The Impact of Wind Turbine Density on Wind Farm Performance

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 2^nd   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

How turbulent winds abuse wind turbine drivetrains
Paul Dvorak, http://www.windpowerengineering.com/