Analysis of a Piezoelectric Floating System for Wave Energy Harvesting in Coastal Area

In this paper, a floating wave energy converter (WEC) employing a piezoelectric power take-off (PTO) for off-grid powering of electronic devices at sea is presented and its electro-mechanical behaviour 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 electricity. As an illustrative example, the system is designed to fit the requirements of being hosted in the Oristano Gulf, located in the west coast of Sardinia Island in the Mediterranean Sea.In recent years, a multitude of wave energy converters (WECs) have been proposed, leading to different engineering solutions in terms of mechanical configurations and power take-off (PTO) systems to better exploit the enormous energy potential carried by surface waves.Among the different ways to convert wave power into mechanical energy, the extraction of energy from the relative motion of mutually and flexibly constrained floating bodies is one of the most promising approaches. This concept has been implemented in DUCK (Falcão, 2010), LEANCON (Martinelli et al., 2011), Pelamis (Henderson, 2006), SeaBeavl (Elwood et al., 2010), SeaPower (SeaPower, 2017), DEXAwave (Martinelli et al., 2011) and M4 (Eatock Taylor et al., 2016). The design of some WECs has been inspired, from the mechanical point of view, by the studies related to wave absorption systems. The Cockerell's device, for instance, consisted in a long train of hinged floating rafts (see Wooley and Platts,1975). Haren and Mei (1979) went a step further by identifying optimal combinations of parameters based on an analytical formulation of system response. In particular, they underlined as no great advantages in energy extraction can be obtained when the number of rafts is arbitrarily increased beyond three. Newman (1979) came to the same conclusion in terms of absorption width and highlighted the practical advantages of elongated bodies with small beam-length ratio. The problem of finding the optimum configuration of an articulated raft system is still present nowadays. For instance, Noad and Porter (2017) focused on number and reciprocal length of the floating bodies as well as on device proportions. They introduced a capture factor (capture width normalized by the raft width) and showed that for three rafts the maximum capture factor is obtained if the raft lengths are in 3 : 3 : 5 proportion. If the PTO system is included in the theoretical model, the detailed representation of the connection between floats allows for a more realistic representation of the response to wave excitation and estimation of the electrical power. Limiting to two-raft systems, the PTO is typically presented with lumped elements as in the case of Zheng et al. (2015), but more accurate descriptions of the PTO can be found for instance in Liu et al. (2017), who proposed a combined hydrodynamic and hydraulic model with increasing levels of details.