Method for detecting the degree of soiling of PV modules

1. A method for detecting the degree of soiling of PV modules of a string, with the following steps:

determination of the deviations of the string power output values from a calculated reference value over the last year;

calculation of a historical trend line from the deviations;

determination of a maximum difference between the trend line and the deviations;

calculation of final deviations of the string power output values through subtraction of the maximum difference from the trend line; and

determination of the degree of soiling through subtraction of the final deviations from the deviations.

2. The method as set forth in claim 1 , wherein the method is based on daily values.

3. The method as set forth in claim 1 , wherein an irradiation sensor is used for the calculation of the string power output values.

4. The method as set forth in claim 3 , wherein a temporal mismatch of the string power output values as a result of alignment errors of the irradiation sensor is determined and considered.

5. The method as set forth in claim 3 , wherein measurement errors as a result of the ageing of the irradiation sensor are determined by comparing all previously observed cloud-free days and considered.

6. The method as set forth in claim 1 , wherein string power output values from a time interval around a maximum elevation angle of the sun are used.

7. The method as set forth in claim 1 , wherein the string power output values are discarded if an output power of an inverter of the string exceeds a threshold, the threshold being greater than 95 percent and less than 100 percent of a rated output power of the inverter.

8. The method as set forth in claim 1 , wherein the string power output values are discarded if an average measured radiation deviates from a simulated radiation by a threshold value, the threshold lying between 5 percent and 25 percent.

9. The method as set forth in claim 1 , wherein the string power output values are discarded if a deviation between two temporally successive measured values of the radiation exceeds a threshold, the threshold lying between 100 watts per square meter and 300 W per square meter.

10. The method as set forth in claim 1 , wherein the string power output values are discarded if an output power of an inverter of the string has been limited.

11. The method as set forth in claim 1 , wherein an entire operating time of the PV modules is observed.

12. The method as set forth in claim 1 , wherein the degree of soiling of at least one string is determined.