Design Requirements for a Common-Shaft Switch-Mode Power Transformer for Ocean Wave-Powered Reverse Osmosis

Wave-powered reverse osmosis (RO) desalination with cogeneration of electricity is promising for freshwater production. The design performance of the default architecture seen in literature and commercialization suffers on metrics of power density, cost, and productivity. The design choices constraining this performance is the choice to drive a seawater pump with the WEC that operates at the same pressure as the reverse osmosis process. There have been studies comparing this default, baseline architecture with architectures operating at higher pressure, either by introducing a series of processes or by including a power transformer. This work contributes three studies that expand on the work on one such architectural choice, the inclusion of a switch-mode power transformer (SMPT). This study expands on the SMPT architecture and introduces a common-shaft power distribution approach, in which the rotary machines are mechanically coupled on a shared shaft. Three studies are performed: a static power-flow study comparing the baseline architecture with two SMPT-based architectures, a shaft-dynamics study quantifying the trade-off between switching frequency, flywheel inertia, and shaft-speed variation, and a pressure-dynamics study evaluating the effects of switch valve area, transition ratio, switching volume, and switching frequency on throttling losses. The baseline architecture delivers 34.97% of the input power to permeate production, whereas the SMPT and SMPT with common-shaft architectures deliver 32.47% and 34.02%, respectively. The shaft dynamics study found that switching frequencies above 15 Hz kept the shaft speed variations below 5%, while lower frequencies require added shaft inertia. A pressure dynamics study shows that switching losses are dominated by valve-opening transients, favoring a large effective flow area and short transition time. The overall findings are that (i) the SMPT enables significant pump downsizing at a small cost in efficiency, (ii) most of the efficiency loss is recovered with the common shaft approach, and (iii) the shaft inertia and valve requirements for the SMPT are reasonable.