WO2020219995A1

WO2020219995A1 - System and method for solar cell array diagnostics in high altitude long endurance aircraft

Info

Publication number: WO2020219995A1
Application number: PCT/US2020/029974
Authority: WO
Inventors: Nader Lotfy
Original assignee: Aerovironment
Priority date: 2019-04-25
Filing date: 2020-04-24
Publication date: 2020-10-29
Also published as: WO2020219998A1; WO2020219997A1

Abstract

In one embodiment a solar array circuit is provided capable of being utilized in a high altitude long endurance aircraft. The solar array circuit includes a solar array string having a plurality of solar cells connected in series. Multiple solar array strings are connected in parallel to form a solar array channel. A plurality of MOSFET switches are provided, each being connected in series to an output of the plurality of solar cells of a solar array string so as to allow each of the plurality of solar strings within the solar array channel to be independently disconnected and connected within the solar array channel.

Description

SYSTEM AND METHOD FOR SOLAR CELL ARRAY DIAGNOSTICS IN HIGH ALTITUDE LONG ENDURANCE AIRCRAFT

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the following applications, which are all herein incorporated by reference in their entireties:

U.S. Provisional Application No. 62/838,937, filed

04/25/2019, by Nader Lotfy, entitled SYSTEM AND METHOD FOR

SOLAR ARRAY DIAGNOSTICS;

U.S. Provisional Application No. 62/893,726, filed

08/29/2019, by Nader Lotfy, entitled SYSTEM AND METHOD FOR

SOLAR ARRAY DIAGNOSTICS IN HIGH ALTITUDE LONG ENDURANCE AIRCRAFT;

U.S. Provisional Application No. 62/893,762, filed

08/29/2019, by Nader Lotfy, entitled SYSTEM AND METHOD FOR

IMPROVED SOLAR CELL ARRAY EFFICIENCY IN HIGH ALTITUDE LONG

ENDURANCE AIRCRAFT;

U.S. Provisional Application No. 62/893,766, filed

08/29/2019, by Nader Lotfy et al . , entitled METHOD FOR TRACKING AND CONTROL OF OPTIMUM OPERATING POINT FOR HIGH ALTITUDE LONG ENDURANCE AIRCRAFT SOLAR CELLS; and

U.S. Provisional Application No. 62/838,783, filed

04/25/2019, by Sechrist et al . , entitled HIGH ALTITUDE LONG ENDURANCE (HALO) AIRCRAFT.

BACKGROUND

[0001] Unmanned Aerial Vehicles (UAVs) are aircraft with no onboard pilot and may fly autonomously or remotely. UAVs require an energy source to power the motor in order to sustain flight. High altitude long endurance may use battery power to stay aloft, and sometimes for take-off. Continuous flight needs to be sustained, and returning to a ground station to recharge is to be avoided as long as possible, or even completely avoided. In such situations, battery powered UAVs are typically charged using onboard solar cells.

[0002] What is needed is a way to ensure that that the solar cells perform optimally. Further, in high altitude long endurance solar powered aircraft, what is need it a way to extract the most amount of solar energy from a solar array.

SUMMARY

[0003] In one embodiment, a solar array circuit is provided capable of being utilized in high altitude long endurance aircraft. The solar array circuit includes a solar array string having a plurality of solar cells connected in series. Multiple solar array strings are connected in parallel to form a solar array channel. A plurality of MOSFET switches are provided, each being connected in series to an output of the plurality of solar cells of a solar array string, so as to allow each of the plurality of solar strings within the solar array channel to be independently disconnected and connected within the solar array channel.

[0004] Typically, the MOSFET switch includes two serial connected MOSFETs.

[0005] In a further embodiment, a boost converter is coupled to the solar array channel. The boost converter may be configured to couple to a high voltage power bus which has a battery coupled thereto.

[0006] In yet another embodiment, a bypass MOSFET switch connected in parallel with one or more of the plurality of solar cells.

