Wind Energy and Wind Power

Wind is a form of solar energy. Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth. Wind flow patterns are modified by the earth's terrain, bodies of water, and vegetative cover. This wind flow, or motion energy, when "harvested" by modern wind turbines, can be used to generate electricity.

What Causes The Wind?

Wind is simply the air in motion. Usually when we are talking about the wind it is the horizontal motion we are concerned about. If you hear a forecast of west winds of 10 to 20 mph that means the horizontal winds will be 10 to 20 mph FROM the west.

Although we cannot actually see the air moving we can measure its motion by the force that it applies on objects. For example, on a windy day leaves rustling or trees swaying indicate that the wind is blowing. Officially, a wind vane measures the wind direction and an anemometer measures the wind speed.

The vertical component of the wind is typically very small (except in thunderstorm updrafts) compared to the horizontal component, but is very important for determining the day to day weather. Rising air will cool, often to saturation, and can lead to clouds and precipitation. Sinking air warms causing evaporation of clouds and thus fair weather.

You have probably seen a surface map marked with H's and L's which indicate high and low pressure centers. Surrounding these "highs" and "lows" are lines called isobars. "Iso" means "equal" and a "bar" is a unit of pressure so an isobar means equal pressure. We connect these areas or equal pressure with a line. Everywhere along each line is constant pressure. The closer the isobars are packed together the stronger the pressure gradient is.

Pressure gradient is the difference in pressure between high and low pressure areas. Wind speed is directly proportional to the pressure gradient. This means the strongest winds are in the areas where the pressure gradient is the greatest.

Also, notice that the wind direction (yellow arrows) is clockwise around the high pressure system and counter-clockwise around the low pressure system. In addition, the direction of the wind is across the isobars slightly, away from the center of the high pressure system and toward the center of the low pressure system. Why does this happen? To understand we need to examine the forces that govern the wind.

There are three forces that cause the wind to move as it does. All three forces work together at the same time.

The pressure gradient force (Pgf) is a force that tries to equalize pressure differences. This is the force that causes high pressure to push air toward low pressure. Thus air would flow from high to low pressure if the pressure gradient force was the only force acting on it.

However, because of the earth's rotation, there is second force, the Coriolis force that affects the direction of wind flow. Named after Gustav-Gaspard Coriolis, the French scientist who described it mathematically

What is wind?

Wind is the movement of air over the surface of the Earth, from areas of high pressure to low pressure. But what causes the changes in pressure? There are a few concepts that we will have to explore to find exactly how this works, but ultimately all the energy on our planet comes from the Sun.

The Sun gives out all sorts of radiation, including heat and light energy, and is so powerful that it radiates 170,000,000 GW of energy to the Earth!  When this energy reaches the Earth, the ground and other surfaces absorb it, and heat the surrounding air. It's these differences in temperature, together with the rotation of our planet, that create the wind.

About 1 to 2 per cent of the energy coming from the sun is converted into wind energy, which is enough to meet the electricity needs of the world three times over, and is a source of power that will never run out.

The density of air

Air, like all substances around us, has a certain density.

Density (kg/m³)= mass (kilograms) /volume (meters cubed)

The density of air is small but not zero. If air didn't weigh anything, the atmosphere would float off into space, which would be bad!

There is only a thin layer of air surrounding the earth, what we know as our atmosphere. This extends upwards more than 50 kilometres above ground level. At this height the density is less than 1% of the ground level value. If the earth were the size of a football, the atmosphere would be equivalent to a 1mm thick layer on the surface of the football.

Air pressure

Because there are miles of air above us and it is all pushing down, the air at the bottom gets squeezed creating a pressure, like the pressure you feel at the bottom of a swimming pool. The size of this pushing force over each unit of area is called the air pressure, or atmospheric pressure.

Pressure (Pascals)= force (Newtons) /area (m²)

(The unit of pressure is called the Pascal or Pa for short, 1 Pascal = 1 Newton per m²)

Atmospheric pressure

The pressure on the earth's surface due to the air above us is about 100000 Pa - 101,325 Pa on average. That's 1Kg pushing on every square cm! 101,325 Pa is also commonly referred to as 'one atmosphere'. The weight of a column of water 10 meters high would be needed to increase the air pressure at the base of the column by 1 atmosphere.

A barometer measures air pressure. If you took a barometer up in a hot air balloon you would see the pressure reading fall the higher the balloon goes. This happens because there is less air above the balloon the higher up into the atmosphere it goes. If you went too high the air pressure would become so low that you would not be able to breathe properly. This is why modern passenger jets have 'pressurised cabins' to keep the conditions similar to that at the earth's surface so the passengers are more comfortable.

There is another unit of pressure called the "milli-bar" or mbar for short. There are exactly 100 Pascals per milli-bar, so 1000 Mb is about one atmosphere.

If you watch the weather forecast on TV you may see a map showing atmospheric pressure. This is referred to as an isobar chart.

