Abstract:
A method for protecting the photovoltaic cells in a photovoltaic (PV) array from reverse bias damage by utilizing a rechargeable battery for bypassing current from a shaded photovoltaic cell or group of cells, avoiding the need for a bypass diode. Further, the method mitigates the voltage degradation of a PV array caused by shaded cells.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 10/440,378 which was filed on May 16, 2003, and incorporated herein by reference in its entirety. 
    
    
     ORIGIN OF THE INVENTION 
     The invention described herein was made by employees of the U.S. Government and may be manufactured and used by or for the Government for Government purposes without the payment of any royalties thereon or therefore. 
    
    
     BACKGROUND OF THE INVENTION 
     This application relates generally to the protection of photovoltaic cells from reverse bias damage. 
     More specifically, this application relates to protecting the photovoltaic cells of a photovoltaic (PV) array from reverse bias damage by utilizing a rechargeable battery for bypassing current from a shaded photovoltaic cell or group of cells. Further, the invention mitigates the voltage degradation of a PV array caused by shaded cells. 
     Photovoltaic (PV) arrays, also known as solar arrays, are typically comprised of a plurality of photovoltaic cells (also known as solar cells) arranged in series in order to increase the voltage level of a PV array to a more usable amount, typically to 28-30 volts or even 120 volts. A plurality of series connected PV cells can then be connected in parallel to increase the current (and power) capability of the PV array. PV arrays are used extensively for terrestrial, orbital, and extra-terrestrial (e.g., planetary or interplanetary) uses. 
     These individual photovoltaic cells are typically constructed of a crystalline or amorphous silicon, or some other semiconductor material, such as the commonly used Gallium Arsenide (GaAs). When exposed to sunlight, these PV cells typically generate a voltage ranging from 0.50 to 2.5 volts each, depending on the materials used. The voltage of the PV portion of the device is determined by the nature of the p-n junction of the photovoltaic cell, or, in other words, the materials used. In the case of a GaAs homo-junction device this will be around 1.0 V. For thin-film a-Si or CuInSe2 (CIS) PV, the voltage generated will be somewhat less (0.4-0.8 V). Accordingly, strings of 30 or more photovoltaic cells are typically strung in a series to form a solar array in order to gain the desired output voltage. 
     Individual arrays of a series connected plurality of PV cells may then be placed in parallel in order to increase the total current and power capacity of the resulting entire PV array. A multiplicity of such arrays could further be combined to increase the power availability even more. The voltage output of individual arrays or the combination of arrays can then be modified using a DC-to-DC converters and/or a DC-to-AC inverters to generate a voltage useful for the typical electrical loads to be powered. 
     However, a problem arises when individual cells of the series connected photovoltaic cells are not generating electricity, such as when some subset of cells is shaded, for example. Because the current through series connected PV cells must pass through each cell in the series, if one or more individual PV cells are shaded, the current generated by the unshaded cells in the solar array must pass through the shaded cells as well. 
     This current through the shaded cell(s) results in a reverse bias across the cell, and can lead to “hot-spot” heating, which can damage the shaded cell. This problem is well-known in the art, and is also called “reverse-bias degradation”, “breakdown”, “shading”, and “shadowing” effects, for example. In the extreme, such “hot-spot” heating can destroy a photovoltaic cell, and thus degrade the array, or make it useless. 
       FIG. 1  shows a graphical example of such “hot-spot” heating, with curve  14  showing the operating points of 30 unshadowed cells (with point  14  representing the operating point with a partially shadowed cell) and with curve  11  representing the single, partially shadowed cell, operating far at the reverse bias point  12 . Line Z represents a constant current line, and line H the nominal operating voltage. Quadrant A represents a reverse-bias, power dissipating area whereas quadrant B represents the power generating forward bias area. 
     Most localized shadowing, however, is transient, lasting only seconds or minutes. Shadowing of the entire solar array is not relevant to the above problem, because only partial or uneven shadowing leads to the “hot-spot” heating effect. 
