Position and properties of the shading device

Exterior shading devices are more effective since the absorbed heat is largely dissipated to the outdoor air. For interior-placed shading, it is essential to try to reflect the shortwave solar radiation since the heat absorbed in the sunshade is trapped by the window and, hence, heats up the room. However, depending on colour, slat angle position, etc., there is a large performance variation among exterior, interpane and interior sunshades. Figure 9.6.1 shows measured gsunshade for different shading devices. For interior shading devices, the reflectance of the fabric is the most important parameter in obtaining low g-values. This is contrary to exterior shading devices. For example, dark-coloured awnings (with low reflectance and high absorptance) provide lower g-values than light ones. On average, exterior shadings are twice as effective as interior shadings in reducing peak cooling loads.

Two additional issues in selecting shading devices are the effect of the devices on transmitted daylight and the view out. Interior shading yielding low g-values admit almost no daylight into the room and totally obstruct the view out, two of the main reasons for having a window.

Figure 9.6.2 shows the total solar transmittance (gsystem) for a window with a beige shading screen. The results are from simulations with the program ParaSol. Monthly means during a year are shown for the cold climate (Stockholm). The window is a sealed, triple-glazed unit with low-e coatings on the inner and the outer panes and filled with argon between the panes. The bare window has a g-value of around 31 per cent to 39 per cent. If the screen is added on the interior side, the gsystem decreases to 26 per cent to 32 per cent. Note that for this position, the gap between window and screen is assumed ventilated to the room (convection). If the screen is positioned between the innermost pane and the mid pane (gap 2), the gsystem is 20 per cent to 24 per cent. If the screen is positioned between the mid pane and the outermost pane (gap 1), the gsystem is 12 per cent to 14 per cent. The best performance is achieved by placing the screen on the exterior side. The total transmittance for the system of window and screen is then only around 6 per cent. Note that vertical shading has a rather constant performance over the year.

Source: measurements from Lund, Sweden, in Wall and Bulow-Hube (2003)

Figure 9.6.1 Measured g-values of different shading devices in combination with a double clear glazing

Source: measurements from Lund, Sweden, in Wall and Bulow-Hube (2003)

Figure 9.6.1 Measured g-values of different shading devices in combination with a double clear glazing

Source: Maria Wall

Figure 9.6.2 The influence of the position of a beige shading screen: The g-value is shown for the system of shading and window; the g-value for the bare window is also shown (south orientation in a cold Stockholm climate)

0 0

Post a comment