Urban Turbines

VAWTs vs HAWTs: The Debate Continues

If you want to start a debate among wind power experts, mention vertical axis wind turbines (VAWTs). If we could harness the energy of people yelling at each other over that topic, we might solve all of our energy needs. So it’s with a bit of trepidation that I introduce the VisionAIR³, the newest VAWT from Urban Green Energy (UGE). I spoke with Nick Blitterswyk, CEO and founder of UGE, about the product. I prefaced our conversation by informing him that, like many in the renewable energy field, I’m skeptical about VAWTs. I am, however, open minded.

Efficiency

I’ll start by acknowledging that a horizontal axis wind turbine (HAWT) - the traditional “propellor” type - is more efficient than a VAWT. I get that. All other things being equal, a VAWT is about 40% to 60% as efficient as a HAWT, depending on where it’s operating on the power curve. According to my calculations*, the VisionAIR³ is roughly 10% efficient at a wind speed of 12 m/s. A comparable HAWT is about 25% efficient at the same speed. But efficiency is only one factor to consider. Let’s look at some others.

Durability Under Varying Wind Conditions

Many people who are fond of VAWTs will say that they’re designed for turbulent conditions like you’ll find in urban settings. I mentioned that to Mr. Blitterswyk, and he clarified that misconception. It’s not that VAWTs are designed for turbulent conditions per se, but that they are designed to tolerate a wide variety of wind conditions. HAWTs need to be placed very high where wind speed and wind direction are fairly consistent. Quick changes in wind direction cause stress on HAWTs, often resulting in premature failure.

In fact, one of UGE’s most recent installations is on an off-shore oil rig near India. (A wind turbine powering an oil rig? I love the smell of irony in the morning!) It seems to me that an off-shore platform would be ideal for a HAWT. I was surprised when Nick told me that those turbines are subjected to turbulent conditions because of their proximity to the platform, as well as frequent helicopters bringing equipment, supplies, and personnel. Some rigs have used HAWTs and experienced failures due to cross winds, so customers sought out UGE and its more rugged, cross-wind tolerant VAWTs.

We all want the most efficient turbines possible, but a very efficient turbine that’s out for repair generates no power at all. A lower efficiency VAWT with less down time may be a better option. (If you’re interested, Sandia National Laboratories is conducting a five-year study on VAWTs for off-shore applications.)

Aesthetics

Engineers like maximum efficiency, but not everyone is an engineer. Some of UGE's customers prefer the look of a VAWT. For what it’s worth, I showed my wife videos of a HAWT and a VisionAIR³, and she preferred the latter, saying it was a more interesting design. (Aesthetics isn't necessarily a deal-breaker for her; she married me, after all.) At a time when some people oppose wind power because they don’t like the looks of a turbine, making a turbine that doubles as a work of art may be the only way to take advantage of wind power at a given location.

Football fans in the USA will see fourteen UGE turbines at Philadelphia’s Lincoln Financial Field, producing up to 56 kW of power. Coupled with a 3 MW solar array, Lincoln Financial is a net-zero energy building. Game days require grid power, but over an entire year the stadium produces six times more energy than it uses. (Granted, the majority comes from solar, not wind.) And speaking of durability, the turbines and the PV array were installed prior to Hurricane Sandy’s arrival; all survived the storm undamaged.

Specifications

I compared the VisionAIR³ with a similar size HAWT, the Bergey Excel 1. Both employ permanent magnet alternators. Here’s a side-by-side comparison:

A Niche Market

When I was a budding amateur astronomer looking for my first telescope, I wanted the biggest “light bucket” that I could afford. Then I read some good advice: a big telescope will probably sit in your closet collecting dust because it’s too difficult to lug around. A small telescope will show more because you’ll take it outside more often. I won’t be doing any “big science” with my small telescope, but it is nice for observing.

In a similar vein, a small amount of wind power is better than no wind power at all. So while it’s ideal to have a HAWT on a very tall tower away from turbulent winds, that’s not always feasible, especially in cities. Since turbulent conditions are the bane of HAWTs, a VAWT might be the best tool for the job in those situations. We probably won’t see utility-scale wind farms made of VAWTs, but urban dwellers might be willing to take these artistic-looking turbines for a spin.

^{Images courtesy of Urban Green Energy}

*To calculate efficiency, I used the Wind Power Formula and plugged in the wind speed and the turbine’s swept area to determine the total amount of wind power available. I then divided the rated power output at that wind speed by the total available power.