[0007] In another implementation in a high altitude long endurance solar powered aircraft, an onboard diagnostic method is provided including providing a series connected switch in series in each of a plurality of solar strings, each solar string being comprised of a plurality of solar cells, using each of the switches to individually connect and disconnect the solar strings while the high altitude long endurance solar powered aircraft is in flight. This implementation further includes detecting a functionality of each of the plurality of solar strings while using the each of the switches to individually connect and disconnect the solar string .

[ 0008 ] In some implementations, the onboard diagnostic method may include providing a series connected switch having two serial connected back to back MOSFETs.

[ 0009 ] In some implementations, the method may include detecting the functionality of one of the plurality of solar strings disposed adjacent to a leading edge of a wing of the high altitude long endurance solar powered aircraft separately from a plurality of solar panels disposed adjacent to a trailing edge of the wing of the high altitude long endurance solar powered aircraft.

[ 00010 ] In some implementations, the onboard diagnostic method may include tracking a performance of each of the plurality of solar strings over time.

[ 00011 ] In some implementations, the onboard diagnostic method may include disconnecting a solar cell string functioning below a predetermined output threshold. In some implementations, the method may include disconnecting a solar cell string that is determined to be non-functioning.

BRIEF DESCRIPTION OF THE DRAWINGS

[ 00012 ] FIG. 1 is a simplified diagram of a circuit having a solar array.

[ 00013 ] FIG. 2 is a simplified diagram of an improved circuit for a solar array.

[ 00014 ] FIG. 3 is a simplified diagram of an improved circuit for a solar array. [ 00015 ] FIG. 4 is a simplified diagram of a string circuit .

[ 00016 ] FIG. 5 is a simplified diagram of an improved solar string circuit.

[ 00017 ] FIG. 6 is a plot showing an example V-I curve of voltage versus current for a typical solar array system.

[ 00018 ] FIG. 7 is a plot illustrating an example V-I curve of voltage versus current for a solar array system for high performance solar cell utilized in high altitude long endurance aircraft implementations.

DESCRIPTION

[ 00019 ] FIG. 1 is a simplified diagram of a circuit 100 having a solar array. Typically, several independent solar arrays or strings 110a, 110b, and 110c connected in parallel to form a channel 120. In some embodiments, a string may have a number of replaceable solar panels, for example. Each of the strings 110a, 110b, and 110c is provided with a serial connected blocking diode 115a, 115b, and 115c. The blocking diode 115a, 115b, and 115c are provided in case one of the strings 110a, 110b, and 110c shorts, to prevent one of the strings 110a, 110b, or 110c from causing shorting, or other failure, in the other non-shorted strings 110a, 110b, or 110c.

Although the blocking diodes add an amount of loss to the system, they are required to reduce failures of associated the solar cells within the array, or other components within the system.

[ 00020 ] The blocking diodes are typically located within a power tracker 180, which also has a boost stage 185 DC/DC converter. The boost stage 185 decouples the supplied voltage and the current from the high voltage power bus 195, i.e. 270V-400V, that is connected to the battery 190, and is configured so that proper voltage is supplied on the high voltage power bus regardless of the voltage and current supplied by the solar array. In one embodiment, the power tracker is a maximum power point tracker or MPPT controller configured to boost voltage from the solar array to the output and to adjust a boost ratio to get the maximum power from the solar array. Examples of MPPT controllers include Outback® FLEXmax 60/80 MPPT, Xantrex® MPPT Solar Charge Controller, and Blue Sky® Solar Charge Controller. Generally speaking, the MPPT controller, which may be programmable, is configured to maximize the available power going into the battery from the solar array. This is important in various high altitude long endurance aircraft applications where the maximum voltage is a function of the temperature and illumination of the solar array, both of which may vary throughout the day.

[ 00021 ] FIG. 2 is a simplified diagram of an improved circuit 200 for a solar array. In this embodiment, in place of the blocking diodes 115a, 115b, or 115c (FIG. 1), a solar string control MOSFET 215 is used in the channel 220. This configuration is more efficient than the circuit of FIG. 1 because it eliminates the voltage drop loss across a diode 115a, 115b, or 116c when the solar string control MOSFET 215 is turned on. This embodiment, however, when the solar string control MOSFET 215 is turned on, it does not inherently provide isolate between parallel connected solar strings in the event there is a short in a solar string 210.

