Windows Optical Performance and Energy Efficiency

Abstract: This thesis treats angle-resolved optical properties and the energy efficiency of windows. A theoretical evaluation of optical and thermal properties of windows is briefly surveyed and the energy performance of a large selection of windows, under different conditions, is examined. In particular, angle dependent optical properties are analysed. A new model assessing angle dependence of the total solar energy transmittance, g, of windows is presented. A comparison of simple models for angle-dependence prediction has been performed, including both fictitious and measured real window glazings. The new proposed model illustrates low errors for both the real and the fictitious glazings. The impact of inaccuracy in the angle dependence of the g-factor has been assessed and found to be clearly noticeable but not necessarily critical. A simple model for comparing the energy efficiency of different windows in different types of buildings and different climates has been further developed and analysed for several conditions. The energy performance of a large number of windows has been analysed using this model, and also by using other building and window simulation models. Typical savings when changing from a standard double glazed window to the optimal window for the investigated case is in the order of 100-150 kWh/m2yr. The annual energy balance of modern low emittance windows illustrates that they can be annual energy savers rather than energy losers, unlike traditional windows. However, it is shown that it is not important to argue about small changes (~0.01) of the thermal emittance value. Furthermore, advance solar control glazings effectively reduce solar transmittance with maintained high light transmittance. AR-coatings and low-iron glazings can increase the transmittance of glazings considerably. In fact, a "super" low emittance window with a U-value below 1 W/m2K can have higher light transmittance than a common double-glazed unit. Windows with variable transmittance, switchable windows, are compared with high-performing solar control windows, illustrating some degree of potential energy savings compared to high performing static solar control windows, depending on the type of control that is used. This is accompanied by the potential for automatic thermal comfort- and glare control. Different models for energy rating of windows have been evaluated, demonstrating that a simple linear rating depending on the U and g-factor of the window may be sufficient with certain restrictions. Division into climate zones is essential. In all, the results demonstrate that energy-efficient windows provide huge energy-saving potentials on a large (regional, national, global) scale.