Environmental Engineering Reference
In-Depth Information
a system failure. Therefore, ethylene glycol, which is used for many technical
applications, is not used for solar energy systems. To avoid corrosion damage,
the antifreeze agent must also be compatible with the materials used in the
system construction.
Thermosyphon systems also have some important disadvantages. The
system itself is inert and cannot react to fast changes in the solar irradiance.
Thermosyphon systems are usually not suitable for large systems with more
than 10 m
2
of collector surface. Furthermore, the storage tank must always be
installed higher than the collector, which is not always easy to realize. The
collector efficiency can also decrease due to high temperatures in the solar
cycle. However, thermosyphon systems are very economical domestic water
heating systems. The principle is simple and needs neither a pump nor a control
system. Therefore, the system cannot fail due to a fault in these components.
Finally, the energy to drive the pump and control system is saved.
Systems with forced circulation
In contrast to thermosyphon systems, systems with forced circulation use an
electrical pump to move the water in the solar cycle. The collector and storage
tank can be installed independently and a height difference between the tank
and collector is no longer necessary. However, the pipe lengths should be
designed to be as short as possible since all warm water pipes cause heat losses.
Figure 3.4 shows a system with forced circulation.
Two
temperature sensors
monitor the temperatures in the solar collector
and the storage tank. If the collector temperature is above the tank temperature
by a certain threshold, the control starts the pump. The pump moves the heat
transfer fluid in the solar cycle. The switch-on temperature difference is
normally between 5 and 10°C. If the temperature difference decreases below a
second threshold, the control switches the pump off again. The choice of both
thresholds must ensure that the pump does not continually switch on and off
during low irradiance conditions.
Conventional circulation
pumps
made for heating installations can be used
for the solar cycle. These pumps are reliable and economic. Most pumps have
various velocity stages to adapt the flow rate to the solar irradiance. Pumps
are usually designed for flow rates of 30-50 litre/h per square metre of solar
collector area. Higher flow rates are chosen for swimming pool absorbers since
the temperature requirement is lower and the water needs less heating.
However, if the flow rate is too low, the temperature in the collector rises and
the system efficiency decreases. On the other hand, if the flow rate is too high,
the energy demand to drive the pump is unnecessarily high.
The pump usually runs at the alternating voltage of the public grid. It is
also possible to use DC motors to drive the pump. A small photovoltaic system
can provide the electrical energy needed. In that case, all of the energy for the
system comes from the sun.
