I tend to be the environmentally conscientious sort. I try to do the right thing in my purchasing decisions and life choices. So, as you might expect, I am of two minds with wind power. On the one hand, I love wind turbines: I think they look cool, make a cool noise, and generate power that is relatively economic and, overall, not too bad in terms of environmental effects. Unfortunately, they also seem to kill bats and birds, which is not nice.

With that in mind, you can understand why a paper about using leaf structures to extract energy from the wind caught my eye. It turns out to be a very cool idea that has a long way to go to become competitive. And, unfortunately, I can't really see where the breakthroughs that might make it competitive will come from.

Everyone is familiar with the rustling of leaves that happens when the wind blows. This occurs because the air flow over a leaf is not stable. Small speed differences in the air flow on either side of the leaf create vortices that detach from the leaf surface, lowering the pressure on one side and causing it to flutter. Each leaf, of course, can't extract much energy, because it is small and rather inefficient. But, as anyone who has stood on the lee side of a windbreak knows, large numbers of leaves are incredibly good at sucking a considerable amount of energy out of the wind. That makes it well worth some effort to explore turning this into energy production.

The simplest way to harvest energy is to create an artificial leaf structure that has a piezoelectric layer. As the leaf flutters, the piezoelectric layer flexes, generating a small, oscillating voltage. Put in some diodes to rectify this (so that you get a direct current), add lots of leaves, and you might be able to get a fair chunk of power.

Researcher Shuguang Li and coworkers have published a variation on this idea whereby, instead of having the leaf wave back and forth as the wind flows along its length, it twists and waves as the wind flows across the leaf. Their results are not really any better than previous implementations, so this paper is more of a vehicle for me to discuss the idea rather than a specific implementation.

This big thing about leaf structures is that, by varying the size of the leaf, you can optimize for power at different wind speeds, so one can imagine an artificial tree that can function quite well over a wide range of wind speeds, or one tailored to typical conditions. The researchers demonstrated that they had leaves that started producing power at wind speeds as low as 0.3m/s, with larger leaves cutting in at speeds of 4m/s and working well at speeds up to 8m/s (they didn't show any data for higher wind speeds). Of course the power per unit is very low, at a maximum of 610 microwatts per leaf—other leaf designs have been able to produce as much as 10mW per leaf.

So can't we just use a lot of leaves? The story is not quite so simple. First, the authors sell themselves short when looking at power per unit volume. Wind turbines are typically separated by 300-500m and have a blade diameter of 52m, giving a power density of 230mW per cubic meter. Taking the volume occupied by one fluttering leaf and doubling it so that each leaf is adequately separated gives a power density of about 650W per cubic meter. I suspect that this is actually more power than is available from the wind, so the hedge of leaves might well extract all available wind energy from any passing drafts—I think the conversion efficiency is already taken into account.

The big problem at the moment is cost, with a typical wind turbine coming in at about 300mW per US dollar, while the leaves are, at best, 0.069 mW per US dollar. That kind of cost difference makes everyone go ouch. Part of the cost might be due to the fact that the piezoelectric polymers are still in small scale production. But no matter what the cause is, that cost has to get better by four orders of magnitude, and that is no easy task.

There are also, of course, questions of longevity and robustness: leaves don't just grow on trees, you know. For traditional wind turbines, the control system feathers the props (adjusting their angle so that they extract the minimum amount of power possible) in high winds, preventing the turbine from tearing itself to pieces. Without a similar level of control, your leafy wind farm might provide the most expensive windfall in history.

Journal of Applied Physics, 2011, DOI: 10.1063/1.3525045