     Conventional approaches for protecting the individual cells of a solar array include putting a “bypass diode” in parallel with each photovoltaic cell.  FIG. 2  shows such an implementation. The bypass diode then shunts the series current of the solar array from the one or more cells that are shaded, protecting the shaded cells from damage. 
     Nevertheless, there are undesirable side-effects to this traditional approach. For example, the entire solar array loses operating voltage whenever one or more cells is shadowed. The amount of this voltage degradation is determined from the voltage no longer generated by the individual shaded cell(s), plus the turn-on voltage of the corresponding bypass diode(s), typically leading to a net voltage drop across the shaded cell, in contrast to the typical voltage rise of a voltage generating, unshaded cell. If the voltage of the solar array drops below the required bus voltage of the solar array, the entire array may not produce useful power. In practice, a shadow of as little as one percent might block one-hundred percent of the solar array output. 
     Accordingly, an approach that can overcome the above identified shortcomings would be desirable. 
     Further, it would be useful to utilized thin-film manufacturing processes for implementing the invention. Thin-film photovoltaic (TFPV) power generation has been under development for some time. TFPV sample cells and panels have flown in space. The principle benefits of TFPV arrays include very high mass specific power (W/kg), radiation tolerance and good stowability. The mission benefits of TFPV solar arrays have been identified, and may be realized when full scale TFPV arrays are constructed and space qualified. 
     In comparison to TFPV power generation, thin-film energy storage (TFES) is a relatively recent development. Very small thin-film lithium-ion batteries have been developed and tested in the lab for use in multi-chip modules (MCMs). With a typical operating range between 3.0 V and 4.2 V, the useable capacity of these initial TFES batteries is very small, ranging from 0.2 to 10 mAh/cm2. The energy capacities of thin-film batteries are typically too low to allow thin-film batteries to serve as primary energy storage for an array, but, can prove useful to solving some of the problems identified above. 
     Because of the similarity in the materials and processes that go into TFPV and TFES devices, it is practical to consider a combination of the two technologies. Further, a solution that in addition to providing protection against hot-spot heating, also enables some energy storage capability for momentary shading of the entire array, would add desirable additional benefit to the design. 
     SUMMARY OF THE INVENTION 
     Provided is a photovoltaic array comprising a photovoltaic battery including a photovoltaic cell and rechargeable battery connected in parallel with the photovoltaic battery, wherein, when the photovoltaic cell is shaded, the rechargeable battery shunts an array current including current not generated by said rechargeable battery from the photovoltaic cell that is shaded. 
     Also provided is a photovoltaic array comprising a plurality of photovoltaic modules connected in series, each photovoltaic module including a photovoltaic battery having a photovoltaic cell; and a rechargeable battery having a rechargeable cell and connected in parallel with the photovoltaic battery. 
     Further provided is a photovoltaic array comprising: a photovoltaic battery including a photovoltaic cell; and a rechargeable battery connected in parallel with the photovoltaic battery. When the photovoltaic cell is shaded, the rechargeable battery is used for compensating for a voltage drop of the photovoltaic battery due to the shaded photovoltaic cell while the photovoltaic array is generating useable power from light. 
     Still further provided is a photovoltaic module comprising a photovoltaic battery including a photovoltaic cell and a rechargeable battery connected to the photovoltaic battery for shunting a current from the photovoltaic battery when the photovoltaic cell is shaded to protect the photovoltaic cell. 
     Even further provided is a photovoltaic array comprising a photovoltaic battery including a plurality of photovoltaic cells connected in series; and a rechargeable battery including at least one rechargeable cell and connected in parallel with the photovoltaic battery. When one or more of the plurality of photovoltaic cells is shaded, the rechargeable battery shunts an array current of the photovoltaic array from the photovoltaic battery to protect the one or more shaded photovoltaic cells from damage from the array current while the photovoltaic array is generating power from light. 
