Thermal Considerations in the Design of Solar Concentrators. Steve Horne Chief Technical Officer February 2008

Transcription

1 Thermal Considerations in the Design of Solar Concentrators Steve Horne Chief Technical Officer February 2008

2 Topics Company Overview Concentrating Photovoltaics Primer CPV Thermal Management Generation 1 Generation 2 SolFocus, Inc. 2

3 SolFocus 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 ) SolFocus, Inc. 3

4 Global Operations Corporate Headquarters Mountain View, CA European Headquarters Madrid, Spain Advanced R&D Center Sunnyvale, CA Glassworks Mesa, AZ Manufacturing: US India China Spain Madrid Suzhou Mesa Mountain View Sunnyvale New Delhi SolFocus, Inc. 4

5 Motivation: Unique Combination of Unprecedented Climate Change Issues Historic Awareness, Movement towards Renewables Economic, Technically Feasible solutions SolFocus, Inc. 5

6 Concentrator 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) SolFocus, Inc. 6

7 Two 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. SolFocus, Inc. 7

8 CPV Has Been Around for Many Years SolFocus, Inc. 8

9 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 SolFocus, Inc. 9

10 Triple Junction Cell 3 coupled cells, each tuned to a different part of the solar spectrum 40% efficiency demonstrated; theoretical max mid 70 s Low T related performance degradation rate ( so higher T op ) Higher efficiency reduces demands on thermal management (+) GaInP 2 GaAs (-) Ge SolFocus, Inc. 10

11 Non Imaging Optics Science of efficient photon transport. Disregards order of light rays. Allows high concentration Compact, low cost optics Wide acceptance angles. ( Kaleidoscope example ) SolFocus, Inc. 11

12 Understanding Advantageous Conditions 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 SolFocus, Inc. 12

13 SolFocus Generation 1 Design Constraints Used in hot, arid climates: 45C. Lack of cooling water Long lifetimes needed: 25 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 SolFocus, Inc. 13

14 G1 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 SolFocus, Inc. 14

15 G1 Approach Largest Reflective Concentrator Unit that is Passively Coolable at: 45C ambient No wind Concentrator horizontal SolFocus, Inc. 15

16 G1: Approach Remove energy by spreading allowed by Cassegrainian optics. Kenji Araki Daido Steel Extensively researched this approach 1 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, 19 th EUPVSEC, 2004, Barcelona Spain SolFocus, Inc. 16

17 G1: 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. SolFocus, Inc. 17

18 G1 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. SolFocus, Inc. 18

19 G1: 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. SolFocus, Inc. 19

20 G1 Thermal Results 24C ambient; Horizontal orientation; 859W/m 2 Backpanel 53 59C measured with IR equipment Max T under cells. Cell T 69 76C SolFocus, Inc. 20

21 G1 Electrical Results Prototype #1 at 93% expected power IV, and power vs V. DNI 763W/m^2. Tamb 24C. Pmax 2060W E E+03 Array Current ( E E E+02 Array Power (W IV Power (W) E E E E E E E E E+02 Array Voltage (V) SolFocus, Inc. 21

22 SolFocus 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 G1 Potentially Lower Cost, Higher Reliability, Higher η Much More Difficult to Bring to Market. SolFocus, Inc. 22

23 Generation 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. SolFocus, Inc. 23

24 G2 in R&D Highly Integrated Monolithic Glass Tile CPV Collector SolFocus, Inc. 24

25 G2 Thermal Modeling Modeling shows improved performance over G1 45C ambient -> Cell temperature 83C SolFocus, Inc. 25

26 G2 Early Electrical Results Power: 114 mw mw Fill factor: 62.3% % Efficiency: 19.3% % Isc: 50.7 ma ma Voc: 2.22 V V Very cursory thermal results corroborate design... but more measurements to be done. SolFocus, Inc. 26

27 Conclusions 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. SolFocus, Inc. 27

28 Test Sites Palo Alto 14kW total, operating > 1 year Damp, industrial, salt environment. Moderate sun. SolFocus, Inc. 28

29 Test Site - APS, Arizona 10kW total, operating < 1 year -- large temperature extremes, dry, high direct sunlight. SolFocus, Inc. 29

30 Test Site - NELHA-1, Hawaii 2.5kW total, operating > 1 year damp, warm ( tropical ), high sulphur content from local volcano. SolFocus, Inc. 30

31 ISFOC, Puertollano, Spain 500kW total, operating, under installation. large temperature extremes, dry, medium sun. SolFocus, Inc. 31

32 Thank you! SolFocus, Inc. 32