Environmental Engineering Reference
In-Depth Information
Figure 3.3
Schematic of a Thermosyphon System
of the tank. This cold water joins the cycle and is heated in the collector in the
same way as before. Due to higher water temperature differences at higher
solar irradiances, the warm water rises faster than at lower irradiances and the
flow rates are increased. Therefore, the water circulation adapts itself nearly
perfectly to the available solar irradiance.
It is very important that the storage tank of a thermosyphon system is well
above the collector; otherwise the cycle can run backwards at night and cool
the water from the storage tank over the collector. In regions with high solar
irradiation and flat-roof architecture, storage tanks are usually put on the roof.
The collector is also mounted on the roof or on the wall of the sunny side of
the house.
With gable roofs, the storage tank must be mounted as high as possible
under the roof if the collector is also installed on the roof. The high weight of
the water-filled tank can sometimes cause structural problems. Furthermore,
integration with a conventional heating system, which is usually placed in the
basement, is more difficult.
A system where the water flows directly through the collector is called a
single-circuit system
. Such a system is only suitable for frost-free regions;
otherwise the water can freeze in the collector and pipes and destroy the
system. In regions with the possibility of frost, a
double-circuit system
is
frequently used, in which the water is kept inside the storage tank. A second
quantity of water is mixed with an antifreeze agent to use as a working fluid
in the solar cycle. A heat exchanger transfers the heat from the solar cycle to
the storage tank, thus separating the usable water from the antifreeze mixture.
Glycols are often used as antifreeze agents; however, antifreeze agents should
be non-toxic because they can contaminate the hot water supply in the case of
