Kollektorfabrik started in June 2006 to incorporate the must-haves of a highly efficient collector and the don’ts of a stagnation vulnerable solar thermal system. In this process we questioned the common water based solar heat transfer medium and put our attention on air.

1.1 Solar fraction

To raise the total share of solar thermal energy, there are different possibilities:

• New fields of solar thermal usages could be implemented. Basically, every process that is in need for heat up to 130 °C can be satisfied or supported by a solar thermal application. Often processes need a huge amount of thermal energy, which leads — if supported by solar thermal systems to huge solarfields with all their necessities. Except for uncovered absorbers for low temperature applications there is not really a wide range of products which can be used for large scaled collectorfields. A field of air collectors is not limited in size, since an inconvenient stagnation behaviour cannot occur.

• The solar fraction of every single application could be raised. The chart in figure 1 is well known as the demand and offer of a solar thermal system for a private household in the run of a year. A big collectorfield enhances the solar fraction but exceeds the heat demand for domestic hot water in summer. This can cause trouble if, for instance due to lack of knowledge, no measures are taken to deal with the energy overrun.

In fact the size of the collector area alone has a minor impact on the system costs. Even systems that support space heating, often cut the size of the collector area due to the maximum content of the installed storage tank and not only to the maximum of the installable collector area.

The solar fraction of an application therefor is often not a question of the affordable collector area, but how to deal with the risks of energy overrun in the summer period and the associated cost of implemented measures.