     And provided is a photovoltaic array comprising: a plurality of photovoltaic modules connected in series. Each photovoltaic module includes: a photovoltaic battery having one or more photovoltaic cells connected in series; and a rechargeable battery having one or more rechargeable cells connected in series. 
     The rechargeable battery is connected in parallel with the photovoltaic battery. The rechargeable battery is for shunting an array current of the photovoltaic array from the photovoltaic battery when at least one of the photovoltaic cells is shaded to protect the shaded photovoltaic cells from damage from the array current while the photovoltaic array is generating power, and the rechargeable battery is also for compensating for a voltage drop of the shaded photovoltaic cells while the photovoltaic array is generating useful power. 
     And even further provided is an integrated power supply comprising a photovoltaic cell; and a rechargeable battery connected to the photovoltaic cell. The rechargeable battery is integrated with the photovoltaic cell on a thin-film substrate. 
     Additionally provided is an integrated power supply comprising a photovoltaic battery including a photovoltaic cell; and a rechargeable battery including a rechargeable cell. The rechargeable battery is connected in parallel with the photovoltaic battery, and the rechargeable cell is integrated with the photovoltaic cell on a thin-film substrate. 
     Also provided is an integrated power supply comprising a plurality of modules connected in series. Each module includes: a photovoltaic battery including one or more photovoltaic cells; a rechargeable battery including one or more rechargeable cells; and a blocking diode for connecting one terminal of the photovoltaic battery to one terminal of the rechargeable battery (such as connecting the diode in series with the photovoltaic battery, for example). 
     The rechargeable battery is connected in parallel with the photovoltaic battery, and the rechargeable battery is for shunting a current of the photovoltaic array from the photovoltaic battery when one or more of the photovoltaic cells is shaded to protect the one or more of the plurality of photovoltaic cells that are shaded from damage from the current while the photovoltaic array is generating power from light. 
     The rechargeable battery is also for compensating for a voltage drop of the shaded photovoltaic cell while the integrated power supply is generating power from light, and the rechargeable cells are integrated with the photovoltaic cells on a thin-film substrate. 
     Each module also includes conditioning and control electronics for conditioning and controlling a charging and/or discharging current of the integrated power supply. 
     And further provided is a photovoltaic array comprising a plurality of PV modules connected in series. Each PV module includes: a photovoltaic battery having one or more photovoltaic cells connected in series; a rechargeable battery having one or more rechargeable cells connected in series, and a blocking diode for connecting an electrode of the photovoltaic battery connected to an electrode of the rechargeable battery. The rechargeable battery has another electrode connected to another electrode of the photovoltaic battery, and the blocking diode prevents the rechargeable battery from discharging through the photovoltaic battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the graph of current as a function of voltage for a photovoltaic array, and shows as a graphical plot the reverse-bias “hot spot” heating on a shaded PV cell; 
         FIG. 2  shows a PV cell protected by a bypass diode; 
         FIG. 3  is a schematic of an embodiment of the invention showing a rechargeable battery in parallel with a PV cell; 
         FIG. 4  is a schematic of another embodiment showing a rechargeable battery connected in parallel with a serially connected pair of PV cells; 
         FIG. 5  is a schematic of still another embodiment showing a plurality of series connected rechargeable cells forming a rechargeable battery connected in parallel with a pair of series connected PV cells forming a PV battery; 
         FIG. 6  is a schematic of a generic embodiment with a to be determined number of PV cells forming a PV battery and a to be determined number of rechargeable cells forming a rechargeable battery. 
         FIG. 7  is a representation of the thin-film PV cell and rechargeable battery cell; 
         FIG. 8  is a representation of the schematic of  FIG. 3  using the thin-film representation of  FIG. 7  and adding a blocking diode. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  shows the traditional means of protecting a solar array from shadowing effects which can cause hot-spot heating as shown in the graph of  FIG. 1 . (Note that a single photovoltaic cell is conventionally represented in a circuit diagram as a current source plus a diode, where the diode represents the p-n junction, an integral part of the cell) In this means of shadow protection, the photovoltaic cell  20  is paralleled with a bypass diode  22 . A number of such cells, each with bypass diode, is then connected in series to form a photovoltaic array. As discussed in the Background section above, however, such an implementation has undesirable side-effects. 
