Biologically Inspired Nanostructures for Smart Windows with Antireflection and Self-Cleaning Properties

Smart thermochromic windows whose insulation properties are tuned by the ambient temperature have been investigated extensively over recent years to improve energy efficiency of commercial and residential buildings. These windows are typically coated with thermochromic materials that exhibit a fully reversible, temperature dependent transition between semiconductor and metallic phases. During hot weather, a smart window passes all or part of the visible radiation incident and rejects the majority of the Sun's near-infrared radiation; thus the need for air conditioning is reduced. During cooler weather, both visible and infrared (IR) radiation is fully transmitted, limiting the need for internal heating. A popular material for such intelligent coatings is Vanadium dioxide (VO2) due to i) the radiation stop-band manifesting in the IR region, ii) the advantage that it can easily be applied to large substrates and iii) the ability to lower its phase transition temperature by doping it with metal compounds, most commonly tungsten. Calculations have shown that a VO2 coating can deliver a 30% reduction in energy consumption of buildings in countries with hot climates such as Italy and Egypt. Nonetheless, the merits of VO2 coatings quickly diminish in colder climates and in places like Helsinki or Moscow they, in fact, deliver a negative energy balance.

One very important factor for this performance reversal is the high refractive index that VO2 exhibits in its cold-transparent phase, which results in a large portion of the incident light being reflected - 30%-35% in the visible for a 50 nm thick VO2 film on glass. This figure compares with <4% reflectivity in conventional glass windows, meaning that a thermochromic window is much darker and colder than its plain glass counterpart in the winter, which in turn translates to an actual increase in the energy required for lighting and heating a building. In addition, dirt and stains further degrade the transmission properties of a smart window.

In order to overcome the above limitations, moth-eye type structures engineered to exhibit broadband and wide-angle antireflection properties are proposed, for the first time, to substantially improve the currently poor transmission properties of thermochromic smart windows and to pave the way for the commercialization of this technology. Our nanopatterned windows potentially have 72% higher transmission compared to existing thermochromic windows and in addition, they exhibit simultaneous self-cleaning properties without additional processing. This challenging, proof-of-concept, 24-month research project focuses on the fabrication and characterization of smart windows enhanced with moth-eye nanostructures and is divided into two research streams: A) Fabrication and characterization of antireflection and self-cleaning moth-eye nanostructures directly onto glass, appropriate for new high-end window products. B) Development of potentially low-cost thermochromic polymer thin-film to retrofit existing non-smart windows.

The main impact that Bionspired Thermochromic Windows will deliver is to improve the energy efficiency of commercial and residential buildings, which currently account for about 40% of the primary energy consumption in developed countries. According to the International Energy Agency (IEA), if no action is taken, the CO2 emissions of the building sector will increase to 15.2 Gt by 2050, nearly double from their 2007 levels. The demand for heat and cooling is the main driving force behind this growth; as an example, the annual sales of room air conditioners in China alone reached 27 million units in 2009, a substantial 35% increase from 2005. Thermochromic windows can potentially make a countable impact in reducing building related CO2 emissions provided that this technology becomes affordable and accessible to the wide public. A recent paper calculated that glass constructions (offices, shopping malls, train stations, airports) can reduce their annual energy consumption by up to more than 30% if they use thermochromic glazing instead of plain glass surfaces. Of course, skyscrapers are not made of plain glass, yet again the same report showed that thercmochromic glass can outperform high-tech static glazing products used these days in the construction sector by anything between 30% and 50%.

In terms of conformance with existing policies, the project's objectives are in close affinity with the EU's famous 20/20/20 SET-plan also adopted by the UK, which amongst others states that advances made in energy efficiency should contribute a 20% reduction in the EU's total energy consumption by 2020 from 1990 levels. It is also aligned with the requirements arising from the UK's own Climate Change Bill, passed through parliament in November 2008, which has set stringent targets for 80% carbon emission reductions by 2050 again from 1990 levels. Along these lines, the Government published the "Building a Greener Future " policy statement in 2007, where it expressed its intentions for all new homes to be zero carbon by 2016, a policy to be enforced by a major progressive tightening of the energy efficiency building regulations. According to this policy statement, insulation technologies, which include smart windows, will play a prominent role in achieving the zero carbon goal.

As about the commercial outlook, smart windows is a rapidly growing market which is expected to exceed the $1B threshold in 2015 and to reach $2.5B in sales by 2018, up from a mere $200M in 2011, although thermochromic windows have not been commercialized yet. The UK already has a significant footing in this market with Pilkington glass - a British company that introduced the first smart window in the market in 2001 - leading the way.