Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 132

− Radiation-concentrating air collectors.
Within these five basic designs there are a number of variations. Fig. 4.4 shows a
selection - however, only very few have been successful as standard solutions in
the market.

Non-concentrating swimming pool liquid-type collectors. This basic design that is
used most frequently in its simplest form consists of an absorber mat with a corre-
sponding system of pipes for the heat carrier (Fig. 4.4, on the top left). This col-
lector design is often referred to as the collector type absorber . It is preferably
used for heating open-air swimming pools. This application needs water at a tem-
perature around ambient temperature. Heat insulation to the ambient is not
needed, because there is no driving force (temperature difference) for heat losses.
Therefore a transparent cover and an insulation on the back side of the collector
are not needed and the optical losses are only due to the reflection coefficient of
the absorber ρ abs . The absorber material is mainly EPDM (ethylene-propylen-dien-
monomers) which is able to withstand UV radiation and temperatures up to 150 C.
Such high temperatures do not occur due to the lack of insulation. This ab-
sorber type is very cheap and results highly efficient for the swimming pool appli-
cation.

“ “

°

Non-concentrating glazed flat-plate liquid-type collectors. Should higher tempera-
ture levels be required, glazed flat-plate collectors are used in many cases (Fig.
4.3 and Fig. 4.4, on the left). They can be built with one or more transparent cover
sheets. In order to further reduce the convective thermal losses from the absorber
to the cover, the space between the two can be evacuated, which turns the collec-
tor into a vacuum flat-plate collector. Due to the pressure difference, the cover
sheet has to be supported from the inside in that case. Heat losses to the back of
the collector are avoided by insulation material. Absorber, cover and insulation
are fixed by a collector case.
The piping in the collector can either be designed with many parallel tubes that
are connected by a distributor and a collector in the absorber or by a single bent
tube covering the whole collector area. In the former case there is a high total
mass flow in the absorber (parallel tubes) but the temperature lift during irradia-
tion is small (high flow principle), in the latter case the total mass flow is low
(only one tube) but the temperature lift is high (low flow principle).
Glazed collectors are built from units down to 1 m² up to units with 16 m². The
advantage of the collectors with bigger areas is the reduced pipe work on site and
therefore reduced possibility of failure. On the other hand such units cannot be
mounted manually but need a crane. The collectors can be integrated into the roof,
which means that the collector is the rain-tight surface of the roof and roof-tiles
can be spared. The frame within the roof can be made out of wood. This is only
feasible, if the roof has the right orientation, but most of the solar thermal collec-
tors e.g. in Germany and Austria are mounted this way. The other possibility is to
mount the collector on the roof as separate element. In this case the whole frame