What is the best PV module for a particular application? Is it one with the lowest cost per watt? Ultimately, it is the amount of energy produced that is the key factor in the economics of investment recovery and profit. The Principal Solar Institute (PSI) has developed a tool for analyzing this key element: The PSI PV Module Rating, an energy assessment tool for comparing the Lifetime Energy Production of PV modules over a 25-year period. Using the PSI Rating, solar energy professionals can finally make easy, meaningful energy-economics comparisons of PV modules between manufacturers or within one manufacturer’s product line. Hear Matt Thompson PhD, Executive Director of the Principal Solar Institute, and Kenneth Allen, COO of Principal Solar, Inc. and Principal Solar Institute Ratings Expert Panelist give an overview of the PSI PV Module Rating and explain how to use the ratings in financial calculations and comparisons of modules and manufacturers. Also, Steven Hegedus, PhD, scientist at the University of Delaware Institute of Energy Conversion, will present an overview of PV module field testing and performance metrics.Then discover specific applications for your business during a LIVE question-and-answer segment following the presentation. PSI has just published a whitepaper detailing the PSI PV Module Ratings. You should download it free of charge here. http://www.principalsolarinstitute.org/uploads/custom/3/_documents/PSIRatingsSystem.pdf

1. Principal Solar Institute
You Be the Judge:
A Ratings Tool for Selecting the Best PV Module
Matthew Thompson, Ph.D. Kenneth Allen Steve Hegedus, Ph.D.
Executive Director Chief Operating Officer Institute of Energy Conversion
Principal Solar Institute Principal Solar, Inc. University of Delaware
Hosted by
Rick Borry, Ph.D.
Chief Scientist, Principal Solar Institute

2. PV Module Rating System
Matthew A. Thompson Ph.D.
December 18, 2012

3. 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.

4. 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.

5. 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

6. 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.

7. 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.

8. 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.

9. 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)

10. 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

11. 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.

12. Application of the
PSI PV Module Rating
Kenneth G. Allen, COO
Principal Solar Incorporated
December 18, 2012

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

14. Energy Cost Comparison – Small Project
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.
Model Type Power PSI Cost # PV 2kW
(W) Rating ($/W) Cost ($)
“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”
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

15. 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

16. kWhr/kW Performance: Comparison of Reported
Field Data from Different PV Module Technologies
Steve Hegedus
Institute of Energy Conversion
University of Delaware
December 18, 2012

17. Outline
Introduction to IEC
Why kWhr/kWSTC as a metric?
Scope of literature review
Critical issues in determining kWhr and kWSTC
Interpretation: measurement variability, TCE, LLE
Summary
17

18. 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) 6 chamber
 Dept of Energy University Center of Excellence PECVD
for Photovoltaic Research and Education (1992)
 Soft funded - government and industry contracts First flexible
 Over 20 deposition systems, complete integrated PV R&D 10% cell
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 4x4” monolithic
interconnected
CIGS minimodule
18

19. 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
19

20. 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 annealing
kWSTC : 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
20

21. 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
21

22. 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)
22

23. 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!!!
From: Hegedus, Review of photovoltaic module energy yield (kWh/kW):
comparison of crystalline Si and thin film technologies,
23
Wiley WIREs Energy Environ 2012. doi: 10.1002/wene.61

24. 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%
24

25. Examples of kWhr/kW data from different
studies: Japan and Cyprus
YF for 20 arrays of 10-30 kW in Hokuto YF for 11 arrays of ∼1 kW in Nicosia,
Japan monitored from 2008-2009*. Cyprus over 2007–2009**
Hegedus WIRE review. Data from
*Ito et al, Prog in Photovoltaics 19 (2011) 878-886.
25
**Makrides et al, Prog in Photovoltaics 20 (2011)

26. 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)
26

27. 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
Technology TCE LLE
c-Si -0.45%/°C 0.95
TF -0.25%/°C 1.05
*From Hegedus WIRE review, data taken from
27
Ransome et al Proc 37th IEEE PVSC Seattle 2011

28. Uncertainty in kWSTC and irradiance
kWSTC Irradiance
 Manu. rated power can  Pyranometers ±2% with
range ±10% or 0/+3% annual calibration
 Flash test individual  Compare results from 2
module can be ±2% for locations same detector:
c-Si, ±3% for TF ±4%
 Stabilization and pre-  Differences in dust
biasing (IEC standard) accumulation, T, shading
 Short vs long term  Si detector more sensitive
degradation to spectrum, T but cheaper
28

29. Effect of ± manufacturers rating on tolerances:
results from Nicosia and Stuttgart, different module
technologies
* Zinnser et al Proc 35th IEEE PVSC 2010
29

30. 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!
range Manu. Flash Field
kWhr/kW 18% 12% 9%
uncertainty ±4-18 ±4- ±4
8%
Zinnser et al Proc 35th IEEE PVSC 2010
30

31. 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)
31

32. Questions and Discussion
Please enter your questions in the chat window.
Matthew Thompson, Ph.D. Kenneth Allen Steve Hegedus, Ph.D.
Executive Director Chief Operating Officer Institute of Energy Conversion
Principal Solar Institute Principal Solar, Inc. University of Delaware
Hosted by
Rick Borry, Ph.D.
Chief Scientist, Principal Solar Institute

33. kWhr/kW Performance: Comparison of Reported
Field Data from Different PV Module Technologies
APPENDIX – EXTRA SLIDES
33

34. Power rating uncertainties
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
34

35. Abstract of Hegedus review paper “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
35

36. Brief comparison of PV module technologies
Module Mono HIT IBC c-Si a-Si CIGS CdTe
Technology or c-Si (Sun- (1J, 2J, (First
multi-Si (Sanyo) power) 3J) Solar)
STC Eff (%) 14-18 18-20 18-21 6-9 8-12 9-12
Manu- Many 1 1 Many 2 1
facturers
TCE (%/°C) -(0.45- -0.30 -0.36 -(0.20- -(0.35- -0.25
0.50) 0.25) 0.45)
Comment Std Si Adv. c-Si Adv. c-Si TF- TF- #1 TF-
wafer heterojnctn all-back many Pilot Single
process contact versions scale Source
36