Measurement of Wind ConditionsMeasurement of Wind Conditions

Knowledge required for the design of wind turbines Knowledge required for the design of wind turbines includes:includes: Wind speed and directionWind speed and direction Inclination to the groundInclination to the ground Turbulence levelsTurbulence levels General weather conditionsGeneral weather conditions

• TemperatureTemperature

• PressurePressure

• HumidityHumidity

• PrecipitationPrecipitation

• . . .. . .

Measurement of Wind SpeedMeasurement of Wind Speed

Wind speed is the magnitude of the wind velocity Wind speed is the magnitude of the wind velocity vector.vector. The wind velocity is a vector, which includes both speed The wind velocity is a vector, which includes both speed

and direction.and direction. Wind Speed is measured with a device called a wind Wind Speed is measured with a device called a wind

anemometer.anemometer. The term “anemometer” strictly refers to a general fluid The term “anemometer” strictly refers to a general fluid

speed measuring device.speed measuring device.• For example “hot wire anemometers” are used to measure For example “hot wire anemometers” are used to measure

turbulence levels in fluid flows.turbulence levels in fluid flows.• In general conversation, when we say “anemometer,” most people In general conversation, when we say “anemometer,” most people

assume we are talking about a wind speed measuring device.assume we are talking about a wind speed measuring device.

Importance of Accurate Wind Speed Importance of Accurate Wind Speed MeasurementsMeasurements

Remember – the power obtained from the wind goes Remember – the power obtained from the wind goes with the cube of the wind speed.with the cube of the wind speed.

A small error in the measurement results in a much A small error in the measurement results in a much larger error in the predicted wind power.larger error in the predicted wind power.

For example, a 5% error at a wind speed of 10 For example, a 5% error at a wind speed of 10 meters/sec leads to a 16% error in predicted wind meters/sec leads to a 16% error in predicted wind power.power. 10% anemometer error leads to 33% errors in power 10% anemometer error leads to 33% errors in power

prediction.prediction.• This could be disastrous if you are monitoring a site for This could be disastrous if you are monitoring a site for

feasibility of wind power development!feasibility of wind power development! This also leads to large errors in efficiency calculations, This also leads to large errors in efficiency calculations,

loads predictions, and so on.loads predictions, and so on.

Perspective – Why are We Measuring Perspective – Why are We Measuring the Wind Speed?the Wind Speed?

In the wind energy field, there are two primary In the wind energy field, there are two primary reasons we might want to measure wind speed.reasons we might want to measure wind speed.

1.1. To determine feasibility of wind power development at a To determine feasibility of wind power development at a site.site.

2.2. As part of a wind turbine control system.As part of a wind turbine control system.• Essentially, to answer the question: “Is it worthwhile to turn the Essentially, to answer the question: “Is it worthwhile to turn the

turbine into the wind and start it?”turbine into the wind and start it?”

The accuracy needed for application 1 is much The accuracy needed for application 1 is much greater than that needed for application 2.greater than that needed for application 2.

Wind AnemometersWind Anemometers The most common wind speed measurement device is the cup The most common wind speed measurement device is the cup

anemometer.anemometer.

This anemometer is heated to prevent ice buildup. This anemometer is heated to prevent ice buildup. It is made by a Danish Company – the spec sheet will be attached to It is made by a Danish Company – the spec sheet will be attached to

the blackboard sitethe blackboard site

Anemometers (continued)Anemometers (continued)

Many cup anemometers have a vane attached Many cup anemometers have a vane attached to measure wind direction.to measure wind direction.

A few Advantages of Cup A few Advantages of Cup AnemometersAnemometers

Low priceLow price FlexibleFlexible

Designs have been developed for all climates.Designs have been developed for all climates. Simple Installation.Simple Installation. Common instrument, most technicians understand Common instrument, most technicians understand

operating principles and necessary connectionsoperating principles and necessary connections Accuracies of 1% can be achieved with calibration of Accuracies of 1% can be achieved with calibration of

higher quality devices.higher quality devices. They remain accurate when the wind has a significant They remain accurate when the wind has a significant

vertical component, even up to 30°.vertical component, even up to 30°.

