted: 
If the solar radiation is such that one photocell array supplies a current 
Ipp which at least equals the nominal motor current Im, the arrangement of 
FIG. 3a is opted for. Starting and operation of the load is then 
guaranteed. If the load is constituted by a motor, allowance being made 
for the selected characteristics, the motor will rotate at the nominal 
speed after the starting period because the nominal voltage appears across 
the motor. 
If Ipp equals Im/2 or is greater than Im/2 but smaller than Im, a starting 
circuit may be designed in accordance with FIG. 3b or FIG. 4. The d.c. 
motor will then receive the nominal current required for starting. 
In the case of FIG. 3b the motor speed will be half the nominal speed and 
the charge of the accumulators will not decrease. 
In the case of FIG. 4 the charge of the accumulators will decrease. 
However, the motor speed tends to increase to the nominal speed. The 
arrangement of FIG. 4 enables the motor to be started and brought to its 
nominal speed in the case of weak solar radiation. 
The diagram of FIG. 6 shows how an arrangement in accordance with FIG. 3a 
or 3b operates as a function of time. In FIG. 6 a period of approximately 
two days has been plotted. The curve Ip gives the total current supplied 
by the photocells. During the night the current is 0. During the day the 
current first exceeds a first level Ipsi and subsequently it exceeds a 
second level Ipss. In the afternoon the current may sometimes decrease, 
e.g. clouds may change the photocell current. 
If the current is greater than Ipss starting of the motor is ensured. If 
the current is smaller than Ipss, relay S.sub.1 is closed by the 
accumulator voltage when it exceeds the upper level, Uass, starting then 
is possible within a specific range of currents. It is obvious that 
starting is impossible when the photocell current is too small. 
The periods during which the relays S.sub.1 and S.sub.2 are open and closed 
are shown in FIG. 6b. The upper level means that a relay is closed. 
In FIG. 6c the speed of the motor pump assembly (wmp) has been plotted. In 
one embodiment of a device in accordance with the invention a 
permanent-magnet d.c. motor was employed because of the efficiency and 
specific power. The speed Wmp was then substantially proportional to the 
voltage Um. 
The motor which was used had the following characteristics: 
nominal voltage 24 V 
power 400 W 
nominal speed 1500 r.p.m. 
The fixed losses are not significant (80% efficiency). It is for example 
possible to couple a piston pump to this motor via a reduction gear, which 
pump operates satisfactorily within a wide range of speeds and pumping 
depths (at least down to 60 m depth with good efficiency). 
FIG. 6d shows the variation of the voltage across the accumulators. These 
accumulators store the surplus energy. At sunset or when the sky is 
temporarily clouded the stored energy is supplied to the motor. 
In this embodiment of the present device conventional accumulators are 
utilized having a comparatively low self-discharge rate. An accumulator 
battery for example comprises four accumulators which in a 
series-arrangement are all four connected in series and which in a 
parallel arrangement are connected in two parallel groups of two 
accumulators in series. 
The curves in FIG. 7 represent the current supplied by a photocell array as 
a function of the voltage across the photocells. The various curves apply 
to different intensities of the solar radiation (1 kW/m.sup.2, 2/3 
kW/m.sup.2, 1/3 kW/m.sup.2). FIG. 7 furthermore shows the knee voltage Uc 
(corresponding to point M) and the voltages corresponding to the 
intersections of the curves with the load line, which in the ideal case is 
a horizontal straight line (shown as a dashed line). 
As particularly in the case of a piston pump (specifically when a correctly 
proportioned flywheel is used) the torque to be produced by the motor 
(permanent magnet d.c. motor) is substantially constant and virtually 
independent of the speed for a given pump installation during stationary 
operation, the current through the motor is substantially constant and 
virtually independent of the motor voltage. In FIG. 7 this is represented 
by the idealized straight load line of the motor-pump system. FIG. 7 is 
divided into quadrants Q.sub.1 and Q.sub.2. Beginning with the arrangement 
of FIG. 4, if one of the curves is traced starting from Q.sub.2, point N 
will be reached where the voltage across the photocell array is zero. At 
this point it is possible to change over from the arrangement of FIG. 4 to 
the arrangement of FIG. 3b. At point M it is possible to change over to 
the arrangement of FIG. 3a. In other words, the parameter for controlling 
the switches I.sub.1 - I.sub.6 of FIG. 5 may be the photocell voltage. 
Means for detecting the photocell voltage may then be provided which 
switches the switches I.sub.1 - I.sub.6 to rearrange the configuration of 
photocells and accumulators. For example, if said voltage is negative, the 
detecting means switches the switches I.sub. 1 - I.sub.6 to set up the 
configuration of FIG. 4. In the range from 0 volts to the knee voltage, 
Uc, the arrangement of FIG. 3b is set up, whereas for photocell voltages 
greater than Uc the arrangement of FIG. 3a is switched in. 
Furthermore, it will be evident from FIG. 7 that without accumulators a 
substantial surplus of current Ip (and thus of power), which is available 
in principle, only yields a comparatively small increase of the motor 
voltage, thus of the motor speed and consequently of the effective motor 
power. 
By using an accumulator battery all the surplus current Ip is temporarily 
stored. The nominal battery voltage has been selected slightly below the 
knee voltage (i.e. paint M in FIG. 7) of the photocells. The curves shown 
in FIG. 7 are typically those of silicon photocells. The electrical energy 
stored in the accumulator battery is delivered during the period in which 
the current Ip has decreased to a value smaller than the nominal current 
through the motor. The capacity of the battery is adapted to the 
periodically available surplus of ampere-hours. 
In an embodiment of the device which was equipped with a motor of 
approximately 400 W, the peak power supplied by the photocells was equal 
to or slightly higher than 400 W. 
The present device for converting solar radiation into motor energy has the 
following advantages: 
Owing to the absence of a separate current limiting system it is of simple 
design. 
As a result of this, better use is made of the available energy and the 
reliability of the device is high. 
The device is furthermore cheap and enables the motor to operate smoothly. 
The invention has been described on the basis of a power supply for a water 
pump. However, numerous other applications of the invention are 
conceivable. In particular the supply of power to a load which exhibits 
switching transients is to be mentioned in this respect.