26-09-2013, 02:39 PM
MAGNETICALLY LEVITATED VERTICAL-AXIS WIND TURBINE
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ABSTRACT
This project dwells on the implementation of an alternate configuration of a wind turbine
for power generation purposes. Using the effects of magnetic repulsion, spiral shaped wind
turbine blades will be fitted on a rod for stability during rotation and suspended on
magnets as a replacement for ball bearings which are normally used on conventional wind
turbines. Power will then be generated with an axial flux generator, which incorporates the
use of permanent magnets and a set of coils. A SEPIC converter will then be used to
regulate the varying voltage from the rectifier to output a steady DC voltage.
INTRODUCTION
Renewable energy is generally electricity supplied from sources, such as wind power, solar
power, geothermal energy, hydropower and various forms of biomass. These sources have
been coined renewable due to their continuous replenishment and availability for use over
and over again. The popularity of renewable energy has experienced a significant upsurge
in recent times due to the exhaustion of conventional power generation methods and
increasing realization of its adverse effects on the environment. This popularity has been
bolstered by cutting edge research and ground breaking technology that has been
introduced so far to aid in the effective tapping of these natural resources and it is
estimated that renewable sources might contribute about 20% – 50% to energy
consumption in the latter part of the 21st century. Facts from the World Wind Energy
Association estimates that by 2010, 160GW of wind power capacity is expected to be
installed worldwide which implies an anticipated net growth rate of more than 21% per
year [2.1].
OVERVIEW
This section introduces and provides a brief description of the major components and
factors that will contribute to an efficiently functioning wind turbine. These factors are
wind power, the generator, magnet levitation and the DC-DC converter. Later sections will
provide an in-depth look into the essence of each factor and its function and importance to
the overall operation of the vertical axis wind turbine.
A. Wind Power
Undoubtedly, the project’s ability to function is solely dependent on the power of wind and
its availability. Wind is known to be another form of solar energy because it comes about as
a result of uneven heating of the atmosphere by the sun coupled with the abstract
topography of the earth’s surface [3.1]. With wind turbines, two categories of winds are
relevant to their applications, namely local winds and planetary winds. The latter is the
most dominant and it is usually a major factor in deciding sites for very effective wind
turbines especially with the horizontal axis types.
Generator
The basic understanding of a generator is that it converts mechanical energy to electrical
energy. Generators are utilized extensively in various applications and for the most part
have similarities that exist between these applications. However the few differences
present is what really distinguishes a system operating on an AC motor from another on
the same principle of operation and likewise with DC motors. With the axial flux generator
design, its operability is based on permanent magnet alternators where the concept of
magnets and magnetic fields are the dominant factors in this form of generator functioning.
These generators have air gap surface perpendicular to the rotating axis and the air gap
generates magnetic fluxes parallel to the axis. In further chapters we will take a detailed
look into their basic operation and the configuration of our design.
DC-DC Conversion
In order to begin the analysis of DC-DC converters it is important to first understand the
concept behind a converter. Over the years, alternating current has been the common
choice of power supply. AC is popular because the voltage can be easily stepped up or down
using a transformer. Due to the inherent properties of a transformer, DC voltage cannot be
altered using this type of equipment. Transformers operate due to a changing magnetic
field in which the change in magnetic flux induces a current. Direct current cannot provide
a changing magnetic field therefore a transformer with an applied DC input would only
produce heat.
The concept of DC-DC conversion emerged after the development of fast switching
transistors. By varying the duty cycle of the pulse that is applied to the gate of the
transistor for switching, these converters can buck or boost the voltage as if it were a DC
transformer. When accurate feedback back is applied to this type of circuit, the converter
will not only transform a supply voltage to the desired output but also maintain it given a
varying input. These qualities of DC-DC converters are the foundation of the circuit that will
be chosen for this project.
WIND POWER AND WIND TURBINES
Wind Power Technology
Wind power technology is the various infrastructure and process that promote the
harnessing of wind generation for mechanical power and electricity. This basically entails
the wind and characteristics related to its strength and direction, as well as the functioning
of both internal and external components of a wind turbine with respect to wind behavior.
The Power of Wind
As mentioned earlier the effective functioning of a wind turbine is dictated by the wind
availability in an area and if the amount of power it has is sufficient enough to keep the
blades in constant rotation. The wind power increases as a function of the cube of the
velocity of the wind and this power is calculable with respect to the area in which the wind
is present as well as the wind velocity [4.1]. When wind is blowing the energy available is
kinetic due to the motion of the wind so the power of the wind is related to the kinetic
energy.
Coil Design
The number of windings per coil produces a design challenge. The more windings will
increase the voltage produced by each coil but in turn it will also increase the size of each
coil. In order to reduce the size of each coil a wire with a greater size gage can be utilized.
Again another challenge is presented, the smaller
smaller the wire becomes the less current will
flow before the wire begins to heat up due to the increased resistance of a small wire. Each
one of our coils has a measured resistance of 40-Ω;
40 Ω; a smaller gage wire would further
reduce this resistance.
When designing a generator the application, which it will be used for, must be kept in mind.
The question must be answered, which property, the current or the voltage, is of greater
importance? The problem that is produced by a larger coil is the field density is decreased
over the thickness of the coil. The thickness of the coil is what reduces the flux magnitude.
In our design we have chosen to sandwich the coils between two attracting magnets.
3-Phase Connections
A 3-phase connection implies that a generator produces three voltages each with their own
phase angle. A major advantage of the 3-phase connection is that the output allows the
current to peak a different times allowing for smaller more frequent peaks as opposed to
one large peak produced in a single phase connection. These smaller current peaks
produce less vibration as the generator spins. Less vibration leads to less wear on the parts
of the generator such as the bearings.
DC-DC CONVERSION
Circuit Selection
Many different types of DC-DC converters have been designed since their conception. Some
well known types of DC-DC converters are the step-down (buck), step-up (boost) and step-
down/step-up (buck boost). These are non-isolated converters which do not use a
transformer to isolate the input from the output like converters such as the “Fly Back”. In
order to choose the right converter for this application, the parameters of input and output
must be known.
The input power will be driven by the wind, so it is clear that the amplitude will be
unstable because it is going to fluctuate with respect to the wind speed. This variable AC
power that will be provided by the wind generator will have to be rectified so that it is a
relatively stable input for the converter that is selected. The desired output is a steady
power source that will have a constant 5 volts DC and be able to supply enough current to
drive an LED. It can be concluded that the type of converter required for this application
must be able to buck and boost the input supply in order to keep the output steady. A
simple buck-boost converter would be a valid solution for this problem but there are a few
drawbacks to this design. As displayed in Figure VII.1, one of the disadvantages of using
this configuration is that the transistor, which will be modeled as an ideal switch for
simplicity purposes, is not terminated to ground. This will make the circuit design more
complicated because a feedback controller will be required which will control the
switching of the transistor and in turn the amplitude of the output. For the controller to
function properly which will be discussed in more detail later, current sensing is required
at the source of the transistor which needs to be terminated directly to ground in order to
minimize circuit components and to get an accurate current reading. The other
disadvantage to this circuit design is the fact that the output polarity is opposite to that of
the input. This characteristic is caused due to the configuration of the circuit where the
diode is reversed bias only allowing current to flow when the switch is open and the
inductor is supplying the output.