Source: Euronews, (2022)
Wind has long been used as a renewable energy source for centuries, from ship sailing to grain milling. And in modern times, wind power is the fastest growing renewable energy technology in the utility sector, exceeding the growth of solar power, hydro power as well as bioenergy. For instance, the global annual wind electricity generation has increased exponentially from 433,770 Gwh in 2011 to 1,588,586 Gwh in 2020, with 93% of the capacity being onshore wind and 7% offshore.
Wind power, as a clean and renewable energy source, is important in reducing reliance on fossil-fuel energy production. Not only that wind power can reduce heavy emissions and pollution of water and soil from the burning of coal and from drilling for gas and oil, it also provides stable and consistent energy generation. Thus, it is becoming price-competitive with fossil-fuel sources, while creating millions of jobs during the global transition to sustainable energy. In addition, wind power encourages decentralized energy production for a resilient energy system and flexibility for remote and regional areas.
To further incentivize the transition to wind power generation, turbine reliability optimization is important for a consistent supply of electricity for consumers, as well as to reduce costs of operation and maintenance, and thereby for an increase in return on investment.
The application of permanent magnetic systems and gear tooth sensing in wind turbines plays an important role in reliability optimization as well as cost reduction in wind power development. Before we learn more about the optimization of turbine technologies, let’s have a look at the basic principles of wind turbines!
What is the principle behind the operation of a wind turbine?
Modern wind turbines take advantage of the aerodynamic forces of lift and drag in order to convert wind energy into electricity. Figure 1 shows the curved shape design of wind turbine rotor blades, with the top side area of the airfoil being larger than that of the bottom side area. When wind blows past the blades, air travels faster on the top side, resulting in a lower pressure level than the bottom side, and thus creating an uplift effect. Meanwhile, the drag force takes place due to the resistance of rotor blades against the wind flow.
For the rotor to spin, the lift force must be greater than the drag force. For instance, a well-designed roller blade should have a maximum lift-to-drag ratio of 400. The generator and the rotor are connected, either directly in a direct drive turbine, or via a shaft and a number of gears in a gearbox turbine. The generator increases the rotation speed and then converts the aerodynamic forces into electricity.
Figure 1: Airfoil profile of a wind turbine blade
Source: Bertagnolio et al., 2001
What are the components of a wind turbine?
Components of a modern wind turbine include:
- Rotor blades: which converts the kinetic energy harvested from the wind into rotational mechanical energy (torque).
- Hub: which holds the rotor blades as well as connects them to the main shaft of the turbine.
- Rotor shaft: which connects the hub and gear as well as transfers the torque of the rotor throughout the power train.
- Gearbox: which transfers slow rotary speed of rotor shaft into fast rotary speed of the generator shaft in a gearbox turbine.
- Coupling: which connects the gearbox and the generator shaft, as well as buffers vibration and excessive torques in the system.
- Generator: which converts rotational energy (torque) into electricity.
- Brakes: which stops the rotor when needed (such as for maintenance or emergency stops).
- Nacelle: which is the housing that contains components such as the generator, shafts, gearbox, brakes, and control electronics.
How is the produced electricity transferred to consumers?
A wind power plant usually consists of multiple wind turbines, and its placement is dependent on factors such as wind conditions, nearby terrain, and accessibility to electricity transmission. Transmission lines can then transport electricity at high voltages from wind turbines and to fulfill electricity demand even in faraway locations. Afterwards, transformers can increase voltage and reduce the current, in order to reduce power losses during the transmission of large amounts of current over long distances by transmission lines. The voltage will be at a safe and usable level by the transformers again when the electricity reaches its consumers.
In order to perform optimization of wind turbine operation, we must first look at the principles of the two types of wind turbine drive trains.
What are the two types of wind turbine drive train technology?
The two types of wind turbine drive trains, the gearbox wind turbine and the direct drive wind turbine, can be differentiated with the presence of a gearbox.
Gearbox wind turbine
The gearbox turbine technology was developed in the 1970s. In this type of turbine, the generator connects with the gearbox through a high-speed rotating shaft (rotary speed of around 1500 rpm), while the gearbox connects to the rotor blades through a low speed (main) shaft (rotary speed of 5-1000 rpm), as shown in figure 2. The gearbox component plays an important role, by converting slow rotating but high torque power from the rotor blades to high speed but low torque power for the generator. Inside the gearbox generator, motor windings rotate through a magnetic field in order to produce electricity.
Figure 2: Simplified schematic diagram of a gearbox wind turbine
Despite the popularity of gearbox turbines in early wind power development, the gearbox has been subject to a high failure rate and high maintenance costs. This is due to multiple stages of gears needed that can result in efficiency loss. For instance, 60% of gearbox failures are the consequences of gear tooth wear, pitting and broken teeth. In addition, tooth cracking may occur due to overload during operation or poor gear cleanliness (inclusions), or that rapid changes in wind speed and direction can also result in a great amount of stress onto the gearbox, indicating that gearbox failures originate at bearing levels. Meanwhile, most gearbox failures lead to the temporary halt of wind power plant operation, which means poor reliability of wind turbine operation will result in an increase of energy cost due to shut down and maintenance.
