As the energy harness for a wind turbine, blades create the link between natural and mechanical energy. The constant quest for increased turbine yield, coupled with machines installed in ever-harder environmental conditions, make blade maintenance a difficult balance between ensuring performance and managing lost revenue. Keeping any energy asset in top condition requires careful consideration of factors that affect their performance, and this is especially true for blades.
Modern wind turbine blades are an incredible engineering challenge. Exposure to a vast array of environmental conditions with a constantly changing load and a typical life expectancy of at least 20 years would be demanding enough, but the complexities of maintaining them makes the task even tougher.
For developers investing in new turbines, managing blade maintenance, especially post-warranty, can be critical to ensuring returns on investments meet expectations. Composite structures have many benefits, but ensuring their long-term health is difficult unless any degradation or damage is obvious.
The diagnostics are complex, and repair methods can be daunting and costly, especially if implemented onsite for offshore projects. However, an understanding of the typical damages, inspection operations and potential degradations of wind blades can help define a strategy that can improve turbine performance while easily justifying downtime costs and lost yield.
Chairing a recent Windpower Monthly conference on blade inspection, damage and repair I noted a much improved understanding of the difficulties of blade maintenance and the effects of "bad blades" on overall turbine performance among turbine OEMs, developers and maintenance organisations. There is also a growing awareness that, post-warranty, some blade types require more maintenance than others, especially in relation to surface coatings and erosion protection materials.
Proactive blade maintenance can offset its upfront costs through performance benefits and increased energy production. New technology for inspection and repair can significantly reduce turbine downtime, and this needs to be considered in maintenance strategies. Interest in optimising installed blade performance is rapidly growing as investment funds and smaller developers look to optimise their investments.
Inspection technologies vary from simple tripod-based zoom cameras and telescopes, which can provide relatively cheap data for onshore turbines, to full-access systems such as inspection platforms with teams performing metre-by-metre checks. Costs for inspections using access systems, either nacelle-hung or ground-based skylift types, rise sharply with tower height increases or in offshore locations, so selecting the right option can have a huge effect on overall maintenance costs. A mixture of technologies employed at different stages in the turbine lifecycle usually provides the most effective route.
A new technology gaining momentum is the use of drones, or flying camera platforms, to take images and, in some cases, video of blade surfaces. This provides close-up images of blades with surprising accuracy, but it does require trained "pilots" and good weather conditions. There are cases where wind-affected drones have collided with blades. Despite this, "stand-off" technologies that provide quality data have significant benefits, not least in reducing health and safety risk.
Performance reviews of operational machines indicate that in a situation with badly maintained blades, erosion and defects can develop to a level where 4-6% annual energy production can be lost from the yield of a wind turbine compared with its theoretical power curve. This can advance to over 8% where the coating of the blade's leading edge has eroded, leaving turbines operating far below their potential.
Clearly, there is a level of rotor performance degradation for each machine where it becomes more cost effective to take it out of service and address surface issues. Typically, this focuses on bringing the leading-edge quality back to a level where aerodynamic losses are reduced and eroded surfaces are reinstated. In the simplest cases, turbines can be accessed by rope systems to allow operatives to apply new leading edge protection systems - usually paint or film - to the blade. In more extreme cases, revitalising surface coatings requires a complete surface preparation and recoating of the blade. This can also be done via rope access methods, but the time and effort involved generally means using larger access platforms and airborne stages to complete the work.
Advancing repair technologies
A critical point emerging from the recent conference was that the amount of information released by blade OEMs is often insufficient to ensure that maintenance is carried out correctly. This means maintenance teams must decide on the best route for repair without the support of design and manufacturing information. This has to be combined with the knowledge required to process composites and the skills to work in exposed conditions. Composite materials require moderate environmental conditions to perform correctly, which adds to the complexity. Using these materials also takes time to a repair, as resins are mixed, applied and cured using heat, a cycle that can take many hours to complete.
The industry is acknowledging these problems, and materials technology is developing to addresses some of the key issues with in-field repairs, such as the Renuvo range from Gurit, which uses UV curing, delivered by neoelectron/UV lamp, to allow a larger environmental operating window for surface and structural repairs. This removes the need for mixing materials and provides a much faster cure cycle, making it attractive for simple repairs.
There is also considerable interest in optimising existing machines beyond just maintaining original performance. A consortium comprising Smart Blade, 3M and the Technical University of Berlin has developed a tool to increase turbine performance by measuring and analysing actual airflow over blades using the Smartviz system. This uses data from cameras and blade-mounted airflow indicators to provide real data on blade-stall performance, which can then be used as the basis for mounting vortex generation devices. Using real blade-stall information provides a level of accuracy that has previously been difficult to achieve, and on installations so far typically reversed the losses from surface degradation and increasing annual energy production on the original power curve performance by more than 2%.
There is an inevitable time lag between the development of an industry, and the standardised provision of technology to meet an inspection, operations and maintenance demand. Blade technology presents a number of challenges to repair operators: first, to understand the materials and processes sufficiently to carry out repair work to a suitable standard; and second, being capable of carrying out that work in tough environmental conditions. The industry's dependence on third-party suppliers of blade maintenance makes this hard to monitor.
In the UK, the Renewables Training Network (RTN) has developed a course for blade repair and inspection designed to create a baseline training standard. This is the first step towards developing a standardised workforce, capable of ensuring wind-farm operators and developers receive the reassurance that their machines, and those working on them, are suitable for the job.
The effect of surface coatings on turbine performance has also been identified by the UK's research group Offshore Renewable Energy Catapult as a major area of research, and plans are being developed for a UK state-of-the-art test facility to better evaluate coating performance.
For most new, large wind farms, especially offshore, the financing models bring significantly more pressure on maintaining asset performance. Reducing maintenance of components such as blades to a minimal, reactive-focused format can be seen as a cost-reduction route. This may bring short-term benefits early into post-warranty operations, but the longer-term performance requires efficient, proactive solutions. As the industry matures, baseline training standards for maintenance operators and a much deeper understanding of coating degradation should help to develop confidence for investors on the long-term performance of wind assets in their portfolio.
Andrew Bellamy is the 8MW blade lead at Areva