Wind turbines aren’t yet as reliable or cost effective as the industry would like. What’s more, most turbines are so new, their maintenance costs are unknown. More than 50% of the national fleet is out of OEM warranty and the figure can only increase.
The huge building boom from 2005 to 2010 launched many new turbines. These units have relatively low maintenance costs, regardless of who’s footing the bill, and relatively high availability. However, look at the lifecycle for a turbine and see operations and maintenance costs for wind power typically double or triple projected costs. To correct this oversight manufacturers have begun offering extended service warranties, some as long as ten years.
“There is no silver-bullet strategy,” says enXco’s Dave Luck. “Because of the diversity of owners and developers, along with the trend of more assets being utility owned, an extended warranty may not be the best strategy.” Wind project owners range from community-owned, one-offs to Fortune 100-owned and power-market focused projects. As such, each owner requires different services from the manufacturer. “If a small or medium-sized owner is relying on bank financing, they are certainly choosing the ten-year warranty option because it limits the conversations with the bank regarding risk,” explains Luck. At the other end of the spectrum, a large-scale project owner such as NextEra Energy or MidAmerican Energy Holdings is better off sourcing its own maintenance contracts. This is where independent service providers add value to a project.
Beyond the who-pays-for-what predicament prevalent in the wind industry, predictive and preventative processes have become more common over the last couple of years. Predictive maintenance, condition monitoring in one form, can provide a substantial boost to turbine availability.
Condition monitoring is a way to keep tabs on power-production equipment mounted on 60 to 100-m towers and often on wind farms in remote locations–not the easiest power plants to check. Condition monitoring equipment assists with vibration sensors on main bearings, gearboxes, and generators by sending vibration signals to a monitoring station. Thanks to the internet, this station could be many miles away. People monitoring the turbines could be responsible for several wind farms. After the vibration signals trend for a few months, it’s possible to tell where trouble is brewing.
Analysis software that assists with turbine monitoring, Fast-Fourier Transforms, can associate a frequency with a vibration so it’s possible to nearly pinpoint an ailing component. Knowing the general speed of a component, say a 300-rpm bearing in the second stage of a gearbox, it’s possible to spot its vibration amplitudes. Because bearings generally do not fail catastrophically, the vibration amplitude can be tracked as it rises, and the bearing scheduled for replacement at the O&M crew’s “convenience” and not as a surprise.
Another trend is for wind-farm owners to look to O&M crews for performance improvements, not just repairs. For instance, to better understand blade aerodynamics, some crews conduct studies in wind tunnels or CFD simulations of proposed blade attachments. Examinations there look at a blade in two configurations. One is smooth, without interruptions in the flow. The other blade has wear conditions that produce a flow disruption on the surface–something to induce a separation or turbulence of flow around the blade. These could be leading-edge erosion or a buildup of debris, or bugs.
A recent idea for restoring performance to wearing blades comes from the aircraft industry but it’s getting attention in wind. Vortex generators (VGs) have been used since the 1940s and are visible on modern passenger aircraft wings. They appear as small rectangular fins placed along the wing’s crest. VGs maintain smooth airflow across the wing for longer periods, thereby increasing its lift. When used on a blade, VGs generally provide greater output for the wind turbine.
The devices have been applied to wind turbines several times over the last twenty years with varying degrees of success. One area where they’ve been helpful is in combatting flow separation, a condition in which the usually laminar and smooth flow across the airfoil detaches at some point so it’s no longer laminar and predictable. The VG steers the turbulent air away from the blade and brings clean (laminar) air back down in contact with the blade, which re-energizes the flow from where it has gone turbulent. This creates more power but only at lower wind speeds. At higher wind speeds and near maximum or rated turbine power, VGs are less effective. WPE