Doubled Power Density from Salinity Gradients at Reduced Intermembrane Distance

TLDR

Mixing of sea and river water can be used as a renewable energy source, with the Gibbs free energy released captured by reverse electrodialysis (RED). The study examines how intermembrane distance and feedwater flow rate affect RED power density, aiming to double output, and identifies reducing pressure drop via a spacerless design as a key improvement. The authors tested RED cells with intermembrane distances of 60, 100, 200, and 485 µm, imposed by spacers. Power densities rise as intermembrane distance decreases, reaching a maximum of 2.2 W m⁻²—almost double previous reports—and energy efficiency also increases; projected outputs above 4 W m⁻² are possible with a spacerless design at small distances.

Abstract

The mixing of sea and river water can be used as a renewable energy source. The Gibbs free energy that is released when salt and fresh water mix can be captured in a process called reverse electrodialysis (RED). This research investigates the effect of the intermembrane distance and the feedwater flow rate in RED as a route to double the power density output. Intermembrane distances of 60, 100, 200, and 485 μm were experimentally investigated, using spacers to impose the intermembrane distance. The generated (gross) power densities (i.e., generated power per membrane area) are larger for smaller intermembrane distances. A maximum value of 2.2 W/m(2) is achieved, which is almost double the maximum power density reported in previous work. In addition, the energy efficiency is significantly higher for smaller intermembrane distances. New improvements need to focus on reducing the pressure drop required to pump the feedwater through the RED-device using a spacerless design. In that case power outputs of more than 4 W per m(2) of membrane area at small intermembrane distances are envisaged.