Interview: keys to durability, and why it counts

Whether it’s a single PV module or even a whole array, the focus of the industry has for many years been the cost in terms of installed peak. However, that may be beginning to change, with the importance of LCOE becoming more widely understood, and materials coming onto the market that can push long term performance boundaries beyond what was considered standard.

The R&D Focal Point for PV encapsulants and backsheet materials within Dow, Brian Habersberger spoke to pv magazine about the current and evolving market dynamics, and the role that encapsulants beyond EVA can play.

pv magazine: How would you describe the awareness of the importance of module durability in the market today?

Brian Habersberger: The market is generally aware that durability is important, but has not valued durability in the same way that it values nominal module power output, even though the desired net result of both – more power at lower cost – is the same.

How has that changed over time?

Appreciation for the value of durability has certainly improved along with general manufacturing quality improvements, as well as with programs such as DOE Sunshot, which outline pathways to grid parity and make it clear that these must include improved module lifetime and reduced annual power loss in order to succeed.

How does that awareness differ between manufacturers and project developers or investors?

Investors should have the strongest interest in high-performance, long-lived PV assets, but there is always a need to balance performance versus cost. Some module manufacturers feel that selling the highest nameplate capacity at the lowest price is an easier marketing strategy than investing in durable materials.

We’ve seen glass-glass modules slowly gain traction modules in recent years. What is your impression of the uptake of glass-glass modules?

Glass is certainly a very durable material once installed, but there are other durability-related material design issues that must be taken into account when a permeable polymer backsheet is replaced with impermeable glass. In this way, durable materials are often synergistic with each other.

How does long module durability and reduced degradation rates impact on project economics over time?

Quantifying the value of long life and reduced degradation rates can be done using the levelized cost of energy (LCOE) metric, which integrates the entire time-discounted energy yield of a PV installation. This quantification will be discussed in the webinar. Of course, such calculations require assumptions about the future performance of the modules, which is very difficult to predict. A spectrum of degradation possibilities can be considered and weighed against the added costs of investment in durable materials.

Do you think this mechanism is well understood today?

There are many mechanisms that contribute to module lifetime and annual power loss rates, some of which are very well understood, and some for which research continues. As engineering solutions that lead to improved lifetime are discovered, new failure mechanisms may be encountered that only occur on longer timeframes.

We’ve touched on glass-glass, but what role does encapsulant play in module durability?

Many well-understood and common causes of module power loss are related to encapsulants, and primarily associated with ethylene vinyl acetate (EVA) copolymers. Potential-induced degradation (PID) is caused by migration of sodium from the glass through the EVA and to the cell, where it causes shunting; sodium migration is blocked by polyolefin materials. Chemical decomposition of EVA naturally produces acetic acid, which may lead to corrosion; polyolefin encapsulants do not contain such corrosive functionalities.

Manufacturers tend to be price sensitive. Can EVA alternatives compete on cost?

EVA is a popular material for encapsulants primarily because of its low cost.

What kind of alterations need to be made to the lamination process to incorporate things like polyolefin encapsulant?

Polyolefin encapsulants can be laminated using the same instruments and processes as are used throughout the PV industry for EVA.

How closely is Dow working with both lamination equipment providers and manufacturers on refining this process and gaining acceptance?

Dow has a wealth of material, formulation, and polymer processing expertise and works closely with film and module manufacturers to address their technical needs. 

What kind of module lifetime and performance do you expect from modules using polyolefin encapsulant?

In spite of the deep desires and significant efforts of the industry, it is not possible to quantitatively predict the performance of a module 20 years from now with an accelerated lifetime test. Nevertheless, it is quite clear that many degradation mechanisms associated with EVA can be prevented with the use of polyolefin encapsulants.

Module manufacturers who use polyolefin encapsulants express their confidence in durability through their warranty terms. A survey of warranty terms shows that modules containing polyolefin encapsulants have an average “annual power loss” rate of 0.33%/year, while conventional EVA modules typically have rates of 0.7 – 0.8%/year.

Are there other similar encapsulants on the market?

Silicone and ionomer-based encapsulants have been considered, and similarly to polyolefins, they can prevent many of the degradation mechanisms associated with EVA. However, these materials are higher in cost with similar performance to polyolefins. 

Dr. Brian Habersberger is the R&D Focal Point for PV encapsulants and backsheet materials within Dow. He is responsible for managing technical projects in product innovation, laboratory capabilities, and quality, as well as interfacing with the PV technical community and external technical collaborations. He will be taking part in the forthcoming “Durability matters” pv magazine webinar, on Wednesday, November 8.