Increasing the efficiency of wind farms

As the world faces the consequences of climate change, the need for clean energy is still increasing. One of the biggest challenges is converting from conventional power sources like coal and oil to renewable energy sources. Another challenge is using these as efficiently as possible. 

“Optimization is key,” NMBU’s PhD candidate Franz Volker Mühle explains.

Efficiency is vital
“Within today’s renewable energies, wind energy plays a major role.” Wind energy installations are large-scale installations and a visible intrusion into the environment.

“They need to be as efficient as possible,” Mühle says.
He has spent the past four years researching them under different conditions.

Experiments in the lab
The goal of Mühle’s work has been to better understand the wake of a wind turbine under different design aspects, flow regimes and operating conditions. He has tested the effect of modifying the rotor design by changing the blade number and by adding winglet devices, similar to those in aviation, at the blade tips. Furthermore, he investigated effect of yaw misalignment, whereat the turbine is not aligned with the wind direction, but inclined to deflect its wake. The experimental tests were conducted at the Norwegian University of Science and Technology (NTNU) in Trondheim.
“I used the wind tunnel in their fluid mechanics laboratory.”

The winglet advantage
Mühle’s experiments showed that there are measures that can be taken to improve single turbines, but also wind farm efficiency. 

“Putting winglets on the wind turbine blade tips, when designed properly, increases the efficiency of a single wind turbine.” 
The winglets used are similar to the ones on airplanes. 

“However, mean velocity in the wake and the available power is not so much affected by such winglets, but the winglets change the strength and durability of the tip vortices,” he says.
“In aviation winglets improve the efficiency around 5%. This is similar to our results.”

There were even bigger improvements of up to 8.9% by attaching winglets.
“However, these findings are based on the wind tunnel environment where I conducted the tests. In reality the improvements would be slightly lower because of wind tunnel wall effects.”

Blade change with no effect
“Changing wind turbine rotor design from 3 blades to 2 blades does no influence the mean velocity in the wake and hence, the performance of possible wind turbines operating in this wake region.”

“If applied properly it has a large potential for the power optimization of a wind farm, but could thereby increase fatigue loads of wind turbines.”

Better predictions
Mühle also compared experimental data with numerical predictions. The results showed that modern computational fluid mechanics (CFD) codes, can predict velocities and turbulence in the wake accurately.
“CFD codes are software codes that can be used to simulate the flow for example around a wind turbine, but also all other things where a fluid is moving,” Mühle explains.
The improved CFD codes from his study can also be applied on other fields of fluid mechanics and aerodynamics.

Improve rotor design
“My findings can be used to develop new rotor designs which limit wake effects in wind farms,” Mühle comments.
Mühle’s findings can aid improvement of wind farm performances. That means higher efficiency, more power, and a reduction of loads.
“In a wider context, it can help the energy transition from conventional to renewable energy sources and improve energy security,” he concludes.

Published 5. September 2018 - 11:14 - Updated 5. September 2018 - 13:02