Abstract
This project will build on advances made by the University of Hawaii (UH) and Indian Institute of Technology Madras (IITM) team during the Department of Energy’s Waves to Water (W2W) Prize. In that competition, Team Nalu e Wai (Hawaiian for “waves into fresh water”), advanced a fixed flap concept – or Oscillating Wave Surge Converter (OWSC) – through extensive numerical modeling and design. The device is fixed to the seabed in the nearshore, with wave-induced flap motion pressurizing a reverse osmosis (RO) water filtration system and generating sufficient pressure to pump generated fresh water to an elevated pier or to shore. Through the competition, the team achieved a sound design based on a fully coupled hydrodynamic/RO model.
In line with the guidelines of the competition, the system was designed to be able to be broken down into components that could be shipped to a remote, potentially disaster-stricken area inside a small container, rapidly assembled, easily deployed, and able to produce fresh water to a specified level of total dissolved solids (TDS).
The coupled hydrodynamic/PTO numerical model creates a sound basis for future prototyping and testing. The modeling results clearly established that in the compact system called for in the competition, the system could produce fresh water with TDS below 500 ppm, at rates from 12 L/hr in lower sea states to over 100 L/hr in higher sea states. The system is capable of pressurizing water to near 70 bars. The flap is mounted on a structure that houses the RO system, which in turn is fixed rigidly to the seafloor. The flap itself is comprised of hollow sections that can be variably inflated – an approach that allows flexibility in deployment options while also allowing flap buoyancy/stiffness to be varied to enhance performance in varying wave conditions – reducing stiffness in longer period waves.
Hydrodynamic analysis was carried out using ANSYS AQWA to determine the optimum flap geometry to meet performance criteria. This analysis also provided insights into pressure developed in the hydraulic cylinder driving the RO system. A suitable mooring was also numerically determined. CFD analysis was carried out to analyze RO membrane performance, feed channel geometry, salt concentration over membrane surfaces, permeate flux, and discharge. This analysis also parameterized freshwater production in terms of feed pressure and flowrate. A WEC-Sim/Simscape Fluids model was then set up that coupled device hydrodynamics and the RO system and included blocks for parametric models for freshwater production.
Although high fidelity numerical modeling has been utilized to develop this concept, sufficient experimental testing has not been carried out. Next steps will also be discussed, including refinement of the RO system design and bench testing of a prototype version of this PTO, leading toward ocean testing of the full system.