Can we imagine life these days without an air conditioner or a heater? Of course, not! Woefully, these creature comforts consume a massive amount of energy (electrical and thermal). The building and construction industries – which air conditioners and heaters fall under – are two of the world’s leading energy consumers.
Society is seeking a net-zero emission target. Renewable energy is the best known way to achieve that goal, however a promising complimentary strategy is optimising the built environment. The built environment is everything we build as humans: the buildings we live in, our roads, bridges and transport systems, and the systems that provide us with water and electricity.
The use of appropriate building materials to minimise the industry’s environmental impact has received increasing research attention. For material scientists, it is challenging to develop sustainable materials that reconcile both human development and climate change mitigation, yet do not compromise people’s well-being or environmental security.
My research at the University of Exeter focuses on smart windows – that is, windows that possess glass or glazing whose light transmission properties can control the passage of solar irradiation into buildings, effectively adapting to diverse climate conditions.
Achieving net-zero targets by employing sustainable building materials, that can both store and release their energy has become an increasing focus in recent years. Applying solar technology to retrofit existing traditional, humdrum windows can help achieve energy-positive built environments. These smart windows achieve this through their heat loss or gain potential, depending on the climate. This has significant implications for the efficiency and green credentials of a building and its ability to hit decarbonisation targets. Smart windows can also reduce energy bills by up to 60%.
Beyond this, IR blocking windows are another innovation that help achieve energy-positive buildings, because infrared (IR) wavelengths are the major player responsible for a house’s high-energy absorption and thermal discomfort.
How do we develop a smart window?
Smart windows can be developed through smart coating materials, which are prepared, rather creatively, for thermal, soiling, anti-reflective, blocking, and wettability applications. Moreover, a window’s ‘smartness’ also depends on its response to several thermal cycles’ spanning several years.
The installation of smart windows can play a pivotal role in controlling a building’s energy usage by reducing the use of energy-consuming areas that devour significant amounts of energy, such as from air conditioning and heating appliances.
Buildings consume energy to create thermal and visual comfort, and transparent envelopes (windows and facades) are necessary to increase daylight, comfort, productivity, health, and well-being. As such, windows are considered an integral component of the built environment and must be modelled to regulate thermal transmittance and reduce the energy consumption of entire buildings.
Current windows’ thermal and optical properties are ‘set’ into glass coatings as part of the manufacturing process. Therefore, the only accurate adjustable variable for local climate conditions is choice of coating. This can be achieved by developing a multi-fold smart composite, combining an optimised matrix of phase change materials (PCM), transparent insulating materials (TIM), transparent infrared absorbers (TIA) to absorb IR radiation, and thermochromic materials (TCM), all at an acceptable cost. Smart coating can be used to control light transmission, along with infrared reflective coating to reduce heat loss/gain through a transparent envelope of the built environment.
New innovations in smart window coating technology
Considering factors like fabrication-installation costs, carbon footprint, and energy security, the coating and surface properties of glazing materials are the only way to meet desired requirements. Glass is unique amongst building materials in that it provides natural light and solar heat with lower thermal conductivity. “Low-emissivity” coatings are the best technology to increase insulation properties. The traditional vacuum-based double-glazing system (two low emissive glass panes sandwiched between a vacuum or with a gas atmosphere inserted), is used in most houses with curtains to maintain daylight activity and heat gain/loss. However, the fabrication and maintenance of double-glazed windows are challenging and expensive.
Developing innovative, multi-fold nanocomposite materials, with a perfect blend of PCM, TIM, TIA and TCM to get a coating on the glass pane is an alternative smart solution. In winter, the multifold nanocomposite coating on windows controls incoming solar radiation to maximise solar gain and minimise heat loss, while the same window ensures optimal natural lighting conditions with indoor thermal comfort. Besides IR shielding properties, the advantage of using a composite of the PCM and a transparent infrared absorber, is that the PCM can also control the heat produced by sunlight, providing better thermal comfort.
Smart nanocomposites such as paraffin incorporated tin oxide and alumina coatings ensure 90% of IR light blocking. They also have improved indoor thermal comfort of 30oC when outdoor temperatures reach 50oC. Other nanocomposites, such as optimized indium oxide-zinc oxide-polymethyl methacrylate-paraffin, exhibit a reduced heat exchange through an innovative combined self-cleaning and energy-saving envelope.
Wider impact of smart windows on emissions goals
In 2019, the United Kingdom became the first of G7 country to enshrine in law the requirement to achieve net zero emissions by 2050. In order to achieve that, policymakers need to address the challenge of energy loss in buildings.
Smart windows will contribute to emissions reduction targets by reducing building energy loads. People living in energy-efficient buildings will also benefit from reduced energy costs, improved internal environmental conditions and comfort, and enhanced health and well-being.
The primary commercial beneficiary of novel smart window technologies will be construction-related industries, including building designers, glazing/material manufacturers, and installation companies. There will be a positive economic impact through the design, production and commercial exploitation of new glazing systems. This is because developing advanced glazing technology can reduce a building’s annual energy consumption by 30-40%.
In summary, there are a plethora of novel materials in smart windows coating technology that offer an exciting range of optical and thermal properties, all acting to save energy while improving human comfort.
“The whole purpose of education is to turn mirrors into windows”, said Sydney J. Harris, a famous American journalist. His quote here embodies a perfect meaning of life: sustainable improvement through the help of science and technology.
