Repellent polymer coating launched for solar surfaces

Materials World magazine
27 Sep 2019

A self-cleaning, highly durable repellent coating has been created for solar photovoltaics using silica nanoparticles. Shardell Joseph reports.

A new polymer coating created for solar photovoltaics (PV) combines a highly repellent surface with durability – a match usually incompatible within a single material. The developers claim that an unlikely combination of materials increases the coating’s capacity for manufacturing on an industrial scale.

The repellent coating, patented as Solar Sharc, was developed by a consortium of partners, consisting of the Advanced Resins and Coatings Technologies Innovation Centre (ARCTIC) – a strategic partnership between London South Bank University, Opus Materials Technologies, PV manufacturer Onyx Solar, research institute CEA – Liten, engineering consultants Millidyne, and The Welding Institute (TWI).

Technology used within the coating joins multi-functionalised silica nanoparticles with a siloxane-based matrix. It is this mix that, according to TWI, has produced a transparent, highly durable, self-cleaning and cost-effective contamination protector.

‘Currently, the few commercially available highly repellent coatings lack mechanical and chemical durability, a fundamental barrier to widespread industrial adoption,’ ARCTIC Director and TWI Functional Coatings and Resins Sectional Manager, Professor Géraldine Durand, told Materials World. ‘Durability is key as you need this coating to be able to withstand harsh environments – sand storms, high UV and temperatures. However, you also need to consider the deposition process of the coatings.’

According to Durand, many coatings are unable to be retrofitted due to them having a very high temperature curing process. The researchers developed the Solar Sharc technology in a way that allowed it to be sprayed directly onto the panel, cured at ambient temperature and retrofitted to existing solar assets.

Functionalising the nanoparticles

Solar Sharc technology started as a research project by TWI on nanoadditives, in order to improve the hardness of polymer coatings. TWI Ltd Technical Fellow, Professor Alan Taylor, told Materials World how the research focused on developing additives for polymeric resin as a way to improve the materials’ functionality and its mechanical properties.

‘In order to achieve functionality such as easy clean properties, you need to combine surface chemistry with the surface roughness and coating,’ said Taylor. ‘The functionalised silica nanoparticles are a way to engineer roughness into the surface and to reduce the surface energy.’

Describing the particles as amorphous, Taylor explained how they are synthesised using the well-known Stöber process. ‘This process allowed us to synthesise silica nanoparticle in various sizes, from 30nm-500mnm, in various solvents including alcohols, acetates and water,’ he said. ‘These nanoparticles are then individually surface treated via a process that allows the nanoparticles to have multiple functionalities.’

There are typically two functionalities – being unreactive, for example hydrophobic, self-cleaning, anti-soiling and anti-icing, and reactive, to enable cross-linking.

‘The reactive functionality will depend on the polymer matrix into which they are blended so compatibility between the two can be tailored,’ said Taylor. ‘Once cured, the nanoparticles are fully incorporated into the polymeric network improving the durability of the nanocomposite.

‘The functionalisation of the particles also reduces the degree of aggregation that can occur when adding silica into organic matrices. This is because the functionalisation shields the nanoparticles from interacting with each other, allowing them to maintain their primary particle nature. By retaining their nano-size and limiting the aggregation phenomenon, we can achieve a high level of incorporation of silica nanoparticles into polymeric resin with little change of viscosity.’

Testing repellence

As part of the project, a number of tests were undertaken to evaluate the coating including the production process for the liquid, the deposition method onto solar PV panels, performance and behaviour of its properties – particularly when panels are subjected to tilting within a large-scale solar PV system – its optical properties, and its ability to withstand and repel surface contamination.

The technology was demonstrated on 48 solar panel modules, grouped into sets of six arrays, which were tested outdoors in operational conditions. Half of the panels were covered in Solar Sharc and the other half were uncoated to determine the impact of the silicon technology on electrical behaviour.

The test results showed that it was able to successfully reduce particle density over time on the surface of the solar PV glass, and that the panels it was deposited onto were able to generate more energy than those without the coating.

‘At a time when the world is facing a severe climate change and where current energy sources are phasing out, Solar Sharc can help decrease power loss, reduce maintenance costs, improve the lifetime of the panels and hence provide higher long-term energy yield,’ said Durand.

‘In addition, by having an anti-reflection capability to the coating we have observed an average of 3.61% in generation increase during our exposure trials.’

Commercialising the Sharc repellent

Solar Sharc is in the process of being commercialised initially in the building-integrated PV (BIPV) sector – Onyx Solar will be integrating the coating into the original equipment manufactuirng process for a BIPV product range. Opus is also working on developing the supply chain capability, aiming to provide market access for the coating in 2020.

Durand and Taylor will continue to develop the technology to enhance repellency and durability and demonstrating performance in increasingly harsh environments.