Smart windows are based on innovative composite glasses and allow to automatically adapt the characteristics of light and heat transmission to different external circumstances. For energy efficiency smart windows are a promising technology since they may shield direct sunlight through the windows and thus save energy for air conditioning.

Technical details:

Smart windows are composed of a multi-layered glass that change colour in a continuously variable fashion. This is obtained by two different effects: gasochromism and electrochromism.

Electrochromism:

Smart windows using electrochromism are made of two glass panes which are laminated together with an ion conducting polymer foil. The glass panes are coated with an electrochromic film. These thin films (e.g. Wolfram-Oxides like HXWO3, LiXWO3) change color in response to an applied electric field due to changes in the light-absorption spectra (especially for visible light and heat in the near infrared spectral range). This effect is triggered by voltage-induced ion transport in the ion-conducting foil situated between the glass panes.

Figure 1: Layers of a smart window

Source: GESIMAT

This way light and heat transmission as well as reflection properties can be altered. This process is regulated automatically. Light transmission may be reduced by up to 70% by means of smart windows.

The energy necessary for the coloration of electrochromic glazing is low (between 20 and 150 Ws/m²). The DC voltage for switching the electrochromic window is between 0.5 and 2.2 V.

Depending on the choice of these parameters full coloration or bleaching of the electrochromic glazings can be achieved in time ranges between 2 seconds and 10 minutes. A multitude of different colors (e.g. blue, green, red, brown, violet, grey) can be realised.

Usually the WO3-films are evaporated onto the glass layer and must therefore be available in a nano-crystalline state, with grain sizes between 1 and 20 nm. This sometimes leads to the categorization of smart glasses as a nanotechnology.

Gasochromism:

Gasochromism is closely related to electrochromism. Instead of an ion-conducting foil, there is an interspace between the two WO3-coated glasses filled by gases. If hydrogen concentration in the interspace is increased the WO3-film turns blue. The backwards process of achromatisation is obtained by regulating oxygen concentration. The gas concentration between the glasses is controlled by a small electrolyser and a pump, which are installed at the window’s balustrade. The gas-circle is closed. The admixing of hydrogen is so small, that there is no danger, even in case of window breaking.

Figure 2: Transmission and reflection on a smart window

Source: GESIMAT

Fields of application:

Buildings, cars (electrochromic glass already in use for car mirrors)

Manufacturer:

Electrochromic windows: Fraunhofer-Institute; GESIMAT-Berlin (Gesellschaft fuer Intelligente Materialien und Technologien)

Gasochrome windows: Interpane in cooperation with Fraunhofer-Institut fuer Solare Energiesysteme (ISE)

• Energy savings
• Improved passenger comfort

Passenger comfort

A continuous shading of windows without impeding sight improves passenger comfort. Smart windows have variable shielding (both technologies to about 30% of the incoming light). No extra sun-blind needed, which saves costs.

Costs

Smart windows as a retrofit measure will probably not be an option due to high costs.

In new stock additional costs will be justifiable by reduced energy costs.

Complexity

Smart windows are high tech applications and defect liability could become a problem.

Electro-chromic windows: For the use in buildings GESIMAT gives a possible energy saving of about 40% of average energy demand for air-conditioning (this number was derived for the very hot Californian climate). The electric energy needed for window switching is about 20-150 Ws/m² and can therefore be neglected.

The corresponding effect for an application in railways is difficult to assess. The following estimate gives a rough potential.

As an annual average, cooling to make up for sunlight induced coach heating will in most cases be less than 10 % of climatisation energy.

Assuming that smart windows reduce incident light by some 50%, they may save about 5 % of the climatisation energy.

This yields a saving potential for total energy consumption of about 1 % (taking into account that climatisation energy comes to 20% of total energy consumption).