These tests and computations may not reflect the requirements for polymeric (and other) collectors if they have measures to limit the temperature of the absorber.
This limiting behavior will arise, for example, in the case of an adequate thermotropic layer that is mounted on the absorber or in the case of ventilation or other limiting cooling measures that are controlled mainly by the absorber temperature.
If the temperature limit is mainly dependent on the absorber temperature then the efficiency curve will more or less abruptly fall to zero at this temperature. This limiting behavior and therefore the area of validity of the formulas (7), (8), (32), (32.1), (D.3), (D.4), (H.1) and (H.2) cannot be expressed as a function of (tm — ta).
Such an efficiency curve can no longer be modeled with a second-order function as a whole. Below this limiting temperature – approximately represented as a limit in tm instead of a limit in the absorber temperature – all the above-mentioned formulas will work correctly with the exception of (C.1), where this simple linear correlation will no longer hold.