Wind turbine reliability is a major factor in a wind project return on investment (ROI). Poor reliability affects the top line i.e., energy revenue, as well as operational and maintenance (O&M) cost. Most publications in the past focused on turbine design, however this is not the only factor. This paper takes a 360-degree approach to these issues, describing unconventional ideas such as reshaping the wind, and tips on how to manage the turbine warranty and later stage service cost issues. It also suggests budgetary tools that will make the operations and maintenance much more predictable.
The Wind
The wind is God’s given gift to the wind industry. But guess what? The wind is also the cause of wind turbine failures. If the turbine would stand still, it will never break. The wind affects the turbine’s wear-and-tear in multiple ways.
Average Wind Speed: The most obvious and simplest one is the turbine “mileage.” Like in a motor vehicle, the turbine wear-and-tear is a direct function of the number of revolutions (driven by wind speed). In other words, the stronger the average wind, the faster wear-and-tear over a given period of time. A turbine owner can increase its productivity by increasing the wind feeding the turbine. This phenomenon puts us in a conflict of interest, which will be discussed in more details farther down below.
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Photo: Wind SL
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Wind Speed Height Differentiation: Large commercial turbines experience few m/s difference between wind speed at the top of the rotor, to wind speed at the bottom of the rotor. This causes significant stress on the rotor shaft and the gearbox. This stress can be reduced in several ways. One technique is adjusting the blades’ pitch to compensate for the speed difference. Another technique is using an external structure that enhances the bottom wind resulting a more uniform wind speeds between top and bottom.
Wind Speed Variability: Like in a motor vehicle, “Stop and Go” is another source of stress to the turbine. The turbine will stop in two conditions: 1) when the wind is below the “cut-in” speed, and, 2) when the wind is above the “cut-out” speed. The second case is addressed, to some extent, by the wind turbine manufacturer, via adjusting the blades pitch to achieve a more uniform rotor speed despite changing wind speeds. In order to reduce the frequency of stops in cases of slow wind speed, one can enhance the wind via devices that enhance the wind speed, thus reducing the amount of times the turbine will experience below “cut-in” speeds.
Wind Direction Variability: The turbine’s yaw mechanism is turning the turbine’s rotor to face the wind. An un-perpendicular wind will cause an additional stress on the rotor because of the uneven forces applied to different blades. The yaw mechanism may not be fast enough to track hectic wind-direction changes.
What can be done about it? Wind-related wear-and-tear can be reduced in several ways. Most modern wind turbines are equipped with pitch and yaw systems. The pitch and yaw control adjust the blades’ pitch and turbine direction to maximize power generation in normal wind conditions. During periods when wind conditions become too stressful to the turbine, the pitch and yaw control will adjust blades’ pitch and turbine direction to minimize the strain on the turbine (Shulte 2009).
The pitch and yaw controller is fed by a weather station on the turbine. This weather station responds to condition as they happen to the turbine. One other technology is real-time tactical prediction of up coming wind variability. This allows turbine’s operations adjustment before the wind event hits the blades (Dakin 2009).
Leviathan Energy is one company that tackles the challenge of improving turbine wear-and-tear by reshaping the wind to be more turbine-friendly (and also improve energy production at the same time). Leviathan’s Wind Energizer reshapes the wind to provide a more uniform wind speed between the high blades and the low blades. Leviathan’s Wind Energizer also reduces the frequency of start-stops.
Maintenance Tool Box
Cumulative maintenance cost can add up quickly to match the initial turbine’s investment cost. This is not a small change. This section will describe multiple tools that help reduce this cost. One gearbox replacement could cost north of $350 K, not including loss of revenue due to downtime (Walford 2006).
Preventive Maintenance
This is usually defined in the manufacturer’s maintenance manual and includes items such as: Periodic inspections, lubricants’ change, minor items periodical replacements, and various calibrations. Major parts replacements are not included in this category because of their high cost.
Lubrication
Lubrication and oil changes are the most important preventive maintenance activities. These are often missed because of various reasons. Harsh weather is one limiting factor, because it occasionally prevents from preventive maintenance to occur at the optimum to the turbine, but, rather, be weather-dependent. The other factor is logistical efficiency. The turbines are often in remote locations, with difficult climbs, and not very human friendly nacelles. Therefore, maintenance technicians often tend to consolidate maintenance activities to the same turbine, which is not always the optimum timing for each specific component. Human errors and lack of training also contributes to lubrication errors. An automatic lubrication system addresses all these issues. An automatic lubrication system provides the right lubricant, at the right amount, with the right frequency, to the right lubrication point (Conley and Shah).
