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. So 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. The 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. The 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
Wind Power, which is measured in Watts, is the power we can 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
So 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. So 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. 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
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you find, at the menu KD-reports, more than fifty reports about different technical aspect of small wind turbines which can be copied for free. At the top of the list with KD-reports there is a note: “Sequence KD-reports for self study” which may be of a help to select the reports which may be of most interest for you. At the menu “VIRYA-folders” there is a separate folder about seven free designs of small VIRYA wind turbines. At page 3 of the folder “Extended specification” there is an overview of the main specifications of these seven wind turbines.