How is Wind Formed

Many people don't know "how is wind formed" on Earth. For those who are eager to learn, an explanation is given below.

Wind is formed by the indirect effect of the sun on the Earth’s surface. There is a reasonable explanation why is it so. As the sun shines, the Sun heats the Earth's surface; the air near ground level gets hot and rises up. Cold air at higher altitudes replaces it. At nighttime the process is reversed. This is  in principal how is wind formed on Earth.

Vertical Wind Formation:

This process of hot air rising and colder air sinking, causes high and low air pressure areas and results in wind formation. The more the pressure changes in an area, the faster the wind speed will be. If two areas are close to each other, pressure difference will cause higher wind speed to be produced compared to areas that are far apart from each other. This is how the wind is formed on Earth and this phenomena is known as vertical wind formation.

How Wind is Formed During the Day and at Nighttime:

Wind is formed on Earth from the air motion. Air over land gets heated faster than air over water. When warm air over land rises, cold air over water replaces it. Land loses heat faster than water, during nighttime, the air above water cools faster than air above land. The same process happens again during nighttime but in reverse. This is how wind is formed on Earth during the nighttime. It is the reverse process of vertical wind formation of daytime.

Interestingly enough, most wind moves horizontally and not vertically, that is; wind moves along the Earth’s surface above the ground. This doesn’t contradict with what was stated earlier about hot air rising up and cold air sinking down as this process only causes two areas of different pressures that is responsible for wind and its motion. Usually, wind moving up and down happens during thunderstorm downdrafts. Vertical winds have low speed. Gravity has a lot of effect. It is the driving force that controls the vertical movement of air.

Then what makes wind travel horizontally rather than vertically?

There is another force that has significant affect on wind patterns. It is the earth’s rotation. Wind is deflected from being in a straight line just like all other flying things including birds and planes. Wind is usually deflected mostly near the Earth’s poles and least near the equator. The amount of air deflection is directly related to both the wind speed and its latitude. Therefore, slow winds are deflected only by a small amount, while stronger winds are much more deflected. This is how is wind formed horizontally.

Other Forces that Affect Wind Behavior:

There are other forces that affect the behavior of wind. Friction for instant influence to a large extent winds near ground level. Friction reduces wind speed near ground level and acts in the direction opposite to the wind direction. Therefore, it affects the flow of wind near ground. That is why most horizontal axis wind turbines are installed at high level above ground level and far away from urban cities.

The rotational force of the Earth has an effect on air speed. Air friction reduces air speed and the atmosphere adjusts by turning the wind from high pressure areas towards low pressure areas. Together the Earth’s rotational force, friction and the horizontal pressure gradient force, leads to a balanced system in the atmosphere with spiral wind motion instead of straight motion from high pressure areas to low pressure areas.

With winds directed from high pressure areas to low pressure areas and air that rises from low-pressure area, water reaches its condensation point in clouds. as a result storms and rain are formed in the atmosphere.

Formation of Jet Stream Wind on Earth:

Also, a change in temperature has direct affect on pressure. With large change in temperature wind gets generated. An area in the atmosphere is created with higher winds, it is known as jet stream areas. These jet streams travel across continents due to the differences between warm and cold air areas in the atmosphere. These atmospheric winds are responsible for moving storms from West to East. Jet streams have a great benefit in driving wind turbines that generates power at selected locations.

Does the information provided above answers "How the Wind is Formed on Earth?" If not, post your question in the comment section below.

Wind Turbine Swept Area

The wind turbine swept area is the area through which the wind strikes the wind turbine blades and causes the blades to spin. The wind spinning the blades causes the rotor to turn and generates power that can be used.

Wind Turbine Swept Area

Swept Area –This refers to the area swept by the blades of the wind turbine when the wind turbine rotates. It is also called the 'capture area' as seen in the diagram to the left. Wind Turbine Radius – it is the distance measured from the center of the turbine blade (hub) to the end of one of the blades. Most if not all wind turbine manufacturers print the blade’s length on the nameplate of the rotor and/or on the manufacturer’s data sheet. The blade’s radius (or diameter which is twice the radius) is simply the diameter the blade’s cover circle wise. For a three bladed wind turbine with radius (r) and diameter (2r), the wind turbine swept area would be the area of the circle (A) = Pi r2 as seen in the diagram. The equation for area swept by the wind turbine is employed as part of the power equation that is used to calculate the power output of a wind turbine.

The wind turbine power output is directly related to the swept area of its blades. Increasing the radius of the wind turbine or the longer the blades would result in an increase of the power that can be extracted from the wind. With larger blades, a lot of wind force will be applied on the wind turbine blades. These blades need to be built to be strong in order to withstand higher levels of centrifugal and varying gravitational loads.

