As turbine capacity increases, so does the need for new designs and components, says co-inventor and Nestor owner Bart van Neerbos, a wind industry veteran who led the development of the 1.5-2MW Zephyros Z72 direct drive turbine. The turbine was one of the first to be fitted with a single rotor bearing.
A single bearing has been incorporated again in the Hydrautrans design, but the substantially larger bearing size required represents a major technological challenge even for component supplier market leaders. The system also incorporates a double-sided industrial precision-made steel crown gearwheel, where teeth project at right angles to the plane of the wheel, resembling the points on a crown. It also uses six (three at each side) patented floating-cup low-speed pumps, a high-speed floating-cup type motor, and a high-speed generator.
"Artisan-type crown-wheels made of wood are best known from mechanical drive systems applied in Dutch four-bladed wooden windmills," said Van Neerbos. "One of the advantages is that wear of the wooden wheels and pinions could be compensated for by allowing axial adjustment of the grinding wheels on their vertical axles to maintain correct teeth engagement."
The large double-sided gear driven by the Hydrautrans rotor has an approximately four-metre outer diameter and the step-up gear ratio with the pump pinions is about 10.
"A major benefit (of crown gears) compared to functionally similar bevel gear systems (coning gearwheels) is that the matching pump pinions can be removed and refitted with little effort and without additional adjustments," explains Van Neerbos.
A life-enhancing feature of the drive train is that the gearbox is not rigidly fixed to the nacelle. An extra main bearing enables a balancing of all forces, whereby only rotor torque acts on the gearbox.
Hydraulic drive systems are often associated with low efficiency of pumps and motors compared to common mechanical-electric wind turbine drive systems with or without gearbox.
"A key pump and motor design feature is that they operate without metal-to-metal contact, offering long service life and better operating efficiency compared to state-of-the-art hydraulic devices. The combined solutions all contribute to our aim of delivering a highly competitive offshore turbine with favourable head mass and superior cost of energy", Van Neerbos said.