Modeling And Design Of Hydraulic Power Take-Offs For Ocean Wave-Powered Reverse Osmosis Desalination

Abstract

Ocean wave-powered desalination of seawater using reverse osmosis (RO) presents an important opportunity for coastal communities as an economical and clean source of fresh water. However, the breadth and depth of study in the design of hydraulic power take-offs (PTOs) for ocean wave-powered RO is not sufficient for reliable high-performance. This work introduces several novel PTO architectures for wave-powered RO systems that take the approach of pressurizing seawater directly using a pump that is driven by the wave energy converter (WEC). These architectures include co-generation of electricity with fresh water to support the system without reliance on a local electrical grid. These architectures are modeled and compared in terms of the size of the WEC-driven pump, the RO membrane module, high-pressure accumulator volume, and the yearly average rate of permeate production. Results show that a parallel-type PTO architecture that closely resembles the state-of-the-art is consistently outperformed by series-type architectures. The series-type architecture, which is examined with and without an integrated switch-mode power transformer, produces as much fresh water as the parallel-type architecture while (1) using a WEC-driven pump that is 30–74 percent smaller without the switch-mode power transformer and 70–92 percent smaller with the switch-mode power transformer and (2) requiring 75 percent less high-pressure accumulator volume. Results also show that varying the active RO membrane area as a function of sea conditions can improve performance in terms of WEC-driven pump size, RO membrane module size, and permeate production, by 7–41 percent. Using model predictive control as an optimal load control method, this work also finds that a variable displacement WEC-driven pump can enhance productivity by 11–29 percent. Pipeline modeling methods are also examined for their use in wave energy systems and results show that a lumped parameter pipeline model that represents a pipeline in multiple segments is sufficient for the design of these systems, subject to a constraint on the length of pipe each segment represents. As a whole, this work provides guidance to the design of PTOs in future projects with insight into selecting the architecture of the PTO, formulation of multi-objective design problems, and models that can be usedeffectively for model-based design.