A few Disadvantages of Cup A few Disadvantages of Cup AnemometersAnemometers

Moving parts wear out.Moving parts wear out. Cheap versions are not very accurate.Cheap versions are not very accurate. Electronic output requires a motor generator, or some Electronic output requires a motor generator, or some

type of counting circuit.type of counting circuit. This isn’t very expensive anymore.This isn’t very expensive anymore.

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

They don’t work well in rapidly fluctuating winds.They don’t work well in rapidly fluctuating winds. On the other hand, neither do wind turbines.On the other hand, neither do wind turbines.

Other Types of AnemometersOther Types of Anemometers

Non-mechanical AnemometersNon-mechanical Anemometers Hot Wire AnemometersHot Wire Anemometers Ultrasonic AnemometersUltrasonic Anemometers Laser/Doppler AnemometersLaser/Doppler Anemometers

Propeller type anemometers:Propeller type anemometers:

Turbulence Measuring Anemometers: Turbulence Measuring Anemometers:

Output from AnemometersOutput from Anemometers

Signal conditioning is usually done within the instrument.Signal conditioning is usually done within the instrument. The output can be an electrical signal to a datalogger or The output can be an electrical signal to a datalogger or

readout device:readout device: Pulse signalPulse signal Voltage signal Voltage signal

• For example, 0-10 V corresponds to the velocity measurement range of the For example, 0-10 V corresponds to the velocity measurement range of the instrument.instrument.

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

distances.distances.

In addition, the output can be internally digitized and directly In addition, the output can be internally digitized and directly sent to a digital data acquisition system. sent to a digital data acquisition system.

Measurement and Specification of Wind Direction,Measurement and Specification of Wind Direction,with respect to a horizontal planewith respect to a horizontal plane

Wind direction can vary both with respect to the horizontal Wind direction can vary both with respect to the horizontal plane and the vertical direction.plane and the vertical direction.

For example, on top of the windward side of an abrupt hill For example, on top of the windward side of an abrupt hill there is usually a significant upward component to the wind there is usually a significant upward component to the wind velocity.velocity.

In tall-tower installations, the turbine is often high enough to In tall-tower installations, the turbine is often high enough to be in nearly-horizontal wind. be in nearly-horizontal wind.

The most common way to measure wind direction is with The most common way to measure wind direction is with a vane or sock attached to a pivot.a vane or sock attached to a pivot.

The vane or sock is forced downstream by the drag force, The vane or sock is forced downstream by the drag force, thus rotating the pivot.thus rotating the pivot.

Large orange socks, supposedly visible to pilots, are used at Large orange socks, supposedly visible to pilots, are used at airports.airports.

A wind from the north is at 0° (or 360°), from the East is at A wind from the north is at 0° (or 360°), from the East is at 90°, and so on.90°, and so on.

Measurement of the Absolute Wind Measurement of the Absolute Wind Direction Direction

Measurement of both horizontal Measurement of both horizontal andand vertical vertical components of the wind velocity vector is much more components of the wind velocity vector is much more complicated than measuring just the horizontal complicated than measuring just the horizontal component.component.

In theory, two vanes, one pivoted about the vertical In theory, two vanes, one pivoted about the vertical axis and an attached vane pivoted about the local axis and an attached vane pivoted about the local (rotated) horizontal axis could do the job.(rotated) horizontal axis could do the job.

Several instruments have been developed that can do Several instruments have been developed that can do the job. the job. An example is given on the next page.An example is given on the next page.

Two Axis Ultrasonic Wind Direction Two Axis Ultrasonic Wind Direction Sensor Sensor

http://www.thiesclima.com/usanemo_e.htmhttp://www.thiesclima.com/usanemo_e.htm http://www.thiesclima.com/ind1e.htmhttp://www.thiesclima.com/ind1e.htm

Measurements of Gusts and Measurements of Gusts and TurbulenceTurbulence

The measurement of wind gusts and turbulence requires that the instrument The measurement of wind gusts and turbulence requires that the instrument and data acquisition system be capable of responding to changes in and data acquisition system be capable of responding to changes in velocity on the time scales of gusts and turbulence.velocity on the time scales of gusts and turbulence.

