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Renewable energy: bearing up to the challenges

01 February 2012

With bearings having a critical impact on the reliability and safety of renewable energy systems, we invite Dr Steve Lacey to describes how one leading precision rolling bearings specialist is working towards improving the performance of wind, wave and tidal energy systems

One of the greatest engineering challenges that we face today is to meet the huge and growing global demand for energy while preserving the earth’s fragile environment. Although solar energy will certainly play its part, here in the UK, wind, wave and tidal energy systems will be the most likely future sources of renewable energy. In the wind energy sector alone, experts predict that by 2020, more than 6,000 wind turbines will need to be installed in the UK if the country’s future energy targets are to be met.

Investment in renewable technologies requires a significant financial commitment. Protecting this investment by ensuring maximum availability of wind, wave and tidal turbines is therefore critical. As a manufacturer of high precision rolling bearings, The Schaeffler Group is committed to playing an important role in improving the reliability and safety of wind, wave and tidal energy systems.

Rolling bearings are a core component of any renewable energy system, and suitably designed bearings are critical in reducing operator costs. The low friction bearings for a wind turbine rotor shaft and gearbox, for example, present many design challenges, as do their associated lubrication systems, their correct mounting and, of course, their maintenance. The effective use of remote condition monitoring and diagnosis techniques will also ensure that maintenance costs are minimised and service life is increased.

A variety of bearings is to be found in a typical wind turbine drive train. Rolling bearings are used to support the rotor, alternator and gearbox, while plain bearings are found in the blade adjusters of pitch-controlled wind turbines and in geared motors that drive the tower slewing ring and pitch control. Over the next decade, the significance of these rolling bearings will increase as new, larger, multi-Megawatt class wind turbines are developed.

This trend towards larger wind turbines, particularly in offshore wind farms, will lead to more radical changes in the design of wind turbine bearings, and ‘single bearing’ designs will become increasingly important.

Single bearings
With single bearings, the wind turbine rotor is supported only by a single, double row tapered roller bearing, typically in a back-to-back arrangement that supports all forces and moments. This concept can take many different forms; for example, with a shaft and gearbox and a high-speed generator, as a hybrid system with shortened gearbox and medium-speed generator, or as a direct drive without a gearbox.

Single bearings always result in more compact designs. For example, the wind turbine nacelle can be fully integrated with the bearing-gearbox-generator unit. This means that it is possible to eliminate drive train components and reduce weight, which, in turn, reduces the head weight of the wind turbine, allowing smaller foundations to be used and simplifying logistics.

Another advantage of the single bearing design is that preloaded tapered roller bearings can be utilised, which prevent axial clearance and small axial misalignments. This narrow, tight guidance of the rotor shaft means that there are fewer movements acting on the system, and thus less adverse impact on the gearbox and generator.

Materials and coatings
Schaeffler’s spherical roller bearings have already found their way into several high-profile European wave and tidal energy systems, including the Pelamis P2 wave energy converter. Contributing to the success of this development is a new joint concept that utilises a low-friction modified PTFE fabric liner developed by Schaeffler, which effectively eliminates the problem of ‘stick-slip’. This has enabled the operating envelope of the P2 machine to be extended beyond the capabilities allowed by standard bearing materials.

Currently in development are bearings with polyether ether ketone (PEEK) segmented cages that reduce friction and increase the efficiency of the turbine. Cages such as these also improve the guidance of the rolling elements and optimise lubricant supply. Single bearing designs can also be supplied with integrated anti-corrosion-protection, by applying a zinc, flame-sprayed surface coating and multi-layer painting. Special hardening processes enable custom material characteristics for integrated functions such as seals.

Dynamic simulation
Rolling bearing calculation software, finite element analysis (FEA) and dynamic simulation tools will also play a critical part in developing next-generation renewable energy systems, and for this to succeed, collaborative development projects between multiple component suppliers will be necessary.

Schaeffler is currently working closely with a gearbox manufacturer, a wind turbine manufacturer and a software developer to develop new simulation software that is able to calculate the dynamic operating loads acting on wind turbine powertrains. Used in combination with FEA tools, this complex, multi-body simulation (MBS) model will enable design engineers to optimise the design of individual powertrain components and to establish their interaction with other powertrain elements.

Simulating dynamic operating loads is a critical factor in the design of wind turbines. Despite new wind turbine designs - which have stemmed from new alternative main bearings and gearbox design concepts, as well as being prompted by demands for better performance and reliability - the load simulations have, up until now, been conducted using relatively simple design calculation models.

These simplified models consider only the effects of load over time for specific internal stress variables, independently rather than for all powertrain components and their interaction. This means that complex units, such as gearboxes, which have multiple dynamic components, are treated as a ‘black box’; the design of the gearbox and how it affects other parts of the powertrain is simply not taken into account by this over-simplified model.

In order to gain a better understanding of the dynamic loads acting on the mechanical powertrain components in a wind turbine, the four project partners have brought together their respective mechanical engineering and software expertise to develop a complex MBS model. One of the core software constituents of this model is ‘BEARINX’, a rolling bearing calculation software tool developed by Schaeffler, which is able to generate dynamic simulation models for complete gearboxes in rapid time. The respective results can then be visualised.

BEARINX enables users to make bearing calculations, including shaft deflections and stress calculations of complex bearing and gearbox arrangements. The software enables rapid visualisation of the effects of any changes, giving users considerable freedom to optimise their designs. It allows various bearing designs and shaft deflections to be considered, and the results to be compared with FEA calculations.

Bearing arrangements can be analysed in huge detail, from single bearings via complex shaft or linear guidance systems, to entire gearboxes and powertrains. The software can model complete gear systems and can simulate the different gearshift conditions of a mechanical or automatic gearbox. Indeed, BEARINX takes everything into consideration, including non-linear elastic deflection behaviour of bearings; the elasticity of shafts and axles; the influence of fit, temperature and speed on bearing operating clearance, preload and contact angle.

The software also takes into account load-related contact angle shifts. Even for complex gears, the contact pressure on each rolling element is considered in the calculation. Lubrication conditions, contamination and actual contact pressure and its effect on fatigue life, are also taken into account. And when used in combination with dynamic simulations and FEA calculations for adjacent components and housings, BEARINX enables powertrain simulations to be accurately and reliably modelled.

The MBS model that has been developed facilitates integrated calculations of fatigue loads or extreme load conditions that may arise during wind turbine operation, including emergency shutdowns or power failures. Having a simulation tool available at an early stage in the design ensures that the development team remains confident as the project progresses, and development costs are well controlled – particularly as modifications can be made much earlier in the design process.

Simulation tools such as these, which can be used at the initial wind turbine design stage, will prove invaluable in helping to make future wind turbine designs more reliable and cost effective under a wide variety of load conditions. Moreover, new bearing designs can be developed and tailored to specific needs, including the requirements of turbines destined for the very different operational environments of onshore and offshore wind farms.

Dr Steve Lacey is engineering manager at Schaeffler (UK)

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