Posts Tagged ‘sodar’

I talked here about the job that myself and my colleagues actually do, in terms of why it would be complex to predict how the wind will flow across a potential site. I’d like to continue that topic a little and talk about how we actually measure the wind.

The first rule of good science is to make good measurements. In the wind industry, that means we want to know as much as possible about the wind which will encounter the rotors of the planned turbines. The most common instrument to use for this is a cup anemometer, which is actually a fairly simple mechanical device. Most commonly it’s made up of three cup-shaped objects which sit horizontally in the wind and spin round a central axis. By counting the number of spins we get a measurement of the wind speed. You’ll most likely have seen them; you see small ones quite commonly by the roadside, near small wind vanes.

Of course, a cup anemometer can only measure the wind speed that it’s actually sitting in. To measure at the heights of a wind turbine we need to mount it on a mast, preferably one which reaches to the turbine hub height. We also need several instruments at different heights, so that we can see how the wind flow changes with height. Then there’s the wind vanes, which give the wind direction, generally we want two of them. Multiple instruments have several advantages: they provide measurements at different locations, they can be used to sanity check measurements, and there is redundancy built in if something fails.

In fact the wind industry has generally had far higher requirements for measurement accuracy than the Met Office when it comes to wind speeds. There are wind industry professionals who visit masts in various locations and assess how accurate the measurements are, how consistent across the dataset and whether the data could be used as a reliable indication of how the wind behaves. The accuracy of the eventual dataset will depend on whether the mast is correctly sited, how the instruments are mounted on the mast, what sort of instruments are used and at what height, and whether the data are regularly checked and maintained.

Recently, lidar and sodar technology have started to really take off in the wind industry. These are alternatives to a mast, to an extent, and they work in a similar way to radar: by bouncing a wave off a moving target and looking for the reflection. Lidar units use light, generally infrared wavelengths, and sodar units use a sound-based wave. They’re collectively termed “remote sensing”, because they can sense the wind speed without sitting in the wind flow.

As it turns out, met masts with their instruments and the less intrusive remote sensing units are complementary technology rather than competitors. Met masts are large and unwieldy and cost a lot to install, but once they’re properly installed they continue to take data and require very little maintenance or additional expense. Temporary met masts which are installed for a wind project often take data for three years or even longer. Lidar and sodar can be bought outright or hired. Their huge strength is that they’re comparatively portable: lidar units in particular can generally be moved across a muddy field by two people, and they are not generally mentioned in planning requirements. A resource analyst might have two or three places on a site where the wind will be challenging to model or which are a long way from the mast but a mast can’t be installed there — in this case a short lidar deployment can really help in forming a full overview of how the wind is behaving.

Met masts are relatively simple things. Wind industry met masts are generally much smaller and less intrusive than the big telecommunications masts you see. However they have their challenges. They can be deployed in incredibly remote locations, which can make getting the required construction vehicles to the required location challenging; sometimes helicopters are required to transport the mast to site. (Note that one of the first things done when constructing the wind farm itself is building the roads. The turbines can then go along the roads. The met mast pre-dates this step, though.)

Remote sensing is obviously also a huge advantage offshore — the wind can be measured from the surface of an oil rig or even on a nearby shore rather than expensive and time consuming offshore met masts being required. The taller wind turbines of today, and the challenging terrain they’re sited in onshore, also benefit from remote sensing measurements which can be made far higher than a mast would support without any increase in cost.

There are technical differences between met masts measurements and remote sensing measurements which the wind industry as a whole are starting to get a handle on. It’s one of the more interesting elements of my job, watching the techniques and the technology changing and evolving. In many respects the wind remains something of a mystery to us.


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