WIND ENERGY SYSTEMS - Cd3wd

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Chapter 3—Wind Measurements 3–15 an increase of pressure in the tube, and that an air flow across the mouth causes a suction. The pressure in a thin tube facing the wind is p 1 = p s + 1 2 ρu2 Pa (15) where p s is the atmosphere pressure. The pressure in a tube perpendicular to the wind is p 2 = p s − 1 2 c 1ρu 2 Pa (16) where c 1 is a constant less than unity. The total difference of pressure will then be ∆p = p 1 − p 2 = 1 2 ρu2 (1 + c 1 ) Pa (17) The combination of parallel and perpendicular tubes is preferred because the atmospheric pressure term can be eliminated. The pressure difference can be measured with a manometer or with a solid state pressure-to-voltage transducer. Air density needs to be measured also, in order to make an accurate computation of u in Eq. 17. This technique of measuring wind speed also lends itself to a fully solid state measurement system with no moving parts. A number of perpendicular pairs of pressure tubes would be required, each with its own differential pressure transducer feeding into a microprocessor. The microprocessor would select the transducer with the largest value, perhaps correct for pointing error, compute air density from atmospheric pressure and temperature measurements, and compute the wind speed. The major difficulty would be keeping the pressure tubes free of moisture, dirt, and insects so that readings would be accurate. Hot Wire Anemometers The hot wire anemometer depends on the ability of the air to carry away heat. The resistance of a wire varies with temperature, so as the wind blows across a hot wire the wire tends to cool off with a corresponding decrease in resistance. If the hot wire is placed in one leg of a bridge circuit and the bridge balance is maintained by increasing the current so the temperature remains constant, the current will be related to the wind speed by i 2 = i 2 o + K √ u A 2 (18) where i o is the current flow with no wind and K is an experimentally determined constant. The wire, made of fine platinum, is usually heated to about 1000 o C to make the anemometer output reasonably independent of air temperature. This anemometer is especially useful in measuring very low wind speeds, from about 0.05 to 10 m/s. Wind Energy Systems by Dr. Gary L. Johnson November 12, 2001
Chapter 3—Wind Measurements 3–16 Rather sophisticated equipment is required to make the hot wire anemometer convenient to use. Rain drops striking the wire may cause it to break, making it difficult to use outdoors on a continuous basis. The power consumption may also be significant. The hot wire anemometer will probably not be important to wind power studies because of these difficulties. Doppler Acoustic Radar Sonic anemometers, orDoppler acoustic radars, as they are often called, use sound waves reflecting off small blobs or parcels of air to determine wind speed. A vertical profile of wind speed is typically determined8 from one receiving antenna and three transmitting antennas, located at ground level and arranged as in Fig. 8. The receiving antenna is pointed straight up. The transmit antennas are aimed toward the vertical line above the receiving antenna. They need to have rather narrow beamwidths so they do not illuminate the receiving antenna directly but just the space above it. Transmit beamwidths of about 50 degrees in the vertical and 35 degrees in the horizontal can be obtained from commercially available high frequency driver and spectral horn speaker combinations. The receiving antenna may consist of another high frequency driver coupled to a parabolic dish, with the entire antenna inside an acoustic enclosure which serves to suppress the side lobes of the receive antenna pattern. The receive antenna beamwidth may be around 15 degrees with a good enclosure. Figure 8: Doppler acoustic radar configuration. Each transmit antenna in sequence broadcasts an audio pulse in the frequency range of 2000 to 3000 Hz. Some of these signals are reflected by atmospheric scatterers (small regions of slightly different density or pressure in the air stream) into the receiving antenna. The frequency and phase of the received signal are analyzed to determine the horizontal and vertical velocities of the atmospheric scatterers. Time delays are used to map the velocities Wind Energy Systems by Dr. Gary L. Johnson November 12, 2001
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