- Smart windows could soon become a key element of sustainable buildings, but their development has faced numerous challenges so far.
- A team of researchers led by Dr Claudio Roscini at the Catalan Institute of Nanoscience and Nanotechnology (ICN2), in collaboration with the group of Dr Jordi Hernando, from the Autonomous University of Barcelona (UAB) are investigating this.
- They have unveiled a thin plastic film which tackles many of these challenges simultaneously.
- Embedded with specially engineered nanoparticles, the film can be easily installed onto existing windows to create highly adaptable smart windows.
In the developed world, buildings account for some 40% of all the energy we consume – around half of which is dedicated to heating, ventilation, and air conditioning. To tackle this problem, researchers are now exploring the energy-saving potential of smart windows, which can actively adjust the amount of sunlight passing through them to control the heating and cooling of indoor spaces.
Roscini’s team developed a new smart window concept based on polymeric films: thin sheets of plastic with unique physical properties.
So far, new advances in this technology have been hindered by high development costs, combined with the difficulty of producing high-performing materials. A team of researchers led by Dr Claudio Roscini (of the Catalan Institute of Nanoscience and nanotechnology, ICN2) and Dr Jordi Hernando (Autonomous University of Barcelona) has developed a new smart window concept based on polymeric films: thin sheets of plastic with unique physical properties, which can be tailored for many different practical applications.
Some help from nanotechnology
In this case, the polymeric films are embedded with nanoparticles made from a low-cost, paraffin-based wax. Depending on the external temperature, these nanoparticles can exist as either a solid or a liquid, altering the film’s optical behaviour.
‘When the wax particles are solid, their refractive index matches the one of the polymer, and the film becomes transparent’. Roscini explains. ‘When the wax becomes liquid, its refractive index mismatches the polymer’s one, causing light to scatter, thus increasing opacity’.
In addition to these temperature-sensitive properties, the team also embedded their polymeric film with photothermal nanoparticles which convert light into heat. By increasing the film’s temperature at high light intensities, these nanoparticles can cause the wax to melt, making the film opaque. Even further, the researchers coated the film with a conductive layer which warms up the film when an electric current is applied – allowing users to adjust its transparency manually at their will.
With this multiresponsiveness, the team’s polymeric film is highly versatile, and can be potentially installed simply by adhering to existing windows. ‘The films do not necessarily need to be installed in the window during its fabrication, and can be removed if desired,’ Roscini says.
Tighter control
Beyond its ease of installation, the film offers a wide array of further advantages – firstly, relating to the level of control they offer over the amount of light entering indoor spaces. ‘The films respond to different stimuli, which can be temperature, light, or electric voltages’, Roscini describes. ‘This makes them self-responding to external weather conditions or applied voltages, thus allowing the user to control the window’s degree of transparency in the same way they would open or close shutters (for antiglare and privacy needs)’.
On top of this, the polymeric films are highly effective at filtering visible and infrared wavelengths which constitute most of the energy contained in sunlight, making them particularly well-suited for regulating indoor temperatures.
In hotter or sunnier weather, the team’s polymeric film can become more opaque to avoid excess heating indoors, minimising the need for power-hungry air conditioning systems. In contrast, the films can become more transparent in cooler, cloudier weather. This allows more energy from the sun inside, minimising the need for lighting and central heating.
Material benefits
A further key set of advantages emerge from the film’s material composition. Since it is made from low-cost polymers and waxes, the team’s design is far more affordable than existing smart window technologies. Added to this, the film is flexible and highly adaptable, adhering to windows of many different shapes and sizes.
The numerous advantages of their approach could vastly increase the availability of smart windows worldwide.
In addition, the polymeric film can be produced through a water-based manufacturing process. This makes it far more sustainable than comparable processes for manufacturing smart window films – which often rely on undesired organic solvents or high-energy processes (eg, extrusion, injection moulding) .
The wax-based composition of the film’s nanoparticles offers a further set of advantages. ‘Since they do not contain dyes like other smart window films, they display less tendency to degradation by sunlight, and will last much longer’, Hernando describes.
