Tobias Rosencrantz, Energy and Building Design, Lund University,

Helena Bulow-Hube, Energy and Building Design, Lund University

The purpose is to evaluate the potential of anti-reflective (AR) coatings on windows. Usually, AR coatings are used on solar collectors and photovoltaic cells to improve their efficiency. However, AR coatings could be used on windows with low-e coated glass to achieve a higher transmittance, while the U-value would remain unaffected. This paper will investigate the effects on daylight availability, solar gain and the resulting heating demand for a typical multi-family dwelling in Sweden with and without AR coatings applied on the windows.

Low-e coated glass based on SnO2 (usually referred to as hard coatings) is an interesting alternative in window renovation. It can already today be used rather cost — effectively to replace one of the panes in ordinary coupled windows, thus considerably improving the energy efficiency of the window, and at the same time preserving the appealing aesthetics of old hand-crafted windows. However, both the solar and visual transmittance will be somewhat reduced and the low-e glass usually has a slight, but distinguishable brownish tint. Previous studies show that both the total solar energy transmittance (g) and the light transmittance (Tvis) can be increased considerably by using AR coatings on windows. Further, the low emissivity of the hard low-e coating remains unchanged. The annual potential for increased solar gain and increased daylight with AR coatings on low-e glass will be studied by using the energy simulation program ParaSol and the light simulation program Radiance. The difference in annual heating demand and absorbed solar radiation will be calculated for windows with and without AR coatings. The study will focus on residential multi-family apartments in various Nordic climates.

The light transmittance can be increased by about 10 percentage units compared to the same type of window without the AR. The g-value can be increased by the same amount, while the emissivity of the low-e coated pane is unaffected. The energy demand is expected to decrease by using AR coatings due to the higher solar transmittance of the window and this potential will be determined in the study. The daylight availability will also be determined by means of changes in the daylight levels.

The potential for improved passive solar energy utilisation and daylight availability by using AR coatings on windows will be discussed. The possible potential for overheating problems will also be analysed. The feasibility of applying AR coatings on windows will also be high-lighted from a more practical point of view.

Introduction

The increasing interest in energy-efficient housing has heightened the need for replacing or improving the existing windows in multi-family houses. One of the possibilities to achieve lower energy consumption is to improve the existing windows by replacing one of the panes to a low-e coated pane. In traditional double-glazed, coupled windows a hard low-e coated pane based on SnO2 can be used for this purpose. Windows from 1950 and earlier often have an excellent wood quality, and architectural details worth preserving.

Previous research shows that the potential to achieve a lower window U-value while preserving the aesthetics of the traditional window is large, (Fredlund, 1999). If the low-e coated pane is applied while the window is being renovated it is also a highly cost-effective measure, (Bulow-Hube, 2001), (Karlsson et al. 2001). One of the problems with low-e coated windows is however a slightly decreasing light transmittance that makes it somewhat darker inside the building. Applying AR-coating (Anti reflective) on the low-e pane could solve this problem and that is what we are going to investigate in this paper. Previous studies show that both the light transmittance (Tvis) and the total solar energy transmittance (g-value) could increase by 2-10 percentage points by applying AR-coating on a single pane (Hammarberg, 2002).