Reliability is critical in making renewables pay

The uncontrolled variability of operating conditions and often remote locations of wind farms can mean that even the smallest fault becomes highly significant in terms of maintenance costs and lost generating revenue.

Reliable by design

Extremes of temperature, humidity, high ozone levels, salt spray, vibration, stop-start frequency - these are the sort of factors that need to be addressed in detail if a unit is to be available for generating and costly unscheduled maintenance avoided.

As with any large item of mechanical equipment, the major 'big ticket' elements of a wind turbine  automatically receive thousand of hours of design input  to eke out the last few tenths of a percent of power and economy. Unfortunately, when equipment is operational it is more often the small components such as seals, bolts and connectors that hold the key to reliability, performance and ultimately profitability.

Bolted joint reliability

With over 1,000 critical bolts in a turbine and 90 - 95% of all bolted joint failures attributed to insufficient bolt tension on installation, it is essential that these components are correctly installed and maintain the correct tension throughout the life of the bolted joint.

This is not as daunting as it may sound if the right bolting technology is specified. Tension control fasteners can be specified, such as the RotaBolt^® from James Walker, , that offer simple, rapid installation to within +/-5% of the design tension plus continual monitoring with tactile or visual indication of any loss of tension for the life of the bolted joint. Combined with the correct installation system, this technology offers significant benefits in faster build times, reduced bolt requirement, improved safety and dramatically reduced maintenance costs.

Sealing, lubrication and protection

Correct seal specification also requires careful consideration; particularly with regard to the environment in which the seal will be asked to perform. Not only does the seal design need to provide a suitable long-term solution in simultaneously preventing the ingress of harmful particles and the escape of precious lubricant, the seal material itself needs to be chosen to suit operational conditions to ensure that seal life and performance is not compromised by extreme cold, ozone damage or high temperature, for example.

Scheduled maintenance

In choosing a seal design it is also important to look ahead to the end of the seal's life and its subsequent replacement. Just how easy will it be to replace a main shaft or blade root seal several miles out at sea? Rapid on-site seal replacement is essential in minimising loss of generating availability, so the ability to replace a seal with the minimum of disassembly is very important. To achieve this requires some form of on-site seal joining or split cartridge assembly offered by major seal manufacturing specialists.

With an on-site seal joining technique such as the Walkersele^® OSJ-2 from James Walker for example, the main shaft seal on a wind turbine could be changed in as little as 30 minutes with no requirement for major disassembly. The new, on-site joined seal offering the same integrity, life and performance as the endless version fitted during manufacture by the OEM.

Vibration elimination

All dynamic machinery generates vibration. A common cause of 'bolt loosening' and metal fatigue as well as a contributor to electronics failures, vibration transmission needs to be minimised or eliminated wherever possible, in the quest to improve performance and reduce maintenance requirements.

Vibration created by shafts and blades is easily transmitted through the structure of the turbine and tower or, in the case of turbines mounted on buildings, into the structure of the building itself. Although materials are available 'off-the-shelf' for mounting duties, successful elimination of vibration and mechanical noise is more effectively undertaken using FEA analysis of the stresses involved along with custom manufacture of pads or structural bearings from materials specifically 'tuned' to damp and absorb the relevant frequency of vibration. Companies such as James Walker company Tiflex are well-proven in providing vibration attenuation and structural isolation solutions.. Tiflex's cork-elastomer composite materials are capable of providing a 'tuned' solution to isolate structures from plant vibration or, conversely, to isolate equipment from structural disturbance.

Insulation and protection

The rapid growth in off-shore wind projects has brought new challenges to the industry, few greater or more frustrating than outages caused by the effects of moisture and salt on electrical and electronic components. The solution of utilising protective mouldings is simple enough, the difficulty being that there are few manufacturers capable of producing the often complex mouldings in materials that are suitable for the task. One such company, James Walker Keaflex, has developed the necessary skills through many years of designing and moulding complex shapes in a range of elastomers ideally suited to the role of protecting electronics at sea. The company and its capabilities are now an integral part of the James Walker renewable energy project team and its work with OEMs to improve turbine reliability and ease of maintenance.

The technology of the renewable energy sector continues to grow at a rapid pace in the quest to generate more power, more efficiently. Pushing the performance envelope in increasingly harsh conditions, such as off-shore wind farms, is all very well, but without reliability the risks to reputation and the bottom line are high. It is critical to be able to generate when the wind blows or tides flow and for that to happen, the part played by the many small but highly critical components that can so easily prevent the 'wheels' from turning should not be overlooked.

As an active partner in the UK's Northern Wind Innovation Project, James Walker is currently investing considerable resource in the areas discussed here with the aim to reduce the costs of maintenance and improve long-term reliability of renewable power generating plant globally.