Abstract
Energy in ocean waves can provide us with clean, predictable energy when and where we need it most. The levelized cost of the energy (LCOE) produced by wave energy converters is still too high for widespread deployment of wave energy despite other advantages. Wave energy must be made more economical by increasing the electrical output while keeping the capital costs down. A key part of achieving this goal is in improving efficiency and cost of the power take-off (or “drivetrain”) of these devices. Power take-offs are responsible for converting between the mechanical power of the hydrodynamic body, and the electrical power to be sent to the grid. It is also responsible for enacting control on the hydrodynamic body in order to absorb mechanical energy in the first place. Improved power take-offs will result in better control (more mechanical energy absorbed) and higher efficiency (more electrical energy sent to the grid).
Our proposed power take-off design, the hybrid hydraulic electric architecture (HHEA), achieves these two goals of control and efficiency by combining the complimentary benefits of hydraulics and electric machines. A set of common pressure rails at various pressure levels equipped with individual accumulators and connected to a common electric generator via hydraulic motors absorb most of the high power from the waves and a small electric machine efficiently provides exact control. The HHEA was originally developed for electrified mobile hydraulic machines like excavators and wheel loaders. Application of HHEA for wave energy differs in that the direction of power flow is reversed to flow from the hydraulic actuator to the electric generator. This architecture was previously shown to have significant benefits over the two most common hydraulic power take-offs: the electro-hydraulic actuator and the hydraulic rectifying circuit.
At the UMERC, we will present a modification to this previous architecture by investigating the effect of removing the small electric machine used for providing fine control. Without this component, the power take-off is only capable of providing discrete forces onto the hydrodynamic body. With 3 common pressure rails at three different pressures, solenoid valves are used to determine the pressure in either end of a hydraulic cylinder, resulting in 9 possible power take-off forces. The forces are selected by computing a continuous power take-off force and rounding to the nearest force option. Both the original HHEA and this simpler power-take off are simulated using WEC-Sim and compared based on energy capture and approximate cost. The results show the discrete HHEA captures roughly the same amount of energy as the full HHEA but converts to electrical energy more efficiently. This suggests that continuous control is not required for wave energy converters and that even coarsely discretized control forces can be just as effective.