For operating a thermal energy exchanger (1) for exchanging thermal energy between a thermal transfer fluid and air, a plurality of measurement data sets are recorded in a control system (40). The measurement data sets include for a different point in time data values which define a normalized energy transfer that represents the thermal energy transferred in the thermal energy exchanger (1) normalized by one or more normalization variables. The control system (40) calculates for each of the measurement data sets a normalized data point defined by the normalized energy transfer. The control system (40) further determines for the thermal energy exchanger (1) a characteristic energy transfer curve which fits the normalized data points. Normalizing the energy transfer makes it possible to operate the thermal energy exchanger (1) more efficiently over a wider range of changing conditions, as saturation can be prevented using more appropriate fixed or variable thresholds.