The National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, announces that a collaborative R&D agreement has been signed with the Nippon Sheet Glass (NSG) on the development of solar heat energy controlling glass.

The target glass automatically controls the transmission of solar thermal energy, to achieve energy saving by deterring entry of solar heat into the room in summer, and by admitting solar radiation to the interior in winter.

Up to now, a variety of functional glasses have been put to the market for windowpanes of housing and automobiles, such as double insulating glass, low-E glass, vacuum-insulated glass, heat-ray cut-off glass, contributing to energy saving and enhancement of comfort. While these glass species are effective for reducing the room cooler burden by shutting out solar radiation in summer, and for reducing the room heater burden by thermal insulation in winter, the transmission of solar radiation through them is fixed throughout a year, and no function has been available to change solar heat intake depending upon the seasonal changes and dwellers' needs.

The target of the present collaborative study is to provide a glass coated with film made of vanadium oxide or its derivative, of which optical property varies with ambient temperatures while keeping the transmission to visible light nearly constant, controlling the solar heat intake in response to environmental temperatures.

The Materials Research Institute for Sustainable Development (MRISD), AIST has been engaged for over a decade in development of preparation process, upgrading the transmission to visible light, and augmentation of solar heat control efficiency. Consequently, it has been successfully achieved to prepare a sample improving the visible light transmission up to 60 %, and controlling the transmission of solar heat from around 60 % to 20 % in response to changes in temperatures from 10 °C to 68 °C.

In order to meet requirements for mass production technology in preparation for future commercialization, the AIST-NSG joint research will be carried out by combining AIST's research achievements with NSG's production technology of functional glasses, making good use of the AIST's Patent Licensing Collaboration Scheme.

It is intended to put to the market solar heat energy controlling glass of size for housing windowpanes within three years.

Figure : Concept of structure and function of solar radiation control glass

At the Third Conference of Parties to the U.N. Framework Convention on Climate Change (COP3), Japan committed to reduce the CO₂ emission by 6 % of the reference level as of 1990 by 2012. According to data provided by the Ministry of Environment, however, carbon dioxide emission from the industry sector will be reduced as promised, but those from the transport sector and the consumer sector involving air-conditioning and housing are expected to increase contrarily. In case of housing, the overall heat flow includes 48 % heat outflow through the window in winter, and 71 % inflow in summer. Accordingly, the windowpane equipped with heat insulation will extensively contribute to the reduction of carbon dioxide emission from the consumer sector. Heat insulating glasses currently available in the market such as double insulating glass, low-E glass, and heat-ray cut-off glass are characterized by fixed optical property incapable of changing in response to seasonal temperature changes. A windowpane to take in much solar heat in winter, and to shut out thermal radiation in summer will make it possible to reduce the room cooling/heating burden extensively. While there have been R&D efforts for electrochromic materials, which change the transmission to solar thermal energy by the control with electric current, no product has been marketed yet for the purpose of energy saving.

The MRISD-AIST has been dedicated to the study on thermochromic glass of which transmission to solar heat varies depending upon ambient temperatures. Using the thermochromic glass for windowpanes of the building, the solar thermal energy will be taken up in winter, and shut out in summer, effectively reducing the air-conditioning load. Moreover, as the optical property of the glass changes automatically, neither additional control system nor operation are needed at all.

Thermochromic glass consists of glass coated with thermochromic materials of which optical property changes reversibly with temperatures. Typical thermochromic materials include vanadium dioxide (VO₂) and that added with metal such as tungsten or molybdenum (V_1-xM_xO₂). The temperature at which the optical property changes can be freely set from the room temperature to 68 °C by adjusting the metal dosage.

In this way, the thermochromic glass may be regarded as an autonomic version of commercially available heat-insulating glass. While it is an innovative functional glass, it involves some demerits for commercialization, such as lower transmission for visible light and tendency of being colored in yellow.

Good prospects of solving these problems have been obtained through recent studies on multiple layer construction and control of film thickness, making a great stride toward the practical application.