Unlike for wind power, no winning design has yet emerged for wave energy capture. New concepts still have a chance. They need first to be scrutinized for feasibility, in particular survivability, the Achilles Heel of many past efforts. Numerical modeling then allows early-stage, low-cost design optimization for a TPL estimate. Because the PIP (Pitching Inertial Pump) WEC - a novel, pitching, self-reacting wave energy capture device - is eminently survivable, a numerical model was developed to predict its performance. The model enables both design and operational parameters to be optimized. First results indicate that significant power can be captured. by this deep-water device.
The PIP device comprises a single, robust and storm- proof pitching body, moored but not tethered to the ocean floor. A pipe/loop coil, fixed inside the body, is filled completely with water. This water constitutes the self-reacting mass for the PTO. Moored with the coil axis parallel to the wavefront, wave induced pitching of the floating body reacts against the inertia of the water-fill to generate an alternating pressure difference at opposite end of the coil. When the pitch induced angular acceleration is sufficient to overcome the pressure difference between two accumulators, check valves open at the ends of the coil and water passes from the low-pressure accumulator, through the coil, to the high-pressure accumulator. Power is generated by a turbine or hydraulic motor connecting the two accumulators.
Python based meshing scripts are used to explore the geometry design space. Capytaine, a Python based fork of the Boundary Element Method Nemoh, is used to develop hydrodynamic coefficients. BEMIO is then used to pre-process the hydrodynamic coefficients and save them to a .h5 file that can be read by WEC-Sim. WEC-Sim, developed by the US National Renewable Energy Laboratory, is a time domain, mid-fidelity WEC numerical modeling tool based on linear potential flow theory. The wave field is approximated as a linear superposition of incident, radiated and diffracted regular wave components.
WEC-Sim uses the frequency-domain coefficients in time-domain formulations of the hydrodynamic forces. This conversion is required to model the WEC system in the time-domain, which is necessary to include non-linearities in the system - such as PTOs, control systems, moorings etc. A version of the model with a simple linear damped PTO was initially developed to check out the WEC-Sim hydrodynamics.
The linear PTO model enabled rapid generation of a power matrix for the baseline geometry. Initial optimization of one geometric parameter increased power capture considerably. This geometry was then used to compare power capture predicted by the linear PTO model with that predicted by the PTO-Sim model. The results were substantially the same.
Rigorous numerical modeling shows promising performance for a novel, highly survivable, single body, self-reacting WEC device. The model is currently being used to optimize design and operational parameters to provide a basis for an initial TPL estimate.