Notably, the materials involved in the polymeric film’s manufacture are highly adjustable, allowing their design to be tailored for the unique requirements of users around the world. ‘The films can be adapted to different geographical regions by changing the type of wax, as well as the type or concentration of photothermal nanoparticles’, Roscini continues. ‘In this way, they can be tuned to change suddenly or gradually their transparency in climates with different temperatures and sunlight intensities’.
Improving life indoors
Finally, alongside their ability to control heating and cooling in buildings, the film’s adjustable transparency could have many practical advantages for the people inside. ‘The films improve comfort and provide a private space, making them ideal not only for use as windows in buildings, but also in awnings and greenhouses, or as space separators’, Roscini says.
Altogether, Roscini’s team is confident that the numerous advantages of their approach could vastly increase the availability of smart windows worldwide. In combining their ease of use, affordability, flexibility in installation, and sustainability in their manufacture, the researchers ultimately hope that the widespread commercial rollout of their technology could already be on the horizon.
When did you first realise that wax-based nanoparticles would be ideal for use in smart windows?
Wax-based materials were already used to vary the refractive index and to change the transparency of films, though the reported examples were actually working in the opposite way, ie, the thermally-induced transition was from an opaque to a transparent state, which is not attractive for smart window applications.
Otaegui: During my thesis, I was involved in a different project related to the preparation of wax-based fluorochromic materials, in which the melting of the wax nanoparticles induced the reversible modulation of the fluorescence properties of fluorophores dissolved in it. However, we realised that when these nanoparticles of a certain size were embedded in films, they not only modulated the fluorescence properties, but also the transparency. From there came the idea of testing the transparency modulation of the wax-based nanoparticles without the fluorophore. As expected, the transparent-to-opaque transition was preserved. Further experiments were then carried out to optimise the system, to provide multi-stimuli response (light and current), and to evaluate the energy saving properties.
How much of an impact do you think your technology could have on the energy consumption of buildings?
In the literature, it’s reported that smart windows can reduce up to 10% of energy consumption in buildings. Our tests in building models revealed that the temperature increment inside the building caused by strong sunlight irradiation (obtained from a solar simulator) at 30 oC (simulating a summer weather) was reduced by 6 oC when our smart window film was employed. This demonstrates that the opaque state formed in these conditions (high temperature and strong irradiation) attenuated the temperature increase inside, as desired. This translates to a 30–38% reduction of solar heat gain.
On the other hand, at lower temperatures (13 oC, which simulates winter weather), the strong sunlight irradiation was not enough to induce the opaque state and the transparent film favoured solar heat gain inside. This is a positive effect since in cold winters it is desirable for the maximum sunlight to pass through the window to heat the interior and minimise the need for artificial heating and lighting. This test was also carried out under real sunlight exposure, providing similar results.
These results show how the technology adapts very well to the different weather conditions, minimising the energy consumption required for heating, ventilation, and air conditioning systems, with the consequent reduction in user costs and CO2 emissions.
What further challenges will you face before this technology can be rolled out commercially?
Despite very promising results, there are some improvements that need to be accomplished before the technology can become commercial:
• Previous stability tests were very positive, though longer-term studies need to be carried out to guarantee the 10–20 years stability required for windows under sunlight and humidity exposure, and daily temperature variations.
• Scalability of the wax-based particles and films preparation need to be demonstrated at an industrial scale.
• The films must be suitable as an after-market product, which could be easily adhered by the user to windows already installed. This implies providing the film of an adhesive layer.
• Commercial partners are required (chemical manufacturers, glass makers, constructors, etc) who can licence the technology.
• Further studies must be carried out in real environments to give a quantitative idea of the energy saving effect. All these challenges are being faced within two competitive R&D projects carried out in collaboration between ICN2 and UAB.
Vallan (the entrepreneur researcher of one of these projects): Significant technical improvements are being obtained in terms of optical properties, scalability, and stability. Moreover, the technology transfer to private companies is being promoted and we have received interest from smart glass companies. We are confident that these actions should position the technology closer to the market.
