The special Energy Challenge issue of Physics Today (April 2002) contained a number of articles that focused strongly on energy production, whereas the possibilities of energy savings were given small attention. So it bears repeating, as mentioned in the article by Arthur Rosenfeld, Tina Kaarsberg, and Joseph Romm (Physics Today, Physics Today 0031-9228 53 11 2000 29 https://doi.org/10.1063/1.1333283 November 2000, page 29 ), that energy efficiency is a vital part of meeting the energy challenge.
The well-known correlation between the Human Development Index, an internationally recognized measure of basic human well-being, and annual per capita electricity use as shown in the figure on Physics Today 0031-9228 55 4 2002 39 https://doi.org/10.1063/1.1480780 page 39 of the April 2002 issue should not be misinterpreted or overrated. Current advances in building technology point toward options for increasing human comfort while diminishing energy consumption. These possibilities are strongly related to advances in materials physics—especially the development of electrochromic materials with optical absorption that can be regulated, reversibly and persistently, by charge insertion and extraction.
Buildings use large amounts of energy for heating, cooling, lighting, and ventilation. Air conditioning has become increasingly important for balancing excessive heat that flows in through glass facades; that excessive heat is a major reason behind the recent energy shortages in California. Architectural trends toward larger glazings—responding to building occupants’ wishes for improved indoor–outdoor contact—are likely to lead to even greater demand for air conditioning.
One emerging technology for diminishing the heat load on buildings is “smart” windows, which have multilayer coatings, including electrochromic materials. These windows can regulate the throughput of solar energy and visible light across a wide range of transmittance values by charging and discharging electricity provided by small solar cells. The first of such products has been on the market for a few years, and strong R&D efforts—in the US and internationally—make it likely that less expensive and more durable smart windows will be available within a few years.
Simple estimates show that the energy savings inherent in the smart windows technology are large. In quantitative terms, the lowered electricity demand for air cooling is of the same magnitude as the electricity that can be generated by today’s best solar cells, given the same area and orientation as for the smart windows.
The smart windows technology can be less expensive than solar cells. Even more important, the ability to regulate the window transparency alleviates thermal and visual discomfort associated with excessive light inflow while maintaining the primary function of the window: visual contact between indoors and outdoors.