Utilisation of Passive Solar Energy - Renewable Energy: Technology, Economics and Environment - page 118

layer, also in case of considerable temperature differences (between the inside and
the outside of the space) only very little heat will penetrate to the inside.
In contrast, a large amount of solar radiation penetrates through elements with
transparent insulation and is converted into heat when striking the black coated
element (absorber) (Fig. 3.9, right). Due to the high thermal resistance of the insu-
lating material, a large amount of heat is transferred into the storage wall.
Fig. 3.10 illustrates the temperature flow and the corresponding heat flow den-
sities as well the U -value (thermal transmittance coefficient) and the equivalent U -
value ( U eq ) of an exemplary solar wall with thermal insulation (TI) on a cold and
foggy winter's day within the course of 24 hours. To provide for good transfer of
the heat created at the absorber (useful heat), and to avoid excessive maximum
temperatures, the wall behind the transparent thermal insulation material (TI) must
have a high thermal transmission coefficient and good storage properties. How-
ever, these properties will result in very low thermal insulation. The total U -values
of transparent thermal insulation (TI) are thus often higher than those of a solely
insulated wall. At night time, when thermal mass has cooled down, the wall will
have higher losses than a solely insulated wall. However, for well-designed walls
with thermal insulation heat losses are in most cases overcompensated by heat
gains; the equivalent U -values (U eq ) (including solar gains) are lower or even
negative (net heat gain). In the example illustrated in Fig. 3.10 the U -value
amounts to 0.527 W/(m 2 K) and the equivalent U -value ( U eq ) to 0.267 W/(m 2 K),
on this unpleasant day.

25

TI Thermal storage wall

20

15 h

9 h

15

U -value 0.527 W/m²K
U eq -value 0.267 W/m²K

10

5

Solar radiation to
absorber 0.2 kWh/m 2 d

0

15 h

-0.1 0 0.1 0.2 0.3 0.4

-5

3 h

Energy flow
0.35 kWh/m²d

Energy flow
0.16 kWh/m²d

7 h

-10

Wall thickness in m

Fig. 3.10 Temperature distribution of a system consisting of glass - transparent thermal
insulation (TI) - air - absorber - concrete wall on a cold and foggy winter's day /3-9/

South-facing solar walls with thermal insulation and without shading devices
allow for annual savings of useful energy within the range from 350 to
400 MJ/(m 2 a), with regard to the solar aperture area and in comparison to com-
mon opaque insulation systems (such as heat-insulation connection system or non-