[ 00022 ] As such, in this embodiment, the voltage drop across the solar string control MOSFET 215 is detected and monitored so as to determine the magnitude and direction of the current in the solar string 210 to determine whether another solar string (not shown) has shorted. If it is determined, based on the monitoring of several parallel connected strings, that one of the solar strings has shorted, the solar string control MOSFET in line with the shorted solar string is opened so as not to damage the other parallel connected solar strings. With the solar string control MOSFET 215 open, it performs as a diode as in FIG. 1, to block current through the shorted solar string.

[ 00023 ] Thus, the embodiment of FIG. 2 provides a more efficient power transfer, but requires monitoring of the current in the solar string 210 to determine if a short has occurred in the solar string 210, or in any of the associated strings. Typically, the monitoring and control of the solar string control MOSFET 215 is conducted by a power tracker, or other associated electronics. The power tracker includes the solar string control MOSFET 215 and the boost stage 285, which supplies power to the power bus 295. [00024] FIG. 3 is a simplified diagram of an improved circuit 300 for a solar array. In this embodiment, in place of the blocking diodes 115a, 115b, and 115c (FIG. 1), MOSFET switches 315a, 315b, and 315c are utilized having back-to-back MOSFETS devices 315ai and 315a₂. The MOSFET switch allows each string 310a, 310b, or 310c in the channel 320 to be individually completely disable or open, even though the output of multiple strings go into a single power stage, such as a boost stage 385 DC/DC converter, which supplies current to the power bus 395 for charging the battery 390. The boost converter 385 controls the MOSFET switches 310a, 310b, or 310c and utilizes the detected string voltages to determine the health of the strings 310a, 310b, or 310c along with all the other strings (not show) and channels (not show) over the entire solar array. An advantage of this it that it allows the system to target each string 310a, 310b, or 310c and perform a lot of diagnostics in flight, such as short circuit current, open circuit voltage, on a per string basis.

[00025] Replacing the protection diodes with MOSFET switches is undesirable in a terrestrial solar system because it increases the cost of the system. Using the MOSFET switches, however, is very desirable in high altitude long endurance aircraft, where it is significant and important to extract the maximum energy from the solar array. MOSFET switches can be selected to have lower power loss across the switch as compared to a protection diode. As such, it increases the efficiency of the charging system, and also allows you to predict (through trend analysis) and detect failures much easier. Imminent failures can be predicted and action taken before a failure becomes critical. This is important in high altitude long duration aircraft, so as to enable avoidance of a critical failure that could otherwise lead to a power off, or even a crash landing. Since strings can be individually tested, it provides much more "visibility" into the functionality and health of the solar array in flight .

[ 00026 ] Moreover, each string can be tested in flight to determine the optimal power output for each string individually according to its V-I and power output characteristic. As such, the string characteristics can be tested over time to determine the health of the string. This is particularly important during long duration flights, and during high altitude flights, so that the need for maintenance and/or remedial measures, such as solar panel replacement, can be anticipated and made when convenient.

[ 00027 ] Performance tracking and health determinations based on power output characteristics may be made with the MPPT, or another onboard system processor. Similarly, some or all of the performance data could be relayed to a ground/sea station, or other aircraft, for tracking and health processing and/or analysis. Further, it allows one or more solar strings that are not performing to an acceptable predetermined threshold to be removed from the system, i.e. within 80%, 75%, 60%, 50%, or lower, such as 0% of its expected output. The solar string may be either temporarily, or permanently, disconnected depending on the functional characteristics of the solar string within a range of operating conditions, i.e. temperature, light intensity, etc.

[ 00028 ] In some embodiments, each string includes a number of solar panels grouped generally spanwise along the wing of the aircraft, for example four, five, or six smaller solar panels are grouped per string. The solar panels are grouped in this way so that the solar panels in a string experience similar environmental and operational conditions together. For example, the solar panels at the leading edge of the wing may be grouped together in a string, while solar panels near the trailing edge of the wing may be grouped together, possibly with one or more strings also extending spanwise or laterally along the span of the wing, between the leading and trailing edge strings.

