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08/03/2010
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Government targets and investment have given a major boost to the renewables market. Paul Fanning looks at some of the ways in which companies in the drives and controls sector are meeting this need.
 The market for renewable energy generation has transformed from a niche application to a major industry. The EU-imposed target of producing 15% of the country's energy from renewable sources by 2020, the recent announcement of the power companies with responsibility for achieving this and the availability of government-funded incentives for companies working in this sector have only served to concentrate minds.
Nowhere has this been more the case than in the drives and controls market, where a number of companies have long been involved in the construction and implementation of technology in a variety of projects and are only too keen to exploit the opportunities provided by this impetus. The major concern is reliability, as ABB's principal UK sales engineer for drives Julian Hobbins points out. "In renewables, it's all about reliability," he says. "These things have got to be rugged and reliable because, when they're working, everything's fine and dandy, but as soon as one of them stops working, it's headline news and can destroy all that good work overnight."
Clearly this need poses considerable challenges when it comes to choosing the drive and control technologies. The difficulty arises in the choice of a system appropriate to the application. One instance of this can be seen from its involvement with wind power generation expert quietrevolution to develop the QR5, a vertical axis wind turbine designed to operate in urban areas. The wind turbine turns an ABB permanent magnet motor, which is linked to an ABB industrial drive acting in regenerative mode to provide the energy conversion and regenerate power back into the grid. The motors themselves are designed exclusively for frequency converter supply. ABB industrial drives are available with permanent magnet motor software to integrate with these motors.
The ABB industrial drive offers a number of benefits to the QR5, including the ability to motor to correct speeds to enhance wind turbine efficiencies very quickly. The ABB industrial drives used in the application feature direct torque control (DTC) technology, which allows variable rotor speed to optimise performance at any given wind speed.
Says Hobbins: "For quietrevolution, the speed of the direct torque control that we operate enabled it to do things like gust tracking, which the company hadn't even considered. That gave more efficiency from the vertical axis wind turbine. Other examples are where people have gone for a DFIG [Doubly Fed Induction Generator] type configuration, where the inverters are not fully rated. In the event of problems, we operate a crowbar; an intelligent switch that enables them to burn the excess energy for the short term while the grid's not available and then go straight back into generating mode without all the wind turbines toppling over like dominoes."
The use of permanent magnet motors of the type employed by quietrevolution is an increasing trend in the renewable sector, but one which brings with it its own set of problems, according to Hobbins. "We've analysed the market and there's a move towards permanent magnet. There are lots of weird and wonderful designs coming along, but, as with anything, there are advantages and disadvantages. The advantages are that across the speed and torque range it's more efficient, but its disadvantages are limited top speed and limited overload capability… we can't dictate to the market, but when we do the analysis dispassionately, then a medium speed gearbox plus a permanent magnet motor look the best all-round option."
Parker Hannifin has also got an established reputation in the renewable market and has recently established a Renewable Energy Team led by Kevin Rowe to meet demand. He believes that the increased demand for more powerful wind turbines is already forcing more sophisticated drive and control options to be employed. "When you look at the original wind turbines," he says, "they were pretty basic hydraulics – a low-cost item, effectively. But as the wind turbines move forward in terms of power output, you've got to look at more sophisticated electronic/hydraulic controls."
Like many, Rowe sees the future lying in direct drive technology. He says: "One of the biggest problem a turbine has is the gearbox. If we can eliminate that in favour of electric drives that will be the way forward. The obstacle, of course, is the cost. You're looking at a couple of thousand pounds in difference at the moment. However, in terms of reliability, when you think about an offshore wind turbine and the costs and possilbe risks involved in sending a man out to maintain or repair a gearbox there, those costs may seem worthwhile."
Moves towards direct drive technology are already afoot. The most notable example has come from Siemens Energy, which recently installed the first prototype of a direct-drive wind turbine. Aimed at the high-end, onshore market, the new SWT-3.0-101 DD is a gearless machine with a power rating of 3MW. Siemens will thoroughly test and validate the performance of the SWT-3.0-101 before the new product will be officially launched for sale in 2010.
Clearly, one of the advantages of the direct drive technology is the reduction of the number of moving parts, with consequent advantages in terms of maintenance time. In the Siemens prototype, for instance, the number of parts was reduced by half when compared with geared machines. Nonetheless, the company has been careful to point out that it will continue to produce wind turbines based on the standard gearbox design as these have "proven to be exceptionally robust and reliable".
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Author
Paul Fanning
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