Generation and Types of Wind

Wind is a clean, abundant, and renewable energy  resource that can be tapped to produce electricity. This article explores how wind  is generated and introduces you to two types of wind — local and global.

We’ll also  explore ways local topography affects wind, introducing you to two key concepts:  ground drag and turbulence.

This information provides the practical knowledge  you will need to select the best site for a wind turbine and the optimum tower  height.

What is Wind?   

Wind is air in horizontal motion across the Earth’s surface. All winds are produced  by differences in air pressure between two regions. Differences in pressure result  from differential heating of the surface of the Earth. Heating, of course, is caused  by sunlight striking the Earth’s surface.    Like most other forms of energy in use today, even coal, oil and natural gas, wind  is a product of sunlight — solar energy.

Some wind advocates, refer to wind as  “the other solar energy” or “secondhand solar energy.” Let’s begin by looking at  two types of local winds:

1. offshore and onshore winds and
2. mountain-valley  breezes.   

Offshore and Onshore Winds: Offshore and onshore winds are generated along the shores of large lakes, such as  the Great Lakes of North America, and along the coastlines of the world’s oceans.  Offshore and onshore winds blow regularly, nearly every day of the year. They are  produced by the differential heating of land and water, caused by solar energy. 

Offshore and onshore breezes operate day in and day out on sunny days, providing a steady supply of wind energy. Because offshore and onshore winds are  fairly reliable, coastal regions of the world are often ideal locations for small (and  large) wind turbines. 

Coastal winds are more consistent than winds over the interior of continents and  also tend to be more powerful because of the relatively smooth and unobstructed  surface of open waters. That is to say, wind moves rapidly over water because lakes  and coastal waters provide very little resistance to its flow, unlike forests or cities  and suburbs, which dramatically lower surface wind speeds.

Mountain-Valley Breezes:    Like coastal winds, mountain-valley breezes arise from the differential heating of  the Earth’s surface. To understand how these winds are formed, let’s begin in the  morning.   

As the Sun rises on clear days, sunrays strike the valley floor and begin heating  the ground, valley walls and mountains. As the ground and valley walls begin to  warm, the air above them warms. It then expands and begins to flow upward. While some of  this warm air rises vertically, mountain valleys also tend to channel the solar-heated air through the valley toward the mountains (Figure 2.2). As the warmed air  moves up a valley, cooler air from surrounding areas flows in to replace it. This  wind is known as a valley breeze.   

Throughout the morning and well into the afternoon, breezes flow up-valley —  from the valley floor into the mountains. These breezes tend to reach a crescendo  in the afternoon. When the Sun sets, however, the winds reverse direction, flowing  down valley.   

Winds flow in reverse at night because the mountains cool more quickly than the  valley floor. Cool, dense air (high-pressure air) from the mountains sinks and flows  down through the valleys like the water in a mountain stream, creating steady and  often predictable down-valley or mountain breezes.  Together, valley and mountain winds are known as mountain-valley breezes. As a  rule, mountain breezes (down-flowing winds) tend to be stronger than daytime valley breezes.   

Mountain-valley breezes typically occur in the summer, a time when solar radiation is greatest. They also typically occur on calm days when the prevailing winds  (larger regional winds, which will be discussed shortly) are weak or nonexistent.   

Mountain-valley winds also form in the presence of prevailing winds — for  example, when a storm moves through an area. In such instances, mountain or valley winds may “piggy back” on the prevailing winds, creating even more powerful  (and hence higher energy) winds. When consistently flowing in the same direction,  such winds can provide a great deal of power that can be tapped to produce an  abundance of electricity. 

Large-Scale Wind Currents   

Local winds can be a valuable source of energy. The winds on which most people  rely, however, are those produced by much larger air masses that result from regional and global air circulation. They create dominant wind-flow patterns, known  as prevailing winds. Prevailing winds, like local winds, are created by the differential heating of the  Earth’s surface, but on a much larger scale. Here’s how they are formed:-

The Earth is divided into three climatic zones: the tropics, temperate  zones and poles. Because the tropics are more directly aligned with the Sun throughout the year, they receive more sunlight and are, therefore, the warmest regions on Earth. 

The temperate zones lie outside the tropics, in both the Northern and Southern  Hemispheres.They receive less sunlight than the tropics and so are cooler.

The  North and South poles receive the least amount of sunlight and are the coolest regions of our planet.   

Global air circulation is created by hot air produced in  the tropics. This air expands and rises. Cool air  from the northern regions — as far north as the poles — moves in to fill the void.  The result is huge air currents that flow from the poles to the equator.    Although air generally flows from the North and South poles toward the equator,  circulation patterns are a bit more complicated.

In the Northern Hemisphere, some  of the warm air moving northward cools and sinks back to the Earth’s surface. It then flows back toward the equator creating the trade  winds. Because the trade winds blow quite consistently, they are a potentially huge  and reliable source of energy for residents and nations fortunate enough to lie in  the winds’ path.   

Other factors influence  the movement of air masses across the surface of the planet. One of the most significant is the Earth’s rotation.To understand why prevailing winds deviate from the expected patterns based solely on convection, let’s start with the trade winds.

The trade  winds in the Northern Hemisphere flow not from north to south, as you might expect, but from the northeast to southwest. Why?    Because they are “deflected” by the Earth’s rotation.   

In reality, the Earth’s rotation doesn’t deflect winds. It makes it appear as if the  winds have been deflected. The apparent deflection in wind direction in the tropics  is a planetary sleight of hand, an illusion produced by the rotation of the Earth on  its axis.

Wind from Storms   

Winds are often associated with storms. Storms, in turn, are produced when high-pressure and low-pressure air masses collide. High and low pressure zones move  across the continents.   

