Switchable Glazing with a Large Dynamic Range in Total Solar Energy Transmittance

Helen Rose Wilson, Andreas Georg, Peter Nitz

Now that windows with good thermal insulating properties have become the "state of the art", as reflected even in some national building codes, attention has shifted to their total solar energy transmittance (g value, formerly TSET) as the property of interest. In contrast to the u value, where it is clear that lower values are advantageous under almost all climatic conditions, the optimisation direction for g values depends strongly on the prevailing climatic conditions. A high value, which allows maximal solar gains to contribute to space heating in winter and the transitional seasons, will cause major discomfort due to overheating in summer. By contrast, a constant low value acts against the primary aim of using solar energy in buildings to replace conventional, fossil-fuelled heating. The solution to this dilemma is to vary the g value according to the requirements, which may well change several times during one day, depending on the location, building orientation and occupation.

Two approaches will be presented in this paper: gasochromic and thermotropic glazing. (A third approach, electrochromic glazing, is discussed in other papers at this conference.) Before the underlying materials are characterised in greater detail, some qualitative differences between the two types of glazing should be noted. These affect the application potential and control strategies, which in turn deter- mine how sensitively the complete system reacts to variation in individual materials properties (Georg et al, 1998).

Gasochromic glazing is switched by introducing the appropriate gas into the system; the triggering signal is external. This means that the switching criteria can and must be determined by a control strategy.

The switching unit consists of both the glazing and the associated gas supply system, which must also be installed. Visual contact to the surroundings through the window is retained even when the g value is minimal.

Thermotropic glazing reacts automatically to a temperature change in the active layer; when it becomes warmer, the transmittance decreases. This intrinsic material property can result in inappropriate switch- ing and must therefore be optimised before installation (Raicu et al, 1998). The switching unit consists solely of the glazing, simplifying installation. However, as it becomes opaque in the switched state, it should not completely fill a window aperture if visual contact to the surroundings is desired.

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