Comparison of Degradatio

of Individual Modules H

Maximum Power

C.R. Osterwald, J. Adelstein, J.A. 

B. Kroposki, D. Trudell, and T. 

NREL/PR-520-39844Presented at the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion May 7-12, 2006 in Waikoloa, Hawaii.
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Outline

• Purpose• Published degradation rates• NREL measurement procedu• Degradation rate results• Discussion and conclusions
Purpose

• Module degradation rates (RD) ne

accurate PV system energy delivecalculations

• Time-consuming measurement• RD data are generally unavailable

• System sizing software PVWATTS

– Has an input for aging loss, but defa– Recommends using the common rul

1% per year

• Attempt to quantify PV module RD
Published Degradation 

• PV literature search for published 

• Only nine references since 2000 f• Indication of measurement difficul• All but two are from modules expo

systems

• R

D values derived from system da

include factors unrelated to moduas:

– Inverter operation; max. power track– Wiring degradation

• System exposures provide more s
Published RD — Syst

† Based on individual module performance measurements
Published RD — Mod

† N. Cereghetti, et. al., 3rd WCPEC, Osaka, May 2003
NREL RD Measureme

• Performance & Energy Ratings T

(PERT) on roof of Outdoor Test F

• Operational since 1994• Currently 35 modules under test
PERT Data Acquisition

• 3 Raydec Multi-Tracer II 

max-power tracking loads

• 15 module channels each• I-V curves every 15 min.• Irradiance and back-ofmodule temperature 

measurements
PTC Power Rating Calcu

• Pmax extracted from I-V curves 

combined with E, T, and s

• For E > 800 W/m

2, 1 month o

Performance Test Conditions 

• Using regression results, pow

@ STC calculated

PTC: E = 1000 W/m

2, T = 20°C, s = 1 m/s
RD Determination

• PTC ratings plotted versus tim• Slope of linear fit gives RD (-0
Pitfalls

• a-Si initial stabilization• Seasonal variations
PERT RD Results — Crys

† Non-commercial prototype modules
PERT RD Results — a

† Non-commercial prototype modules
Discussion and Conclu

• Many Si R

D values < 1%/year

• Some thin-film R

D values < 1%

• Recommend 0.5% per year fo• R

D > 2%/year likely indicative 

module or system problems

• R

D values vary over wide rang

accurate data should be availsystem designers