Abstract
Our technical approach is to develop a point-absorber type of WEC to provide power for an ocean observing buoy. WEC is integrated into a separate buoy that is tethered to the ocean observing buoy by a compliant member that transmits power and data but does not influence buoy motion or interfere with measurements. The WEC power buoy takes advantage of the heaving motion of waves to create an oscillating water column to drive a mechanical piston linked to an electrical generator. (See Figure 1).
The oscillating water column (OWC) is created by a tube that extends from the WEC buoy to a depth where the effects of surface waves have been largely attenuated. This is referred to as the “wave base” and is typically a depth greater than ¼ of the wavelength of incident waves for waves in “deep water” (defined as where the ocean depth exceeds ½ of the wavelength). Pressure below the wave base is nearly equal to the constant hydrostatic pressure of still-water at that depth and is not significantly influenced by the passing wave trough or crest.
As the WEC buoy-tube body is acted upon by passing waves the lower open end of the tube experiences an oscillating pressure. The upper end of the tube is open to the atmosphere at constant pressure, so the water column experiences an oscillating pressure differential that causes the water column to rise and fall within the tube. (This principle is used to create the sound in whistle buoys.) A piston located at the water free surface in the tube can be connected to a power take off (PTO) to extract energy from the OWC motion. The piston at the water free surface in the tube will oscillate through an elevation change equal to the wave height. The piston reacts against a linear to rotary converter that drives a rotary generator. The generator provides a reacting force that is proportional to the piston velocity. The proportional factor is referred to as the generator damping coefficient, which is related to the load on the generator.
The concept of a standalone WEC, was developed based on user feedback. This addon system would be installed in tandem with an existing ocean buoy to provide power without making major design modifications to the main buoy. This configuration was explored due to the prevalence of scientific measurements happening directly beneath ocean buoys. An integrated wave energy converter would disturb these sensors. An integrated WEC would also most likely require major Figure 2: Oscillating Water Column Buoy Motion Figure 1: Triton Power Buoy tethered to ocean buoy Biofouling Analysis 3 design modifications to the main buoy, leading to a reduced chance of adoption. The Triton team evaluated and simulated the concept and determined that the WEC buoy would not negatively impact the motion of the main buoy and would maintain scientific measurement fidelity. (See Figure 3).
Technical Assistance Objectives
Triton would like to test the performance of the seal and component materials within the Power Take Off (PTO) assembly, specifically with regards to biofouling. Biofouling has been identified as the biggest risk to the performance of our Wave Energy Converter. Biofouling is also one of the most difficult aspects to test, as it often requires a prolonged deployment in situ, which is expensive and time consuming.
Support being requested through TEAMER
Triton is requesting technical support for the testing of a half-scale PTO prototype to study the effects of biofouling. This testing will take place within controlled biofouling tanks at PNNL, which removes the excessive costs and complexity of open water tests for biofouling.
The study will focus on periodic evaluation of the piston assembly to characterize the growth of biofouling. This evaluation can be done through visual inspection and other quantitative methods, such as weighing of the assembly to calculate added biofouling weight. Caliper measurements can also be taken of the inside surface. In depth biofouling analysis can be performed on material coupons periodically. At the end of the test period, the prototype can be deconstructed and all parts analyzed. An integrated load cell can monitor the resistance experienced by the piston. The current of the actuator can also be monitored as a secondary measurement of resistance or friction.
Two prototype test articles will be compared, one with a biofouling mitigation seal, and one without. The wear and tear on the main dynamic seal will also compared.
This support from PNNL helps meet the Technical Assistance Objectives by providing a controlled environment for evaluating biofouling, which will inform future design improvements and iterations.