How to harvest plentiful low-level winds on existing wind farms

This paper will be submitted for peer review to Renewable Energy Focus  after receiving comments from the publication of excerpts in the April issue of Windpower Engineering.  Send comments and suggestions to Kevin Wolf, kwolf@windharvest.com

Wind farms in California and other regions of the world exist only in relatively small geographic regions.[i] Most of these resource areas have reached their physical or political[ii] limits in their ability to install additional propeller-type, horizontal axis wind turbines (HAWTs).[iii] Nonetheless, many have topographies that create excellent near-ground wind speeds.

To profit from the energetic wind below their HAWTs, wind farm owners need cost-effective vertical axis wind turbines (VAWTs) that operate efficiently in high turbulence and that do so without wake[iv] from the added rotors negatively impacting their existing turbines. They also need turbines that are wildlife friendly.

Near-ground turbulence

The good-to-excellent average annual wind speeds (6 to 9 m/s, 14 to 20 mph) found at 10–25m above ground level in wind farms in California[v] and other regions are well known to wind industry meteorologists.[vi] Passes and ridgelines accelerate near-ground wind and cause wind shears to decrease, often significantly. Meteorological data also document that thermal and obstacle-induced turbulence in the high-energy, near-ground wind is found in many wind farms, including in four of California’s five Wind Resource Areas.

       Three Windstar 530G VAWTs among HAWTs in the San Gorgonio Wind Resource Area, California

One reason near-ground wind resources haven’t been developed is that HAWTs have increased failure rates when their blades pass through turbulence.[vii] As a result, rows of HAWTs are hundreds of meters downwind of each other, and the bottom tips of their blades range between 20m and 50m above ground level.

HAWT turbulence-loading problems arise primarily from their long blades connecting to the drive shaft at only one end and their large rotor having to operate in changing wind speed and direction. The blades and bearings used in modern HAWTs would have to be substantially strengthened to withstand the high peak and cyclic loads from the near-ground layer of extreme turbulence.[viii]

Why VAWTs now 

VAWTs are intrinsically less sensitive to turbulence than HAWTs because their blades are attached to the rotating shaft at two or more locations. Another beneficial outcome from their geometry is that VAWTs don’t have to yaw and turn into the changing wind direction.

At least one such wind turbine (i.e., Wind Harvest International’s (WHI) Harvester VAWTs[ix]) is ready for certification and operation underneath HAWTs.[x]  Other turbines could also soon be capable of achieving a 20+ year service life in high turbulence and be ready for industry-scale sales(e.g., Stanford/Dabiri’s VAWTs), once they can comply with the IEC 61400 certification process and become UL listed.

Historically, VAWTs have had trouble with mechanical design and durability because they lacked field-validated, aeroelastic modeling that HAWT engineers use. That has been resolved by building on VAWT modeling developed by Sandia National Labs and advanced at Delft and Danish Technical University.   The engineers of the WHI Harvester used a suite of a prototype-validated finite element, frequency response, and fatigue analysis models that together function as an aeroelastic model.[xi]

Aerodynamic modeling funded by a 2010 California Energy Commission (CEC) EISG grant[xii] to WHI proved that modern VAWTs, when placed close together would also create the “coupled vortex effect”. The one-meter close spacing and counter rotations let them produce 20 to 30% more energy per pair than from two VAWTs operating separately. This offsets the problem VAWTs face that HAWTs don’t: Their blades create drag as they return into the wind. Historically, this increase in drag prevented them from realizing more than a 45% efficiency,[xiii] whereas HAWTs can achieve 50%. With the coupled vortex effect, VAWTs in arrays can theoretically realize the efficiencies of HAWTs.


 

 

Another problem hindering VAWT development is that smaller VAWTs like WHI’s Harvesters use more steel and material per rotor-swept area and MW of installed capacity than do large HAWTs.  However, with large-scale use possible in wind farms:

  • The mass manufacture of the smaller VAWTs offer significant savings.[xx]
  • Their shorter towers use less material[xxi] and smaller and easier to build foundations.
  • They make dual use of valuable land and infrastructure[xxii] when installed in existing wind farms.

An additional benefit modern inverter-based VAWTs have for repowering wind farms is that they can help solve the grid harmonics and reactive power problems that are caused by older HAWTs using “induction generators”.  A megawatt of VAWTs like the WHI Harvester 70 with inverters similar to the ones in Northern Power Systems’ 100kW HAWTs can, independently of wind speed instantaneously source or sink 450 KVARs[xxiii] of the problematic reactive power produced by the older HAWTs.  Solving the reactive-power problems of older wind farms can increase their power quality and real output.

VAWT impacts on HAWTs

Aerodynamics predict that the wake from VAWTs won’t harm HAWTs, and may in fact help them.  The wake and vortices shed from an array of tightly spaced VAWTs should stay in the same wind layer that passes through their vertically spinning rotors. Modeling shows that downwind by five rotor heights[xxiv] or ~eight rotor diameters[xxv] , the wake of VAWTs is gone, their vortices have disintegrated, and the wind speed has recharged, in part due to the vertical mixing that their spun-off vortices create.

