Wind Energy Turbines
Wind Energy Turbines Convert Wind into Electricity
As well as using the power of the sun to heat water, living spaces or produce electricity using photovoltaic cells, we can also use the suns resource in the form of Wind Energy to generate electrical power as it is the suns solar energy that controls our weather.
The sun heat’s our planet unevenly, making the air hotter around the equator as it absorbs more energy and colder near the poles. Air expands when it is warmed and contracts when it is cooled. These differences in temperature cause convection currents to flow around the globe as the denser air from the colder regions moves to the warmer regions were the air is lighter. This movement of air within the atmosphere of the earth from a hotter place to a colder place is what we call the “wind”, and which can be weak or strong depending upon the solar energy striking the earth at that time.
Also, since both the Earth’s land mass and its oceans absorb and release solar energy back into the atmosphere at different rates, there is a constant shift of air from between the Earth’s surface and the atmosphere causing the air to move around in currents, again generating “wind energy”. The Earth’s rotation also plays a major role in wind energy production.
We can define wind as “air in motion”, which varies from zero velocity to high gusts. In theory, the world has an inexhaustible supply of free wind energy as every corner of the earth receives the effects of the wind at some time during the day.
Also, due to annual variations, such as winter or summer time, or geographic locations, such as flat desert or mountain ranges, some parts of the world receives more of the winds energy than others. With fossil fuel supplies running out, Wind Energy and Wind Power are now becoming an important renewable energy source.
As we now know, “wind energy” is a free and renewable secondary form of solar energy, due to the uneven distribution of temperatures in different areas around the world, and people have been harnessing this free wind energy resource since windmills and sailing boats were first used in ancient times.
Windmills harnesses the energy contained in the moving air to produce mechanical power in the form of torque which is then used directly for pumping water or grinding corn, but windmills could also be modified to generate electrical power for heating and lighting by attaching an electrical generator to the rotating shaft connected to the windmills sails.
The kinetic energy (kinetic energy is the movement or motion of substances and objects) contained in the wind can be converted into both mechanical and electrical energy by a windmill. A modern type of windmill that uses the kinetic energy of the wind to produce an electrical energy output is called a Wind Turbine.
Wind turbines that are in use today are far more likely to be a type of wind generator which operates differently and more efficiently from a conventional sail windmill. Multiple wind turbines that are arranged together in clusters to capture large amounts of wind energy at the same time and convert it into electrical power feeding this power into the electrical grid are known as Wind Farms. These wind farms can be located on flat land, mountain tops or offshore in the sea.
Wind turbine technology may look simple but there are many mechanical parts to a modern wind turbine. The wind turbine rotor blades rotate around a central hub, which turns a low speed gearbox shaft, which rotates a generator at a higher speed and makes electricity.
The electrical generator converts the kinetic energy of the rotating blades into electrical energy were electrical cables carry this energy to an electrical sub-station for distribution to the utility grid. Modern wind turbines have a number of air foil shaped rotor blades resembling aeroplane propellers, unlike windmills which usually had several flat blades or sails. A combination of both lift and drag causes the turbine blades to rotate in the wind.
Although there are many different configurations of wind turbines available today, most of them can be classified as either “vertical-axis wind turbines” (VAWTs), which have blades that rotate about a vertical axis, or “horizontal-axis wind turbines” (HAWTs), which have blades that rotate about a horizontal axis parallel to the wind. Both have their good and bad points in how they extract the wind energy but both designs can generate electricity from a few hundred watts to many thousands of watts but both types contain the same major components such as:
- A tower or support mechanism that supports the rotors, gearbox, generator and axillary equipment.
- The wind turbine with two or more rotor blades which captures the wind energy.
- A Mechanical gearbox to increase the rotational speed of the generator.
- An Electrical generator or alternator to produce the electrical power.
- Speed sensors and control electronics to monitor and regulate the speed and output.
- Electrical cables connecting the wind turbine to the area grid.