[ 00029 ] In high altitude applications, the grouping of the solar panels into strings is significant. This is because the temperature can vary greatly from the leading to the trailing edge of the wing. Further, the orientation of the aircraft with respect to the sun, in elevation, azimuth, rotation, etc., as well as having a greater curvature from leading to trailing edges of the wing, can further exaggerate the temperature differential. In high altitude long endurance solar powered aircraft, the temperature can range across the wing from -60 degrees Celsius on the leading edge to +60 degrees on the trailing edge of the wing. As such, grouping solar panels into strings combined with being able to individually switch on or off individual strings based on the performance of a string allows for more efficient solar power generation .

[ 00030 ] Turning to FIG. 4, shown is a simplified diagram of a string circuit 400. In FIG. 4, the string 400 typically has a bypass diode 445 in parallel with two or more solar cells 405a and 405b. The bypass diode 445 allows the other solar cells 405m to supply current around the solar cells 405a and 405b when one or more of the solar cells becomes non-functional or an open circuit, such as by being cracked, or broken.

[ 00031 ] With reference to FIG. 4, in a further embodiment, the bypass diode 415 is replaced by a bypass

MOSFET switch 560 as shown in the simplified diagram of a solar string circuit 500 of FIG. 5. Such an embodiment, allows further efficiency over the diode when a solar cell

405a, 405b, ...or 405m (FIG. 4) fails, due to the lower power loss associated with a MOSFET switch 560 as compared to the bypass diode 445. Furthermore, it allows for closer monitoring and predictive analysis of individual, or a small group of solar cells for better predictive analysis of the string. As the maximum allowable current for the string 510 is restricted by the restriction of the lowest individual solar cell, being able to bypass only one or several individual solar cells 505a and 505b, or others, can be used to optimize power output of the string 510.

[ 00032 ] One advantage of various embodiments over string circuits using blocking diodes, is that the blocking diodes can contribute a loss of about 0.7%, whereas MOSFET switches can reduce those losses. Though discussed above with respect to a MOSFET switch, other comparable type switch, i.e light weight, low loss switch, could be utilized in other embodiments. Furthermore, although shown in FIG. 5 with only three solar strings in a channel for illustration purposes, embodiments may contain two or more strings, and multiple channels .

[ 00033 ] Various embodiments enable, or expand the capability to run in-flight diagnostics. In high altitude long endurance solar power aircraft, factors such as turbulence, frequent and extreme thermal, motor vibrations can increase the possibility of failure. Various embodiments, provide performance tracking over time, with trend analysis, and can enable eminent failure detection, more flexible scheduling of service/periodic maintenance, and avoidance of lack of airborne network capability/coverage or surveillance capability/coverage in the coverage area. This is particularly important if the high altitude long endurance aircraft is being used as a cellular repeater or for other network communications in area that would otherwise be without coverage should the platform be missing from the network.

[ 00034 ] FIG. 6 is a plot 600 showing an example V-I curve 610 of voltage versus current for a typical solar array system. The power curve 620 for the solar array system is superimposed on the plot 600. It is desirable to extract the maximum power from the solar array system. As such, it is desirable to operate along the V-I curve 610 where the power for the system is at its peak.

[ 00035 ] FIG. 7 is a plot 700 showing an example V-I curve 710 of voltage versus current for a solar array system for high performance solar cell utilized in high altitude long endurance aircraft implementations. In various high performance solar cell implementations which may be utilized in high altitude long endurance aircraft, the V-I curve 610 and the power curve (not shown) have a very steep slope as they approach the maximum current. Thus, the optimum operating point of the system lies with a narrow operating range. If the current is too great by even a slight amount, the voltage goes to zero or short circuits very easily. For example, the difference between optimum power output and short circuit can be as low as 100mA of current per channel 720 (FIG. 7) . Depending on solar cell and channel configuration, this could be even lower in some embodiments, such as 75 mA, 50 mA, 25mA, or less. To avoid this, while achieving the highest power output, a voltage loop is utilized to monitor voltage while determining the peak power operating point, as well as monitoring the current and power. This is because the change in voltage is much bigger than either the power or the current near this point.