The graphic shows a vertical and horizontal staggering of VAWTs upwind and downwind of a 2-MW HAWT. The distances between VAWTs and their heights are described in Table 1 with the exception that the Harvester 70 VAWT distance immediately upwind of the HAWT is 100m and not 70m. The faster-moving wind that is upwind and above the HAWT will be drawn down toward the ground by the vertical mixing and energy extraction of the VAWTs below.

 

 

 

VAWT placements are theorized to increase the wind speeds entering the rotors of the HAWTs above them in two major ways.

Lowering the wind shear

A growing body of field data and research, led in large part by Dr. John O. Dabiri, has demonstrated how counter-rotating VAWTs lowers wind shears by bringing higher, faster-moving wind toward the ground and replenish the wind speeds lost to the energy and turbulence the VAWTs produce.[xxvi]  As a result, faster moving wind from above will drop down into HAWT rotors and increase their energy output.[xxvii].

Stanford University doctoral candidate Anna Craig led a study that modeled various VAWT arrangements. Their results[xxviii] indicate that VAWTs can interact positively when placed in close proximity to one another. Craig noted that “We think that the VAWTs can have blockage effects causing speedup around the turbines that help downstream turbines. They can also have vertical wind mixing in the turbine’s wake region, which assists in the wind velocity recovery.”

The complete 12-page article including a discussion on VAWT impact on wildlife is available from the News and Updates section on windharvest.com home page


Footnotes and further reading

[i] In 2014, WHI conducted a cursory review of wind farms around the world to evaluate them for topographies and roughness that were conducive to creating good near-ground wind speeds. At that time, approximately 20-25% of wind farms had the topographies, wind shears and wind speeds that should produce 15-20 mph average annual wind speeds at 10-20m above ground level.

[ii] The politics of zoning and permitting are influenced by concerns over views, habitat, aviation and wildlife impacts. There are many peer reviewed articles documenting this. Here is one that covers many of the issues – https://journal.gnest.org/sites/default/files/Submissions/932/932_published.pdf

[iii] The large setback requirements needed by rows of HAWTs are well documented. New HAWTs cannot be installed within most existing wind farms without reducing the wind speeds or increasing the turbulence realized by their neighbors.

[iv] The rotating blades of wind turbines create wake and turbulence in the wind in a similar way that a boat and its propeller create wake and waves in the water. The wake created by VAWTs is very different from the wake created by HAWTs.  HAWTs with their blade tip speeds often exceeding 150 mph create circular wakes extending and mixing together long distances downwind.

[v] Almost all the data used to produce the “Wind Atlas published by the CEC in April 1985 was derived from reports written in part by some of the consultants below from 1980-1984 who collected and found near-ground wind data, most often at 30’ above ground level. For example, it shows seven sites in the Altamont Wind Resource Area have average annual wind speeds varying between 15.4 – 19.7 mph and averaging close to 17 mph at 30’ above ground level (agl).  The Atlas also shows six sites in the Solano Wind Resource Area with wind speeds averaging ~15-18.5 mph at ~33’ agl. One of the reasons for collecting near ground wind data in the 1970s and 1980s is shown on the inside back cover of 1983 CEC Report – Wind Energy, Investing in our Future. There is a photo of a DAF-Indal Darrieus-type VAWT with hub height of around 30’ agl.  At that time, VAWTs were expected to be major players in the future of wind energy production and data collection for potential wind farms was oriented to their hub heights.

[vi] The following wind industry meteorologists and companies will confirm that there are good to excellent average annual speeds and high turbulence in the near-ground wind in California’s Wind Resource Areas. Note that titles and associated organizations are used for identification purposes only:

Allen Becker, Consulting Meteorologist
John Bosche, President and Principal Engineer at ArcVera Renewables
Neil Kelley, Applied Meteorologist (retired)
Pep Moreno, CEO, Vortex
Ron Nierenberg, Consulting Meteorologist
Lucile Olszewski, General Manager, Ensemble Wind
Richard Simon, Consulting Meteorologist
John Wade, Senior Meteorologist, Ensemble Wind
ArcVera Renewables,  Wind Prospecting and Resource Assessment

WindSim,  CFD Wind Resource Assessment

[vii] Turbulence problems created in HAWT blades, gearboxes, and bearings HAWTs are documented in multiple places in this wind engineering textbook, “Wind Energy Explained: Theory, Design and Application,” J.F. Manwell,  J.G. McGowan, A.l. Rogers; John Wiley, U of Mass Amherst, 2002.

[viii] David Malcolm, PhD, structural engineer, retired from Det Norske Veritas/Gemanischer Lloyd

[ix] WHIHarvester VAWTs have ~170 square meters of rotor-swept area and 50 to 100 kW+ generators, which vary based on the wind resource. For specifications see: http://windharvest.com/harvester-vawt/

[x] WHI’s Harvester 70 design files have been sent to the  Small Wind Certification Council, which follows the IEC 61400-2 certification requirements for small wind turbines under 100kW in size.

