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A Few Potential Design Alternatives and System-level Reassessment
In recent years, increasing evidence of failures has been reported from spherical roller main bearings used in three-point mounting (TPM) drivetrains of wind turbines. One of the leading causes has been micropitting, a failure mode that is possibly overlooked by design, selection and life-prediction tools. It remains to be seen if retrofitting problematic spherical roller bearings (SRBs) with improved bearing design solutions can improve their durability. Questions to ask might be: ‘Are the operating conditions of the main bearing well understood?’ and ‘Are the failures caused by deficient design practice or other unidentified external sources within the system?’ These questions fundamentally challenge the underlying design basis and encourage the need for a system analysis approach that is currently being undertaken by researchers from the National Renewable Energy Laboratory (NREL). Specifically, this article discusses a few potential design alternatives and system-level reassessment to circumvent micropitting in main bearings used in TPM drivetrains.
By Latha Sethuraman, Yi Guo and Shuangwen Sheng, National Wind Technology Center, National Renewable Energy Laboratory, USA
Conditions Leading to Micropitting
Most common main shaft arrangements for TPM drivetrains in wind turbines rated 1.5–2MW employ SRBs. These bearings exhibit a high tolerance to system deflection and misalignment but limited tolerance to thrust loads (in most bearing designs the axial loads cannot exceed 10‒38% of their two-row radial reaction). Preliminary studies by the authors [ref. 1] were carried out using a system analysis approach for a representative TPM wind turbine with a 230/600 series SRB (having a design axial load limit that is 22% of the radial loads). Modelling results (see Figure 1) showed that this design limit is exceeded for a majority of the turbine’s operating conditions.
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Development of a Collision Risk Model
The US Fish and Wildlife Service (FWS), in conjunction with the US Geological Survey and Washington State University, have developed a statistical model that enables a wind facility to predict its expected number of bird fatalities in advance of construction. Avian fatalities at wind facilities are a serious consideration for both wildlife and wind facility managers. Many local, regional and international laws protect various bird species, making an understanding of a facility’s potential impact invaluable for planning and conservation purposes.
By Dr Leslie New, Washington State University, Vancouver, USA
The new model builds on existing approaches, making use of best available biological knowledge and directly incorporating uncertainty so that the risk to the facility and avian species can be fully assessed.
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The Effect on the Bottom Line
One of the most difficult jobs facing project managers tasked with the mobilisation of critical resources in the renewables industry is planning ahead for what are often referred to as medium range weather impacted events. Forecasting weather over longer periods (typically up to 15 days in advance, often termed medium range forecasts) is extremely difficult to predict with any degree of accuracy due to the volatile and chaotic nature of the atmosphere. Very small variations in the initial conditions of a computer forecast model can lead to huge variations in the forecast – a phenomenon known as the ‘Butterfly Effect’. This is why forecasters can typically only forecast conditions up to roughly three days ahead with any degree of precision. Beyond this timescale, conditions become significantly more influenced by these tiny initial variations.
By Polly Kirk, Regional Marketing Executive, MeteoGroup, UK
Understanding Uncertainty
The key to understanding medium range weather forecasting lies in knowledge of how to deal with uncertainty. Weather is a risk-related activity because forecasters are dealing with uncertainty. One of the best ways of addressing this, whether it be related to weather prediction or any other risk assessment activity, is to use probability as the mechanism of measurement.
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A Crane-Less Solution for Great Heights
Concrete towers have become an increasingly popular choice in the wind industry around the world because of their superior ability to support larger turbines at higher hub heights. However, this market is being constrained when it comes to increasing tower height because of the limited availability of the powerful cranes needed to erect such tall towers.
By Ramón López Mendizábal, Director, Esteyco, Spain
Some years ago there were just a few units and a single turbine manufacturer using precast concrete towers. Since then, the market has seen a spectacular increase in the demand for precast concrete towers, with more than two thousand already built. This has allowed for a crucial reduction in the cost of energy because of cost efficiencies in increasing the hub height. Many of the more important markets, each of them with their own particular demands, have begun to adopt these structures, and nowadays all the major turbine manufacturers are looking at the possibility of using concrete towers.
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Optimising Aerodynamic Efficiency at the Wind Farm Design Stage
Reduced wind turbine performance, as compared to the manufacturer’s design or warranted power curve, is common outside of the turbine design conditions. On many sites, such as those with steep slopes or considerable forestry, or simply those that experience certain atmospheric conditions, turbines will regularly operate outside the ideal operational conditions without falling outside the operational envelope. This typically results in reduced wind turbine performance and can have a major impact on overall project performance. Prevailing has applied established angle of attack based aerodynamic theories to produce a method of modelling the aerodynamic efficiency of a wind turbine for supplied wind conditions. The presented methodology can be used to provide turbine performance predictions. Accurate turbine performance predictions contribute to better wind farms both by optimising turbine layouts and providing improved preconstruction energy yield estimates.
By Alex Head, Prevailing Analysis, USA and Joel Manning, Prevailing Analysis, UK
Minimising wind farm underperformance requires turbine performance to be quantified at every potential turbine location at the project design stage. This requires both known wind conditions, and a method of calculating turbines’ response to these conditions. Modelling and analysis can be used to site turbines optimally to reduce risk of decreased performance.
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Weather Patterns in the USA during 2015
So far, the year 2015 has undoubtedly been one of challenge for many US wind project operators. In terms of performance, the first quarter was one of the lowest on record for large portions of the country with some areas seeing wind speeds up to 50% below average.
By Dr Jim McCaa, Manager of Advanced Applications, Vaisala, USA
Recent reports of low production both at individual projects and even across entire grid systems such as California and Texas captured a lot of attention and raised a great deal of concern for utilities and project owners, a number of whom reported expected shortfalls in quarterly and annual wind production.
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Doing Without Blades
Vortex Bladeless is a technology that uses the vortex effect. Basically, Vortex Bladeless consists of a conical cylinder fixed vertically on an elastic rod. The cylinder oscillates in the wind, which then generates electricity through a linear alternator’s system.
By David J. Yáñez Villarreal, David Suriol Puigvert and Raúl Martín Yunta, Vortex Bladeless, Spain
In Vortex Bladeless the outer inverted conical cylinder is designed to be effectively rigid while still being able to oscillate freely because it is fixed at its base to a flexible supporting rod. The top of the cylinder is unconstrained and has the maximum amplitude of the oscillation. The structure is built using resins reinforced with carbon and/or glass fibre, materials used in conventional wind turbine blades.
