GB2189097A

GB2189097A - Solar power systems

Info

Publication number: GB2189097A
Application number: GB08608436A
Authority: GB
Inventors: James Douglas Haslam
Original assignee: POWAMATE Ltd
Current assignee: POWAMATE Ltd
Priority date: 1986-04-07
Filing date: 1986-04-07
Publication date: 1987-10-14
Also published as: GB8608436D0

Abstract

An electrical load 3, or electrical storage means, is connected to a solar panel 1 via a D.C.-A.C. or D.C.-D.C. converter 2 which may be directly connected to the solar panel 1 and may be structurally integrated therewith. The load 3 may be switched between the converter output and a mains supply via a switch operated manually, or by a timer or mains failure sensor or solar power availability detector (Figs. 4, 5, 10, 11). A battery may be associated with the system and this may be built into the solar panel structure, or a first solar panel with built-in battery may be electrically coupled to a second solar panel with built-in converter (Figs. 7 to 9). The converter may have a toroidal transformer driven by push-pull MOSFET's (Fig. 12). The load may be a fan, water purifying, lighting, refrigeration or air-conditioning apparatus. <IMAGE>

Description

SPECIFICATION Solar Power System The present invention relates to a solar power system which is capable of generating electrical power direct from solar energy.

Solar panels which generate electrical power from sunlight are in widesread use. Solar panels generally produce low voltage direct-current electrical power. In some applications this can be utilised directly, but there are other desirable applications of electrical power derived from solar energy for which a low voltage direct-current electrical supply is not suitable. Electrical appliances such as a.c. motors, lighting and so forth normally require a high voltage alternating-current supply such as domestic mains electricity. In many parts of the world, mains electricity is either unavailable or is unreliable, and in some countries it is very expensive, and so it is highly desirable to be able to use solar power to supply such appliances.

There is known a solar power system in which a solar panel is connected to a battery, which stores electrical energy generated by the solar panel, and supplies power to an inverter for conversion to a suitable form for consumption by an electrical appliance. Such a system has disadvantages; in particular the battery is typically large, heavy and expensive, making the system difficult to transport and too costly for use in poorer regions of the world.

In addition, the battery may be unreliable and will in any case require maintenance, which is not always possible or convenient.

According to a first aspect of the present invention, there is provided a system for generating electricity, comprising a photovoltaic panel for generating electrical power from light incident upon the panel, and an inverter or a converter having an input and an output for converting the electrical power from the panel into an electrical supply, wherein the input of the inverter or converter is connected to receive electrical power directly from the panel and the inverter or converter is operable to provide a useful electrical supply from its output to an electrical appliance or other consumer apparatus, orto electrical storage means.

Advantageously, the inverter or converter is located adjacent the panel, and preferably is arranged in an integral unit with the panel, for example located within a housing for the panel.

Conveniently, the panel is one adapted to produce electrical power in the form of a direct current, and the inverter is adapted to provide an alternatingcurrent supply.

In order to extend the use of a system according to-the invention, the system may further include a switch having one input connected to the output of the inverter, another input connected to an external source of electrical supply, and an output connectable to an electrical appliance, wherein the switch is operable to be switched between the output of the inverter and the external source of electrical supply, whereby alternative supplies are available to the appliance.

Thus, in one construction, a battery is connected between the panel and the inverter input in parallel with the direct connection between the panel and the inverter input, operable at times when the panel is generating the electrical power to be charged from the panel, and operable at other times to supply the electrical power to the inverter.

Means may be provided for detecting a failure of the external source of electrical supply (e.g. mains or battery), so that in the event of a failure of the external source, the switch is switched to the output of the inverter for supply from the solar panel, thereby ensuring a continuity in supply of electricity to an electrical appliance; and/or means may be provided for detecting a useful supply of electrical power from the inverter, so that in operation, when the useful supply of electrical power is available from the inverter, the switch is switched to the output of the inverter to direct that useful supply to an electrical appliance.

A second photovoltaic panel having an output connected to the inverter or converter, may also be provided for supplying electrical power generated by the second panel directly to the inverter or converter.

The second panel may be electrically connected to an integral unit housing the first panel and the inverter or converter; and/or a battery, for storing electrical power generated by the second panel and for supplying electrical power to the inverter or converter, may be arranged together with the second panel in an integral unit.

In a particularly advantageous embodiment of the present invention, the system is used to drive a fan, connected or connectable to the output of the inverter. However, a system in accordance with the invention may find applications for lighting or for water purification, or possibly in large power supply installations in which many solar panels are used to charge storage batteries, when a converter would be used rather than an inverter.

