Every few months I read something about the benefits of vertical axis wind turbines (VAWTs): they’re smaller, they work better in turbulent winds (i.e. urban environments), they can be mounted on rooftops, etc. I’ve seen two of them actually mounted to rooftops, and both caused so much noise and vibration that the residents complained. They also generated very little energy compared to their horizontal axis counterparts. So I’m generally skeptical about VAWTs. But I have to admit, this video intrigues me because instead of looking at individual VAWTs in urban environments, this research focuses on small wind farms and the interaction among groups of turbines. It’s a 42 minute video, so I’ll give the highlights with approximate timestamps. The presenter is Dr. John Dabiri, a professor of aeronautical and bioengineering at Caltech.
(1:33) Engineers measure individual turbine efficiency, which has a theoretical limit (the Betz Limit) of about 59%. The more efficient a wind turbine is, the more wind it captures and the less wind there is behind the turbine. That means that in a wind farm, the next turbine has to be placed quite far behind. The presenter introduces the concept of wind farm power density, a measure of how much power you can actually generate on a given plot of land. In some cases, concentrating on individual turbine efficiency causes a loss of overall power density.
(4:19) Professor Dabiri compares horizontal axis wind turbines (HAWTs) and VAWTs, acknowledging that modern utility-scale HAWTs are near the theoretical maximum for efficiency, which is why the industry focuses on that type of turbine. He then questions the assumption that if one turbine is more efficient, then an entire farm of these turbines is also more efficient.
(7:00) Dabiri shows that most wind farms see the bulk of the wind reaching the first two or three rows of turbines, and those are the ones that generate the most electricity. The remaining turbines get most of their wind from turbulence that actually moves vertically more than horizontally. (That was established by research done at Johns Hopkins University.)
(12:50) We see a picture of a HAWT wind farm on a foggy day, which gives a nice view of the wake and turbulence caused by the front row of turbines. The modern solution is to space the turbines far apart, which wastes a lot of potential space.
(16:00) Dabiri discusses some of the real and perceived problems with large HAWTs: dangers to birds and aircraft, aesthetics, manufacturing issues, maintenance, etc.
(18:00) The proposed approach is to use smaller VAWTs spaced closely together. Smaller turbines are easier to manufacture, they’re lower to the ground (which lessens the impact on birds and aircraft and decreases the maintenance costs) and they’re more scalable. Compared to farms of HAWTs, the VAWT farm has three times the power density at one tenth of the height.
(22:50) To determine the optimal placement of VAWTs in such a farm, Dr. Dabiri went to a school … of fish! (His research interests include bio-inspired engineering.) By studying the movement of fish tails as they swim in a school, Dabiri developed a model that describes the rotation of VAWTs as they spin. One noteworthy conclusion was that adjacent turbines should spin in opposite directions to reduce the impact on their neighbors.
Image: California Institute of Technology
(27:40) We visit Caltech’s experimental VAWT farm (pictured above), the proving grounds for Professor Dabiri’s theory. (Spoiler alert: it works well!)
(35:30) High wind speeds produce more energy, but too high and the turbines can be damaged. Once again looking to nature for solutions, Dabiri studies trees and how they respond to wind gusts.
(36:00) Dabiri discusses the manufacturing issues related to wind turbines. Large utility-scale HAWTs are very expensive to produce and they require enormous manufacturing facilities. Smaller VAWTs can be produced in more modest facilities, and the mass production using standard techniques will lead to lower overall costs.
Great innovations usually come from people who question the basic assumptions. It remains to be seen whether Dr Dabiri's ideas will change the prevailing winds or just cause a passing breeze, but I'm happy that someone out there is challenging the status quo.