Validation of pre-construction energy assessment estimates of wind farms vs real production

1. 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.

2. Validation of Wind Farm Production Estimates White Paper, March 2012
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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

3. Validation of Wind Farm Production Estimates White Paper, March 2012
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12MJD019.R1
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
NumberofWFyears
Actual annual production / MJ predicted P50
RealProd. - Windiness and
Avail. adjusted
MJ Predicted
P90

4. Validation of Wind Farm Production Estimates White Paper, March 2012
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12MJD019.R1
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
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www.youtube.com/MEGAJOULEConsultants
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megajoule@megajoule.pt