WO2020219994A1

WO2020219994A1 - Aircraft power bus architecture and power bus stabilization

Info

Publication number: WO2020219994A1
Application number: PCT/US2020/029973
Authority: WO
Inventors: Thaddeus MATUSZESKI; William SECHRIST; James Daley
Original assignee: Aerovironment
Priority date: 2019-04-25
Filing date: 2020-04-24
Publication date: 2020-10-29

Abstract

In one embodiment, a solar powered high altitude long endurance aircraft power bus architecture is provided which includes a main DC power bus, with a battery connected to the main DC power bus. This embodiment includes a plurality of solar panels coupled to the main DC power bus via a plurality of DC to DC converters such that each of the plurality of solar panels is coupled to the main power bus via one DC to DC converter of the plurality of DC to DC converters. A plurality of propeller drive units are coupled to the main DC power bus via a plurality of inverters such that each of the plurality of propeller drive units is coupled via an inverter to the main DC power bus.

Description

PCT Patent Application

FOR:

AIRCRAFT POWER BUS ARCHITECTURE AND POWER BUS STABILIZATION

BY: Thaddeus Benjamin MATUSZESKI, James Gallager DALEY, and William Stuart SECHRIST

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. 62/838,783, filed 04/25/2019, by William Sechrist et al . , entitled HIGH ALTITUDE LONG ENDURANCE (HALO) AIRCRAFT;

U.S. Provisional Application No. 62/838,936, filed 04/25/2019, by Thaddeus Matuszeski et al . , entitled AIRCRAFT POWER BUS ARCHITECTURE AND POWER BUS STABILIZATION; and.

U.S. Provisional Application No. 62/897,985, filed 09/09/2019, by Thaddeus Matuszeski et al . , entitled AIRCRAFT POWER BUS ARCHITECTURE AND POWER BUS STABILIZATION.

BACKGROUND

[ 0001 ] In high altitude long endurance aircraft, there is a need to ensure that the power bus is stabilized in flight. With fuel powered high altitude long endurance aircraft, a power module configured to provide power via a fuel cell, such as disclosed in U.S. Patent 8,011,616, by MacCready et al . , issued Sept. 6, 2011, which is herein incorporated by reference in its entirety .

[ 0002 ] Aircraft power management is important in aircraft that utilize fuel to provide electrical power for the aircraft system, such as the hydrogen powered fuel cell system disclosed in U.S. Patent 8,457,860, by Matuszeski et al . , issued on June 4, 2013, which is herein incorporated by reference. Such a system has the advantage of a continuous supply of power for the system. In solar powered high altitude long endurance aircraft this is not the case, as clouds, aircraft shading/orientation, sunlight position/angle, and darkness cause the aircraft to have diminished or no capability to generate power.

[0003] What is needed is a bus architecture and power stabilization for solar powered aircraft.

SUMMARY

[0004] In one embodiment, a solar powered high altitude long endurance aircraft power bus architecture is provided which includes a main DC power bus, with a battery connected to the main DC power bus. This embodiment includes a plurality of solar panels coupled to the main DC power bus via a plurality of DC to DC converters such that each of the plurality of solar panels is coupled to the main power bus via one DC to DC converter of the plurality of DC to DC converters. A plurality of propeller drive units are coupled to the main DC power bus via a plurality of inverters such that each of the plurality of propeller drive units is coupled via an inverter to the main DC power bus .

[0005] In another embodiment, a solar powered high altitude long endurance aircraft power bus architecture is provided which includes a main DC power bus with a battery connected to the main DC power bus via a relay. This embodiment also includes a solar panel coupled to the main DC power bus via a maximum power point tracker, the maximum power point tracker having a DC to DC converter coupling the solar panel to the main DC power bus. An AC propeller drive unit is coupled to the main DC power bus via an inverter.

[0006] In one implementation, a method to provide high voltage bus stability is provided. This includes maintaining a high voltage within an allowable voltage range on a high power bus by coordinating the maximum power point tracker and the propeller drive unit and causing the maximum power point tracker to reduce a power supplied to the high voltage bus upon reaching a maximum voltage setpoint corresponding to a battery full charge, and by controlling the propeller drive unit to either consume additional power or to reduce consumption upon reaching either the maximum voltage setpoint or a minimum voltage setpoint so as to stabilize the high voltage bus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 is a simplified schematic of a possible power bus system architecture for a solar powered high altitude long endurance aircraft.

