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.