Introduction

In the last decade, we have heard more and more about the need of renewable clean energy, but not much has been done. Today, pollution and greenhouse gases are at its peak all over the world and the price of oil is higher than ever. Our earth is getting very sick, and it is time that we start contributing in fighting pollution by exploiting renewable clean energy. These clean energy consist of solar energy, wind power, geothermal heat, hydroelectricity, biofuels, and more. Currently, the wind power energy is the most popular of all of these green technologies. Thousands of wind turbines are being invested and installed everywhere worldwide. Thus, many questions arise. What is a wind turbine? What are the different types of design? What are the advantages and disadvantages? How do the aerodynamic blades work? Are these wind turbines worth investing? Is it feasible? Will they help us significantly in reducing greenhouse gases? And what are impacts of wind turbine in our environment? These are the questions that I will attempt to answer.

What is a Wind Turbine?

The definition of a wind turbine is a machine that converts the wind’s kinetic energy into rotary mechanical energy and this rotary mechanical energy is then converted by the generator into electricity.

Wind Turbine Components

Wind turbine usually has six main components: the rotor, the gearbox, the generator, the control and protection system, the tower and the foundation. These main components can be seen in the figure below.

Rotor

The rotor takes the wind and aerodynamically converts its energy into mechanical energy through a connected shaft.

Gearbox

The gearbox increases the rotational velocity of the shaft for the generator. In some turbines, the gearbox is not needed because the rotational velocity or the torque from the shaft high enough.

Generator

The generator is a device that produces electricity when mechanical work is given to the system.

Control and Protection System

The protection system is like a safety feature that makes sure that the turbine will not be working under dangerous condition.

Tower

The tower is the main shaft that connects rotor to the foundation. It also raises the rotor high in the air where we can find stronger winds.

Foundation

The foundation or the base supports the entire wind turbine and make sure that it is well fixed onto the ground.

Wind Turbine Components

Type of Wind Turbine

There are mainly two types of wind turbine: horizontal axis and vertical axis. The horizontal axis wind turbine (HAWT) and the vertical axis wind turbine (VAWT) are classified or differentiated by the axis of rotation the rotor shafts.

Horizontal Axis Wind Turbine

The wind turbine shown in figure below is a horizontal axis wind turbine. It is the most popular wind turbine used and it can be seen all around the world because of its higher efficiency than vertical axis turbines. The HAWT usually have their generator place on the top and they have a propeller-type rotor.

Horizontal Axis Wind Turbine

Vertical Axis Wind Turbine

The vertical axis wind turbine however isn’t as popular as HAWT, but because the axis of rotation is vertical, it allows a better originality or creativity in the design of the configuration of the blades. There are many VAWT sub-types: Darrius (shown in figure below), Neo-aerodynamic, Novel, Savonius, Zephyr, etc. These sub-types all have different blade configuration. The VAWT have their gearbox near the ground.

Darrius Vertical Axis Wind Turbine

Advantage of Horizontal Wind Turbine

An advantage that most HAWT have is a wind vane attached on the rotor blades which would orient the blades towards the wind, allowing thus the best angle of attack for the blades. In doing so, this would give the wind turbine the maximum amount of wind energy possible. Another advantage is the towers of HAWT are relatively taller than VAWT, which allows the blades to face much higher velocity winds in high altitude. In some places, the power output of the wind turbine could increase up to 34% every ten meters in altitude because the wind speed is increased by 20%. Taller towers wind turbine can also be placed on uneven land or in offshore sites. Some financial benefits of HAWT is that it becomes cheaper at high production volumes and larger sizes. They are also more efficient and have higher capacity factors than VAWT.

Disadvantage of Horizontal Wind Turbine

For the disadvantages of the HAWT, most turbines are not efficient when it is installed close to the ground where there isn’t much wind. Also, the tall tower and long blades are very expensive and difficult to transport and install. Another disadvantage of HAWT is that it is difficult to perform repairing operations to HAWT because the gearbox, generator and the rotor are all placed at the top of the tower.

Advantage of Vertical Wind Turbine

The advantage of VAWT is that it doesn’t need to always be faced against the wind’s direction like the HAWT. The blades of the vertical axis wind turbine can intercept the wind from any directions because of the blade design. The VAWT are usually of smaller scale turbine than most HAWT and therefore it is easier to be transported and installed. The VAWT can also be use in residential area on roofs because of its smaller size. It is also easier to repair VAWT because the gearbox and generator are placed at the bottom of the turbine.

