Evaluation of optical and thermal properties of coatings for energy efficient windows

Over the past couple of decades a number of coated glazing products have appeared on the market, converting the window from an energy drain to a possible resource in the building's energy supply system. In this paper the light transmittance, total solar energy transmittance, and thermal transmittance of coated glazing currently available on the market and future dynamic electrochromic coatings are reviewed and their function in heating or cooling dominated climates discussed. Electrochromic coatings can switch between a transparent state and an absorbing or reflecting state when a small electrical potential is applied, and hence adapt their optical properties to the present condition. Energy balance calculations for the window component stress the importance of selecting the window depending on climate. In a heating dominated climate a low-e coating with a high g-value can result in a net energy gain on a south facing façade and thus be better than an ideal wall.