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Emerging trends in wind turbine drivetrains and towers

A wind turbine looks rather simple in appearance. However, it has several complex components that cumulatively build and support its overall functioning. A drivetrain is one such component, wh­ich converts mechanical energy into el­ectricity. It comprises the entire power conversion system of the turbine, including the main bearing, shafts, gearbox, generator and power converter. A range of wind turbine drivetrains with various costs, techno­logies, physical features, efficiencies, deg­rees of reliability and materials currently exist in the market. However, with wind turbines constantly undergoing technological upgradation to keep pace with the growing de­mands of the global clean energy transition, there is an increasing em­phasis on innovations in drivetrains. Fur­th­er­more, the wind energy market, globally, is witnessing a move to­wa­r­ds offshore wind, creating further sco­pe for adaptation and improvement in drivetrains. Offshore wind projects differ from la­nd-based projects in terms of not only their geographical environment but also their ability to deploy wind turbines of larger size and capacity. Thus, a considerable increa­se in the ca­pa­city of projects may be supported by creating more robust drivetrains.

Another crucial component of a wind energy system is the turbine tower. The material, height and size of turbine towers are key determinants of the power outcomes and economic returns of wind energy projects. Recent trends reflect a growing interest among developers in taller wind turbine towers. No doubt, such ende­avours are not short of challenges.

This article highlights the current and em­erging market trends in wind turbine drivetrains and towers.

Emerging drivetrain focus areas

Drivetrain innovations in recent years have focused largely on improving the power outcome for wind turbines. A 2021 study by Nejad et al in Wind Energy Science id­entified key emerging areas of focus in the drivetrain segment.

Floating wind turbines are becoming more prominent, exposing the turbines to additional damage from water, apart from wind loads. As a result, drivetrains will have to become more resilient to potential damage. The study highlights that the ma­in bearing, which carries the axial loa­ds, is particularly prone to damage from waves. Air gap management for direct-dri­ve generators on floating platforms has also been suggested.

Minimising the “wake effect” of turbines has been identified as another area of fo­cus. The wind speed often gets reduced be­hind a turbine, impacting the generati­on capacity of adjacent turbines. The drivetrain may often face turbulence, which can impact the loading on it. Thus, design control that can relatively even out the wi­nd speeds for surrounding wind turbines may be crucial to improving the reliability of wind projects.

Digitalisation of drivetrains is also emerging as a key focus area due to the importance of real-time data management in su­pporting wind turbine functioning and efficiency. Digitalisation in drivetrains is primarily associated with the sensors and actuators installed on them, but may be extended to other turbine systems to create a more robust control and monitoring system. Digitalisation has also been cited as an enabler for the adoption of digital twin models in wind turbines, which utilise real-time data along with cloud computing to support decision making.

Wind turbine towers – materials and features

Wind turbine manufacturers are now ex­ploring new material such as concrete in order to reduce the costs of towers. To­wers are being built with new material not only to reduce the amount of steel requir­ed, but also utilise steel more efficiently. Moreover, new tower technologies are em­­­erging as a response to hedge against steel-price volatility. Tower designs are, mo­reover, being altered to enable on-site manufacturing, in order to negate the tra­nsportation challenges and costs associated with large towers. However, such co­st savings may be offset – slightly, if not significantly – by the larger labour requirement on-site.

At present, the wind turbine market is dominated by steel towers, particularly tu­bular steel towers – the most popular choice for a majority of large wind turbines with hub heights above 120 m. These towers are conical, with a broader base to strengthen the structure of the wind turbine and make it more resilient to handle the heavy weight of larger turbines as well as external shocks. Concrete towers are also being developed to reduce developers’ dependence on costly steel with vola­tile pricing. However, despite the lower material cost, the relatively large amount of material utilised in such towers may aff­ect the intended cost savings. As per a 2019 National Renewable Energy Labo­ratory (NREL) study, the process of field-casting and erecting concrete towers may also be costly in terms of labour and time.

Lattice towers, also known as space fra­me towers, are manufactured using wel­d­ed steel profiles, which require about half as much material as tubular towers. The­se towers were widely set up in the 1990s and early 2000s. The low material requirement and the resulting lower costs of ma­nufacturing are the biggest advantages of such towers. These towers can al­so be transported easily using conventional ve­hicles and along regular transport routes, thereby lowering their transportation costs significantly. However, despite such ad­va­n­tages, lattice towers have almost been completely phased out of the wind energy market, primarily due to their appearance and lack of visual aesthetics. Interactions with birds has also been cited as a reason for their disappearance.

Another type of tower is the bolted steel tower, which involves the stacking of individual steel tubes that are bolted together vertically at the site of installation. A key advantage of such towers is their capacity to attain heights that are not usually re­a­ched by other types of towers. They also enable swift assembly, making them an attractive choice for developers. For sm­a­ll­er wind turbines, guyed pole towers, whi­ch are narrow and supported by guy wires, are often cheaper due to their wei­ght. However, this advantage is often offset by the overall fragility of such towers.

Trends in tower heights

In the recent years, the focus in the wind power segment has shifted towards improving wind turbine efficiency by increasing the tower heights of turbines. The 2019 study by NREL highlights key trends in the height of wind turbine towers. The research suggests that, consistent with industry experience, the regions most abundantly endowed with wind in the US prefer the lowest possible tower height, while higher hub heights are preferred in regions with moderate wind. The study also suggests that there are several factors that may determine the height of towers, such as blade tip clearance re­q­uirements, turbine nameplate capacity, cost of balance of station, and specific power ratings. The size of the turbine may also determine the output capacity of towers, as larger turbines tend to enable economic advantages and taller towers come with potentially greater cost savings.

NREL recommends assessing the ideal height for towers for a given project using a quantitative evaluation of measures su­ch as breakeven costs, capital expenditure and the levelised cost of electricity. Par­ticular on-site features should also be considered for each individual project due to the variability in geospatial conditions for different projects. Moreover, a comparison between the up-front cost of tall towers and the overall capital expenditure may also provide a clearer picture for choosing the optimal height for a wind turbine tower. How­ever, in the quest for increasing tower heights, it is also crucial to simultaneously improve the efficiency per turbine. Inc­re­a­sing tower heights and rotor diameters provides several advantages, but also brings certain limitations that cannot be ignored. The transportation and installation of larger towers and blades may be­co­me challenging, especially in land-ba­s­ed wind projects. These towers cannot be dismantled into smaller parts once cons­tructed, thereby li­miting the means as we­ll as routes for tra­ns­portation. As a res­ult, taller towers may not be feasible in regions with narrow roa­ds, which is especially common in the re­mo­te regions of India.

Outlook

Going forward, improvements in drivetrains are expected to emerge in tandem with the overall growth in the wind turbine sector. New developments may include single-stage gearboxes, direct drive systems and permanent magnet generators. Increasing the height of towers is also becoming an attractive tool to improve the capacity and power output per wind turbine. Over time, land constraints are likely to be amplified, especially in countries such as India that are urbanising rapidly. Therefore, improving turbine efficiency so as to generate greater energy per turbine would reduce the total number of turbines required at a particular site. This can significantly reduce the cost of turbine transportation and installation while also reducing the overall O&M costs throughout the project lifetime. India is also actively exploring the offshore wind segment. This would enable the de­velopment of bigger turbines with greater capacities, thereby creating further scope for improving drivetrain efficiency and increasing tower heights for larger wind turbines.

By Kasvi Singh

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