Solar energy use in buildings - Solar Energy Sciences and Engineering Applications - page 611

Figure 18.10.3 Diurnal variation of irradiance on different façade orientations and transmitted
irradiance by a sun-protected south facade on a day in August (Stuttgart).

Table 18.10.1 Energy reduction coefficients of internal and external sun protection

Sun shading system Colour Energy reduction coefficient [-]

External sun shades Bright 0.13-0.2

External sun shades Dark 0.2-0.3

Internal sun shades Bright 0.45-0.55

Reflection glazing - 0.2-0.55

the south-facing office were between 200 and 300 Wh m − 2

compared to 400 and

500 Wh m − 2 in the north-facing office.
External loads caused by solar gains depend greatly on the surface area of the glaz-
ing as well as the sun-protection concept. Also, as demonstrated by Figure 18.10.3,
the façade orientation plays a significant role. On a south-facing façade, a maximum
irradiation of about 600 W m − 2 occurs on a sunny summer day. The best external sun
protection reduces this irradiation by 80%. Together with the total energy transmis-
sion factor (g-value) of low-E coated double-glazing of typically 0.65, the transmitted
external loads are about 78 W per square metre of glazing surface. In the case of a
3m 2 glazing surface of an office room, the result is a load of 234 W, which creates an
external load of 20 W m − 2 for a room surface area of 12 m 2 . This situation is illustrated
for south-, east- and west-facing façades in the summer.
The shading coefficients of sun-protection devices depend particularly on their
arrangement: external sun protection can reduce the energy transmission of solar radi-
ation by 80%, whereas with sun protection on the inside a reduction of at most 60%
is possible. Table 18.10.1 describes this relation.
External loads depend on the relation of window surface to floor space, as well
as the chosen shading system. For an area ratio between 0.1 and 0.7, the typical
external loads are between 8 and 60 W m − 2 . Together with the internal loads there are
25-90 W m − 2 total cooling loads, as can been seen in Figure 18.10.4.