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Harnessing the Sun:Extend Your UAV’s Endurance with GaAs Solar Power

Chris FranceMTS, Product Development

April 29th, 2015

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Presentation Outline

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‣ Solar energy 101:• How solar cells work• Comparing solar technologies• Different solar environments

‣ Solar for UAVs• Alta Device’s AnyLight™ Technology• Photon Recycling• System Considerations

‣ Case Studies• Puma UAV• Cyclopes-C

AeroVironmentSolar Puma

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Solar Energy 101

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Science of a Solar Cell (Photovoltaic)

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‣ Photo-voltaic:• Light → electricity• A semiconductor device

(diode) that absorbs light to energize electrons that are extracted as current

• Material properties determine: Amount of light absorbed Voltage produced by solar cell Energy lost at high temperature Sensitivity to environment

(moisture, UV, heat, light, etc.) Ability to be thin, light and

flexible[2]For details, see: http://www.altadevices.com/technology-videos.php

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Band Gap – Tradeoff Between Voltage & Current

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‣ The fundamental solar material parameter is band gap• Low band gap PV

Absorb most light, but low cell voltage Silicon (single & multi-crystalline)

• Medium band gap PV Optimum balance of absorption and

voltage GaAs, CdTe, amorphous Si

• High band gap PV Only absorb high energy light (blue →

UV) CdS, InGaP, AlGaAs Only used in multi-junction cells

Low Bandgap

High Bandgap

Absorption (current)

↑ ↓

Potential (voltage)

↓ ↑

[3]

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Major Solar Technology

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‣ UAV application require:1. Flexible material that can easily be integrated into the wing,2. High efficiency solar due to limited surface area,3. Lightweight material to maintain payload capability

‣ Thin-film GaAs is the only solution to fit all 3 criteria!

10% 12%

14%

16%

18% 20% 22

%24%

26%

28% 30%

Organic• Flexible• Moisture

sensitive• Low cost

(theory)• Low

efficiency

a-Si• Flexibl

e• Low

cost• Low

efficiency

CIGS• Flexible• Emergin

g thin-film tech.

• Moisture sensitive

• Low cost (theory)

CdTe• Rigid• Establish

ed thin-film tech.

• Cost competitive

µc-Si• Rigid• Cost

competitive

• Very common material

c-Si• Rigid• Industr

y standard solar material

GaAs• Flexible• Emerging

thin-film tech.

• World record efficiency

• 1 W / gm• Great

temperature coefficient

• Well established in space environment

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Click to edit Master title styleSolar environments

‣Terrestrial Solar• Solar irradiance or insolation• 1000 W/m2

• Wide spectral distribution• 300-2500nm

• Varying angle• sun and UAV move

• Varying intensity• sunny, clear sky 600 - 1000 W/m²• sunny, partly cloudy 300 - 600

W/m²• cloudy, fog 100 - 300 W/m²

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http://en.wikipedia.org/wiki/Solar_insolation

‣ Solar for UAVs• Unique because solar power

plant always moving• Solar surfaces directly facing

the sun = More power• More solar power at higher

altitudes• Shading based on craft

orientation

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Solar for UAVs

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GaAs – The Benchmark Solar for Efficiency

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Pros:‣ GaAs has been the industry

standard in single- and multi-junction space qualified solar• Robust to moisture, radiation

and UV• Highest efficiency technology• Great temperature coefficient

(minimal power loss at high temp)

• Ideal band gap for terrestrial solar

Cons:• Cost: GaAs wafers

are >200x more expensive than Si

• Rigid: Single wafer technology like c-Si (also heavy)

3J GaAs solar cells on MidSTAR-1 satellite[wikipedia]

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And the data to prove it

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4J RecordsGaAs Based

3J RecordsGaAs Based

2J RecordsGaAs Based

1J RecordsGaAs

Alta Devices

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Alta Devices’ AnyLight™ Power Technology

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‣ Alta Devices is the leader in thin-film GaAs technology• Goal:

Bring the efficiency of GaAs to a broad market

• Technique: Develop highest throughput GaAs MOCVD reactor in the world Transfer solar film from wafer template to flexible light-weight carrier

– Reuse the expensive wafer– Utilize efficient light-trapping of thin-film to beat thick-film GaAs 1J efficiency!

Develop flexible product sizes to fit broad customer base

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Photon Recycling in AnyLight™ Power Technology

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Reflection

Radiative recombination

Strong external fluorescence

Th

in F

ilm

cel

l

Photon recycling

• Single-crystal thin films provide new solar cell design opportunities

• Minimizing optical and electrical losses is key to high performance• Maximize absorption of incident sunlight• Maximize external fluorescence yield

• Minimize non-radiative recombination• Minimize loss of recycled photons

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Click to edit Master title styleSystem Considerations

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‣ Voltage Conversion• Voltage from solar can range from 0.3V (aSi) to 1V (GaAs) • Need multiple cells in series to match electrical system voltage

• Series connection multiplies voltage• Parallel connection multiplies current

• Use diodes to protect against leakage currents / shading• Max power point tracking (MPPT) increases system efficiency

across wide-range of illumination

‣ Energy Storage• Needed to use power when sun may not be shining

• Commercial solar uses the grid as a giant battery• Mobile solar needs something local!