Disadvantage of Vertical Wind Turbine

The disadvantage of the vertical wind turbine comparatively to the horizontal wind turbine is the efficiency. In some cases, the efficiency of the energy produced of a VAWT can be less than the half of the production of a HAWT. The VAWT designs are also limited in height because of the sweep area available. The VAWT can only be installed on flat surfaces. Many VAWT types need to be started manually by giving an initial momentum or by an automatic starting mechanism because they have low starting torque.

Aerodynamics of Rotor Blades

The aerodynamics principles of the two types of wind turbine are different. The HAWT and VAWT can have many different configurations of blades; therefore the aerodynamic concept may be different. In most cases, HAWT blades are aerodynamically based on the lift. The higher the lift forces, the higher power we get. On the other hand, the VAWT is mostly based on the drag forces.

Aerodyanamics of Horizontal Wind Turbines

The horizontal wind turbine types of blades are usually made of two or three airfoils such as a propeller. In these types of blades, it is the lift force which makes the rotor turn. As shown in figure 4, when the wind hits the airfoil, the wind gets separated into two. It will pass over the top and the bottom side of the airfoil. Since the top distance is longer, the wind velocity will increase, therefore creating lower pressure on the top of the airfoil. The lower portion will therefore have a higher pressure than the top of the airfoil and this difference in pressure will result in the force known as the lift.

Principles of HAWT Aerodynamic Lift

The lift force can be calculated from the following equation:

L=1/2ρV^2ACL

Where
ρ=Density of air [kg/m^3]
V=Velocity of the wind [m/s]
A=Surface area [m^2]
CL=Lift coefficient

The drag force acts perpendicular to the lift force due to the resistance of the airfoil from the wind and would counteract the rotation to rotor. The higher the lift-to-drag ratio, the higher the torque output would be for the wind turbine.

Aerodynamics of Vertical Wind Turbines

For the vertical axis wind turbines, there are more creativity and variety in the design of the blades. Some sub-types turbines use blades that are based on lift forces such as the Darrius. But most of the VAWT rely on drag force to rotate the rotor shafts such as the Savonius and the Zephyr types wind turbine. When the wind hits the blade, the resistance of the blade would create a force called drag. The drag applied on the blades would create a torque and rotate the rotor shaft. The drag force can be obtained from the below equation:

D=1/2ρV^2ACd

Where
ρ=Density of air [kg/m^3]
V=Velocity of the wind [m/s]
A=Surface area [m^2]
Cd=Drag coefficient

The drag coefficient is dependant on the geometry of the blade. The table below gives the coefficient of various shapes. In order to obtain a higher torque output, the drag must be maximized. The design of the blade must therefore be made of geometry of high drag coefficient such as a hollow semi-cylinder or a long flat plate at 90°. The blade must also be as large as possible since the drag is directly proportional to the surface area.

Drag Coefficient

Economy

Wind power is the world’s fastest-growing energy source and has an average annual growth rate of 29% over the last decade. There are many thousand of wind turbines operating in different parts of the world.

Large Scale

On the large-scale utilization of wind turbines, Germany, Spain, United States, India and Denmark have invested the most in wind electricity. From table 2, Germany leads the world with about 21,000 total wind power capacity, then follows Spain with 12,800, the U.S. with 13,800, India with 7,200 and Denmark with 3,000 MW.

Installed Windpower Capacity

Canada is starting to do very well compare to other countries by doubling the wind power capacity in 2006, giving them the 10th position with 1,670 MW wind power capacity presently. More investments into wind electricity generation are undergoing nationwide. For example, the federal government has just invested 53 Million over ten years to the Prince Wind Energy Farm located in Sault Ste. Marie. This investment will allow the installations of 126 wind turbines which could generate 189 MW of electricity for 40,000 homes. Another example, a wind farm of 200 MW is being installed for 2008 near Kingston in Ontario. Approximately 86 wind turbines with a capacity of 2.3 MW each will be installed for this Wolfe Island Wind project.

Small Scale

On the small-scale utilization of wind turbines, we can find small residential wind turbines which generate about 400 watts to wind farms and offshore turbines producing a few megawatts of electricity. These small wind turbines are used in conjunction with grid energy storage and diesel systems to compensate for the irregularity of the electricity output.