[xi] WHI used a Frequency Response and Fatigue Model first created and validated by Sandia National Labs on its Darrieus-type VAWTs. Using strain gauge data from the Harvester 70 prototype in Denmark, WHI validated the loads predicted in its Midas FEA model and these other two models.

[xii]Modeling Blade Pitch and Solidity in Straight Bladed VAWTs”, Iopara Inc, Bob Thomas and Kevin Wolf, February 12,2012, Final Report to the California Energy Commission’s Energy Innovations Small Grant Program.

[xiii] Sandia National Labs’ field research on a Darrieus-type VAWT showed it was capable of achieving a maximum of a 45% efficiency or Cp max. See “A Retrospective of VAWT Technology”, Herbert J. Sutherland, Dale E. Berg, and Thomas D. Ashwill, SANDIA REPORT (SAND2012-0304), January 2012 and “The Sandia Legacy VAWT Research Program.”

[xiv]  “Land-use Requirements for Wind Turbines in the U.S.”,  Paul Denholm, Maureen Hand, Magdalena Jackson, and Sean Ong, NREL, August 2009.  Note that NREL’s study covers “Total Wind Plant Area” which is considered the “footprint of the project as a whole” and includes more land than the Rule of Thumb methods of determining land use per MW of wind turbines.

[xv]  Six rotor diameters between turbines in a row and 7 rotor diameters between rows.

[xvi] Three rotor diameters between turbines in row and 10 rotor diameters between rows.

[xvii] Satellite imagery of the 750kW NEG Micon turbines. Mountain View Power Partners LLC, Riverside County, California. Note that this specific location might not be appropriate without hydrological modeling for many more VAWTs because of their roughness impact on flood events from the Whitewater River.

[xviii] Assumes four VAWTs in a 13-m long array with two rotor diameters between each 12kW array in a row.

[xix] The blade tips of and vortices shed from the VAWTs create turbulence, vertical mixing and roughness should bring faster-moving wind into the taller Harvester 70 arrays. An array of the 9m tall VAWTs only meters upwind of Harvester VAWTs on 8-10m tall towers would create a blockage effect, and if positioned correctly send more into the VAWTs above and downwind. Together, the field of VAWTs could have a greater positive impact on the wind speed increases realized by the HAWTs.

[xx] WHI’s Harvester VAWTS are made of extruded, aircraft quality (6061-T6) aluminum. Bob Thomas, a founder of the Wind Harvest Company and the inventor and lead engineer of their Windstar turbines determined that a NACA 0018 blade profile to be effective and easy to extrude with internal walls.

[xxi]   HAWT towers not only use more material because they are taller but they have to be strengthened further to handle the oscillations that come from their offset rotors. VAWT blades rotating evenly around the top of its tower (discussion with Herb Sutherland, retired Sandia National Laboratory engineer who worked on the mechanical aspects of wind turbines.

[xxii]  For a full build-out of a wind farm’s understory, new transmission lines and substations will be needed.  For a “capacity factor enhancement” project where VAWTs are added to the wind farm but are turned off as the substation reaches capacity, no new transmission lines or substations are needed.

[xxiii] “Reactive Power Compensation – Using a Northern Power® NPS 100TM or NPS 60TM wind turbine to manage power factor”, NPS Engineering Bulletin

[xxiv] In Dr. Marius Paraschiviou’s letter to the CEC in support of WHI’s grant application, he stated “…after the CEC Innovations grant was completed, we conducted additional aerodynamic modeling on downwind wakes that showed VAWTs like WHI’s Harvesters will be able to be placed about six  rotor heights downwind of an upwind VAWT array and realize the full wind speed that entered the rotors of the upwind array.”

[xxv]  A number of Dr. John O. Dabiri’s papers show that VAWTs when placed as in their field studies can regenerate ~95% of the full wind speed at 7 rotor diameters downwind. (Kinzel M, Mulligan Q, Dabiri J., Energy exchange in an array of vertical-axis wind turbines. Journal of Turbulence 2012; 13: 1–13).  Note that in his field studies, Dabiri’s placement of VAWTs are as close together as they are in Paraschivoiu’s modeling.  The tighter spacing and the resulting increase in wind speed in the gaps between the VAWTs that were used in Paraschivoiu’s modeling probably is the reason for the difference

[xxvi]Potential order-of-magnitude enhancement of wind farm power density via counter-rotating vertical-axis wind turbine arrays”. John ODabiri, Journal of Renewable and Sustainable Energy 3, 043104 (2011) and Benefits of Co-locating Vertical-Axis and Horizontal-Axis Wind Turbines in Large Wind Farms.

[xxvii]  The energy in the wind is the cube of the wind speed so a small increase in wind speed results in a significant increase in the energy available for the turbine to convert to electricity.

[xxviii]  “Low order physical models of vertical axis wind turbines”, Anna E. Craig1,a), John O. Dabiri2, and Jeffrey R. Koseff3, Journal of Renewable and Sustainable Energy, Feb. 2017

 

 

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