Horizontal and Vertical Wind Turbine Designs
Wind Speed and Direction
It is all well and good to have a shiny new wind turbine at the bottom of the garden or fixed onto a roof, but if the wind does not blow the wind turbine will not rotate and produce any electricity. Likewise, if the wind energy is to strong the wind turbine may rotate so fast as to damage itself or overheat because it is producing too much output voltage and/or electrical current. So it is important when siting a wind turbine as part of a wind energy system to know beforehand how much wind there is available and how fast the wind is actually blowing.
At any given site, the wind speed can vary from zero to high gusts, so wind speed is important because the amount of electricity that wind turbines can generate is determined by the winds actual speed or velocity.
Wind speed can be measured using a weather vane or a type of wind gauge known scientifically as an Anemometer. An Anemometer is a device used to measure the velocity and direction of the wind giving us an idea of the amount of wind energy available at a particular location. Anemometer consists of an arrow-shaped metal or wooden vane mounted on a shaft high in the air and is designed to point in the direction of the wind.
Wind direction is the direction from which the wind blows. It generally has three arms with cup shaped bowls at the end that spin on top of a shaft. The cups catch the wind and spin the shaft in proportion to the velocity of the wind. The harder the wind blows, the faster the shaft spins. These cup and propeller anemometers are the most common type of wind measuring device as they are cheaper than most other types.
An electronic circuit inside the anemometer counts the number of rotations per minute and converts that figure into a miles per hour (mph), kilometres per hour (kph) or metres per second (m/s) signal. Some form of display on the anemometer itself or a computer link shows the actual speed of the wind. You can also measure the speed and the amount of wind energy available very cheaply without buying an expensive anemometer by using a standard desktop or house fan and counting the number of revolutions per minute.
Nearly all commercially available wind turbines are designed to operate from a minimum rotational speed called the “cut-in” wind speed at which they begin to produce electrical power once the wind speed is above a certain mph or kph. Obviously, if the wind speed is very low or non-existent the ability of the turbine to produce any useful output power will be zero. Likewise, while high winds feel strong and contain large amounts of available wind energy, the amount of power that can be captured is very small as these strong winds do not occur very often (such as in storms).
Then the turbine has a way of regulating or limiting the peak power produced by having a maximum rotational speed called a “cut-out” wind speed at which the turbine will be shut down to prevent it from being damaged if the wind speed is too excessive. Therefore, a wind speed or wind velocity window exists for a wind turbine between the cut-in speed and the cut-out speed that allows them not only to generate large amounts of free solar electricity but operate at a safe rotational speed.
So before we can purchase or install our wind turbine we need to measure and understand the wind speed and strength for our given location and use this data collected by an anemometer or other device to produce a wind speed distribution graph as shown:
Wind Speed Distribution
The first graph on the left shows the actual wind speed for our location varying over a period of time. Each time period used can be anywhere from a few days to many years and will give us the raw data to plot our site distribution and site characteristics curves. Wind speed data can be collected every minute, every hour or every day depending upon our location.
However, this data must be collected on a regular basis and not just one day of data every hour then nothing for two weeks, etc. Also the more data that is collected the more accurate will be the resulting graphs allowing the correct wind turbine to be purchased. By collecting wind speed data in this way, we may find that our original and possibly convenient location next to a building has less wind energy potential or available wind power than another location away from the building, so think about data collection around different points.
Once we have our raw site data we can then use it to generate a “wind speed distribution” graph, (second graph). This shows the probability of the various wind speeds for our proposed site(s). Using this graph we can define the maximum and average wind speed (the centre of the graph) and the width of the wind speed window from the minimum cut-in speed to the maximum cut-out speed.
The final graph on the right shows the number of hours a particular wind speed or wind velocity is available to us throughout the data period giving the characteristics of the site or location for our renewable wind energy system. On this graph we can also draw or super-impose the proposed wind turbines operating window (cut-in to cut-out speed) given by the manufactures data sheet to see the average time and how much power will be generated by the turbine.
