Wind Index Calculation

One of the most important purposes of the wind energy index is the ability to long-term correct local short-term measurements, which has taken place on a specific site.

Another important purpose of the wind energy index is to calculate the expected long-term production of a WTG or wind farm based on a shorter production period in order to find out if the wind power plant can be expected to obtain the calculated production on an early stage.

Finally, the wind energy index can also be used for calculation of production losses in periods where the wind power plant has been out of operation, e.g. for calculation of insurance covered loss calculation.

Calculation of wind energy index
There are a least 4 different types of sources for creating a wind index:
1 Based on actual wind turbine production figures
2 Based on measured wind data from a meteorological measure station or other measurement station with a relative long measure period.
3 Based on NCAR data. The NCAR data are Upper Air data (850 mB) – approx 1 500 m height, simulated with a global climate model and available for each 6 hours for free at the Internet for each 2.5-degree latitude and longitude.
4 Based on NCAR data filtered through a MESO-scale model, e.g. the World Wind Atlas (Sander & Cube), which gives more accurate data than the raw NCAR data.

Based on the different sources, different methods will be applied.

It is not uncomplicated to create a well working wind energy index, and we normally recommend using more data sources and testing more filtering methods in order to try out the sensitivity of the different data sources/methods.

EMD has a comprehensive experience in calculating a wind energy index based on all of the above methods and also in combining the different methods e.g. using known production data from existing wind turbines to validate a wind energy index based on measured wind data from a reference station.

The wind energy index will typically be presented on monthly basis. The wind energy index will be delivered in an Ms excel sheet ready to use for different correction purposes. More detailed index than monthly can be delivered on request if the data sources allow it.

We can also provide a excel sheet set up so the user can continue the calculation month by month in the sheet based on adding new data each month. Finally, we can provide a report describing the methods and the analyses behind the calculated wind energy index.

The cost of a wind energy index calculation can vary from just a few hour of work (100-300€) for a very simple rough index, e.g. from NCAR of WWA data to more comprehensive analyses/verifications based on more detailed data. Here is no “upper limit”, so feel free to contact us for a discussion on needs, data sources and recommendations.

Performance of Wind Energy Index

The 2 figures above show how large deviation from long term production there is taking the min/max values of e.g. 1 year of successive data from any part of the 14 year production record without (upper) and with (lower) index correction. With just half a year of production, the wind index correction method is within +/- 15% while the non-corrected is within +/- 50%.
The table below gives some typically guidelines based on experiences from many cases, taken from the report: Case studies calculating wind farm production

Worst case long-term production estimates based on 14 year actual data	1 year of data	2 year of data	3 year of data
Actual production figures	+/- 30 %	+/- 20 %	+/- 10 %
Wind index corrected production figures	+/- 15 %	+/- 10 %	+/- 5 %

 

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EMD International A/S  Niels Jernesvej 10  9220 Aalborg Ø Denmark  
Tel.: +45 9635 4444  Fax: +45 9635 4446  E-mail: emd@emd.dk