Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 134

sation. Thus, heat can be transferred at very low temperature differences. In spite
of these and further advantages (e.g. self-regulation, no overheating), this concept
has not been widely accepted so far due to its comparably costly production.
Non-concentrating air collectors. Fig. 4.4 also shows different design types of
non-concentrating air collectors. Due to the low heat transfer coefficient between
the absorber and the air, the contact area between absorber and air flow has to be
large. This is for example ensured by ribbed absorbers, multi-pass systems or
porous absorber structures.
As no frost, overheating or corrosion problems can occur, air collectors have a
simpler design when compared to liquid-type collectors, for example. Even leak-
ages of the heat carrier are comparatively uncomplicated. Disadvantages are the
large channels and the often significant drive capacities required for fans.
The reason why air collectors are not widely used for the heating of buildings
or the supply of domestic hot water in Central and Northern Europe is that heating
systems based on hot water distribution networks are commonly applied. Never-
theless they are used in individual cases, e.g. for solar food drying systems and
low-energy houses with exhaust air heat recovery that are already equipped with
air distribution and collector systems and thus do not require a water heating sys-
tem.
Concentrating liquid-type or air collectors. These collector types reflect the direct
share of solar radiation through mirror areas and thus concentrate the direct radia-
tion on the absorber area. The level of concentration of solar radiation is the con-
centration ratio or the concentration factor C . It is defined as the ratio of the opti-
cally active collector area to the absorber area impinged on by radiation. The
maximum theoretical concentration ratio of 46,211 is the result of the distance
between sun and earth, and the sun radius. Technically, concentration factors of
up to a maximum of 5,000 can be achieved at present /4-1/, /4-2/, /4-3/.
Mainly, the temperature that can be achieved in the absorber depends on the
concentration factor (Fig. 4.5). The theoretical maximum absorber temperature
just equals the surface temperature of the sun in the case of a maximum concen-
tration ratio (approximately 5,000 K). The temperatures that can be realistically
achieved in the absorber are significantly lower. Rotation parabolic mirrors, for
example, can achieve absorber temperatures of a maximum of 1,600 °C /4-2/.
Concentrating collectors can be divided into three different groups: fixed, plus
one-axis and two-axis tracking systems (Fig. 4.4, right). Fixed concentrating col-
lectors have the lowest concentration ratios, whereas two-axis tracking systems
have the highest.
Which heat carrier is used mainly depends on the achievable temperatures. Liq-
uids are preferred in the low temperature range, whereas gaseous media are also
used with increasing working temperatures.