vertical axis wind turbine - a study
TRANSCRIPT
A Study of Vertical Axis Wind Turbines
Contents
1. Introduction 1
2. Wind is a Natural Phenomenon 2
3. History of Vertical Axis Wind Turbines 3
4. Why Vertical Axis turbine? 4
5. Energy in Wind 5
6. Types of Vertical Axis Turbines 6
7. Materials Used 9
8. Areas of application 10
9. Conclusions 10
10. References 11
1. Introduction:
As we all know, the demand for energy is increasing exponentially as more and more people
inhabit this earth. At the same time, the energy source that met all of our needs in the past
decades, non-renewable in nature, seems to decrease in equal pace leaving us at the door steps of
renewable energy sources for help as we go forward in our journey in to the future. Due to this
inevitable need, many scientists and researchers are turning back to the essential elements of
nature which before the oil and gas came forcefully into the picture, helped humanity meet many
of their energy needs. Water, Sun and Wind have been proven to be excellent sources of energy
in the past centuries. Besides being renewable in nature, these energy sources do not seem to hurt
the nature and seem to keep the chemical balance of the atmosphere intact.
In ancient times sailors used wind energy to propel their ships and farmers used windmills to
pump ground water and power grain grinding apparatus. In recent times, technology has shown
us more ways to use this wind energy including producing electricity which can be used for
variety of applications. Over the past few years the use of wind energy in this manner has
increased approximately 25 percent a year. However, compared the energy available in wind that
encompasses our earth, what humans harness today is a very small. Wind energy is captured
using wind turbine technology where the wind energy is converted in to mechanical energy and
to generate electricity. Configuration of wind turbines can be categorized into two main types,
Horizontal Axis Wind Turbines (HAWT) and Vertical Axis Wind Turbines (VAWT). In
HAWTs the axis of rotation of the turbines is horizontal and these are ideal for producing high
rotational speeds and low torques. HAWTs are typically used for producing large quantities of
energy. On the other hand, VAWTs are used for producing various quantities of energy with low
rotational speed and high torque turbine rotation.
2. Wind is a Natural Phenomenon:
Though wind energy seems to be originated from a source not related to the sun, on the contrary,
it all starts from sun heating the earth surface and the elements around it namely land, water and
the wind. This heating of the earth causes pressure differences between different areas on land
and becomes a reason for wind movement between these areas. Besides the temperature
difference between different areas on land, different thermal properties of land and the sea also
contributes to this phenomenon. Seas
ability to maintain the temperature for
longer times than that of land caused a
substantial temperature difference
between the land and the sea during both
daytime and night time. This then causes
sea breezes which contributes to the
above mentioned wind movement.
Moreover, the altitude differences
between the valleys and the mountains
due to density differences in the air also cause wind flow between these two areas. Therefore one
of the early installations of wind turbines took place near mountains. Due to above mentioned
reasons, wind moves in different direction in different parts of the earth surface. Typically this
Figure 1: Wind generation [1]
wind movement could occur in different cell patterns namely, Hadley Cell which are also called
trade winds, Ferrel Cell and Polar cell [1]
. A typical wind turbine may experience one or more of
these wind cell patterns at a given time. For the consistence of the power generation, one should
carefully evaluate the presence of these patterns in the area of proposed turbine installation.
3. History of Vertical Axis wind turbines:
Windmills are not a new technology, they are found
to be used in Persia as early as 200 BC. The wind
wheel of Heron of Alexandria is the earliest
documented machines that were powered by wind[2]
.
However, the earliest working windmills were
Vertical Axis Wind Mills that are believed to be
used in the region next to present day Iran in around
7th
century. These windmills were primarily used for
pumping water and grinding grain.
In11th and 12th
centuries, Europe began using
windmill technology which then started spreading to countries such as Denmark and Syria [3]
.
However, the first wind turbine that was used for battery charging and in essence used for
generating electricity was installed in July 1887 by Scottish scientist James Blyth. Shortly,
following the above, an American inventor built first wind turbine that could produce electricity
and can be operated automatically. By 1900s Denmark built approximately 2500 wind turbines
which are intended to use to provide mechanical power to various pumps and mills. These
turbines are estimated to produce a combined peak power of 30 MW. Each of these turbines was
Figure 2: Various turbines used in history [1]
approximately 80 ft in diameter. At this same time US built many electricity generating wind
turbine with capacities ranging from 5kW to 25kW. By the end of World War 1, US was
producing approximately 100, 000 farm wind mills each year that can be used for pumping water
and electricity. In 1930s fist 100kW generator was built by Russian scientist and then in 1940s,
first Mega Watt generator was built by an American which then was successfully synchronized
to a utility grid.
