Photovoltaic Degradation Analysis

Cracked PV cells

Photovoltaic modules degrade over time as they are exposed to the elements. While they are exceptionally durable—as shown by the 60-year-old modules that are still producing power and 30-year power production warranties that are becoming more commonplace—there is always room for improvement. Technology continues to improve as researchers study how it fails and take steps toward ameliorating the causes.

At FSEC, we work with academic and industry collaborators to study how modules of different technologies degrade. Not only do we study modules that have been in the field, but also degraded via accelerated age testing.

Accelerated Age Testing

Accelerated age testing exposes modules to environmental stressors that are as severe or more severe than what is experienced in the field. These tests are designed to simulate environmental conditions without requiring years of outdoor exposure. Accelerated age testing induces failures that would realistically occur in the field.

FSEC is capable of both static and cyclic mechanical load testing, in which positive or negative pressure is applied to the back of the photovoltaic panel. FSEC also works with collaborators to study the effects of other accelerated condition testing, such as:

  • humidity freeze (module is put in a chamber with very high humidity and in freezing temperatures)
  • thermal cycling (-40 to +85° C temperature changes), and
  • damp heat (85% relative humidity at 85° C).

Inducing Field Failures

Failures are induced in modules to study how their performance changes. Inducing failures in the field reduces the time necessary to wait for failures to naturally occur. It also mitigates the need for finding data corresponding to specific failure modes. At FSEC, we have induced soiling, interconnection failures, crack formation and opening, external resistive loads, and high voltage conditions for studying potential induced degradation. Studying how these conditions impact performance gives insight into what aspects of module design need to be optimized.

Studying interconnection failures, which can occur in the field via corrosion and solder joint failure, has revealed the importance of having multiple busbars on resilience. Studying cell cracking has shown that cracks themselves do not significantly impact performance early in the life of the module, but will present problems as years go on. Studying high voltage conditions revealed the need for high voltage module qualification tests for verifying reliability.

Years of Research: 2013-2014, 2018-Current

Sponsored by: Solar Energy Research Institute for India and the United States, U.S. Department of Energy

Suggested Publications

Electrical Characterization

Illuminated (left) and dark (right) I-V curves of a module.

A module’s performance characteristics change over time as it is exposed to the elements. Illuminated and dark current-voltage (I-V) measurements describe these characteristics and are used to pinpoint how the module’s performance changes. Analyzing this data provides insight into the underlying mechanisms behind this change, or degradation.

Illuminated I-V is the standard technique for characterizing a module’s performance. The most notable parameters extracted from I-V are fill factor (FF), short-circuit current (Isc), open-circuit voltage (Voc), maximum power (Pmp), and current and voltage at the maximum power point (Imp and Vmp, respectively). Knowing which parameter changes gives an idea as to what type of degradation is occurring (optical, resistive, or recombination).

Suns-Voc is a technique that measures intensity versus Voc and is often used to build a pseudo-I-V curve. This curve essentially describes what a module’s performance would be if it had no series resistance. Coupling I-V and pseudo-I-V gives a more accurate series resistance value, and also provides carrier density and lifetime information.

Dark I-V data can be used to describe the recombination and junction characteristics of the module. It could also be used to obtain a series resistance value. Outside of the context of degradation studies, its usefulness is debatable. Series resistance values obtained for dark I-V curves measured over time could give information about a module’s resistive degradation mechanisms.

Years of Research: 1980-Current

Sponsored by: U.S. Department of Energy, Sandia National Laboratory, National Renewable Energy Laboratory

Suggested Publications

 

Electroluminescence Imaging

Electroluminescence (EL) imaging allows us to see features within modules that are invisible to the naked eye. Using EL imaging alongside I-V measurements provides powerful insight into how a module degrades. For example, interconnection failures reduce power and increase series resistance. While those parameters can be tracked using I-V, it is unsure what the cause of those changes is unless EL images are available. These changes are not only detectable by visual inspection, but by quantitative analysis.

The value of EL images used to be restricted to visual, qualitative analysis. In recent years, much work has been done to show the value of quantitative EL image analysis, including work done at UCF’s Orlando, Fla. campus and FSEC. There are subtle changes in the module’s performance that are not visible to the naked eye when viewing a module or its EL image. By peering into the actual pixel intensities of the images, their changes over time reveal information that is not obvious. For example, cells could darken so subtly that the images look the same but the analysis reveals a performance change.

Suggested Publications

 

Thermography Imaging

Infrared thermography image of two photovoltaic modules superimposed over photography of actual modules.

Thermography imaging allows us to visualize the temperature profiles of modules. Different module technologies and assemblies (packaging schemes) exhibit different profiles. Studying how these profiles relate to performance and reliability gives information that helps improve the state of the art.

Thermography imaging quickly pinpoints troubled regions within modules. It is often used to inspect fields to detect under-performing modules. As modules degrade in different ways, spatial temperature profiles may change. Degradation mechanisms that contribute to this include delamination and hot spots.

Delamination reduces the encapsulant’s ability to transfer thermal energy away from cells. Cells can overheat and not only suffer in performance but also can present a safety risk if hots pots form. Hot spots are local shunts through which elevated levels of current transfer. These regions are at an especially high temperature and are easily visible with a thermography camera. If left unchecked, a hot spot could melt polymeric components (encapsulant and backsheet) and eventually start a fire.