Increased solar energy and daylight utilisation using anti-reflective coatings in energy-efficient windows

Glass with low-e coatings based on SnO2 (usually referred to as hard coatings) provides a cost-effective replacement of one of the panes in ordinary double-pane windows. It considerably improves the energy efficiency of the window and at the same time preserves the appearance of old hand-crafted windows. Adding a low refractive index anti-reflective (AR) coating on both sides of the low-e coated pane in such a double-glazed window makes it possible to achieve high light and solar transmittance, while the U-value remains unaffected. In this study the influence on the daylight factor, solar factor and annual heating demand when AR-coated low-e glass is used instead of normal low-e glass and ordinary clear glass has been investigated for a typical multi-family dwelling in Scandinavia using energy and daylight simulation. double-glazed window with one low-e hard coating, light transmittance was found to increase by as much as 15 percentage points, from 74 to 89% transmittance, if both panes were AR-treated, while the emissivity of the low-e coated pane was virtually unaffected. Compared to clear double glazing, the visual transmittance was increased by 7 percentage points. The simulations show that the monthly average solar factor (g-value) increased by 7 percentage points compared to the low-e double-glazed window without AR-coatings. The annual heating demand decreased by 4% due to the higher solar transmittance of the window. The AR-coating increased the daylight factor by 21% according to the simulation. The study has shown that the main benefit of using AR coatings in a low-e window is the improvement of visual transmittance and the resulting increase in the daylight factor.