Direct drive wind turbine
Since the 1990s, another turbine technology has been developed against gearbox failures and transmission losses, which is known as the direct drive wind turbine. Instead of utilizing a gearbox to generate electricity, the excitation of generator poles is induced by permanent magnets, which are layered throughout the rotor pipe. Magnets are then used to spin the rotor to produce electric currents as they flow through stationary copper coils. In addition, this technology connects the rotor directly to the generator, such that the generator speed is synchronized with the rotor speed. Due to the lower rotational generator speed, a large amount of magnetic poles is needed to achieve a high enough output frequency. In addition, magnetic properties have to be resistant to high heat and influences of the external magnetic fields. Therefore, Neodym-Iron-Boron (NdFeB) magnets are usually used.
Figure 3: Simplified schematic diagram of a direct drive wind turbine
The “technology battle” – gearbox and direct drive technologies
To further incentivize transition to wind power generation, it is important to perform reliability optimization of turbine operation as well as to induce its price-competitiveness against conventional power production. Let’s take a look at the advancements in both direct drive and gearbox technologies over the years, and how they achieved wind turbine optimization with stimulated competition and innovation!
Technological advancements in direct drive technology with permanent magnetic systems
In early development, direct drive wind turbines had a heavier weight and higher cost.
However, through technological advancements, permanent magnetic systems and new equipment mounting designs of direct drive wind turbines have improved to be lighter in weight, requiring less copper and manufacturing costs. It is also suggested that the weight of the generator and the gearbox is a critical factor that affects other components in the system as well as the installation costs of the turbine. In addition, significant cost reduction for permanent magnets in recent years has also made direct drive technology more price competitive with its gearbox counterpart.
Furthermore, a simplified nacelle system of the direct drive system has been suggested to increase efficiency and reliability, by reducing energy losses through power conversions, as well as gearbox failures and the resulting maintenance and shutdown time.
Along with its reduction in cost and complexity in operation, the direct drive technology has made onshore wind application more affordable and practicable, especially for communities in remote areas with limited accessibility to electricity infrastructure and the manpower needed for maintenance. It also serves as a better option for offshore wind power due to less maintenance required and a lower weight and cost for the turbine structure.
Figure 4: NdFeB magnets used in direct drive wind turbines
Source: Roo72, (2006)
Technological advancements in gearbox technology with gear tooth sensing and permanent magnets
Despite continuous technological advancements in direct-drive systems, improvements in gearbox technology can also be observed, particularly in a reduction of gearbox failures with the help of sensors, such as gear tooth sensors.
As previously mentioned, gearbox failures or gear-tooth design deficiencies occur at specific bearing locations and later advance into bearing debris, surface wear and misalignments of gear teeth. Meanwhile, gearbox turbines experience challenges such as the need to schedule shutdown time for maintenance, as well as the high cost and difficulty in exchanging a gearbox that requires a crane. Thus, gear tooth defect detection, which monitors the speed and position of the ferrous gears, became a proactive measure for turbine operation optimization.
With Hall Effect Gear Tooth sensing, the flux variation in the air gap between a magnet and passing ferrous gear teeth can be detected using a Hall element (read more about Hall elements). The signal from the Hall element is then converted into a digital value for signal processing of the condition monitoring system (early fault detection system in wind turbine drivetrains). A permanent magnet is placed in a way such that the axial magnetization is pointing towards the surface of the gear teeth, in order to create a periodic magnetic flux change when the target wheel rotates. The Hall sensor then detects the magnetic flux change and outputs a square wave for the rotational speed measurement.
Hall Effect Gear Tooth sensing is important in assisting real-time status monitoring, fault diagnosis and the forecast system, which can reduce maintenance costs and overall reliability of the system.
Wind power is an important source of energy production against environmental degradation from fossil-fuel sources. Reliability optimization and increasing price-competitiveness of wind turbine operation is thus vital for inducing wind power development. Advancements in wind turbine technology can also be observed in recent years. With permanent magnetic systems in direct drive turbines and gear tooth sensing in gearbox turbines, the optimization of wind turbine reliability and efficiency can be enhanced through stimulated competition and innovation, while improving the cost-effectiveness of the system.
ChenYang Technologies offers a wide range of Hall Effect sensors and permanent magnets for multiple applications. Based on the requirements, ChenYang Technologies provides customers with the best solution for their applications. Even custom-made products with special requirements can be provided by us.
Learn more about the principles of Hall Effect current sensors.
Learn more about the application of Hall Effect sensors in the automotive industry.
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