Solar Ceils, 28 (1990) 145 - 150 145

THIN-FILM PV PERFORMANCE AT PG&E

CHRISTINA JENNINGS, CHUCK WHITAKER and DAVID SUMNER

Pacific Gas and Electric Company, Department o f Research and Development, San Ramon, CA 94583 (U.S.A.)

Summary

The outdoor performance of photovoltaic (PV) modules is monitored at PG&E's PV Test Facility in San Ramon, CA. This paper evaluates the per- formance through mid-1989 of commercially available amorphous silicon (a-Si) PV modules manufactured by ARCO Solar, Chronar, Solarex, Sovonics, and Utility Power Group. All of the modules have exhibited effi- ciency degradation on initial sun exposure. Module efficiencies are initially 5 - 6% and appear to stabilize around 4% after 1 yr of outdoor exposure. The a-Si modules installed in 1985 which initially operated at 5% efficiency have maintained 4% efficiency for the last 3 yrs.

1. Introduction

Photovoltaics (PV) is a viable electricity-generating option for many applications. One promising PV technology is thin films. Considerable research effort is directed towards reducing capital costs by developing thin- film PV devices which use less material than crystalline devices and have attractive automated manufacturing potential. At PG&E's PV Test Facility, the outdoor performance is monitored for more than 50 PV modules, includ- ing thin-film modules [ 1 - 4]. This paper evaluates the performance through mid-1989 of the thin-film PV modules installed at PG&E's PV Test Facility.

2. Experimental procedure

Commercially available thin-film PV modules manufactured by ARCO Solar, Chronar, Solarex, Sovonics, and Utility Power Group (UPG) were pur- chased (Table 1). These thin-film modules are made of silicon-hydrogen alloy, commonly referred to as amorphous silicon (a-Si).

Panels at PG&E consist of four current- and voltage-matched modules, wired in parallel to raise the signal-to-noise ratio in current measurement. Each of the four modules in a panel is nameplate rated at 5 W for ARCO Solar and Solarex and 4.5 W for UPG. Mounted at a 30 ° tilt facing due south, the modules and panels are connected to an electronic load that maintains

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146

T A B L E 1

Thin-f i lm PV m o d u l e descriptions

Manufacturer Model Configuration Namepla te Aperture Month rating (W) area (m 2) installed

ARCO Solar G 1 0 0 Panel a 20 0 .3391 6 /85 ARCO Solar G 1 0 0 Panel a 20 0 .3391 6 /86 A R C O Solar G 4 0 0 0 Module 30 0 .4942 1/88 A R C O Solar G 4 0 0 0 Module 30 0 .4942 5 /89 Chronar Module 9 0 .2667 6 /86 Chronar CM1-3 Module 12 0 .2639 1/88 Solarex SA-5 Panel a 20 0 .3788 12 /86 Sovonics R 1 0 0 Module 32 0 .6622 6 /86 Sovonics R 1 0 0 Module 32 0 .6622 12 /86 UPG UM10 Panel a 18 0 .3265 5 /89

apanel consists of four modules wired in parallel.

their operation at peak power except when current-voltage (I-V) curves are taken. The data acquisition system automatically takes I - V curves every 2 h during the day if weather conditions are stable.

3. Results

All of the a-Si modules installed at PG&E have exhibited efficiency degradation on initial sun exposure. Monthly averaged midday efficiencies and corresponding temperature conditions are plotted in Figs. 1 and 2, res- pectively. Periods of missing dam were caused by data acquisition system (DAS) problems.

ARCO G100

ARCO G100

ARCO G 4 0 0 0

CHRONAR

,----. CHRONAR CM1-3

- - SOLAREX SA-5 \ ..... ..: sovo. s::

:,, ......

7

6

~s

m_ o_

2

1

O

June 1985

j

I I I I

June 1988 J u n e 1 9 8 9 June 1986 June 1987

Fig. 1. PV m o d u l e efficiency.

w CHRONAR CM1-3 SOLAREX SA-5

..... SOVONICS R100 SOVONICS R100

o . E ~B

Q I .

0

Ambient Temperature ARCO Gt00

m ARCO G4000

CHRONAR

- V

O I

June 1985 June 1986

147

I I I

June 1987 June 1988 June 1989

The first a-Si modules at PG&E's PV Test Facility, four ARCO Solar G100 modules in a panel, have operated at 4% efficiency for the last 3 out of 4 years. Operating initially at 3% and leveling at 2% efficiency, the older Chronar module was disconnected in June 1988 and sent to SERI for analysis. ARCO Solar G4000 and UPG UM10 PV modules recently installed in May 1989 are operating at about 5% efficiency (Figs. 3 and 4). Although initial efficiencies of 5% to over 6% are observed by PG&E for most a-Si modules, efficiencies appear to stabilize around 4% after the first year of sun exposure.

Summer spectral solar irradiance conditions favor a-Si performance. Midday back-of-module temperatures are about 25 °C greater than ambient temperature. The seasonal change in midday air mass and possible thermal annealing are among the factors that cause increased efficiencies during the summer. These factors contribute to the variations around 4% efficiency.

2.0

I n s t a l l e d M a y 1 9 8 9 3 . 0 ,

$0

80

tu 40 n-

!,o

10

J u n e 2, 1 9 8 9

Irradlance 974 W/m2 T ambient 28 C T module 49 C Power 23.1 W Efficiency 4.8% Fill Factor 56%

15

1.0

.5

.0 P I 5 10 15 20

Voltage (vol ts) Fig. 3. Initial performance of ARCO Solar G4O00 module.

