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.

Lighting accounts for the second-largest amount of residential power consumption after air conditioning, and efforts to improve the energy efficiency of lighting are being accelerated. Figure 1 shows the trend of efforts toward the energy saving of lighting. Currently, efforts are being made to improve the efficiency of existing light sources and use them effectively. Light-emitting diode (LED) lights, have been introduced on the market, expecting higher efficiency. Organic electroluminescent (OEL) lamps are also at the stage of commercialization. In addition to the replacement of existing light sources, there is an increasing need to create a new comfortable lighting environment using a system that takes advantage of new light sources, such as the directivity, thinness, space savings, and easy and quick output control of LED lights and the flexibility and thinness of OEL lights. Under these circumstances, building components such as walls and windows will inevitably be integrated with electric devices such as lights and displays in the near future. We will also need to develop a method to effectively use sunlight, which has a spectral distribution comfortable for human beings, as a light source in combination with these new light sources.

In order to achieve the above goal, it is important to functionalize glass using light-emitting devices such as LEDs, without compromising the transparency of glass windows as openings to let sunlight in. As a materials approach, we are attempting to produce a new light source that combines fluorescent transparent glass with an LED. Fluorescent glass can convert the glaring light of an LED light, which is a point light source, to the soft light of a surface light source that the human eye can readily adjust to. A flat light source transparent to visible light can be made by placing a near-UV LED and a fluorescent glass as shown in Fig. 2. The glass that we have produced is a small 3-inch piece. If we can produce larger sizes, which is technically challenging, we can develop windows with a light-emitting capability to correct diurnal and weather-related variations in the intensity of sunlight using LED lights. As wall openings become larger, heat loss increases and the power consumption of air conditioning rises. Therefore, other insulation techniques and thermal design during air conditioning must be considered at the same time.

Large size and low cost required for building materials are challenging targets for functional materials in developing lights and displays of the future. We will continue our research to develop future lights and displays as well as light-emitting, transparent glass windows.

We are developing and demonstrating energy-saving ceramic exterior wall materials, including water-retaining ceramics and exterior wall tiles with controlled sunlight reflectivity. Water-retaining materials absorb water from rain and watering and allow the water to evaporate when the surface temperature rises. These materials cool surfaces by the evaporation of water and help to mitigate the heat island effect. Water-retaining materials have been tested and commercialized for road applications and demonstrated to be effective. They are also expected to be applied to residential building materials. Low cost, high functionality, and high quality are required for residential building materials. We have developed a water-retaining ceramic material that can be used in residential construction, in collaboration with companies with the technical capability to manufacture porous ceramics without sintering as well as public research institutions.

Indicators for the evaluation of water-retaining ceramics include water-retaining capacity, evaporation rate, water absorption rate, water absorption coefficient (porosity), density, and strength. A porosity of more than 30 vol% and sufficient strength were achieved by optimizing the grain size of the raw material, the material selection, and forming conditions. The relationship between the microstructure and water-evaporation characteristics of the ceramic material was analyzed to control its water-evaporation properties.

Water-retaining ceramics have the inherent disadvantage of being vulnerable to frost damage. We are endeavoring to overcome this vulnerability by eliminating unevenness in forming ceramics. The development and demonstration of water-retaining ceramics are being carried out at the same time to examine problems in commercializing the material and evaluate their capability to mitigate the heat island effect and the energy saving effect (photo).