GB1580513A

GB1580513A - Self regulating power system using solar cells

Info

Publication number: GB1580513A
Application number: GB1051278A
Authority: GB
Original assignee: Tideland Signal Corp
Current assignee: Tideland Signal Corp
Priority date: 1977-03-16
Filing date: 1978-03-16
Publication date: 1980-12-03
Also published as: DE2811285A1; AU510527B2; JPS53114041A; AU3330778A; IN149355B

Description

(54) A SELF-REGULATING POWER SYSTEM USING SOLAR CELLS (71) We, RDELAND SIGNAL CORPO- RATION, a corporation organized under the laws of the State of Texas, U.S.A., of 4310 Directors Row, Houston, Texas, P.O. Box 52430, Houston, Texas 77052, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a self-regulating power system of the type comprising a solar photovoltaic energy converter, such as a solar array, and a chargeable battery.

Various power systems have been used which combine solar photovoltaic energy collectors with electrochemical storage batteries such as U.S. Patent No. 2,780,765.

However, it has been standard practice in solar powered systems designed for medium to high power consumption to use regulating devices. That is, numerous regulating devices have been proposed to protect the electrochemical storage system or battery from damage due to overcharging during periods when energy collection exceeds the combined usage and storage capacity, such as shown in U.S. Patent Nos. 3,426,263; 3,541,422; 3,350,618; 3,816,804; and 3,921,049. Generally, these devices have added failure prone components and by their presence have increased the load requirements for the solar photovoltaic converter. As a consequence, solar collection arrays have been necessarily oversized to supply the added voltage and power required by the regulating devices.

In accordance with a first aspect of the invention there is provided a self-regulating power system comprising a plurality of solar photovoltaic cells connected in series, a battery connected in parallel with the series connected cells, a blocking means connected between the cells and the battery for allowing the cells to charge the battery, and wherein the number of solar cells connected in series is such that the full charge voltage of the battery is greater than the reverse bias voltage required to reduce the current output from the series connected cells, but is less than the reverse bias voltage to entirely stop the current output from the cells without overcharging the battery thereby providing self regulation preventing overcharge of the battery.

In accordance with a second aspect of the invention, there is provided a self-regulating power system comprising a plurality of solar photovoltaic cells connected in series, a reversible electrochemical charge retaining storage battery connected in parallel with the series connected cells, electrical blocking means connected between the cells and the battery for allowing the cells to charge the battery and wherein the number of solar cells connected in series is such that the full charge voltage of the battery is greater than the breakdown voltage of the series connected cells, but is less than the reverse bias voltage to entirely stop the current output from the series connected cells without overcharging the battery thereby providing self regulation preventing overcharge of the battery.

It will be seen that in both the above aspects, the number of solar cells connected in series are matched to the type of battery being charged such that the charging current from the solar cells will automatically decrease when the battery voltage increases to a predetermined amount, but will increase when the battery voltage decreases thereby providing automatic self regulation of the power system.

In one embodiment the use of sixteen silicon solar cells connected in series to a nominal six-volt lead acid battery is able to provide the necessary output for delivering electrical energy to the battery during periods of low energy storage of the battery, but to automatically reduce its output to prevent overcharging the battery without the necessity of additional regulating devices.

By ensuring that the conditions required by the invention exist over the range of temperature under which the power system operates, self regulation may be achieved regardless of the temperature conditions encountered.

In order that the invention may be better understood, an embodiment thereof will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a block diagram of one embodiment of a self-regulating power system according to the present invention having a solar array and a rechargeable battery; Figure 2 is a graph illustrating the current versus reverse bias voltage characteristics of solar cells; Figure 3 is a graph of battery voltage versus the percent of energy stored; and Figure 4 is a graph illustrating the matching of a specific solar array with a specific battery for achieving automatic self regulation of the power system.

Referring to the drawings and particularly to Figure 1, the numeral 10 generally indicates the self-regulating power system of the present invention and generally includes a solar photovoltaic collector array 12, electrical blocking means 14, a charge retaining electrical chemical storage battery 16 and a power output 18. The system 10 is provided to deliver a daily requirement of electrical energy at the battery voltage to a load connected to the output terminals 18 for various applications as mentioned above.

