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DWD TRY 2011

Using irradiance data of TRY 2011 data of the Deutscher Wetterdienst (DWD) for thermal simulation could yield to strange results for sloped surfaces facing east/west. For TRY-Region 10, the yearly irradiance on an east facing vertical surface is much higher (720 kWh/(m²a) ) than on a west facing surface (543 kWh/(m²a)). In general, the irradiance for east / west facing surfaces is expected to be approximately the same.

This strange effect seems to be due to a structural change in the official weather data set TRY 2010 which is NOT documented in the manual. The irradiation data doesn’t refer to the same time base as in the previous TRY-Data sets.

Based on our studies and our communication with the DWD the following time base seems to be used by the irradiance data sets:

  • Measured irradiance data refers to solar time as an average of the previous hour
  • Computed irradiance data refer to CET, as an average value of the interval 0.5 hour before and 0.5 hour after the data point. 

Unfortunately, these assumptions aren’t officially confirmed by the DWD, why we have not adapted Type 15-7 yet.

Meanwhile, the following measures can be taking to avoid an incorrect east/west irradiance shift:

  • For data sets with computed irradiance data the entered longitude of Type15-7 has to be shifted 0.5 hours east (which means -7.5 degrees) such that the time interval refers to the previous hour.
  • For data sets with measured irradiance data, the time base has to be set to solar time. This can’t be done by Type15-7. However, Type 99 (user format) can be used for solar time calculation.

For TRY-Region 10 (computed irradiance data), the simulation with a modified longitude of -19.38 degrees (original longitude: – 11.88 degrees) yields to the expected result: the yearly irradiance for east and west are approximately the same east: 627 kWh/(m²a) ; west 609 kWh/(m²a).

Unfortunately, the used time convention of TRY 2011 irradiance data is very confusing and error-prone. For example, in DIN V 18599-10:2011-12 „Anhang E (informativ) Meteorologische Daten für 15 Klimaregionen in Deutschland“ the irradiance data for TR2010 doesn’t seem to be calculated with correct time convention: east 715 kWh/(m²a) and west 548 kWh/(m²a).

You can also download a TRY-Converter here:

    Login:          TRY2011_Converter
    Password:   *N3!C$WFsx%8