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Definition of Uncertainty in the Predicted Energy Production

Uncertainty analysis is an important part of any assessment of the long-term energy production of a wind farm.  Although an uncertainty analysis needs to be considered on a site-specific basis, the process can be described as follows:

• Identify the different inputs to and processes within the analysis;
• Assign an uncertainty to each of these elements, both in terms of the magnitude of the uncertainty and the shape of the distribution;
• Convert each of the uncertainties into common units of energy;
• Combine the various uncertainties to define a total uncertainty for the entire prediction; and
  
• Present uncertainty statistics at requested levels.
  

Research work reported in Raftery et al (1999) defined a comprehensive risk register for wind power projects and included detailed Monte Carlo-based analysis techniques to assess the uncertainty in the results obtained.  Based on the results of this work, use of an uncertainty analysis with a number of simplifying assumptions can be justified.  The main simplifying assumptions are that it is reasonable to consider a relatively small number of key uncertainties and that these individual uncertainties can be assumed to be normally distributed.  Making these assumptions, it is possible to define energy production levels with a defined probability of those levels being exceeded. 

It is common to present uncertainty results for both a long future period of, say, ten years and also for a shorter future period of one year.  It is now normal practice for such figures, in parallel with a central estimate for the production of a wind farm, to be used to inform investment decisions for projects. 

The uncertainty analyses presented within energy assessments typically assume that the turbines will perform exactly to the defined availability and power performance levels.  The power performance and availability levels are usually covered by specific warranty arrangements, and hence any consideration of the uncertainty in these parameters needs machine-specific and contract-specific review, which is generally outside the scope of a ‘standard’ energy analysis.  However, it is increasingly the norm to assign a moderate uncertainty to the estimated availability, loss factor and power performance factors, to reflect that small deviations from expected availability and power performance levels may not be sufficient to trigger damage payments under the warranty. Uncertainty in the energy estimates is a vital part of the result.

 

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