cell-to-module gains and losses in crystalline silicon pv

Gabor Photovoltaics Consulting, LLC

10Jul2013 - Intersolar NA 1

Cell-to-Module Gains and Losses in Crystalline Silicon PV

Andrew Gabor


July 10, 2013 - Intersolar NA



Some material sourced from



Outline

• Background

• Loss/Gain Types

• Optical and Format Losses/Gains

• Electrical Losses

• Summary

• Crystalline Silicon Short/Long-term Challenges

• Metrology Short/Long-term Challenges



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



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



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



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)



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



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)



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



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



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



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



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



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



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)



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]



Light-Induced-Degradation (-0.5%)

• A quick decrease of ~0.5% absolute [NREL-2012] due to B-O defects in p-type CZ cells.



Summary

• ~2% absolute module efficiency gain with combined improvements



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



Long-term (4+ years) metrology challenges

• New performance metrics on specs sheets

– Predict kWh/yr for a few different standardized locations and install conditions



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



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

cell-to-module gains and losses in crystalline silicon pv

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