About a thousand years ago, the Persians had the bright idea of building several sails around a central mast. The entire contraption rotated and provided enough power to grind grain. It became the prototype of today’s vertical axis wind turbines (VAWTs).

Can VAWT technology create green electricity? Or rather, can it do so efficiently and economically?

The history of the technology doesn’t inspire confidence. After all, the horizontal axis windmill came along only a couple of centuries later. Nothing like the Persian sails continued to develop. All advances in wind technology had a horizontal axis until after the First World War. Today’s wind industry depends almost entirely on horizontal axis wind turbines (HAWTs).

Unfortunately, HAWTs have become so large—and so controversial—that developing the technology may be reaching a point of diminishing returns. I have recently written about the trouble with HAWTs.

VAWT technology has recently attracted renewed attention. Perhaps some innovative idea will give it a role in generating wind energy.

The first generation of VAWT technology

Finnish engineer Sigurd Johannes Savonius introduced the first modern VAWT in 1922. In principle, he reinvented the ancient Persian concept.

He attached two half drums to a central shaft in opposing directions. One of them catches the wind and begins to rotate. The back of the other moves against the wind, but as the contraption turns, the drums take turns catching wind and backing into it.

This design works on impulse (drag). It rotates at the same speed as the wind. An anemometer, used to measure wind speed, is a variation on Savonius’ design. Its very low efficiency makes it unsuitable for generating electricity except in very small applications.

French engineer Georges Darrieus patented the first aerodynamic VAWT in 1927. The patent covers any arrangement of vertical airfoils.

A Darrieus VAWT can have a shape something like an eggbeater or a so-called H-rotor. It spins at a speed faster than wind speed. Darrieus “eggbeater” blades are especially complex to make.

In any case, the operation of its blade resembles a bicycle in that it creates an apparent head wind. It resembles an airplane propeller in that the “angle of attack” on the blade creates lift. Since the blade rotates around a vertical shaft, it doesn’t matter which direction the wind is blowing.

The trouble with VAWT technology

VAWT technology has three main advantages over HAWTs:

• It has fewer component parts.With no need to keep it oriented into the wind, it has no need to control pitch and yaw. Among other things, this simplicity means VAWTs can work better in gusty conditions.
• It is safer for workers. Since the gearboxes and other components are closer to the ground, no one needs to climb a tower to perform maintenance.
• It can be scaled down. VAWTs will work better on rooftops and smaller plots of ground in densely populated areas than will HAWTs.

But VAWT technology’s drawbacks have so far kept it from being competitive for use in large wind farms.

• It is less efficient. While all the blades of a HAWT contribute to making electricity, the VAWT blades returning into the wind do not. With only a fraction of the machine producing power, a VAWT cannot generate power as efficiently.
• It is less sturdy. While a VAWT has fewer parts, it experiences more wear and tear and therefore requires more maintenance.
• It can’t be scaled up as easily. The larger they’re built, the less sturdy they become. Eventually, they cannot remain upright without multiple long guy-wires to keep them in place.

HAWT blades maintain a steady torque, which changes only when the wind changes. VAWTs have an unavoidable “torque ripple.” That is, torque varies depending on whether the blade is in the upwind or downwind position. A VAWT also has a resonant frequency at a certain rotational speed. When it reaches that speed, it is subject to violent vibrations that cause blade fatigue and eventual failure.

The Darrieus VAWT has a further drawback; it requires some external force to make it start to spin. No matter how fast the wind blows, the wind itself can’t start the rotor. A Savonius rotor on the same shaft provides enough power to start it.

New developments in VAWT technology

Darrieus apparently gave up on following up on his VAWT patents. Some Canadians began to explore the concept again in the 1960s, but research didn’t start in earnest until after the Arab Oil Embargo of 1973.

The federal government conducted extensive research in the 1970s, but basically discovered all kinds of approaches that proved unworkable, both with HAWT and VAWT technology.

Lacking Thomas Edison’s patience in the face of finding out what doesn’t work, the government pulled the plug on wind energy research in 1981. It started again on a smaller scale in the 1990s.

The Sandia National Laboratory worked extensively with Darrieus rotors in the 1980s. In 2011, it began to build on its earlier findings specifically for application in offshore windfarms. So far, only small VAWT machines have been commercially viable.

