SPONSORS OF GEARBOX V DD
MORE POWER FROM THE WIND
When considering generation costs for wind turbines, it is a simple matter of principle. Dr. Eng. Andreas Vath, Sebastian Schmidt and Stuart Williams of Bosch Rexroth recommend transmission concepts that rotate at medium speeds and use differential gearboxes.
Growing electricity consumption in Europe has increased the demand for wind energy. Since the number of suitable onshore sites is limited, many offshore projects are currently being carried out. Modern wind turbines can generate up to eight megawatts of power and coming generations are to deliver as much as ten megawatts. The operators are always conscious of keeping generation costs as low as possible and this is arrived at from the sum of all the costs required to convert the wind’s energy into electrical power. The key to greater profitability is to be found in a highly efficient and reliable drive concept. Here operators of offshore wind farms will have to make a fundamental decision when selecting the drive concept. The choices are:
• Direct drives, without gearing • Medium-speed drive concepts with gearboxes
• High-speed drive concepts with gearing
The decision needs to be based on solutions with high efficiency levels, but also with the lowest possible installation and maintenance costs.
COMPARING EFFICIENCY If the wind speed, rotor blades, tower, and system controls are identical, then the energy yield for offshore wind farms will depend exclusively on the degree of efficiency for the power train. Direct drives achieve high efficiency, particularly at lower wind speeds. But this falls off with stronger winds. This is due to a rise in thermal losses in the copper windings as the power levels rise. High-speed concepts are attractive, although they, in turn, have a problem with idling losses in the gearing stages when less wind is available.
If, for the purpose of comparison, we assume a 25-year service life and an average wind speed of nine meters per second, then using a medium-speed concept is advisable. Eliminating the high-speed gearing stage will reduce the losses incurred and the efficiency of the remaining two-stage transmission rises to more than 98 percent. Thanks to the low thermal losses of the medium- speed generator, it achieves 98.3 percent efficiency. Given the 97.5 percent efficiency for the frequency inverter, the overall efficiency for the entire power train is 94 percent – the top value in the wind speed range relevant for offshore facilities. Even at lower wind speeds, the gearing losses are reduced thanks to the elimination of the high-speed stage. Seen over the entire life of the system, the operators of offshore wind farms will profit from the maximum energy yields.
Comparison of the investment costs In offshore facilities, power generation costs will basically fall as the size of the wind turbine increases due to the high fixed costs for planning, installation and maintenance. But despite all of this, there are differences in the individual concepts. The investment costs are the highest for direct drives, this being due to the large amount of raw materials used – regardless of whether a separately excited or a permanent-magnet concept is implemented.
Direct drives with separately excited synchronous generators require large amounts of copper and steel, while permanent-magnet generators need expensive rare earths. Due to reduced materials consumption and the lower investment costs that entails, the electrical generation costs for concepts using transmissions are lower than for direct- drive solutions. Just as was the case in comparing the degree of efficiency, the medium-speed drive train fares better than the high-speed solution.
INFLUENCE OF THE STEP-UP RATIO IN MEDIUM-SPEED CONCEPTS The gear ratio plays an important part in reducing power generation costs. There are two major concepts on the market today, in competition with each other: the conventional design with two planet sets and the REDULUS GPV-D differential gearboxes made by Rexroth. This latter concept, with a power split, comprises two planetary stages and one differential stage. This gearing concept makes possible a particularly broad range of transmission ratios. As a result, it satisfies all the requirements of today’s system concepts and is distinguished by great reliability, since there are no components which rotate at high speeds and would thus be susceptible to wear.
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www.windenergynetwork.co.uk
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