Wind Harvest International's recent patents focus on the physics that result from the placement of Vertical Axis Wind Turbines (VAWTs) in closely spaced arrays. The benefits include an expected minimum 20% increase in Annual Energy Production (AEP) for pairs of VAWTs plus, when placed properly around HAWTs, an increase in the wind speeds entering their rotors and in the AEP of both types of turbines.
Coupled Vortex Effect
Wind Harvest International Patent: US6784566
“Coupled vortex vertical axis wind turbine”, Robert Nason Thomas
A pair of vertical axis wind turbines are arranged in close proximity to one another so that their vortices interact with each other to provide enhanced aerodynamic efficiency.
History of the Concept
In the 1990s, while field-testing two Windstar Model 1066 VAWTs, WHI’s founder and turbine inventor Bob Thomas observed increased wear on the external aerodynamic sheathing of the turbine’s downwind stators (aerodynamically shaped support columns). This was unusual because it indicated the regular occurrence of a significant and energetic wind force that could not be attributed solely to the dominant wind direction. Thomas was intrigued, and it led him to hypothesize that the turbine blades themselves contribute to the creation of this force, a type of “flow field” that results in higher wind speeds in one of the quadrants of the rotor.
In order to capture this energy, Thomas suggested placing WHI VAWTs close together in an array. In 2001 and 2002, Thomas and his team tested this concept with a three-turbine array of Model 530G turbines installed in the highly energetic wind farms of San Gorgonio Pass, California. First, a single turbine (T1) was installed and operated through a range of wind speeds. Daily average energy production for each wind speed was recorded over seven months. Two additional turbines were then installed on either side of T1, labeled T2 and T3. The averaged daily power of T1 for each wind speed was then collected. As the data was tabulated, it became apparent that there was a significant increased energy capture by the array configuration with a doubling of power performance in.these 33% solidity VAWTs. The two resulting power curves were compared to derive a quantitative assessment of the observed effect for each recorded wind speed. Thomas named this newly discovered property the “coupled vortex effect” (CVE). This resulted in the awarding of international patent US6784566 in 2004.
In 2008, WHI hired the renowned aerodynamic modeler Ion Paraschivoiu and his team at IOPARA Inc. to use the data from the three 530G turbines to model the coupled vortex effect. After his initial evaluations, Dr. Paraschivoiu reported to WHI, “I have been modeling VAWTs for many years, and it never occurred to me that the coupled vortex effect would exist, but now our modeling shows that it truly does increase the output of pairs of closely spaced VAWTs.” IOPARA’s analysis corroborated Thomas’ field data, finding “excellent agreement” between it and their modeling program.
In 2010, WHI applied for and received a grant from the California Energy Commission to further study the CVE and its potential benefit in increasing the efficiency of VAWTs with lower solidities and other attributes. IOPARA was selected to build upon the modeling they had already done to further analyze VAWTs in the CVE positions. Their work suggested that a lower solidity VAWT, at 16.5% solidity, would result in the highest power coefficient (Cp max) when placed in the CVE position.
IOPARA’s modeling and analysis also helped clarify the physics behind the coupled vortex effect.
Blockage Effect. All turbines create some blockage, forcing wind to go around them. Paraschivoiu commented in a memo dated February 3, 2010, that “Due to the fact that the flow is confined by the proximity of another rotor, the wind speed increases not only between the blades of two adjacent rotors but also through the rotor.” The higher the turbine solidity and rotations per minute, the greater the blockage, and the more wind flows around them. A single VAWT creates the same blockage but there isn’t a neighboring turbine to realize the benefits of the increase in wind speed.
Stream-tube Contraction Effect. Wind speed increases in the space between two adjacent VAWTs because their blades and the flow field of their rotors deflect wind into the gap. IOPARA’s modeling showed that the wind speed in the gap can be 1.75 times faster than the incoming (free flow) wind speed (Vw). Basically, the “stream-tube contraction effect” follows Bernoulli’s Continuity Principle, where a fluid entering a constricted space speeds up. The lift on a blade (and the resulting torque) is the square of the wind speed across its surface. Thus an increase in wind speed in the gap will significantly increase the torque and resulting energy output (see schematic at right). “The results (of the modeling) therefore confirm that to get maximum velocity augmentation, the preferred rotor configuration is the one with blades moving opposite the free stream direction in the region between the two rotors.”
Pressure Difference. The blades, when moving with the wind in the space between the adjacent rotors, create vortices behind the paired VAWTs. Vortices create a lower air pressure. When shed immediately downwind of the VAWTs, they increase the pressure difference between the upwind and downwind sides of the array of VAWTs. This pressure difference increases the wind speed through their rotors.
In 2010, Dr. John Dabiri (then with CalTech, now with Stanford) and his researchers, after studying the increased efficiency of fish swimming in school formations, proposed that closely spaced and counter-rotating VAWTs might exhibit similar efficiency-boosting capabilities. Subsequent field-testing of VAWTs in pairs and different configurations corroborated their hypotheses.
More recently, in 2014, a French and Belgian team studied the effect of closely spaced VAWTs in a series of wind tunnel experiments coupled with field measurements of flow field effects. They concluded that the spacing, rotor-rotational direction and synchronization of two-bladed VAWTs could increase efficiency of each turbine in the pair between 10% and 20%.
Patent number: 4115027
Abstract: An omnidirectional windmill employing lift type airfoils mounted about a vertical axis. The windmill includes a support frame which defines the vertically oriented axis about which the elongate airfoils rotate. Five vertically oriented stators are positioned outwardly of the airfoils about the windmill to form an omnidirectional diffuser. These stators extend radially from the vertical axis to substantially enhance the efficiency of the windmill. The stators also make the windmill self-starting. A friction heater is also disclosed in association with the windmill.
Filed: 7 January 1977
Date of Patent: 19 September 1978
Inventor: Robert Nason Thomas
Vertical Windmill with Omnidirectional Diffusion
Patent number: 5332925A
Abstract: A vertical windmill employing aerodynamic lift includes stators that form an omnidirectional diffuser and can rotate out of the wind to reduce the destructive tendencies in high winds. A braking mechanism included in the windmill uses rotation of the airfoils to reduce the lift caused by the wind and disengagement of the airfoils to reduce nearly all lift on the airfoils. Centrifugal force isused to activate the brake in high winds, both to slow the rotor speed and, in extreme winds, to stop the rotor. A motor is provided to drive the windmill to simplify controls and increase energy production.
Filed: 16 September 1991
Date of Patent: 28 July 1994
Inventor: Robert N. Thomas