4. Why Vertical Axis Wind Turbines:
Vertical axis wind turbines (VAWTs) in addition to being simpler and cheaper to build have the
following advantages [4]
:
They are always facing the wind - no need for steering into the wind.
Have greater surface area for energy capture.
Are more efficient in gusty winds
VAWT s can be installed in more locations such as on roofs, along highways and in
parking lots.
Do not kill birds and wild - life - slow moving and highly visible.
VAWT s can be used to produce electricity in various amounts, from mill watts to
megawatts.
Can be significantly less expensive to build
Can have low maintenance downtime.
Produce less noise - low speed means less noise
Are more esthetically pleasing
Most wind farms consist of horizontal axis wind turbines (HAWTs) due to the high power
coefficient (mechanical power output divided by the power of the free-stream air through the
turbine cross-sectional area) of an isolated turbine. However when in close proximity to
neighboring turbines, HAWTs suffer from a reduced power coefficient. In contrast, previous
research on vertical axis wind turbines (VAWTs) suggests that closely spaced VAWTs may
experience only small decreases (or even increases) in an individual turbine’s power coefficient
when placed in close proximity to neighbors, thus yielding much higher power outputs for a
given area of land.
Wind turbines with vertical axes have been identified to be superior to wind turbines with
horizontal axes in urban wind conditions that are characterized by winds of high turbulence and
frequent change of wind direction. Results from a computational study have been presented that
indicate that a vertical-axis wind turbine with helically twisted blades can significantly reduce
the environmental impact through noise and vibration compared to a vertical-axis wind turbine
with untwisted blades.
5. Energy in Wind:
We know that Kinetic Energy = 0.5 m*V
2
Where
m = mass (kg)
V = velocity (m/s)
Energy (Joules) = kg m2 / s2
Energy in a finite mass of air moving at a certain
velocity:
Kinetic Energy of a finite mass of air = 0.5 mV2 Figure 3: Energy in wind [1]
Power produced by a finite mass of air:
Energy (Joules) / time = power (Watts)
Let us assume that the finite mass of the air in consideration is passing through a hoop where it
transfers its energy to an imaginary plate place at the entrance of the hoop.
Mass of air going through the hoop per second: ρ*A*V
Where
A = cross sectional area of the hoop
ρ = air density (mass/volume)
Power of a mass of wind blowing through a HOOP at a certain rate (velocity): 0.5 ρ*A* V3
Energy Conversion Efficiency: We cannot convert all of the power in the wind to Mechanical
(then electrical) power. According to Belts, Maximum theoretical power conversion efficiency
(wind to mechanical) is 59% (0.59, Beltz limit).
Maximum mechanical power extracted from wind blowing through a HOOP at a certain rate
(velocity): 0.59 * 0.5* ρ* A* V3
The losses are typically caused by aerodynamic drag, frictional losses due to high wind velocity
and wind turbulence.
6. Types of Vertical Axis Wind Turbines:
Vertical Axis machines were the first wind turbines that are used in ancient time due to its
simplicity and ease of building. As the centuries passed, various researchers and inventors
refined and reinvented the designs to meet various types of energy needs. Following are the few
broad categories that a typical Vertical Axis Wind Turbine may fall into [1]
.
1. Drag type: Where the thrust force of the wind is used to push the wind turbine to rotate.
This is inefficient way of converting wind energy into mechanical energy.
2. Lift type: This type of wind turbines use blades with aerofoil design and therefore are
very efficient. They use the lift created by the interaction of blade profile and the wind
force to convert wind energy into mechanical energy.
3. Hybrid: This type of wind turbines try to take advantage of elements that come from both
the above mentioned technologies.
Figure 4: Various types of Vertical Axis Wind Turbines [1]
Moreover, Vertical Axis Wind Turbines are also categorized
into the following types depending on the types of blades
used on the turbine and these types are named according to
their makers [5, 6]
.
Eggbeater or Darrieus wind turbine: These turbines were named after the French inventor,
Georges Darrieus. Though they are very efficient, they produce large torque ripples and cyclical
stress on the tower, which contributes to poor reliability. They also generally require some
external power source, or an additional Savonius rotor to start turning, because the starting
torque is very low. The torque ripple is reduced by using three or more blades which results in
greater solidity of the rotor. It consists of blades which are
shaped like loops which connect to the top and bottom of the
axle.
