In-water testing demonstrated the feasibility of a combined wave energy converter - uncrewed underwater vehicle system (WEC-UUV). This project aims to show that wave energy converters can act as the power source and charging station for remotely operated underwater vehicles. The main project objectives are to 1) show the capability of this WEC to power and support remote operation and docking of a UUV, 2) generate data to characterize the wave energy converter’s hydrodynamics and compare to simulation, and 3) to characterize the generator response in both a lab setting (using a dynamometer) and in natural waves.
This WEC consists of a floating body at the surface, constructed by CPower (Corvallis), and a submerged heave plate/docking station, constructed by APL-UW. The surface body is made up of three parallel cylinders - two floats, each connected to a generator housed inside the central nacelle. Wave motion causes rotation of the floats with respect to the nacelle, driving the generators to output power.
Testing has shown that the wave energy converter is capable of remote operation and docking of a UUV. We successfully piloted and docked a BlueROV2 in the heave plate below the WEC during deployments in Lake Washington. Operation was conducted wirelessly from a nearby support vessel with the UUV tethered to the heave plate at a depth of 30 m. A submersible winch, also controlled via WiFi from the support vessel, managed the 50 m of tether connected to the UUV. Alignment pins in the docking station control the position of the UUV when docked, facilitating wireless recharge of the UUV in future sea trials.
In addition to demonstrating docking operations in calm sea states, the system was deployed in a natural wave field in both Lake Washington (Hs = 0.25 m, T = 2 s) and Puget Sound (Hs = 0.35 m, T = 2.5 s). Data collected includes heave plate loads and motion, measurements of the incident waves(used to reconstruct a time series of sea height at the position of the WEC), and the response of on-board power take-off units. This data is used to both assess the performance of the system, and to validate numerical models of the device, which will be used for system optimization prior to the next deployment.
Four main factors ultimately limited power production in the field, providing valuable lessons learned. Limited power output was a result of lower than expected mass (causing the system to sit higher in the water than intended), higher than expected friction, a higher than expected start-up torque (due to the power required to initially turn on the power electronics), and a slightly lower than expected sea state. In-lab dynamometer testing identified this start-up torque, but using velocity control for these tests makes the results less representative of the response seen in the field. Improvement on all four factors will undoubtedly improve power generation in future tests.