Functional coatings: “An exciting period of creativity and innovation
What are the latest developments in anti-soiling coatings?
David Hannan: Developments in anti-soiling coatings are being driven by the sustainability agenda and the need for clean power. A generation ago it would have been difficult to envisage the exponential growth in the use of renewable energy – in particular solar. In addition, the application and marriage of nano-technology and renewables would have been considered blue sky thinking. However, as these technologies have evolved, that union has been realised and we are in an exciting period of creativity and innovation. In terms of the challenges we face, dust, dirt and fouling of solar panels are major sources of inefficiency and loss in solar generation, resulting in lost generating capacity to a value in excess of EUR 40bn p.a. In turn this causes over 100Mtonnes of CO2 emission through fossil fuel generation in order to make up the shortfall. Current so-called ‘self-cleaning’ coatings suffer from drawbacks including a short lifetime (2-3 years), poor transparency and high cost (over €260/litre). This means that they are not usually cost-effective and are not deployed, with losses accepted as the lesser economic impact for the operation of the plant.
The accumulation of dust on the surface of photovoltaic modules decreases the amount of solar radiation reaching the cell and results in power loss. This can lead to a reduction in power output from a PV system of 15-30% even in moderate dust conditions. Data from 186 solar farms in California has shown that over an average 145-day summer, a 7.4% loss in power generation was experienced. This is larger than losses due to cell degradation. The emerging PV cleaning sector estimates losses in the UK are 10-15% annually if modules are left unattended. The light that is absorbed by the dust is converted to heat, increasing temperatures of the cell and reducing solar panel efficiency. This is a big impact, especially in hot climates.
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Currently, manual cleaning is the most widespread technique and competitive solution to anti-soiling coatings. It requires manual access so is inappropriate or too hazardous for building applications (i.e. BIPV) and is expensive. In fact, many industrial users have carried out a cost-benefit analysis concluding that in most situations it is not worthwhile to clean at all and to take the loss in impaired efficiency. In effect, this is reducing the efficiency of the globally installed generating capacity by around 10%. Cleaning also uses large amounts of water which precludes its use in areas of water scarcity such as the Middle-East and hinders efforts in the EU to improve water use efficiency.
Automated and robotic cleaning systems are also available but they do not really solve the problem. They are also expensive (€50k-€100k) and require installation of sprinklers and plumbing. Automated cleaning still requires water which is in short supply particularly in regions of high insolation where large-scale solar generation is most likely to be installed. There are a number of coating agents sold on the consumer market for repelling water e.g. from car windscreens. These are not durable and not suitable for large area solar panel / BIPV applications.
A key development in this area is the prevention of soiling build-up in the PV sector. Recently, a number of nano-technology products have come to the market, but a major limitation of all of these coatings is their limited durability and high cost. In addition, many of them are not transparent so not suitable for solar PV. Through the H2020 public funded project “SolarSharc”, we have developed a new, patented coating technology that employs multi-functionalised silica nano-particles bonded strongly to the coating polymer matrix to provide a highly transparent, low cost, durable and robust self-cleaning coating. The success of this coating will help towards the economic and environmental challenges in implementing safe, secure and more sustainable low carbon energy supplies in the UK, Europe and beyond.
What is the current state of the art in anti-ice coatings?
Business Development Director, Opus Materials Technologies
Hannan: Creating anti-icing coatings that have long lasting ice repellent properties has been an area of research for many institutes across the globe for some time, and yet to date there has been very little in the way of true breakthroughs in this area – until now. One of the major focus areas for anti-icing coatings is the aviation industry. In Northern latitudes the icing of aircraft continues to compromise flight safety. It is at best a major cause of delays, at worst the cause of accidents. Ice-related incidents during the last 20 years have resulted in the loss of more than 250 lives. As aircraft fly through clouds with a temperature between zero and minus twenty degrees C, ice builds up on the surface of the aircraft, increasing its weight and potentially compromising its ability to stay in the air. At present, larger aircraft use warm engine-bleed air piped to ice-prone areas. Recently, the Boeing 787 has been using electric heating for de-icing, but the power required (e.g. up to 200kW for the 787) reduces efficiency and increases CO2 emissions.
