Wind power surged last year, with enough turbines installed globally to generate over 54 gigawatts of power. Nearly all of those turbines were the good old-fashioned kind: a tall tower with three propeller-like blades rotating on top.
But researchers and some small companies have been working on another turbine concept for years: vertical-axis wind turbines (VAWTs), which are cylindrical and typically look like an egg-beater or a weather vane. Vertical-axis turbines are cheaper to make and maintain, take up less space, and safer for birds and bats. The problem is that they are not very efficient.
Now, researchers at Stanford University have created a state-of-the-art lab model that could make it easier to increase the productivity of VAWTs. The model, reported in the Journal of Renewable and Sustainable Energy, makes it easy to test how large arrays of vertical turbines function together.
The power output of a wind farm depends on how its turbines are arranged. That’s because individual turbines interact with each other and affect the flow of wind across the array. When conventional wind turbines are placed close to each other, for instance, upstream turbines slow down wind and cause turbulence, which decreases power output from neighboring turbines. This is why such turbines have to be placed hundreds of feet apart.
VAWTs, on the other hand, affect each other positively when placed close to each other. “We think that the VAWTs can have blockage effects causing speedup around the turbines that helps downstream turbines,” said Anna Craig, Stanford mechanical engineering PhD student and the study’s leader author, in a press release. An individual VAWT typically produces a fraction of the power produced by a same-sized conventional horizontal-axis wind turbine (HAWT). But if you pack VAWTs closer together, you could in theory produce 10 times as much power from the same acreage of land than an array of HAWTs.
But testing the best arrangement of turbines is a challenge. Field tests are best, but they are expensive, time-consuming, and only involve a few turbines. So engineers and wind farm developers typically test turbine arrangements using computer models. But today’s models only consider wind flow in two dimensions and do not take into account the vertical flow of wind around VAWTs.
So Craig designed and built a sophisticated lab-scale model of a VAWT array to test 3D wind flow around the turbines. Her setup involved 1,300 interconnected 1-inch gears carrying 300 cylinders on top to create a 10 foot-long array. She set this system in a water flume at the university’s Fluid Mechanics Laboratory. The idea was that the cylinders emulated VAWTs while the water flow approximates wind flow.
By mixing and matching how the cylinders were spaced, different heights of the cylinders, and their rotation direction, Craig tested a total of 10 different arrangements. She was able to observe a net vertical flow that started above the array, went down into it, and then out the sides.
Further insights from the model could be valuable for improving computer models and could aid field tests of VAWTs.