Meteorological Field Measurements at Potential and Actual Wind Turbine Sites
TABLE OF CONTENTS
6. SUMMARY OF FIELD EXPERIENCE

7.1  TOWER AND EQUIPMENT CONFIGURATIONS

Based on our experiences in the Meteorological Validation Program, we offer the following recommendations for equipment installations at sites being evaluated for large wind turbine utilization:

• If possible, measurements at least as high as the hub of the machine should be obtained. One, and preferably two additional levels, should be installed to allow data to be extrapolated to the hub height or higher, and to provide important information on the vertical shear characteristics at the site.

• Nevertheless, it is preferable to keep the height of the tower below 62 m (200 ft), since the Federal Aviation Administration (FAA) requires a beacon light on towers higher than this (in some cases, a beacon light would be required even for shorter towers, and after the turbine is installed a beacon light might be required at the height of the blade tip). The operation of the beacon lights requires ac power and, at least early in the site evaluation stage, electrical power may not be available at the site.

• The climatological conditions at the site should be considered before ordering equipment. If severe gustiness or icing is possible, more rugged sensors should be utilized (at the expense of the capability to obtain long-term, high-resolution turbulence data). If severe moisture could be a problem, special instrument enclosures and circuitry designs should be considered to assure that the data logging equipment stays absolutely dry and operational. If severe cold is a problem, the data logging equipment should be placed in a heated enclosure, or some type of resistance heat element installed in the enclosure.

• It is preferable to install towers at locations where icing conditions are minimized. However, if icing is a problem at a chosen location, special care should be taken in the tower installation to ensure that a heavy load of ice will not buckle the tower or cause a guy wire to break loose. In the MVP, some problems were encountered with standard guy grips at sites where severe icing was experienced. Water would work its way inside the grip during thaws, and then expand and loosen the grip during refreezing. The grip should be designed to eliminate this problem.

• Although it is preferable to power the data loggers with ac power, in many cases none will be available. Some type of reliable onsite battery charging system should be a part of the equipment, such as solar cells or windchargers. (These were not available in this program, but would have simplified the utility personnel's weekly procedures.)

• Some type of remote data transfer system is recommended (using telephone lines, communications satellites, or telemetry) and a system to automatically incorporate the data into a computer data base is recommended. This will greatly improve data recovery rates and allow for more efficient site maintenance procedures. Nevertheless, it is still recommended that local people be utilized as much as possible to maintain a visual check on the equipment. An onsite data logger should still be used as a backup recording system.

7.2  SAMPLING AND DATA PROCESSING PROCEDURES

In Section 5 we discussed an expanded data collection program at the turbine installation sites. One feature of this program is to provide better guidelines on data collection procedures at sites where turbines could be installed, so that the data base more closely resembles the turbine operating logic. Detailed analysis of data collected at these sites has begun, and at this time we can offer the following preliminary recommendations on data collection procedures (see also Renne and Corotis 1981, Corotis 1982):

• The two-minute instantaneous samples provide an adequate sampling scheme for climatological purposes. However, they do not resemble the logic used by the wind turbines for start-up/shut-down procedures.

• For machine performance evaluation, some type of data averaging is appropriate. A suggested sampling rate of once every 10 minutes is made, with the following pieces of information recorded:
  • a 1-second average wind speed

  • a 15-second average wind speed (experience has shown that this correlates well with turbine performance)

  • a 1-minute average wind speed (correlating with the sampling procedure followed at routine National Weather Service Stations)

  • a 10-minute average wind speed (for machine operating procedures)

  • the variance of all 15-second averages within the 10-minute average

  • maximum wind gust during the 10-minute sampling period.

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