According to a second aspect of the present invention, there is provided a system for generating electricity, comprising a photovoltaic panel for generating electrical powerfrom light incident upon the panel, and an inverter or converter arranged in an integral unit with the panel, for supplying electrical power to an appliance or other consumer apparatus orto electrical storage directly or indirectly from the panel.

The invention may be put into practice in many ways but reference is now made, by way of example, to the accompanying drawings in which: Figure lisa schematic diagram of the basic arrangement of a solar power system in accordance with a first aspect of the present invention; Figure 2 is a schematic diagram of the basic arrangement of a solar power system in accordance with a second aspect of the present invention; Figure 3 is a schematic diagram of a first embodiment of the solar power system of Figure 2; Figure 4 is a schematic diagram of a second embodiment of the solar power system of Figure 2; Figure 5 is a schematic diagram of a third embodiment of the solar power system of Figure 2; Figure 6 is a schematic diagram of a fourth embodiment of the solar power system of Figure 2;; Figures 7 and 8 are schematic diagrams of a fifth embodiment of the solar power system of Figure 2; Figure 9 is a schematic diagram of a sixth embodiment of the solar power system of Figure 2; Figure 10 is a schematic diagram of a seventh embodiment of the solar power system of Figure 2; Figure 11 is a schematic diagram of an eighth embodiment of the solar power system of Figure 2; and Figure 12 is a circuit diagram of an inverter suitable for use in a solar power system embodying the present invention.

In the first aspect of the invention shown in Figure 1, a solar panel 1, more specifically a polycrystalline silicon photovoltaic array, generates low-voltage direct current (d.c.) electrical power when light is incident upon its surface. The electrical power is fed from an output of the panel to an inverter 2. The inverter functions to convert the electrical power from the panel to a form more suitable for consumption by an electrical appliance. More particularly, the inverter 2 converts the electrical power to alternating current (a.c.) form, preferably at a higher voltage and supplies this converted power from its output. For example, the output of the inverter may be similar two a domestic mains supply.

In the simplest arrangement of the solar power system, the output of the inverter is supplied directly to an external appliance or other electrical consumer 3. For some types of appliance it will be preferable to interpose regulating means, such as a voltage regulating circuit, between the inverter and the appliance, but the main point is that the output from the inverter in accordance with the present invention is a useful supply of electrical power and contrary to known systems it is not necessary to employ a battery connected between the inverter and the appliance. This is made possible by efficient design of the inverter and its connections with the panel, as explained below.

Figure 2 schematically illustrates the second aspect of the present invention. A solar panel 10 has a "built-in" inverter 1 or, that is, the inverter is located adjacent the panel preferably in an integral unit with the panel or within a housing for the panel.

The inverter 1 OA will usually be electrically connected direct to the panel output but it could be connected indirectly via other devices or a battery if required. The solar panel with built-in inverter 1 OA will also usually be connected directly to an electrical appliance 3 producing an arrangement corresponding to Figure 1; however, regulating means or a battery may be interposed between them.

In a solar power system in which the inverter is an appreciable distance from the solar panel, it would be necessary to employ expensive, low-loss cable to provide the connections between the two, in order to avoid considerable power losses in the lowvoltage d.c. power generated from the panel. The solar panel with built-in inverter according to the second aspect of the invention has the advantage that due to the physical proximity of the inverter to the solar panel, such power losses are minimised and only a short distance of connecting cable, if any, is required. This is particularly beneficial in a system in which the electrical appliance is supplied directly from the inverter (the Figure 1 arrangement) because the resultant improved efficiency enables a useful power supply to be provided under a wide range of operating conditions (incident light conditions).

Another important factor in determining the efficiency of the solar power system is the design of the inverter. Figure 12 is a circuit diagram of an inverter suitable for use in a solar power system embodying the present invention, connected to a polycrystalline silicon photovoltaic array panel SX1 and to a 220/240 v a.c. ceiling fan.

The components used in this inverter design are as follows: D1: silicon diode, C1 and C2: electrolytic capacitors, C3 and C4: polyester capacitors, IC1: regulating Integrated Circuit (l.C.), IC2: CMOS Multivibrator IC. with complementary outputs, R1 to R4: 1% resistors, DZ1 to DZ4: Zener Diodes, TX1 and TX2: MOSFETTransistors, and XF1: Toroidal Transformer.

The circuit shown is suitable for use with a single solar panel of maximum output power 45 watts at an incident light level of 1 kilowatt per square meter.

