Smart glass

02 / 2013, Industry & Suppliers | By:  Sara Ver-Bruggen

Anti-reflective coatings: A new wave of anti-reflective technology is emerging with some of the most promising products in development having multi-functional properties that could benefit solar PV cover glass.

Research groups around the world have been using the principal of moths’ compound eyes that do not reflect incoming light, as the basis for developing highly efficient anti-reflective solar glass coatings.

An elegant way of reducing the cost of electricity generated by a typical solar PV power plant would be to make panels more productive whilst decreasing maintenance costs. That pushes down the levelized cost of electricity (LCOE), which not only takes into consideration initial capital expenditure (CapEx) but also the costs of continuous operation, maintenance and other associated expenses over an energy plant’s lifetime. This could be achievable in future as a new generation of smart coatings that combine anti-reflective (AR) with self-cleaning properties becomes commercially mature. Anti-reflective coatings (ARC) for solar glass have a significant boost on a PV panel’s ability to turn sunlight into electricity. They work by reducing the reflectivity of glass, allowing more light to pass through, increasing the energy output of modules by a few percentage points.

The majority of anti-reflective glass for PV cover glass is produced by depositing nano-porous solid silica particles, or sol-gel, coatings, as the method is cost-effective and can be integrated into production lines.

The unprecedented growth of the global PV industry in recent years has encouraged R&D efforts to develop more advanced AR technologies, some of it now in production.

A recent example is Dutch chemicals producer Royal DSM N.V., which has managed to push the performance of ARC designed for PV glass with its KhepriCoat technology, by increasing light transmission by 6%, resulting in 4% more panel efficiency.

In response to rapidly growing demand, mainly from the solar industry, DSM ramped capacity for KhepriCoat at its site in Galeen, in the Netherlands, late last year. The material is applied as a very thin layer, up to 150 nanometers in thickness, by standard coating processes such as roll or slot die coating. Besides its excellent anti-reflective properties, KhepriCoat is extremely durable, able to perform outdoors for 25 years, to match panel lifetimes.

In 2012 the global flat glass industry, which mainly supplies building and construction markets, produced over six billion square meters of glass. Accounting for 4% of the end-user market, PV glass is niche but expanding.

AR coatings and solutions have tended to be the result of internal development and production by solar manufacturers, or sometimes as collaborations with PV companies and glass manufacturers. That DSM has emerged as the largest independent supplier of ARC coatings for solar glass proves there is lots of potential for advanced anti-reflective glass technologies, compatible with industrial PV production processes. Many of the most promising next-generation AR technologies in development also have additional properties, such as self-cleaning, or anti-fogging.

PV panel efficiency is affected by soiling and dirt on the cover glass. Potential energy loss can be as high as 30% for very dirty panels and can reduce the power conversion of solar mirrors by 40% or more depending on the thickness of dust and particle layers. For utility-scale solar plants, the LCOE – which must take into consideration the operations and maintenance (O&M) costs over a plant’s lifetime – using a self-cleaning solar glass can help reduce the LCOE.

Multi-functional coatings

Several independent R&D efforts are using nanotechnology to develop glass coating technologies with multi-functional properties. These can both enhance module output by reducing the amount of light reflected from the glass surface and prevent the glass from getting dirty and dusty. Combining these different properties in one material to create bifunctional coatings, or so-called “smart coatings”, could also reduce production costs as only one coating product would need to be applied.

Researchers at Technical University Clausthal, in Germany, have examined AR coatings for solar cover glass and photocatalytic coatings for self-cleaning glass, each based on a different type of nano-functionalized thin film. Broadband AR coatings in wide use tend to exploit the low refractive index of nano-porous silica, whereas the most established photocatalytic coatings consist of high refractive index materials, such as titania. The potential compatibility of these two functional materials was investigated using sol-gel dip-coating technology.

A bifunctional coating technology has been achieved by Centrosolar Glas GmbH Co. KG, Germany. The company claims its Centrosol HiT Nano Power AR coating improves transmission properties, boosting the annual energy yield of a PV system up to 6%. In addition, low-iron solar glass treated with the coating exhibits hydrophilic surface properties, resulting in a degree of self-cleaning.

NanoSonic Products Inc, in the U.S. is a private research company that works closely with the University of Virginia to commercialize technologies based on nanotechnology and polymer chemistry, such as foams, sensors and smart fabrics integrated with sensing and conductive properties. The company has been testing a solar coating with multifunctional properties – including anti-reflective, self-cleaning and anti-fogging – on outdoor PV modules and is looking to bring its coating to market with potential partners in the solar industry.

