Reinventing urban wind power

Innovation: Reinventing urban wind power
16:49 06 August 2010

With the environmental movement gathering momentum, many are thinking of installing wind turbines to generate their own electricity. Unfortunately, wind speeds in urban areas are usually too slow and turbulent to make micro wind generation cost-effective.

So while the strict planning regulations that have prevented homeowners from erecting domestic turbines in the UK are expected to be relaxed next month, city-dwellers may find manufacturers reluctant to sell them their turbines for fear that poor performance will reflect badly on a young and vulnerable industry.

However, researchers at Cornell University in Ithaca, New York, believe that the problem is not with the low wind speeds after all, but with the methods used to harvest wind power. Cities have plenty of wind energy we can use, they say, but to harness it requires a different tack. It's time to reinvent the urban wind turbine.

Moving away from traditional electromagnetic generators and turbines may seem like a radical step, but on a small scale and with low wind speeds, piezoelectric generation looks like an attractive option.

Ephrahim Garcia, a mechanical engineer at Cornell, attached a flexible aerofoil to the end of a pole made out of a piezoelectric material. When air passes over the aerofoil it flutters, causing the pole to flex and generate a small alternating current. "The inspiration came from fish tails," Garcia says.

Garcia and colleague Matthew Bryant tested aerofoils that were 13 centimetres long in a wind tunnel, and found that they generated power in the milliwatt range from wind speeds of just 2 metres per second. With many devices operating in parallel,
the amount of power generated could quickly add up, they say.

Leaf out of nature's book

Hod Lipson, also at Cornell, and Shuguang Li, now at the Northwestern Polytechnical University in Xi'an, China, have been working on the same principle. Taking a leaf out of nature's book, they have devised a tree-like configuration that uses lots of flapping leaves as generators.

The leaves are attached to vertically hanging piezoelectric branches by a hinge. As air flows over the leaves, instabilities create turbulent vortices first on one side and then on the other, causing it to flap.

To make the technology as cost-effective as possible, Lipson and Li built their branches from a piezoelectric material called polyvinylidene fluoride, or PVDF. However, while this is cheap it is relatively insensitive, "so we had to find ways to make it shake more vigorously", says Lipson. For this reason the leaves are designed to twist as well as bend the branch, increasing the strain acting on it.

Each leaf can generate nearly 0.3 milliwatts of power, the team say. Although considerably less than Garcia's arrangement, at just a few centimetres long they are smaller and potentially cheaper, says Lipson.

Another solution is to increase the wind speed. Borrowing a trick from the world of concentrated solar power, Kevin Pratt and Francis Charles Moon, both at Cornell, have designed honeycomb-like arrays of funnels designed to accelerate wind as it passes over fluttering piezoelectric strips just a few centimetres across.

Like a lens

"The amount of energy contained within moving wind is determined by the amount of air and its speed," says Pratt. So by forcing the same volume of air through a smaller aperture you can increase the speed. "It's the wind equivalent of a lens," he says.

Computer simulations have shown that some concentrator designs should be able to increase the wind speed by more than 50 per cent. The pair are now building the first prototype for wind tunnel tests. They envisage the final product containing arrays of 30-centimetre-wide concentrators, each housing several dozen piezoelectric strips.

Wind concentrators are not a new idea, but have proved impractical for standard turbines because of their large size. By shrinking the technology the researchers hope to achieve an output power of 5 watts per square metre, roughly one-third of that created by solar power. So if they can be made for a third of the price of solar panels, then the technology could be competitive.

Journal references: Garcia and Bryant's work is published in the Proceedings of SPIE, DOI: 10.1117/12.815785; Lipson and Li's work is published in the Proceedings of the ASME 2009 Conference on Smart Materials, Adaptive Structures and Intelligent Systems


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I love the idea of alternative sources of power, but I do believe that the engineers behind this technology have been slightly blinkered.... Engineers aren't very good at wearing designers hats! I'm in no doubt that their ideas will work, but can you imagine the noise that an array like this would generate? The urban area is loud enough already, so having millions of flapping "leaves" wouldn't really help!

I have millions of flapping leaves in my woods, another tree giving me power would do me just fine

I've watched trees bending in the wind and wondered about a rope and pulley to wind a dynamo. Might even try it one day.

But I am puzzled about urban wind. There is clearly plenty of wind here, why can't it turn a fan?
I'm thinking that it needs to be steady enough to overcome the inertia of the alternator, so why not have many smaller turbines instead?

One day I was in my garden & watched something hanging from a bit of spiders web that was going like the clappers, it was screw shaped, it seemed very efficient as there was hardly any breeze, always remember it, meant to have some deep & meaningful pondering time on it, but never got around to it...