1. VAWT Model
Design ConditionsProject Definition
Vertical Axis Wind Turbines (VAWTs)
The purpose of a wind turbine is to transform the kinetic energy of
the wind into mechanical energy by rotating a rotor, and then transfer
this mechanical energy into electrical energy with the help of a
generator. VAWTs are characterized for having several blades parallel
to a central vertical shaft, around which they spin.
Typically, all modern wind turbines work using airfoil-shaped blades
to generate lift as the wind passes over them. A component of this lift
is translated to rotational force in the shaft (torque), allowing the
movement to take place.
In VAWTs, the direction of the relative wind coming to the blade is
always changing due to the rotation of the airfoil around the main
axis. This results in a constant variation of aerodynamic forces, lift and
drag. However, the overall torque provided is positive.
Both VAWTs and HAWTs (Horizontal Axis Wind Turbines) have advantages and are best suited for different
installation 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 the
ground.
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, which
reduces 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 smaller
and more compact, are generally best suited for urban environments, where available space is limited, and
the wind speed is low with frequent changes in direction due to surrounding buildings and other obstacles.
Testing
The main objective of this project is to test the performance of a vertical axis wind turbine (VAWT) model in
urban environment conditions, taking as a case of study the city of London (for which average
meteorological data will be used as design parameters), and study possible modifications that could be
implemented in order to increase the system efficiency. Consequently, the primary goal was to accumulate
enough experimental data to use analytically to support any operational or design changes.
The model itself is a 3-blade
darrieus VAWT which was
previously constructed and
modified in former university
projects. The system inte-
grates a cowling augmenter
which is designed to direct
the airflow into the turbine in
a more optimal angle for the
blades, improving with this
the overall efficiency. The
system will be tested with
and without this augmenter
to verify its functionality. A
preliminary design of the
model was done using
SolidWorks computer design
software.
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 at
the university aerodynamic laboratory. Hence, another way of measuring the output torque that the turbine develops was necessary in
order 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. These
conditions will be related with the average winds in the area where the system is going to be installed, in
this case an urban environment. Therefore, the design may vary slightly from city to city, as the wind
conditions 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 buildings
or trees, which can be of great importance in cities while setting up wind turbines, due to a reduction in the
wind speed of the area.
Measures of the model
Theoretical estimation Conclusions
The calculation of the theoretical power the turbine can extract from the wind starts
with calculating the kinetic power contained in the free flowing wind stream itself. The
most relevant factor is the wind speed (V). Slight variations in the forthcoming wind
deeply modify the power available in the current.
The kinetic power in the wind stream that goes through the turbine, considering an
average 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 kinetic
power in the wind for the coefficient of performance, or power factor. The maximum
physical achievable power factor for wind turbines is 59%, and it is designated as the
Betz limit. Nevertheless, in practice, values of obtainable power from the wind are in the
range of 45% for HAWT and 35% for Darrieus rotor VAWTs as the one of this project.
The solution adopted was adding a resistive torque
in the driveshaft with the help of a slip belt (or
friction belt) made with a string and loads hanging
from its end. This way, the belt acts as a band
brake.
The resistive torque added via the friction belt
simulates the resistive torque produced by the
generator during power extraction in a real
application.
The torque exerted by the friction belt is calculated according to the principles of
band 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 the
coefficient of friction, which was suppose of 0.3, the contact angle (several ones were
used during testing) and the tension in one of the ends of the belt.
Once the problem of the torque and power
measurement was resolved, and the design
conditions selected, the rig was set up for testing
in the wind tunnel. The turbine was tested at
different blade’s angles of attack, loads and wind
speeds, with and without the augmenter.
RESULTS
The performance of the turbine is characterized for a slow increment of
the turbine’s rpm with wind speed at the beginning, near the starting
wind speed, which then, after a certain point, changes to an each time
faster acceleration of the turbine as the wind speed continues rising.
Many times, the experiment had to be stopped for safety due to the risk
of breakdown that was possible from the high vibrations that appeared
in the turbine when it surpassed 300 rpm. This behavior varies
substantially 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 new
torque and power measurement devices convenient to the scale of the turbine). Although the attained results are likely to be highly imprecise, some good
ideas 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 around
1000rpm, for an operational wind speed limited to 10m/s, would have the best performance for this model according to estimations. Future development of
the 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 same
amount 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 rotating
axis 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 more
designs 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 Reynolds
number. In addition, benefits would be obtained from mounting VAWTs on top of tall buildings instead of at ground level, where wind speeds are higher and
turbulences and obstacles fewer. Considering the design situation of the system outdoors, white would be an ideal color for the system since it avoids many
thermal problems and does not stand out in an urban environment.
Therefore, our model should theoretically
be able to generate a maximum power of
5W under the same average wind speed of
6m/s as before.
For this project, design conditions for the city of
London will be used, for which a lot of
meteorological 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