Principal Solar Institute

You Be the Judge: A Ratings Tool for Selecting the Best PV Module

Kenneth AllenChief Operating Officer

Principal Solar, Inc.

Steve Hegedus, Ph.D.Institute of Energy Conversion

University of Delaware

Matthew Thompson, Ph.D.Executive Director

Principal Solar Institute

Hosted byRick Borry, Ph.D.Chief Scientist, Principal Solar Institute

PV Module Rating SystemMatthew A. Thompson Ph.D.December 18, 2012

Why Principal Solar Created a PV Module Rating

Principal Solar, Inc. needed a simple, comprehensive metric to use in their due diligence process.– Lifetime Energy Production is only one item in a long

checklist for potential acquisitions.– It is essential for a utility operator to understand Lifetime

Energy Production.Any purchaser of PV modules can use the PSI PV

Module Rating in their own comparative cost-benefit analysis.

History of the Approach

Lifetime Energy Production – The quantity of energy a

PV module is expected to produce in 25 years.

– A calculated quantity based on PV module characteristics, and a model of irradiance and temperatures.

The Seven Characteristics

Available to the public through organizations such as the California Electric Commission, and datasheets provided by PV module manufacturers.

Seven characteristics of PV modules that affect energy production:– Actual Maximum Power vs. Advertised: “Actual” is Vmp x Imp, STC– Negative Power Tolerance: Actual maximum power is reduced– Nominal Operating Cell Temperature: NOCT– Temperature Coefficient at Maximum Power: Gamma– Power at Low to High Irradiance Ratio: Low power is 200 W/m2 incident – Total Area Efficiency: Energy produced divided by module area– Annual Power Reduction: Manufacturer’s 25 year power warranty

Modeling the LEP

PV module characteristics that affect energy production depend on Irradiance and Temperature.

The Phase 1 model samples representative ranges of values for these conditions.

PSI PV Module Rating is Comparative

The impact on energy of each PV module characteristic is calculated hourly through the model of irradiance and temperature over a 25 year period.

The result is divided by the energy that would be produced by an ideal PV module to determine the PSI PV Module Rating: PSI Rating = Calculated Module LEP / Calculated Ideal Module LEP

The Rating is a comprehensive metric for comparing the energy performance of PV modules.

Phase 2 Model

Models under development will include regional insolation levels and historical temperature data. This enhancement will show in a comparative way how PV modules’ performance is affected by region.

The Comprehensive PSI Rating

The PSI Rating is most useful as a side-by-side comparison of energy production performance.

The PSI Rating does not include: – Manufacturer financial strength– Pricing– Delivery– Reliability– System components– Regional differences (until Phase 2)

Ranking and Rating PV Modules

The Principal Solar Institute Rating webpages provide:– Two classifications: Crystalline and Thin Film– Rank within classification– PV Module Rating, a comparative number based on LEP– Percentile Rank: across 10,000 modules, both classifications

Conclusion

The PSI PV Module Rating is a comparative score based on Lifetime Energy Production.

PSI does not currently test PV modules, but uses publicly available test data.

Developing a model with regional conditions.Working to develop interactions with University,

Industry and Government. We welcome feedback and suggestions.

Application of the PSI PV Module Rating

Kenneth G. Allen, COO Principal Solar IncorporatedDecember 18, 2012

Direct Comparison

Model Type Power (W) PSI Rating

“A” …275 Poly-crystalline 275 8.6

“B” …275P… Poly-crystalline 275 7.8

Lifetime Energy Production from “A” is (8.6/7.8) = 110% of “B”A 10% difference is a substantial difference to a utility operator

Nameplate rating of PV modules does not provide the detail needed to differentiate Lifetime Energy Production.

Energy Cost Comparison – Small Project

Model Type Power (W)

PSI Rating

Cost ($/W)

# PV 2kW Cost ($)

“A” …250p… Poly-crystalline 250 8.7 1.0 8 2000

“B” …P200A… Mono-crystalline 200 6.9 0.9 10 1800

Lifetime Energy Production of “A” is (8.7/6.9) = 126% of “B”

Assume you have a list of available PV modules. Go to the PSI Ratings website and pick the highest rated one, then make a comparison to others of interest.