Most domestic roof mounted wind turbines are designed to operate with a wind speed from about 10mph (15kph) up to a maximum of about 60mph (100kph) giving a wind speed window of about 50mph or 85kph. The rotational speed of the wind turbine also plays an important role in the production of wind energy.
Generally, on a calm still day, the turbine sits idle and the blades are not spinning. As the wind picks up it eventually reaches the cut-in speed of the turbine (usually around 10 mph). At this wind speed, the turbine blades will spin up to their cut-in operating speed and start generating electricity and as the wind speed increases, the rotor blade velocity increases so the generator output increases.
Wind turbines deliver maximum power at a wind speed of around 30 – 35mph (varies by turbine model) so a generator that has a name-plate rated capacity of 100kW, will be outputting 100kW at the rated wind speed, but will deliver less than a quarter (1/4) of their rated power at the lower cut-in wind speeds of only 15mph.
Wind speeds above 30 mph, the generator maintains its rated capacity (i.e. 100kW) until wind speeds reach 55 to 60 mph, and then the turbine reaches its cut-out speed and its built in safety circuit stops generating electricity.
So we can see that the wind speed or wind velocity, is a very important factor to consider for the correct and safe operation of a wind turbine generator. Wind speed data is used to calculate the winds power when deciding where to erect a suitable wind turbine, whether on the ground or on a roof.
The Power in the Wind
Then we can see that Wind Power, which is measured in Watts, is the power we can expect to extract from the wind to drive our turbine. Wind power is determined by the size of the rotor blades, the wind velocity and the air density. Then the theoretical power in moving air is the flow rate of kinetic energy per second by a wind turbine and is given by the equation:
Wind Power Equation
Where: P is the Wind Power, ρ (rho) is the air density in Kg/m3, A is the circular area in m2 swept by the rotors, V is the air velocity in m/s or mph and Cp is the power coefficient (efficiency) which is the percentage of power in the wind that is converted into mechanical energy, typically 0.35 to 0.45, (35 – 45%).
You will notice from the equation that if the rotor area in m2 is fixed, and the air density is fixed for a given location, the energy contained in the wind is only dependant upon the wind speed. Then we can simplify the above equation to give K.V3 where K is a fixed constant representing the combined fixed rotor blade area, air mass and efficiency of the turbine. This means then that the “available wind energy is proportional to the cube of the wind speed” or wind velocity, and this statement is very important as a small change in wind speed makes a big change in the power contained within it.
Wind Energy Example No1
Lets assume that we live in an area slightly above sea level that has an air density of 1.225Kg/m3 and we have installed a 40% efficient wind turbine which has a rotor blade radius of six ( 6 ) metres. Calculate the output power from the turbine at a wind speed of 8 metres/second, ( 8m/s ) and again at double the velocity of 16 metres/second ( 16m/s ).
1. at 8 metres/second:
2. at 16 metres/second:
Then we can see that at a wind velocity of 8m/s the theoretical output power is calculated at 14.2kW and at 16m/s is calculated at 113.5kW. Since the wind power, P and therefore the wind energy vary with the cube of the wind velocity, ( V3 ) doubling of wind velocity from 8m/s to 16m/s results in eight times (x8) the amount of available power being produced. By plotting different values of wind speed against theoretical power output calculated from the above equation we can produce a simple power curve of any wind turbine given the manufacturers operational characteristics of the turbine.
Wind Energy Curve
Finding a good windy site to install a wind turbine and maximising the wind speed becomes an important part of making renewable wind energy economical. Wind speed histograms can be purchased, used or drawn for any particular site to show the number of hours, days or weeks, or whatever time period is used, that the wind blew for each sampled period of time.
Since the movement of the wind mass varies from seconds to years, wind power and wind energy will also vary over the same time scale. Therefore, by taking the data first of “how windy” is the proposed site for a wind turbine, helps decide what size and type of turbine best suits the location. Increasing the rotor blade length, or increasing the height of the wind turbine above the ground will also increase the power output.
