Rethinking wind energy?

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New research at Harvard University conducted by David Keith suggest that we may not have access to wind energy as much as scientists thought because the world's capacity for large-scale wind farms has been overestimated.

"People often think that there is no better energy source than wind power and that it is one of the most evolutionary sources of energy," says Harvard applied physicist David Keith. After all, gusts and breezes don't seem to be lacking on a global scale .

However, the latest research in atmospheric modelling, published today in the journal Environmental Research Letterss, suggest that the generating capacity of large-scale wind farms has been overestimated.

Each wind turbine creates a "wind shadow" behind it, in which the air is slowed down to slide over the turbine blades. The ideal wind farm would be made up of land-based islands of wind turbines spaced far enough apart to reduce the impact of these "wind shadows". But as wind farms grow larger, they begin to interact with each other and the regional wind regime becomes increasingly important.

David Keith is trying to show that the generating capacity of very large wind energy facilities (more than 100 square kilometres) can peak at between 0.5 and 1 watt per square metre. Previous estimates, which ignored the effect of wind turbines on slowing the wind, had overestimated this figure by between 2 and 7 watts per square metre. In short, we cannot have access to wind energy as much as scientists thought.

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As an internationally renowned expert on climate science and technology policy, Keith is as well known as Gordon McKay, Professor of Applied Physics at the Harvard School of Engineering and Applied Sciences (SEAS) and as a professor of public policy at Harvard Kennedy School. Another eminent scholar, Amanda S. AdamsAssistant Professor of Geography and Earth Sciences at the University of North Carolina in Charlotte, says: "One of the challenges with wind energy is that once you start developing wind farms and harvesting the resource, you change the resource, making it difficult to assess what is actually available.

How can we have a really accurate estimate on this issue of carbon neutral energy resources? Solar, wind and hydropower: all could play a role in meeting the energy needs currently met by coal or oil.

If we were to cover the entire Earth with wind farms, according to Keith, "the system could generate huge amounts of energy, well in excess of 100 terawatts, but at that point, according to our climate modeling, the effect on global winds, and therefore on the climate, would be severe, perhaps greater than the impact of doubling CO2. « 

"Our results don't mean we shouldn't pursue wind power, which is far better for the environment than coal, but its geophysical limitations may make sense if we really want to scale up wind power to provide a third alternative to our primary energy," Keith adds.

And the climate effect is not the only constraint, geography and economics also matter: "Clearly, the theoretical upper limit for wind energy is huge, if you don't care about the consequences of covering the whole world with wind turbines," Keith explains. "What isn't clear, and this is a topic for future research, is what is the practical limit to wind energy, considering all the constraints of the real world? One would have to assume that wind turbines are located relatively close to where people live and where there is a fairly constant supply of wind, in order to cope with environmental constraints. You can't put them everywhere. « 

The real problem," he adds, "is that if you can't get much more than half a watt, and if you accept the fact that you can't put them everywhere, then you can begin to understand where their usage limit is. « 

"In order to stabilize the Earth's climate, Keith believes the world will need to identify the sources of several dozen terawatts of carbon without jeopardizing human life. At the same time, decision-makers must also allocate resources to develop new technologies to harness this energy. »

To conclude, Keith believes that "it is worth asking questions about the scalability of each potential energy source: will it be able to supply, say, 3 terawatts, which would correspond to 10% of our overall energy needs, or only 0.3 terawatts, or 1% of our needs? « 

"Wind energy is in a field of consensus," he says again. "It is still one of the most evolving renewable energies, but our current research suggests that we will need to pay attention to its limitations and climate impacts".

 (Source: Caroline Perry / Harvard Gazette - Feb. 25, 2013) 

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