South Africa is celebrated as the wind capital of the world, so why the need for wind measurement?

South Africa has one of the greatest wind sources in the world and coupled with its enormous areas of open terrain and infrastructure, has the potential to develop into a "wind powerhouse". This is the observation of the leaders in the wind power revolution in South Africa, who say that in one province alone, wind has the possibility to create 10 times the official national wind energy estimates.

A study which was completed in 2008, found that there is enough wind in South Africa to provide an unexpected 35% of South Africa’s electricity.

So just why is it important to measure wind speeds when we are being told that the wind speeds are already bountiful. In choosing the location of turbines it is essential to base any decisions on precise and accurate wind data. Government data in the form of wind maps (such as NOABL) will undoubtedly exist for your proposed site, but it is not uncommon to find an error of up to 10 percent in the predicted long term wind speed. A change of just 1 percent in average wind speed can spell success or failure for the project, whether it be a full scale wind farm or micro generation venture. For a 1 percent increase in wind speed, the typical energy yield would increase by up to 2 percent, with the Internal Rate of Return (IRR) expected to increase by up to 2.4 percent. Therefore the need for collecting wind data at the exact proposed turbine location is critical for accurately evaluating the expected IRR of the investment.

There are now a wide range of sophisticated devices designed to gather accurate wind data but it is essential that these are combined with expert knowledge and experience so that the uncertainty associated with the data is minimised. Met masts are normally erected on the proposed site and anemometers and wind vanes are used to assess wind speed, direction, turbulence intensity, and shear. The correct installation of these sensors is essential if the uncertainty of the data is to be minimized. They should only be installed by qualified wind engineers. A range of other sensors can be used collect data on temperature, air pressure and relative humidity. These data sets further reduce the uncertainty associated with the final energy yield calculations.

However, with this improved accuracy you inherit an increased met mast purchase price. Other methods of increasing data accuracy include using first class anemometers, positioning the mast at the proposed turbine location, and having the top anemometer at the expected hub height of the desired turbine. If measuring at the hub height is not possible then extrapolation techniques are used to adjust the data. As soon as complex extrapolation and terrain models are used during the data analysis the uncertainty of the final energy yield increases. The energy yield that you can be 90 percent of achieving each year will reduce as a result. This is the value that banks use in assessing the viability of the project, so minimising the data’s uncertainty should always be the main aim of wind feasibility studies.

The quality of anemometers varies enormously. Generally the more expensive equipment will give more accurate readings. Class one anemometers are advised as they provide the greatest accuracy. The calibration of the anemometers also varies between manufacturers. It is essential to check with banks precisely what data they require and ensure compliance with this: most will expect MEASNET calibration on some/all of the anemometers.

There is considerable evidence to show that if the separation of the anemometers from the met masts, booms and other sensors is not sufficient the subsequent wind speed data will be inaccurate due to sheltering effects. There are British Standards that need to be adhered to when deciding on the exact set up of a met mast. However, these standards are only a guideline and they themselves have limitations. Data accuracy can be further enhanced by the use of LiDAR (Light Detection and Ranging) equipment. This is used to measure the wind profile from 10m up to 200m depending on the proposed turbine configuration. Such devices can provide precise data on wind speed and direction, but are not as advanced as some cup anemometers in measuring turbulence intensity. Wind turbines are designed to operate in winds with a maximum turbulent level. When combined with met masts LiDAR is considered an excellent indicator of long-term energy yield.

Banks, planning authorities, and wind turbine manufacturers, usually insist that developers provide a minimum of one year's full data and sometimes up to three years to prove the feasibility of the proposed site. It is vital therefore that any disruption or delay in data collection is avoided.