Experiments with deployment of VAWTs

John Dabiri used some off-the-shelf VAWTS in pioneering research at Caltech. Based on his understanding of shed vortices produced in the wake of schools of fish, he tested the hypothesis that counter-rotating arrays of VAWTs can actually benefit from interactions with nearby turbines.

When HAWTs are grouped together, the performance of each turbine decreases. The closer they are, the more the decrease in performance. When a HAWT extracts energy from the wind, it hinders the air speed. Turbines downstream therefore receive slower moving air and extract less power.

A single VAWT has a lower power coefficient and costs about a third more than a single HAWT that puts out the same power. But counter-rotation of adjacent VAWTs orients the airflow from them in the same direction and therefore reduces the energy dissipation and turbulence in the space between them. Since turbulence does not dissipate energy, turbines further downwind can operate on full wind speed.

VAWTs can therefore achieve higher power densities as defined by the power extracted per unit of land area.

Now teaching at Stanford University, Dabiri continues to experiment with the ideal placement of arrays of VAWTs, relying on commercially available turbines. Unfortunately, not a one of them can last for an entire season in actual field conditions without some kind of mechanical or electrical failure. His group has not been able to modify them to make them more robust, so it is working on its own design. It focuses on eliminating wobble and make flexible blades that can bend in strong wind.

A remote village in Alaska serves as the testing ground for new designs. It is almost entirely dependent on diesel fuel brought there by air. A working wind farm would benefit it and similarly remote areas. Currently the team has deployed a pair of turbines. If they work reliably, the team will add additional machines and install them in a pattern that will maximize synergy and provide enough electricity that the village will no longer need to import diesel fuel.

Experiments with new VAWT designs

I haven’t found a description of Dabiri’s new design ideas, but I have found a couple of patents that do not rely on standard Darrieus technology.

One, called the blinking sail, more nearly resembles a Savonius rotor. In fact, it even more nearly resembles what the ancient Persian array of sails must have looked like. But instead of a single piece of fabric, the blinking sail has horizontal bars across each frame.

Separate sheets of fabric (or plastic, or aluminum) hang on these horizontal bars in such a way that when a sail faces into the wind, the fabric blocks the wind. It acts as the downwind side of a Savonius rotor. But on the upwind side, the wind will blow the fabric away from the frame and pass through.

The following video explains its working and also has a good description of the nacelle in a HAWT.

A team of mostly Italian engineers has described application of the blinking sail principle to building facades. They claim that the technology can use the outer structure of buildings to generate both wind and solar power.

Another patent describes a structure that does away with the central axis. Instead, it supports the blades on a circular frame.  Since I have not conducted a thorough patent search, there are undoubtedly others. If you know of anything interesting, please link to it in a comment below.

I have already written about alternative wind turbines that either have no blades at all or do away with towers in favor of some kind of airborne mechanism.

Will any of these ideas work?

The history of invention suggests that most will work only on paper, or only in laboratory conditions. It also suggests that someone will eventually find something that will work as well as HAWTs, but at a more human scale and less expensively.

Sources:

Illustrated History of Wind Power / Darrell M. Dodge, TelosNet. Last updated 2015
Offshore use of vertical-axis wind turbines gets closer look / Sandia Labs news release. July 30, 2012
Potential Order-of-Magnitude Enhancement of Wind Farm Power Density Via Counter-Rotating Vertical-Axis Wind Turbine Arrays / John O. Dabiri, Journal of Renewable and Sustainable Energy 3, 043104 (2011)
Vertical-axis wind turbines: what makes them better? / Andrew Merecicky, Wind Power Engineering & Development. October 31, 2014
Vertical axis wind turbines / Magdi Ragheb, University of Illinois at Urbana-Champaign. March 21, 2015
Wind turbine design and positioning can improve efficiency / Kevin Wilcox. American Society of Civil Engineers. March 15, 2017 [link no longer works]

Other suggested reading:
How VAWTs could replace HAWTs / Peter Allen Sharp, Wind Power Engineering & Development. June 27, 2016
Vertical axis wind turbine technology continues to improve / John Yan, Wind Power Engineering & Development. August 7, 2017

Photo credits:
H-rotor Darrieus turbine. Public domain from Wikimedia Commons
Savonius turbine. Some rights reserved by OiMax
Darrieus rotor with Savonius rotor. Public domain from Wikimedia Commons
Darrieus eggbeater in wind farm. Some rights reserved by Guillaume Paumier
VAWT for home use. Source unknown