The solar photovoltaic collector array 12 consists of a plurality of solar cells connected in series. While any suitable type of solar cells may be used, silicon cells including either a P-type zone shallow diffused into N-type body or an N-type zone shallow diffused into P-type body has been found to be satisfactory. If desired, additional cells may be connected in parallel to provide the required electrical output from the array 12.

The battery 16 may be any suitable type of reversible, electrochemical type with individual cells connected in series and/or parallel to form a battery. Lead acid or NiCd type batteries have been found to be satisfactory.

The power output from the solar array 12 is connected to the battery 16 for charging the battery 16 through the electrical blocking means 14, which is preferably a forward biased blocking diode of a type having the lowest voltage drop in the forward direction, such as the Schottky type. The blocking means or diode 14 allows the array 12 to charge the battery 16 during periods but prevents the battery 16 from losing energy to the array 12 during periods of darkness.

The above described power system is generally conventional. However, in order that the battery 16 retain its design energy storage capacity, it is necessary that the battery 16 not be overcharged during periods of peak solar radiation presence. At the same time, the system must have sufficient cells to deliver electrical energy to the battery 16 from the solar array 12 during periods of low energy storage state of the battery 16. This requires both minimum and maximum charging of the battery 16 from the array 12.In known systems, these conditions are generally met by providing an excess number of series connected solar cells to ensure that sufficient energy could be supplied to the battery 16 and at the same time by providing various types of regulators to protect the battery 16 from damage due to overcharging during periods when the energy collection of the array 12 exceeded the combined usage at the output terminals 18 and the storage capacity of the battery 16.

The present power system 10 provides automatic self regulation by selecting the number of solar cells connected in series in the array 12 to properly coact with the voltage characteristics of the battery 16 to automatically self regulate the power system 10. That is, by matching the current versus voltage characteristic of a number of solar cells to the type of battery being charged, the charging current from the array 12 will automatically decrease when the battery voltage increases to a predetermined amount, and will automatically increase when the battery voltage decreases, thereby providing automatic self regulation of the system 10.

Referring now to Figure 2, a graph of output current from an array of series connected solar cells versus the reverse bias voltage on the array is illustrated. A graph 20 is shown for a temperature of 80"F. and a graph 22 is shown for a temperature of 140"F. However, both graphs 20 and 22 illustrate that the output current supplied by a group of series connected solar cells at a particular light level is nearly constant for low bias voltages at least up to the so-called knee point 24 and 26 of the graphs 20 and 22. However, as the reverse bias voltage is increased, the current output from the array decreases and eventually ceases.

Referring now to Figure 3, it is noted that the electromotive force or voltage of an electrochemical battery, such as battery 16, increases rapidly as the fully charged state (100%) is approached. Therefore, referring again to Figure 2, if the number of cells in the solar array 12 are selected, such that the full charge voltage 30 of the battery 16 falls between the knees 24 and 26 and zero current output from the array 12, self regulation may be achieved. That is, as the full charge state 30 of the battery is approached, the reverse bias voltage on the solar photovoltaic array 12 increases thereby reducing the current delivered to the battery 16 preventing overcharging of the battery 16.

However, as a load connected to the output terminals 18 of the system 10 removes energy from the storage battery 16, its voltage output will decrease, as indicated by Figure 3 thereby lowering the reverse bias applied to the solar array 12 which increases the current delivered from the array 12 to the battery 16.

Furthermore, as indicated by the differences in the graphs 20 and 22 due to differences in temperature, as indicated in Figure 2, the zero current reverse bias voltage decreases as the junction temperature is increased. This effect improves the self regulation of the silicon solar array since the temperatures generally tend to increase when the solar radiation flux is greatest, that is, during the summer months when the energy output from a solar cell 12 would normally be greater. The increased temperatures cause the reverse bias voltage to shift to graph 22 to further insure that overcharging of the battery 16 will not occur.