Lifetime Energy Cost of “B” is (8.7/6.9)x( $1800/$2000) = 113% of “A”In this case lower upfront cost, but higher cost of energy produced

Conclusion

The PSI PV Module Rating:– is a comprehensive, comparative metric based on

Lifetime Energy Production– is a performance metric, unlike the nameplate

wattage rating– allows for a quantified cost-benefit analysis

kWhr/kW Performance: Comparison of Reported Field Data from Different PV Module Technologies

Steve HegedusInstitute of Energy ConversionUniversity of DelawareDecember 18, 2012

Outline

Introduction to IECWhy kWhr/kWSTC as a metric?Scope of literature reviewCritical issues in determining kWhr and kWSTC

Interpretation: measurement variability, TCE, LLESummary

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Institute of Energy Conversion at U of Delaware

Founded in 1972 to perform thin-film PV research World’s oldest continuously operating solar research facility First 10% efficient thin film solar cell (1980) Dept of Energy University Center of Excellence for Photovoltaic Research and Education (1992) Soft funded - government and industry contracts Over 20 deposition systems, complete integrated PV R&D

lab: film growth, device fab, characterization 2012 staff: 15 professional/tech, 5 post doc, >14 grad students (4 depts) Fundamental science, engineering, tech transfer and workforce supply

First flexible10% cell

4x4” monolithic interconnectedCIGS minimodule

6 chamber PECVD

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IEC Technology Thrust Areas

Thin film polycrystalline CuInGaSe2-based (CIGS) solar cells Wide bandgap alloys (Ag, S), flexible R2R vs glass, high temp substrates

Thin film polycrystalline CdTe solar cells Higher temperature substrate + TCO

Silicon-based solar cells Front and back heterojunction (a-Si/c-Si): first SHJ-IBC cell

Thin film tandem a-Si and nc-Si at higher growth rate

Reliability and stability; D-H under light and voltage bias

Characterization: in-line monitoring, device imaging/mapping

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I. Why kWhr/kWSTC as a metric?

kWhr: energy produced over time (1 vs 20 yr)Directly related to how much $ someone will be paid (PPA,

contract) or will save (offset electric bill) Includes effects of varying weather, degradation, shading,

dust, seasonal annealingkWSTC : power produced by module at STC

Directly relates to how much someone paid for modules typically $/WSTC based on initial power rating

Standard Test Conditions (STC: 1 kW/m2, 25°C module, AM1.5 spectra) may occur a few minutes a year

Largest source of uncertainty has been initial kWSTC

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II. Why kWhr/kWSTC as a metric?

kWhr/kWSTC units of ‘hours’: equivalent to # hours the array produced STC rated power

Must be specified over specific time (typ. 1 year) Sometimes called final yield YF

kWhr can be DC or AC (after inverter)Typical kWhr values: assume 5 hrs equivalent ‘1 sun’

irradiance per day, 20% system and module losses

kWhr= (5 hrs ‘1 sun’ per day) x 365 day/year x 0.8 = 1460 kWhrs/year

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Scope of Literature Survey Widely reported for >20 years that thin film (TF) PV modules

have higher kWhr/kW performance compared to c-Si modules Consistent with some well-established fundamental

differences but magnitude of advantage often too large Most obvious difference is that TF devices have smaller negative

temperature coefficient of efficiency (TCE); i.e. lower loss in efficiency at higher module temperature

Many different TF materials, processes and device structures Recently new c-Si device architectures

Higher efficiency (>20%) and lower TCE Asked to write a review of published performance data

comparing TF and c-Si module field data for new Wiley WIRE Energy and Environmental (abstract at end)

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Typical data from 2 widely referenced studies from 1990’s: 3 European cities, 4 technologies

I grouped all different modules by technology

Mallorca: sunniest, hottest Lugano: moderate Oxford: cloudiest, coolest Mono and multi Si similar TF a-Si and CIGS similar TF a-Si and CIGS higher by

16-20% all 3 locations!!!

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From: Hegedus, Review of photovoltaic module energy yield (kWh/kW):comparison of crystalline Si and thin film technologies, Wiley WIREs Energy Environ 2012. doi: 10.1002/wene.61

Select sources of field data comparing module technologies – not complete!

2007-2010 study by Univ Cyprus (Nicosia) and Univ Stuttgart: compared 13 module technologies in both locations (Makrides, Georghiou, Zinsser, Schubert) Detailed reports of YF, degradation, detectors, tracking, STC rating issues

Consultant Steve Ransome Consulting Ltd (SRCL, UK) Wide range of EU and US data, variability, uncertainty, predictive models

Two Japanese studies of different PV technologies Ito et al: Hokuto City: 20 systems of 10-100 kW Ueda et al: Ota City: 553 residential rooftop systems

Arizona Public Service and Tucson Electric Power (c-Si only) Moore, Post et al: detailed YF, O+M costs, fixed vs 1 axis vs 2 axis tracking, 2 kW to 3.5 MW

PV Trade Mag Photon International (Aachen DE) 130 modules on test, 97% are c-Si, very small difference in YF :~5-7%

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Examples of kWhr/kW data from different studies: Japan and Cyprus

YF for 20 arrays of 10-30 kW in Hokuto Japan monitored from 2008-2009*.

