VAWT

The vertical axis wind turbine presentation

Un rotor monté sur roulement, une structure fixe, et une génératrice équipée de sa régulation

Modern Darrieus VAWT

Introduction to the vertical axis wind turbines

The vertical axis wind turbines are fitted with a rotor which turns around an axis that is orthogonal to the fluid flow. At the opposit to an horizontal axis wind turbine, the fan rotation axis is vertical. This technology exists since the begining of the XX century, within different topologies: Darrieus, Savonius, Cycloturbine...

These particular wind turbines have some advantages (sillentfull, easy maintenance, tolerant to wind changes) that make it very suitable for self consumption and domestic applications.

Vertical axis wind turbines and horizontal axis wind turbines have many common behaviour : aerodynamically, mechanically, control and conversion systems. This article does not compare technologies, but specifically describes different vertical wind turbine architectures, constraints and applications.

The fundamental principle of the "vertical axis" systems is to convert the mechanical power of the flow by a movement of a solid, the rotor, which rotates along an axis which is generally orthogonal to the velocity vector of the air flow. This feature makes the system isotropic regardless of the direction of fluid flow in the plane perpendicular to the axis of rotation. The application in wind power conversion is to set the axis of rotation vertically, in order to catch the wind energy whatever the direction of the wind, without empennage. The mechanics are simplified : there is no nacelle.

Nevertheless, in some specific installation, the rotor can be fitted along a not vertical axis. For instance on the ridge of a roof. In this case the energy conversion works for a single wind direction. These special cases are included in the families of vertical axis wind turbines.

Un rotor monté sur roulement, une structure fixe, et une génératrice équipée de sa régulation

Vertical axis wind turbine architectures

Only one single rotation part : the fan is linked to the stator via bearings.
Generator is powered by the fan shaft.

Principle

All vertical axis wind turbines are fitted with a structure, a rotor and a conversion system.

-The structure is fixed and supports all the mechanisms, allowing to withstand all the forces and realize the pivot connection with the rotor. The structure, in some cases, can incorporate a mast in order to elevate the equipment.

-The rotor is the rotating part. It is fitted with specifical blades which will harvest and convert energy from wind to mechanical movement. Usually, blades are fixed to the rotor. But some vertical axis wind turbines (the cycloturbines) can change the pitch of the blades during the rotation.

-The conversion system is in charge to convert the mechanical power (torque * rotating speed) in an usefull power. As for solar panel conevrsion systems, the device shall both convert and regulate. The rotor speed shall be optimised permanently, depending on wind speed, wind variations and current rotating speed. The system tracking the best running point is called MPPT (Maximum power point tracking) as for solar panel inverters. Some very vulgar systems exists with a simple connexion to a pump, or to batteries. In this case, the system runs on a unneficient running point. Without MPPT conversion system, the running point cannot be close to the optimum due to very particular efficiency curves of the vertical axis wind turbines.

Darrieus wind turbine

Savonius wind turbine

Cycloturbine

Various architectures

There are 3 main different types of vertical axis wind turbines :

-Darrieus wind turbines, with thin and efficient blades (NACA profiles).The shape of the blade generates lift , similar to plane wings. The blade is fitted so that the lift will drift the blade and speed up the fan. The efficiency of these machine can theoritically reach 59% (Cp=0.59). Best machines have harvested 35% and improvement are expected thankd to new technologies.

-Savonius wind turbiens, fitted with 2 "half pipes". This shape generates a difference for draft between the two round parts, and so a torque on the shaft. Advantage of this technology is the very simple way of manufacturing the rotor. But efficiency and performance are very poor. M. Savonius demonstrated that the best design consists to overlap the 2 half pipe of 1/6 of the diameter. Usually, the efficiency is around 15%. Some tests in laboratory show that the maximum that we can expect is 25%.

-The cycloturbines, fitted with variable pitch system, have a not-isotropic behaviour. Effectively, the blades switch or orientation depending on the wind direction. A complex mechanical system shal actuate the blade pitch depending on the position of the blade along the rotation of thee fan (close to an helicopter rotor). The wind turbine shall have an additional empennage or wind sensor. Only a few individuals tried to design such machine, and there is no clear performance recorded on such cycloturbines.

The efficiency of the wind turbines (Cp) is the ratio of thee harvested power on the power going through dthe wet area (½ ρ S V³ with ρ=the mass of air, S=the wet area, et V=the wind speed). The way to compare the technologies is to draw the Cp curve along the "Tip Speed Ratio" (TSR, or λ) variable, which is the speed of blades / wind speed. Tipically, the wellknown noise of wind turbines is due to the very high λ of the horizontal axis wind turbines. The lower λ of other technologies make them quieter.

sketch-savonius

Les éoliennes de Savonius : 2 tôles roulées emboîtées

Le vent s’engouffre dans la partie concave, et se déverse en partie vers l'autre pale, réduisant sa traînée lorsque la pale remonte le vent.