• Li-ion, Li-Polymer batteries – already used on non-solar UAVs

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Click to edit Master title style‣ Flexibility / Weight

• Crystalline silicon is thick (>200um), heavy and not flexible• Thin-film technologies can use a flexible substrate

• Weight driven by metal or glass substrate and encapsulation

‣ Encapsulation requirements• Depends on environment and material

• Product lifetimes:– Rooftop solar - 25 year outdoor exposure– Indoor appliances - 2 year indoor product– UAVs - X number of landings?

• Some solar constructions are inherently moisture sensitive (CIGS, CdTe, organic)– GaAs is not

‣ Layout considerations• Fill the wings with as much solar as possible

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Material Considerations for UAVs

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Case Studies‣ 2-4x Increase in Endurance‣ AV: Puma

• Application: Hand Launched Military UAV• ~160 W of thin film GaAs solar on wing• 2-3 hr → 8 hrs

‣ Aerial Vista: Cyclopes – C• Application: Anti-poaching (WC-UAV

Challenge)• 60 W of thin film GaAs solar on wing, and

control surfaces• 6 hr endurance → 10 hrs• Come see an example at the Alta booth

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Thanks for listening!

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Supplemental Material

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Click to edit Master title styleAdditional Resources‣ Alta Devices: www.altadevices.com (homepage)

• UAV Application Info: www.altadevices.com/applications-uavs.php• YouTube Channel: www.youtube.com/user/altadevicesvideos

‣ AsiaTech Drones: www.asiatechdrones.com/• Manufacturer of the Cyclops-E used in this presenation

‣ sUAS: http://www.suasnews.com/‣ PV Education dot org: www.pveducation.org

• All you ever wanted to learn about solar and more

‣ Sustainable Energy: Without Hot Air: http://www.withouthotair.com• Free, e-textbook on sustainable energy

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Click to edit Master title styleReferences

1. http://www.altadevices.com/pr-2013-08-12.php

2. https://www.youtube.com/watch?v=HWBVi0FcrdM

3. http://solarcellcentral.com/limits_page.html

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ASIATECH DRONES – CYCLOPS E

A solar UAV example:

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Which UAVs are best for solar?

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‣ Fixed-wing UAVs• Have large surface area and require less

power to remain airborn

‣ Many wing sizes can provide useful power

‣ Must consider many factors:• Battery voltage• Use conditions• Payload• Available wing area• Desired flight extension time

‣ Alta Deivces’ product engineers can help optimize a solar design to your UAV

Example Layouts:

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AsiaTech Drones Cyclops E – Specifications

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‣ Manufacturer’s Specs:• Cruise Speed ~ 32 knots• Max speed ~ 110 kph• Stall speed @5.1 lbs ~ 12 knots• Climb Rate > 2000 ft/min• Wingspan ~ 101 in• Wing area ~ 598 sq in• Fuselage Length ~ 52 in• Materials:

EPO Foam - Wings, Fuselage and Tail Carbon Fiber - Tail boom, Wing spars and Tail

support components Plywood - Internal structure & support Wood, Plastic – Misc. parts

‣ Additional Configuration:• Assuming a 6S (25.8V) 5.5Ah LiPo

battery• ~ 3lbs additional payload with battery

‣ 2.56m wingspan high-efficiency fixed-wing airframe

‣ Wing shape allows for easy solar integration

‣ AsiaTech Drones is a sponsor of wcUAVc and University of British Columbia Aerodesign team

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Basic Layout and Power Calculations

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Assumptions for Cyclops-E:• Large wing area for cells• Optional area:

V-tail Large control surface on wing

• 6S (25.8V) LiPo battery 18V when discharged

• 2A draw at cruise• 15A draw at take-off

Required Info: Dimensioned top-down schematic Hold-out areas

High curvature Movable / detachable parts Wiring difficulties

Battery voltage Battery capacity Average power consumption or flight time

‣ Find usable solar area on UAV:• Mostly flat, horizontal surfaces• Avoid shading from fuselage / tail• Must be able to route wiring back to

battery area

‣ Estimate flight-time gains:• Based on power draw of UAV• Usable solar area

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Detailed Layout and Design

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‣ Design:• String length determined by

system voltage Advanced charging electronics

allows more flexibility in string length

• Fit maximum number of panels on surface to maximize power

• Might require larger holdouts if: Protection diodes in panel Certain types of lamination

methods used– See pg 20

‣ Result:• Product design specs:

Size and number of matrices Diode install / wiring plan Lamination method that works

with customer integration

Conservative: 41W

Aggressive: 51W

Aggressive w/ flaps: 61W

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Cyclops E – Conservative 41W Design

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‣ Range extension estimate• 2 A cruise for 2 hrs = 4 Ah.

1.5Ah remaining for take-off and climb– @ 15A draw, that is 6 min of climb

Reasonable estimate?

• This solar array produces > 1.5A of current. Cruising consumption dropped from 2 to

0.5A 4 Ah / 0.5A = 8h is the new cruising time!

• Since the 2A @ 25.8V is ~50W the more aggressive configuration could sustain flight without battery draw.

‣ 26 cell strings• Voc=27.0 V

• Vmp=23.6V

• Assuming blocking diode

‣ Require at least 2 bypass diodes• Can be placed on outer

strings on wings.

Qty Size Power Notes

2 26x2

10.2 W

2 26x1

5.1 W

2 26x1

5.1W Split in 3

Total: 40.8 W

Solar BOM: