During operation of real solar thermal systems, the following dynamic processes are relevant.

• Fast fluctuations of irradiance on a time scale of a few minutes,

• fast decreases of the operating temperatures caused by draw-offs (also on a time scale of minutes),

• slow increases of operating temperatures over several hours due to the warming of the contents of the storage tank,

• the switch-on process of the solar circuit pump.

Which value of an effective one-node collector capacity best describes these four processes? Is there a unique value at all for all the processes?

In order to answer these questions, the mentioned processes are (just like the procedures for determination above) analysed qualitatively, based on fundamental physical arguments. Again the dynamic behaviour of Tabs and TF is investigated, as their amplitudes correspond with the effective capacity (see also figure 2).

Fast fluctuations of irradiance are similar to the processes of the J.3-procedure. Consequently, for the amplitudes ATabs >> ATF holds. A high effective thermal capacity best describes the process.

After a draw-off during collector operation, Tabs and TF decrease. As at the same time the collector performance will increase slightly (as the point of operation on the collector
efficiency curve is shifted to the left), Tabs-TF also increases, and hence the amplitude ATabs will be smaller or almost equal to ATF; so the result is ATabs < ATF. This process is best described by a low effective capacity.

When the storage tank is charged, all the system temperatures (including Tabs and TF) will slowly rise. The relation between ATabs and ATF depends on the (slow) dynamics of the irradiance G. If G is more or less constant or decreases, the collector thermal performance will slowly decrease, and ATabs < ATF holds. For increasing irradiance, the amplitudes will be similar: ATabs « ATF. Hence the corresponding effective capacity is low.

The switch-on process shows parallels with the draw-off, since colder fluid suddenly enters the collector. Again the result is ATabs < ATF, and the best suiting effective capacity is low.