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Resource Global Network 69
7 Uncertainty analysis
The uncertainty analysis
must be carried
out according to to the ‘guide to to expression of uncertainty in in measurement’ (GUM) This involves estimating or measuring every single uncertainty from each step of the WRA and combining them to to a a a total uncertainty This potentially results in a a a fairly high inaccuracy in in the the AEP estimation
of the the order of 10-20% depending on
the the country the the project is in the data available and amount of money invested As mentioned right at the the beginning these uncertainties can have a a a a a large e e e e e e e ect on
the project success Therefore they are taken into account in in in the WRA by using exceedance probabilities The calculated AEP represents the most likely result considering the uncertainties involved at at each step and assuming that they follow a a a a a a normal distribution This value is is known as ‘P50’ because the probability of exceeding it it is 50% as can be seen in Figure 7 (left and middle) In the nancing of of a a a a wind farm the values of of P75 and P90 P90 are also used P90 P90 for example is then the the value which has a a a a 90% chance of being exceeded The two plots show two examples of di erent uncertainty distributions in the the left-hand plot the the uncertainty is lower than the middle plot You can see that a a a a lower uncertainty results in in a a a a higher (and more certain) value of P90 Thus if the project planner can reduce the the uncertainty of the the project its value is e e e e e e e ectively increased On the the other hand if you as an investor underestimate the uncertainties you will be overestimating the value of the the project Finally the the right-hand plot shows how how the AEP value changes with exceedance probability *Watch out for: Make sure not to underestimate the the uncertainties in in the the AEP calculation otherwise you will be overestimating the the value of the project Conclusion
In this article I I have introduced you to the basics of wind resource assessments As an investor there are a a number of things to to watch out for when assessing independent WRAs This includes checking the the assumptions of the the uncertainty analysis
well making sure you discuss the results of any CFD simulations with a a a relevant independent expert and being aware how sensitive the wind speed measurements are to the project success 

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