[0008] FIG. 2 is a simplified schematic of a power bus system architecture embodiment for a solar powered high altitude long endurance aircraft.

[0009] FIG. 3 is a plot showing an example I-V curve of current versus voltage for a typical solar array system.

[00010] FIG. 4 is a plot illustrating an example I-V curve of current versus voltage for a solar array system for high performance solar cell utilized in high altitude long endurance aircraft implementations.

[00011] FIG. 5 is a simplified flow diagram depicting maximum power point tracking in accordance with an implementation of the present invention.

[00012] FIG. 6 is a simplified flow diagram depicting an implementation for power bus stabilization. DESCRIPTION

[00013] FIG. 1 shows a possible power bus system architecture 100 for a solar powered high altitude long endurance aircraft. In the architecture 100, one or more propeller drive units or PDUs 120 are connected to the main power bus 110. One or more solar cell arrays 130 are also connected to the main power bus 110. One or more batteries 140 are connected to the bus via a DC/DC converter 145. Since the voltage output of the solar cell array 130 supplied to the bus 110 can fluctuate based on its temperature, the DC/DC converter 145 regulates the voltage to the battery 140. The PDUs 120 are controlled in response to the voltage supplied on the bus 110, to adapt to the voltage being supplied on the bus 110.

[00014] Turning to FIG. 2, in an alternate power bus system architecture embodiment 200 for a solar powered high altitude long endurance aircraft, the battery is connected to the bus 210, while the solar arrays 230 are connected via DC/DC converters 235. The PDUs (motors) 220 are also each connected via an inverter 225 to supply alternating current to the PDUs 220. If there is a payload 250, such as a camera, sensor array, deployable or launchable device, or other device that requires bus power, it would be connected via a converter 255, such as DC/DC converter.

[00015] Optionally, the battery (s) 240 may be connected to the bus via a relay 245, or a solid state switch, so that one or more of the batteries 240 may be disconnected from the bus when desired, such as due to its functional/operational health, for example removing non-functioning batteries, or to prevent damage to the battery, for example to prevent overcharging, thermal degradation, etc., or to prevent damage to the main power bus or other components on the bus. In some embodiments, a battery may include several battery packs, which may be made up of multiple battery cells. In such embodiments, the individual battery packs may be connected to the bus via its own relay, or solid state switch. In still further embodiments, the individual battery cells may be connected via its own switch, or relay to the main high voltage power bus. The main power bus is a high voltage power bus that typically operates in a range of from about 270 volts to about 400 volts.

[00016] In some embodiments, there may be multiple solar array panels 230a and 230b, such as more than one hundred solar panels, or more, for example there may be one hundred and ten panels, two hundred, three hundred, or more. As such, regardless of the output of each of the panels, the DC/DC converter for each of the one hundred and ten panels, or for strings of panels, for example, would convert the voltage for each solar array panel 230a or 230b, or string of solar array panels, to the voltage of the main power bus 210. As such, there may be twenty- two DC/DC converters, one for every solar string of five solar panels, for example. The solar panels need not be equally divided among the DC/DC converters, so in some embodiments there may be twenty eight DC/DC converters employed, for example.

[00017] Additionally, there may be multiple PDUs or motors to propel the aircraft. In some embodiments, there may be ten PDUs, which would each have an inverter 225. Although it is preferred to have separate inverters to supply power and control separately to each of the PDUs, in an alternate embodiment, a single inverter 225 may supply phased voltage to all of the PDUs 220.

[00018] In some alternate embodiments, DC motors could be utilized so that the inverter (s) would not be necessary. In such an embodiment, a DC/DC converter could be used instead of the inverter (s) 225, depending on the motor characteristics and desired operating profile of the PDUs 220.

[00019] In some embodiments, the DC/DC converter 235 are contained within a power tracker 260. In one embodiment, the power tracker is a maximum power point tracker 260 or MPPT charge 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 charge controllers include Outback® FLEXmax 60/80 MPPT, Xantrex® MPPT Solar Charge Controller, and Blue Sky® Solar Charge Controller. Generally speaking, the MPPT charge controller, which may be programmable, is configured to maximize the available power going into the main power bus from the solar array. This is important in various high altitude long endurance aircraft applications where the optimum voltage is a function of the solar array temperature which is a function of illumination of the solar array, altitude, airspeed, etc., all of which may vary throughout the day.

