Experimental and numerical modeling of a piezoelectric floating system for wave energy harvesting

In this paper, a floating wave energy converter (WEC) employing a piezoelectric power take-off (PTO) for offgrid powering of electronic devices at sea is presented and its electro-mechanical behavior is investigated both experimentally and numerically. The prototype floating structure consists of two box-shaped bodies joined by flexible beam elements. Due to the wave excitation, the floaters experience relative pitch rotations and vertical displacements, which dynamically deform the piezoelectric linking element, producing a superficial charge distribution. A small-scale prototype, built in an innovative way with LEGO™ bricks, is tested under regular wave excitation in a small basin. The recorded relative motion between the floaters is used as input to calculate via multiphysics software the voltage and power output of a piezoelectric device having the same flexibility of the passive element employed in the experiments. As a starting point for more accurate simulations, a 4-dof reduced-order model (ROM) of the floating system, including a simplified piezoelectric model, is developed and the wave induced response is compared with the experimental data. The elastic connection is represented through a torsional spring and potential linear theory is employed to calculate the hydrodynamic loads. Thus, Cummins equations for the floating system are formulated in pure differential form by introducing additional equations for radiation forces. This numerical approach allows for performing in an efficient and robust way the computation of the specific response amplitude operators, characterizing the WEC’s electro-mechanical response. Introducing system modes, the electro-mechanical equations can be further simplified and, by means of their frequency-domain analytical solution, facilitate a sensitivity analysis of the system response, capture-width ratio and power output with respect to relevant parameters, highlighting optimal conditions and performance enhancements by using arrays of piezoelectric elements, both in ideal (regular waves) and realistic (irregular sea) conditions.