Vertical Axis Wind Turbine (VAWT) wind farm array
performance benefit phenomenon explanation
It is our belief that the phenomenon of improved performances achieved by VAWT’s when sited within close proximity to each other in a wind farm setting is due to the following.
A wind turbines wake is the disturbed air flow that is the result of the wind blowing through its rotating blades. After having passed through a turbines rotor, the wind has a lower velocity, is very disrupted and turbulent, in turn this has a detrimental effect on the efficiency of other downstream turbines in a wind farm.
In all cases, weather vertical axis or horizontal axis, the efficiency of all wind turbines is dependent on two main factors, the aerodynamic efficiency of the blades and the mechanical losses of the machinery for converting the rotation of the blades to electrical energy. It is widely known that in isolation the aerodynamic efficiency of a HAWT is generally about 10% - 15% better than that of a VAWT, although this margin has been slowly eroded with recent developments in VAWT aerodynamics.
The key to the statement above is “in isolation”. In recent years, a number of studies have proven that when more than one VAWT is sited in close proximity to another, the performance of each individual machine can be enhanced by up to 15%. This is the opposite for HAWT’s, which studies in recent years have indicated that the spacing’s between machines should be extended due to the detrimental effect of a single machines wake on other machines within the vicinity.
This wind farm wake phenomenon and the differences in performance between HAWT’s and VAWT’s in a wind farm scenario, has been widely reported in recent months. Although, there seems to be a lack of understanding as to what and why it’s happening.
Studies of HAWT wake show that the turbulence (disturbed air flow) created by the wind blowing over and through the machines rotating blades spirals and funnels out from behind the machine. This disrupted air flow has a detrimental effect on the performance (up to 30% - 40% lower) of other machines downstream. Latest guidance recommends a spacing of 8-10 diameters apart in the prevailing wind direction and 4-5 turbine diameters apart in the cross-wind direction to allow sufficient recovery of the air flow to optimize the overall output of the wind farm.
In stark contrast, other studies into the wake of VAWT’s show that the performance of individual machines can be enhanced by up to 15% when sited 2.5 – 3 machine diameters apart.
Imagine that the wind flow sees a HAWT as a rotating disk with gaps in it that some of the wind can pass through. The wind that hits the blades is deflected and caused to increase its velocity as it is diverted around the airfoil shape of the blade. Exiting the wind turbine, the through flow wind and the increased velocity deviated wind collide, causing the spiraling turbulent wake. Some of the wind is also diverted along the leading edge of the blade and spills off of the blade tip, which then encourages the funneling of the created spiraling wake
Now imagine that the wind flow sees the rotating blades of a VAWT as an upright cylinder, with vertical airfoil profile blades constantly proceeding on one side, receding on the opposing side and crossing perpendicular to the direction of flow in front and behind. More of the wind flow can pass through the cylinder relatively undisturbed on one side and the wake created by the blades vortex shedding on the other side forms a snake like wake that curves to the left or right, dependent on the direction of the machines rotation. Some of the wind flow that doesn’t pass through the cylinder of the turbine
is diverted around it and like with an airfoil,
the velocity of this wind
is increased as it rushes
to get around the obstacle
and fill the void created behind.
By placing a second VAWT (another cylinder) with a counter rotating blade configuration, slightly to one side and behind the first, it not only sits in the area of higher velocity wind created by the first machine, but it creates its own area of increased wind speed. This provides an additional draw on the already increased velocity of the wind flow of the first machine. By having the second machine counter rotating, also means that the snake like wake from the second machine curves the opposite way and in a short space collides with the wake from the first machine. This collision of the wake from the two counter rotating machines appears to have a canceling effect and the wake dissipates quickly. This closely sited arrangement and increased wind velocity between machines has been shown to improve the performance of each individual machine by up to 15%.
Therefore, a wind farm made up of clusters or rows of counter rotating VAWT’s angled to the predominant wind direction, can provide a much denser power to area output (estimated to be at least a factor of 6 times higher) than a conventional HAWT wind farm. In turn this would provide a more efficient wind farm overall, at lower cost and with less impact on the environment.
See some links to reference material below: