Wave-Energy/-Device Modeling: Developing A 1:17 Scaled Model

Many theoretical studies show that additional energy can be captured through control of the power-conversion chains (PCCs) of resonant wave-energy converter (WEC) devices. The numerical models employed in these studies are, however, idealized to varying degrees. The objective of the Resilient Nonlinear Controls (RNLC) project is to validate the extent to which control strategies, given real-world limitations, can increase rigid-body WEC device energy production. RNLC includes both numerical and experimental components, in order to make the difficult leap from idealized, theoretical paper studies to deployable WEC hardware.

A 1:17 scaled WEC, shown in Figure 1 (named T3R2: three-translations, two-rotations), has been designed and is currently being constructed for this project’s the experimental component. The device’s operational principle was selected to provide a control-strategy testbed, in which any specific control strategy’s effectiveness (and the parameters on which its effectiveness depends) can be empirically determined. The team employed numerical design studies to determine device geometry—to maximize testing opportunities in the Maneuvering and Seakeeping (MASK) Basin at the Naval Surface Warfare Center’s David Taylor Model Basin (see Figure 2).

The 12.2-million-gallon Maneuvering and Sea Keeping (MASK) Basin at the Naval Surface Warfare Center, Carderock.

To better approach the reality of ocean-deployed WECs, many sources of nonlinearity were intentionally included in the T3R2’s design. Additionally, the device was designed such that the presence of these nonlinearities, which include dynamics, viscous losses, overtopping and breaching events, nonlinear hydrostatics/ hydrodynamics, as well as motion constraints, can be largely controlled, either via mechanical “locks” or through the input to the system (i.e., basin waves).

T3R2 is selectively allowed to move in three dimensions. T3R2’s float is connected to the power-conversion mechanism through a lockable universal joint (see “U-joint” in the detailed float representation, Figure 3). Detailed views of the planned and as-currently-constructed PCC are shown in Figure 4, below. The motions of the body in roll and pitch (yaw is not allowed) nonlinearly couple into the heave degree of freedom, thus affecting power conversion. A planar motion table (PMT) allowing the entire device (body plus power-conversion mechanism) to translate in the horizontal plane was selected to emulate the mooring systems of most deep water devices (see “PMT” in Figure 1). The PMT is also selectively controllable such that the device can be locked in a given x-y location. The restoring force provided by the PMT will mimic a compliant mooring system and will result in large natural resonances in surge and sway that are outside of the model-scale waves.

The sensor suite developed for T3R2 is broad in nature and will result in a wealth of large-scale experimental data. Sensors have been chosen that allow for real-time control, model validation, and health monitoring. All device motions will be tracked redundantly through on-board sensors and an external motion-tracking system. Force in all the translational degrees of freedom will be measured. A series of sensors, shown in Figure 3, will monitor the pressure distribution on one quadrant of the float. Lastly, impact events (slam) will be recorded with fast-rise pressure sensors as well as in-house developed slam panels.