25

1 4 8

3.0

2.5

i 2.0

1.5

1 . 0

. S

Installed May 1989 June 2, 1989

Irradiance 9 7 4 W / m 2 T a m b i e n t 2 8 C T m o d u l e 4 7 C Power 16.0 W Efficiency 5 . 0 % Fill Factor 6 0 %

V o l t a g e ( v o l t s )

.0 r 0 5 10 18 2O 25

Fig. 4. Initial performance of Utility Power Group UM10 panel.

Monthly averaged midday efficiencies for the ARCO Solar G4000 and Chronar CM1-3 modules installed in January 1988 and the Sovonics R100 module installed in December 1986 are given in Fig. 5. From January 1988 to May 1989, the Sovonics module operated at about 4% efficiency and the ARCO Solar and Chronar modules followed typical a-Si degradation trends.

To account for seasonal variations affecting the performance of these ARCO Solar and Chronar modules, the Sovonics module is designated as a reference module. The reference is assumed to have non-degrading perfor- mance during the period under analysis (January 1988 - May 1989) and assumed to have spectral and temperature responses similar to those of the ARCO Solar and Chronar modules. The ARCO Solar module efficiency is divided by the Sovonics module efficiency for each month (Fig. 6). Similar- ly, the Chronar module efficiency is divided by the Sovonics module effi-

7

5

~ 4

3

sam ARCO Solar G 4 0 0 0 2 mm Chronar CM1-3

m Sovonlcs R100 1

0 I I I I I I I I I I I I I I I I

Jan 1988 Jan 1989 May 1989

Fig. 5 . P V module efficiency.

149

1.1 T a m ARCO~ovonk:s 1.4 - ~ mm ChroMc/Sovonlcs

1.5

1.2

1.1

1

O.9

O.8 Jan 11188 Jan 198g May 1969

Fig. 6. Ratio of module efficiencies.

ciency for eath month. The relative degradation rates between the ARCO Solar and Chronar modules are indicated by dividing the ARCO Solar effi- ciency by the Chronar efficiency. Except for the first month, the rates of degradation for the ARCO Solar and Chronar modules are very similar, indicated by the fairly straight line of the ARCO/Chronar efficiency ratio. The September 1988 discontinuity in the Chronar cmwe is attributed to DAS software problems.

Several factors could cause these different degradation rates during the first month of installation. As the most degradation occurs during the first month, light exposure of modules before performance monitoring can signif- icantly affect the first month's measured performance degradation. If the Chronar module had been exposed more than the ARCO Solar module at the factory or during installation at PG&E (but before data collection), the modules may have identical degradation rates that are not indicated by com- paring the first month of performance data.

Degradation rates can be compared in Fig. 7, where the ARCO Solar G4000 and Chronar CM1-3 curves in Fig. 6 are normalized based on the second month. Except for the first month, the two modules have similar

2 1.8

1.6

1.4

_~o~1"21

0.8 ~ 0.6

~ 0.4

0.2

0

-0.2 '~A C

Jan 1988 Jan 198g May 1989 Fig. 7. Normalized module efficiencies.

150

degradation rates. Module performance appears to stabilize after about 1 year of ou tdoor exposure.

A degradation time constant could be used to estimate stabilized per- formance for new modules. Assuming that a-Si performance approximates an exponential decay, a time constant for initial performance degradation can be determined. (In one time constant, 63% of the degradation to stabilized performance has occurred.) Time constants should be calculated after accounting for seasonal variations affecting performance. Degradation time constants to stability of 2 - 4 months are indicated for the ARCO Solar and Chronar modules. As stated earlier, the time constant of the Chronar module actually may be closer to 2 months, depending on the light exposure history before performance monitoring.

Although bar-graph delamination is not occurring for any of the modules, visible changes in appearance have been observed for the 2% effi- cient Chronar module and the Sovonics modules. Resembling small frost formations initiated at the grid lines, a-Si has delaminated from the glass near the center of the Chronar module. On the first Sovonics module, two rust- colored lines about 2 cm long have appeared. The newer Sovonics module has light blue splotches, each about 1 cm in diameter, on a few of the cells. Although these visible irregularities probably do no t affect module perfor- mance significantly compared with the Staebler-Wronski effect, the 2% effi- cient Chronar module was sent to SERI for further analysis.

4. Conclusions

PG&E's PV Test Facility has installed a-SiPV modules f rom five manufacturers. Module efficiencies are initially 5 - 6% and appear to stabilize around 4% after 1 year of ou tdoor exposure. Degradation time constants to stability between 2 and 4 months are indicated. The a-Si modules installed in 1985 which initially operated at 5% efficiency have maintained 4% effi- ciency for the last 3 years.

By monitoring, evaluating, and supporting PV development, PG&E can determine and be prepared for the time when this and other promising PV technologies qualify as an energy resource for utilities.

References

1 C. Jennings, Proc. 20th IEEE Photovoltaic Specialists Conf., Las Vegas, NV, 1988, IEEE, New York, 1988, p. 1225.

2 C. Jennings, D. Sumner, C. Whitaker and F. Duner, Models of Photovoltaic Module Performance, EPRI Final Report AP-6006, 1988.

3 C. Jennings, Proc. Stability of Amorphous Silicon Alloy Materials and Devices, Palo Alto, CA, 1987, American Institute of Physics, New York, 1987, p. 334.

4 C. Jennings, Proc. Photovoltaic Thin-Film Module Testing Workshop, Golden, CO, 1986, SERI, Golden, CO, 1986, p. 244.