Utility/Lab Workshop on PV Technology and Systems

November 8-9, 2010

Tempe, Arizona

NREL/PR-5200-49854

Survey of PV Field Experience

Dirk Jordan

NREL

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Outline

2

♦ Introduction

♦ Historical component failures

20 years ago – Modules ; Today - Inverters

♦ Historical degradation rates (Rd) 

Most modules degrade at 0.5%/year & are improving

♦Connection Degradation rate uncertainty & risk

Higher uncertainty leading to higher risk
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Growth of PV Industry

3

Alamosa Plant in Colorado

Photo credit: Steve Wilcox, NREL PIX 15548

Sources:

International: PV News, April 2009

USA: http://www.eia.doe.gov/emeu/international/contents.html

Reliability required to sustain exponential growth of industry

10

100

1000

10000

1995 2000 2005 2010

PV Shipments (MW)

Time (Years)

USA

International
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Reliability & Durability

4

♦ Reliability: Ability to perform designed task without failure  discrete, disruptive events

♦ Durability: Ability to perform task without significant deterioration  continuous, 

gradual decline

0.70

0.75

0.80

0.85

0.90

0.95

1.00

700

800

900

1000

1100

1200

1300

Dec-02 Apr-04 Sep-05 Jan-07 Jun-08

Inverter Efficiency (%)

DC Power (W)

Date

DC Power

Efficiency

Degradation Fit

Rd=(-0.14±0.16) %/year

Inverter 

Replacement

Both important for cost of electricity

Extreme example of inverter failure
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

PV for Utility Scale Application (PVUSA)

5

Location: Carrisa Plains

Size: 5.2 MW

Data: 1988 

“CARRISA PLAINS PV POWER PLANT 

PERFORMANCE”, Wenger et al., PG&E, 

PVSC 1990.

Panels showed the highest maintenance

The plant was originally constructed by the Atlantic Richfield oil company (ARCO) in 1983.

Provided electricity, research opportunity, data & experience through the 1980s and 1990s.

Plant was dismantled in the late 1990s.

Plant contained engineering modules.

Some Research Publications
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Tucson Electric Power - Springerville

6

“Five Years of Operating Experience at a Large, Utility-scale Photovoltaic Generating 

Plant”, L. M. Moore et al., Prog. Photovolt: Res. Appl. 2008; 16:249–259

Inverters seem to dominate O&M cost now

Module stability has improved over the last 20 years  the next component 

requiring improvement is the inverter.

Unscheduled maintenance costs for PV system operation 

69% 

10% 

21% 

Inverters

System

PV

Category

No.

Events 

(%)

Cost 

(%) Notes

Inverter 37 59 25% from 1 

lightning storm

DAS 7 14 90% from 1 

lightning storm

AC 

Disconnect 21 12 50% due to dirt accumulation

Module/

J Box 12 3 60% due to failed blocking diode

PV Array 15 6 45 % from 1 

lightning storm

System 8 6 All utility meter
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Maximum Warranties - Inverters

7

0.5

10

500

0

10

20

30

40

50

60

70

80

1 2 3 5 10 11 12 15 20 25

Power (kW)

Number of Models

Warranty (years)

Source: 

Photon International, 

April 2010 

Inverters suffer from early failures in the field due temperature-related issues, mismatch between PV 

voltage and inverter window. 

Qualification and performance standards for inverters and BOS are not well-defined

Inverters are improving but still have wide distribution
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Maximum Warranties - Modules

8

Source: 

Photon 

International, 

Feb 2010

Module maximum warranties typically greater than inverters

PV modules show smaller distribution

“Long Term Photovoltaic Module 

Reliability”, J.Wolgemuth, NCPV 

and Solar Program Review Meeting 

2003

Date Length of 

Warranty

Before 1987 5 Years

1987 – 1993 10 Years

1993 – 1999 20 Years

Since 1999 25 Years

Solarex/BP module 

Warranty Period

60

76

80

83

90

0

50

100

150

200

250

300

350

400

450

5 10 15 20 25 26 30

Power (%)

Number of Models

Warranty (years)
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Degradation Rate (Rd)- Discrete Points 

9

1. Translation to reference conditions (IEC60891)

2. Time series to determine degradation rate

0

10

20

30

40

50

60

70

0 50 100 150 200

Pmax (W)

Time (Months)

Rd= (0.33±0.07) %/year

Quarterly taken I-V curves for degradation

0

10

20

30

40

50

60

70

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 10 20

Power (W)

Current (A)

Voltage (V)

Current

Power

Irradiance

Temperature

Isc

Voc
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

0

10

20

30

40

50

60

70

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 10 20

Power (W)

Current (A)

Voltage (V)

Current

Power

Irradiance

Temperature

Isc

Voc

Degradation Rate - Discrete Points 

10

1. Translation to reference conditions (IEC60891)

2. Time series to determine degradation rate

0

10

20

30

40

50

60

70

0 50 100 150 200

Pmax (W)

Time (Months)

Rd= (0.33±0.07) %/year

sc oc sc VI oc

VI

VI

P FF ⋅⋅ = ⋅ = max maxmax

Quarterly taken I-V curves for degradation
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Degradation Rate - Discrete Points 

11

88

90

92

94

96

98

100

102

0 50 100 150 200

Isc of initial (%)

Time (Months)

Short-circuit Current Open-circuit Voltage Fill Factor

Degradation is due to decline in Isc, (Voc & FF are stable)  clues to failure mechanism

