Instrumentation

An anemometer is a device for measuring airflow. There are a number of measuring devices for wind speed: pitot tube, cup, vane, propeller, hot wire, hot film, sonic, and laser Doppler anemometers. The common devices are the cup and propeller anemometers, since they are cheaper. However, their response times to changes in wind speed are slower. Wind turbines also have a response time to changes in wind speed, so cup anemometers are adequate for determining the wind energy potential. Sonic and hot wire anemometers have the advantage of no moving parts and no response time in contrast to mechanical sensors. However, their higher cost has kept them from much penetration into the wind resource assessment market.

An anemometer can be obtained to measure the amount of wind that has passed, a wind run. From the wind run, the average wind speed can be calculated for the time period. An anemometer can also be obtained to measure the fastest mile, the maximum wind speed.

Previously, meters and strip charts, which give analog outputs, were used. However, analyzing strip chart data becomes quite tedious, and the time resolution is fairly coarse unless the paper feed rate is large. Today the major difference is the availability of microprocessors for sampling, storing, and even analyzing data in real time. Also, personal computers alleviate most of the problems in analyzing large amounts of data.

Wind resurces over open sea (more than 10 km offshore) for five standard heights

=

10 m ms-i Wm-J

25 m ms-i Wm-J

50 m ms-i Wm-J

100 m ms-i Wm-J

200 m ms-i Wm-J

>8.0 >600 7.0-8.0 350-600 6.0-7.0 250-300 4.5-6.0 100-250 <4.5 <100

>8.5 >700 7.5-8.5 450-700 6.5-7.5 300-450 5.0-6.5 150-300 <5.0 <150

>9.0 >800 8.0-9.0 600-800 7.0-8.0 400-600 5.5-7.0 200-400 <5.5 <200

>10.0 >1100 8.5-10.0 650-1100 7.5-8.5 450-650 6.0-7.5 250-450 <6.0 <250

>11.0 >1500 9.5-11.0 900-1500 8.0-9.5 600-900 6.5-8.0 300-600 <6.5 <300

FIGURE 4.7 European offshore wind resources at five heights for open sea. (From I. Troen and E. L. Petersen, European Wind Atlas, Riso National Laboratory, Denmark, 1989. With permission.)

FIGURE 4.7 European offshore wind resources at five heights for open sea. (From I. Troen and E. L. Petersen, European Wind Atlas, Riso National Laboratory, Denmark, 1989. With permission.)

Digital instruments or analog inputs, which are digitized, typically have sample rates of 0.1 to 1 Hz (Hertz = number/second). Values can be stored in a histogram of wind speeds, or wind speed and other selected variables can be stored for selected averaging time periods, along with standard deviations. Events such as maximums and time of occurrence can also be recorded and stored. Micro data loggers were designed specifically for wind potential measurements and record time sequence data (averaging time is selectable) on chips. The chips can store data from a number of channels, and the data loggers can even be queried by telephone (cell or direct), radio link, or satellite, so data are transmitted directly to the base computer. Now Internet connection is available. More detailed information on instrumentation and measurement can be found in Rohatgi and Nelson [17]. Also see the Wind Resource Assessment Handbook [24] for detailed information on wind measurement, instrumentation, and quality assurance.

The advantages of sonic detection and ranging (SODAR) and light detection and ranging (LIDAR) are that the instrumentation is at ground level and no tower is needed, and wind speeds can be measured to 500 m (SODAR) and even out to several kilometers (LIDAR). The disadvantage is the cost; however, the cost for met towers over 60 m is substantial and the cost for a met tower of 150 m, a height to the top of the rotor for large turbines, is quite expensive. A short-term study [25] compared the relative accuracy of high-resolution pulsed Doppler LIDAR with a mid-range Doppler SODAR and direct measurements from a 116 m met tower that had four levels of sonic anemometers. The primary objective was to characterize the turbulent structures associated with the Great Plains low-level nocturnal jet. The actual measuring volumes associated with each of the three measurement systems vary by several orders of magnitude, and that contributed to the observed levels of uncertainty. The mean differences were around 0.14 m/s.

There are three general types of instrumentation for wind measurements: (1) instruments used by national meteorological services, (2) instruments designed specifically for determining the wind resource, and (3) instruments for high sampling rates in determining gusts, turbulence, and inflow winds for measuring power curves, stress, fatigue, etc., for wind turbines.

The data collection by meteorological services is the most comprehensive and long term; however, in much of the world, the data are almost worthless for determining wind power potential. The reasons are the following: few stations; most locations are in cities and airports, which are generally less windy areas; sensors are mounted on buildings and control towers; the quantity of data actually recorded is small (one data point per day, or sometimes monthly averages); and lack of calibration after installation. As an example of the problem of using meteorological data, the annual mean wind speed for Brownsville, Texas, is 5.4 m/s, compared to 2.8 m/s for Matamoros, Mexico, which is just across the Rio Grande River.

There are several types of instruments at varying costs for measuring wind speed: handheld anemometers, $400; data loggers, $1,500; and data loggers with cell phones, $3,000. Companies sell instruments that sample at rates of 0.1 to 1 Hz and with the output displayed on analog devices (meters and recorders) or digital devices (stored on tape or chips). Instruments will record and analyze time sequence data, as not only wind speeds and direction can be stored for selected time intervals, but the power can be calculated and selected events such as maximums, gusts, and time of occurrence are also available. Companies that sell instrumentation specifically for wind measurements also sell digital readers and provide software for analyzing the data. Pole towers are available specifically for wind measurements from 10 m, $500, to 60 m (with gin pole), $10,000. Guyed lattice towers can be obtained for higher heights. Pole towers of 50 and 60 m are normally used for the following reasons: tower can be raised and lowered with gin pole, tall enough to obtain the higher nighttime wind speeds, and tower is below height (61 m, 200 ft) for required lights per U.S. Federal Aviation Administration.

In many countries, mechanical anemometers were the norm; however, they require more maintenance and more frequent calibration. The power from the cup anemometers drove the strip chart recorder or a counter. Because of the small number of data points, the Weibull distribution was widely used to estimate wind power potential. As an example of the problem, wind run data were collected three times a day from an anemometer at less than 2 m height at a national meteorological station in Jujuy, Argentina (Figure 4.8), to determine daily average wind speed. Due to height and of course blockage of trees and buildings, the wind power potential would be vastly underestimated.

Data from Mexicali Airport, Mexico [26], provide an example of a trend in wind speed data over time (Figure 4.9). The number of observations, 1 h values, was fairly consistent from 1973 to 1999, when the airport was operating. The downward trend indicates degradation in the anemometer (not maintained or recalibrated) or less exposure due to increased vegetation or other obstructions. The wind power changes from 170 W/m2 at the beginning to 25 W/m2 at the end, a factor of 7.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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