     Disclosed herein is a means of preventing hot-spot heating during transient shadow by placing a rechargeable battery  31  in parallel with the photovoltaic cell  30 , as shown in  FIG. 3 . In the resulting design, the “hot-spot” destruction of a shadowed photovoltaic cell can be avoided without using a bypass diode. The battery is charged by the normal, sunlit operation of the photovoltaic cell, but when the photovoltaic cell is shadowed, the majority of the solar array current flows through the battery instead of the photovoltaic cell, thereby protecting the PV cell. 
     For example, a typical transient shadow on a spacecraft (such as an antenna shadow) may last for about two minutes. The eclipse time for a spacecraft in low orbit, for comparison, is about 40 minutes. If the array provides one amp of current, shadow protection by a diode will require a battery of storage capacity 33 milliamp-hours, while providing a storage for eclipse power requires a battery of storage capacity 667 milliamp hours. Thus, it is clear that the shadow protection function can be accomplished by a battery of considerably lower storage capacity than that required for eclipse power. (However, if the battery also is large enough in capacity to provide eclipse power, this would be an added benefit). For use on the surface of the Earth, the situation is even worse. Using a battery to provide 12 hours of night-time power would require 12,000 milliamp-hours of storage, considerably more than the 33 milliamp-hours required to provide protection for a two minute shadow. 
     In addition, because the rechargeable battery  31  generates a voltage of its own, the degradation of the voltage of the series connected array can be greatly reduced to only the difference between the typical shaded PV cell voltage when in sunlight and the battery voltage. This voltage difference can be minimized by closely matching the battery voltage of the chosen battery to that of the chosen individual photovoltaic cell generating voltage. Because there are alternative battery and solar cell designs available, many potential embodiments exist. Close matching also ensures that the photovoltaic cell  30  does not overcharge, and thus damage, the rechargeable battery  30 . Alternatively, protective circuits incorporated within the device (such as within the battery) could prevent battery overcharging or maintain the desired battery voltage. 
     Of course, this lack of voltage degradation lasts only as long as the rechargeable battery can maintain its charge. As the battery charge is depleted, the array voltage will begin to degrade. However, because most cell shadowing during array use is transitory, by choosing batteries of sufficient energy storage capability, the array can be designed to avoid such degradation under most circumstances. 
     Unfortunately, matching battery voltages to photovoltaic cell voltages can be problematic. Thus, examples of alternative configurations are shown in  FIGS. 4 ,  5  &amp;  6 . These configurations all show a basic photovoltaic module using a photovoltaic battery with one or more photovoltaic cells connected in series. Further, the photovoltaic battery may have additional protection and/or conditioning circuits. The photovoltaic module also uses a rechargeable battery having one or more rechargeable cells connected in series. Again, the rechargeable battery may have additional conditioning and/or protection circuits. The rechargeable battery is then connected in parallel to the photovoltaic battery. Additional electronics could be added to the module for conditioning and/or protection instead of, or in addition to, any additional electronics in either of the batteries. 
       FIG. 4  shows a particular photovoltaic module with a rechargeable battery  41  placed in parallel with a pair of series connected photovoltaic cells  40  (the PV cells  40  thereby forming a photovoltaic battery  42 ). Thus, a single rechargeable battery  41  protects the photovoltaic battery  42  comprising the series-connected pair of photovoltaic cells  40 . This approach allows the rechargeable battery voltage to be approximately double the individual photovoltaic cell voltages. 
     As an example of an implementation of  FIG. 4 , one 4.2 Volt Lithium CoO 2  rechargeable battery could protect a pair of 2.1 Volt dual-junction series connected photovoltaic cells. 
