Alex Bradley, DuPont Photovoltaic Solutions

Investigating Module Performance in the Service Environment

Alex Bradley is a principal investigator for DuPont Photovoltaic Solutions, studying material reliability in service environments over time. He obtained a PhD in chemistry from the University of New Hampshire in 1997 and was a postdoctoral fellow with the Council on Science and Technology at Princeton University before joining DuPont in 2001. Dr. Bradley is an industry representative on the Standards Technical Panel for UL 1703, the equipment safety standard for flat-plate PV modules and panels, and serves as the American Chemistry Council’s alternate representative on the National Electrical Code–Making Panel No. 4 regarding PV installation, the panel responsible for NEC Articles 690 and 705.

At Solar Power International 2012, Bradley moderated a panel called “Standardization of Codes for Solar Project Development.” SolarPro staff met with him afterward at the DuPont booth on the floor of the Orlando Convention Center.

SP: While DuPont is a household name, some people may not be familiar with its role in the solar industry. Where does DuPont fit into the value chain?
AB: DuPont has a historic place in the solar industry. The company was an original supplier to the Jet Propulsion Laboratory research program sponsored by the US Department of Energy, which developed many of the original specifications for PV modules. We donated our Tedlar polyvinyl fluoride [PVF] film for PV backsheet and EVA encapsulant materials. The goals of the study were to develop a 30-year module—one that met the functional, safety and reliability requirements for large-scale terrestrial deployment of PV—and to derive the appropriate balance between system cost and system life and reliability.

Now, more than 30 years later, DuPont products are used to manufacture both crystalline silicon and thin-film PV cells and modules. We provide films, resins, encapsulation sheets, flexible substrates, conductive pastes and silicon inks, as well as high-performance seals for solar cell manufacturing equipment and wet-etch additives for semiconductor texturing.

SP: What kind of work do you do as a principal investigator?
AB: I’m heading up our Fielded Module program, in which we’re going out into the service environment and looking at how our materials and our competitors’ materials have performed over time. In our minds, that’s the true test of performance.

This is a unique initiative, one that provides us and system owners with a significant amount of information. By virtue of being a 200-year-old company, our material science capability is well established, and we can leverage our experience from other industries. DuPont PV materials have been out in the service environment for well over 30 years. Since we haven’t changed the recipes for some of our materials very much, we can learn a lot by understanding how these materials perform over an extended period of time and throughout the module lifetime.

SP: How does this approach differ from other methods of performance testing?
AB: For the most part, the industry uses accelerated aging tests to try to predict the quality and reliability of PV modules. These tests can include thermal cycling, damp heat exposure, humidityfreeze cycling, hail impact analysis, static and dynamic mechanical loading, and so forth. I have colleagues at DuPont who perform accelerated aging tests on a mini mockup structure, and they beat the heck out of the test modules to establish relative durability.

The limitation to this approach is that we can generate failure modes that aren’t seen in the service environment. What we want to understand is how modules perform in the real world. If accelerated aging tests generate other types of failures, this isn’t really helping us or helping the industry. We still lack a fundamental understanding of how these results relate to module performance in the service environment. We can test for anything, but what do the results truly mean?

SP: What is the process used for the Fielded Module program?
AB: Basically, I’m always looking for modules that have been deployed in different environments and in different applications—for example, roof mounted versus ground mounted—and have been in service for different periods of time. All of these things have an effect on the rate of degradation.

It’s an interesting dynamic. When I first started the program, no one would let me in the door. They’d say, “You can’t come inside the fence,” and “No, you can’t do that.” So, we now have ten PV systems installed at DuPont locations worldwide. Some of these installations function as living labs, where we’re using a systems approach for our own learning, so that we can understand module degradation and material performance in detail. At the same time, we’re gaining access to other facilities that will provide additional learning opportunities.

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