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Content archived on 2024-04-19

Development of a building glazed component with electrically controlled optical properties


The aim of the programme is to study and develop an industrial glazed component of large dimensions (1 m2 or more) including an active film made from a mixture of chiral liquid crystal polymer in the form of a gel, or from micro-inclusions evenly distributed in a polymer matrix. This component will be used in building facades to reduce the solar input in summer. Under the action of a low level voltage on this film, the glazing will reflect solar irradiation in the visible and infra red parts of the solar spectrum. This reflection can be predetermined by the proper selection of the constituents. Electrical control will be achieved by using transparent conductive layers (ITO).
The glazed element will be multifunctionnal and will include other characteristics such as shock resistance, anti-condensation and anti-frosting.
The work programme covers all the complementary aspects concerning the definition of the active films, the window panes including the manufacturing process involved and their development and characterization in the form of small samples or pre-industrial products, tests and the evaluation of energy performance once thermal and energy computer programme is developed, and the design of tools and adaptation of existing machines for their fabrication.
The deliverables will be : - Technical information on the active film - Softwares for the design and the computation of energy performances of electrically controlled glazing - Prototypes of glazed components (1 m2 or more) - Experimental data.

Research on new materials and processes for the construction of glass panels with adjustable optical properties for the building industry, has been intensive during the last few years. Existing technologies use several types of organic and inorganic, thermochromic, photochromic and electrochromic materials.
As regards the various usable chromogenic materials available, the best known avenue of investigation has concerned devices using inorganic electrochromic materials such as tungsten and molybdenum oxides in which the electrochromic layer darkens when ion-enriched and loses colour in the contrary case. The active layer of tungsten oxide is sandwiched between an electron-providing layer and a solid liquid or polymer layer able to provide ions.
One of the advantages of the system is the low voltage that is required (just a few volts), but a major drawback is its long response time (a fraction of a minute to several minutes) which depends on the surface area of the whole. In this type of component the voltage applied does not enable light to be reflected in a chosen wavelenght band.
Liquid crystals are well known for their electro-optical properties. However a continuous film of low molar mass liquid crystal between two large area glass substrates, coated with ITO so as to allow a voltage to be applied, is not used because of fluidity reasons and the pressure differences that occur between the lower and upper parts of the glass panel. Pure liquid crystal polymers in the form of solid films, side chain types for example, would frequently have long response time.
This is why either liquid crystals forming mico-inclusions in polymer matrices or mixtures forming gels or open-pore composites are required so as to obtain a solid film with good electro-optical properties. These liquid crystal films forming micro-inclusions in polymers are currently under intensive development because of technical and economic considerations chiefly focused on display technologies.
R&D work on this process is developing mainly in the United States (Taliq, General Motors, Polytronix, Hoechst Celanese, Hughes Aircraft...) and Japan (Mitsubishi, Seiko, Nippon Seiki, Nippon Victor, Asahi,...). Most of this work is focused on nematic liquid crystal. More recently, a similar process, using an anisotropic gel obtained by photopolymerization of disfunctional monomers in a liquid crystal, was developped simultaneously by Philips at Eindhoven and by the Kent University team in the United States. A Joule programme (JOUE/0045) on solid state variable transmission windows has been carried out. Its objective was to develop and to demonstrate the technical feasibility of electro-chronic windows of 30 x 30 cm2 with a transmission range of better than 50 %, a switching time of 30 sec. and a lifetime of 5000 cycles. The process is based on coating the window with a stack of thin films.
A glazed component with electrically controlled optical can increase the energy performance of fenestrations and can be easily integrated in intelligent solar building design.
Thermal computations have been performed on buildings using ideal electrochromic glazings in Louisiana's hot and sunny weather conditions. This type of glazing should lead to a reduction of energy consumption of 50 % compared to a standard glass. This gain increases with the glazed surface area.

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Commissariat à l'Energie Atomique (CEA)
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Centre d'Études de Cadarache Sere-Ders
13108 Saint-Paul-lez-Durance

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