[ 00036 ] Thus, to find the optimum operating point of the system, the current is regulated, while monitoring the voltage as well as the power. The commanded current is varied by power point tracker circuitry, while monitoring the power. Additionally, the voltage is also monitored to determine when the power output is maximized because the rate of change of the voltage is greater than the rate of change of the power at near the maximum power output operating point.

[ 00037 ] To achieve the most efficiency in some embodiments, the voltage is monitored at a faster rate than the current and power. In some embodiments, the voltage may be monitored ten time faster than the current or power. For example, the current and/or power may be monitored at 10 times a second, while the voltage is monitored at 100 times a second .

[ 00038 ] This enables various embodiments to extract the most amount of solar power from the solar panels for in high altitude long endurance aircraft applications, without drawing too much current and sending the voltage to zero, thereby shorting the solar cell.

[ 00039 ] It is important to note that factors such as the solar panel temperature and the amount of solar exposure can shift the maximum power operating point. In high altitude long endurance solar powered aircraft, these factors are more significant because the temperature range across the solar cells is more extreme, as discussed further below, and the shading or shadowing is typically experienced more frequently and to a greater degree. Thus, monitoring and adjusting the operating point is especially important in high altitude long endurance solar powered aircraft. Fig. 7 depicts example V-I curves for hotter temperature 710 and colder temperature 711 for bright solar exposure 710, and shadowed solar exposure 712.

[ 00040 ] Thus, in a high altitude long endurance solar powered aircraft, a method for controlling a solar cell array includes determining an operating point of the of the solar cells in an array. This may include regulating a current output of the solar cells, monitoring a power output of the solar cells, monitoring a voltage output of the solar cells, and varying the current output of the solar cells in response to the monitoring of the voltage output to maximize the power output of the solar cells.

[ 00041 ] As discussed above, the method may include monitoring the voltage output at a faster rate than the monitoring of the current output. This may include monitoring the voltage output at ten times faster rate than the monitoring of the current output. In some implementations, the voltage output may be monitored at one hundred times a second with the current output being monitored at ten times a second .

[00042] It is worthy to note that any reference to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment may be included in an embodiment, if desired. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment.

[00043] The illustrations and examples provided herein are for explanatory purposes and are not intended to limit the scope of the appended claims. This disclosure is to be considered an exemplification of the principles of the invention and is not intended to limit the spirit and scope of the invention and/or claims of the embodiment illustrated.

[00044] Those skilled in the art will make modifications to the invention for particular applications of the invention.

[00045] The discussion included in this patent is intended to serve as a basic description. The reader should be aware that the specific discussion may not explicitly describe all embodiments possible and alternatives are implicit. Also, this discussion may not fully explain the generic nature of the invention and may not explicitly show how each feature or element can actually be representative or equivalent elements. Again, these are implicitly included in this disclosure. Where the invention is described in device-oriented terminology, each element of the device implicitly performs a function. It should also be understood that a variety of changes may be made without departing from the essence of the invention. Such changes are also implicitly included in the description. These changes still fall within the scope of this invention.

[00046] Further, each of the various elements of the invention and claims may also be achieved in a variety of manners. This disclosure should be understood to encompass each such variation, be it a variation of any apparatus embodiment, a method embodiment, or even merely a variation of any element of these. Particularly, it should be understood that as the disclosure relates to elements of the invention, the words for each element may be expressed by equivalent apparatus terms even if only the function or result is the same. Such equivalent, broader, or even more generic terms should be considered to be encompassed in the description of each element or action. Such terms can be substituted where desired to make explicit the implicitly broad coverage to which this invention is entitled. It should be understood that all actions may be expressed as a means for taking that action or as an element which causes that action. Similarly, each physical element disclosed should be understood to encompass a disclosure of the action which that physical element facilitates. Such changes and alternative terms are to be understood to be explicitly included in the description.

