The researchers from Malta University proposed a fancy way to improve performance of the offshore wind farms by using fastened inflatable parafoils allowing for attracting more powerful wind streams from big heights and accelerate the recovery of the wind downstream of wind-driven turbines. According to their calculations, implementing such system may significantly cut down the installed capacity losses, which today are inevitable due to turbines’ aerodynamic trails and increasingly limit further development of the offshore wind farms.
Wind-driven turbines produce energy from wind streams forming downstream lengthy zones of slowed-down turbulent air. In dense offshore clusters, where the plants are relatively close to one another, such “wind shadows” may be dozens of kilometers long, especially in the stable atmosphere. It results in the situation, when the turbines locate downwind receive weaker air flows and generate 10-30% less electricity. The calculations for the North Sea show that by 2030 the aerodynamic trails between wind farms will result in more than 5% decrease in energy generation at 25 wind farms out of 69, and 13 facilities will have losses exceeding 10%, even up to 18% in certain zones.
The economic implications of this effect are already quite well measurable today. In the scenarios with strong impact of such trails, LCOE (levelized cost of energy across the entire life cycle of the plant) grows approximately from EUR 105-110 up to EUR 130-140 per MW*h. At the same time, even a moderate decrease of the average wind speed by 5% causes the annual generation fall approximately by 7%. On an industry-wide scale the effect is even more noticeable: only 1% increase of the installed capacity utilization coefficient for 650 GW global wind farm is equivalent to additional revenue of more than USD 1.1 bn per annum.
In this context the researchers from Malta proposed to go beyond the traditional methods of counteracting aerodynamic trails, such optimizing the turbines positioning, turning nacelles or enforced capacity restrictions. Their idea is to capture the wind at the height of several hundred meters. According to the European Center for Medium-Range Weather Forecasts (ECMWF), at the height of 500 m the available wind energy density for European marine areas is on the average twice as high as at the height of 100 meters. And in the range of 200-400 meters the wind is not only stronger, but is more sustainable.
The proposed solution is to use fastened inflatable parafoils, i.e., effectively controllable aerodynamic airfoils fastened directly on the offshore wind-driven turbine. Such “kites” are not designed for generating electricity. Their main function is to actively control the air streams: creating the lifting capacity, the parafoil forms a directional descending current and a vortex structure, which suck in the quicker air from the above heights into the trail zone downstream of the turbine. The vertical momentum transfer is key here – the process, which is naturally suppressed in the stable marine atmosphere making the trails of the turbines to last for a long time.
In their preliminary calculations the researchers used a simplified model of the turbine and rigid parafoil placed directly above and slightly behind the turbine (by 0.8 of the rotor diameter above the axis and by 0.25 diameter behind the turbine). Even at such close distance they observed noticeable change of the speed profile: due to the lifting capacity the parafoil captured quicker air from above and channeled it downward, into the weaker stream zone behind the turbine. Eventually, the air downstream of the wind-driven turbine recovers faster, and the next downwind plants receive a more powerful stream, not “squeezed out” by the upstream plants.
The received data are of preliminary character and do not allow for exact evaluation of future incremental generation. But they prove that the mechanism per se is workable. And this is what makes this study interesting: due to dense positioning the offshore wind farms have practically reached their limit, hence, it is important to search for new solutions to control the wind streams.
