Sorry Mr. Wolf, but after reading your article, I remain unconvinced of the value of vertical axis wind turbine or VAWTs. Let me explain why.

The shortcoming of the vertical axis design was made clear with personal experience. A neighbor bought and mounted an expensive version not far from here, a high-wind location in California. Sadly, I photographed its wreckage less than a year later. I think water skiing through rough water is similar to flying through turbulent air, so I imagine the poor blades which were beaten by hitting a constantly-reversing air at high speed.

Tower height is another issue. The article suggests “lower height” as an advantage. Why deploy at a lower height if the wind is found up higher? Vertical-axis turbines are almost never installed at a decent height because anyone bothering to put up a tower will not bother with a vertical-axis turbine. Try finding a vertical-axis machine on a tall tower. I know of none.

Another statement says blades attached at two points make them better able to withstand turbulence. Personal experience with the turbine in the first paragraph says otherwise. Blades on conventional or “propeller-type” turbines (horizontal axis wind turbines or HAWTs) are aligned with the main force involved, which is centrifugal force. So they do not need multiple mounting points and need not be as stiff. Also, “propeller-type” blades are a better use of material.

In addition, HAWTs outperform vertical-axis versions for similar reasons that boat propellers outperform paddle-wheels on old riverboats. Propellers are proven the best choice for kinetic-energy exchange in an open current flow, whether for propulsion or for energy capture.

The thrust forces on propeller-type blades are more constant than those experienced by vertical-axis blades because the blades always face the wind. A vertical-axis blade experiences reversing aerodynamic forces twice with every rotation, so the machines must be built ten times as strong as a regular turbine just to avoid ripping itself apart.

Unfortunately, VAWTs do destroy themselves, which is why you rarely see one operating in a wind farm, even though many had been installed in the 1980s.

The article further advocates placing vertical-axis turbines in wind farms where the geography funnels wind into a small area. But slowing the wind near the ground has the same effect as lowering all of the towers for the regular (productive) wind turbines, thereby reducing their output.

Wildlife is another concern. Many birds and bats spend most of their time near the ground. What evidence is that VAWTs are less hazardous to them than HAWTs?

We all want to see wind energy advance, and it is good that someone continues to at least research vertical-axis machines.
 
Mr. Wolf responds:

Thanks for your comments, Mr. Selsam. Let me address your concerns.

1. Near ground wind resources can be excellent in areas like all of the CA wind farms where topographies create a near ground, speed-up effect and low-to-negative wind shears. See references 5 and 6 in the article and speak with most any industry meteorologist with experience in California. You can also find data on how exceptional the 10m above ground level wind speed data is in CA’s Wind Resource Areas in the CEC 1985 Wind Atlas.

2. You are right that VAWTs will inherently use more material per rotor swept area than a HAWT. However, being able to use shorter towers to capture the excellent near ground wind resources and making double use of land and infrastructure should more than compensate for the cost difference of more rotor material. We don’t anticipate competing with tall HAWTs but there are no reasons why repowered wind farms with HAWTs and VAWTs shouldn’t be less expensive per MW installed and kWh produced than at a wind farm repowered by HAWTs alone.

3. Let’s not ignore the problems HAWT blades have with turbulence. Several references in my paper proved that HAWT blades cannot effectively operate in near ground turbulence and that is the first fundamental pushing HAWT blades to stay out of near-ground turbulence. Talk to HAWT manufacturers about problems their turbines have with turbulence. As a technically savy person, you should have faith in the ability of good engineering to solve problems such as turbulence over blades. Don’t be so pessimistic believing that VAWTs can never overcome the problem of operating well in turbulent winds.

4. Don’t ignore the coupled vortex effect (CVE) that occurs with two H-type VAWTs placed about 1-meter apart. Bernoulli’s Continuity Principle and the modeling done by Canada-based CFD consultants at IOPARA (www.iopara.ca) show the CVE to be solid.

5. Please present references that VAWTs must be made 10 times as strong as HAWTs to withstand the same wind forces. Our strain-gauge data doesn’t show that. VAWTs will take more material per rotor swept area, but not 10 times as much. From experience, the figure is closer to two and less depending on factors such as solidity, rotor swept area etc. Additional material costs are easily offset by shorter towers, and lower transportation, installation, and maintenance costs per installed MW.

6. The history of Sandia VAWTs tells that it only created field validated aeroelastic models (e.g. fatigue, frequency response) after they tested their 500kW Test Bed Darrieus type VAWT in the early 1990s. They never used what they learned on a new VAWT. The FloWind VAWTs were not designed using aeroelastic models. Modern VAWTs can now be engineered with validated models. You seem to be a person who would understand how problematic the lack of good models has been to getting VAWT designs to be as mechanically sound as HAWT designs.

7. The modeling done at CalTech and Stanford shows that rows of VAWTs would pull energy out of the wind but when the VAWTs are installed as counter-rotating pairs, the vortices they shed create vertical mixing that quickly recharges near ground wind speeds and brings faster moving wind closer to the ground.

8. If people believe that determined engineers can to overcome problems and companies want to exploit excellent renewable natural resources, they should also acknowledge that wind farms with 15 to 18+ mph average annual wind speeds in the layer of wind below HAWT rotors will one day result in a technology that will let them economically harvest that valuable resource. We think our VAWTs will be first to enter this market and a surge of competitors will follow because making the VAWTs is pretty straightforward with good modeling. No one is making more windy land in California, and existing wind-farm land increases in value with each passing year.

9. Regarding wildlife, we explained why VAWTs are likely to be friendly to wildlife (Animals evolved ability to better see 3D versus 2D objects). We say that VAWT developments should come with camera technology that can evaluate whether or not birds see and avoid the new turbines. If some species don’t, then developers should calculate into the project IRR, the lost energy that would occur when the VAWTs need to slow when problematic birds or bats are detected nearby.

10. Comparing small Lift-type VAWTs or any size Savonius turbine with large VAWTs like ours is an unfair comparison. Size matters as does the type of VAWT. The failures of small turbines or any past turbine model does not make it a given that all future VAWTs will fail. This is especially true when one considers the problems early VAWT manufacturers faced by engineering their turbines without field validated aeroelastic modeling based on near-exact prototypes.

I encourage you to read the paper again and all its footnotes. I’ll be glad to hear what you have to say.

Best regards,
Kevin Wolf, author of the paper