Questions & Answers

 

Technical questions

What is the wake effect?

A wind turbine extracts energy from the wind to convert it into electricity, slowing the wind down after it passes through the turbine. Additionally, the turbulence is increased. This zone of low wind speed and high turbulence behind the rotor is called the wake. Now imagine that a wind turbine is placed in the wake of another turbine as it would be in a farm. It will produce less electricity because the wind it receives is slower. This power loss due to wake is called the wake effect.

  

How can flow or wake control, coordinated by a wind farm controller, result in a higher energy yield of the wind farm?

When wind turbines are gathered in wind farms, many of them operate in wakes of other turbines. The purpose of wind farm flow control is to reduce the wake effect and so limit the power losses. This results in higher energy yield for the wind farm.

  

What is the impact of time-varying atmospheric conditions?

Whenever you go out in the wind, you can feel it, the wind changes over time: its direction is not always the same, it can be stronger or slower wind, and turbulent gusts may also happen. These time-varying wind conditions that you feel are the ones wind turbines need to work with. And they cannot “work” equally in any wind conditions. Just as you do not ride your bike the same way uphill as downhill, or when turning as in a straight line. The wind turbine controller must therefore be able to adapt to changes in wind direction, wind speed, or turbulence.

  

What is closed-loop active wake steering (based on yaw control)?

Wake steering uses the yaw actuators of the nacelle to misalign the whole rotor from the wind. By doing so, the wake of a turbine can be steered away from the one downstream of it. Open-loop wake steering consists of imposing a given misalignment angle for each turbine in a wind farm and for each wind condition. The misalignment angles are taken from loop-up tables, whose values are computed beforehand using numerical models. With closed-loop wake steering, the impact of these pre-computed misalignment angles is also evaluated online so that adaptations can be made to further optimize the energy yield: that is closing the loop.

  

What is the novel HELIX active wake mixing technology (based on pitch control)?

Normally, an uncontrolled wake propagates downstream in line with the wind direction. With wake steering, the wake is laterally deflected. With the HELIX, the wake propagates downstream following a helical pattern. This is the result of actuating the blade pitch dynamically. Each turbine blade is pitched with a different angle and changes over time. The HELIX works in two ways: it deviates the wake from the downstream turbine by displacing it horizontally and vertically and also weakens the velocity deficit.

  

What happens when you pitch the blades?

The pitch actuators are located at the base of the blade. They allow the blades to rotate around their axis, thus changing their orientation towards the wind. By doing so, they modify the force that the wind exerts on them. Just as you, when you put your hand out of the car window on the highway: if you pitch your hand, you feel that the force exerted by the wind onto your hand changes.

 

Project questions

What does the project aim for?

The project aims at advancing the field of wake effects minimization for modern wind farms. To do so, the partners develop the novel active wake mixing technology called the HELIX and work on the implementation of closed-loop wake steering. The main goal is to mature these technologies and demonstrate them at the offshore wind farm Hollandse Kust Noord in time-varying atmospheric conditions.

  

Will Dynamic Wind Farm Flow Control develop a new technology?

The HELIX strategy is novel and testing it in the field will be a worldwide first. Wake steering has been used in the past and this project will take it to the next step by closing the loop.

Demonstrating these two novelties in the field will open new doors for better energy harvesting in wind farms.

  

Why will the project include wind tunnel experiments? What will be tested in the wind tunnel?

With wind tunnel experiments, the control strategies can be tested on model wind turbines in a closed environment. This allows to choose the wind conditions that are tested, enables reproducibility of the experiments and eases comparison between methods. Testing in wind tunnel is also advantageous when it comes to measurements: the model turbine can be equipped with load sensors and the flow field can be measured with high precision thanks to techniques like Particle Image Velocimetry (PIV).

In the wind tunnel, we have successfully tested the HELIX and the Pulse.

  
Why will the project test the technology on an onshore wind turbine first at NREL?

There are several reasons for testing first at NREL. The turbine is onshore, which means it is much easier to access it. It is also easier to equip it with the required equipment like blade load sensors or LiDARs to measure the wake. NREL is a research lab with years of experience when it comes to field tests of scaled wind turbines. Their expertise will be a key element for the first demonstration of the HELIX on a real turbine.

  
What experiments are going to be executed in Hollandse Kust Noord?

Hollandse Kust Noord is the ultimate testing environment. It is a full wind farm, offshore, with modern large turbines. It is the target for the control strategies that are developed in the group at TU Delft. The tests will include the HELIX control and closed-loop wake steering.