by Susan Kraemer, Solarplaza
Accepted wisdom is that thin film will perform best in hot and humid environments. But will modules last a typical 25-year contract?
Solar developers exploring new market opportunities in Southeast Asia face a difficult choice when choosing PV technology. The hot and humid climate, with plentiful rainfall, can impact on PV modules performances for as much as a 20% power loss due to a modest 25°C increase.
The accepted wisdom in the solar industry is that thin film will perform best in hot and humid environments, at least right out of the box.
But Dave Williams, CEO and founder of Dissigno, which advises solar developers and investors on risk, suggests that mono-crystalline silicon solar may actually be the safer bet over the long term.
This is due to the fact that, as an older solar technology, mono-crystalline has been tested in real-world conditions over time, for example on buoys and telco towers around the world.
It is actually the real-world testing that can provide the best estimate of long-term performance at this point, according to Williams.
This is because there is a lack of tough standards in International Electrotechnical Commission (IEC) lab testing of hot and humid condition endurance, Williams told Solarplaza. The standard IEC test is 1,000 hours at 85°C and 85% relative humidity.
For higher confidence, companies commonly double, triple or quadruple this test time. This equates to about 24 weeks of exposure. But it has not been well established whether surviving the longer test gives the desired confidence.
Modules may fail for many reasons and the extended damp heat test may not be the best indicator. Said Williams: "IEC tests may not in fact be indicative of performance in harsh environments."
In addition, because thin film is more of "a delicate cocktail" than mono-crystalline PV, Williams suspects it is more likely to experience accelerated degradation and fail in humid conditions.
Finally, thin film has a wider degradation range from module to module, which can affect output since overall performance is a function of the worst performer on each PV string.
It is true that thin-film PV has long enjoyed a privileged position in India, which in places has a similar climate to markets such as Thailand or Vietnam.
However, it is unclear whether thin film’s dominance in India is due to technical superiority or an exemption from local content rules and support from foreign funding bodies such as the Export-Import Bank of the United States.
In other Asian markets, thin film is unlikely to have a local content or funding advantage. This may force thin-film makers to push for more stringent IEC tests so their products can face up to mono-crystalline PV in technical terms.
Dr Sarah Kurtz, research fellow with the National Center for Photovoltaics and a principal scientist at the US Department of Energy’s National Renewable Energy Laboratory (NREL), said this is already starting to happen.
“In my observation, many of the manufacturers are lobbying for tougher standards,” she said. “The bigger reason why the current standards don’t include everything is that they lag our knowledge.
“It takes time to identify and solve a problem, then define the best way to include the solution in the standard and get everyone to agree; the PV industry is growing very fast, so it’s challenging to keep standards aligned with the latest innovations.”
Her PV Module Reliability Test and Evaluation Group at NREL is working to identify and solve problems with earlier IEC standards, with specific testing to determine panel performance in humid heat.
If successful, the work could make life easier for solar developers in Southeast Asia. "Sarah's work is revolutionary, developing a higher bar," said Williams.