Swept Area and Output Power Relationship:

So what is the relationship between the wind turbine swept area and the power output? Looking at the equation for output power you can see that the capture area of the wind turbine blades is utilized to calculate how much power can be extracted from the wind turbine.

Power Output (Watts) = ½ ρ C_P A V³ = ½ ρ C_P (pi r²) V³

Where:

Air density is: (ρ)

Betz limit (Cp): 35% (for a good design)

Swept area is: (A) = Pi r²

and Wind speed is: (V)

See examples on Output Power Calculations

Have you found what you where looking for about "wind turbine swept area"? If not, post your needs in the comment section below and you will see a response within a couple of days.

Wind Turbine Power Output

Everybody talks about wind turbine power output, however; few people know what wind turbine output power really means or "how to calculate wind turbine power output". Formula for calculating output power of wind turbine is a simple equation given towards the end, but general explanation about wind turbine power output will be explained first.

 

Wind Turbine Power Output Overview

The output power of a wind turbine has to be one of the most misunderstood and over quoted aspects of a wind turbine. In order to shed some light on this subject you have to know that wind turbines convert kinetic energy (rotational energy) to mechanical energy and the mechanical energy will be transformed to electrical energy.

In other words, when the alternator (also known as generator) spins and rotate, power will be produced. This is usually typical for all power generating equipments whether it is used for renewable energy or conventional energy such as the alternator that charges your car’s battery (Note: Car's alternator shouldn't be used as a wind turbine alternator without modifications).

However, this does not mean that the alternator can run at any speed and produce the same output power; there is a limitation to how much each and every generator can produce.

Power Output Production and Speed:

When the alternator starts spinning, very little power will be produced. With increased speed, the output power increases and a point will be reached where the power output will be the rated power of the alternator. However, the wind turbine generator can spin faster than its rated power and can produce much more output power.

Eventually, a shutdown value will be reached at higher speed which is usually assigned by the wind turbine alternator manufacturer and will force the generator to stop delivering output power by applying the internal breaking system in order to slowdown the wind turbine and prevent damage to the alternator.

How to Calculate Wind Turbine Power Output?

The wind turbine power output can be calculated using one formula that relates the wind turbine blade diameter (swept area) to the wind speed. You don’t have to concern yourself with how this equation was derived which will be explained somewhere else, however; you can use it to calculate the wind turbine power outlet of three bladed wind turbine.

Formula for Calculating Wind Turbine Power Output

Example: Calculating Wind Turbine Power Output:

Assume you have the below data and you want to calculate the output power of your wind turbine.

Wind speed (V): 3 m/s

Air density (ρ): 1.23 Kg/m3

Betz limit (Cp): 35% (for a good design)

Swept area (A): 5.73 m2

If you plug these values in the power equation, your calculated wind turbine power output would be:

Power Output= ½ (1.23) (.35) (5.73) (3x3x3) = 33.2 Watts

The power output is really too low for you to use, the 3 m/s is usually around the cut-in wind speed, however; if the wind speed was 7 m/s then the wind turbine power output would increase dramatically and can be calculated with the same equation and would produce:

Power Output = ½ (1.23) (.35) (5.73) (7x7x7) = 421.6 Watts

So your turbine can extract about 423.05 watts when the wind speed is 7 m/s which would lead to about 12.8 times more power. However, what would you get if the wind speed was 12 m/s?

Plugging the values in the previous equation while changing only the wind speed value, the wind turbine power output equation yields:

Power Output = ½ (1.23) (.35) (5.73) (12x12x12) = 2,123.8 Watts

What is unique about wind turbine power output is that doubling the wind speed will give 8 times power output while tripling the wind speed will give 27 times the power output and so on. Why is that? It is basically because the wind speed (velocity) is cubed in the power formula. 

Below the graph represents the wind turbine power output curve at different wind speed. The values plotted where taken from the above power equation formula.

 

Data for Calculating Wind Turbine Power Output:

 

Wind Turbine Power Output Data
Wind Speed (m/s)	1	2	3	4	5	6	7	8	9	10	11	12	13
Power Output (Watts)	1.2	9.8	33.2	78.7	153.6	265.5	421.6	629.3	896.0	1,229.1	1,635.9	2,123.8	2,700.3

Data Plot: Wind Turbine Power Output Curve

Have you found what you where looking for about "wind turbine power output"? If not, post your needs in the comment section below and you will see a response within a couple of days.

Wind Turbine Rated Power

Wind Turbine Rated Power – Wind turbine are basically rated in terms of how much power they can produce at a particular wind speed. Most manufacturers if not all will rate their wind turbines at a particular wind speed.

If you take a look at a wind turbine power curve below you would notice that the Y-axis has values starting from zero Kilowatt to some maximum Kilowatt value. These Power rated value are usually the turbine output  power that corresponds to different wind speed values on the X-axis.

