cell-to-module gains and losses in crystalline silicon pv
TRANSCRIPT
Gabor Photovoltaics Consulting, LLC
10Jul2013 - Intersolar NA 1
Cell-to-Module Gains and Losses in Crystalline Silicon PV
Andrew Gabor
Gabor Photovoltaics Consulting, LLC
July 10, 2013 - Intersolar NA
Gabor Photovoltaics Consulting, LLC
10Jul2013 - Intersolar NA 2
Some material sourced from
Gabor Photovoltaics Consulting, LLC
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Outline
• Background
• Loss/Gain Types
• Optical and Format Losses/Gains
• Electrical Losses
• Summary
• Crystalline Silicon Short/Long-term Challenges
• Metrology Short/Long-term Challenges
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Background
• What really matters? – Module tester results (somewhat) – Energy delivery in field under real conditions
• Variable temperature • Variable intensity (temporal, spatial) • Variable angle of incidence • Variable soiling
• Most effort goes into optimizing cell tester results – Sometimes an improvement that gives a big boost at the cell level,
gives a much smaller boost at the module level – Sometimes an improvement that only gives a small boost at the cell
level can give a larger improvement at the module level – Some silicon PV road-mapping activities stop at the cell level – More low hanging fruit at the module level
• Module efficiencies lower than cell efficiencies – Cell-to-Module (CTM) Losses typically > 12% relative – Higher balance of system costs
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Cell-to-Module Gain and Loss Factors
• 14% relative CTM loss • A few additional factors • More factors for field
performance
Source: Fraunhofer ISE, Schmid - 2012
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Module Border Area (-1.33%)
• Wide variation in percentage of border area between manufacturers – Higher labor to hide
bussing wire behind cells – Different sensitivity to
water ingress, dirt accumulation, and frame shading at steep angles
• Materials cost savings by reducing border area
• Larger modules Smaller percentage border area
• Frameless modules – Eliminate frame area
Large module Small border
Small module Large border
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Cell Spacing Area (-0.56%) • Higher percentage with smaller
cells • Higher amount with pseudo-
square vs full square cells • String spacing influenced by
– Quality of stringing and layup automation
– Quality of manual adjustment of string positions
• Potentially closer spacing with back contacted cells (MWT and IBC) – No need for stress relief bends
between cells in strings – “pick and place” with MWT
eliminates tolerances related to string “effective width” and string placement
• Separate from backsheet coupling (later slide)
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Glass Reflection/Absorption (-0.73%)
• Improve with AR coatings or glass texturing – Both help energy
delivery (kWh/kWp) due to higher gains at steep angles
• Improve with thinner glass or polymer coversheets – ~0.2% relative
efficiency gain with 2mm glass
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Encapsulant Absorption (-0.33%)
• Improve with better encapsulants – Higher grade EVA
– Ionomer
– Silicone
• Low blue/UV absorption increasingly important – Better cells have better spectral response in the
blue/UV
• Improve with thinner encapsulant, enabled by – Back contacted cells (no wires)
– Higher resistivity encapsulants? (reduced PID)
– Stronger cells (need less cushioning)
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Reflection – Cell Active Surface (+0.54%)
• High reflectivity at an abrupt cell/air interface – Reduced reflectivity by encapsulating with an intermediate
index of refraction material
• The better the texture, the smaller the gain upon encapsulation – Texturing optimization needs to measure encapsulated cells or
can be fooled
Source – ISE,2012
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Reflection – Fingers (+0.16%)
• ~50% effective width due to light trapping for typical fingers
– Narrow effective width for better aspect ratios
• Optimize based on encapsulated results
Source – Fraunhofer ISE, 2008
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Coupling Backsheet (+0.27%)
• Some light reflects from white backsheet, undergoes total-internal-reflection at the glass/air interface, and then hits the cells – Larger gains for larger cell
spacings
• Typical gains over black backsheets are in the 1-3.5% relative range
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Structured Backsheets
• Could have further gains over white backsheet both between the cells and in the border area by directing light to the cells with V-grooves or other optical approaches
Source – Exxon, 1981
Source – U.S.Patent #5,994,641 – ASE Americas
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Resistive Loss – Interconnect Wires (-0.57%)
• Reduce loss by – Wider wires
• Generally no – increased shading losses
• Possible with light trapping interconnect wires from Ulbrich or Schlenk
– Thicker wires • Generally no – increased cell cracking • Possible with conductive epoxy
instead of solder
– Conductive backsheets and back-contacted cells
– Wire arrays
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Reduce resistive losses in wires without increasing “dead” area
• Shorter wires – Lower I2R losses
• Cut rectangular bricks – Even possible from mono
• Challenge - expensive to redesign all equipment to handle a new shape
1.6m
1.1m
Gen5 – 25 square bricks
Gen5 – 21 rectangular bricks
60-cell module
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U. Konstanz
Reflection – Interconnect Wires (+0.16%)
• Other measurements for coupling gain on standard wires are lower
• Wire/busbar shading losses in cells are ~ 2.5% relative
• Multiple ways to reduce shading losses – Structured interconnect wires
(Ulbrich, Schlenk) • ~2% relative Eff improvement • White paint (iPV Stuttgart)
– Round wire arrays (Day4/Meyer-Burger, Schmid)
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Cell to cell mismatch losses
• Calculations of losses can be in the range of 0.03% to 0.06 absolute for narrow cell sorter bins or > 0.1% for wider bins [PV Measurements – 2004, Mobil Solar – 1981] .
• Losses can be much larger at low light intensities, especially when shunt resistance is not factored into the binning algorithm [RWTH Aachen - 2012]
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Light-Induced-Degradation (-0.5%)
• A quick decrease of ~0.5% absolute [NREL-2012] due to B-O defects in p-type CZ cells.
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Summary
• ~2% absolute module efficiency gain with combined improvements
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Near-term (1-3 years) metrology challenges
• Reflectivity measurements, Cell IV testing, and Module IV Testing should simulate in-laminate conditions and field conditions – Index matching oil or encapsulated coupons for R&D – Probe differently to simulate wire resistive losses – Predict energy delivery for different applications at variable light
intensity, temperature, and angles of incidence – Separate standardized conditions and testing methods for
bifacial cells/modules
• Cell IV testers that can contact fingers – A bit silly to have busbars just for ease of cell testing – Conductive adhesives or wire arrays can contact fingers directly
• Cell-sorter binning algorithms – Take into account performance at nonstandard conditions
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Long-term (4+ years) metrology challenges
• New performance metrics on specs sheets
– Predict kWh/yr for a few different standardized locations and install conditions
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Near-term (1-3 years) challenges - c-Si PV feedstock/crystal growth/wafering
• Eliminate LID for p-type mono
– Lower O?
– Ga doping?
– Hydrogenation solution?
• Inherently crack-resistant wafers/cells
– e.g. Solexel design
– Or cell designs without soldered busbars
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Long-term (4+ years) challenges - c-Si PV feedstock/crystal growth/wafering
• Rectangular wafers and updated tooling/process/handling equipment – Cut rectangular wafers from CZ ingots along
growth axis or from multi bricks • Reduce dimension in direction of current flow
• Increase other dimension for larger cell size
– Reactive ion etching or other advanced texture
– Reduce resistive power loss • thinner wires and reduced cracking for standard
cells
• reduced Cu thickness for back contact cells