Savonius Wind Turbines

Sigurd Savonius (1884-1931) was a finland architect who has made a very complete and detailed analysis of a wind turbine designed with 2 half pipes.

The aerodynamism of the system is apparently simple : the drag difference between the pipes descending or going against the wind (Cd≈1.2 and Cd ≈0.5) makes the wind turbine turn. But M. Savonius overlapped the 2 pipes, in such a way that the air can partially go from one pipe to another, and demonstrated it reduces the drag of the pipe going against the wind.

The Savonius wind turbines have an optimum TSR of 1.0 (λ≈1.0). The wind turbine is in the "low TSR VAWT families" that are suitable for water pumping rather than electric production.

The main disadvantage of Savonius VAWT is the strong wind resistance of the turbine, even when rotor is stopped. A wind turbine shall be designed to resist to tempest, which is very hard for this technology : a safety brake shall be designed to resist to high torque, and the structure shall wistand to tempest wind thrust.

For istance, a 1KW savonius wind turbine will be sized to produce 1KW at 40km/h of wind ( 1 000 =Cp ½ ρ s V³ ) (ie s=8 m² if we consider a standard Cp of 0.15). We can consider a standard rotor of 2m x 4m for this exemple. The 200Km/h tempest design case calculation results in a thrust of more than 3 tons(ρ s V²= 35.8KN), a folding torque of 70KN.m and a brake designed to resists to 400N.m (T=ρ D*H*V²(Cd1-Cd2)*D/4=406N.m).

To resist, manufacturers usually add a large shaft at the center of the two pipes, which decreases the air flow between pipes, and makes the efficiency even lower.

The blade pitch is modified during fan rotation.

The cycloturbines

The cycloturbines are mechanically very complex. The pitch of each blade is permanently adjusted by a system of links and balls. Many inventors have tried to develop machines of this type with varying degrees of success. The kinematic of the pitch actuator looks like the helicopter rotor. At the difference of industrial horiontal axis wind turbines that adjusts the blade pitch a few times per hour, the cycloturbine mechanical actuators will change the pitch thouthands of times per hour. It results in wear, maintennace, failure, noise...

But the main advantage of these wind turbines is the optmise the pitch of the blade all along the fan rotation. Theoritically, target is to reduce the drag and increase the lift obviously.

These machines works with reduced Reynolds number (Re <10⁶), like Savonius VAWT, but the design shall be design to obtain high lift forces. the speed of the blade shall be high enough to increase the lift, but not too high due to centrifugal forces that fragilianical system. Usually, the speed of the blades is close to the wind speed (0.8<λ<1.5). This speed is high enough to make the machine difficult to design. For instance, a cyclo turbine with a diameter of 2m designed for a wind speed of 11m/s will have blades rotating at Vb=16m/s (if TSR=1.5), resulting to a centrifugal force of (Vb²/R=256m/s²) ie 25G.

Centrifugal forces, complexmechanical system, heavy actuators and maintenance costs are the main barriers of this technology.

Troposkine shape

H shape (straight)

Hélicoïdal shape

Hyperboloïdal shape

The Darrieus wind turbines

Georges Darrieus (1888 - 1979) is a french engineer who has designed the patented vertical axis wind turbine fitted with NACA shape blades. The blades catch and harvest wind energy when going through the wind,

This technology is in the range of fast wind turbine (2<λ<5), that enable to work with high Reynolds numbers (> 10⁶). This permits to optimise the aerodynamic behaviour of the blade (high lift, low drag). The resulting lift-to-drag-ratio can easily reach a value of 20 which is tipically the range of ratio in aircraft, fan or large industrial wind turbines. This ratio enable to get correct efficiency of the system : the lift is the force that will speed up the rotor, although the drag  is the force that will generates losses.

Thanks to this high Reynolds, number, the global efficiency of the wind turbine is improved compared to the "low speed wind turbine family" (savonius, windmill, western farm pumping wind turbines...) and the technology suits with efficient wind power harvesting solutions and electricity production. But large Reynolds number implies relative high blade speed and so high centrifugal forces.

In 1920, When Mr Darrieus developped the first prototype, the engineer uses cables and wood to make the blades, in a bowed shape called "troposkine". The centrifugal forces were correctly absorbed thanks to this shape.

Then the new high performance aluminium alloy permits to make the blades in a straight shape (H shape wind turbines), then in helicoidal shape thanks to composit, and now within an hyperboloidal shape using high density composit structures.

The advantage of the wind turbines is the global efficiency, that could theoritically reach 59%. The rotating speed enable to actuate directly a genrator, which is mandatory for electric energy production. And the architecture, with only one rotating part, without nacelle, make it suitable when maintenance is a criteria, for instance in decentralised energy production, or in offshore applications.

