Efficient flow energy harvesting
Coupling mechanisms between the dynamics of flexible structures and surrounding flows can lead to the self-sustained motion of a solid body. Several important fluid–solid instabilities have been hitherto studied to assess the feasibility and efficiency of such systems to produce renewable energy from geophysical flows. These systems have attracted a great deal of attention, especially since conventional wind or water turbines cannot provide efficient power conversion to low-power applications. In the EU-funded project FLOWENERGY, scientists adopted a different strategy to the hitherto used approaches. Instead of reducing flow-induced instabilities and vibrations, they boosted vibrations of the solid to maximise the amount of the produced energy from a given steady fluid flow. A fraction from the extraction of the flow energy into solid kinetic energy can be converted into electricity. This energy conversion involves impacting on the solid vibration properties — instability threshold, amplitude and frequency. FLOWENERGY showed that energy extraction can benefit from the fundamental properties of these fluid–solid systems, fostering vibrations at lower flow velocities and with larger flapping amplitudes. Scientists also placed focus on studying the interactions amongst the fluid flow, the vibrating solid and the electric output circuit on a piezoelectric flag. The circuits were found to significantly impact harvesting efficiency by fostering tuning or lock-in phenomena. Another task was to consider complex assemblies of vibrating solids and determine the relation between local arrangement and their vibration properties. Experimental studies involved irregular arrays of vibrating cylinders. A simple model was proposed to determine the vibration amplitude of the different cylinders as a function of their local arrangement. This study is crucial to designing energy harvesting farms or assemblies of flow energy harvesters. The problem of hydrodynamic interactions in assemblies of self-propelled systems led to establishing several collaborations within the project. FLOWENERGY helped to further enhance understanding of the energy exchange mechanisms in vibrating fluid–solid systems, and determine their energy harvesting potential.
Keywords
Flow energy harvesting, geophysical flows, fluid–solid instabilities, piezoelectric flag, vibrating solids
