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Photovoltaics for the Terawatt Challenge
Christiana Honsberg Department of Electrical Computer and Energy Engineering Director, QESST ERC Arizona State University
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Outline Terawatt Challenge Future prospects Education 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 /01/2014C. Honsberg 2
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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 /01/2014 C. Honsberg 3
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Terawatt Challenge Why is compound annual growth rate important?
ASU-UA-NAU Student Solar Conference /01/2014 C. Honsberg
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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 (NREL data) NREL,2012 Renewable Energy Data Book ASU-UA-NAU Student Solar Conference /01/2014 C. Honsberg
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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 /01/2014 C. Honsberg
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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 /01/2014 C. Honsberg
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Potential for PV in the US
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Photovoltaic Milestones
ASU – reached 50% of total electricity supplied by PV ASU-UA-NAU Student Solar Conference /01/2014 C. Honsberg
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Arizona Context ASU-UA-NAU Student Solar Conference /01/2014C. Honsberg 10
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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
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Duck Curves Power after sun goes down a concern for utilities.
Can mitigate by load management. ASU-UA-NAU Student Solar Conference /01/2014C. Honsberg 12
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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 /01/2014 C. Honsberg
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Present State of PV: efficiencies
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Fraction of Efficiency Achieved
APS Tutorial Nanostructured Photovoltaics C. Honsberg
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Types of PV Systems Optical configuration of photovoltaic systems: One-sun or flat plate; concentrating systems; tracking APS Tutorial Nanostructured Photovoltaics C. Honsberg
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Scope of QESST ERC 4/13/2018 QESST Second Site Visit - Overview
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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
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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 solar spectrum (i.e. tandem) No entropy penalty Efficiency controlled by existence of materials Beneficially circumvent one of the assumptions in thermodynamics # junctions in solar cell 1 junction 2 junction 3 junction 1 sun h 30.8% 42.9% 49.3% junction 68.2% Max con. h 40.8% 55.7% 63.8% 86.8% APS Tutorial Nanostructured Photovoltaics C.Honsberg
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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 are poorly lattice matched;5 band gaps and six band-gaps are better matched APS Tutorial Nanostructured Photovoltaics C.Honsberg20
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Ge-based tandem solar cells
Metamorphic solar cell reached 40.7% at ~200X. APS Tutorial Nanostructured Photovoltaics C.Honsberg21
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Carrier-Selective Contacts
Carrier-selective contacts enable ideal VOC Shouldn’t the external contacts contact the fermi levels. In the diagram, the inversion is too strong
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CSC Implementation: a-Si/c-Si solar cell
Demonstrated 746 mV on 50 µm wafers
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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
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Experimental GaAsSb/InAs QD material
Doping of QD layers to control occupancy of the QD. GaAsSb (20nm) S.I. GaAs substrate InAs QDs δ-doping GaAs (50nm) (b) (c) 8nm GaAsSb/GaAs interface
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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 means small area (and lower cost) needed for solar cells Trade balance of systems and solar cell cost. APS Tutorial Nanostructured Photovoltaics C.Honsberg
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Experimental GaAsSb/InAs QD material
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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 /01/2014 C. Honsberg 28
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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
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Questions? ASU-UA-NAU Student Solar Conference /01/ C. Honsberg 30
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