[ 00020 ] Regarding power tracking, FIG. 3 is a plot 300 showing an example I-V curve 310 of current versus voltage for a typical solar array system. The power curve 320 for the solar array system is superimposed on the plot 300. It is desirable to extract the maximum power from the solar array system. As such, it is desirable to operate along the I-V curve 310 where the power for the system is at its peak.

[ 00021 ] FIG. 4 is a plot 400 showing an example I-V curve 410 of current versus voltage 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 I-V curve 310 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, for some extremely high efficiency solar arrays, an increase of 5% in current from the maximum power point will cause about 50% reduction in voltage which would cause a significant reduction in power. 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 the change in either the power or the current near this point.

[00022] Turning to FIG. 5, to find the optimum operating point of the system, the output current is regulated 515, while monitoring the output voltage 535, as well as the output power 525. The commanded current is varied 545 by maximum power point tracker circuitry, while monitoring 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 near the maximum power output operating point.

[00023] 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 time a second.

[00024] This enables various embodiments to extract the most amount of solar power from the solar panels in high altitude long endurance aircraft applications, without drawing too much current and sending the voltage to zero, thereby shorting the solar cell.

[00025] 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 maximum solar intensity at any given time changes with aircraft heading in addition to time of day. Thus, monitoring and adjusting the operating point is especially important in high altitude long endurance solar powered aircraft. Fig. 4 depicts example I-V curves for hotter temperature 410 and colder temperature 411 for higher solar intensity 410, and lower solar intensity 412.

[ 00026 ] 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 solar array. This may include regulating a current output of the solar array, monitoring a power output of the solar array, monitoring a voltage output of the solar array, and varying the current output of the solar array in response to the monitoring of the voltage output to maximize the power output of the solar array.

[ 00027 ] 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.

[ 00028 ] High voltage bus stability is provided by maintaining a safe bus voltage at all times during operation, preventing the battery packs from over-charging or under charging, balancing power generated vs power consumed, and handling transient conditions like battery failure, PDU failure, or fuse clearing.

[ 00029 ] To achieve high voltage bus stability, the responsibility is distributed between power subsystems, i.e. the MPPTs and the PDUs. The MPPTs will taper off the chart current upon reaching their high voltage setpoint. Generally, this will correspond to battery full charge. The setpoint will never be set higher than the max safe battery pack voltage. The PDUs will throttle up/down upon reaching bus voltage extremes to stabilize the bus.

[ 00030 ] Thus, in some implementations, a method to provide high voltage bus stability 605 as depicted in FIG. 6. FIG. 6, is a simplified flow diagram depicting an implementation for power bus stabilization. The implementation of FIG. 6 includes maintaining a safe voltage on a high power bus by coordinating the maximum power point tracker and the propeller drive unit and causing the maximum power point tracker to reduce the power it supplies to the high voltage bus upon reaching a maximum voltage setpoint corresponding to a battery full charge, and by controlling the propeller drive unit to either consume additional power or to reduce consumption upon reaching either the maximum voltage setpoint or a minimum voltage setpoint so as to stabilize the high voltage bus. The implementation of FIG. 6 also includes the ability to reduce the throttle command to the propeller drive units below zero throttle if the bus voltage is still at its minimum voltage setpoint the propeller drive unit consumption has been reduced to zero. In this case commanding negative throttle settings will make the propeller drive units generate power (extracting energy from the motion of the aircraft through the air like a wind turbine) to stabilize the high voltage bus even if the battery state of charge is zero and there is no power coming from the solar array.

[00031] This is of particular importance in embodiments where there is no resistor load bank to dissipate power on the bus .

[00032] The maximum voltage setpoint is the maximum safe battery pack voltage allowable without causing damage to the battery. Upon reaching the maximum voltage setpoint the battery may be disconnected from the high voltage bus.

[00033] As discussed above in some embodiments, the battery comprises a plurality of battery packs. Thus, disconnecting the battery from the high voltage bus may include disconnecting a full battery pack from the high voltage bus while continuing to charge a non-full battery pack of the plurality of battery packs. So, a full battery pack may be disconnected from the high voltage bus while continuing to charge one or more non-full battery packs. [00034] 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.

[00035] 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.

[00036] Those skilled in the art will make modifications to the invention for particular applications of the invention.

