Field experiments on dielectric elastomer generetors integrated in a U-OWC wave energy converter

Dielectric Elastomer Generators (DEGs) are a class of electro-mechanical transducers, which are nowadays being increasingly investigated for Power Take-Off (PTO) applications on Wave Energy Converters (WECs). Functionally, DEGs are variable electrostatic capacitors based on deformable elastomeric dielectric layers, which enable the conversion of mechanical work involved in their deformation into electrical energy. Thanks to their architectural simplicity and the low cost of constituent materials, DEGs might provide a boost towards cost reduction, reliability and market-scale deployment of future WECs. To date, the application of DEG-PTO systems has been limited to theoretical analyses and wave-tank tests on small-scale models. This paper presents the first experimental activity on the integration of DEGs on an Oscillating Water Column (OWC) WEC model based on a U-shaped collector (namely, a U-OWC) installed in a benign natural basin. For this purpose, a U-OWC, has been considered. Tests have been carried out at the Natural Ocean Engineering Laboratory (NOEL) in Reggio Calabria (Italy), which is a test-site characterized by naturally scaled wave conditions, favorable for model testing of small/medium scale models. A U-OWC model available at NOEL has been equipped with a set of circular diaphragm DEGs mounted on top of the device air chamber, that undergo bubble-like deformation induced by air pressure variations driven by the water column oscillations. The presented experimental campaign has been addressed to observe the dynamical behavior of the coupled U-OWC-DEG system and to assess modifications in the U-OWC dynamics owing to the DEGs elasticity without the influence of electrical activation and control applied on the DEGs. Tests have been carried out using different combinations of installed DEGs number and DEG parameters (pre-strain, thickness etc.). Several relevant variables describing the system dynamics have been measured, including water column displacement, air chamber pressure, excitation pressure at the collector inlet, and DEGs deformation. Further, analysis of the system natural frequency response has been conducted. In general, results collected over a wide variety of sea states have shown repeatable and stable behavior of the U-OWC-DEG system, free from undesired side effects, thus paving the way towards future tests with full electrical generation capability.