Kun Shan University Undergraduate school of Mechanical Engineering Final Project A New Venturi Type - Bladeless Wind Turbine Design Coupled to a Permanent Magnet Generator For Small Scale Electricity Generation Undergraduate Students: Rodyn Gilharry 吉洛庭 Carlos Campos Saravia 甘伯斯 Advisor: Song-Hao Wang 王 松 浩 June, 2014
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final project 2ir.lib.ksu.edu.tw/bitstream/987654321/21639/3...1 Abstract: This Paper presents a design of a bladeless wind turbine that uses the Venturi Tube/Bernoulli's principle
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Kun Shan University
Undergraduate school of Mechanical Engineering
Final Project
A New Venturi Type - Bladeless Wind Turbine Design Coupled to a Permanent Magnet Generator For Small
This Paper presents a design of a bladeless wind turbine that uses the Venturi Tube/Bernoulli's principle to amplify the ambient wind speed so that it may drive a permanent magnet generator. Thus producing energy in areas where harvesting wind power would have not been profitable before. This design incorporates a direct drive method so that the total number of moving parts are reduced, eliminating the need for a gear box and making the maintenance of the wind turbine much easier. The magnet generator used is a coreless one, reducing magnetic drag (cogging torque) thereby increasing the energy output. This design also address many of the environmental concerns presented by conventional wind turbines such as the production of noise and the threat to birds and wildlife.
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1. Introduction:
Today most of the electricity produced in the world is obtain from the burning of fossil
fuels (see figure 1). This source of energy is not only non-renewable and rapidly depleting but
also causes numerous environmental concerns that in the long run cause more problems than
benefits. Now, because global warming is no longer a warning but an occurrence and its
disastrous effects are being felt globally, most countries are investing more heavily in green
energy. Green energy is obtained from renewable sources and even though they are described as
'Green' that does not necessarily mean that they have no environmental impact. Nuclear energy
produces radioactive waste that takes thousands if not millions of years to decompose. They have
to be stored in a secure facility in boxes that are tested to be 'indestructible', all of this leads to a
lot of spending in the R&D departments. Hydro power is also another great source of energy but
is accompanied by numerous environmental effects. Large Valleys and fields need to be flooded,
completely eliminating the ecosystems in those areas, and also with water shortages and longer
droughts occurring each year, water might soon become too valuable for it to be used as a source
of green energy. The best other renewable energy source to be exploited is Wind Energy. Wind
energy is a very clean energy that poses little environmental risks as can be seen in Table 1.
Fig. 1. Comparison of Energy Sources and Their Usage in Terawatts, 1965-2005 [3]
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Table 1
Comparison of habitat impacts of wind energy to other energy sources [3].
The world's energy demands are ever increasing (see figure 2), and so the demand for
more reliable and efficient wind turbine systems are also increasing. The clean energy trend that
started mainly in the United States and Europe is now spreading to countries all over the world
with millions of watts of clean energy being supplied to large cities (see figure 3).
Fig. 2. World Energy Demand Growth [3]
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Fig. 3. Top Ten Wind Power Generating Countries by December, 2011 [2]
One of the main advantages of wind power generation systems in their adaptability, they
can be built in for industrial purposes and many large turbines can be built together side by side
forming large wind mill fields, or they can be made as smaller individual units for domestic use.
This way towns, villages, farms, factories or any person in a remote location can install their own
domestic wind mill and harvest wind power. Individual power units or domestic power units
might be one way in which energy is generated in the future removing the numerous problems
associated with a centralized power station and eliminating miles of power cables that are
currently criss-crossing large cities and towns. Most of the recent designs of small scale wind
turbines incorporate the uses of a Permanent Magnet generator.
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1.1 Permanent Magnet- Direct Drive Generator
About 1.6 billion people lack access to electricity and many of these are in rural areas so
there is a large potential market for isolated small wind turbines, these wind turbines are usually
less than 5KW and are located in remote areas which are not connected to the main power grid
system [30] [17]. Permanent magnet generators are suitable for small scale wind turbines
because they have a high efficiency, they are categorized into two types because of the direction
of the magnetic flux, the radial type and the axial type (see figure 4).
Fig. 4. The direction of the magnetic flux from the rotor of axial and radial types [22].
Permanent Magnet generators are usually more efficient because of the fact that field
excitation losses are eliminated resulting in major rotor loss reduction, thus high power density is
obtained. Also PM generators have small magnetic thickness which results in smaller dimensions.
For the sake of mechanical simplicity most designs incorporate a direct couple or direct drive
system thereby eliminating the need for gears or complex mechanical parts, eliminating gearbox
failures prolongs the life of the wind turbine and also saves on cost and weight [14] [16] [22] .
