Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 150

This, however, requires a heat transfer medium that normally releases the heat in
the collector circuit to a storage that can distribute the heat further.
As the circuit is sealed off from the environment, it is normally under greater
pressure. In order to operate it safely, an expansion tank and a pressure control
valve have to be installed within the primary circuit. If such systems are used in
areas exposed to frost, frost-resistant heat carriers have to be used and the storage,
the cold and the hot water service pipes must be protected against frost.

Open natural circulation (thermosyphon)

Closed natural circulation (thermosyphon)
Pressure-relief valve

Cold water

Hot
water

Hot

Expansion tank

water

System without
circulation

Heat
exchanger

Hot
water

Cold

Heat consumer
(e.g. unpressu-

wa ter

Heat consumer
(e.g. pressurised
storage)

rised storage)

b c

Open forced circulation Closed forced circulation

Pressure-relief
valve

Cold
water

a

Expansion tank

Hot

wat er

Heat
consumer
(e.g. pressur-
rised
storage)

Heat consumer Heat exchanger

(e.g. swimming pool)

Pump

d e Cold water

Fig. 4.11 Basic concepts of active solar thermal systems (see e.g. /4-6/)

Open forced circulation systems (Fig. 4.11, d). If the heat sink (e.g. heat store,
swimming pool etc.) cannot be installed above the collectors, circulation of the
heat transfer medium has to be forced by integration of a pump into the circuit.
The given advantage of orientating collectors and heat sink independent from
each other is of importance for heating open-air swimming pools, where the col-
lectors are normally positioned on roofs or free spaces above the heat sink.
If the fluid in the collector cools down more quickly than in the collector loop
pipes, the circulation might be reversed during the night if the pump is not run-
ning. In that case, the cold liquid is pressing downwards from the collector and
extracts hot fluid from the storage or the heat exchanger. This can be prevented by
e.g. integrating a check valve into the collector return flow pipe.

Closed forced circulation systems (Fig. 4.11, e). For open forced circulation
systems the medium flowing through the collector circuit is generally ordinary
water. Therefore these systems are exposed to the same frost and corrosion risks
as open natural circulation systems. In order to avoid freezing, the forced circuit is