Long read: Reflections on a soiled module

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Since emerging around 2013, anti-reflective (AR) coatings for PV glass have been able to quickly establish themselves as a surefire way to gain a significant performance boost from a module, without making major changes in production. Typical anti-reflective coatings are reported to provide a 2-3% boost to module output, by altering the surface of the glass to trap and transmit more light through to the cell.

The 2018 International Technology Roadmap for Photovoltaics (ITRPV)report expects AR coated glass to remain the standard material for the front of a module, retaining a market share above 90% in the foreseeable future. The report also notes that the challenge for coating suppliers now is to continue meeting the ever-lengthening requirements on module lifetime and durability. The 2018 report notes that current coatings in mass production have a lifetime of 15 years, while coatings with a lifetime beyond 20 years exist, but are still at the interim stage and not optimized for mass production.

“One of the key challenges has been durability, which we define as the retention to acceptable levels of the primary material characteristics after exposure to aggressive mechanical or environmental conditions”, explains David Hannan, CEO at Opus Materials, a U.K.-based company which is part of a consortium that plans to launch a new module glass coating to the market in 2019. Hannan adds that the coating, ‘Solar Sharc,’ has proven its performance in accelerated durability tests based on the IEC 61215 standard for PV modules, including 1,000 hours of damp heat cycling and 1,000 hours of UV exposure.

Dutch chemical company DSM has been a leading producer of module glass coatings for several years, with an estimated 70 GW of installed capacity featuring its technology. The company’s new anti-reflective coating, designed for application on existing installed modules, uses a ‘closed surface’ structure (see diagram on p. 60), which it says makes it more durable than competing coating materials.

“Compared to other coatings, the DSM ARC for aftermarket application consists of a layer with an internal porosity and a closed surface,” explains Paolo Tusa, Commercial Manager at DSM Advanced Solar. “This feature ensures excellent durability and makes the coating intrinsically mechanically more robust and less prone to degradation or ingress of dirt.”

Anti-soiling

Coating suppliers today are also looking beyond anti-reflective features, at what else a glass coating can do to optimize module performance. In 2017, DSM launched its anti-soiling coating, designed to repel dirt and dust from accruing on the glass surface while also maintaining anti-reflective properties. This coating has begun to gain traction in PV markets, as falling module costs mean O&M is responsible for an increased share of overall projects costs, and viewed by developers as a key area for cost optimization.

“Our anti-soiling coating business is developing very well. As with all new technologies, it takes time to not only convince our direct customers, but also our customers’ customers down the value chain,” explains Tusa. “Small and medium-sized parks and recently also a large-scale park with our anti-soiling coating have now been installed in countries in Asia, Russia, and the MENA region.”

Given that the anti-soiling property can benefit from a smooth surface, free of bumps and craters where dirt and dust can collect, while anti-reflective relies on surface roughness with features to trap light inside, developing a coating that does both is another challenge for suppliers. Solar Sharc is marketed primarily as an anti-soiling coating, which also has anti-reflective properties. Hannan explains that by engineering the coating at the nano scale, they are able to incorporate both properties. “Solar Sharc incorporates functionalized silica nanoparticles that provide improved mechanical properties in the coating. Using these particles as the source of surface roughness promotes both enhanced repellency and anti-soiling capability,” he says. “This has not compromised the coating’s attributes, which also include anti-reflectivity; high light transmittance, UV, and chemical resistance; hydrophobic [properties]; and of course durability.”

DSM reports that its anti-soiling coating has the same anti-reflective properties as its other coating, and that in testing conducted at a TÜV SÜD installation in Dunhuang, China, the coating was shown to provide a similar 3% boost to module output (measured in flash testing, in comparison to uncoated panels). In addition, panels with anti-soiling could maintain optimal performance for longer periods, resulting in a further 1% average increase in output over time. Opus materials also says that its ongoing field and laboratory testing suggests that the Solar Sharc coating will be able to provide a generation increase of approximately 4%.

Cleaning

While anti-soiling coatings promise to reduce the need for cleaning, none have yet promised to eliminate it completely, particularly in the dry and dusty regions that tend to offer otherwise ideal conditions for solar generation.

This is another area where glass coatings, as the top layer of a module left exposed to the elements, must prove their durability. Cleaning solutions new and old can put a lot of stress on coatings – whether water free cleaning relying on airflow – which can blast abrasive dust particles onto the coating layer, or water-based cleaning which threatens to wash it away.

The ITRPV report notes that some coatings have struggled to remain effective and stable in outdoor conditions even for 10 years, but that there is a clear trend towards better durability.

This is reflected in the testing applied to the next generation of glass coatings. DSM points out that it has tested its anti-soiling coating with leading suppliers of automated cleaning equipment and found that performance for modules featuring the coating remained stable after 1,000 cleaning cycles – equivalent to almost 20 years of bi-weekly cleaning.

Aftermarket application

A key innovation in both DSM’s latest offering and in the Solar Sharc coating is that they can be applied at ambient temperature, with no need for additional heated drying/curing. DSM points out that PV glass is tempered, and both its anti-reflective and anti-soiling coatings can be heat cured during this stage. So, there is no real cost reduction for production applied coatings, but the development does bring about one interesting possibility.

DSM is preparing to launch its anti-reflective coating for retrofit application – to be sprayed onto modules in the field – offering a performance boost to older PV installations where modules were produced and installed without any AR coating. “The composition is based on the same technology platform used for our industry-leading anti-reflective coating,” says Tusa. “The only difference is that it dries at ambient temperatures.” He goes on to explain that standard AR coatings contain polymer nanoparticles, which are burned out in the tempering process to leave a nanoporous coating, and that DSM has developed an alternative method to ensure porosity of the coating and adhesion to the glass, as well a high-speed application method.

The application method consists of an automated applicator with an array of spray nozzles, which DSM says can coat a single module in one to two seconds, and full rows in one to two minutes. The coating also dries in less than one minute at a temperature of 20°C. The plan is for DSM to collaborate with partners as ‘certified installers,’ and offer a turnkey solution including supply of the coating and its application.

The retrofit AR coating is set for a commercial launch in European markets in the second quarter of 2019, with expansion into further regions set for the following year. DSM estimates a potential 40 GW of uncoated solar panels in large-scale solar parks around the world, with China, Germany, and Italy host to the largest numbers. It notes that many of these older installations were installed under attractive feed-in tariffs, so the 2-3% performance promised by the retrofit coating could prove extremely attractive to asset owners. “The extra power boost is key, it is the holy grail in the solar PV world,” states Paolo Tusa. “We expect typically less than three years’ payback time and an attractive IRR. Leading asset owners have shown a very high level of interest.”

Opus materials tells pv magazine that it is initially partnering with solar glass producer Onyx Solar to integrate the Solar Sharc coating into its production processes, and that the coating will also be available for use in the retrofit market.

Like any other new technology, retrofit anti-reflective coatings will have to prove their worth to various stakeholders before being accepted by the market. DSM currently has around 1 MW of test sites where the coating has been applied, some of which have been in operation for more than 18 months. “PV park owners have to be convinced by a demonstration of stable gain in the field over time,” concludes Paolo Tusa. “So far, the results in terms of performance gain and durability are excellent, meeting our expectations.”