Comparatively to the Savonius wind turbine that shall be oversized to resist to tempests, the Darrieus wind turbine can be light, and better resists to high speed winds. Following the same calculation as for Savonius, a Darrieus wind turbine developping 1KW at 40km/h with an efficiency of 25% will have a wet area of approx 3m². assuming a rotor of 1.8m x 1.8m, fitted with thin blades with low drag, the 200km/h tempest will blow and generate a thrust of approx 1140Kg, a folding torque of 600N.m and it will require a brake of 20N.m.

For the same power, a Darrieus wind turbine will be much more light and the structure of the machine is downsized compared to Savonius system.

Complex aerodynamic behaviour of Darrieus wind turbine

Dynamic stall

In the blade system, the wind comes from a point ahead the blade, that describes a circle.

Aerodynamic description of Darrieus VAWT

The Darrieus-type VAWT have a very particular aerodynalic behaviou, largely different from horizontal axis design. Blade crosses the wind, and harvest energy during this step. Then the blade turns and come back to the original point with nearly no energy captation.
Some theoritical approaches make think that blade can caught energy when coming back, but that is false. There is no energy in the wind downstream the first path due to turbulence generated by the blade.

The best way to explain the aerodynamical behaviour of the blade is to consider a the coordinate system of the blade. In this system, the wind is see as the addition of the vectof dispacement (-Rω) and the vector wind (V) that rotates on the blade system : .

Thnaks to this consideration, we clearly understand that when rotative speed is high enough, (Rω >> V), the blade will see an incident flow globally arriving from a point ahead the blade. The lift of the blade will generate the mechanical torque on the shaft.

This analysis results in some main conclusions for the Darrieus wind turbines :
-They are not designed to be self start.
-They will see oscillating efforts.
-Regulation is complex
-Cp curve is asymetric : the effciiency is better at high TSR (λ)
-H shape Darrieus wind turbines shall resists to strong variation of torque.

Starting from the previous theoritical approach, we can estimate the lift and the drag of the blade, resulting in the following Cp formula :

With i( θ) = incidence 

R(x) : The radius of the bade, l is the Cord, Cl and Cd the lift and the drag.

Unfortunately, the formula is useless since the Cl and Cd are curves baed on stationary studies of profiles. For Darrieus Wind turbines, the air flow are permanently changing and implies non-stationary calculation. Worst, the optimum working situation is above the stall limit, which is considered as the limit of models.

Effectively, when stall limit is reached, there is a short time during which the lift is very high. This lift bonus is called delayed stall. This effect has been understood for 10 years and explain how the insect fly.

The same effect is used in Darrieus Wind Turbines. Progress in understanding of this phenomena opens large improvement in Darrieus technology.

The blade materials

The difficult design of the Darrieus VAWT comes from the blade, that shall resists to centrifugal forces. The aerodynamic models os these turbines usually considers that the air flow vectof shall oscilate between -20° and +20°, which correspond with a TSR close to 3 (λ=3).

For a VAWT that is optimised on a wind of 11m/s, like most of the wind turbines, the speed of the blade will turn at 33m/s (TSR=3, wind = 11m/s => Rω=33m/s).

So, the centrifugal forces applied on the blade will be very high  : a = Rω² = (Rω/V)²*V²/R = (λV)²/R ≈ 1100/R. And the centrifugal force will be applied in flexion (and not as a tension in horizontal axis wind turbines). For a VAWT with a diameter of 2m, the centrifugal acceleration will be above 100G.

In the begining of the XX century, Mr Darrieus designed a wind turbine in troposkine shape (like a jumping rope curve), and placed steel cable inside wooden blades. This was the best solution to resists to centrifugal forces. New materials (high resistance aluminium alloy, composit...) make possible to design H shape wind turbines, then helicoidal and now hyperboiloidal shape vertical axis wind turbines.

EOLIE developepd a unique and patented manufacturing process to obtain resistant, thin and efficient blades.

End of blades of horizontal axis wind turbines moves at 280Km/h

The wingtip vortex is the origin of noise. Hyperboiloidal VAWT avoid this noise.

The most sillentfull wind turbines.

The noise of wind turbines mainly comes from the wingtip vortex. This phenomena is well known in aircraft. It consists in a vortex generated at end of wing, due to air in intrados (overpreasured) that wants to move to extrados area (underpreasured). this vortex makes a lot of noise, and losses.

In horizontal axis wind turbines, the TSR is around 7 (λ≈7). ie : for a wind of 11m/s, the blade speed will be 77m/s = 280Km/h. The very high speed of end of blade is the origin of a big wingtip vortex, and so noises and losses.

In verital axis wind turbines, the TSR is around 3 (λ≈3). ie : for a wind of 11m/s, the blade speed will be 33m/s = 120Km/h. THe lower blade speed results in very sillentfull technology. Moreover, the VAWT designdes by EOLIE are fitted with a housed generator, in such a way to reduce electrical noises. This particula design makes the EOLIE wind turbines much more sillentfull than others.

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