From a flapping flag to wind energy

Home Technologist Online From a flapping flag to wind energy

Flutter-driven system for harvesting wind energy. What if all the flags you can see all over the city could generate electricity?

Challenges of wind power

Wind is an inexhaustible resource with a potential for reducing the CO2 emission. Technologies to harvest electrical energy from wind have vast potentials because wind is one of the cleanest and most sustainable energy sources that nature provides.

Busting the wind energy myth (Infographic: EWEA)

Busting the wind energy myth (Infographic: EWEA European Wind Association)

Small windmills have been around for some decades, but in recent years the focus has shifted on developing their potential use in an urban environment. However, new wind technology, always receives some skepticism: most wind-harvesting technologies only work at a fraction of their most efficient output, and wind turbines need smooth, laminar airflow. Therefore, some will assert that the wind flow is not sufficient in the cities, or that the cost of installing and repairing these wind turbines might not worth the money. Although wind power plants have relatively little impact on the environment compared to other conventional power plants, there is some concern over the noise produced by the rotor blades and aesthetic impacts. Recently, flow-induced vibration of flexible structures has been suggested as an alternative to overcome the drawbacks of wind-turbine-based energy generators.

The wind power capacity installed by the end of 2014 would, in a normal wind year, produce 284 TWh of electricity, enough to cover 10.2 percent of the EU’s electricity consumption.

A French flag on study

Integrating windmills to the cityscape has become an undeniable challenge. Some companies offer to paint flags on rooftops wind mills. Three researchers from ENSTA ParisTech and Polytechnique reversed the problem by turning flags themselves into turbines, able to capture the wind power.

The spontaneous flapping of a flag in a steady wind flow could be used to convert the wind’s mechanical energy into electrical power, by using piezoelectric elements positioned at its surface.

What does ‘piezoelectric elements’ mean? The generation of electricity or of electric polarity in dielectric crystals subjected to mechanical stress, or the generation of stress in such crystals subjected to an applied voltage.

Yifan Xia (École Polytechnique), Olivier Doaré (ENSTA ParisTech) and Sébastien Michelin (École Polytechnique) are studying numerically the effect of inductive circuits on the dynamics of this fluid-solid-electric system and on its energy-harvesting efficiency. In particular, a destabilization of the system is identified, leading to energy harvesting at lower flow velocities.

Also, a frequency lock-in between the flag and the circuit is shown to significantly enhance the system’s harvesting efficiency. These results suggest promising efficiency enhancements of such flow-energy harvesters through the output circuit optimization. The three researchers have now calculated that with an optimized resonant circuit, stable flapping motion, robust to velocity fluctuations, can be achieved at lower flow velocities than in previous studies. The scheme could open the way to efficient piezoelectric technologies that can harvest energy from a wide range of fluids (wind, tides, rivers, etc.).

More examples of new windmills concepts in Europe:

Rotterdam, WindWheel project, will exhibit the EWICON (electrostatic wind energy converter).

Paris, tree shaped wind turbines by New Wind.

Spain, Vortex Bladeless has produced a wind turbine that takes advantage of the vortices produced when wind moves around an obstacle.


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