YF for 11 arrays of 1 kW in Nicosia, ∼Cyprus over 2007–2009**

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Hegedus WIRE review. Data from*Ito et al, Prog in Photovoltaics 19 (2011) 878-886.**Makrides et al, Prog in Photovoltaics 20 (2011)

I. Meteorological conditions are coupled: complicated trade-off on module output

What happens when clouds/humidity reduce incident solar irradiance on the module?1st order effect: ~ linear decrease in output (-) Less light: lower module T, less T-related loss (+) Less light: less current, less I2R power loss in module (+) More scattered light: collection of indirect light (±)More scattered light: spectra shifts to blue, advantage for

cells with high bandgap, high blue response (±)Changes with low light intensity grouped together as Low Light Efficiency (LLE) =Eff (200 W/m2) / Eff (1000 W/m2)

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II. Meteorological conditions are coupled: trade-off between LLE and TCE

Model output with real weather data for hot sunny and cool cloudy locations*

Compare typical values of LLE and TCE for TF and c-Si PV

Graph: 5-6% gain in kWhr/kW performance for TF over c-Si

Trade-off between LLE, TCE

27*From Hegedus WIRE review, data taken from Ransome et al Proc 37th IEEE PVSC Seattle 2011

Technology TCE LLE

c-Si -0.45%/°C 0.95

TF -0.25%/°C 1.05

Uncertainty in kWSTC and irradiance

kWSTC Manu. rated power can

range ±10% or 0/+3% Flash test individual

module can be ±2% for c-Si, ±3% for TF

Stabilization and pre-biasing (IEC standard)

Short vs long term degradation

IrradiancePyranometers ±2% with

annual calibrationCompare results from 2

locations same detector: ±4%

Differences in dust accumulation, T, shading

Si detector more sensitive to spectrum, T but cheaper

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Effect of ± manufacturers rating on tolerances: results from Nicosia and Stuttgart, different module technologies

29* Zinnser et al Proc 35th IEEE PVSC 2010

Effect of uncertainty in determining initial kWSTC

Three methods of measuring initial kWSTC Manu. rating, flash test, and

field rating (over 1 yr) Data for Stuttgart below for

range of kWhr/kW Cannot distinguish using

manu rating!

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range Manu. Flash Field

kWhr/kW 18% 12% 9%

uncertainty ±4-18 ±4-8%

±4

Zinnser et al Proc 35th IEEE PVSC 2010

Summary

Comparison of field data complicated by uncertainty in module kWSTC rating and irradiance

Experts estimate ±5% is best uncertainty we can achieve at present (same location) Comparing data from different locations has much larger uncertainty

due to detector, calibration, module rating procedures, weather Response to meteorological conditions complicated

Depends on module technology and location TCE and LLE 2nd order, weaker compared to linear dependence on

irradiance, most important in hotter climates But responsible for ~3-6% advantage for CdTe, advanced Si (HIT, IBC)

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Questions and Discussion

Please enter your questions in the chat window.

Kenneth AllenChief Operating Officer

Principal Solar, Inc.

Steve Hegedus, Ph.D.Institute of Energy Conversion

University of Delaware

Matthew Thompson, Ph.D.Executive Director

Principal Solar Institute

Hosted byRick Borry, Ph.D.Chief Scientist, Principal Solar Institute

kWhr/kW Performance: Comparison of Reported Field Data from Different PV Module Technologies

APPENDIX – EXTRA SLIDES33

Power rating uncertainties

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Quoted from paper by Zinsser* (Univ Stuttgart) titled “Rating of Annual Energy Yield More Sensitive to Reference STC Power than Module Technology”

“If we assume an error of ±3% in STC power measurement (calibration) and ±2% for the energy determination (detector), there could be a difference of 10% between the annual yield of two PV systems at the same location. . . . For thin film technologies, the error is even bigger due to nominal power variations. The worst case would be comparing two thin film technologies on the basis of rated power at different locations. The tolerance of (±10%) plus flasher measurement error (±6%) plus energy measurement error (±2%) plus irradiance measurement error (±2%) sum up to a possible total difference of 40%.”

* Zinnser et al Proc 35th IEEE PVSC 2010

Abstract of Hegedus review paper “Review of photovoltaic module energy yield (kWh/kW): comparison of crystalline Si andthin film technologies” Wiley WIREs Energy Environ 2012. doi: 10.1002/wene.61

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Brief comparison of PV module technologies

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