Incentives

Many countries, like U.S., Canada and Germany, provide tax incentives to promote clean energy over traditional energy sources to reduce the emissions of greenhouse gases. For Canada, we have an incentive called Wind Power Production Incentive (WPPI); this is a $ 260 million program of 15 years duration that started in 2002. The WPPI will support financially the installation of 1,000 MW of new capacity over a period of five years, and will pay incentives of about 0.01$ per kilowatt hour of production for the next ten years. In 2005-06, six projects have been commissioned for a total of $150 Million investment over 10 years which will give 436 MW of wind power capacity. Ever since the WPPI was introduced in 2002, $239 Million have been invested, which is about 670 MW of wind power capacity in approximately 18 projects. This incentive applies to electric utilities and power producers and covers approximately half of the cost of wind turbine installation. Wind power is therefore very desirable, mostly in global, national and company scale to reduce the emission of greenhouse gases.

Economic Feasibility

Wind power is economically feasible. It is a very competitive energy compared to traditional energy source such as fossils fuel and nuclear energy if we take into account the full environmental and health cost. Usually when we look at economic feasibility, we only look at the direct economic cost, such as the cost of the turbines and the amount of energy it can generate compared to nuclear power plant; but we fail to look at the indirect cost of these tradition energy sources, for example, emission tax penalties and medical health cost. In addition, the costs of wind turbine are becoming much cheaper because of technological development and scale enlargement. Since the last decade, wind turbine’s cost has reduced to nearly 50%. Wind power is also economically feasible because the cost of installation would become even more inexpensive from government incentives (WPPI). It has also been suggested that private firm can also sell their excess electricity back into the grid. Overall, wind energy can be economically advantageous in comparison with conventional energy.

Economic Development

The installation of wind turbines not only generates extra clean energy, but it would also contribute to the creation of jobs. Gamesa, a Spanish wind turbine maker, will be building a blade manufacturing plant in the US, creating 1,000 new jobs over the next five years. Also, wind energy would also contribute to the increase of personal income, tax income and landowner net revenues from leasing their land for turbine installations. For example, at Lake Benton in Minnesota, approximately $1 million of annual property tax revenue are generated for each 100 MW of wind development.

Environment

This green energy has no negative effects on our environment in short or in long term since it is a renewable and pollution-free form of energy. When the wind hits the wind turbine, a part of its kinetic energy is transformed into mechanical and electrical energy. This energy is environment-friendly because the fuel it uses to produces electricity, the wind, is produced from the sun. About 1 to 3% of the sun energy that hits the earth is converted into wind energy and this energy is replenished everyday. It also doesn’t need to be mined, treated, transported or burned to the atmosphere.

Air Impacts

In short term, wind energy has a positive effect on the air because it does not produce any air pollutants or greenhouse gases like other energy sources such as fossil fuel, coal, natural gas and oil. Wind power can therefore help attaint and meet the goals of Kyoto Protocol. Kyoto Protocol is an international agreement among industrialized countries to reduce greenhouse gas emissions negotiated in Kyoto, Japan in December 1997. Canada is committed to reduce its greenhouse gas emissions by 6 percent of 1990 levels between 2008 and 2012. In long term, the environment impacts it can have on the air are slowing down of climate change and global warming or even completely reduce the greenhouse gas emissions.

Water Impacts

Many wind turbines are sited offshore because of potent winds and they cannot be seen from shore and therefore do not contribute to visual pollution. Even though, offshore wind turbines are installed on water, it does not have any environmental effect in short or in long terms on water.

Land Impacts

Onshore wind turbines are installed on land. Similarly to offshore wind turbines, they do not pollute the soil but it will need the clearing of trees, roads, foundation and constructions of power lines. Wind turbines will require at least a distance of ten times the height of the turbine without any obstruction in order to receive sufficient wind to function properly. Some of supporters of wind power argue that wind turbine can be installed on farms where trees and other obstacles have been already cleared. Farmers would receive lease payment from companies for the installation of each turbine and they would still be able to farm on it.

Wind energy does not pollute air, land or water but it cause three main concerns: visual impact, noise pollution and avian mortality.

Visual Impact

The first problem with wind turbines is visual impact. Many homeowners oppose to the idea of wind farm because they believe that wind turbines would cause visual pollution. The aesthetic problem can be solved by building offshore wind farm, but this alternative is expensive to maintain and much of the energy will be loss through transmission.

