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Photovoltaics for the Terawatt Challenge

Photovoltaics for the Terawatt Challenge

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Photovoltaics for the Terawatt Challenge

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  1. Photovoltaics for the Terawatt Challenge Christiana HonsbergDepartment of Electrical Computer and Energy Engineering Director, QESST ERCArizona State University

  2. Outline • Terawatt Challenge • What is it? • Photovoltaics for the TW challenges • Importance of rapid growth • Recent milestones in PV • But what about ….. • Myths of photovoltaics: land area; efficiency; energy payback time; materials availability; time to impact; duck curves, etc • Future prospects • Education ASU-UA-NAU Student Solar Conference 04/01/2014C. Honsberg 2

  3. Terawatt Challenge • Terawatt Challenge: Encapsulates the dichotomy surrounding energy– essential for improved quality of life, but also tied among the most serious global challenges. ASU-UA-NAU Student Solar Conference 04/01/2014 C. Honsberg 3

  4. Terawatt Challenge • Why is compound annual growth rate important? ASU-UA-NAU Student Solar Conference 04/01/2014 C. Honsberg 4

  5. Terawatt Challenge • In the nearly two decades since the TW challenge paper, renewables have reached multiple milestones • In US, renewable compound annual growth rate 4.8% from 2000-2012 (NREL data) NREL,2012 Renewable Energy Data Book ASU-UA-NAU Student Solar Conference 04/01/2014 C. Honsberg 5

  6. Photovoltaic Milestones • Germany, Spain, Italy have yearly installed PV capacity > yearly increase in electricity demand. • In Germany, PV is 50% of summer peak electricity demand ASU-UA-NAU Student Solar Conference 04/01/2014 C. Honsberg 6

  7. Learning Curves for Photovoltaics • PV learning curves show compound annual growth rate (CAGR) of ~30% over the last several decades • Extending the growth rates shows ability of PV (renewables more generally if these are included) to make a substantial impact on electricity generations ASU-UA-NAU Student Solar Conference 04/01/2014 C. Honsberg 7

  8. Potential for PV in the US

  9. Photovoltaic Milestones • ASU – reached 50% of total electricity supplied by PV ASU-UA-NAU Student Solar Conference 04/01/2014 C. Honsberg 9

  10. Arizona Context ASU-UA-NAU Student Solar Conference 04/01/2014C. Honsberg 10

  11. Photovoltaics “FAQ” • Energy payback time • Land use • Cost • What do you do at night for power? • Materials availability • For silicon, limitation is silver in grids, which cause a limitation at 2 TW • Availability subject to efficiency, thickness APS Tutorial Nanostructured Photovoltaics C. Honsberg 11

  12. Duck Curves • Power after sun goes down a concern for utilities. • Can mitigate by load management. ASU-UA-NAU Student Solar Conference 04/01/2014C. Honsberg 12

  13. PV for the Terawatt Challenge • PV technology must be high efficiency, efficient use of materials, scalable, reliable, and enable path for future improvements • High efficiency; overcome limits; thin ASU-UA-NAU Student Solar Conference 04/01/2014 C. Honsberg 13

  14. Present State of PV: efficiencies

  15. Fraction of Efficiency Achieved APS Tutorial Nanostructured Photovoltaics C. Honsberg 15

  16. Types of PV Systems • Optical configuration of photovoltaic systems: One-sun or flat plate; concentrating systems; tracking APS Tutorial Nanostructured Photovoltaics C. Honsberg 16

  17. Scope of QESST ERC

  18. Multiple Junction (Tandem) Solar Cells Concentration or stacking multiple solar cells increases efficiency To reach >50% efficiency, need ideal bandgap 6-stack tandem, (assuming ~75% of detailed balance limit). Hard to get compatible materials with the right bandgaps. APS Tutorial Nanostructured Photovoltaics C.Honsberg 18

  19. # junctions in solar cell 1 sun h Max con. h 1 junction 30.8% 40.8% 2 junction 42.9% 55.7% 3 junction 49.3% 63.8%  junction 68.2% 86.8% What do efficiency calculations tell us? Approaches to high efficiency: • Concentrate sunlight. “One sun” = 1kW/m2, max concentration ~46,000. • No entropy penalty for concentrating sunlight, but etendue limits to acceptance angle and concentration. • Optically split solarspectrum (i.e. tandem) • No entropy penalty • Efficiency controlled by existence of materials • Beneficially circumventone of the assumptionsin thermodynamics APS Tutorial Nanostructured Photovoltaics C.Honsberg 19

  20. Tandem Solar Cells • Key issue for III-Vs: need precisely controlled band gaps which are lattice matched • “Missing” low band gap material • Approaches: • Lattice matched; Ge-GaAs-GaInP • Metamorphic;Ge-GaInAs-GaInP • Metamorphic; GaInAs-GaAs-GaInP • Band gaps for 4-tandem arepoorly lattice matched;5 band gapsand six band-gaps are better matched APS Tutorial Nanostructured Photovoltaics C.Honsberg20

  21. Ge-based tandem solar cells • Metamorphic solar cell reached 40.7% at ~200X. APS Tutorial Nanostructured Photovoltaics C.Honsberg21

  22. Carrier-Selective Contacts Carrier-selective contacts enable ideal VOC

  23. CSC Implementation: a-Si/c-Si solar cell Demonstrated 746 mV on 50 µm wafers

  24. InAs QDs on GaAsSb barriers • InAs QDs achieved on GaAsSb material • Increasing Sb composition decreases QD size and increases QD density InAs QDs on GaAs (5 ML) / GaAs1-xSbx (5nm) buffer layers with x = 23%, with density 2.6 x 106 cm-2 InAs QDs on GaAs APS Tutorial Nanostructured Photovoltaics C.Honsberg24

  25. GaAs (50nm) GaAsSb (20nm) δ-doping InAs QDs GaAsSb (20nm) S.I. GaAs substrate 8nm (b) (c) Experimental GaAsSb/InAs QD material • Doping of QD layers to control occupancy of the QD. GaAsSb/GaAs interface

  26. Tandem Solar Cells • Monolithic III-V tandem solar cells; Series connected; three junctions • High efficiency used in high concentration, two-axis tracking systems • High concentration meanssmall area (and lower cost) needed for solarcells • Trade balance of systemsand solar cell cost. APS Tutorial Nanostructured Photovoltaics C.Honsberg 26

  27. Experimental GaAsSb/InAs QD material

  28. Path for Continual Improvement • Ideal solar cell consists of a light-trapped, thin solar cell • Nanostructured surfaces allow light trapping and advanced concepts (e.g., multiple exciton devices) ASU-UA-NAU Student Solar Conference 04/01/2014 C. Honsberg 28

  29. Student Led Pilot Line • Silicon pilot line capabilities for interaction among students, industry and researchers • 10 Fulton Undergraduate Research Initiative Projects • 2 honors thesis • 4 capstone projects

  30. Questions? ASU-UA-NAU Student Solar Conference 04/01/2014 C. Honsberg 30