Performance of Energy Efficient Windows and Solar Shading Devices

Abstract: This Licentiate dissertation deals with windows and solar shading devices and how they could be designed and used to save energy for heating and cooling in buildings. Parametric studies using the dynamic energy simulation software ParaSol v 2.0 were performed for different windows and solar shading devices. One study showed that both the cooling load and the annual cooling demand could decreased by a factor of two by using external solar shadings. For internal solar shadings the cooling load and the cooling demand decreased only by one third. The general conclusion of this study is that external shadings are much more efficient than internal shadings. When low-e windows are used the daylight transmittance decreases compared to clear glass windows. To avoid this effect a study of anti-reflective coatings on low-e windows was made. Simulations of the annual energy demand were performed in ParaSol and the daylight distribution was studied in Rayfront. Rayfront is a user interface to the light simulation software Radiance. It was shown that low-e windows with an anti-reflecting coating increase the daylight transmittance so that it becomes even higher than a clear glass window. However, the anti-reflective coating did not decrease the heating demand in any significant way. Instead it was the low-e coating which accounted for the largest energy saving. In a study of solar-control windows and internal solar shading devices measurements were performed in the solar laboratory at Energy and Building Design. The measurements were compared with simulations in ParaSol. The results showed that the efficiency of the solar shadings increased with decreasing window absorption. Simulations of an ideally reflecting solar shading proved that internal solar shadings can be effective if they are highly reflecting and used in combination with non-absorbing clear glazings. A recommendation is that the g-value, or solar factor, should be given for the whole system (window plus shading device). If it is given only for the internal solar shading, the g-value of the window that it is combined with must also be given. In a study of angle dependency of solar shadings a new model for characterizing the g-values of asymmetric and symmetric shadings was introduced. The new model was verified by outdoor measurements, indoor measurements and ray-tracing. The model worked well for venetian blinds, screen and diffuse film. For the awning the model had to be modified slightly to account for edge effects that occurred for incidence angles larger than zero degrees.

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