[00037] 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.

[00038] 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.

[00039] 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 WE CLAIM IS:

1. A solar powered high altitude long endurance aircraft power bus architecture comprising:

a) a main DC power bus;

b) a battery connected to the main DC power bus; c) a plurality of solar panels coupled to the main DC power bus via a plurality of DC to DC converter such that each of the plurality of solar panels is coupled to the main power bus via one DC to DC converter of the plurality of DC to DC converters; and

d) a plurality of propeller drive units coupled to the main DC power bus via a plurality of inverters such that each of the plurality of propeller drive units is coupled via an inverter to the main DC power bus .

2. The bus architecture of Claim 1, wherein the battery is connected to the main DC power bus via one of: (a) a relay; or (b) a solid state switch.

3. The bus architecture of Claim 2, wherein the

plurality of solar panels comprises about one hundred solar panels.

4. The bus architecture of Claim 3, wherein the main power bus is a high voltage power bus with a range of from about 270 volts to about 400 volts.

5. The bus architecture of Claim 4 further comprising a payload coupled to the main power bus via a DC to DC

converter .

6. The bus architecture of Claim 5 further comprising a plurality of maximum power point trackers, and wherein the plurality of power point trackers each comprise a respective one of the plurality of DC to DC converters.

7. The bus architecture of Claim 1, wherein the

plurality of solar panels comprises about one hundred solar panels

8. The bus architecture of Claim 1, wherein the main power bus is a high voltage power bus with a range of from about 270 volts to about 400 volts.

9. The bus architecture of Claim 1 further comprising a payload coupled to the main power bus via a DC to DC

converter .

10. The bus architecture of Claim 1 further comprising a plurality of maximum power point trackers, and wherein the plurality of maximum power point trackers each comprise one DC to DC converter of the plurality of DC to DC converters.

11. A solar powered high altitude long endurance aircraft power bus architecture comprising:

a) a main DC power bus;

b) a battery connected to the main DC power bus via a relay;

c) a solar panel coupled to the main DC power bus via a maximum power point tracker, the maximum power point tracker comprising a DC to DC converter coupling the solar panel to the main DC power bus; and

d) an AC propeller drive unit coupled to the main DC power bus via an inverter.

12. The bus architecture of Claim 11, wherein the main power bus is a high voltage power bus with a range of from about 270 volts to about 400 volts.

13. In a solar powered high altitude long endurance aircraft, a method to provide high voltage bus stability, the method comprising maintaining a high voltage within an

allowable voltage range on a high voltage bus by coordinating a maximum power point tracker and the propeller drive unit and causing the maximum power point tracker to reduce power supplied to the high voltage bus upon reaching a maximum voltage setpoint corresponding to a battery full charge, and by controlling the propeller drive unit to either consume additional power or to reduce consumption upon reaching either the maximum voltage setpoint or a minimum voltage setpoint so as to stabilize the high voltage bus.

14. The method of Claim 13 further comprising reducing the throttle command to the propeller drive unit below zero throttle if the bus voltage is still at its minimum voltage setpoint and the propeller drive unit consumption has been reduced to zero so as to cause the propeller drive unit to generate power by extracting energy from a motion of the aircraft through the air so as to stabilize the high voltage bus when the battery state of charge is zero and there is no power coming from the solar array.

15. The method of Claim 13, wherein the maximum voltage setpoint is a maximum safe battery pack voltage allowable without causing damage the battery.

16. The method of Claim 15 further comprising

disconnecting at least a portion of the battery from the high voltage bus after the maximum voltage setpoint is reached.

17. The method of Claim 13 further comprising

disconnecting the battery from the high voltage bus after the maximum voltage setpoint is reached.

18. The method of Claim 17, wherein the battery comprises a plurality of battery packs, and wherein

disconnecting the battery from the high voltage bus comprises disconnecting a full battery pack from the high voltage bus while continuing to charge a non-full battery pack of the plurality of battery packs.

19. The method of Claim 13, wherein the battery

comprises a plurality of battery packs, and further comprising disconnecting a full battery pack from the high voltage bus while continuing to charge a non-full battery pack of the plurality of battery packs.

20. The method of Claim 13, wherein maintaining a high voltage within an allowable voltage range comprises

maintaining the high voltage on the high voltage bus in a range from about 270 volts to about 400 volts.