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Anemometers: Measuring Wind Speed Debajyoti Bose Lt. Col. Ashis Khare Pravin Badarayani

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Anemometers: Measuring Wind Speed

Debajyoti Bose

Lt. Col. Ashis Khare

Pravin Badarayani

Anemometer: Basics

A common weather station instrument

The term is derived from the Greek word anemos, meaning wind

Describe any air speed measurement instrument used in meteorology or aerodynamics

Invented in 1846 byJohn Thomas Romney Robinson

Image: National Weather Service (NWS) Collection, Baltimore, Image ID: wea00920

Anemometer: Reason For Use

Two primary reasons to measure wind speed:

To determine feasibility of wind power development at a site As part of a wind turbine control system

“Is it worthwhile to turn the turbine into the wind

and start it?”

Animation: How the air moves forming Winds, V. Ryan © 2005

Anemometer: Types

Mechanical Type: Cup Anemometer

Non-Mechanical Type: Hot Wire Anemometers Ultrasonic Anemometers Laser/Doppler Anemometers

Pressure Anemometers Plate Anemometers Tube Anemometers

Propeller Type Anemometers

Image: Digital Anemometer, www.everflowscientific.com

Cup Anemometers

Most common wind speed measurement device

It consists of hemispherical cups, each mounted on one end of horizontal arms

And the arms were mounted at equal angles to each other on a vertical shaft

Image: www.directindustry.com

Cup Anemometers: Continued

The air flow past the cups in any horizontal direction turned the shaft in a manner that is proportional to the wind speed

On counting the turns of the shaft over a set time period produced the average wind speed for a wide range of speeds

Animation: Anemometer 2 from Barani Design and Wind101.net

In a Nutshell: Cup Anemometers

Many cup anemometers have a vane attached to measure wind direction

Advantages Disadvantages

Low Price Flexible Design Simple Installation

Most technicians understand operating principles and necessary connections

Moving parts wear out

Without provisions for heating, they don’t work well in snow or freezing rain

They don’t work well in rapidly fluctuating winds

Image: http://www.photolib.noaa.gov/nssl/nssl0161.htm

Hot Wire Anemometers

Uses a very fine wire (on the order of several micro-metres)

Electrically heated up to some temperature above the ambient

Air flowing past the wire has a cooling effect on the wire

“The electrical resistance of most metals is dependent upon the temperature of the metal”

Using above a relationship can be obtained between the resistance of the wire and the flow speed

Probe Specifications

“Tungsten is a popular choice for hot-wires”Tungsten or Platinum filament~1 mm long4-10 mm diameter

BenefitsGood spatial resolutionFlat frequency response

LimitationsFragileRequires clean flowCost (start at $300-400)

Image: www.tungstenringsco.com

Typical Specifications

Parameters Handheld/Economy Industrial Grade

Measurable velocities 0.2-20 m/s 0.2-90 m/s

Operating temp ranges 0-50 °C -40-200 °C

Velocity Accuracy ± 3% reading ± 1% reading

Time constant 200 ms 100 ms

Interfacing options Handheld reader, RS232

RS232, RS485, voltage, 4-20 mA, Modbus,

Profibus, etc.

Source: http://www.extech.com/instruments/product.asp?catid=1&prodid=43

Ultrasonic Anemometers

First developed in the 1950s Use of ultrasonic sound waves to

measure wind velocity They measure wind speed based

on the time of flight of sonic pulses between pairs of transducers

Measurements from pairs of transducers can be combined to yield a measurement of velocity in 1-, 2-, or 3-dimensional flow

Figure: 2D ultrasonic anemometer with 3 paths

Image: Anémomètre / girouette ultrasonique à trois chemins

Ultrasonic Anemometers

Advantage Disadvantage

The lack of moving parts makes them appropriate for long-term use in exposed automated weather stations and weather buoys where the accuracy and reliability of traditional cup-and-vane anemometers is adversely affected by salty air or large amounts of dust

Main disadvantage is the distortion of the flow itself by the structure supporting the transducers, which requires a correction based upon wind tunnel measurements to minimize the effect

Figure: 3D ultrasonic anemometerImage: Google

Laser/Doppler Anemometers

Laser/Doppler anemometers use a beam of light from a laser that is divided into two beams, with one propagated out of the anemometer

Particulates (or deliberately introduced seed material) flowing along with air molecules near where the beam exits reflect, or backscatter, the light back into a detector, where it is measured relative to the original laser beam

When the particles are in great motion, they produce a Doppler shift for measuring wind speed in the laser light, which is used to calculate the speed of the particles, and therefore the air around the anemometer

Plate Anemometers

Modern anemometers

Are simply a flat plate suspended from the top so that the wind deflects the plate

The pressure of the wind on its face is balanced by a spring

The compression of the spring determines the actual force which the wind is exerting on the plate

This is either read off on a suitable gauge, or on a recorder

Plate Anemometers: Continued

Advantage Disadvantage

They are used on these high places because they are in a plate shape; has a good measurement status on higher altitudes

Instruments of this kind do not respond to light winds, are inaccurate for high wind readings, and are slow at responding to variable winds.

Image: www.stormdebris.net

Tube Anemometers

A tube anemometer uses air pressure to determine the wind pressure, or speed

A tube anemometer measures the air pressure inside a glass tube that is closed at one end

By comparing the air pressure inside the tube to the air pressure outside the tube, wind speed can be calculated

Figure: Invented by William Henry Dines in 1892, The movable part (right) to be put on top of the fixed part (left)

Image: Wikipedia

Propeller Type/Vane Anemometers

The axis on the vane anemometer is parallel to the direction of the wind and therefore horizontal 

Since the wind varies in direction and the axis has to follow its changes

Combines a propeller and a tail on the same axis to obtain accurate and precise wind speed and direction measurements from the same instrument

Image: nssl0161, National Severe Storms Laboratory (NSSL) Collection

Output from Anemometers

Signal conditioning is usually done within the instrument The output can be an electrical signal to a data-logger or 

readout device: Pulse signal Voltage signal For example, 0-10 V corresponds to the velocity measurement range of

the instrument.

Current signal Typically, 4-20 mA corresponding to the instrument range. Eliminates voltage drop when the signal is transmitted over larger

distances.

Importance of Accurate Wind Speed Measurements The power obtained from the wind goes with the cube of 

the wind speed A small error in the measurement results in a much larger 

error in the predicted wind power

For example, a 5% error at a wind speed of 10 meters/sec leads to a 16% error in predicted wind power

10% anemometer error leads to 33% errors in power prediction

This could be disastrous if one is monitoring a site for feasibility of wind power development!

Thank You Very Much Indeed !!!

A Presentation by M. Tech Renewable Energy Engineering

Debajyoti Bose: R120214007

Lt. Col. Ashis Khare: R120214006

Pravin Badarayani: R120214005

Department of Electrical Engineering