Commentary: Spectrally selective coatings on glass: solar-control and low-emissivity coatings

Abstract

Increased living standards in the developed world, as well as in developing countries, have made air conditioning very popular. It is estimated that energy consumption for air conditioning in developing countries will continuously increase from 115 TWh in 2005 to 757 TWh in 2030 [1]. In comparison, energy consumption for cooling residential spaces in the United States will level off and stay constant at around 200 TWh during the same period of time. Glass is widely used in commercial and residential buildings, and automobiles. For these applications, there is a tendency to increase the area covered by windows aiming at increased visual comfort and aesthetics. However, glass does not allow control over infrared (IR) radiation, and thus heat control through the windows is not possible. This way, IR radiation entering buildings and automobiles results in increased temperature. In the case of cars, windows contribute up to about 70% of the total heat, and nearly half of this figure is from windshields [2]. Moreover, energy used for room heating is lost by radiation through the windows. As such, efficient use of energy in air conditioning, i.e. control of IR radiation through windows, becomes desirable since it would result in reduced energy consumption, resulting in energy savings and contributing to reduce global warming. Highly transparent thermally reflective coatings are deposited onto glasses to be used in windows for the purpose of saving energy. These are generally termed spectrally selective coatings, and include solar-control coatings and low-emissivity coatings. Before describing the fundamentals of spectrally selective coatings, it is necessary to keep in mind that all matter emits radiation. To describe radiation from matter the concept of blackbody is introduced. A blackbody is usually defined as a perfect radiator which absorbs all radiation incident upon it [3]. Planck’s Law describes the amplitude of electromagnetic radiation emitted (spectral radiance) from a blackbody. If the wavelength, λ, is given in microns and temperature, T, in Kelvin, Planck’s Law takes the form [3]