The circuit is particularly suitable for the application of the solar power system of driving a fan.

The noteworthy features of the circuit with regard to efficiency are as follows: 1. MOSFETtransistors are employed in the push pull output drive circuit.

2. A CMOS integrated circuit is used as the oscillator and MOSFET gate drive device.

3. A toroidal transformer is employed, to reduce both the magnetising currentto a low value and the copper and iron losses to a low value.

The toroidal transformer also has the advantage of small physical size and weight, both important for improving the portability of the solar power system.

In particular, the toroidal transformer enabies the inverter to be small enough for incorporation with the solar panel in an integral unit or within a housing for the solar panel, in accordance with the second aspect of the invention to minimise transmission losses and cable size.

High efficiency is not the only requirement for the inverter used in the solar power system. It is also important that the inverter can accept the wide voltage range of the power generated by the solar panel.

As the input voltage to the inverter can vary from zero to +22 volts depending on sun conditions, the circuit has to be able to accommodate this.

The drive l.C. chosen will operate from 3 volts to 15 volts d.c., but in order to protect it from the extremes of voltage it is supplied from a voltage regulator l.C. which stabilises the voltage at 8 volts.

The inverterwill automatically shut down when the voltage falls below -5 volts due to insufficient drive voltage on the MOSFET gates. In the application of the solar power system of driving a fan, this is an advantage in that there would be insufficient torque developed by the fan to rotate at this input; the power being supplied, although small, would only be able to manifest itself in creating heat.

Some embodiments of the solar power system will now be described with reference to Figures 3 to 11. Although these Figures illustrate systems in accordance with the second aspect of the invention, i.e. systems including a solar panel with a built-in inverter, it will be understood that the embodiments would generally be applicable to systems in which the inverter is not actually built-in but is merely physically adjacent the solar panel.

Figure 3 illustrates a first emodiment, in which the solar panel 10 with built-in inverter is directly connected to a fan 5. With an efficient fan, the power output from the inverter is sufficient to keep the fan turning continuously for long periods under favourable conditions. Such a system would be used for example for cooling purposes in hot, sunny conditions.

Explaining this embodiment in more detail with reference to Figure 12, a suitable type of fan is for example a Crompton Parkinson type CV3 fitted with 4-foot diameter blades. This requires a 220/240 v a.c.

supply and has a power requirement of 55 watts at full speed.

The design of the transformer is crucial in this application, since the power taken by the fan at full speed, if connected to its usual mains supply of 240 volts a.c. will be 55 watts. Given that the maximum output power from one solar panel is approximately 45 watts, full speed will never be developed.

However, in practice, full speed is rarely needed, except perhaps in such places as railway booking halls, airport lounges, etc. A range of speed, equivalent to an input power of from 7 to 45 watts, is quite acceptable.

The 10 watt short-fall in power input has two implications.

Firstly, there is no need to regulate the inverter output voltage, because if the inverter voltage tends to rise too high, the electrical load represented by the fan will pull it back down again. This simplifies inverter design considerably and keeps efficiency at a maximum.

Secondly, maximum power will be supplied by the solar panel when the internal impedance of the solar panel and the external impedance of the connected circuit are equal, for any given incident light condition. Or, in other words, the peak power point will move up and down depending on the sun conditions. By careful selection of transformer ratios the speed of the fan and hence the power consumed can be made to follow the peak power point of the solar panel, giving maximum power transfer.

For example, if the solar panel was producing 10 watts of power, the fan would be rotating at a speed proportional to this, neglecting the very small inverter losses. If sun conditions improve and the panel now produces 20 watts, this would manifest itself in the voltage increasing at the input of the inverter. As the a.c. output voltage from the inverter increases, so the voltage at the fan increases.

The torque of the fan motor is proportional to V2 (volts squared), so the motor accelerates, increasing the power taken from the supply, the increased current causing the voltage to drop. This will continue until a new balance is achieved between the power the system can supply and what the system is trying to take from the supply. Therefore, provided that the transformer voltage ratio is selected carefully, the fan system will regulate itself to the peak power point of the solar panel. Due to the inertia of the system, the changes in speed occur smoothly and slowly not causing annoyance.

Thus, the inverterwill accept a wide range of input voltages and when connected to the fan matches the peak power point of the solar panel to the fan.

