When we consider energy saving in buildings, the windows and other openings are important in that a large amount of heat goes through them. Particularly in Japan with its large seasonal variations in climate, the cooling and heating load can be significantly reduced if solar heat is well controlled by window glass. Glass that can transmit and block sunlight reversibly to save energy is called switchable glass. Such glass is expected to be the next-generation glass that provides greater energy savings than low-E double glass, which is increasingly being used.
The most typical switchable glass is electrochromic glass that can electrically switch between transparent and light-blocking states. Electrochromic glass using a thin tungsten oxide film as the switchable layer has been studied for more than 40 years. However, such glass requires the process of forming multiple thin layers on glass with a large vacuum deposition system and has the disadvantage of high cost. The Nanosystem Research Institute has developed a new technology to make Prussian blue pigment into nano-ink and produce electrochromic glass by simply applying the ink to the glass surface (Fig. 1) using various wet processes. This technology has the potential to significantly reduce the cost of electrochromic glass.
Conventional electrochromic glasses control light transmission by switching between transparent and colored states. The Materials Research Institute for Sustainable Development has developed switchable mirror glass that can efficiently block sunlight by switching to a mirror state (Fig. 2). The switchable mirror glass has been demonstrated to reduce cooling loads by more than 30 % compared with ordinary transparent glass.
There is another type of switchable glass called thermochromic glass. It is always transparent to visible light and reflective to near-infrared light at high temperatures, and automatically becomes transparent to near-infrared light at low temperatures. In the summer, thermochromic glass reflects the heat of sunlight and prevents a rise in indoor temperature. In the winter, it allows solar heat in. This switching is carried out automatically. The Materials Research Institute for Sustainable Development has developed thermochromic glass using a vanadium oxide thin film and is conducting research to commercialize the glass.
In addition to the glass, the window frame is also important in improving the insulation performance of a window. Wood has a low thermal conductivity about 1/2,000 that of aluminum, though it is inferior in strength, dimensional stability, flame retardancy, and durability. We are working to improve the material properties of wood. We are characterizing the molecular-level microstructure of wood and developing techniques, such as compression, chemical impregnation, heat treatment, and chemical modification, to improve the reliability of wood quality and make wood an environmentally friendly industrial material.