Optimal size for wind farms revealed by computational study
Optimizing the placement of turbines within a wind farm can significantly increase how much energy can be extracted – but only until the installation reaches a certain size. So say researchers in the US who conducted a computational study on wind turbines’ effects on the airflow around them, and consequently the ability of nearby turbines – and even nearby wind farms – to extract energy from that airflow. The researchers hope their analysis will assist in better designs of wind farms (Proc. Natl Acad. Sci. 118 e2103875118).
Wind power could supply more than a third of global energy by 2050 but it is well known that the efficiencies of turbines in a wind farm can be much less than that of a single turbine on its own. While small wind farms can achieve a power density of over 10 W/m2, this can drop to as little as 1 W/m2 in large installations. That’s because turbines inject turbulence into the air flow, which can make it more difficult for downstream turbines to extract energy.
Enrico Antonini and Ken Caldeira of the Carnegie Institution for Science in California have now used fluid dynamic models to examine the airflow over turbines. They considered multiple, idealized simulations of both small and large wind farms.
First, they looked at the individual wakes of each turbine, studying different arrangements. Reducing the turbine density in small wind farms provides a higher output per turbine, they found – but arranging the turbines in rows facing the wind or in a mosaic pattern provides 56% more power output than if they are in wind-facing columns.
These gains largely vanished, however, as wind farms grew very large. To find out why, the duo used meteorological simulations of wind speed. They found that in a small wind farm, a turbine loses most of its output due to the wake of its near neighbours, which can be mitigated by careful design. But in a large wind farm the surface wind speed is slowed down by the greater drag of that region. “For large wind farms, the limit on how much energy can be replenished is on the order of 1 W/m2,” says Antonini. “[The turbines] create a kind of uniform wake across the wind farm.” The wakes of these large farms could extend tens of kilometres downstream, potentially affecting other wind farms.
There is, however, no clear dividing line between a small wind farm (where the arrangement of turbines is important) and a large wind farm (where it is not). It generally occurs once the farm is about 30 km in size but depends on several factors such as the wind speed in the upper atmosphere and the latitude of the wind farm affecting the calculations. The researchers are developing this concept further to help energy system planners build better wind farms for the future.
Tim Wogan