Isobars are similar to contour lines. Instead of the lines showing areas where the ground is the same height above sea level, the lines show areas where the atmospheric pressure is the same. The closer the lines are together the more rapidly the pressure changes from one place to another. This is similar to contour lines on a map, the closer they are together the more steep the slope.

Why does the pressure vary from place to place and from day to day?

There are two causes:

1) the rotation of the earth
As the earth spins on its axis it drags the atmosphere round with it. However, the air higher up in the atmosphere is less affected by this dragging/stirring effect. The difference in the air speed at different levels in the atmosphere causes the air to mix, forming turbulence, which causes wind at the earth's surface.

The rotation of the earth causes another related phenomenon, the Coriolis force.This is best demonstrated by example. Take a piece of paper and pin it onto something which will not get damaged, e.g. a carpet. Rotate the paper anti clockwise (to represent the movement of the earth), whilst at the same time trying to draw a straight line. The line you draw will appear curved.

A similar effect occurs when air is moving over the surface of the earth as it rotates. Instead of travelling in a straight line, the path of the moving air veers to the right. As a result instead of the air (or wind) moving in a straight line from areas of higher pressure to areas of lower pressure, it moves almost parallel to the isobars. The result is that the wind circles in a clockwise direction towards the area of low pressure. In the Southern hemisphere, the wind will circle in an anti-clockwise direction and clockwise in the Northern hemisphere.

2) the heating effect of the sun
The warming effect of the sun varies with latitude and with the time of day. Warmer air is less dense than cooler air, and rises above it, so the pressure above the equator is lower than the pressure above the poles.

The warming effect is greater over the equator as the sun is directly overhead. Nearer the earth's poles the angle at which the suns rays hit the earth is more acute, so the same amount of energy is spread over a greater area.

Temperature Differences Drive Air Circulation

NASA Satellite: Terra Sensor: MODIS

The regions around equator, at 0° latitude are heated more by the sun than the rest of the globe. These hot areas are indicated in the warm colors, red, orange and yellow in this infrared picture of sea surface temperatures . Hot air is lighter than cold air and will rise into the sky until it reaches approximately 10 km (6 miles) altitude and will spread to the North and the South. If the globe did not rotate, the air would simply arrive at the North Pole and the South Pole, sink down, and return to the equator.

The wind rises from the equator and moves north and south in the higher layers of the atmosphere. Around 30° latitude in both hemispheres the Coriolis force prevents the air from moving much farther. At this latitude there is a high pressure area, as the air begins sinking down again. As the wind rises from the equator there will be a low pressure area close to ground level attracting winds from the North and South. At the Poles, there will be high pressure due to the cooling of the air. Keeping in mind the bending force of the Coriolis force, we thus have the following general results for the prevailing wind direction:

Prevailing Wind Directions

Sea Winds

Credit: NASA JPL Satellite: QuikSCAT Sensor: SeaWinds

Wind Energy

The terms "wind energy" or "wind power" describe the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity to power homes, businesses, schools, and the like.

Global maps of average wind speed help researchers determine where to develop wind energy. Wind turbines (high-tech windmills) can generate power in places far from power plants and without an electricity grid—but planners need to know where there is sufficient wind for the turbines to operate efficiently. A team at NASA’s Langley Research Center developed these maps, and maps of solar insolation, and provide them free of charge. Private companies are using these data to design, build, and market new technologies for harnessing this energy.

How Wind Power Is Generated

The terms "wind energy" or "wind power" describe the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity to power homes, businesses, schools, and the like.

Wind Turbines

Wind turbines, like aircraft propeller blades, turn in the moving air and power an electric generator that supplies an electric current. Simply stated, a wind turbine is the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity.

Wind Turbine Types

Modern wind turbines fall into two basic groups; the horizontal-axis variety, like the traditional farm windmills used for pumping water, and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor. Most large modern wind turbines are horizontal-axis turbines.

Turbine Components

• Anemometer: Measures the wind speed and transmits wind speed data to the controller.
• Blades: Most turbines have either two or three blades. Wind blowing over the blades causes the blades to "lift" and rotate.
• Brake: A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies.
• Controller: The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds.
• Gear box: Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gear boxes.
• Generator: Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.
• High-speed shaft: Drives the generator.
• Low-speed shaft: The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.
• Nacelle: The nacelle sits atop the tower and contains the gear box, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on.
• Pitch: Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.
• Rotor: The blades and the hub together are called the rotor.
• Tower: Towers are made from tubular steel (shown here), concrete, or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity.
• Wind direction: This is an "upwind" turbine, so-called because it operates facing into the wind. Other turbines are designed to run "downwind," facing away from the wind.
• Wind vane: Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.
• Yaw drive: Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don't require a yaw drive, the wind blows the rotor downwind.
• Yaw motor: Powers the yaw drive.

• other equipment, including controls, electrical cables, ground support equipment, and interconnection equipment.

Turbine Configurations

Wind turbines are often grouped together into a single wind power plant, also known as a wind farm, and generate bulk electrical power. Electricity from these turbines is fed into a utility grid and distributed to customers, just as with conventional power plants.

