Mount Lucas and the proposed Cloncreen Wind Farm - The Impact of Wind Turbine Density on Wind Farm Performance
by John Dooley, an industrial engineer
Building larger and more efficient turbines mean fewer turbines overall. For some larger
commercial turbines, a 10-15% increase in turbine height can increase the energy yield by up
to 50%. These more efficient turbines increase our ability to meet targets, reduce the
amount of turbines needed and reduce the amount of raw materials required. When
delivering the least cost solution to society, the grouping or clustering of wind projects in
relatively close proximity on sites with suitable resource is crucially important to reducing
cost to both developers and consumers. Well planned cluster developments enable
developers to achieve lower average connection costs and reduce the costs and timelines of
infrastructure delivery for the Grid Operator - IWEA, 2013.
On July
2nd this year, 2015, I attended an Information
Presentation on the proposed Cloncreen Wind Farm. I had a particular interest in
Cloncreen Wind Farm as I suspected that I suspected that BNM would populate the
proposed wind farm with a similar density as they had done in Mount Lucas. In
April 2014 I went to observe Mount Lucas to confirm to myself that BNM
were actually going to commission wind turbines with over 100 meter diameter
hub 500 meters a part. This was a surprise to me as the rule of thumb for most
wind farms on the Continent was 8 times the hub diameter. This would have meant
that the wind turbines on Mount Lucas would have been over 800 meters no matter
what size wind turbine was installed. However these multi megawatt wind
turbines with larger extended blades to extract more wind energy in lower wind
speed areas have different spacing requirements.
Research carried out by Johns
Hopkins University and the Catholic University Louvain concluded that to get larger wind turbines to operate to the Manufacturers specified output
curve the separation distance between wind turbine’s should be 15
times the hub diameter or at least 1.5 kilometers apart in the case of
Mount Lucas. This research has recently being reinforced by research into the
impact of wind wake on wind speeds, carried by the Jena- Max Planck Institute, in Kansas,
and published by The National Academy of Sciences in the USA. It was
discovered that wind hitting the first wind turbine at 7 meters per second had
dropped to 1 meter per second at the wind turbine in the lee of the first. Other research, carried out by Texas Tech
University using Doppler Radar discovered that the wind speeds can drop to
minus figures for wind turbines in the lee of the first wind turbines.
The cut
in speed for the Mount Lucas wind turbines is apparently 3.5 meters per second.
If wind turbines are too close together they are impacted by wind wake and can
lose up to 40% of their output. Wind turbines so close together are also
afflicted by wind shear and can be severely damaged by reverse torque. Reverse
torque can wreck the drive train and usually happens in extreme wind conditions
when the wind turbine reaches cutout speed and is stopped by the braking
system. This has recently been recognized as the main reason for a significant
increase in O&M Costs. Had Bord Na Mona populated the Mount Lucas wind farm as recommended by
The Johns Hopkins/Catholic University Louvain research just over 9 wind
turbines would have been commissioned on the site costing in the region of €38m.
Instead 28 three megawatts wind turbines at 84MW were commissioned at a cost
of €115m. This means that €77m was over
invested in the site.
The
recently published Environmental Impact Statement indicates that 75 megawatts of
wind turbine capacity, or 25 three megawatt wind turbines, are to be installed on
Cloncreen Bog. Cloncreen bog is slightly smaller than Mount Lucas suggesting
that BNM are sticking to what they call as the Irish Standard Separation. Which
they consider to be 500 meters. This suggests that the construction costs of
Cloncreen will be in the region of €105m at that density. However if the density
was informed by recent research, the construction costs would be in the region
of €35m. The total actual likely investment by BNM in these sites is likely to
be close to €220m. Had the density been informed by research, the total
investment would be €73m a difference, adding in something for inflation, of
€150m+.
Wind Over Power Ratio (WOPR)
The failure of BNM to apply research findings to their layout has other
significant implications for these sites. The wind turbines installed in Mount
Lucas and likely to be installed in Cloncreen have a Wind Over Power Ratio of
9. This ratio quantifies the ability of the wind turbine to dissipate
the force of the energy/output to be
reduced caused by the wind turbine having to stop at cut out speed or at an
emergency stop. These larger wind turbines with increased blade size generate
more energy but at emergency shut down or at cut out speed you also have
increased energy to loose or dissipate.
The correct WOPR for wind turbines with large blades that exceed 101
meters should be in the region of 4 or 5 at the highest. Wind Turbines with a high WOPR are more likely to be damaged by reverse torque significantly
increasing maintenance and shortening their lives. Much of this wind shear is being generated by the wind turbines themselves. To get wind turbines, with a rated output of
12 meters per second and cut a out speed of 25 meters per second, down from a
WOPR of 9 to 4 to 5 the cutout speed will have to be reduced to 20 meters per second.
Unmodified these wind turbines should have a fairly short operational life. The
average operating lives of wind turbines, many of them large multi megawatt
wind turbine’s, commissioned and decommissioned in Denmark between 2000 and
2014 was 6.2 years.
References :
Two methods for estimating limits to large-scale wind
power generation
Lee M. Millera,1, Nathaniel A. Brunsellb
, David B. Mechemb
, Fabian Gansa
, Andrew J. Monaghanc
, Robert Vautardd
,
David W. Keithe
, and Axel Kleidona
Wind Farm Operators are going to have to space turbines further apart
Johns Hopkins University researcher