I-V curves provide clues to underlying failure mechanism 

Problem: 1. Labor-intensive, has to be clear sky

2. Large arrays  portable I-V tracer may not be available

3. Typically not available

97

97.5

98

98.5

99

99.5

100

100.5

101

101.5

0 50 100 150 200

FF of inital (%)

Time (Months)

96

97

98

99

100

101

102

0 50 100 150 200

Voc of initial (%)

Time (Months)
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Degradation Rate - Continuous Data 

12

Standard Test Conditions (STC): E=1000 W/m2, Tmodule=25oC

PVUSA Test Conditions (PTC): E=1000 W/m2, Tambient=20oC, wind speed=1 m/s

DC, AC Power

0

10

20

30

40

50

60

0 20 40 60 80 100 120

DC Power (W)

Time (Months)

1. Translation to reference conditions (use a multiple regression approach)

2. Time series to determine degradation rate

( aTaEaaEP ws) ambient 321 4 ⋅+⋅+⋅+⋅=

Seasonality leads to required observation times of 3-5 years  long time in today’s market

PVUSA – multiple regression

Long time required for accurate Rd
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Most modules degrade by ca. 0.5 %/year

Historical Degradation Rates

13

0

20

40

60

80

100

120

Frequency

Degradation Rate (%/year)

Median: 0.5 %/year

Average: 0.7 %/year

N = 784 Technology, age, 

packaging, geographic 

location

Published Rd in literature
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

PERT – Degradation Rates

14

Performance Energy Rating Testbed = 

PERT

More than 40 Modules, 

> 10 manufacturers, 

Monitoring time: 2 yrs-16 yrs

Appears that CdTe, CIGS & poly-Si improved Deg.Rate (%/year)

0

1

2

3

4

1 2 1 2 1 2 1 1 2

a-Si CdTe CIGS c-Si poly-Si

Date of Installation w ithin Technology

Pre Post Pre Post Pre Post Pre Pre Post

Pre: Installed before year 2000

Post: Installed after year 2000
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Historical Degradation Rates

15

 Deg.Rate (%/year)

0

1

2

3

4

5

1 2 1 2 1 2 1 2 1 2

a-Si CdTe CIGS c-Si poly-Si

Date of installation w ithin Technology

Pre Post Pre Post Pre Post Pre Post Pre Post

Appears that CdTe, CIGS & poly-Si improved

Historical degradation rates are analyzed in a similar way

Similar tendency found as with the PERT modules

While the Si technologies remain stable, thin-films seem to have improved.

c-Si and Poly-Si show an uptick in Rd  could be from new manufactures pushing into market*

*G. TamizhMani et al., “Failure Analysis of 

Module Design Qualification Testing”, Proc. 

35th PVSC, Honolulu, HI, June 20-25 2010.
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Degradation Rate Uncertainty

16

Modeling: (i) Classical Decomposition

(ii) ARIMA**

*Osterwald CR, Adelstein J, del Cueto JA, Kroposki B, Trudell D, Moriarty T. Proc. of the 4th IEEE World Conference on Photovoltaic Energy Conversion, Hawaii, 2006.

** D.C.Jordan et al., ”Analytical Improvements in PV Degradation Rate Determination“, Proc. 35th PVSC, Honolulu, HI, June 20-25 2010. 

Seasonality Irregular

Trend

Original Data

800

850

900

950

1000

1050

1100

0 40 80 120

DC Power (W)

Time (Months)

800

850

900

950

1000

1050

1100

0 40 80 120

DC Power (W)

Time (Months)

-50

-40

-30

-20

-10

0

10

20

30

40

50

0 40 80 120

DC Power (W)

Time (Months)

-40

-30

-20

-10

0

10

20

30

40

50

60

0 40 80 120

DC Power (W)

Time (Months)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

R

D Uncertianty (%/year)

Time (Months)

a-Si

c-Si

poly-Si

CdTe

CIGS

Traditionally: need 3-5 years to determine Rd*.

PERT Data Original

Accurate Determination of Rd takes time

Modeling can shorten required time
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

Consequences of Rd Uncertainty

17

2 examples from NREL:

Different observation lengths, seasonality etc.  Leads to different uncertainties

0

5

10

15

20

25

30

35

40

45

0 50 100

DC Power (W)

Time (Months)

Module 1

Module 2

Rd (Module 1) = (0.8 ±0.2) %/year

Rd (Module 2) = (0.8 ±1.0) %/year

Same Rd but very different uncertainty
Utility/Lab Workshop, Nov. 2010, Tempe, AZ

RD Uncertainty Impact on Warranty

18

0.00

0.05

0.10

0.15

0.20

0.25

Probability

Power Production after 10 Years (%)

Rd=(0.8±0.2) %/year

Rd=(0.8±1.0) %/year

0.00

0.02

0.04

0.06

0.08

0.10

0.12

Probability

Power Production after 25 Years (%)

Rd(0.8±0.2) %/year

Rd=(0.8±1.0) %/year

Manufacturer Warranty often twofold: 90% after 10 years, 80% after 25 years

Probability to invoke warranty:

1.0 %/year uncertainty = 46%

0.2 %/year uncertainty = 4%

Probability to invoke warranty:

1.0 %/year uncertainty = 57%

0.2 %/year uncertainty = 24%

( )

( ) ∑

= +

−⋅ =

N

n n

n D

N

r

Energy Year R Energy Year

1

1

1

1)( )(

Higher Rd uncertainty significantly increases warranty risk
Utility/Lab Workshop, Nov. 2010, Tempe, AZ 19

Thank You!