Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 149

− systems without circulation (storage collector),
− natural circulation systems (Thermo-siphon-systems) and
− forced circulation systems
can be distinguished. If the formation of the solar circuit is used to differentiate,
− open systems and
− closed systems
can be distinguished. On the basis of these criteria, five basic principles of solar
systems can be defined. They are shown in Fig. 4.11 which describes the essential
system components for the functionality and safe operation.
Systems without circulation (Fig. 4.11, a). In this most basic of all possible prin-
ciples, heat transfer medium and the liquid actually used by the consumer are the
same medium. Within the normal drinking or domestic water circuit, a suitable
collector is integrated. When flowing through the collector, the water is heated up
and can be used afterwards. This basic principle is used in storage collectors, to
give one example.
Open natural circulation systems (Fig. 4.11, b). This most basic of all circula-
tion concepts consists of the collector, the flow and the return pipe and a pressure-
free, open storage. The reason for the natural circulation is the decrease in density
in a liquid with increasing temperature. To give an example, water density at
20 °C is 998 kg/m 3 and at 80 °C only 972 kg/m 3 . These density differences be-
tween the hot liquid in the collector and the cold fluid in the storage and the col-
lector flow pipe create circulation within the system if the storage with the colder
medium is positioned above the collector.
The driving forces of these differences in density are opposed by the pressure
drop caused by pipe friction. Lift pressure and pressure losses caused by flow are
the same when stationary. This results in the mass flow of the fluid. If radiation
intensity increases, the collector outlet temperature increases, too, and with it the
temperature difference between the storage and the collector. The mass flow is
increased, an increasing amount of heat transfer medium, and with it heat, are
transported to the storage and released to the storage medium. As a result, the
temperature in the collector sinks again. Therefore, this is a self-regulating system
that, at least in its basic form, can work without sensors and control instruments.
The natural circulation system is open in this case. The same liquid flows
through the collector that is directly passed on to the user and is utilised in its
heated-up condition. As there is generally no danger of frost in southern countries,
and thus the heat transfer medium cannot freeze within the collector circuit, such
systems are widespread in those regions. The collector circuit has to be resistant to
corrosion as drinking water generally flows through it.
Closed natural circulation systems (Fig. 4.11, c). In order to prevent freezing
and corrosion, the collector circuit can be closed in natural circulation systems.