"Blue energy" from ion adsorption and electrode charging in sea- and river water

TL;DR

This paper explores a sustainable energy source from the mixing of fresh and seawater, using cyclic electrode charging and discharging to convert ionic entropy into electrical energy, and proposes optimal cycles for energy extraction.

Contribution

It introduces a modified Poisson-Boltzmann model to calculate ionic adsorption and proposes optimal electrochemical cycles for energy harvesting from water mixing.

Findings

Calculated ionic adsorption/desorption using a new free-energy functional.

Proposed optimal electrochemical cycles for energy extraction.

Suggested a modified cycle for limited freshwater streams.

Abstract

A huge amount of entropy is produced at places where fresh water and seawater mix, for example at river mouths. This mixing process is a potentially enormous source of sustainable energy, provided it is harnessed properly, for instance by a cyclic charging and discharging process of porous electrodes immersed in salt and fresh water, respectively [D. Brogioli, Phys. Rev. Lett. 103, 058501 (2009)]. Here we employ a modified Poisson-Boltzmann free-energy density functional to calculate the ionic adsorption and desorption onto and from the charged electrodes, from which the electric work of a cycle is deduced. We propose optimal (most efficient) cycles for two given salt baths involving two canonical and two grand-canonical (dis)charging paths, in analogy to the well-known Carnot cycle for heat-to-work conversion from two heat baths involving two isothermal and two adiabatic paths. We also suggest a slightly modified cycle which can be applied in cases that the stream of fresh water is limited.