OPEN THE DOORS FOR SMART WINDOWS

Windows provide a soft transition between interior and exterior spaces by allowing daylight, ventilation and outside views essential for occupants’ well being. They also help in energy conservation by bringing in natural light and thereby reducing the need for electric lighting. Natural light impacts the visual quality of a space and also thermal needs of occupants. Many studies have shown that the visual and thermal comfort factors affect the productivity of occupants.

Natural light changes throughout the day. Sometimes, we face glare due to too much daylight which makes it difficult to perform tasks such as reading and typing on screens, which reduces productivity. This problem is more prevalent in office buildings as people spend most of their time on computer screens. Excess of light also means more heat, and consequently increased energy consumption as air-conditioners stay on longer.

Current market solutions to overcome these issues are window shades such as curtains and interior blinds. However, they cannot meet the two objectives simultaneously: maintaining the outside view and controlling the amount of daylight entering the space according to the changing outdoor environment.

What we need are smart windows that can adapt to the outdoor environment and provide comfort to the occupants of buildings, while also saving energy.

The smart window glass switches between transparent and opaque states, and can change its tint as per the occupant’s need and outdoor conditions. Recent studies show that smart windows can reduce energy consumption for lighting and cooling up to 20 per cent by controlling the entry of daylight.

The presently available smart window solution employs electrochromic technology that provides dynamic tinting of glass by either automatic or manual control. These smart windows are fabricated by sandwiching layers of electrochromic material between two glass panes that can change its optical properties as per applied voltage thus allowing control over the amount of light passing through them.

The most successful electrochromic material used commercially is transition metal oxide, and the most common of these is Tungsten Trioxide WO3. Although several companies are trying to commercialise electrochromic windows, this technology is expensive.

At present such windows are used in airplanes and luxury cars and are seldom used in buildings. These windows take a lot of time to switch from one transparency level to another, and it is difficult to colour tune this material as desired. Further, they manage heat poorly because they re-radiate the absorbed light as heat and their life is comparatively short due to degradation at each on/off cycle.

Researchers at Stanford University have recently developed a smart window with a completely different approach. Their prototype is based on reversible metal electrodeposition of Copper-Lead or CopperSilver on a transparent conducting electrode that switches uniformly between transparent and opaque states in less than three minutes.

The devices consist of an ITO (Indium Tim Oxide) working electrode, modified with platinum nano particles, anchored via a self-assembled monolayer (SAM) of 3-mercaptopropionic acid and a metal counter-electrode frame.

The key to uniform reversible metal deposition is nucleation of metal on ITO electrode surface by using platinum nano particles. This technique is similar to that of a flat panel display. It utilises metal as an optically active material in the dynamic window for daylight modulation.

Another important feature of this technique is that it uses only 20nm of metal for getting complete opacity. As the metals have reflective properties, this technique works better in heat management than the electrochromic materials. The lab prototype of this window contains a Cu-Ag gel electrolyte instead of liquid because it is less prone to leaks, has easier fabrication process and keeps the electrolyte confined in case of any accident. Therefore, it can be commercialised more easily.

As compared to the electrochromic window, they can go up to more than 5,000 cycles of switching with minimum degradation and thus are more durable. Smart windows based on reversible metal electrodeposition exhibit fast switching times, high durability, tuneable transparency and chemical stability.

India has abundant amount of daylight, and new technologies in smart windows such as technology based on reversible metal electrodeposition might be a possible solution. Such windows will filter out all wavelengths equally and will provide the uniform colour of daylight. They will block up to 90 per cent of the light, reducing glare and heat while facilitating the outdoor views.

The writer is a Professor in Building Science, IIIT, Hyderabad