Giromill: This type actually is a subtype of Darrieus turbine with straight blades. Among the
different varieties, the cycloturbine variety has variable pitch to reduce the torque pulsation and
is self-starting. The advantages of variable pitch are: high
starting torque; a wide, relatively flat torque curve; a lower
blade speed ratio; a higher coefficient of performance; more
efficient operation in turbulent winds; and a lower blade speed
ratio which lowers blade bending stresses.
Savonius wind turbine: These are drag-type wind turbines with
two or more scoop type blades, these scoops look like Flettner
vents. They are always self-starting if there are at least three
scoops. They sometimes have long helical scoops to give a
smooth torque. Savonius Wind Turbine mills usually consist of
two scoops which convert wind power into torque on a rotating
shaft.
Figure 5: Darrieus Vertical-Axis
Wind Turbine
Figure 6: Giromill Vertical-
Axis Wind Turbine
Figure 7: Savonius Vertical-
Axis Wind Turbine
Besides these basic types, in recent times several evolution of these basic types with enhanced
mechanisms have arrived on the scene. Just to mention few, Vertical Axis Turbines with aerofoil
profile and turbines made of sails which can be magnetically controlled in order to control the
rotational speed of the turbine.
7. Materials Used for Wind Turbine Manufacturing:
Various types of materials are used in the making of wind turbines. There are substantial
differences between small and large machines and there are projected changes in designs that
will accommodate the introduction of new material technologies and manufacturing methods.
The estimated materials use in small and large turbines is shown in table below. To arrive at a
total, the material usage is weighted by the estimated market share of the various manufacturers
and machines types. The trends in design and manufacturing differ between small and large
turbines. Small machines tend to use lighter weight castings in an effort to reduce costs. Many
parts are die cast aluminum in small turbines, while in large machines steel castings or forgings
are needed to meet strength and structural fatigue requirements. The size of steel castings for
large turbines, especially the blade hub units, is one of the manufacturing challenges [7]
.
Table 1: Percentage of materials used in components [7]
Notes:
1. Small turbines with rated power less than 100 kW- (listed in italics where different)
2. Assumes nacelle is 1/3 gearbox, 1/3 generator and 1/3 frame & machinery
3. Approximately half of the small turbine market (measured in MW) is direct drive with no
gearbox
4. Rotor blades are either glass reinforced plastic, wood-epoxy or injection molded plastic with
carbon fibers
Material fatigue properties are an important consideration in wind turbine design and materials
selection. During the expected 30 year life of a wind turbine, many of the components will need
to be able to endure 4 x 108 fatigue stress cycles. This high cycle fatigue resistance is even more
severe than aircraft, automotive engines, bridges and most other man-made structures.
8. Applications:
Typical vertical axis wind turbines can be used in variety of applications including:
1. Domestic Electricity production
2. Electricity generation at locations such as camping sites
3. Water pumping applications
4. Wind farm applications
5. Urban area commercial power generation
6. Portable wind turbine applications
7. Power generation in rural areas where extending electric conventional electric grid is very
expensive.
9. Conclusions:
Vertical axis wind turbines are very promising systems that are practical and clean sources of
energy especially for small scale energy needs and are also ideal for places where there are space
constraints. As more and more institutions are involved in research to make these wind turbines
affordable, one could hope that the people groups in poorer nations, who do not have access to
electricity today would one day enjoy the benefits of electricity. In the process we all, one day
may considerably minimize the size of our energy use carbon foot print.
References
1. Boyle, G. "Renewable Energy Power for a Sustainable Future." 2004.
2. Drachmann, A.G. "Heron's Windmill." Centaurus 1961: 145–151.
3. Hill, Donald Routledge. "Mechanical Engineering in the Medieval Near East." Scientific
American May 1991: 64-69.
4. Sankar, Terrence C. "The Case for Vertical Axis Wind Turbines." n.d.
5. Agarwal, Abhishek. www.articlebase.com. 17 February 2009.
<http://www.articlesbase.com/environment-articles/7-types-of-vertical-axis-wind-
turbine-benefits-available-774696.html>.
6. Smith, Rich. www.symscape.com. 4 June 2007.
<http://www.symscape.com/blog/vertical_axis_wind_turbine>.
7. McVeigh, Dan Ancona and Jim. "Wind Turbine - Materials and Manufacturing Fact
Sheet." n.d.