This limiting behavior can be observed, for example, in the case of an adequate thermotropic layer that is mounted on the inner side of the glazing of the collector. This type of temperature limitation is only satisfying in the case of non-selective absorbers and double glazed collectors because a good thermal coupling from the thermotropic layer to the absorber and a higher thermal resistance to the ambience is needed.
This behavior is illustrated here for a modeled polymeric collector with an idealized thermotropic layer that switches in the temperature range 55-60 °C (Figure 18.1) and with different transmittances in the opaque state (Figure 18.2) [1].
As the temperature ofthe thermotropic layer is a function ofboth the absorber and the ambient temperature the limit of the absorber temperature depends on the switching temperature range ofthe thermotropic layer and the ambient temperature
0.5
c
a>
о
0.4
a)
о
0.3
a>
0.1 0.0
(left-hand side of the graphs). Two discontinuities can be observed if the residual transmittance in the opaque state is not low enough (right-hand side).
Because of these dependencies care should be taken to define the validity limits of all the functions mentioned above. These limits can be dependent strongly on the ambient temperature and the absorber temperature.
The stagnation temperature tsg (C.1) is dependent on the ambient temperature and measurements have to be made with different ambient temperatures.