[00047] Having described this invention in connection with a number of embodiments, modification will now certainly suggest itself to those skilled in the art. The example embodiments herein are not intended to be limiting, various configurations and combinations of features are possible. As such, the invention is not limited to the disclosed embodiments, except as required by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. In a high altitude long endurance aircraft, a solar array circuit comprising:

a) a solar array string comprising a plurality of solar cells connected in series;

b) a solar array channel comprising a plurality of the solar array strings connected in parallel; and

c) a plurality of MOSFET switches each being connected in series to an output of the plurality of solar cells of a solar array string so as to allow each of the plurality of solar strings within the solar array channel to be independently disconnected and connected within the solar array channel.

2. The circuit of Claim 1, wherein the MOSFET switch comprises two series connected complementary MOSFETs.

3. The circuit of Claim 2 further comprising a boost converter coupled to the solar array channel.

4. The circuit of Claim 3, wherein the boost converter is configured to couple to a high voltage power bus having a battery coupled thereto.

5. The circuit of Claim 4 further comprising a bypass

MOSFET switch connected in parallel with at least one of the plurality of solar cells.

6. The circuit of Claim 5, wherein the bypass MOSFET switch is connected in parallel with multiple solar cells of the plurality of solar cells.

7. The circuit of Claim 6 wherein the bypass MOSFET switch comprises two series connected complementary MOSFETs.

8. The circuit of Claim 1 further comprising a boost converter coupled to the solar array channel.

9. The circuit of Claim 8, wherein the boost converter is configured to couple to a high voltage power bus having a battery coupled thereto.

10. The circuit of Claim 1 further comprising a bypass

11. The circuit of Claim 10 wherein the bypass MOSFET switch is connected in parallel with multiple solar cells of the plurality of solar cells.

12. The circuit of Claim 11 wherein the bypass MOSFET switch comprises two series connected MOSFETs.

13. In a high altitude long endurance solar powered aircraft, an onboard diagnostic method comprising:

a) providing a series connected switch in series in each of a plurality of solar strings, each solar string being comprised of a plurality of solar cells;

b) using each of the switches to individually connect and disconnect the solar strings while the high altitude long endurance solar powered aircraft is in flight; and

c) detecting a functionality of each of the plurality of solar strings while using the each of the switches to individually connect and disconnect the solar string .

14. The onboard diagnostic method of Claim 13, wherein providing the series connected switch comprises providing two serial connected back to back MOSFETs.

15. The onboard diagnostic method of Claim 13, wherein detecting comprises detecting the functionality of one of the plurality of solar strings comprised of a plurality of solar panels disposed adjacent to a leading edge of a wing of the high altitude long endurance solar powered aircraft separately from a plurality of solar panels disposed adjacent to a trailing edge of the wing of the high altitude long endurance solar powered aircraft.

16. The onboard diagnostic method of Claim 13, further comprising disconnecting a solar cell string determined to be non-functioning .

17. The onboard diagnostic method of Claim 13, further comprising disconnecting a solar cell string functioning below a predetermined output threshold.

18. The onboard diagnostic method of Claim 13, further comprising tracking a performance of each of the plurality of solar strings over time.

19. The onboard diagnostic method of Claim 18 further comprising further comprising disconnecting a solar cell string functioning below a predetermined output threshold.

20. In a high altitude long endurance aircraft, a solar array circuit comprising:

c) a single MOSFET transistor connected in series to each output of the plurality of solar cells of a solar array string so as to allow each of the plurality of solar strings within the solar array channel to be independently disconnected and connected within the solar array channel.

21. The circuit of Claim 20 further comprising a boost converter coupled to the solar array channel.

22. The circuit of Claim 21, wherein the boost converter is configured to couple to a high voltage power bus having a battery coupled thereto.

23. The circuit of Claim 20 further comprising a bypass MOSFET switch connected in parallel with at least one of the plurality of solar cells.

24. The circuit of Claim 23 wherein the bypass MOSFET switch is connected in parallel with multiple solar cells of the plurality of solar cells.

25. The circuit of Claim 24 wherein the bypass MOSFET switch comprises two series connected complementary MOSFETs.