Efficiency of interacting wind turbines

Context

The development of renewable energies is a major challenge for decarbonizing our society and our lifestyles. Wind power is one of the most promising forms of renewable energy: wind turbines are expanding rapidly and now account for a growing share of the French and global electricity mix. In France for example, while annual electricity production has fluctuated between 500 and 550 TWh since the early 2000s, the share of wind power has risen considerably, from virtually zero in 2000 to 50.8 TWh in 2023, i.e. almost 10% (source: RTE-France).

In 2023, almost all of this production will come from onshore wind power (49 TWh). However, it does present a number of limitations, in terms of power (rarely exceeding 3 to 4MW), production duration (linked to intermittence and wind speed), and social acceptance.

Offshore wind power provides an answer to these limitations: larger and therefore more powerful wind turbines (up to 18 MW), greater available space enabling the deployment of extended wind farms, but also stronger and more regular winds. For example, the capacity factor – defined as the ratio between the energy actually produced and the energy that would have been produced if the turbine had been operating at full power for the whole year – is higher at sea: 24% onshore, compared with 38% offshore (Source: Wind Europe – Wind energy in Europe in 2019 – p.18).

Objective

The development of offshore wind farms poses a number of engineering challenges. One of these is the question of interaction between wind turbines, which can be summed up as follows: depending on wind direction, speed and atmospheric class, as well as on the type of turbine (size, blades, etc.), what effect does the distance between turbines have on their respective efficiencies?

As this issue alone could justify one or more PhD theses, the subject was restricted to the present study, which focused specifically on 2 wind turbines in tandem (wind direction parallel to the vector linking the two masts) and in stable atmospheric class.

Simulation and results

By combining fluid mechanics, heat transfer and chemical kinetics, numerical methods can be used to accurately assess the aerodynamic efficiency of wind turbines and other energy production systems.
Today, they are an essential link in new energy development programs, enabling the optimization of these complex systems.

Here’s a video example of CFD modeling showing the impact of interactions on wind turbine efficiency. For this study, the rotational speed of each wind turbine is calculated using a force balance, integrating the driving force of the wind and the resistive torque of the alternator. It can be seen here that the downstream turbine rotates slightly slower than the upstream one. This is due to the modification of the wind field incident on the downstream turbine by the upstream turbine, resulting in a loss of efficiency. Coupled with a good understanding of a site’s wind potential, these simulations can lead to the optimization of turbine layout to maximize electricity production.