Utilisation of Passive Solar Energy - Renewable Energy: Technology, Economics and Environment - page 120

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Absorber Rock storage system

Fig. 3.12 Thermally decoupled solar system attached to the building envelope (see /3-8/)

Decoupled systems are characterised by good controllability. Thanks to the
thermal insulation between absorber and the internal space, at night heat losses are
very low. However, these systems present the disadvantages of high construction
costs, a propensity towards defects (e.g. leakages) and high absorber tempera-
tures.
Thermally decoupled systems are most suitable to compensate considerable
time lags between solar radiation and heat demand. They also result advantageous
if separate heat accumulators are already available or if they can be easily inte-
grated into the building structure.

Sunspaces. Sunspaces are another variant of functional systems. Most popular are
unheated sunspaces, whose connecting doors to the internal living space are left
open if heating is needed and the temperature inside the adjacent sunspace be-
comes warmer. Furthermore, sunspaces of two or more stories also serve for ven-
tilation of houses (Fig. 3.13). In wintertime the minimum temperatures amount to
0 °C, whereas in summertime heat needs to be evacuated to the outside to avoid
overheating (temperatures above 50 °C are possible). For this reason slanting
windows should be avoided and the roof should be well-insulated. Also, orienta-
tion toward the east and west is unfavourable, as the incident solar radiation will
be very little in winter, and in summer shading can only be provided by window
blinds, but not by roof overhangs.
During the heating season a well designed sunspace, with optimum control,
supplies the house with just as much or a little more energy than it receives from
the house. Besides the utilisation of passive solar energy, unheated sunspaces also
cut the heat load of a building, as the system wall - sunspace - wall usually pre-
sents a lower U -value (thermal transmittance coefficient) as the outer wall. A
heated sunspace, however, will result in higher heat losses.
Even shaded sunspaces provided with roof overhangs are often overheated in
summertime. Fig. 3.14 illustrates the temperature curves of the sunspace (θ ss )
shown in Fig. 3.13, the living space temperatures (θ i ), the ambient temperatures
(θ e ) and the floor (θ fl ) and ceiling temperatures (θ ce ) in a house equipped with a
floor heating system on three nice days in summer. In spite of a high ventilation