A new generation of turbines

Faced with the need to cut fuel costs, an Oxford professor has invented a cheap wind turbine that uses an induction motor
Michael Pollitt
guardian.co.uk, Wednesday 19 August 2009 19.30 BST
Professor John Gregg at the University of Oxford is an international expert in the fields of spin electronics, spintronics and magnetic instrumentation. But he has designed and built something for homeowners facing high energy costs: a new-generation wind turbine. He is testing the turbine, which features a standard induction motor as a generator, in his mother's garden in Ireland.
It all started thanks to rising energy prices. "About five years ago, the prices got so bad that we agreed we needed a wind turbine to heat the water for the shower," says Gregg. He was "knocked sideways" to find he would have to pay ¤38,000 (£33,000), giving an unrealistic 50-year payback time.
One problem, of course, is the wind. Velocities drop dramatically as you come down in height, and planning laws – though thanks to Irish MP Dick Roche, Ireland has good exemptions for domestic wind turbines – make it difficult to erect tall wind turbines in gardens. The high costs are attributable to custom-built generators, invertors, storage batteries and complex circuitry.
Question time
Although he had initially dismissed wind turbines, an Oxford student's question three years ago led to a rethink. Gregg teaches an electrical power and machines option, including a study of "induction machines" (a class of electric motors).
The student's question was this: how can an induction motor work as a generator? Do it with an ordinary motor and you will get out the right voltage and frequency. If you turn an induction motor's rotor expecting to induce a current in the primary windings (normally energised to power the rotor), what you get out varies with the speed.
"So, pride suitably dented, I spent a long time puzzling out the answer for him," says Gregg. In doing so, he was able to spot a novel – and very cheap – method of using an induction motor (found in everything from domestic appliances to industrial machines) for wind turbines.
The electricity generated by using an AC induction motor as a generator doesn't appear at the mechanical speed like an ordinary motor. Instead, the electricity appears at the frequency which would make the total impedance of the generator plus its load equal to zero. All you do is connect a load, correctly switch the primary windings, and turn.
"Then you'll see a voltage appear on the load and the harder you wind, the bigger that voltage gets," says Gregg. "The bigger the load you put on, the lower the voltage you get for given wind conditions."
Wind turbines came back on Gregg's agenda when he realised that hot-water tank heater elements don't mind variable voltages or frequencies. "That's why we can make it cheaply and why it performs well because we are not handcuffed by the necessity to deliver 240V 50Hz," he says.
To make the new turbine work efficiently, Gregg designed a patented electronic control method, which draws inspiration from Swiss locomotives. "There is no mechanical gearbox but as the train pulls out of a station, you can feel it 'changing gear' electrically as the field windings on the motor are switched to give maximum acceleration at all speeds. Well, our generator works in a similar fashion."
And that is the big advantage here. "Because the generator is configured as a constant power source and acts effectively as a generator and a continuously variable electronic gearbox, the turbine aerofoils operate on the peak of their performance curves at all times, and all the power they deliver is harvested and channelled to the load. So, the diminished wind power that you get at low altitude is used to maximum effect."
The wind turbine (which has six-metre diameter blades and a standard 7.5kW induction motor used as a generator) in his mother's garden provides electricity for a heat exchanger tank, which in turn feeds the domestic hot-water tank. "On windy days, the hot tank is pre-fed with hot water so less mains electricity is used. The heat exchanger tank also has a heat exchanger coil that ploughs surplus heat into the domestic central heating system, so saving on heating oil," says Gregg.
His early results already show the equivalent of 1kWh continuous power — a useful reduction in his mother's electricity bill. By combining a large diameter blade with an inexpensive generator system, the wind turbine is expected to pay for itself in three to seven years.
With his co-inventor Dr Mazhar Bari, Gregg is now proposing a spinout company, Renewox, through Isis Innovation, the technology transfer company of the University of Oxford.
Wind of change
AbuBakr Bahaj, professor of sustainable energy at the University of Southampton, has been undertaking the data analysis work for the UK national micro-wind trials funded by the Energy Saving Trust. A key issue for prospective wind turbine users is understanding their resources.
"Regardless of whether the wind turbine produces electricity or heat, if the wind resource is poor, the device will perform inadequately," he says. "It is important to consider for a realistically sited micro-wind turbine what the wind resource is, rather than considering performance at for example a 12m/s wind speed [how commercial large-scale turbines are rated] – this is very misleading for micro-wind."
It also comes down to the user's usual source of energy. For example, the home heating oil used in rural areas is more expensive than gas, with 60% of the domestic energy budget being consumed by heating.
"I assume that the market for this device is housing which is not on the national gas network," says Bahaj. "This obviously helps the economics because the heating costs are much higher than normal."
As the world invests in renewable resources, every little counts.