A Simple Answer to Complex Terrain
Complex terrain causes complex wind flow — but there is a simple way to improve your wind measurement campaign
Ask a North American wind energy developer what makes wind development work challenging, and at some point the topic of complex terrain will come up.
North America is a mature wind energy market, which is another way of saying that a lot of turbines are already in the ground here, and have been working for years and even decades. The "low-hanging fruit" have been picked and made into jam, and the flat, wind-swept plains of Kansas and West Texas are dotted with wind farms. Now, among other efforts, the industry is increasingly looking at more rugged, topographically complex sites as the next frontier of wind energy development.
Besides the obvious challenges — it’s already hard enough to put up a 100-meter turbine in a cornfield, and it’s really hard to get it up on a ridge-line — complex terrain, which can include rolling land and nearby mountains and even forests, makes it a lot harder to accurately measure wind to estimate the energy production.
Why? Because complex terrain causes complex wind flow. Think of a stream of air blowing across flat land — all of a sudden it hits a hill. Now, instead of blowing horizontally, it bounces off the hill and goes in all directions. Add in a few more terrain features and all of a sudden you've got the wind playing bumper cars.
Until a decade ago, wind was almost always measured with instruments mounted on met towers. Remote sensing systems are now used as primary measuring devices in wind resource assessment campaigns (complemented by met towers), as they can measure wind at higher heights and can be much more easily deployed and re-located – this last a feature that is especially valuable in complex terrain.
But the problem with using remote sensing systems is that they rely on an assumption that the horizontal flow will be uniform across the several locations that the remote sensor samples to calculate wind speed. We’ve omitted the technical explanation, but convex wind flow (the most common type of upward curvature in wind flow) generally causes the calculated horizontal wind speeds to be lower than they actually are. In practice, this has been ignored since the measurement is "conservative", but the fear of complex terrain causes an additional degree of uncertainty to be assigned to the remote sensing measurements by independent engineers. And of course, a low measurement bias will also cause the overall estimate of energy production to be low – something you don't want if you’re trying to borrow money to build a wind farm.
In practice, wind developers do use remote sensing systems to measure in complex terrain because the value they add by measuring at higher heights and being able to easily measure at multiple locations more than makes up for any uncertainty that is assigned to the measurements. As one of Vaisala's customers recently wrote on LinkedIn: "Remote sensing in very complex terrain. Good day."
It's been recognized for some time that wind flow modeling has the potential to be used to adjust remote sensing measurements in complex terrain. This process used to cause an extra cycle of processing and headaches.
Now, Vaisala and WindSim have partnered to provide a model simulation that can be used to adjust remote sensing data in any location. We validated the simulation by running it on a large dataset consisting of Tritons deployed in different types of complex terrain all over the world, and found that:
- Running the simulation eliminates nearly all the bias in the remote sensing data
- It also reduces the uncertainty of the remote sensing data. It is still higher than flat terrain, but has been shaved down to 2% — compared to 1% in a similar study done using data from flat terrain.
- The uncertainty tends to decrease with measurement height.
- The adjustment process can be run at any time during the measurement campaign, with or without met towers located nearby.
The complex terrain correction service is easy to use — our production team, with tons of experience analyzing remote sensing data, will run the simulation for you on order. And it is available for measurement campaigns done with the Vaisala Triton Wind Profiler — thus making the Triton remote sensing system even more useful in complex terrain.