Means of Improving the Utilization of Energy Available From a Solar Electric Generator

This invention relates to a method of, and apparatus for, improving the utilization of energy available from a solar electric generator.

Solar cells, including silicon and other semiconductor solar cells are increasingly used in extraterrestial and earthbased uses. Such cells singly or connected in an array are herein referred to as a solar electric generator.

A problem associated with the application of a solar generator for driving an electric motor or other load arises from the characteristic of these cells which causes their output power to vary with the intensity of the sunlight falling upon them.

It is known that in order to transfer the maximum amount of power from a generating source to a load, the source impedance or resistance must be equal to the load impedance or resistance. (The term impedance will be used to mean impedance or resistance).

When the output of a solar electric generator varies with the light intensity, its effective source impedance varies also. This means that the maximum power transfer from the solar electric generator cannot take place at all light levels when connected to a given fixed load. The problem of maximum power transfer is further compounded by an initial impedance mismatch, if any, between the load and the solar electric generator or by any variation in the load.

An object of this invention to match the varying impedance, at varying light levels, of a solar electric generator to the fixed, or unrelated varying, impedance of a load being driven therefrom.

According to one aspect the invention consists in apparatus for matching the electrical impedance of a solar electric generator and a load driven therefrom, comprising:

circuit means for comparing the generator voltage and a predetermined voltage to produce a difference signal indicative of the difference if any in said voltages,

control means for repetitively switching on and off the current flow from the generator to the load, the control means being responsive to the difference signal to vary the duration that said current is switched on relative to the duration that said current is switched off so as to maintain the generator voltage at a substantially constant level, and

storage means for storing energy generated by the generator during the periods that current is switched off the load.

For preference the control means is a semi-conductor switch operated by a repetitive signal which is pulse width modulated according to the voltage difference between the generator output and a reference. The energy storage means may be a storage capacitor.

By repetitively interrupting the current to the load on the generator it is possible to maintain the generator terminal voltage at or near an optimum constant level and the transfer of power from the generator to the load can be maximized nothwithstanding variation in load impedance or in light intensity.

By way of example only an embodiment of the invention will now be described with reference to the accompanying drawings wherein:

FIG. 1 shows schematically the voltage/current characteristics typical of a solar generator.

FIG. 2 is a block diagram showing an embodiment according to the invention.

FIG. 3 is a schematic circuit diagram of an embodiment of the invention.

The voltage/current characteristic of a typical silicon solar cell or array thereof is as shown in FIG. 1. It can be shown that with this type of curve, the maximum power output ie. E.times.I occurs on the knee of the curve. Thus if the load is so adjusted as to maintain a nearly constant voltage from the cell at any light level, then the load will always receive close to the maximum power available. In other words if the cell voltage remains constant there is an impedance match between the cell and the load.

FIG. 2 is, by way of example only, a block diagram of one type of regulating circuit which achieves this. The output of solar generator 1 goes to the load 2 via terminals 3A and 3B of an electronic switch 3 and also to one input 4A of a voltage comparator 4. The other input 4B of the voltage comparator 4 is connected to a voltage reference source 5 such as a zener diode. The resulting difference signal at 4C is then used at 6 for pulse width control, or pulse position (pulse interval) control, of a square wave signal used at terminal 3C in turn to control the on-off cycle of electronic switch 3. Switch 3 may be a transistor, a MOSFET, an SCR, or any other type of semiconductor switch. In order to supply load 2 with the required current pulses, storage means such as for example a capacitor of sufficient size is preferably connected across solar generator 1 to store its energy during the off time and so smooth the pulsing load on the generator.

By way of further example, a schematic circuit diagram of a typical regulator for driving an electric motor load is shown in FIG. 3. A reference voltage is provided by Zener diode Z1 and this is compared with a proportion of the solar generator's output voltage, the error difference between the two being amplified by the operational amplifier U2A. The potentiometer RV1 is used to set the operating voltage to the optimum level.

Integrated circuit U1 is a voltage controlled oscillator used to produce a triangular wave output which by means of operational amplifier U2B has its dc offset modified so that the triangular wave output extends from zero upwards. This triangular wave is fed to one input of comparator U3 while its other input is fed with the difference signal from U2A. The output of U3 now becomes a pulse width modulated signal, the duty cycle of which becomes a function of the error signal.

The modulated output of U3 goes to U4A, a high voltage open collector buffer, which provides the drive to turn on the electronic switch Q1. Diode D2 carries free wheeling current caused by the motor inductance. Components D1, Z4, C14, and R17 absorb transient switching spikes.

The components Z2, R16, R15 and U4B permit the control circuit to start up correctly without any significant load being connected to the generator.

Although the invention has been described with reference to an electronic switch operated by a square wave signal which is pulse width modulated, it will be apparent that a switch operated by a pulse interval modulated control signal or a switch operated by a fixed frequency signal together with a frequency modulated signal or other control signal means could be employed to interrupt current from the generator to the load and it will be apparent to those skilled in the art from the teaching hereof that circuits for carrying the inventive concept hereof into effect could be implemented in a variety of ways without departure from the inventive concepts hereof.