Condition Monitoring Techniques
Turbine wear-and-tear can be tracked by several monitoring techniques. One group of techniques that is becoming very popular is vibrations monitoring. Vibration sensors monitor the vibrations of different parts of the machine. A centralized control system is programmed to distinguish between normal vibrations and abnormal vibrations. This control system will flag if a subsystem needs service so the appropriate maintenance can be scheduled. This technique facilitates, just in time, maintenance of expensive parts, avoiding premature replacement on one hand, or expensive down times due to unexpected breakdown. Condition Monitoring also allows consolidation of maintenance calls, thus, sharing some of the overhead expenses among multiple repairs. Another type of monitoring technique include off line and on line fluids contamination tracking (Walford 2006). The knowledge base required to perform such a task is accumulated over a long period of time. Although non of the suppliers can provide hard proof and quantitative benchmark of one system versus another, several important factors need to be considered when shopping for a condition monitoring system: 1. The experience of the supplier, 2. The amount of data provided to the customer, and 3. The independence of the condition-monitoring supplier from the equipment provider.
WindSL capitalizes on its parent’s company RSL Electronics’ 30-years success doing condition monitoring for the aviation industry. WindSL implements RSL’s technology to support wind turbines. Every time you fly, you are likely to put your life in the hands of RSL/WindSL technology.
Planning and Budgeting Tools
The condition-monitoring tools described above are good for tactical maintenance: Providing an advanced warning of a soon-to-happen major part failure, so replacement service can be ordered before the turbine goes down. The condition monitoring system does not provide a macro data to plan spare parts production volumes, spare parts scheduling, and maintenance budget for out of warranty farms (Vachon 20006; Gates 2007; Hansen 2007). These are software tools that have long-term statistical models that will help the relevant parties in this planning process (Teboul and Vittal 2004). Vestas made maintenance planning one of its key initiatives for 2009 (Vestas 2008).
The Turbine Supplier
The wind energy industry is a young one. The turbine manufacturers are on a steep learning curve to improve the durability of their machines. They also improve the maintainability of the turbines to allow mechanisms for advance warning to an upcoming failure, and easier access to system components. These technologies will be covered in more details in the next section.
In order to improve reliability the turbine manufacturers are analyzing failure modes, determining the root causes, and implement corrective actions in the form of adjusting maintenance guidance, as well as future design improvements.
Generators and gearboxes are the most costly items to maintain. Their complexity contributes to high-failures rate as well as very expensive repairs. The gearbox and generator challenge is to produce constant frequency (50 Hz or 60 Hz) out of unpredictable variable input speed. Turbine manufacturers are working on developing less complex generators architectures that hopefully will be more reliable and less expensive to maintain. Some of the stresses on the gearbox are direct results of the wind behavior, and discussed in the previous section (Gates 2007; Hansen 2007; Butterfield, McNiff, et. al 2007).
Poor reliability is sometimes a result of poor quality. Vestas launched a major quality control initiative in 2008. As a result of this initiative, they reported a 99% reduction in the need of rework glue during the blade manufacturing process (Butterfield, McNiff, et. al 2007).
The Wind Farm Owner
Since O&M cost is such a significant factor in an overall wind project investment, one needs to make a careful consideration predicting O&M costs of different turbine options, different warranty plans, and different add-on O&M products and services. These items could make a big impact on the overall ROI of a project.
One golden rule for the buyer: Negotiate the O&M coverage before you commit the purchase of the turbine.
Here are some pieces of information you may want to obtain before making a decision:
Historical Reliability Data: Some manufacturers may be reluctant to share this with potential buyers, in which case, your alternative is to obtain it from third-party sources.
Condition Monitoring: Find out what type of condition monitoring mechanism is provided by the turbine manufacturer. It may be a std feature or, for some suppliers, an optional one. Do you get access to the condition reporting system? Or only the manufacturer service people do? The built-in monitoring system is often limited to basic parameters. Even if the turbine comes with a built-in monitoring system, you may want to consider adding a third-party one, which is usually more features-rich, and you will have full access and control on.
Spare Parts Price List: Even the first years are covered by warranty; this is the best time to negotiate spare parts price list.
Different Warranty Options: As part of your turbine purchase negotiations, the developer may be presented different warranty options. The developer does not have total freedom here, because the project financers are likely to require a certain level of warranty coverage. Another issue is the level of visibility the wind farm owner is allowed by the turbines suppliers. The turbine vendors tend to keep the O&M information close to their chest. This is not a problem in the short run, but the farm owner needs to get good visibility as the warranty period approaches the finish line.
Summary
Keeping a wind farm running is an overall ecosystem teamwork. This paper just touched briefly on the different topics and different considerations.