Noise Pollution

Another problem that wind turbines create is sound pollution. This sound pollution comes from the generator and the gearbox which produces mechanical noises but most importantly it comes from the 35-m-long rotor blades each time it passes through the air. These blades would create a deep subsonic noise and cause sound pollution. But now, from technological advancement, we have turbine that produces a lot less sound pollution compared to conventional turbines. These improvements have eliminated most of the mechanical noise and have decreased the aerodynamic noise that the rotor produces from improved rotor designs. We can now have a conversation right under these huge turbines without the need to shout or raising our voice. Furthermore, turbine noises are insignificant as wind increases because of other background sound. This table compares the pressure level of turbines noise to other daily urban activities.

Sound Pressure of Urban Activities

From this table, we can see that the wind farm would create less noise pollution than a truck, a car, a busy general office, a pneumatic drill and jet aircraft.

Avian Mortality

Finally, the last major concern of wind farms is the killing of birds. From a survey from California performed by California Energy Commission in 2004 demonstrate that approximately 4,700 birds are killed each year at Altamont Pass. And about 1,300 of those birds are birds of preys. Wind turbines not only kill birds but they also kill lots of bats. A study conducted in 2004, at two US eastern wind farm, have found that the 63 turbines have killed more than 2200 bats within 6 weeks and have showed that these wind farm are dangerous for bat population. On the contrary, some study indicate that the killing of birds from turbines are insignificant compared to other human activities like traffic, hunting, power lines, tall buildings and pollution caused from the burning of fossils fuel. For example, a study in the United Kingdom (UK) conducted by the Royal Society for Protection of Birds (RSPB) showed that about 1 bird is killed per turbine annually whereas about 10 millions birds were killed by cars. Another study, shows us that migrating bird are a lot smarter than we think and can fly through wind turbines easily without the need to changing their route. They have concluded that good positioning of wind turbine will not endanger the birds and that climate change is a much greater threat to birds and to other living species. They therefore support wind energy. Figure 5 is a bar chart that compares the deaths of birds from wind farm and those from human structures in the US annually. We can see that wind turbines are not the major cause of bird fatalities with only 0.1-0.2% comparatively to the other structures.

Percentage of Annual Bird Fatalities by Source

Future of Wind Turbine

In the near future, wind turbine could be sited in high altitudes in the air because of the strong and constant winds there. This idea has been suggested by professor Bryan Roberts in Australia from the University of Technology. The airborne wind turbine would look like a turbine with airfoils. It would be able to “float” in the air due to the lift the winds will act on the wings. The problem with this concept is that when the winds aren’t strong enough to maintain the turbines in the air, the turbine will fall. It has been suggested that helium balloons can be attached to the turbines to compensate for the low winds. Again, there is a downfall to the helium idea because as time passes, helium would dissipate from the balloons and more helium should be added back into the balloons. This task can be extremely difficult in high altitude with the presence of high winds. Another problem with airborne wind turbine is the requirement of very long power lines and much of the power would be lost through transmission lines. The last disadvantage of airborne wind turbines would be an aerial restriction zone for these turbines to prevent the airplanes from flying into them. Presently, an Ontario and a San Diego company, Magenn Power Inc. and Sky WindPower are trying to make these airborne turbines available on the market.

Conclusion

In conclusion, a wind turbine is a machine that converts the wind kinetic energy into electricity. The major components of a wind turbine are: the rotor, the gearbox, the generator, the control and protection system, the tower and the foundation. Wind turbines are classified into two types of category: horizontal axis wind turbine and vertical axis wind turbine. The major advantage for a HAWT is the high efficiency it has; the disadvantage is the maintenance and repair at high altitude. The advantage of a VAWT is that the wind can come from any direction; the disadvantage is the height limitations. Aerodynamically, the wind turns the rotor blades of the HAWT because of the pressure differential between the top and the bottom of the airfoil. For the VAWT, it is the drag that acts on the blades and turns the rotor blades. Today, wind power is economically competitive compared to traditional energy because the cost of wind turbines is getting cheaper because of technology advancement and government incentives. It also creates jobs and generates extra personal and tax income. Wind energy is also a renewable and pollution-free energy which can help us reduce the emissions of greenhouse gases. I believe that wind energy can become an important asset to solve climate change and global warming issues in the future.