Figures 4 and 5 illustrate second and third embodiments, in which there are added to the system of Figure 3 a manual change over switch 6, speed regulation means 7, and a mains input 8. In this system, mains power is provided as an alternative to the supply from the inverter, selection between the two being made as desired by switching the manual change over-switch 6. The speed regulation means 7 shown in Figure 4 is positioned on the mains input side of the switch 6so that it acts to regulate the speed of the fan 5 in a desired manner only when on mains supply. Speed regulation is desirable when on main supply, as the fan would normally run at full speed from mains power, whereas speed regulation is not so important when on the inverter supply, because as mentioned above the fan does not run at full speed from that supply.Also, it is preferable not to employ the speed regulation means 7 with the inverter supply as it entails a slight loss of power. However, the speed regulation means 7 could alternatively be interposed between the change over switch 6 and the fan 5 as shown in Figure 5 so as to be operable with either power supply, or else omitted altogether.

Also, if an electrical appliance other than a fan was used in such a system, a different type of regulating means might be appropriate.

Figure 6 illustrates a fourth embodiment, in which the system basically comprises the solar panel 10 with built-in inverter, an additional solar panel 11, and an electrical appliance such as a fan 5. The additional solar panel does not have associated with it an additional inverter; rather, it supplies its d.c.

generated output to the solar panel 10 with built in inverter, for conversion by that inverter. For the reasons explained above it is therefore preferable for the additional solar panel 11 to be near to or directly adjacent the panel 10.

In a solar power system employing two or more solar panels, it may be desirable to provide voltage regulating means for the power supply from the inverter. For example, in the application of driving a fan mentioned above, the power output with two solar panels under favourable conditions will exceed the power requirement of the fan.

Figure 7 shows a fifth embodiment, being a modification of the embodiment of Figure 6. Here, an additional solar panel 12 has associated with it a battery for storing the power generated by the panel 12 and for supplying stored power to the inverter of the panel 10. Thus, in this system the power generated by panel 10 could be fed to the inverter to provide a supply under favourable conditions (e.g.

daytime) with the panel 11 meanwhile charging the battery. Then during unfavourable conditions (e.g.

nighttime) the power stored in the battery could be fed to the inverterto maintain the supply. The battery for the panel 11 is preferably located adjacent panel 11 for example by being built into an integral unit with the panel or into a housing for the panel. As with the built-in inverter, this helps to minimise power losses in connecting cables.

Figure 8 shows a possible arrangement of the two panels 10 and 12. The panel 10 has two output terminals 10B which are connected to the built-in inverter 1 OA. The panel 12 has two output terminals 12B which are connected, via charge regulating means 12C, to a built-in battery 12A. The output terminals 12B are also connected to the inverter 10A of panel 10.

With this arrangement, both panels normally supply power to the inverter 10A. In the example of driving a fan mentioned above, the power output from the inverter with a single panel 10 is 45 watts, and the power requirement of the fan is 55 watts.

Therefore, when the solar power system of Figure 8 is applied to driving a fan 5, the power output from the inverter 10A under favourable conditions exceeds the power requirement of the fan, and the resultant excess of power is stored in the battery 1 2A. The charge regulating means 1 2C ensures that the battery 12A does not become over-charged.

A battery may also be employed in a solar power system having a single solar panel. Figure 9 illustrates a sixth embodiment of the solar power system in which a solar panel 10 with built-in inverter 10A has connections both to an electrical appliance 3 and to a battery 9. The electrical appliance is connected to the (high voltage, a.c.) output of the built-in inverter 10A, and the battery 9 is connected directly to the (low voltage, d.c.) output of the solar panel via terminals 1 OB.

At times when the appliance 3 is drawing less power than is being generated by the panel 10, the excess power can be stored in the battery. At times when no power or insufficient power is being generated by the solar panel 10, the battery 9 may supply its stored power to the inverter in order to ensure an adequate supply to the electrical appliance 3.

The battery 9 could also be built into the panel 10 along with the inverter 1 or, for minimising power losses and enhancing the portability of the system.

Figure 10 illustrates a seventh embodiment, which is similar two the arrangement of Figure 4 in that a mains supply 8 is provided as an alternative to the supply from the inverter. However, instead of a change over switch 6 which would normally be operated manually or by a timer means, a mains failure detector 15 controls the selection of the two supplies. The electrical appliance such as fan 5 is normally operated by the mains supply. In the event of a failure or deterioration in the mains supply such that the appliance is insufficiently powered, the mains failure detector recognises this and switches over to the inverter supply. As indicated in the Figure, the solar panel part of the system may be that in accordance with any of the preceding embodiments.

