Measurement of Wind Conditions • Knowledge required for the design of wind turbines includes: • Wind speed and direction • Inclination to the ground • Turbulence levels • General weather conditions • Temperature • Pressure • Humidity • Precipitation • . . .
Measurement of Wind Speed • Wind speed is the magnitude of the wind velocity vector. • The wind velocity is a vector, which includes both speed and direction. • Wind Speed is measured with a device called a wind anemometer. • The term “anemometer” strictly refers to a general fluid speed measuring device. • For example “hot wire anemometers” are used to measure turbulence levels in fluid flows. • In general conversation, when we say “anemometer,” most people assume we are talking about a wind speed measuring device.
Importance of Accurate Wind Speed Measurements • Remember – 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 you are monitoring a site for feasibility of wind power development! • This also leads to large errors in efficiency calculations, loads predictions, and so on.
Perspective – Why are We Measuring the Wind Speed? • In the wind energy field, there are two primary reasons we might want to measure wind speed. • To determine feasibility of wind power development at a site. • As part of a wind turbine control system. • Essentially, to answer the question: “Is it worthwhile to turn the turbine into the wind and start it?” • The accuracy needed for application 1 is much greater than that needed for application 2.
Wind Anemometers • The most common wind speed measurement device is the cup anemometer. • This anemometer is heated to prevent ice buildup. • It is made by a Danish Company – the spec sheet will be attached to the blackboard site
Anemometers (continued) • Many cup anemometers have a vane attached to measure wind direction.
A few Advantages of Cup Anemometers • Low price • Flexible • Designs have been developed for all climates. • Simple Installation. • Common instrument, most technicians understand operating principles and necessary connections • Accuracies of 1% can be achieved with calibration of higher quality devices. • They remain accurate when the wind has a significant vertical component, even up to 30°.
A few Disadvantages of Cup Anemometers • Moving parts wear out. • Cheap versions are not very accurate. • Electronic output requires a motor generator, or some type of counting circuit. • This isn’t very expensive anymore. • Without provisions for heating, they don’t work well in snow or freezing rain. • They don’t work well in rapidly fluctuating winds. • On the other hand, neither do wind turbines.
Other Types of Anemometers • Non-mechanical Anemometers • Hot Wire Anemometers • Ultrasonic Anemometers • Laser/Doppler Anemometers • Propeller type anemometers: • Turbulence Measuring Anemometers:
Output from Anemometers • Signal conditioning is usually done within the instrument. • The output can be an electrical signal to a datalogger 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. • In addition, the output can be internally digitized and directly sent to a digital data acquisition system.
Measurement and Specification of Wind Direction,with respect to a horizontal plane • Wind direction can vary both with respect to the horizontal plane and the vertical direction. • For example, on top of the windward side of an abrupt hill there is usually a significant upward component to the wind velocity. • In tall-tower installations, the turbine is often high enough to be in nearly-horizontal wind. • The most common way to measure wind direction is with a vane or sock attached to a pivot. • The vane or sock is forced downstream by the drag force, thus rotating the pivot. • Large orange socks, supposedly visible to pilots, are used at airports. • A wind from the north is at 0° (or 360°), from the East is at 90°, and so on.
Measurement of the Absolute Wind Direction • Measurement of both horizontal and vertical components of the wind velocity vector is much more complicated than measuring just the horizontal component. • In theory, two vanes, one pivoted about the vertical axis and an attached vane pivoted about the local (rotated) horizontal axis could do the job. • Several instruments have been developed that can do the job. • An example is given on the next page.
Two Axis Ultrasonic Wind Direction Sensor • http://www.thiesclima.com/usanemo_e.htm • http://www.thiesclima.com/ind1e.htm
Measurements of Gusts and Turbulence • The measurement of wind gusts and turbulence requires that the instrument and data acquisition system be capable of responding to changes in velocity on the time scales of gusts and turbulence. • Technically, wind gusts are essentially turbulence phenomena – but in this context, by turbulence we means smaller time scale phenomena. • Turbulence time scales range up to a few 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 measurement in about 20 milliseconds – it is a good turbulence measuring device. • Most installed monitoring systems average the readings over a period of several minutes. • Some weather monitoring stations output both mean and peak gust values over a given period.
A Complete Weather Station • Other properties of interest in wind applications include temperature, atmospheric pressure, humidity and precipitation. • Many companies offer a complete set of measurements in a package that is usually called a “weather station.” • For example, access www.omega.com search for “weather station” • The unit to the right is $2190 from Omega
Subtleties of Measurements in Atmospheric Air • The temperature of atmospheric air must be carefully measured in the shade, in a ventilated enclosure protected from all radiation • If it is measured in the sun, the temperature of the thermometer will increase until it is in equilibrium with the atmospheric and radiant environment – a much higher temperature than the actual air temperature. • 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 climates. On the other hand, once we are out in the sun, the heat load we absorb is much higher than humid climates. You can’t survive very long in the desert sun!
Subtleties of Measurements in Atmospheric Air (continued) • Most weather stations measure relative humidity, but there are several ways to do this. • Some stations measure wet bulb temperature and/or dew point. Take MET 432 for a complete explanation. • Local variations in barometric pressure are not too important in terms of how much power the wind can produce. • Temperature and Humidity have a greater effect on the air density. • Pilots – see “density altitude.” • 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 production is more significant. • 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. • Violent storms and hail can result in inaccuracies. • Most weather stations don’t measure snow depth, and very few measure the moisture content of snow.