Technically, wind gusts are essentially turbulence phenomena – but in this Technically, wind gusts are essentially turbulence phenomena – but in this context, by turbulence we means smaller time scale phenomena.context, by turbulence we means smaller time scale phenomena.

Turbulence time scales range up to a few seconds.Turbulence time scales range up to a few seconds. Wind gusts may vary over on the order of several seconds. Wind gusts may vary over on the order of several seconds.

The two-axis instrument on the previous page is capable of taking a The two-axis instrument on the previous page is capable of taking a measurement in about 20 milliseconds – it is a good turbulence measuring measurement in about 20 milliseconds – it is a good turbulence measuring device.device.

Most installed monitoring systems average the readings over a period of Most installed monitoring systems average the readings over a period of several minutes.several minutes.

Some weather monitoring stations output both mean and peak gust values over Some weather monitoring stations output both mean and peak gust values over a given period.a given period.

A Complete Weather StationA Complete Weather Station Other properties of interest in wind Other properties of interest in wind

applications include temperature, applications include temperature, atmospheric pressure, humidity and atmospheric pressure, humidity and precipitation.precipitation.

Many companies offer a complete Many companies offer a complete set of measurements in a package set of measurements in a package that is usually called a “weather that is usually called a “weather station.”station.” For example, access For example, access www.omega.comwww.omega.com

search for “weather station”search for “weather station” The unit to the right is $2190 from The unit to the right is $2190 from

OmegaOmega

Subtleties of Measurements in Subtleties of Measurements in Atmospheric AirAtmospheric Air

The temperature of atmospheric air must be carefully The temperature of atmospheric air must be carefully measured in the shade, in a ventilated enclosure measured in the shade, in a ventilated enclosure protected from all radiationprotected from all radiation If it is measured in the sun, the temperature of the If it is measured in the sun, the temperature of the

thermometer will increase until it is in equilibrium with the thermometer will increase until it is in equilibrium with the atmospheric atmospheric andand radiant environment – a much higher radiant environment – a much higher temperature than the actual air temperature.temperature than the actual air temperature.

• This is why Arizona is so dangerous in the summer! As long as we This is why Arizona is so dangerous in the summer! As long as we stay in the shade, we are more comfortable than more humid stay in the shade, we are more comfortable than more humid climates. On the other hand, once we are out in the sun, the heat climates. On the other hand, once we are out in the sun, the heat load we absorb is load we absorb is muchmuch higher than humid climates. You can’t higher than humid climates. You can’t survive very long in the desert sun!survive very long in the desert sun!

Most weather stations measure relative humidity, but there are several ways to do Most weather stations measure relative humidity, but there are several ways to do this.this.

Some stations measure wet bulb temperature and/or dew point. Take MET 432 for a Some stations measure wet bulb temperature and/or dew point. Take MET 432 for a complete explanation.complete explanation.

LocalLocal variations in barometric pressure are not too important in terms of how much variations in barometric pressure are not too important in terms of how much power the wind can produce.power the wind can produce.

Temperature and Humidity have a greater effect on the air density. Temperature and Humidity have a greater effect on the air density. • Pilots – see “density altitude.” Pilots – see “density altitude.”

However, barometric pressure is a good indicator of approaching changes in the weather. However, barometric pressure is a good indicator of approaching changes in the weather. As the atmospheric density changes with increasing elevation, the effect of power As the atmospheric density changes with increasing elevation, the effect of power

production is more significant.production is more significant. Standard density at 7000 feet is about 80% of that at sea level.Standard density at 7000 feet is about 80% of that at sea level.

Precipitation is easily measured as long as it is in the form of calm rain.Precipitation is easily measured as long as it is in the form of calm rain. Violent storms and hail can result in inaccuracies.Violent storms and hail can result in inaccuracies. Most weather stations don’t measure snow depth, and very few measure the moisture Most weather stations don’t measure snow depth, and very few measure the moisture

content of snow.content of snow.

Subtleties of Measurements in Subtleties of Measurements in Atmospheric Air (continued)Atmospheric Air (continued)