A modification of this system is the eighth embodiment shown in Figure 11. Replacing the mains failure detector 15 is a solar power availability detector 16 which determines which of the two supplies is selected. During unfavourable conditions, the mains supply is selected; however, when a sufficient power supply is available from the inverter, the detector 16 recognises this and switches to the inverter supply. This allows economical operation of the electrical appliance whilst ensuring a proper supply at all times.

The invention could also be applied to a largescale solar power system including an array of a large number (maybe hundreds) of solar panels. In such a system, a bank of storage batteries would probably be used: these would be very large and would of necessity be some distance from the solar panels.

The large-scale solar power system could employ solar panels having built-in converters, rather than built-in inverters. The built-in converter of a solar panel would receive the low-voltage d.c. power generated by the panel and convert it to a high voltage d.c. output, such as 1 1O240 v, for transmission to the bank of storage batteries. The d.c.-to-d.c. conversion would reduce 12R losses in the transmission cables and reduce the necessary cable size, and hence could result in a considerable saving in both cable cost, weight, and efficiency.

The weight saving in the transmission cables would be particularly advantageous for an array of solar panels which is rotated to follow the sun.

Although in the above embodiments a fan has been used as an example of an electrical appliance, it will be understood that the solar power system can supply many other types of electrical appliance or other consumer apparatus. In particular, appliances and consumers for use in remote regions of the world, or regions in which there is no reliable mains supply can be conveniently powered by the solar power system provided there is sufficient sunshine. Water purifying apparatus and lighting and refrigeration or air-conditioning, are examples of such appliances and consumers. In these cases regulating means to regulate the output voltage from the inverter would be provided.

Claims

1. A system for generating electricity, which system comprises a photovoltaic panel for generating electrical powerfrom light incident upon the panel, and an inverter or a converter having an input and an output for converting the electrical power from the panel into an electrical supply, wherein the input of the inverter or converter is connected to receive electrical power directly from the panel and the inverter or converter is operable to provide a useful electrical supply from its output to an electrical appliance or other consumer apparatus, orto electrical storage means.

2. A system according to claim 1 in which the inverter or converter is located adjacent the panel.

3. A system according to claim 2 in which the inverter or converter is arranged in an integral unit with the panel.

4. A system according to any one of the preceding claims in which the panel is one adapted to produce electrical power in the form of a direct current and in which the inverter or converter is adapted to provide an alternating current supply.

5. A system according to any one of the preceding claims and including a switch having one input connected to the output of the inverter or converter, another input connected to an external source of electrical supply, and an output connectable to an electrical appliance, wherein the switch is operable to be switched between the output of the inverter or the converter and the external source of electrical supply, whereby alternative supplies are available to the appliance.

6. A system according to claim 5 in which a battery is connected between the panel and the inverter or converter input in parallel with the direct connection between the panel and the inverter or converter input, operable at times when the panel is generating the electrical power to be charged from the panel, and operable at other times to supply the electrical power to the inverter or converter.

7. A system according to claim 5 or claim 6 and including means for detecting a failure of the external source of electrical supply, so that in the event of a failure of the external source, the switch is switched to the output of the inverter or converter for supply from the solar panel, thereby ensuring a continuity in supply of electricity to an electrical appliance.

8. A system according to any one of claims 5 to 7, and including means for detecting a useful supply of electrical power from the inverter or converter, so that in operation, when the useful supply of electrical power is available from the inverter or converter, the switch is switched to the output of the inverter or converter to direct that useful supply to an electrical appliance.

9. A system according to any one of the preceding claims and including a second photovoltaic panel having an output connected to the inverter or converter.

10. A system according to claim 9 in which the second photovoltaic panel is for supplying electrical power generated by the second panel directly to the inverter or converter.

11. A system according to claim 9 or claim 10 in which the second photovoltaic panel is electrically connected to an integral unit housing the first panel and the inverter or converter.

12. A system according to any one of claims 9 to 11 in which the second photovoltaic panel is connected to a battery for storing electrical power generated by the second photovoltaic panel and for supplying electrical power to the inverter or converter.

13. A system according to any one of the preceding claims and including a fan, the fan being connected to the output of the inverter or converter.

14. A system for generating electricity, which system comprises a photovoltaic panel for generating electrical power from light incident upon the panel, and an inverter or converter arranged in an integral unit with the panel, for supplying electrical power to an appliance or other consumer apparatus or to electrical storage directly or indirectly from the panel.

15. A system for generating electricity, substantially as herein described with reference to the accompanying drawings.