thermal considerations in the design of solar concentrators solfocus.pdf · 2013-03-27 ·...

Thermal Considerations in the Thermal Considerations in the Design of Solar ConcentratorsDesign of Solar Concentrators

Steve HorneChief Technical OfficerFebruary 2008

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TopicsTopics

Company OverviewConcentrating Photovoltaics PrimerCPV Thermal Management• Generation 1• Generation 2

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SolFocus OverviewSolFocus Overview

ission: Solar Energy at Cost Parity with Fossil Fuels

ounded in late-2006

eadquartered in Mountain View, CA; 120 People

ombination VC and Strategic Backing

irst Product Line: High Concentration Photovoltaic Arrays

edium Volume Manufacturing

Field Test Sites ( CA, HI, AZ, Spain )

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Global OperationsGlobal Operations

Madrid

Corporate Headquarters – Mountain View, CAEuropean Headquarters – Madrid, SpainAdvanced R&D Center – Sunnyvale, CAGlassworks – Mesa, AZManufacturing:• US• India• China• Spain

New Delhi

Mountain View

Mesa

Sunnyvale

Suzhou

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Motivation: UniqueMotivation: Unique Combination ofCombination of……

Unprecedented Climate Change IssuesHistoric Awareness, Movement towards RenewablesEconomic, Technically Feasible solutions

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Concentrator PhotovoltaicsConcentrator Photovoltaics

Goal - Reduce the Cost of Energy Generation by…

• Replacing photovoltaic material with inexpensive optics

• Using established high volume industries for technology & methods (automotive, electronics)

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Two Fundamental ApproachesTwo Fundamental Approaches

Refractive:• Uses at least one lens to

concentrate energy on the cell.

Reflective:• Uses at least one mirror to

concentrate energy on the cell.

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CPV Has Been Around for Many YearsCPV Has Been Around for Many Years

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So why now?So why now?

They were:• Unreliable

▫ Thermal management problems▫ Materials problems

• Not economical▫ Low efficiency cells▫ Expensive tracking requirements▫ Concentrator “tax” – DNI only

Breakthrough:• Triple Junction cells• Non Imaging Optics• Better understanding of advantageous conditions

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Triple Junction CellTriple Junction Cell

3 coupled cells, each tuned to a different part of the solar spectrum40% efficiency demonstrated; theoretical max mid 70’sLow T related performance degradation rate ( so higher Top)Higher efficiency reduces demands on thermal management

(+)

(-)

GaInP2

GaAs

Ge

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Non Imaging OpticsNon Imaging Optics

Science of efficient photon transport.• Disregards order of light

rays.• Allows high concentration• Compact, low cost optics• Wide acceptance angles.

▫ ( Kaleidoscope example )

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High average DNI• Well correlated with population centers• Satelite – based assays now available, and are encouraging• But....in areas of low average water availability

Medium granularity ( commercial scale, leading to utility )• Fractional megawatt to multimegawatt• Less effective at residential level

Understanding Advantageous Understanding Advantageous ConditionsConditions

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SolFocus Generation 1 DesignSolFocus Generation 1 Design

Constraints• Used in hot, arid climates: 45C. Lack of cooling water• Long lifetimes needed: 25 years @ 80% output• Cell operating temperatures

▫ <= 100C when on sun, drawing power▫ <= 150C, short term survivability when on sun, no power

draw • Electrical isolation to 3kV• Very high volume manufacturing

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G1 Design GoalsG1 Design Goals

Arid Climes.• Must use air cooling only.

Long Lifetime• Passive cooling. No fans, plumbing, chilling, heat exchangers• absolutely stable cooling. Not conditionally stable.

