Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 129

heat exchangers plus, for certain designs, pumps with a drive to maintain the heat
carrier cycle, sensors and control instruments are required.

4.2.1 Collectors

Collectors are part of solar thermal systems, partly converting solar radiation into
heat. Part of this heat is subsequently transported by a heat carrier flowing
through the collector. For that purpose, a collector consists of several components
described in detail in the following.

Collector components. Fig. 4.3 shows the main components of a liquid-type flat-
plate collector. Accordingly, a collector consists of the absorber, the transparent
cover, the frame and the heat insulation. Additionally, heat carrier inlet and outlet,
plus fixing methods are shown. Depending on the collector design, it does not
include all of the illustrated components. The absorber, however, including the
appropriate pipes for the heat carrier, is an absolutely essential part. For most
designs, the other components described below are also collector parts.

Cover
Transparent
material:
Single pane
Solar compound
pane
Plastic plates or
sheets

Box
Material:
Aluminum
Galvanized steel
plate
Plastic
Wood

Transparent,
single or multi-layer
cover

Direct

radiation

Heat carrier outlet

Fixing device

Insulation
Material:
Polyurethane
Mineral wool
Fiber-glass wool

Frame with

Diffuse

radiation insulation

Selective coating

of the black surface

Absorber
black
Material:
Metall (Cu, Al, Fe)
Plastic (e.g.
Polypropylene,
Polyethylene,
EPDM)

Heat carrier
Material:
Water (with
antifreeze)
Air

Absorber wath channel system

Heat carrier

Heat carrier inlet

Fig. 4.3 Main components and materials as well as the schematic design of a flat-plate
liquid-type collector (according to /4-6/ and various other sources)

Absorber. The absorber converts short-wave radiation into heat (photo-thermal
conversion). The function of “radiation absorption “ is carried out by a type of
absorber material with quite a high absorption capacity within the luminous spec-
trum. On the other hand, a low absorption and thus emission capacity, is aimed for
in the thermal radiation wave spectrum. In addition, the absorber has to enable a
good heat transfer to the heat carrier and also be temperature-resistant, as nor-
mally temperatures of up to 200 °C occur in an insulated absorber with glass