REALTIME CONTROL

Wind power site assessment

This page describes how to determine the wind power which would have been generated at a site using the data logged by a DIY Rotorvane anemometer and the power curve of the wind turbine.

Mount the anemometer at the site at the same height as would be the centre of the hub of the proposed wind turbine. Power up the anemometer and reset the anemometer log by holding down the pushbutton for more than five seconds.

As frequently as considered necessary, return to the site with a portable computer or remove the anemometer display unit to a location where a computer is available.

Connect the anemometer serial port to the computer serial port. Power up the anemometer.

Start Rotorvane for Windows.
Select "Connection" and add the site location name to the list.
Select "Connect".

Select "Logs", "Instrument Log", "Read Instrument".
The wind speed and direction distributions logged by the anemometer will be displayed in graphical form.
Select "Save file".
The wind speed and direction data logged will be save to a file called I(location)(date).txt in the working directory.
Confirm the file has been saved successfully, then reset the anemometer log by holding down the pushbutton for more than five seconds.

Start Excel or a similar spreadsheet program. Open the saved file. You should then see a screen similar to this:

In column C, add the mid speed of each speed range.
If necessary, convert the speed range mid points to the units used for the wind turbine power curve.

From the wind turbine power curve, enter the power from the turbine at the middle of each speed range.

In the next vacant column, for each speed bin, multiply the power at that mid range speed by the corresponding speed bin contents, then divide by six as each speed bin count corresponds to 1/6 hour. The result is the KW.hr generated while the wind was in that range.
Add the KW.hr figures for each of the speed ranges to get the total energy which would have been generated during the logging period. (In the example below, bins with 0 counts have been deleted for clarity.)

In this example the total of the speed bins is 6892 ten minute periods, so the logging period was 6892/6 = 1148.67 hours, about 48 days. The file is named IWilliamstown20010206, so it relates to our site at Williamstown and the data was downloaded from the anemometer on 6 Feb 2001, and so corresponds to the period from 20 Dec 2000 to 6 Feb 2001.
The total power output during that period would have been 73.74 KW.hr using the wind turbine with the power curve shown above. The average power output was thus 73.74 / 1148.67 =  0.0642 KW. This site is clearly extremely poor for wind power as the 10 KW (nominal) turbine would on average produce only about 0.6% of its theoretical maximum.

Further down the spreadsheet are the direction bins:

It is apparent that most of the wind energy is provided by winds from between South East (135 degrees) and South West (225 degrees), and there is also a significant contribution from winds betwen North West (315 degrees) and North East (045 degrees). New trees or buildings in these directions could severely adversely affect power generated. If possible the turbine should be placed so there are no existing structures in these directions, and none are likely or planned there.
North east through Easterly winds contribute little, as do West through North West winds, so new obstructions in these quarter would have little effect. If several turbine were to be installed at this site then they should be on a line running East West for minimum power loss from wind-shadow interference.

Direction bins are incremented on the basis of distance run, not time. Each 3.2 km (approx) of distance run, the direction is read and the corresponding counter incremented. Consequently the direction bins give a much more accurate idea of the wind directions which provide the most power than is obtained with a time-based wind direction record.

Back
Index