Use of SODARs as a reliable tool for wind power resource evaluation
Met masts with mechanical anemometers and wind vanes have being the standard for wind power assessment during many years. This measuring principle relies on the conversion of the wind energy into mechanical energy that anemometers and wind vanes convert to an electric signal proportional to wind speed or wind direction. Like any other measuring technique, anemometers and wind vanes have their advantages and disadvantages. One of the limitations of anemometers and wind vanes is that that they need a solid structure to be held at a certain height, the other is that they take samples of wind almost in a single point. In the last decades, size of wind turbines have grown considerably. With this increment on rotor height, a more precise assessment of the wind profile is then necessary in order to make a better production forecasting. It is possible to use tall towers with sensors all their way up to 100 meters or more, but the increase of the costs of such towers increase considerably. This fact along with others like reliability, installation and maintenance costs and environmental impact make necessary to consider other measurement techniques that do not need towers. One option is to use a SODAR (SOund Detection And Ranging). They use the heterogeneities found on air to reflect ultrasonic sound pulses and retrieve their velocity using the Doppler effect theory. SODARs are at ground level but can reach very high levels of the atmosphere, depending on their configuration. SODARs specially focused on the first hundreds of meters of the atmosphere have become in the last years very helpful on the wind energy resource area.
The objective of this project was to set up a solar powered, remote controlled SODAR platform to check reliability, comparability with anemometric tower and analyse added values in relation with wind power resource assessment.
A REMTECH PA0 SODAR was used (remtech.com). This phased array SODAR was one of the smallest in market for that time and had good evaluations at other aplications (air pollution, air traffic).
Since PA-0 was designed not for remote operation, a powerful solar powered system needed to be designed and installed to satisfiy the high power demand from the antenna, electronics and processing units. In order to check reliability and consumption a data logging system with shunt resistors and voltage dividers was used to monitor SODAR consuption, batteries charge and solar panels eficiency.
SODAR data and ancillary information stored at the data logger was downloaded using a GPRS router and was available almost on-line.
The SODAR was installed in a location characterized by extreme temperatures, with freezing temperatures during winter lasting for days and temperatures above 30ºC in summer. It was installed at a reasonably distance of a 80 m anemometer tower in order to perform the comparisons.
The whole system was running for more than two years with a more than satisfactory data completness. This was probably one of the first SODAR applications to wind power in the country and helped our client to start using this technique many years in advance from its competitors.
The SODAR used in this project can be considered portable and useful for field deployment. This makes this instrument, and others of their family, suitable for:
- Preliminary wind power assessment
- Extrapolation of results obtained by towers.
- Completion and verification of assessments done using towers.
- Assessment of wind power resources at locations where tower cannot be installed due to technical, environmental, economical or legal constrains
- Wind profiling up to 200 m and more
Some disadvantages are inherent to this measuring technique like noise interferences influence of precipitation and power consumption but these are being solved with newer instruments.