     A further alternative would be to use more than one series connected rechargeable cell (forming a rechargeable battery) to protect a single photovoltaic cell. Thus, even more flexibility can be provided for engineering an optimum solution. An example of this implementation would be using two 1.2 Volt series connected NiCd or NiH battery cells to protect a single 2.5 Volt triple-junction photovoltaic cell. 
       FIG. 5  shows another alternative photovoltaic module using two series-connected photovoltaic cells  50  (forming a photovoltaic battery  52 ) with three series-connected rechargeable cells  51  (forming a rechargeable battery  54 ). The photovoltaic battery  52  is connected in parallel with the rechargeable battery  54 . For this implementation, each rechargeable cell  51  should have a charged voltage of about  2 / 3  of the voltage of a single photovoltaic cell  50 . Thus, the rechargeable battery  54  comprised of the series of three rechargeable cells  51  should have approximately the same voltage as the photovoltaic battery  52  comprised of the series of two photovoltaic cells  50 . 
     Finally,  FIG. 6  shows a flexible generic configuration of a photovoltaic module having a to-be-determined number of PV cells  60  and a to-be-determined number of rechargeable cells  61 . The number of PV cells and battery cells, which do not have to be equal, is determined using the design constraints discussed above and below. However, a design using a single PV cell and/or a single rechargeable battery cell as shown in  FIG. 3  could also be utilized. 
     Thus,  FIG. 6  allows for additional variations to utilize various numbers of photovoltaic cells connected in series to form a photovoltaic battery, and then connected in parallel to one or more rechargeable cells connected in series forming a rechargeable battery. In this manner, rechargeable battery voltage  64  can be accurately matched to the photovoltaic battery voltage  62 , allowing a wide variation of rechargeable cell and/or photovoltaic cell design materials to be utilized and voltages to be closely matched. An optional blocking diode  65  can be made part of the photovoltaic battery, for example, to prevent the rechargeable battery  64  from discharging through the photovoltaic battery  62 . A blocking diode could be utilized in any of the embodiments discussed above in a similar manner for the same reason. 
     Still, care must be taken to ensure that the final approach does not result in too many photovoltaic cells in series being protected by a rechargeable battery because of the potential of hot-spot heating. If only a single photovoltaic cell of a protected series is shadowed, there would be a reverse-bias voltage on that shadowed cell equal to the voltage generated by the unshadowed photovoltaic cells of that series. If too many photovoltaic cells are utilized in series, then damage to the shadowed cell is again possible due to hot-spot heating. 
     Accordingly, there will likely be an upper limit on the number of photovoltaic cells that can be safely and serially connected together to be connected to a rechargeable battery. That upper limit will depend on the type of photovoltaic cell and its material composition, for example. Thus, care must be taken in determining how many serial photovoltaic cells should be protected by a single rechargeable battery cell or series of rechargeable cells. The optimum number will depend on the materials chosen for the photovoltaic cells and the desirable rechargeable battery choice. Hence, engineering tradeoffs must be made. In practice, then, the greatest protective benefit is likely to be obtained when the number of cells series connected and protected by a single battery or string of series-connected cells is about five or fewer. 
     Finally, a photovoltaic array is created by stringing any number of photovoltaic modules together in series, forming a series array. Further, any number of series arrays could also be connected in parallel to form an even higher current/power array. In this manner, the photovoltaic modules become building blocks for building photovoltaic arrays, and thus provide great flexibility in forming a variety of array sizes and capacities for various applications. At the same time, any shaded photovoltaic cells in a given module are protected from hot-spot heating damage by the current bypassing action of the corresponding rechargeable battery. In this manner, no bypass diodes need be integrated with the photovoltaic cells to protect them. 