Wind Turbine Size and Power Ratings

Wind turbines are available in a variety of sizes, and therefore power ratings. The largest machine has blades that span more than the length of a football field, stands 20 building stories high, and produces enough electricity to power 1,400 homes. A small home-sized wind machine has rotors between 8 and 25 feet in diameter and stands upwards of 30 feet and can supply the power needs of an all-electric home or small business. Utility-scale turbines range in size from 50 to 750 kilowatts. Single small turbines, below 50 kilowatts, are used for homes, telecommunications dishes, or water pumping.

Advantages and Disadvantages of Wind-Generated Electricity

A Renewable Non-Polluting Resource

Wind energy is a free, renewable resource, so no matter how much is used today, there will still be the same supply in the future. Wind energy is also a source of clean, non-polluting, electricity. Unlike conventional power plants, wind plants emit no air pollutants or greenhouse gases. According to the U.S. Department of Energy, in 1990, California's wind power plants offset the emission of more than 2.5 billion pounds of carbon dioxide, and 15 million pounds of other pollutants that would have otherwise been produced. It would take a forest of 90 million to 175 million trees to provide the same air quality.

Cost Issues

Even though the cost of wind power has decreased dramatically in the past 10 years, the technology requires a higher initial investment than fossil-fueled generators. Roughly 80% of the cost is the machinery, with the balance being site preparation and installation. If wind generating systems are compared with fossil-fueled systems on a "life-cycle" cost basis (counting fuel and operating expenses for the life of the generator), however, wind costs are much more competitive with other generating technologies because there is no fuel to purchase and minimal operating expenses.

Environmental Concerns

Although wind power plants have relatively little impact on the environment compared to fossil fuel power plants, there is some concern over the noise produced by the rotor blades, aesthetic (visual) impacts, and birds and bats having been killed (avian/bat mortality) by flying into the rotors. Most of these problems have been resolved or greatly reduced through technological development or by properly siting wind plants.

Supply and Transport Issues

The major challenge to using wind as a source of power is that it is intermittent and does not always blow when electricity is needed. Wind cannot be stored (although wind-generated electricity can be stored, if batteries are used), and not all winds can be harnessed to meet the timing of electricity demands. Further, good wind sites are often located in remote locations far from areas of electric power demand (such as cities). Finally, wind resource development may compete with other uses for the land, and those alternative uses may be more highly valued than electricity generation. However, wind turbines can be located on land that is also used for grazing or even farming.

All renewable energy (except tidal and geothermal power), and even the energy in fossil fuels, ultimately comes from the sun. The sun radiates 100,000,000,000,000 kilowatt hours of energy to the earth per hour. In other words, the earth receives 10 to the 18th power of watts of power. About 1 to 2 per cent of the energy coming from the sun is converted into wind energy. That is about 50 to 100 times more than the energy converted into biomass by all plants on earth.

Wind Energy Projects Throughout the United States of America

Installed MW for each state 

After reaching 1,000 MW of wind energy in 1985, it took more than a decade for wind to reach the 2,000-MW mark in 1999. Since then, installed capacity has grown fivefold. Today, U.S. wind energy installations produce enough electricity on a typical day to power the equivalent of over 2.5 million homes.

Wind energy continued its growth in 2008 at an increased rate of 29 %. 

• All wind turbines installed by the end of 2008 worldwide are generating 260 TWh per annum, equalling more than 1,5 % of the global electricity consumption.

 • The wind sector became a global job generator and has created 440’000 jobs worldwide. • The wind sector represented in 2008 a turnover of 40 billion !. 

• For the first time in more than a decade, the USA took over the number one position from Germany in terms of total installations. 

• China continues its role as the most dynamic wind market in the year 2008, more than doubling the installations for the third time in a row, with today more than 12 GW of wind turbines installed. 

• North America and Asia catch up in terms of new installations with Europe which shows stagnation. 

• Based on accelerated development and further improved policies, a global capacity of more than 1’500’000 MW is possible by the year 2020.

The market for new wind turbines showed a 42 % increase and reached an overall size of 27’261 MW, after 19’776 MW in 2007 and 15’127 MW in the year 2006. Ten years ago, the market for new wind turbines had a size of 2’187 MW, less than one tenth of the size in 2008. In comparison, no new nuclear reactor started operation in 2008, according to the International Atomic Energy Agency. 

Leading wind markets 2008 

The USA and China took the lead, USA taking over the global number one position from Germany and China getting ahead of India for the first time, taking the lead in Asia. The USA and China accounted for 50,8 % of the wind turbine sales in 2008 and the eight leading markets represented almost 80 % of the market for new wind turbines – one year ago, still only five markets represented 80 % of the global sales. The pioneer country Denmark fell back to rank 9 in terms of total capacity, whilst until four years ago it held the number 4 position during several years. However, with a wind power share of around 20 % of the electricity supply, Denmark is still a leading wind energy country worldwide.