Module Reliability Trends

The ability to accurately predict power delivery over the course of time is of vital importance to the growth of the PV industry. Two key cost drivers are the efficiency with which sunlight is converted into power and how this relationship changes over time. An accurate quantification of power decline over time, also known as the degradation rate, is essential to all stakeholders—utility companies, integrators, investors and researchers alike.

Outdoor field testing has played a vital role in determining PV field performance and lifetime for at least two reasons: It is the typical operating environment for PV modules; and it is difficult to correlate indoor accelerated testing to outdoor results to forecast field performance. In this article, we present degradation rates for more than 40 modules tested under the same environmental conditions at National Renewable Energy Laboratory (NREL) and compare them with results from other publicly available sources, such as research institutions, testing laboratories and professional organizations.


Figure 1 shows a compilation of degradation rates reported in the extensive literature regarding outdoor field testing of PV modules. While this histogram needs to be constantly updated as new information becomes available, some general insights can be drawn from it.

First, the distribution is highly skewed, with the most frequently occurring degradation rates below 1% per year. One possible reason is that modules with a large power loss are often considered to have failed and therefore are not included in calculations of degradation rates. Second, Figure 1 also shows a few occurrences of “negative degradation rate” points—modules that appear to exhibit improvement. Short field exposure in combination with seasonal performance variation can result in this observation (see the description of thin-film performance transients in Rick Holz' article). The third, and most important, fact shown by Figure 1 is that the most frequently reported degradation rate is about 0.5% per year.

Field Testing at NREL

It is difficult to extract details from the module field test literature due to the wide variety of testing conditions, module types, manufacturers, dates of installation, length of monitoring times and geographical locations. The Performance and Energy Rating Testbed (PERT) at NREL is ideally suited for detailed field testing because it can accommodate a large variety of modules in a limited footprint. More than 40 modules from more than 10 different manufacturers were compared side by side, under the same atmospheric conditions, for their long-term outdoor stability on the PERT system. Module installations varied greatly, with the earliest installations occurring in 1993. There was an equally large variation in the monitoring times, from merely a few months to more than 16 years of continuous data. Due to increased uncertainty, no degradation rates were calculated for monitoring times below two years. Module technologies tested include amorphous silicon (a-Si), monocrystalline silicon (mc-Si), polycrystalline silicon (pc-Si), cadmium telluride (CdTe) and copper indium gallium selenide (CIGS).

The modules, mounted at a latitude tilt of 40° facing south, are held at maximum power with IV curves taken every 15 minutes. The PVUSA method was used to translate the measured power to the PTC reference state. The PVUSA test conditions are: irradiance = 1,000 watts per square meter; ambient temperature = 20°C; wind speed = 1 meter per second. Subsequently, the monthly normalized data are graphed as a time series, and the degradation rates are determined from a linear least-squares fit, as shown by the example in Figure 2 (above).

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