The National Renewable Energy Laboratory (NREL) is also evaluating the Hybridsil Solar Coating. The technology can be applied by a range of coating methods, during manufacturing, as retrofit or in the field. In pilot-scale quantity the coating costs about US$1 per square foot, which should drop to about $0.10 per square foot in large-scale quantities. The product is ready for testing and evaluation by commercial companies in the solar industry, ahead of rollout.

Inspired by nature

Biomimetics – human-made processes, substances, devices, or systems that imitate nature – is a field of science and technology broad in practical and commercial application. Research groups around the world, including Max Planck Institute for Intelligent Systems in Germany, have been using the principal of moths’ compound eyes that do not reflect incoming light, as the basis for developing highly efficient AR coatings. A group of researchers at the Massachusetts Institute of Technology (MIT) in the U.S. took inspiration from lotus leaves – which repel water – and moth eyes to develop a method of producing glass with anti-fogging and self-cleaning properties combined with low reflectivity. Such glass has a broad range of applications including lenses for cameras and microscopes suitable for even the most humid conditions and smart phone touchscreens as well as AR solar glass.

The team developed a process that began with depositing very thin layers of material on a surface. This is then selectively etched away to produce a surface covered with tiny cones, each five times taller than their width of about 200 nm across. The pattern both prevents reflections and repels water from the surface. This could result in a dual functioning PV cover glass able to help maximize module output. The glass is able to keep itself clean of dust and dirt and also absorb more light, especially during times of the day when the light hits the panel at a sharp angle of incidence, known as broadband omnidirectional transmission. For the solar glass application, the MIT team has been working on the improvement of mechanical robustness and the development of a scale-up version of nanotextured surfaces at lower cost.

The nanotextured coating could be applied in two ways. “One is using nanotextured glass itself as solar glass. The other is attaching polymeric nanotextured film to the existing solar glass. In terms of cost, the latter option would be better, while the first option would provide higher performance,” explains MIT researcher on the project Hyungryul Choi.

The performance of the nanotextured surface has already been demonstrated for commercialization with lab-scale nanoimprinted nanocones that are compatible with scaling up technology. Pre-commercial demonstrators could be developed within one to two years through collaboration with companies – ideally glass producers – that have suitable fabrication equipment and facilities for large-scale, low-cost production of nanotextured surfaces. MIT has already had some commercial enquiries concerning the technology. In the meantime work is focusing on augmenting superoleophobicity to enable enhanced self-cleaning and to provide fingerprint resistance in a broader range of applications.

In Singapore, an AR nanotechnology-based coating has been developed by the government-funded Institute of Materials Research and Engineering (IMRE) and its Industrial Consortium On Nanoimprint (ICON) partner companies. The ICON team, in a follow-up project, is now piloting roll-to-roll nanoimprinting to produce two types of patterned plastic films with low reflectivity and better viewing angles. The films are also tough, scratch-resistant and have self-cleaning surfaces. ICON’s six partners include the state’s DSO National Laboratories, Yong Chang Chemical, and Innox Corp. The piloting project is focused on ramping up substrate size, targeting a throughput of 30 meters per minute. The project should be completed by mid-2013, and the initial application is for mobile display electronics. Solar glass requires further work such as scaling of substrate sizes. However a solar company has already been in touch with the project team to ask about the potential robustness of the film, particularly its abrasion-resistance, according to IMRE’s Hong Yee Low.

All components that go into making a solar panel have their price tag, so it makes sense that a key component such as glass, in addition to its main function of protecting the modules within, should be engineered, or treated, to do more to maximize panel output. Falling costs in PV modules and increasing efforts by governments to implement PV at the utility scale, particularly in sunbelt markets such as the Middle East, will help to drive demand for some of these advanced bifunctional and multifunctional glass coatings and solutions in the coming years. Maintaining a solar farm in dusty desert conditions includes keeping panels as free from dust and dirt as possible. Such smart coatings could help to reduce these costs over the lifetime of the plant. A PV plant with lower operating and maintenance costs leads to lower LCOE, making PV cheaper in future.

A piece of glass, one half with a normal flat surface and the other half with a patterned surface, shows the anti-fogging properties of MIT’s nanotextured coating.

Through a selective etching process on extremely thin layers of material deposited on glass or other surfaces, MIT researchers produced a surface covered with tiny cones, creating a pattern that prevents reflections and also repels water from the surface.