International commitments for reduction in CO2 emissions are driving manufacturers and operators to comply – 2005 levels by 2050 – and improve fuel efficiency by 1.5% per annum through seeking improvements in all aspects of aircraft efficiency. Improvements in de-icing technology would be a significant contributor to this. For the next generation of aircraft, Natural Laminar Flow (NLF) wings will reduce drag by 20% and provide 4% fuel savings. A smooth wing surface is essential for NLF – ice and contamination from insect debris are major challenges. De-icing systems add up to 5% to the aircraft weight and create potential failure points. The de-icing system is normally left off and only used under icing conditions, so the decision to activate it must be made by the aircrew, and this is prone to error.
In addition, ice is a major problem on the ground. De-icing during freezing conditions is mandatory for take-off, expensive (up to £10,000 for a large aircraft) and consumes up to 1000 litres of fluid. With strong consumer demand for low cost air travel operators need to reduce costs through savings on ice-protection and improved efficiency. These problems could all be avoided by preventing ice from sticking to the aircraft in the first place by using an ice-repellent coating. This has been recognised by the UK’s Aerospace Technology Institute as a priority but is still in a “blue-sky” phase. Researchers have developed super-hydrophobic coatings, low energy surfaces, and flexible surface coatings, but in all cases, they lack durability, particularly with regard to rain drop erosion.
Of course, it’s not just the aviation industry that faces the icing problem – the automotive, renewable energy and marine sectors also have similar challenges. Accordingly, there is a major unmet need for a coating system that is both ice-repellent and durable. As part of the Innovate UK public funded project “Icemart” we are developing an ice-repellent coating to prevent ice formation and adhesion without the need for active ice-management. This will have far-reaching impact across a wide range of sectors, including aviation and energy where it could save hundreds of lives, eliminate the discharge of over 100 million litres of de-icing fluid, contribute to £14bn fuel and 150Mtonne CO2 saving annually from aviation and improve wind energy generation efficiency by 17%.
Could you expand on your approach to incorporate nanoparticles to improve the functionality of coatings?
Hannan: Repellency and durability are usually incompatible within a single material. Repellent surfaces are difficult to bond to substrates, making them prone to rapid erosion from weathering (particularly sand) or erosion under flight conditions. The appoach of “SolarSharc” is to co-locate water repellent functional groups alongside active functional groups on nano-structured particles. These then bond to the resin matrix, cementing the silica nano-particles into the resin for a tough, durable and transparent coating. The nano-particles are smaller than the wavelength of visible light, so do not scatter light, providing a high degree of transparency. Hence the multi-functionalised silica nano-particles provide a unique combination of repellency and durability needed for long effective lifetime of low/zero maintenance solar PV. In addition, the nano-particle structure provides high transparency, improving generating efficiency by 4%.
This technology permits high loading of nano-particles (up to 50%) into coating resins with no aggregation or increase in viscosity. The much larger surfaces provide vastly more possibilities for multi-functionalisation i.e. water repellency. This is further enhanced by the nano-scale roughness of the surface due to gaps between the nano-particles, preventing contact with the surface (the ‘lotus leaf’ effect). This has been demonstrated with acrylic resins:
- 120° contact angle (sessile drop assessment), a 40° increase in water repellency compared to unmodified resins;
- 3500 cycles (Taber rotary test CS10 abrasive) abrasion resistance compared to 150 cycles with no additive;
- 2 months’ damp heat test (45°C/95%RH) with no loss of repellency.
Research in the US (NASA, Michigan) and commercial products from Ross (US) and NTTAT (JP) have demonstrated water-repellency, but the coatings have shown to demonstrate a lack of durability. Multi-functionalising silica nano-particles addresses this problem and represents a major opportunity for the development of novel coatings that can that could have cross sector applications.
If you want to learn more about smart coatings, the European Coatings Seminar Functional Coatings is the place to be.