Wind Turbine Power Curve

However, this is not the wind turbine rated power. Usually the manufacturer will rate the output power at around 50%-75% of the maximum power output of the alternator and would consider that point to be the wind turbine rated power. Having said that, it means that the wind turbine can produce more power than its rated power. But, why is that?

There is a reasonable explanation why manufacturers rate their turbines lower than the maximum output power. The wind speed is never constant at the same particular value; rather it is fluctuating all the time. Sometimes wind speed with small fluctuation can’t be noticed and some other times large fluctuation can be noticed.

For small fluctuating wind speed, the power output from the wind turbine would stay around the average wind speed of that location, however; for large fluctuating wind speeds, the power output can reach the maximum peak value and even exceed that point.

It is interesting to know that when the wind speed exceeds the maximum peak value its output power starts to drop down and can bring the output power to the wind turbine rated power value or even below. However, if the wind speed becomes excessive, the wind turbine has to protect itself by applying the internal breaking system to slow down the turbine blades and prevent damage to its equipments.

Example:

To simplify things, look at the wind turbine power curve in the graph below. You will notice that the wind turbine rated power is around 6.5 kilowatts at 8.5 meters per second (the red lines). While the maximum output power (peak power) is 10 Kilowatt accruing at around 10.5 meters per second (dotted pink line).

However, if wind speed increases without being turbulent (above 17 m/s), the output power will start to drop down as the wind speed increases till it reaches a pre-assigned value (6 Kilowatts) by the manufacturer. At that point the wind turbine will apply its internal breaking system to slow down the wind turbine from over speeding (to the right of blue dotted line).

Wind Turbine Rated Power Graph

Note that the wind turbine breaking system of the wind turbine are applied in case of over speeding in order to keep the output power near the wind turbine rated power but on the upper limit of the power curve.

Wind Turbine Cut-out Speed

Wind turbine cut-out wind speed is much more important than cut-in wind speed. Wind turbine cut-out speed means, the highest predicted wind speed at which an operating wind turbine stops producing power due to excess wind. The cut-out wind speed is designed to apply the breaking system to slow down or stop the turbine from spinning and protect your equipment from damage.

The wind speed is usually monitored by the turbine control system and no power will be generated above the cut-out wind speed. There are several ways to stop a wind turbine in high wind. In some wind turbines, speed sensors are installed to automatically apply a built-in braking system. Other (large) turbines twist the blades away from incoming wind also known as "pitching the blades" to allow the wind to pass through thus reducing the wind turbine speed.

Some wind turbines use drag flaps that are mounted on the blades of the wind turbine to reduce blade speed or the hub itself will automatically activate drag flaps at high rotor RPM's. Also, some wind turbines are mechanically activated by spring loaded devices which turns the machine away from the wind stream. Usually wind turbines resume operation when the wind speed drops back to a safe level below the cut-out wind speed.

Wind turbines make use of cut-out wind speed for damage prevention. Regardless of the method used, if your rotor or power electronics are not equipped with cut-out wind speed function, serious damage may occur to your wind turbine or electronic equipments. You should not be confused between wind turbine cut-out wind speed and wind turbine cut-in speed.

Wind Turbine Cut-in Speed

Wind turbine cut-in speed means, the lowest wind speed at which the wind turbine will start to generate noticeable electrical output power. If you were to search for a wind turbine, don't consider cut-in wind speed as a primary goal. It is not desirable nor is it preferable to buy a wind turbine with low cut-in .

The reason why you should ignore cut-in speed is that the generator of the wind turbine won't start to charge your battery or battery banks till the output voltage from the alternator is higher than the battery voltage or battery bank.

However, if you were to find a suitable wind turbine and the turbine has a low cut-in wind speed you may consider it since this can produce at least some kind of electrical output power most of the times when wind speed is low and your battery or battery bank is fully discharged.

On the other hand, if you’re designing your own wind turbine, you may not need to worry about cut-in wind speed since you might be able to utilize funneling technique to your wind turbine to increase the wind speed.

Wind Turbine Power Curve

Take a look at the wind turbine power curve below; you will notice that there are two axes, the X-axis and the Y-axis. The X-axis represents the hub wind speed in meters per second (m/s) and the Y-axis represents the expected power output in kilo Watts (kW) of the wind turbine.

Wind Turbine Power Curve

If you have taken wind speed data at the location you intend to install your wind turbine, then you can predict the average power output from your wind turbine generator from the power curve. Just make sure that the wind speed data recorded is taken at the same height as the hub height, otherwise your wind speed data would be incorrect.

For example, if you are planning to install a 10kW wind turbine at your location and the average wind speed recorded over a week period during summer time was 5 m/s at ground level, then your site location may be ideal for installation of a wind turbine. You might not believe this if you were to look at the wind turbine power curve above since at 5 m/s the wind turbine would produce about ~2kW.

