Utilisation of Passive Solar Energy - Renewable Energy: Technology, Economics and Environment - page 113

30

28

θ i internal window blind

θ i without shading equipment

26

θ i external window blind

24

22

20

18

16

θ e

14

12

10

4080 4104 4128 4152 4176 4200 4224 4248

Time in h

Fig. 3.5 Shading by internal and external window blinds (θ e ambient temperature, θ i room
temperature) /3-7/

In contrast, reliability is considerably enhanced with shading systems incorpo-
rated into the glass pane. These systems include the following operating princi-
ples.
− Thermotropic glazing becomes opaque at defined outside or system tempera-
tures, as molecules tend to accumulate to a pane-incorporated gel layer.
− Electrochromic glazing is characterised by a special coating which converts
from transparent to opaque at a defined voltage.
− Glazing covered by holographic foils reflect the irradiation from the high-angle
sun, so that sun-rays incident at small angles reach the absorber without any
obstacle.

Absorber and heat storage. While absorber and heat storage are individual com-
ponents in active solar systems, they are integrated into the building structure of
passive systems.
Within direct gain systems, the room envelopes with solar radiation exposure,
serve as absorber surfaces. Passive solar systems should thus offer well-absorbing
outer surfaces and a heat-storing building structure which is well-adapted to the
solar system.
The "classic" passive energy system is not equipped with any control. The
thermal mass of a house, which is heated up by solar radiation, releases the heat
back into the internal space with a certain time lag and reduced temperature with-
out any user intervention. It is thus essential to prevent passive accumulators from
overheating the rooms. For this purpose, the time lag and heat flow reduction by
passive storage need to be known factors. Also, in most cases, additional (active)
shading devices need to be provided to reduce energy absorption in summertime.
Indirectly heated thermal mass (e.g. unheated internal walls) can only be used
sensibly if the corresponding room temperature variations are permitted. In case