Measuring Wind

On-site wind measurements should be taken prior to deciding to purchase a wind turbine. The data collected will determine the wind resource and help with wind turbine selection and economic value. If you live in an area already known to have low average winds (less than 12 mph), it probably is not worthwhile taking measurements, let alone economical to consider buying a wind turbine. There are exceptions to this, however, and you may have good reason to believe that your site is much windier than the surrounding region. You should consider several questions about taking measurements:

  • Who should take them?
  • Exactly what should be measured?
  • What instruments should be used, and at what cost?
  • Where should the measurements be taken?
  • For how long should they be taken?
  • How will the data be analyzed?

Before answering any of these questions, you need to back up and examine how the collected data will be used. This depends on the intended purpose of the wind turbine. If a wind turbine is being considered for annual electric power production, then the wind data will be used to estimate that production. In this case, mean wind speeds and wind speed distributions will be important. If, however, a wind turbine is intended to furnish power during periods of the peak demand, a more detailed evaluation will be needed. At a minimum, the variation of speed with time-of-day and season should be estimated, and possibly the duration of windless periods for applications where battery storage is to be used.

Who should take the measurements?
There are two options. You can take the measurements yourself or you can hire a specialist. The second option costs more, but a professional is better equipped to do the job right and can be a valuable resource for other wind energy information. A list of consultants and wind turbine dealers providing this service can be found in the Resources section.

What should be measured?
Obviously, wind speed must be measured. Wind direction measurement is optional, but is recommended if you can afford it, or if the site is influenced by obstructions or irregular terrain. How frequently measurements are taken will depend on the intended use of the contemplated wind turbine. Cost considerations once again play a role. As a general rule, the more frequently data is recorded, the more expensive the recording equipment is and the more data there is to analyze. Hourly recording will suit the needs of virtually any application. Both short-term information, such as the day-night (diurnal) speed variations and the duration of calm spells, and long-term information, such as monthly and annual averages, can be provided in this way. Daily recording is usually sufficient to characterize average wind conditions where hourly detail is unnecessary.

What instruments should be used?
Anemometers measure wind speed. Most are designed with cups mounted on short arms connected to a rotating vertical shaft. Some use a small propeller. The anemometer rotates in the wind and generates a signal proportional to wind speed. In most cases the signal is electrical, although some anemometers produce mechanical signals. Wires lead from the anemometer to an indicator (display) or recorder that is made for indoor or outdoor use.

Indicators give current information on a dial or digital display or with blinking lights. They present only visual wind values and do not have any storage capability. If data are to be collected, they require a person to monitor the system and manually record the data into a logbook. Indicators are impractical for most wind feasibility studies and in general are not recommended.

Recorders store past information using counters (mechanical or electronic), paper strip charts, magnetic tapes or electronic data chips or cards. Electronic recorders and other storage devices can be easily interfaced with a personal computer for subsequent data analysis. The simplest recorder is a counter that accumulates “run-of-the-wind”. It totals the miles of wind that blow past the anemometer, just as a car odometer records accumulated road mileage. If 240 miles of wind passage are recorded in a 24-hour period, the average speed for the period is 10 mph. This recorder, known as a wind-run recorder odometer or accumulator, can be read at any time interval-hourly, daily or monthly-to suit the needs of the intended wind turbine application. A logbook is required to enter the recorded data. Electronic recorders offer the greatest power and flexibility among recorder types. They can provide a variety of statistics such as sequential hourly wind speed averages and wind speed frequency distributions. More sophisticated recorders may be used to record wind speed by direction and/or time of day, as well as compute statistical parameters such as standard deviations and maximum/minimum values.

The equipment for complete wind speed measurement costs from several hundred dollars for the simple indicators and recorders to several thousand dollars for the higher quality, sophisticated recorders. PC-compatible software is also available from some equipment suppliers to automatically analyze your data. Some systems can be rented. If you hire a specialist to do the wind study, he or she will provide the necessary equipment, but you should inquire about the type of equipment the specialist is using, and the amount and quality of data to be gathered.

Wind direction is senses with a wind vane, which also sends a signal to an indicator or recorder. Costs are comparable to those incurred in measuring wind speed. Some wind equipment is designed to measure speed and direction. A list of manufacturers and distributors of wind vanes can be obtained from the Iowa Energy Center. Some manufacturers are listed in Businesses.

The anemometer should give readings no less accurate than 5% of the actual wind speed. Wind vanes should be within 10 degrees of the true wind direction. Inexpensive hand held wind sensors should not be used for wind turbine studies. Generally, they are inaccurate and must be held too close to the ground or one’s body.

Where should measurements be taken?
Measurements should be taken at the intended wind turbine location and at the anticipated hub height, the distance above the ground at which the turbine rotor hub will be located. If the instruments cannot be placed there, they should be placed near the intended location in a similar environment. Measurements should be taken in an open area and not above the roof of a building where the wind flow is likely to be disturbed. Wind instrument towers, much less substantial and costly than towers, can be purchased, rented or provided by a wind specialist.

For how long should measurements be taken?
The longer the monitoring period, the more confident you can be of the results. Ideally, one full year, and preferably several years, of wind measurement is recommended. This will provide data for all the seasons, and on interannual changes, should the period be more than one year. Because of cost or scheduling problems, some people may not wish to make lengthy measurements. Shorter-term measurements can still be of value, but they also will have greater limitations. The next section provides some tips for short-term monitoring.

Tips for Short Term Monitoring

  1. The measurement period should be no less than three months, and preferably 6 to 12 months. Measurements should be targeted to spring or fall when winds can be measured from both southerly and northerly wind directions.
  2. Because only part of the year will be measured, the data for the rest of the year must be estimated. This can be done by comparing the readings at the wind turbine site with those taken at the same time at a nearby IEC station. The difference between these short-term measurements is assumed to represent the difference between the actual average wind speeds at the two locations. Where the long-term average at the IEC station is known, the average at the site can be estimated. Because of the IEC’s extensive, long-term wind monitoring project, people in Iowa can most likely obtain adequate information during a 3-month time period and establish the correlation between a test site and the nearest IEC monitoring tower. By comparing your own winds on an hourly or daily basis with the IEC measurements, you may quickly notice a relationship between the two sites.
  3. The accuracy of the wind instruments should be checked at the end of your measurement program. Anemometers may lose accuracy with prolonged operation.