What Is Sodar? A Snapshot of Wind Development Applications
A technology older than the modern wind industry, sodar (an acronym for Sonic Detection and Ranging) was invented in the early 1960s -- before the invention of weather radar.
Sodar is important to wind meteorologists because it is ideally suited to measuring the lower boundary layer (the lowest layer of the earth's atmosphere) -- below the sweep of radar and above the ground. This, of course, is where the wind is that matters most to wind turbines. Sodars have gained commercial acceptance in wind development applications because they measure the wind at higher heights than is practical using meteorological towers.
Like its cousin technologies, radar (radio detection and ranging) and lidar (light detection and ranging), sodar uses waves for measurement; and, as the name suggests, it uses sound waves.
Sodars historically have been used to research atmospheric conditions ranging from insect migration patterns to pollution. Commercial sodars in today's wind industry are ground-based systems that send up focused sonic beams in rapid succession, producing an audible chirp. Wind turbulence sends a portion of the sound back towards the ground as an echo. By precisely measuring the frequency and time delay of the echo (aka the Doppler shift), the sodar device -- using trigonometry -- measures the wind speed and direction at any height up to around 200 meters. This is why the sodars are also called Doppler sodars.
Types of Sodar
A monostatic sodar (like a monostatic radar) is one where the transmitter and receiver are in the same place. Second Wind's Triton is a monostatic sodar whose speakers are also receivers. A bistatic sodar is one whose transmitter and receiver are in different locations.
Wind Industry Applications
Commercial sodars and other remote sensing systems were developed and commercialized for the wind industry to reduce the uncertainty inherent in measuring the wind below a turbine's hub height. Sodars are now used commercially in wind resource assessment and wind farm operations, either alone or in tandem with meteorological towers. They offer the following advantages:
- They measure up to a turbine's hub height -- now usually 80 meters and above -- without the necessity of erecting a tall met tower. The higher the turbine's hub height, the less practical and more costly it is to measure at height using a met tower.
- They are much more portable than a met tower and thus can be moved around during wind prospecting or micro-siting.
- They can measure at many station heights, replacing anemometers and wind vanes mounted at many different heights on a met tower.
Sodars lend themselves well to wind resource assessment, the formal phase of wind measurement that is part of a wind project financing package. As long as a sodar or other remote sensing system has gained acceptance in the industry, the hub-height data it provides is used to reduce the uncertainty of annual energy projections (AEP) and thus provide more attractive, profitable financing terms.
Beyond Wind Resource Assessment
Because of their mobility, sodar systems are also well suited to more specialized wind development applications, including:
- Wind prospecting (also known as scouting or site finding). This phase of wind farm development has fewer formal requirements than wind resource assessment. It involves a wind developer gathering enough data to justify further study of a site. If the data from a wind site show that the wind resources do not justify further development activities, the sodar can be inexpensively moved to a more attractive location.
- Micro-siting. Before the advent of sodars and other remote sensing systems, developers relied on modelling (otherwise known as horizontal extrapolation) to characterize the wind across a large prospective wind farm site. Using a remote sensing system allows the developer to use measured data at locations around the site, reducing the topographic uncertainty inherent in turbine placement and gaining a much more thorough understanding of the energy output of a particular wind farm layout.
- Wind shear measurement. Because of their ability to measure wind at many different heights, sodars can provide valuable information about the amount and type of wind shear and veer (a.k.a. wind direction shear). Because extreme differences in wind speed and direction over time are believed to cause extra wear and tear on a turbine, information about shear is an important part of understanding the suitability of a site for a turbine.
Since their introduction to the commercial wind industry, sodars have been firmly established as a valuable part of the wind developer's measurement toolkit. Although much attention has focused on their use in formal wind resource assessment studies, they save time and enhance the project value in many other ways during the development stage of wind farms.
