Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 136

less impact. If the characteristic curve is drawn across the temperature difference
related to radiation, the curves for the different radiation intensities almost merge
into one. Therefore this form of representation is preferred in many cases (see e.g.
/4-1/, /4-2/, /4-3/).

1.0 800

Optical losses

τ cov α abs

0.8

600

Heat losses

0.6

400

0.4

200

0.2

real

0.0 0

0 20 40 60 80 100 120

Temperature difference θ abs - θ e in K

Fig. 4.6 Characteristic curves of single flat-plate collectors ( τ cov α abs = 0.82; G .

g global radi-
ation on horizontal receiving area; Approxim. Approximation; see e.g. /4-1/, /4-2/, /4-3/)

Fig. 4.7 shows the courses of the characteristic curves for a number of different
non-concentrating liquid-type collector designs. A single absorber might have a
significantly steeper course of the characteristic curve and nevertheless achieve
high specific energy yields, if it is only used in cases in which the difference be-
tween the absorber and the ambient temperature, on average, is very low. This is
for example the case for absorbers for solar open-air swimming pool heating, as
they are only run during the summer and the temperature level of the required heat
is also low for this kind of application. Due to the missing cover (τ cov = 1) the
optical collector efficiency is higher than for the other collector types at these
small temperature differences. Collectors used all year round would generally
show flatter courses of the characteristic curves as efficiency must not decrease
too much with larger temperature differences.
Some typical parameters and important areas of use for non-concentrating liq-
uid-type collectors that are most often used in Central and Northern Europe are
shown in Table 4.3. The temperatures of the heat carrier in the collector are -
depending on the meteorological conditions and the collector design - between 0
and approximately 100 °C during operation. Solar open-air swimming pool heat-
ing and the partial cover of the demand for domestic water are typical areas of
application. Coupled solar thermal supply of domestic water and space heating
(solar combined system) is also increasingly used. In Austria and Switzerland
50 %, and in Germany 30 % of the collector areas are solar combined systems
/4-4/.