ECN Active Wake Control on track to boost wind farm lifecycle performance

Tuesday May 24, 2016 14:40

The Netherlands flagship energy R&D centre ECN is a recognised pioneer in wind farm Active Wake Control (AWC) research and implementation. Patents for the potentially game changing technology were granted already in 2003. Since then, much R&D has occurred and recently five commercial feasibility studies have been performed for leading wind farm developers and OEMs.

AWC is the method to control (and alleviate) the wake effects in a wind farm and is achieved by pitching and/or yawing the wind turbines in a special configuration. The result is an increase of the energy production and decrease of the fatigue loads experienced by the turbines from turbulent wind. AWC control can be applied in offshore wind farms and also flat terrain. The results of the feasibility studies are very promising (highlighting yield increases of up to 2%) and bring the wind sector a step closer to welcoming more intelligent wind farm control strategies.

ECN’s AWC boost annual energy production, elongate turbine life, and most importantly contribute to lowering the levelised cost of energy.

Traditional approach to wind farm control
Stoyan Kanev has played a leading role in ECN’s wind farm control R&D team since 2006 and comments on the still common practice to maximize power output of individual wind turbines and not consider the wind farm as one unit: “Such traditional strategy ignores the impact of turbine wake effects and is thus not optimal for maximising power output at wind farm level. This inadequate way of thinking triggered ECN to develop a new cooperative approach for operating windfarms.”

Yaw-based AWC, “wake redirection control”, involves turning rotor planes of upstream turbines within a given row relative to the wind direction. This creates a skewed wake deflection directly behind these rotors and away from the turbine row right behind them. A direct positive result is increased wind flow velocity, less performance loss and more favourable rotor/turbine loading. Kanev: “The same control principle is applied to all other turbine rows further downwind. The misalignment angles created purposely between individual rows can be either clockwise or anti-clockwise depending upon wind farm layout and wind direction.” A challenge of yaw-based AWC is the asymmetric rotor loading. However ECN is confident that the positive impact of AWC at loads downstream outweigh those from asymmetric loading. Overall lifetime loads at turbines in such windfarms thus decrease.

Dedicated
The second wind farm wake control strategy is called pitch-based AWC. The leading principle is that by increasing the pitch angle of upstream turbines at wind speeds below rated, their output level is being reduced. And because wind velocity in their wake increases, the downwind turbines generate more power, said Kanev. He added: “The outcome is a net increase in wind farm yield. Furthermore, because turbulence intensity in the wakes reduces, downstream a decrease in turbine load level can be expected. Finally, pitch-controlled variable speed is already widely used semi-standard wind turbine technology, and adding pitch-based AWC therefore has lower wind industry overall risk profile.”

ECN has conducted multiple feasibility studies using its advanced FarmFlow wake modelling tool for wind farm owners. These commercial projects were characterised with different scales, layouts, turbine sizes and numbers, also with different interspacing. The studies involved yaw-based as well as pitch-based AWC and showed promising results regarding loads reduction and in boosting wind farm output.

“We continuously optimise our software tools and ECN’s experience grows in parallel. We are now in a rapidly maturing product development stage and by using either yaw-based or pitch-based AWC, up to 2% higher AEP and 1-2% lifetime extension are both feasible and realistic. Moving forward, we will continue to work closely with industry to further mature the technology and work towards large scale implementation and acceptance,” said Kanev.

More information
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Category: Corporate, Wind Energy