Coupled Vortex Effect

Bob Thomas, a co-founder of Wind Harvest, discovered the Coupled Vortex Effect. When arranged in close proximity to one another, a pair of H-type wind turbines create a synergistic effect. They increase wind speeds through their own and their neighbors’ rotors and enhance aerodynamic efficiency. Field data followed by computer modeling and analysis shows that placing two or more turbines in a correctly spaced pattern can increase efficiency by ~20%.

Wind Harvest Patent

Patent Number: 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 1066s, Wind Harvest’s founder, chief engineer, and inventor Bob Thomas observed increased wear on the turbine’s downwind stators (aerodynamically shaped support columns). The increased wear 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 creating this force, a type of “flow field” that results in higher wind speeds in one of the rotor quadrants. He envisioned that neighboring turbines would create a synergistic effect.

Three Windstar model 530G H-type vertical axis wind turbines in Palm Springs, Calif. provided the data needed to secure the Coupled Vortex Patent.

To capture the observed energy, Thomas and his team tested the coupled vortex effect concept in 2001 and 2002. They installed a three-turbine array of Windstar 530Gs in one of the highly energetic wind farms of San Gorgonio Wind Resource Area in California.

Step One

First, they installed a single turbine (T1). It operated through a range of wind speeds, modeling the typical operation of the turbine. They recorded the daily average energy production for each wind speed.

Step Two

After one year of data collection, they installed two additional turbines one meter away on either side of T1, labeled T2 and T3. They measured the average daily power of T1 for each wind speed. As they tabulated the data, it became apparent that there was a significant increase in energy capture with the array configuration.

Results

They compared the two resulting power curves, which confirmed the increased energy output from the array. Thomas named this newly discovered property the “Coupled Vortex Effect” (CVE). This discovery resulted in the awarding of International Patent US6784566 in 2004.

The advantage of most low solidity H-type vertical axis wind turbines is that the wake is smoother. The peak Cpe is realized at a high TSR, where the blade speed is much higher than the wind speed, so the wake is smoother. In this condition, the flow stays attached to the blades for a more extended period, creating the smoother wake behind the turbine. The result is that a second row of turbines can be placed much closer to the upwind row (e.g., 4-5 rotor diameters versus 10-15 rotor diameters) than with H-type turbines with peak Cpes at lower TSRs.[1]

[1] Letter of Support for Wind Harvest’s VAWT Wake Research Proposal. Dr. Marius Parashivoiu, Concordia University

Bob Thomas, a co-founder of Wind Harvest, discovered the Coupled Vortex Effect. When arranged in close proximity to one another, a pair of H-type wind turbines create a synergistic effect. They increase wind speeds through their own and their neighbors’ rotors and enhance aerodynamic efficiency. Field data followed by computer modeling and analysis shows that placing two or more turbines in a correctly spaced pattern can increase efficiency by ~20%.

Wind Harvest Patent

Patent Number: 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 1066s, Wind Harvest’s founder, chief engineer, and inventor Bob Thomas observed increased wear on the turbine’s downwind stators (aerodynamically shaped support columns). The increased wear 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 creating this force, a type of “flow field” that results in higher wind speeds in one of the rotor quadrants. He envisioned that neighboring turbines would create a synergistic effect.

This figure shows where in the rotational cycle Wind Harvesters realize more lift and thus more torque in the coupled vortex position (compared to a turbine operating alone). 90 degrees in the rotation is when the blade is upwind and fully face on to the wind. It is on the downwind side of the cycle, that the CVE results in the higher energy output.

[1] Aerodynamic Model of the Wind Harvest Model 1500 Wind Turbine, Dr. Ion Paraschivoiu, (with comments by Robert N Thomas, Dr. Farooq Saeed, and Norbert V. Dy), IOPARA Inc., May 18, 2009

[2] Modeling Blade Pitch and Solidities and Straight Bladed VAWTs –Final Report to Wind Harvest and the California Energy Commission, IOPARA Inc.,2 August 2011

[3] Analysis of the coupled vortex effect, Letter to WHI by Ion Paraschivoiu, Feb. 3, 2010

Fish Schooling hypothesis

In 2010, Dr. John Dabiri and his researchers at CalTech University studied the increased efficiency and synergistic effect of fish swimming in school formation. They wondered if closely spaced and counter-rotating H-type wind turbines might exhibit similar efficiency-boosting capabilities. Subsequent field testing of small turbines corroborated their hypotheses[1].

French and Belgian team

In 2014, a French and Belgian team studied the effect of closely spaced H-type turbines. They conducted a series of wind tunnel experiments coupled with measurements of flow field effects. They concluded that the spacing, rotor rotational direction, and synchronization of two-bladed turbines could increase efficiency by 10% to 20% for each turbine in the pair [2].

Nenuphar wind turbines graphics showing coupled vortex effect.

[1] Fish Schooling As A Basis for Vertical Axis Wind Turbine Farm Design, Robert W Whittlesey, Sebastian Liska, and John O. Dabiri, 11 February 2010, and Vertical-Axis Wind Turbine Arrays, John O. Dabiri, Journal of Renewable and Sustainable Energy, 19 July 2011

[2] Efficiency Improvement of Vertical-Axis Wind Turbines with Counter-Rotating Lay-Out, Nicolas Parneix, Rosalie Fuchs, Alexandre Immas, Frédéric Silvert, EWEA 2016

Increased wind speed in the gap between the turbine

Bernoulli’s Principle dictates that a fluid such as wind will speed up in a narrower space. Modeling shows that the one-meter gap between each turbine pair can realize a 1.75X increase in wind speed. The blades’ flow field and the blockage effect of the rotor’s blades are the primary reason more wind flows faster through the gap.

The energy in the wind is the cube of the wind speed. The increased wind speed through the gap only affects 1/4th of the blades’ rotation, but a 1.75X increase wind speed in this quadrant is powerful.

Change in air pressure

The blades are rotating at 50-80 mph. This speed is 3 to 5 times faster than the incoming wind. As a result, the blades shed vortices downwind. The video to the right shows the modeling of this effect where the outsides of the vortices are spinning at near rotor speed, and air pressure drops with increased wind speed. Thus, the vortices produce a lower pressure zone downwind of the rotor. This pressure change causes the wind to increase speed through each rotor that has shed vortices behind it.

Blockage effect

All wind turbines create a blockage effect. The more blade area to rotor area a turbine has, the more wind will be blocked. The blockage causes some wind to flow around, over, and under any turbine’s rotor. When installed in close proximity, some of the wind blocked by H-type turbines spills into the neighboring turbine [1]. This spillage increases the wind speed through that turbine. A single turbine operating alone does not realize this benefit.

Produced by IOPARA Inc for Wind Harvest.

Produced by IOPARA Inc for Wind Harvest.

[1] CFD Analysis of Vertical Axis Wind Turbines in close proximity, Marius Paraschivoiu, Chad X. Zhang, Selvanayagam Jeyatharsan, Norbert V. Dy, Farooq Saeed, Robert N. Thomas, Ion Paraschivoiu, IOPARA Inc. 2011