Low-pressure air masses originate in the tropics. They are created by the huge in-flux of solar energy in these regions. Huge masses of low-pressure air frequently  break away and migrate northward, sweeping across the North American continent.   

High-pressure air masses originate in the North and South poles, regions of  more or less permanent cold, high-pressure air. Like warm tropical air, huge masses of cold Arctic air also break loose and drift southward, sweeping across the  Northern Hemisphere.   

High-pressure and low-pressure air masses, often measuring 500 to 1,000 miles  in diameter, move across continents. The movement of high- and low-pressure air  masses across continents is steered by prevailing winds and by the jet stream  (high altitude winds). As these air masses collide, they produce an assortment of  weather, often accompanied by winds.

As with all other forms of wind, storm  winds are created by differences in pressure between high- and low-pressure air  masses. The greater the difference in pressure between a high-pressure air mass  and a “neighboring” low-pressure air mass, the stronger the winds. In some cases,  these winds contain an enormous amount of energy. 

Friction, Turbulence and Smart Siting   

Wind does not flow smoothly over the Earth’s surface. It encounters resistance,  known as friction. This results in a phenomenon called ground drag. Ground drag  is caused by friction when air flows across a surface.  When wind flows  across land or water, friction occurs. This reduces the speed at which air moves  over a surface.   

Ground drag due to friction, however, varies considerably, depending on the  roughness of the surface. The rougher or more irregular the surface, the greater the  friction. As a result, air flowing across the surface of a lake generates less friction  than air flowing over a meadow. Air flowing over a meadow generates less friction  than air flowing over a forest.   

Friction extends to a height of about 1,650 feet (500 meters). However, the greatest effects are closest to Earth’s surface — the first 60 feet over a relatively flat,  smooth surface. Over trees, the greatest effects occur within the first 60 feet (18  meters) above the tree line.   

Friction has a dramatic effect on wind speed at different heights. For instance, a  20-mile-per-hour wind measured at 1,000 feet above land covered with grasses  flows at 5 miles per hour 10 feet above the surface. It then increases progressively  until it breaks loose from the influence of the ground drag or friction.

Ground drag dramatically influences wind speed near the surface of the ground  where residential wind generators are located. Because the effects of friction decrease with height above the surface of the Earth, savvy installers typically mount  their wind machines on towers 80 to 120 feet high (24 to 37 meters), or even as  high as 180 feet (55 meters) in forested regions, so their turbines are out of the  most significant ground drag. At these heights, the winds are substantially stronger  than near the ground.

A small increase in wind speed can result in a substantial increase in the amount of power that’s available from the wind  and the amount of electricity a wind generator produces. Mounting a wind turbine  on a tall tower therefore maximizes the electrical output of the machine. Placing a  turbine on a short tower has just the opposite effect. It places the generator in the  weaker winds and is a bit like mounting solar panels in the shade. 

Another natural phenomenon that affects the output of most wind turbines is  turbulence. Turbulence is produced as air flowing across the Earth’s surface encounters objects, such as trees or buildings. They interrupt the wind’s smooth laminar flow, causing it to tumble and swirl, the same way rocks in a stream interrupt the flow of water.

Rapid changes in wind speed occur behind  large obstacles and winds may even flow in the direction opposite to the wind. This  highly disorganized wind flow is referred to as turbulence.   

Turbulent wind flows wreck havoc on wind machines, especially the less expensive, lighter-weight wind turbines often installed on short towers by cost-conscious  homeowners. Buffeted by turbulent winds, wind machines hunt around on the top  of their towers, constantly seeking the strongest wind, starting and stopping repeatedly. This decreases the amount of electricity a turbine generates. 

Turbulence also causes vibration and unequal forces on the wind turbine, especially the blades, that may weaken and damage the machine. Turbulence, therefore,  increases wear and tear on wind generators and, over time, can destroy a turbine.  The cheaper the turbine, the more likely it will be destroyed in a turbulent location. 

When considering a location to mount a wind turbine, be sure to consider turbulence-generating obstacles such as silos, trees, barns, houses and other wind turbines. Proper location is the key to avoiding the damaging effects of turbulence.  Turbulence can also be minimized by mounting a wind turbine on a tall tower.

In  sum, then, mounting a wind generator on a tall tower offers four benefits:

1. it  situates the wind generator in the stronger higher-energy-yielding winds, substantially increasing electrical production,
2. it raises the machine out of damaging  turbulent winds,
3. it decreases the wind turbine’s maintenance and repair requirements, and
4. it increases the wind turbine’s useful lifespan substantially, perhaps  tenfold.

Longer turbine life means less overall expense — and more electricity from your investment.   

To avoid costly mistakes, installers recommend that wind machines be mounted  so that the complete rotor (the hub and the blades) of the wind generator is at least  30 feet (9 meters) above the closest obstacle within 500 feet (about 150 meters), or  a tree line in the area, whichever is higher. Don’t listen to those who  recommend lesser heights. Many unhappy customers will attest to that! 

If your home or business is in an open field surrounded by trees, the wind turbine needs to be well above the tree line. Remember, too, to account  for growth of trees over the 20- to 30-year life span of your wind system when  determining tower height. 

Closing Thoughts   

Wind is a tremendous resource available in many parts of the world thanks to the  Sun’s unequal heating of the Earth’s surface. Depending on your location, you may  be able to take advantage of offshore and onshore winds or perhaps mountain-valley winds.

Prevailing winds and winds that are created between high-pressure  and low-pressure air masses could become your ally in reducing your carbon foot-  print and meeting your own needs sustainably. Don’t forget to take into account  ground drag that slows wind speed and robs you of additional energy and damages  your wind turbine. Site wisely and you’ll be repaid day after day after day.