Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 144

ing to evaporate. When started up again, the circulation pump presses the gase-
ous volume from the collector to the envisaged tank again. It thus needs to have
a larger pressure head than ordinary circulation pumps. If the gaseous volume
is dimensioned in a way that all system components exposed to ambient
temperature are filled with gas at standstill, the collector circuit can even be run
without antifreeze.
− By running the circulation pump during the night, the collector circuit can also
be used to cool the storage. As the thermal losses of the collector circuit are
much higher than in the storage, the storage cools down below a defined tem-
perature limit during the night. This temperature has to be at a level that pre-
vents the collector from heating the storage up above its maximum temperature
if the next day is warm and sunny. The disadvantage is that this type of cooling
is dependent on the use of auxiliary energy and thus no heat removal can take
place if a power breakdown occurs. Additionally, the decision of how much
cooling down the storage needs during the night needs to be based on knowing
the weather forecast for the next day.
− A system can also have its own integrated heat removal system that is switched
on through the control instrument on demand (e.g. swimming pool, heat ex-
changer on the roof, auxiliary boiler that is cooled by natural convection
through the chimney), but again having to use auxiliary energy is problematic.
Apart from these two tasks - regulating the circulation pump and maintaining the
temperature limits - a suitable regulation also has to ensure additional heating in
the case of low radiation.
Heat transfer medium. Some of the requirements of a heat transfer medium are:
− high specific heat capacity,
− low viscosity, i.e. good flow capability,
− no freezing or boiling at operating temperature,
− non-corrosion in the conduit system,
− non-flammable and
− non-toxic and biologically degradable.
Water fulfils most of these requirements very well. However, the danger of freez-
ing at temperatures below 0 °C can cause problems. Water without additives can
thus only be used in the warmer zones of the earth without the risk of frost.
In Central and Northern Europe mainly mixtures of water and antifreeze are
used. Normally, an anticorrosive agent is also added to the antifreeze as mixtures
of water and antifreeze are more corrosive than pure water. The most commonly
used substances are ethylene glycol and propylene glycol; for domestic hot water
supply systems normally the food-safe propylene glycol is used. The disadvan-
tages of this additive are the lower specific heat capacity compared to water,
higher viscosity and the reduced area tension. The mixture can thus permeate
pores that pure water cannot get through. Furthermore, pressure losses are higher
and the heat transfer worse. Thus the main components (pumps, cross-sections of
pipes, heat carriers) have to be adjusted to this mix. Recently, heat carriers with a