Calculation Tool for Determining the Net Energy Gain

The introduction of an energy rating and marking system of fenestration products in Denmark has contributed to an increased interest in developing and using windows with improved energy performances. Formerly the evaluation of energy performance of windows was based only on the heat loss coefficient. But as the heat loss coefficient for windows during the last two decades has improved considerably (especially for the glazing part) it has become more relevant also to include the solar energy gain that is transmitted in through the windows and contributes to the space heating in the building. A proper and direct way to describe the energy performance of windows is by the net energy gain, E, which expresses the energy balance for the window. It is defined as the solar heat gain transmitted in minus the heat loss transmitted out through the window during the heating season. The net energy gain is dependent on both the U-values and the g-values. Beyond this it is dependent on the orientation of the windows and the climate and the actual period. This makes it difficult to choose the glazings and windows that are optimal with regard to energy performance in a given case. These facts have aroused a need for simple and accurate methods to determine and compare the energy performance of different window products. When choosing windows for new buildings or retrofitting a calculation tool that in a simple way determines the net energy gain from the specific windows in the actual building will ease the selection of the best window solution. Such a tool combined with a database with window products can make calculations of the heat loss or energy demand corresponding to the requirements in the new building code easier and more correct. In the paper, methods to determine energy performance data and the net energy gain for windows are presented. Based on these methods a program has been developed that determines the heat loss coefficient, U, and the total solar energy transmittance, g, for windows compounded of specific window components selected from a database. The program calculates the net energy gain for specific windows inserted in an actual building.