15.07.2015   EuroSun2008-4

Window products

There are a large number of coated and uncoated glass panes available on the market, and all of these can be combined into numerous different window combinations. To help customers select suitable windows among this almost infinite number of glazing combinations, the International Glazing Data Base (IGDB) has been set up by the Lawrence Berkeley National Laboratory in California [12]. The database contains glazing products manufactured by most major glass manufacturers in the world, and their main optical properties are provided, usually together with reflectance and transmittance spectra. It is a complex task to choose the most suitable window for a certain building and location out of all these products. The important parameters for the function of the window vary within wide limits, and if we for example consider the U-value and the g-value, we can see in Fig. 1, that windows with many different values of these parameters are available. Each point in the diagram represents a window made up from two panes found in the IGDB. The figure only includes a small selection of double glazed configurations air filled insulated glazing units. Triple glazed configurations with argon-fill would extend the graph down to U-values of around 0.6 W/m2K. Depending on the function of the window and the type of building, orientation and climate, different combinations of high or low U — and g-values would be the best choice. For optimum performance in a certain situation we may want to look for a window close to one of the corners in the graph. Often a compromise has to be selected since summer and winter may require quite different properties for optimum performance.

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g-value

Fig. 1. U-value vs. g-value for over two thousand Fig. 2. Visible transmittance vs. g-value for twelve double pane windows. double pane windows.

These considerations lead to the conclusion that we often would want windows with variable optical and thermal properties depending on the time of year, time of day or weather. The electrochromic coatings mentioned in the introduction bring us a few steps towards this situation. Existing products on the market or prototypes made up in the laboratory show that we can identify window products with variable optical properties. A few examples are shown in Fig. 2 where the product specifications have been plotted in a graph with tv versus g-value. Table 1 gives a brief description of the double glazed windows presented in this graph. This type of graph is mainly useful for solar control glazing, for which both a low g-value is desired to prevent over-heating and a low Tv-value is desired to prevent glare. The problem is that we also want high tv for the day­lighting and visual contact with the surroundings. Thus, the switchable electrochromic glazing could be the ideal solution. In the graph we can see that the shown optical properties can be varied within quite wide limits, which makes it possible to optimize the performance for different weather conditions and according to the needs of the occupants. The non-physical area in the graph is due to the fact that around 50% of the solar radiation is visible light.

Table 1. Description of pane configuration used for double pane windows in Fig. 2

Hard Low-E

SnO2 low-e coating on surface three combined with an outer floatglass pane.

Soft Low-E

Single silver coating on surface three combined with an outer float glass pane

Soft SC

Double silver coating on surface two combined with an inner float glass pane.

Abs. SC

Absorbing outer pane combined with an inner float glass pane

SS clear

Solid state electrochromic film in its clear state on surface two combined with an inner float glass pane.

SS dark

Solid state electrochromic film in its dark state on surface two combined with an inner float glass pane.

Foil/float C

Electrochromic plastic foil in its clear state on the inside of an outer float glass pane combined with an inner float glass pane.

Foil/float D

Electrochromic plastic foil in its dark state on the inside of an outer float glass pane combined with an inner float glass pane.

Foil/Low-E C

Electrochromic plastic foil in its clear state on the inside of an outer float glass pane combined with an inner low-e pane.

Foil/Low-E D

Electrochromic plastic foil in its dark state on the inside of an outer float glass pane combined with an inner low-e pane.

MH trans.

Metalhydride electrochromic film in its clear state on surface two combined with an inner float glass pane.

MH refl.

Metalhydride electrochromic film in its reflective state on surface two combined with an inner float glass pane

3. Method

3.2. Energy simulations in WinSel

The energy simulations were performed using WinSel, a simulation tool developed at Uppsala University [7,8]. The program was designed as a window selection and energy rating tool, hence the acronym WinSel. Hourly climate data of direct and diffuse radiation and temperature are used for hour-by-hour calculations of the annual energy balance. Window in-data are the total solar energy transmittance (g-value), the thermal transmittance (U-value), and parameters controlling the

angular dependence of the g-value. The building input data are limited to the thermal mass dependant time-constant and the balance temperature. The balance temperature is defined as the outdoor temperature when neither cooling nor heating is required to maintain the set indoor temperature. Consequently, the heating season is defined as all hours of the year when the outside temperature is lower than the balance temperature. The cooling season is defined with a temperature swing allowing the indoor temperature to increase to a value above the set indoor temperature before any cooling is considered.