Actually, you may not have noted that you have taken your wind speed data at ground level. Usually the hub height is at least 10 meters above ground level and wind speed tend to approximately double every ~12 meters provided obstacles such as buildings and trees are not in close vicinity to your wind turbine.

However, 5 m/s at ground level during summer time (winter time wind speed is higher) is a good indication for a suitable location for wind turbine installation and you should take measurements at higher heights to be certain. It is highly recommended that you take several readings around the same location as wind paths can be a few meters away from where you are.

For homeowners who intend to install small permanent magnet generators (alternators) it is extremely important to get the power curve from the manufacturer since this power curve is different from one manufacturer to another. Furthermore, a wind turbine power curve is usually corrected if your site location is more than 300m above sea level or ambient temperature is far above 20 degrees C.

Wind Turbines

Wind turbines are devices that transform wind energy into mechanical energy to generate electricity. Wind turbines can have one blade or multi-blades depending on their area of application.

There are two types of wind turbines; the vertical axis wind turbine (VAWT) and the horizontal axis wind turbine (HAWT). Today, most of the wind turbines use a horizontal axis with two or three blades due to their high tip speed and efficiency which contribute to good reliability.

There are two types of applications for wind turbines. Either stand-alone, or synchronized and connected  to the utility power grid. Also, wind turbines can be integrated with a photovoltaic (solar cell) system, batteries, and diesel generators and in such case they are known as hybrid systems.

Water pumping wind turbines are examples of a stand-alone application that doesn’t produce electricity and use multi-blades similar to the ones used in the old west. These days a large number of electricity generating wind turbines are being erected in close vicinity to each other in designated areas known as wind farms to harvest the wind energy and feed the utility power grid. However, in windy areas far away from the utility power grid, homeowners and farmers are utilizing wind turbines to generate electricity.

There are several factors that can be used to predict electrical power output that can be generated from a  wind turbine. These factors can be extracted from manufacturer’s data sheets and power curve. The most important of these factors include:

• Wind Turbine Power Curve
• Wind Turbine Cut-in Speed
• Wind Turbine Cut-out Speed
• Wind Turbine Rated Power
• Wind turbine Power Output

Based on these factors that can be extracted from the manufacturer’s data sheet  contribute to the wind turbine capacity factor.

Using Small Wind Turbines in Underground Mines

In underground mines, it has been recognized that there is a requirement for an independent battery charging system other than the AC power source that can be used in case of emergency since the radios located through the underground mines can’t stay charged for sufficient periods to insure the safety of trapped mineworkers.

Since it was a necessity to always have air distributed throughout the mine in order to supply oxygen and remove unwanted gases, it was suggested to use this air (wasted energy) to charge the batteries using small wind turbines that can be installed in the path of moving air that is forced into the underground mine.

It was necessary to determine the feasibility of the idea of using small wind turbines that are placed in the airflow path to charge batteries especially since the airflow will not be much variant. It was found that it was feasible and a numerical model was developed utilizing Blade Element Momentum theory to produce optimal battery charging system to aid in case of emergency mine communications.

The power requirements to charge the batteries were 4 Watts and several rotor configurations have been found to produce the amount of power required. However, due to safety issues and structural concerns in an underground mine, the required wind turbine should be designed and shielded to protect mine workers. It was possible to provide the structural protection with the use of a diffuser shroud which also helped in increasing the power output of the wind turbine system.

Results from the model and the physical parameters needed to efficiently design a wind turbine system for underground mines was addressed by this thesis and has been published in this book: Feasibility and optimum design study of a low speed wind turbine rotor system for underground communication power.

Electronic Wind Speed Meter

There are a lot of electronic wind speed meters on the market, however; the Davis Turbo Electronic Wind Speed Meter measures wind speed and provides maximum sensitivity and accuracy for your convenience which is vital for your wind turbine site. The unit measures wind speed from 0 - 99.9 mph . The turbine rotation is sensed by an infrared light beam which adds no friction to your reading and gives you the accuracy you desire.

Lebanon's Vertical Axis Wind Turbine

The first phase of a homemade prototype Vertical Axis Wind Turbine (VAWT) that was constructed in Lebanon. It stands near 6 meters high (~19.68 feet ). Made of no more than galvanized sheets welded together, galvanized water pipes and a few self lubricated bearings.

Background...

In September 2009 I have moved back to Bayssour-Lebanon to settle and live their permanently with my family after being away working in the Arabian Gulf most of my life. Empowered with a university degree from the University of Arizona in Electrical Engineering and 15+ years of diversified  and extensive experience in the electrical field. I found myself in a new community that lacks continuous electrical power due to power rational which was the result of the more than 15 years of civil war.