Power Absorption in Oscillatory Flows: Experimental Setup and Preliminary Results

Ocean wave energy conversion (WEC) has been the subject of research since the oil crisis of the 1970s. A popular wave energy device is the Oscillating Water Column (OWC), which involves very few moving parts. The OWC generates power by guiding ocean waves into a chamber and harnessing the energy of the resulting pulsating flow. In the OWC, the low-frequency wave oscillation is forced through a pneumatic nozzle which accelerates the flow to higher speeds, resulting in better impedance matching with an air turbine connected to an electrical generator. For efficient energy extraction, it is useful to understand the interaction among the various subsystems - hydrodynamics, pneumatics, turbine dynamics, and electrical systems. The power take-off (PTO) of a WEC is one of the most important features as it characterizes the conversion of energy from hydro to pneumatic to mechanical, and then to electrical power. Investigation of the PTO is difficult to perform analytically, and experimental studies are needed for verifying and validating numerical computations. Clearly, it is most cost-effective to perform experiments on small-scale models for data gathering and testing design elements before deploying full-scale prototypes. Small-scale testing is challenging as the different contributing features-hydrodynamics (wave dynamics), mechanical systems (turbine performance), and electrical conversion-scale differently with size variations. Turbines have their own scaling laws, and for simplicity of analysis or experimentation, a porous membrane or orifice plate is used to simulate a linear or nonlinear turbine, respectively. The overarching goal of this study is to build and utilize an oscillatory piston rig to examine damping, simulating the turbogenerator at small scales. The current study employs porous membranes and orifice plates to compare and contrast their features as PTO components. A key objective is to examine the piston amplitude and frequency in response to the input wave signal and measure the corresponding phase lag between the pressure and flow rate across the various dampers. Preliminary results indicate that orifice plates experience higher pressure drops compared to porous plugs in the range of chosen parameters. Such knowledge is critical for developing better scaled model damping proxies for PTO in oscillatory flows, ultimately leading to improved OWC converters.