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

Chris France

MTS, 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

AeroVironment

Solar 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•Flexible

•Low

cost

•Low

efficiency

CIGS•Flexible•Emerging thin-film tech.•Moisture sensitive•Low cost (theory)

CdTe•Rigid•Established

thin-film tech.

•Cost

competitive

µc-Si•Rigid

•Cost

competitive

•Very

common

material

c-Si•Rigid

•Industry

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

ce

ll

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

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Total: 40.8 W

Solar BOM: