**Outdoor Monitoring and High Voltage Bias Testing of Thin Film PV
Modules**

Neelkanth G. Dhere, Ashwani Kaul, Santosh D. Khatri, Shirish A. Pethe,
and Bhaskar Kumar

Florida Solar Energy Center

1679 Clearlake Road, Cocoa, FL 32922-5703

<dhere@fsec.ucf.edu>

#### ABSTRACT

Limitations of accelerated testing to predict all possible degradation
modes and mechanisms in the photovoltaic PV modules necessitate that
actual outdoor monitoring and testing of PV modules be performed
out-doors. For this reason, thin film PV modules from leading US thin
film PV manufacturers namely, First Solar (Glass/CdTe/Glass), Shell
Solar Glass/CIS/Glass), Shell Solar New (Glass/CIGS/Glass), United Solar
(a-Si:H on flexible substrate), Energy Photovoltaics
(Glass/a-Si:H/Glass) and Global Solar (CIS on flexible substrate) with
additional one crystalline silicon module are being tested. The goal is
to assess their performance in the hot and humid climate of Florida and
to correlate the PV performance with the meteorological parameters
namely, solar irradiance, temperature, relative humidity, wind speed,
etc. Statistical data analysis of the recorded data is carried out on a
daily basis and on a monthly basis with PVUSA type regression analysis.
Current-voltage characteristics (I-V) of module arrays taken on a
regular basis complement the results obtained with continuous data
monitoring. Moreover, high voltage bias testing of the modules is
carried out to study behavior of leakage currents and detect any
packaging material and processing flaws and consequently the module
reliability.

**1. Objectives**

The development of cost effective photovoltaic modules with 30 year
useful lifetime is the goal of the U.S. Department of Energy. It is
therefore essential to understand the failure modes and mechanisms in PV
modules and recommend improvements in the manufacturing technology so as
to avoid premature degradation of field-deployed modules and thus
improve their performance, reliability and durability.

**2. Technical Approach**

Modules from each company are divided in two sets. Within each set they
are connected in series so as to build maximum open circuit voltage of
less than +600 V and -600 V with respect to the ground. The arrays are
connected across fixed load resistors and current measuring shunts. Two
modules from each company are also biased at +600 V and -600 V
individually in order to study the leakage currents from the module
frames to the ground. The datalogger measures the data every five
seconds and averages over 15 minutes intervals are recorded.

**3. Results and Accomplishments**

The data is recorded on continuous basis and the power versus solar
irradiance graphs are plotted daily. PVUSA type regression analysis of
the recorded monthly data is carried using a LABVIEW program developed
in-house. The program provides power value for each module array under
performance test conditions (PTC) i.e. at 1000 W/m^2^ and 25°C. The
variation of PTC power calculated for each month is plotted and studied
over a period of time. The quality of data has certainly improved which
can be seen from the PVUSA regression plots of a modules array (Figure
1). The plots show very little scatter in the data compared to the
scatter that used to be seen previously.

![](media/image1.png){width="3.38125in" height="2.1868055555555554in"}

Figure 1: Plot of PV USA Regression for United Solar negative module
array for Dec 2006

Central Florida region being a hurricane prone area, hurricane
protectors were designed, fabricated and tested by the FSEC team. These
hurricane protectors have reduced DAS downtime to few days from few
months during 2004 which was the year with the maximum hurricane
activity in over 100 years.

Figure 1 shows consistency in the performance of EPV positive module
array. The plot is expected to remain horizontal if there are no signs
of degradation. Any downward shift in these curves would indicate an
onset of degradation.

Current-voltage measurements are also carried out on the module arrays
to corroborate the results obtained with continuous data monitoring.
Current-voltage measurements provide characteristic data,

![](media/image2.wmf){width="3.372916666666667in"
height="1.886111111111111in"}

Figure 2: Variation of PTC power for EPV positive module array.

such as open circuit voltage, short circuit current, peak power and fill
factor of PV arrays. The peak power is easier to relate of all the PV
characteristics since the values are normalized at 1000 W/m^2^ and
25^0^C using the module temperature coefficients provided by the
manufacturers. Additionally, this data is plotted over time for all
module array to understand the performance trend. Taking the analyses
based on the current-voltage measurements and the monthly PTC power
variation together has prove to be considerably more effective than
taking either of them alone. An indicator of this fact is the observed
trend for shell solar modules that pointed toward some significant
performance loss. This performance loss was due to degradation in some
older generation modules that were part of the array, which was
confirmed after the modules were tested at NREL.

The Shell Solar modules were redeployed in May 2006 and since then no
further degradation has been observed in their performance. I-V
measurements are being carried out on a monthly basis since May-2006 in
order to gather more data points to fine tune the observed performance
pattern. No degradation has been observed for the module arrays of other
companies. New large-area CIGS modules received from Shell Solar were
installed at the outdoor site in September 2006.

The study of leakage currents in high-voltage biased thin film PV
modules fabricated by FS, SS, USSC and EPV is also being studied at
FSEC. Data is being collected on a continuous basis. It is plotted on a
daily basis from midnight to midnight and analyzed, to study the
correlation of leakage currents with the relevant meteorological
parameters such as solar irradiance, module temperature and relative
humidity over the entire diurnal cycle (Figure 3).

**4. Conclusions**

The PVUSA regression model is a very efficient and accurate tool for
studying performance degradation in PV module arrays. Regression
analysis complemented with I-V characteristics verify results. No signs
of module degradation have been observed in modules of First Solar,
United Solar, Energy Photovoltaics and

![](media/image3.wmf){width="3.3652777777777776in"
height="2.122916666666667in"}

Figure 3: Leakage Current trend for Shell Solar modules on a cloudy day

new-generation Shell Solar modules. Even though initially a few
old-generation Shell Solar modules showed some degradation, no further
degradation has been observed in their performance after being
redeployed in May 2006. Stable performance of PV module arrays over an
extended period of time can provide basis for providing warrantees. Any
loss of performance can assist in pin-pointing processes used in
specific production batches that need to be improved. Further, in-depth
study and analysis of the modules can also be carried out for
understanding failure modes and mechanisms using detailed analysis^1-3^.

Overall, data monitoring carried out over an extended period of time
will provide an insight into module performance, useful life-time and
degradation modes and mechanisms.

## ACKNOWLEDGEMENTS

This work is supported by the NREL and the Thin-Film Partnership under
the contract \# ZDJ 33360002. The authors are thankful to Bolko von
Roedern, Carl Osterwald, Harin Ullal, Ken Zweibel, NREL and colleagues
from leading US PV companies for their useful suggestions and
discussions.

## REFERENCES

^1^N. G. Dhere and N. R. Raravikar, Solar Energy Materials and Solar
Cells, vol. 67, (2001), pp 363‑367.

^2^N. G. Dhere, Proc. 31^st^ IEEE Photovoltaic Specialists' Conference,
Orlando, FL, (2005), pp. 1570-1576.

^3^N. G. Dhere, N. R. Raravikar, A. Mikonowicz and C. Cording, Proc.
29^th^ IEEE Photovoltaic Specialists' Conference, New Orleans,
Louisiana, (2002), pp. 231-234.

### **MAJOR FY 2006/2007 PUBLICATION**

A. Kaul, B. Kumar, S. Khatri, and N. G. Dhere, "Outdoor Monitoring High
and High Voltage bias Testing of Thin Film PV Modules", Poster
Presentation, FLAVS, March 2006.