In figure 5 the solar gross heat gain of the different collector fields and the solar irradiation on collector plane (15 °) are shown. The solar heat gain varies in the different years due to the following reasons:

In 2000 the control unit failed and the collectors were operated manually for several months. In addition, the second buffer tank was installed which caused service interruption. Furthermore some components such as a heat exchanger and a pump failed which was not immediately detected.

In 2001 the duct heat store was extended during summer and therefore no heat storage was possible. Since the heat demand in the heat distribution net in summer is less than the heat delivery from the solar collectors some collector fields were manually taken out of operation.

In 2002 only minor operational and technical problems occurred and therefore the solar heat gain was higher than in the previous years.

In 2003 the highest solar heat gains were reached. This was the result of a solar irradiation which was significantly above average. In figure 6 it is seen that the solar gross heat gain for the same solar radiation values tends to be lower in 2003 than in 2002. This is caused by higher preheating temperatures of the collectors due to higher return temperatures from the heat distribution net and the duct heat store than in 2002, see figure 7. Furthermore some minor problems occurred, for example collector damage due to heavy storm. The heat exchanger of the collector field “shopping centre” was cleaned since it was clogged.

The differences in heat gain, in 2003 between 336 and 432 kWh(m2a), of the different collector fields are mainly caused by different return temperatures from the heat distribution net from the buildings to the collector fields.

01 23456789 10

Solar irradiation on collector plane (15 °) in kWh/(m2-day)

Figure 6: Gross heat gain of solar collectors versus solar irradiation on collector plane