Figure 5 shows the general setup of an axial flux permanent magnet generator.
Fig. 5. General arrangement for an Axial Flux Permanent Magnet Generator [17]
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However, PM generators also have some disadvantages, they do not posses field
excitation control and therefore voltage regulation can be a problem. Using external voltage
control such as large capacitor banks or choosing the turns on the stator windings properly to
produce the anticipated required nominal voltage can be used to correct the problem. Also since
the permanent magnet fields cannot be turned off, there exists the risk of excessive currents in
the case of an internal fault. This could also be corrected by the incorporation of a turbine
governor or dynamic breaking [19]. The Comparison of a PM generator to a conventional wound
rotor type is shown below in Table 2.
Table 2.
Comparison of Wound Rotors and PM Generators
Another Disadvantage of PM generator is Demagnetization. The magnets can become
partially demagnetized by over current or excessive temperature or a combination of both [30].
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1.2 Negative Effects of Wind Turbines
Even though wind turbines are not thought to pose any major environmental hazards,
there has been extensive studies done on the allegation of Noise Pollution and other assessments
on the effects of large wind turbines on people and their surrounding area. Studies have
concluded that the main negative effects can be listed as follows:
(1) Visual Impact
(2) Infrasound and Noise
(3) Impact on Wildlife especially birds
(4) Shadow Flicker and Blade Glint
(5) Electromagnetic Radiation and Interference
1.2.1 Visual Impact
Although the idea of wind energy is generally well received in public, there is still a
tendency for people not to want a wind farm located close to where they live or other residential
areas. Papers written by Saidur [3] indicate that the visual impact varies according to the wind
energy technology such as colour or contrast, size, distance from residencies, shadow flickering
and the times when the turbines are operational. Recently more and more larger wind turbines
are being built so that they reduce their carbon footprint, this adds to the visual impact since the
towers become more visible from a further distance. The increase of wind turbine size over the
years is represented in Figure 6 with 20MW towers predicted to be built in the near future.
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Fig. 6. Growth in size of commercial wind turbines [11]
1.2.2 Infrasound and Noise
The most critical environmental impact of wind turbines is noise pollution. Noise is
defined as unwanted sound and sound is characterized by its sound pressure level (loudness) and
its frequency (pitch) which are measured in decibels (dB) and Hertz (Hz), respectively. The
normal human hearing range is from 20Hz - 20,000Hz and anything below 20Hz is referred to as
infrasound [3] [29] [31]. Wind turbines generate sound through mechanical and aerodynamic
means.
The aerodynamic noise is present in all frequencies from infrasound to the audible range,
producing the characteristic 'swishing' sound. Mechanical noise is produced from the motor or
gearbox, but if the turbine is working correctly this noise is reduced to a minimum and should
not be an issue. The main concern with wind turbine noise pollution is its impact on human
health, various studies have been carried out and showed that the health problems associated
with infrasound are [31]:
(a) Effects include annoyance, nuisance and dissatisfaction.
(b) Interference with activities such as sleep, speech and learning
(c) Physiological effects such as anxiety, tinnitus or hearing loss.
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The relations between wind turbine and its health effects on the general populous is shown in
figure 7.
Fig. 7. Model of the possible relationships between sound exposure, annoyance, sleep
disturbance and psychological distress [6]
1.2.3 Impact on Wildlife especially birds
Wind energy is one of the sources of energy that is most compatible with wildlife and
people around the globe, however there are some reports of bird fatalities by researchers. The
wildlife impacts can be separated into two categories, direct and indirect impact. Direct impact is
when there are bird fatalities due to collisions with wild turbines while the indirect impact are
avoidance, habitat destruction and displacement. Table 3 shows the bird fatality rate in the
United States in 2011.
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Table 3
Regional and overall bird fatality rates in the United States [3]
1.2.4 Shadow Flicker and Blade Glint
Shadow Flicker occurs when the wind turbine blades rotate in sunny conditions, casting
moving shadows on the ground that results in alternating changes in light intensity that appear to
flick ON and OFF. Blade glint occurs when the surface of the blade reflects the sun's light [29]
[31]. Close to 3% of people with epilepsy are photosensitive and sunlight at flash frequencies
greater than 3Hz has the potential to provoke photosensitive seizures.
1.2.5 Electromagnetic Radiation and Interference
Electromagnetic radiation is a wave of electric and magnetic energy that are
perpendicular to each other and are moving together. Electromagnetic interference from wind
turbines can affect radio communication signals, broadcast radio and television, mobile phones
and radar. Transmission signals from radio or television can get distorted when passing through
moving wind turbine blades. This effect was more present in the first generation of wind turbines
when the blades where made of metal. Today however, most if not all wind turbine blades are
made completely of synthetic material and so this problem is not as pronounced.
