Vertical axis wind turbine: A Compact, Efficient Revolution in Wind Energy

In the ever-evolving renewable energy sector, traditional horizontal axis wind turbines(HAWT) may soon be replaced by more streamlined and higher-productivity vertical axis wind turbines(VAWT). A study conducted by Oxford Brookes University shows that these vertical designs are significantly better than traditional designs in a wide range of wind farm settings. In addition, when deployed in series, such vertical turbines can increase each other’s output by 15%.

Vertical axis wind turbine: A Compact, Efficient Revolution in Wind Energy 7

Traditional wind farms usually use horizontal axis wind turbines on a large scale. As wind flows towards the initial line of turbines, it spawns turbulence in its wake, a phenomenon that adversely impacts the operational efficacy of the succeeding turbine rows. The foremost row of turbines manages to transmute roughly half of the kinetic energy from the wind into electrical power. However, for the trailing rows, this conversion rate plummets to a mere 25-30%, illustrating the cascading effect of turbulence on energy yield.

Only when the distance between the two wind turbines exceeds 20 times the diameter of the wind wheel can the airflow interference be completely eliminated, which greatly increases the cost of laying cables and wastes a lot of land resources.

Types of vertical axis wind turbines

Vertical axis wind turbine

Vertical Axis Wind Turbines (VAWTs) come in a variety of designs, with the two most common being the Savonius and Darrieus types. The Darrieus rotor has several subtypes, including helical, disc-shaped, and the H-rotor with vertical blades. These turbines usually feature three sleek rotor blades that operate through lift forces, enabling them to reach high rotational velocities.

Simple designs are prevalent among vertical wind turbines, as described here. In practice, numerous modifications and hybrid designs are encountered, reflecting the innovative approaches of developers in creating diverse VAWT forms.

Savonius turbines are drag-based VAWTs, often featuring a central shaft with scoops that capture the wind’s energy. Their straightforward and robust design, despite lower efficiency, makes them suitable for applications where reliability is paramount. One efficiency issue is that only part of the turbine generates useful torque, as the opposite side moves into the wind, creating negative torque. An innovative variant is the Harmony turbine, with helical blades and an automatic braking mechanism for high winds.

Darrieus turbines are lift-based VAWTs, traditionally with curved airfoil blades on a top-mounted shaft. Some designs use straight, vertical airfoils, known as H-rotor or Giromill types. Helical blade shapes can also reduce torque fluctuations by distributing force more evenly throughout rotation.

As lift-type mechanisms, Darrieus turbines can harness more wind energy than drag-based models like the Savonius.

Characteristic analysis

Advantages:

  1. Omni-directional Performance: VAWTs can capture wind from any direction without the need for complex mechanisms to yaw or pitch the blades, simplifying design and reducing maintenance needs.
  2. Ease of Maintenance: With the gearbox located at ground level, maintenance and replacement are safer and more efficient than with HAWTs, where operators often have to work at great heights.
  3. Foundation and Installation: Some VAWT designs can utilize screw pile foundations, which reduce the need for transporting concrete and minimize environmental impact during installation. These piles can also be fully recycled at the end of the turbine’s life.
  4. Supplementary Power: VAWTs can be added to existing HAWT wind farms, enhancing power output without the need for additional land.
  5. Operability in Challenging Conditions: VAWTs, particularly the Savonius type, can operate in irregular and slow wind conditions, making them suitable for remote or unattended locations.
  6. Reduced Noise: VAWTs typically generate less noise than HAWTs, making them more suitable for urban or noise-sensitive environments.
  7. Bird Safety: The design of VAWTs poses a lower risk to birds compared to HAWTs, which can be lethal to avian wildlife.

Disadvantages:

  1. Variable Power Output: VAWTs may experience a drop in power output as rotational speed increases, requiring the use of disc brakes to control speed, which can potentially lead to overheating and fire if not managed properly.
  2. Dynamic Stall: The blades of VAWTs can experience dynamic stall, a phenomenon where the lift on an airfoil suddenly drops due to the angle of attack changing rapidly, leading to decreased efficiency.
  3. Fatigue and Durability: The blades of VAWTs are subject to greater stress due to the variation in forces experienced during each rotation, which can lead to twisting, bending, and a shorter lifespan.
  4. Reliability: Historically, VAWTs have been less reliable than HAWTs, though modern designs have made significant improvements in addressing early reliability issues.

Neodymium-iron-boron magnet in wind turbine

Neodymium-iron-boron magnets play a crucial role in wind turbines, as they are the core components that generate magnetic fields in the motor. The high remanence and coercivity of these magnets enable wind turbines to convert wind energy into electricity more efficiently. The use of neodymium-iron-boron magnets significantly improves the energy conversion efficiency of generators, reduces their weight, and reduces their volume, which is of great significance for improving the cost-effectiveness and expanding the application scope of wind power generation.

In wind turbines, the type of neodymium magnet commonly used varies depending on specific design and application requirements. Common models include N42, N38UH, N38EH, etc. These numbers represent the maximum energy product (BHmax) of the magnet, while the letters represent the temperature resistance of the magnet. For example, the N38 model has a maximum energy product of 38 MGOe. Choosing the appropriate model requires consideration of factors such as the power requirements of the generator, operating temperature, and cost-effectiveness.

Small wind turbines often do not require high temperature resistance due to their lower operating power. The magnet assembly in the figure is used in a 10KW wind turbine with a performance level of N42, which can ensure normal operation below 80℃. However, large wind turbines require the use of UH and EH grade neodymium magnets, which can operate at a working temperature of 180 to 200℃.

Reference:Numerical modelling and optimization of vertical axis wind turbine pairs: A scale up approach. (2021). Available at: https://www.sciencedirect.com/science/article/pii/S096014812100344X

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