Referring to Figure 4, a more specific example is shown in which the graph 40 illustrates the current versus reverse bias voltage characteristic of sixteen series connected silicon cells. It is noted that the charging current from the array is relatively constant up to the knee 42, that is, up to approximately seven volts. However, beyond the knee 42 the graph 40 indicates that as the reverse bias voltage is increased above seven volts, the current output quickly decreases. Graph 44 indicates a typical battery electromotive force in volts of a conventional six-volt lead acid battery.

Actually, a nominal six-volt lead acid battery has a fully charged voltage of approximately 7.2 volts. Therefore, it is seen from the graphs 40 and 44 in Figure 4 for the particular array and battery combination that self regulation of the power system will automatically occur. That is, as the full charged state (100%) of the battery is approached, the reverse bias on the solar cell array increases thereby reducing the current delivered to the battery. On the other hand, as energy is removed from the storage battery from time to time its battery voltage will tend to decrease thereby decreasing the reverse bias voltage on the solar array causing the current from the array to increase. However, by using twenty cells as is conventional, the battery is in danger of being overcharged and thus regulating devices must be added to the system.

In actual tests employing a power system 10 utilized to power a marine navigational aid, it was found that utilizing a lead acid battery consisting of three series cells, commonly designated as six volts, a selfregulating power system 10 was achieved using a silicon photovoltaic collector consisting of sixteen silicon cells in series. Simi larly, 12-volt lead acid batteries may be used in a self-regulating system 10 by utilizing thirty two silicon cells. And other types of batteries may be utilized in a self-regulating system by properly selecting the number of solar cells such that the full charge voltage of the battery selected is greater than the reverse bias voltage required to approach the breakdown voltage of the array, but less than the reverse bias voltage to entirely stop the current output.Systems have been tested in which the daily power consumption ranges from 2.5 to 25 amp per hour per day in which different daily power consumption capacities were obtained by varying the total solar photovoltaic cell area and the storage capacity of the battery. The system 10 supplies adequate power to the load during periods of low solar energy presence and the batteries recover to a full charged state without degradation of the battery by overcharge during periods of peak solar energy presence.

There has been described a self-regulating power system having a plurality of series connected solar cells and a chargeable battery designed for high reliability, cost effec- tive usage, and unattended applications, requiring moderate to large amounts of power delivered to a load from time to time.

The load requirements may be on a constant, a periodic or an irregular consumption basis. Typical applications are marine aids to navigation, signalling systems for railroad or pipeline traffic control, communication systems, safety equipment, cathodic protection of metallic structures in pipelines, and supply of other essential power requirements where reliable conventional power sources are unavailable.

WHAT WE CLAIM IS:- 1. A self-regulating power system comprising a plurality of solar photovoltaic cells connected in series, a battery connected in parallel with the series connected cells, a blocking means connected between the cells and the battery for allowing the cells to charge the battery, and wherein the number of solar cells connected in series is such that the full charge voltage of the battery is greater than the reverse bias voltage required to reduce the current output from the series connected cells, but is less than the reverse bias voltage to entirely stop the current output from the cells without overcharging the battery thereby providing self regulation

