Double-row slewing-type pitch bearings are currently a semi-standard wind turbine solution, but as power ratings and rotor diameters continue to grow, blade-induced loads are also increasing, creating potential for larger deformations in these. These slewing bearings with rolling elements are now seen by some experts, for a variety of reasons, as a sub-optimal solution for pitch bearings.
In response, bearing manufacturer SKF has developed a groundbreaking blade pitch system inspired by the movement of the human wrist. "Individual pitch control has increased pitch action intensity," says Jascha van Pommeren, the system's architect. "Meanwhile, the industry requires enhanced pitch system reliability and reduction in operation and maintenance costs to drive down the cost of energy. For offshore applications, where turbine access is restricted during autumn and winter, the demands for reliability and robustness require even more reliable solutions."
Around the end of the last decade, SKF significantly increased spending on breakthrough technologies and operational productivity, an overall strategy aimed at strengthening the company's global leadership, said van Pommeren. The key focus is on three areas: emerging technologies, adding value through exploring breakthrough development opportunities; growth potential, driving innovation through technology; and volume, through incremental customer-driven engineering.
It took three attempts to reach the final and most uncomplicated solution for the pitch bearing, which consists of a coning cantilever-type structure with two interconnected frames that cross each other, one outer stationary and an inner body capable of rotating with a maximum stroke (rotation) of about 90 degrees.
"Our project task was to develop a new pitch system without rolling elements," says van Pommeren. During normal operation, pitch systems rotate only a few degrees, and in exceptional conditions can turn up to a maximum of about 90 degrees. Yet, most pitch system are capable of rotating a full 360 degrees. "Breakthrough developments require, above all, unconventional thinking, and our first study object was the complex movement of the human wrist.
"In an effort to maximise the value of an improved pitch system, we further evaluated the bearing, the actuator system, operations and maintenance, and availability. The next step involved a quantitative comparison between a standard slewing bearing and the new solution that included defining functional and performance specifications in an approach to reduce cost of energy."
The main differences between the conventional and SKF's new specifications are a substantial reduction in peak internal friction, which in turn results in a greatly reduced pitch drive system size requirement. The reduction in bearing size enables individual up-tower bearing replacement, which will realise a substantial cost saving in operating expenses. The reduction in friction also aims at doubling the lifetime potential of the pitch system, again cutting operating costs. The minimum required pitch stroke was set at less than 90 degrees, matching the pitch action evaluation.
SKF's analysis of its journal bearing pitch system shows a cut of several per cent in the turbine's cost of energy. Half of this saving is bearing life-related (50%) a substantial part of which is for bearing exchange, approximately 35% of the saving is bearing friction-related, and the remaining 15% is bearing cost-related.
In the laboratory at SKF's business and technology park in Nieuwegein, the Netherlands, Van Pommeren shows the rather modestly sized journal bearing used at each end of the pitch system structure, calling them a standard product used in heavy-duty applications such as earth-moving equipment, he says. Key is a self-lubricating Polytetrafluoroethylene fabric liner without either grease or oil, which gives these bearings their life long self-lubricating performance characteristics.
While it can be developed for a variety of sizes, the concept pitch system was created to fit a 2MW-class wind turbine. "We decided on this size because of the extensive track record with a wide variety of different installations, and the extensive database on reliability and other data available," he says. "So far, results look promising, but some major steps remain to be taken in terms of mass reduction and, for example, in exploring possibilities for integrating the blade and pitch system."
It will likely be a few years before we see one on a turbine, but the technology is sufficiently developed for the company to already be talking to potential customers.