Extracting the winds kinetic energy and using it to produce electrical power is a very attractive option. Wind technology has grown in scope, and in most places wind is becoming a feasible source of energy but is vulnerable to weather conditions. However in certain locations, mainly in coastal or offshore regions and at high altitudes, there is a steady stream of wind to drive a turbine.
The main benefits of wind power are that it is clean, safe, and endlessly renewable but the biggest advantage of generating electricity using the winds energy is that the wind that powers the wind turbines is completely free. Using wind energy has many other advantages as well as disadvantages over other forms of renewable energy sources as listed below.
Wind Energy Advantages
- Wind energy is a clean and renewable technology that does not release pollutants, emissions or by-products into the atmosphere during operation since there are no chemical processes involved in its electrical generation.
- Modern turbines produce very little mechanical noise when operating except for a low “whooshing” sound.
- Wind energy, which is actually a secondary component of solar energy, is a “renewable energy” in the sense that there will always be wind as long as the sun continues to heat the earth unevenly, and the earth continues to rotate.
- Although the strength of the wind varies from one day to another, the total output of energy over a set period of time, varies by only a small percentage as wind turbines are designed to operate within a wind speed window which is usually between 10mph and 60mph or 4m/s to 25m/s.
- Although wind turbines and especially wind farms take up a lot of land space, the land on a wind farm can be used simultaneously for wind generation, crops growing, animal grazing or anything else below the vanes of the turbines.
- Wind generation can be done in remote areas and on any scale from small personal and domestic use to large full size wind farms, which means that even remote mountainous places that might otherwise be regarded as “off grid” can generate power.
Wind Energy Disadvantages
- The environmental impact of wind energy as many people consider wind farms unsightly and as man-made structures, the turbines may have a negative visual impact or be viewed as a form of visual pollution.
- Wind farms require large areas of land or have to be placed in environmentally sensitive areas such as deserts, on top of hills and mountain ridges or offshore in the sea were the winds force is stronger and constant.
- The wind turbine is like a giant propeller and as such needs the kinetic power of the wind to rotate it meaning that at low wind speeds or prolonged levels of no wind (calm weather), the turbine does not generate any useful electricity.
- Wind farms injure, kill and disturb the flight patterns of migratory birds and predatory birds. Some birds and even bats have been killed by flying into the wind turbine blades when rotating, but these numbers are very small.
- Wind turbines cause noise pollution because they produce a low frequency “whooshing sound” as the blades rotate which itself is largely masked by the noise of the driving wind.
- The initial investment in the turbines, transportation and ground works makes wind energy costs higher than for conventional fossil-fuel generators.
- The best locations for converting wind into electricity are far away from populated urban areas, which often means that the electricity must be stored and/or transported over long cable distances.
- Although annual winds and power output from a wind turbine are relatively predictable, hourly and daily wind energy output levels are not as wind speed does not remain constant giving little power output in low winds.
Wind Energy is another type renewable energy that can be used to power the home. Wind energy offers both environmental and economic benefits as it produces zero emissions, is renewable, and the fuel itself is free, local, and will never fluctuate in cost.
But before you install a wind turbine you need to study the proposed site and height to determine the general wind power potential that exists there as well as the predicted energy production from the new wind turbine. After all, if you buy a new wind turbine you will want it to rotate as much as possible generating electricity and providing a quick return on your investment.
The general wind power potential can be determined from using wind maps and wind speed data available for your specific area and most local authorities and local airports can provide this data at heights from sea level up to several hundreds of meters above ground level.
However, you should not only depend on this type of historical and regional data to determine the feasibility of installing a small wind turbine. Using an anemometer first will help to select the ideal site for placement of a wind power turbine.
In our next tutorial about Wind Energy, we will look at the operation and design of a typical wind turbine, and hopefully we will see that for the standard wind turbine design, three turbine rotor blades are better than two at extracting the winds power for generating electricity.