Cell Temperatures• Require short thermal path to thermal exhaust

Electrical Isolation• Electrical insulation/Thermal conduction layer needed

High Volume Manufacturing• Low parts count

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G1 ApproachG1 Approach

Largest Reflective Concentrator Unit that is Passively Coolable at:• 45C ambient• No wind• Concentrator horizontal

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Remove energy by spreading – allowed by Cassegrainian optics.• Kenji Araki – Daido Steel

▫ Extensively researched this approach1

• Complex materials stack functionality:▫ Cell attach, isolation, spreading, backpan attach

• Materials choice very important▫ Cte, bond strength and buckling

• Geometric design also important▫ Flux management

1: Araki et. al, 19th EUPVSEC, 2004, Barcelona Spain

G1: ApproachG1: Approach

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G1: ApproachG1: Approach

Highly Reflective Secondary• Sits in a difficult location to cool• Operates above 1 sun• Set the limit to min size for secondary

mirror, minimum shading.

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G1 ApproachG1 Approach

“Receiver unit” robotically assembled• Highest flux paths most

accurately assembled• Eases assembly of complete

system – less variability in overall thermal path.

• Makes use of standard electronics industry practices.

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G1: ApproachG1: Approach

Array of concentrator units• Small Bill of Materials.• High replicability – suited to compact automation cells• Higher reliability than complex systems

Well suited to high volume manufacturing.

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G1 Thermal ResultsG1 Thermal Results

24C ambient; Horizontal orientation; 859W/m2

• Backpanel 53 – 59C▫ measured with IR equipment▫ Max T under cells.

• Cell T 69 – 76C

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G1 Electrical ResultsG1 Electrical Results

Prototype #1 at 93% expected powerIV, and power vs V. DNI 763W/m̂ 2. Tamb 24C. Pmax 2060W

-2

0

2

4

6

8

10

12

0.00E+00 5.00E+01 1.00E+02 1.50E+02 2.00E+02 2.50E+02 3.00E+02

Array Voltage (V)

Arra

y Cur

rent

(

-5.00E+02

0.00E+00

5.00E+02

1.00E+03

1.50E+03

2.00E+03

2.50E+03

Arr

ay P

ower

(W

IVPower (W)

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SolFocus Generation 2 DesignSolFocus Generation 2 Design

Feasibility Study: Advantages to Smaller Optics & Cell (1/10 linear dimensions).Opportunity to Exploit New Processes:• Optical quality glass pressing – totally solid optics (no air)• Deposition techniques from semiconductors, protective

coating industries• Assembly processes from electronics industry

Same Constraints as for G1Potentially Lower Cost, Higher Reliability, Higher ηMuch More Difficult to Bring to Market.

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Generation 2 DesignGeneration 2 Design

Thermally• Solid optics – 3 dimensional thermal energy migration

▫ Allows radiation to sky as well as out the rear• Deposition techniques

▫ inexpensive heat spreaders• Assembly processes

▫ thermal management materials from high power electronics (IGBT’s, LED’s)

• Small unit size▫ more uniform energy generation across the array area.

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G2 in R&DG2 in R&D

Highly Integrated Monolithic Glass Tile CPV Collector

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G2 Thermal ModelingG2 Thermal Modeling

Modeling shows improved performance over G1• 45C ambient -> Cell temperature 83C

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G2 Early Electrical ResultsG2 Early Electrical Results

Power: 114 mW - 116.5 mWFill factor: 62.3% - 70.05 %Efficiency: 19.3% - 25.37%Isc: 50.7 mA - 52.1 mAVoc: 2.22 V - 2.23 V

Very cursory thermal results corroborate design ... but more measurements to be done.

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ConclusionsConclusions

Thermal management is one of the largest constraints to concentrator design• Must be simple & inexpensive• Must be reliable• Must use air

Requires combination of good architecture, optical design, materials choice. Robust, inexpensive, manufacturable designs possible.

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Test Sites Test Sites –– Palo AltoPalo Alto

14kW total, operating > 1 year• Damp, industrial, salt environment. Moderate sun.

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Test Site Test Site -- APS, ArizonaAPS, Arizona

10kW total, operating < 1 year-- large temperature extremes, dry, high direct sunlight.

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Test Site Test Site -- NELHANELHA--1, Hawaii1, Hawaii

2.5kW total, operating > 1 year• damp, warm ( tropical ), high sulphur content from local volcano.

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ISFOC, Puertollano, SpainISFOC, Puertollano, Spain

500kW total, operating, under installation. • large temperature extremes, dry, medium sun.

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Thank you!Thank you!