Annual Energy Production Estimates Validation of Wind Farms (March 2012)

Validation of Wind Farm Production Estimates White Paper, March 2012

Validation of Wind Farm Production Estimates, White Paper, March 2012 page: 1/1 12MJD019.R1

Validation of Wind Farm Annual Energy Production Estimates

During 2011 MEGAJOULE has completed its first comprehensive validation program for Wind Farm

pre-construction estimates.

The estimated average annual wind farm energy production and corresponding uncertainty, from

previous wind assessments, was compared with the actual production from a number of wind

farms.

Resulting statistics can be used to assess the reliability of estimates.

Wind Farm database considered

The validation was based on 12 Wind Farms located in Portugal with a total of 45 independent

Wind Farm Years1.

Wind Farm operational data was supplied by the Wind Farm owners, from which MJ selected

those with a previous wind study and at least 2 years of reliable operational data.

Total monthly produced energy was taken from the substation meter. Wind farm balance of

plant monthly availability was also considered.

Each monthly Wind Farm production was corrected for Availability.

For historic reasons, all 12 Wind Farms are located in Portugal and the operational periods range

from 2007 to 2011 with an average of about 4 years of operation per Wind Farm.

Table 1 summarizes the Wind Farms characteristics.

1 A Wind Farm Year represents an individual civil year (January through December) with operational data (Energy Production and

Plant Availability) for a given Wind Farm.



Table 1 - Summary of Wind Farm data

Summary of Wind Farm data

Nbr. of Wind Farms 12

Total nbr. of Wind Farm Years (Range per WF) 45 (2 to 5)

Period 2007 – 2011

Total Wind Capacity (Range per WF) 237 MW (6 MW – 50 MW)

Mean Monthly WF Availability (Range per WF) 93.9 % (31% - 99%)

Windiness Index

In order to exclude the impact of each year windiness

from the analysis, the Portuguese Windiness Index2

published by APREN (Portuguese Renewable Energy

Association) and MJ was considered.

The annual energy production for each Wind Farm Year

was corrected for the corresponding windiness, based

on the Wind Farm location.

As an example, the following figure shows the Windiness

Indexes for Portugal in 2011.

Validation results

Figure 3 shows the frequency distribution of the ratio

between pre-construction long-term central estimates

(P50) and the actual Wind Farm production for each

Wind Farm Year (adjusted for Windiness and

Availability).

Table 2 summarizes the results for all tested scenarios

(original, adjusted for Windiness and adjusted for Windiness and Availability).

If P50 estimate is correct, the ratio should be 100%. If the actual production is below estimate

the ratio will fall below 100%, and vice versa.

2 - The monthly Portuguese Windiness Index is a joint publication of the Portuguese Renewable Energy Association (APREN) and

MEGAJOULE with the support of a large number of Portuguese WF owners. It’s available at (http://www.apren.pt)

Figure 1 – Windiness Index for 2011



Table 2 also shows the percentage of Wind Farm Years that fell below the estimated P90 annual

production. If uncertainty assessment is correct, 10% of the Wind Farm Years will fall below the

P90.

Figure 3 – Frequency distribution of the ratios between Real Production and MJ P50 estimates

Table 2 - Summary of validation results

Summary of Validation Results

Wind Farm Years database

No adjustment (45 WFY)

Windiness adjusted (44 WFY)

Windiness and Availability adjusted (44 WFY)

Mean Ratio (Real Production/P50)

94.8% 96.1% 98.9%

Median Ratio (Real Production/P50)

96.4% 94.3% 97.7%

Standard Deviation 12.3% 11.2% 10.6%

WFY below P90 14.6% 14.6% 7.3%

Globally, figures show a good agreement with actual wind farm production.

The statistics show a small systematic over-estimate in pre-construction estimates. In average,

the real annual production for the studied Wind Farm Years, already corrected for Availability

and windiness, were 98.9 % below estimates.

The percentage of Wind Farm Years below the estimated P90 is 7.3% (or 3 WFY). This can

evidence an over conservative uncertainty assessment, however, given the still small number of

WFY used in validation, the representativeness of this figure is somewhat limited.

0

2

4

6

8

10

12

Nu

mb

er

of

WF

year

s

Actual annual production / MJ predicted P50

Real Prod. - Windiness and Avail. adjusted

MJ Predicted

P90



Further work

MJ is now undertaking a revision of this validation, with the inclusion of new wind farms and

additional years of data for the existing ones.

MJ is also pursuing possible motives for the slight over-prediction of annual energy production

and for a conceivably conservative uncertainty assessment.

Further info.:

www.megajoule.pt

www.facebook.com/MEGAJOULE

www.youtube.com/MEGAJOULEConsultants

www.linkedin.com/MEGAJOULE

www.twitter.com/MEGAJOULE

[email protected]

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Annual Energy Production Estimates Validation of Wind Farms (March 2012)