Environmental Engineering Reference
below the collector, the water has to be pumped through the collector. A pump moves
the water in a solar circulation through the collector and a control mechanism ensures
that the pump only starts when the temperature of the collector is higher than that
of the storage cylinder (Figure 6.12). A conventional boiler heats up the water during
the transitional seasons and in winter so that a hot shower is possible all year round.
It can happen that during the summer a solar collector will heat up an entire storage
cylinder to a predetermined maximum temperature. This maximum temperature is
usually set at 60 °C to prevent large deposits of lime from forming. If the cylinder
is full, the control interrupts the incoming supply from the collector. Despite full
irradiation from the sun, no more water will fl ow through the collector. The collector
can then heat up to temperatures well over 100 °C and evaporate the water. If the
expansion tank is large enough, it absorbs the expansion of the water volume.
Figure 6.12 Pumped solar thermal system for heating domestic hot water.
6.3.2 Heating with the Sun
Solar thermal systems can be used to provide not only hot water but also heating.
In principle, this only requires increasing the sizes of the collector and the storage
cylinder and connecting them to the heating cycle. As no provision is normally made
for domestic hot water in the heating cycle, two separate heat storage cylinders are
needed - one for hot water and the other for drinking water. Combination boilers
can integrate both storage elements and reduce heat losses (Figure 6.13).
In places like Germany and Great Britain the sun is only able to meet heating
demand during the transitional periods of spring and autumn. Consequently, these
systems are usually designed so that solar energy only serves as a supplemental heat
supply. The output of the collectors is not suffi cient to cover the heat needed during
the winter. In principle, the sun could be used to cover all heating requirements.
However, this would necessitate either an extremely large collector or an extremely