VAWT Model

Design ConditionsProject Definition

Vertical Axis Wind Turbines (VAWTs)The purpose of a wind turbine is to transform the kinetic energy ofthe wind into mechanical energy by rotating a rotor, and then transferthis mechanical energy into electrical energy with the help of agenerator. VAWTs are characterized for having several blades parallelto a central vertical shaft, around which they spin.

Typically, all modern wind turbines work using airfoil-shaped bladesto generate lift as the wind passes over them. A component of this liftis translated to rotational force in the shaft (torque), allowing themovement to take place.

In VAWTs, the direction of the relative wind coming to the blade isalways changing due to the rotation of the airfoil around the mainaxis. This results in a constant variation of aerodynamic forces, lift anddrag. However, the overall torque provided is positive.

Both VAWTs and HAWTs (Horizontal Axis Wind Turbines) have advantages and are best suited for differentinstallation scenarios.

Advantages of VAWTs:

- They can be packed closer together since they generate less surrounding turbulence.- They are omnidirectional and therefore do not require orientation.- The generator can be located at ground level.- They usually start generating power with lower wind speeds, allowing them to be installed closer to theground.

Disadvantages of VAWTs:

- Generally less efficient than HAWTs.- More sensitive to off-design conditions, sometimes presenting stalling and dynamic stability problems.- The oscillatory nature of the torque results in vibration problems and fatigue in its components, whichreduces the life span.- They are not self starting (this problem is solved when they include an augmenter)- Design complications to add a pitch control system, for which the operational range is reduced.

Vertical axis systems, since they normally work the best in reduced wind speed conditions, and are smallerand more compact, are generally best suited for urban environments, where available space is limited, andthe wind speed is low with frequent changes in direction due to surrounding buildings and other obstacles.

TestingThe main objective of this project is to test the performance of a vertical axis wind turbine (VAWT) model inurban environment conditions, taking as a case of study the city of London (for which averagemeteorological data will be used as design parameters), and study possible modifications that could beimplemented in order to increase the system efficiency. Consequently, the primary goal was to accumulateenough experimental data to use analytically to support any operational or design changes.

The model itself is a 3-bladedarrieus VAWT which waspreviously constructed andmodified in former universityprojects. The system inte-grates a cowling augmenterwhich is designed to directthe airflow into the turbine ina more optimal angle for theblades, improving with thisthe overall efficiency. Thesystem will be tested withand without this augmenterto verify its functionality. Apreliminary design of themodel was done usingSolidWorks computer designsoftware.

OUTPUT TORQUE AND POWER MEASUREMENT (PROBLEM SOLUTION)

At the beginning of the experimentation stage, a problem was found in the lack of appropriate torque and power measurement devices atthe university aerodynamic laboratory. Hence, another way of measuring the output torque that the turbine develops was necessary inorder to calculate the power that could be extracted from it.

The VAWT has to be optimized to generate the maximum possible power in working conditions. Theseconditions will be related with the average winds in the area where the system is going to be installed, inthis case an urban environment. Therefore, the design may vary slightly from city to city, as the windconditions depend much on the geographical zone of the world in which the city is located.

As can be observed, average wind speed in the zone of London is between 5 and 7 m/s at 25 meters height.Nevertheless, this is without taking into consideration the presence of several obstacles, such as buildingsor trees, which can be of great importance in cities while setting up wind turbines, due to a reduction in thewind speed of the area.

Measures of the model

Theoretical estimation ConclusionsThe calculation of the theoretical power the turbine can extract from the wind startswith calculating the kinetic power contained in the free flowing wind stream itself. Themost relevant factor is the wind speed (V). Slight variations in the forthcoming winddeeply modify the power available in the current.

The kinetic power in the wind stream that goes through the turbine, considering anaverage altitude of 50m above sea level and wind speed of 6m/s, is approximately 15W.Theoretical power that the turbine may generate is calculated multiplying this kineticpower in the wind for the coefficient of performance, or power factor. The maximumphysical achievable power factor for wind turbines is 59%, and it is designated as theBetz limit. Nevertheless, in practice, values of obtainable power from the wind are in therange of 45% for HAWT and 35% for Darrieus rotor VAWTs as the one of this project.

The solution adopted was adding a resistive torquein the driveshaft with the help of a slip belt (orfriction belt) made with a string and loads hangingfrom its end. This way, the belt acts as a bandbrake.

The resistive torque added via the friction beltsimulates the resistive torque produced by thegenerator during power extraction in a realapplication.

The torque exerted by the friction belt is calculated according to the principles ofband brakes. For this, the Eytelwein’s Formula, more known as the ‘Capstan Equation’is used. This expression allows us to calculate the brake torque as a function of thecoefficient of friction, which was suppose of 0.3, the contact angle (several ones wereused during testing) and the tension in one of the ends of the belt.

Once the problem of the torque and powermeasurement was resolved, and the designconditions selected, the rig was set up for testingin the wind tunnel. The turbine was tested atdifferent blade’s angles of attack, loads and windspeeds, with and without the augmenter.

RESULTS

The performance of the turbine is characterized for a slow increment ofthe turbine’s rpm with wind speed at the beginning, near the startingwind speed, which then, after a certain point, changes to an each timefaster acceleration of the turbine as the wind speed continues rising.Many times, the experiment had to be stopped for safety due to the riskof breakdown that was possible from the high vibrations that appearedin the turbine when it surpassed 300 rpm. This behavior variessubstantially at different blade’s pitch angles and brake torques applied.

The data obtained by this primitive technique resulted in having more inaccuracy than anticipated (it would be suggested that the university purchases newtorque and power measurement devices convenient to the scale of the turbine). Although the attained results are likely to be highly imprecise, some goodideas regarding overall performance of the system were extracted:

- The performance of the system could be much higher if the turbine was able to reach a higher rpm, and also tsr (tip speed ratio). Turbine speed around1000rpm, for an operational wind speed limited to 10m/s, would have the best performance for this model according to estimations. Future development ofthe VAWT model should either aim to achieve this range or build a bigger model (which, due to the scale, would not have to spin so fast to achieve the sameamount of tsr).

- For designed wind speed average of 5 to 7 m/s, positive pitch angles seem to achieve higher efficiencies, this is, with the leading edge further to the rotatingaxis than the trailing edge.

- The efficiency of the system improves with the use of the augmenter in this tsr range. However, further study should be done at higher rpm and moredesigns considered in order to find the optimal solution.

- Should future installation of the prototype done in urban environments, a bigger model would present better performance due to a higher Reynoldsnumber. In addition, benefits would be obtained from mounting VAWTs on top of tall buildings instead of at ground level, where wind speeds are higher andturbulences and obstacles fewer. Considering the design situation of the system outdoors, white would be an ideal color for the system since it avoids manythermal problems and does not stand out in an urban environment.

Therefore, our model should theoreticallybe able to generate a maximum power of5W under the same average wind speed of6m/s as before.

For this project, design conditions for the city ofLondon will be used, for which a lot ofmeteorological data is available:

VAWTs for urban environments open the door to greener cities in the future

Cleanfield’s 3kW VAWT

CAD model

Real model in the aerodynamic lab

A Darrieus wind turbine