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. full charge voltage 30 of the battery 16 falls between the knees 24 and 26 and zero current output from the array 12, self regulation may be achieved. That is, as the full charge state 30 of the battery is approached, the reverse bias voltage on the solar photovoltaic array 12 increases thereby reducing the current delivered to the battery 16 preventing overcharging of the battery 16. However, as a load connected to the output terminals 18 of the system 10 removes energy from the storage battery 16, its voltage output will decrease, as indicated by Figure 3 thereby lowering the reverse bias applied to the solar array 12 which increases the current delivered from the array 12 to the battery 16. Furthermore, as indicated by the differences in the graphs 20 and 22 due to differences in temperature, as indicated in Figure 2, the zero current reverse bias voltage decreases as the junction temperature is increased. This effect improves the self regulation of the silicon solar array since the temperatures generally tend to increase when the solar radiation flux is greatest, that is, during the summer months when the energy output from a solar cell 12 would normally be greater. The increased temperatures cause the reverse bias voltage to shift to graph 22 to further insure that overcharging of the battery 16 will not occur. Referring to Figure 4, a more specific example is shown in which the graph 40 illustrates the current versus reverse bias voltage characteristic of sixteen series connected silicon cells. It is noted that the charging current from the array is relatively constant up to the knee 42, that is, up to approximately seven volts. However, beyond the knee 42 the graph 40 indicates that as the reverse bias voltage is increased above seven volts, the current output quickly decreases. Graph 44 indicates a typical battery electromotive force in volts of a conventional six-volt lead acid battery. Actually, a nominal six-volt lead acid battery has a fully charged voltage of approximately 7.2 volts. Therefore, it is seen from the graphs 40 and 44 in Figure 4 for the particular array and battery combination that self regulation of the power system will automatically occur. That is, as the full charged state (100%) of the battery is approached, the reverse bias on the solar cell array increases thereby reducing the current delivered to the battery. On the other hand, as energy is removed from the storage battery from time to time its battery voltage will tend to decrease thereby decreasing the reverse bias voltage on the solar array causing the current from the array to increase. However, by using twenty cells as is conventional, the battery is in danger of being overcharged and thus regulating devices must be added to the system. In actual tests employing a power system 10 utilized to power a marine navigational aid, it was found that utilizing a lead acid battery consisting of three series cells, commonly designated as six volts, a selfregulating power system 10 was achieved using a silicon photovoltaic collector consisting of sixteen silicon cells in series. Simi larly, 12-volt lead acid batteries may be used in a self-regulating system 10 by utilizing thirty two silicon cells. And other types of batteries may be utilized in a self-regulating system by properly selecting the number of solar cells such that the full charge voltage of the battery selected is greater than the reverse bias voltage required to approach the breakdown voltage of the array, but less than the reverse bias voltage to entirely stop the current output.Systems have been tested in which the daily power consumption ranges from 2.5 to 25 amp per hour per day in which different daily power consumption capacities were obtained by varying the total solar photovoltaic cell area and the storage capacity of the battery. The system 10 supplies adequate power to the load during periods of low solar energy presence and the batteries recover to a full charged state without degradation of the battery by overcharge during periods of peak solar energy presence. There has been described a self-regulating power system having a plurality of series connected solar cells and a chargeable battery designed for high reliability, cost effec- tive usage, and unattended applications, requiring moderate to large amounts of power delivered to a load from time to time. The load requirements may be on a constant, a periodic or an irregular consumption basis. Typical applications are marine aids to navigation, signalling systems for railroad or pipeline traffic control, communication systems, safety equipment, cathodic protection of metallic structures in pipelines, and supply of other essential power requirements where reliable conventional power sources are unavailable. WHAT WE CLAIM IS:-

1. A self-regulating power system comprising a plurality of solar photovoltaic cells connected in series, a battery connected in parallel with the series connected cells, a blocking means connected between the cells and the battery for allowing the cells to charge the battery, and wherein the number of solar cells connected in series is such that the full charge voltage of the battery is greater than the reverse bias voltage required to reduce the current output from the series connected cells, but is less than the reverse bias voltage to entirely stop the current output from the cells without overcharging the battery thereby providing self regulation

preventing overcharge of the battery.

2. A power system as claimed in claim 1 wherein the battery is of the lead-acid type.

3. A power system as claimed in either one of claims 1 to 2 wherein sixteen silicon solar photovoltaic cells are connected in series and wherein the battery has a nominal voltage of six volts.

4. A self-regulating power system comprising, a plurality of solar photovoltaic cells connected in series, a reversible electrochemical charge retaining storage battery connected in parallel with the series connected cells, electrical blocking means connected between the cells and the battery for allowing the cells to charge the battery and wherein the number of solar cells connected in series is such that the full charge voltage of the battery is greater than the breakdown voltage of the series connected cells, but is less than the reverse bias voltage to entirely stop the current output from the series connected cells without overcharging the battery thereby providing self regulation preventing overcharge of the battery.

5. A power system as claimed in any one of the preceding claims wherein the number of cells is such that self regulation occurs over the range of temperatures under which the power system is to operate.

6. A power system as claimed in any one of the preceding claims wherein said blocking means comprises a blocking diode.

7. A self-regulating power system substantially as hereinbefore described with reference to the accompanying drawings.