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1.3 Advantages of Venturi - Type Design
There are many reason why the venturi - type design (if functional) would be a much
better wind turbine design than conventional designs. The first is that it does not need high wind
velocity. Since the purpose of the design is to accelerate slow moving air then these type of wind
turbines would be able to be placed in areas where wind energy was not considered before due to
slow wind speeds. The second major advantage over the conventional design is that, due to the
bladeless nature of this wind turbine, the noise pollution factor would be basically eliminated, as
well as the shadow flickering and blade glint. The design allows for the total containment of the
turbine rotor which would lead to less aerodynamic noise and also, if noise is still present, to the
addition of noise insulating materials. The third advantage is the elimination if the wildlife
environmental hazard as well. Since there won't be any blades exposed to the environment, bird
fatalities can be reduced drastically. The fourth major advantage is its mobility and life
endurance. Since this design incorporates a housing that contains the rotor and electric generator,
shielding them from the elements, the life of the unit is expected to be prolonged and the unit
very mobile. Making this design type perfect for domestic applications, by placing it on their
roof tops or it can also be scaled up, making large venturi cones out of concrete to withstand high
wind velocities generated inside. There are also many other minor advantages such as low
manufacturing cost, low maintenance etc. but the main advantages for which this design was
created are stated above.
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2. Theory
2.1 Wind Power
The wind power Pw in an air stream with a density and a velocity through a cross -
area is:
A wind turbine will develop a power Pt, and Pt/Pw is the conversion efficiency, sometimes
referred to as the power coefficient, mathematically [26] [30]:
So the Larger the swept area the more wind power will be generated. In the Venturi-Type
design the swept area is not the same as in conventional designs, the area is not like that of a
circle but more of a donut, and also the angle through which the air does work is dependent on
the position of the inlet and outlet pipes as shown in figure 8 below. The swept area would then
be: A = (θ/360) π [(R1)2 - (R0)
2]
Fig. 8. Cross section area for working air in venturi - type wind turbine
R1
R0
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2.2 The Venturi tube
Two equations that were used most extensively throughout this project when coming up
with the design and troubleshooting errors was the Bernoulli's theorem and the equation of mass
conservation or continuity equation as it is sometimes referred to. Bernoulli's theorem is a
statement of conservation of energy for fluid flow and in the case of the venturi tube's internal
flow, this equation is not perfectly accurate as the flow is not completely incompressible but it's
an excellent approximation for the purpose of this design [27]. Mass flow simply explains that
the mass inflow must equal to the outflow and for so if a parameter is increased or decreased
then another parameter must decrease or increase to maintain the conservation of mass.
Therefore if the areas for a given mass flow with velocity V and Pressure P is decreased (i.e. the
flow is constricted), then for the same quantity of mass to pass through a given section in the
same period of time, the velocity of the flow must increase. This in turn reduces the pressure at
the point where velocity is increased.
For and incompressible fluid the Bernoulli's theorem is:
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where (P/γ) is the static pressure head; (V2/γ) is the velocity pressure head and z is the potential
energy head. If the potential energy head is zero then Bernoulli's equation is reduced to
A typical Venturi Tube has a converging cone on one end and a diverging cone on the other (see
figure 9) . The section where the cone is constricted the most is where the velocity is highest.
Fig.9. Venturi Tube [15]
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2.3 The Permanent Magnet generator
A PM generator consists of two rotor discs mounted on either side of a non-magnetic,
non conducting stator. the magnets are generally arranged in a N-S-N-S manner
circumferentially round each rotor plate with the North magnet on one plate facing the South
magnet on the other. Magnetic flux permeates the air space between the rotor disc and travelling
one pole pitch before coming back across the air gap. The flux density distribution is
approximately sinusoidal in both the radial and circumferential directions so that the flux density
profile can be described as a sinusoidal hill, described by the equation [14] :
where ĵn is the magnet equivalent current density given by:
and Brem is the magnet remanence, dm the magnet diameter, τ the pole pitch and un = (πn/ τ). This
flux density profile can now be used to derive the flux between the centre of the armature coil
and the radius ra as:
If it is assumed that the armature coil is concentrated at its mean axial position but that
the coil is divided into three segments a,b and c in the radial direction. Then the total flux linkage
and the coil emf are given by:
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3. ANSYS Settings
All simulations for the wind cones were done using ANSYS fluent. The geometries were
imported from Solidworks as (.IGS) files and a fine tetrahedron mesh was created from the
imported geometries using the mesh modeler. Inlet and Outlet selections were chosen and named
to make the setting of boundary conditions easier when using fluent. The setting for fluent are