     A further enhancement of the invention is to use rechargeable thin-film battery technology in conjunction with photovoltaic cell fabrication processes to integrate the thin-film battery with the photovoltaic cell on a substrate as shown in  FIG. 7 , which shows a photovoltaic cell  70  having the semiconductor layer(s)  71  covered by a front metalization layer  72  and a back metalization layer  73  for providing the battery electrodes. This photovoltaic cell  70  can be combined with a thin-film battery  75  having a negative (anode) layer  76 , a electrolyte layer  77 , and a positive (cathode) layer  78 . These can be combined as shown in  FIG. 8  to form an Integrated Power Supply (IPS), with electrical connections  82  and  83  to allow the integrated power supply to be connected in series with additional IPS units to form an array. Optionally, a blocking diode  81  may be used to prevent the battery from discharging through the PV array during eclipse. A blocking diode is most useful to protect across multiple IPSs and less beneficial if one is put on each IPS. A tab  79  is shown indicating the electrical connection to the center layer of the sandwich. 
     The battery in this example  FIG. 8  is shown with the negative (cathode) layer on the top side in contact with the solar cell back metallization; however, the configuration of battery with the positive layer connected to the solar cell back metallization can also be used, and is preferable for the n on p polarity of cell. If an electrically insulating layer is used between the solar cell back metallization  73  and the battery, then either configuration (anode on top or cathode on top) will function. If the solar cell back metallization is electrically connected to the battery, then the preferred configuration for a p-on-n type solar cell is to have the negative battery electrode on the side in contact with the solar cell; and for the n on p polarity of cell the configuration of battery with the positive layer connected to the solar cell back metallization is preferable. 
     Because of the similarity in the materials and processes that go into TFPV and TFES devices, it is practical to consider combination of the two to practice the invention. It is feasible to combine a TFPV cell on a substrate material (such as Kapton® made by DuPont, for example) with a Li-ion thin-film battery sandwiched in the substrate material. With the further addition of very small power conditioning and control electronics, a compact and useful Integrated Power Source (IPS) is possible. 
     The voltage of a Li-ion battery is based on its chemistry and is primarily determined by the material used in its cathode. A vanadium pentoxide or manganese oxide battery will have an open circuit voltage of 3.0 V, whereas a nickel cobalt cell will be 4.2 V. 
     In a way similar to PV cells, Li battery cells can be connected in series configurations to produce different voltages. However, the amount of energy that can be stored in a cell, its capacity, is determined primarily by its volume. Thus for a thin-film Li-ion battery, the capacity will be determined in the same way the current capability of the PV cell is determined—by the area of the device. The size also impacts the rate at which a battery can be charged and discharged (i.e., the smaller the battery the smaller the charging and discharging currents it can handle). 
     Ideally, in order to minimize the control electronics associated with a battery, the photovoltaic array should be designed such that its output voltage matches the voltage needs of the battery and its current output is sufficient to charge the battery while simultaneously providing power to the load. The precise sizing of the array and battery will also be dependent on the duration of shadow. 
     The matching of the solar array and batteries for these small power systems is essential as the parasitic power loss in a conventional charge controller normally used in a larger power system might actually exceed the output of a small IPS. Once the PV and battery are matched, the only additional components required are a blocking diode if it is desired to prevent the battery from discharging through the PV array during eclipse. 
     The Li-ion batteries play a large role in determining the temperature regime in which these systems are suitable. Li-ion cells will deliver a sizeable fraction (i.e. 80%) of their capacity at temperatures as low as −20° C. Below such a temperature they do not perform well. However, they do not exhibit permanent damage if they are cycled between larger temperatures regimes (i.e., plus or minus 80° C.). The high temperature performance is much less of an issue with thin-film Li-ion batteries as they have been shown to operate well at temperatures up to 60° C. 
     The invention has been described hereinabove using specific examples and embodiments; however, it will be understood by those skilled in the art that various alternatives may be used and equivalents may be substituted for elements or steps described herein, without deviating from the scope of the invention. Modifications may be necessary to adapt the invention to a particular situation or to particular needs without departing from the scope of the invention. It is intended that the invention not be limited to the particular implementation described herein, but that the claims be given their broadest interpretation to cover all embodiments, literal or equivalent, covered thereby.