Windows and glass facades often let in or out too much energy so that cooling or heating must be used in order to achieve a good indoor environment. Chromogenic fenestration can diminish the energy expenditure substantially by being able to adjust the transmittance of solar energy and visible light and is also able to improve indoor comfort. This paper outlines recent progress in thermochromics, which allows the transmittance to be lowered reversibly upon temperature increase, and in electrochromics which permits transmittance modulation between widely separated limits by the application of electrical voltage pulses. Thermochromics makes use of vanadium-dioxide-based thin films or nanoparticles; this technology has advanced strongly during the past few years and now appears viable for practical fenestration.
Electrochromics is also under rapid development and the first products are currently on the market; they are normally based on thin films of nanoporous tungsten oxide and nickel oxide. Design of thermochromic and electrochromic devices, related materials, thin film deposition technology, and achievable performance limits are discussed in some detail.