Speed control boosts energy output

Written by: Tom Shelley | Published:

A study of the underlying physics and implementation of simple control can greatly enhance machine efficiency, as Tom Shelley discovers



By controlling speed so as to keep vertical axis wind turbine blades away from stall conditions, energy extraction can be enhanced by 42%.
As well as making this kind of ‘green’ energy option much more economically attractive, it also demonstrates what can be achieved by some fairly basic analysis of what is physically going on in a piece of machinery, followed by the adoption of a control strategy to take advantage of what has been discovered.
The study was undertaken by PhD student Simon McIntosh under the supervision of Dr Holger Babinsky, reader in aerodynamics in the Department of Engineering at the University of Cambridge. Babinsky described its results to one of the university’s Horizon seminars, ‘Energy in Cities’. The department was approached by Quiet Revolution, which makes vertical axis wind turbines. Vertical axis wind turbines are more suitable than horizontal axis wind turbines for an urban environment, as they are easier to mount on rooftops without putting excessive strain on structures. They also look distinctly more elegant and they do not have to be turned to face the wind. This is important, because, in a town or city, wind speeds vary by a ratio of about 2:1 over both the short and long term, and they come from directions that typically vary within 60 degrees.
The approach led to a three-year PhD study, supported by an EPSRC Case Award, to look at the aerodynamics of wind turbines and see whether they could be improved.
A typical turbine made by Quiet Revolution produces a peak power of 7.5kW, an annual output of 10,000kWh, costs £25,000 and is designed to last for 25 years. The study made no attempt to change the shape of the spiral blades, but what it did find was that the crucial parameter that decides power output is the tip speed ratio, which is the blade speed divided by the wind speed. When the tip speed ratio drops below 3.7, power output falls sharply when the blade goes into what would, in an aircraft, be a stall condition. The solution to improving the energy output dramatically is therefore to ensure that this happens as rarely as possible.
Although a motor is not driving the turbine, its speed can be controlled by regulating the amount of energy that is drawn from the generator. This means that, when it slows down and approaches stall conditions, the energy draw is reduced until the wind can speed it up again. “You need to track the gusts, but only if they last more than a few seconds,” says Babinsky. “The controller looks at what has happened in the last 300 seconds and then predicts what is likely to happen in the next five minutes, to ensure that the turbine operates on the right-hand side of the peak. The end result is a 42% increase in energy extraction.”
The control strategy is a fairly classic one, with the measured, filtered wind speed and the power curve from the turbine input into a mathematical model, which is used to calculate the optimum rotation speed at any one time. According to Babinsky, a similar strategy could also be applied to horizontal axis wind turbines. And, it might be argued, to many other kinds of machinery, whether extracting energy from a varying source or using it to move a varying load.

Pointers

* The strategy has been to understand the physics of the problem, develop a model of what is happening and then keep the turbine operating under those conditions where it is most efficient

* Applied to vertical axis wind turbines, it has resulted in a 42% improvement in efficiency by ensuring that they rotate fast enough to keep away from stall conditions


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