Post on 07-Aug-2015
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Harnessing the Sun:Extend Your UAV’s Endurance with GaAs Solar Power
Chris FranceMTS, Product Development
April 29th, 2015
Confidential and Proprietary Information of ALTA Devices, Inc. a company
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: