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Accueil > Productions scientifiques > Séminaires à PHENIX > 2012 > Séminaire 08.11.2012 à 14h00

Séminaire 08.11.2012 à 14h00

par Benjamin Rotenberg - 6 octobre 2012

Rene van Roij, de l’Université d’Utrecht, présentera un séminaire le 8 novembre 2012 à 14h00 dans la bibliothèque du laboratoire PECSA (7e étage, bâtiment F, porte 754) intitulé :

Harvesting "blue" energy from mixing river- and sea water with carbon (super)capacitors


In this talk we will discuss recent advances in the harvesting of renewable "blue" energy from the spontaneous irreversible mixing of river- and sea-water. A thermodynamic analysis shows that a free-energy dissipation of about 2kJ takes place with every liter of river water that flows into the sea, i.e. the equivalent of a waterfall of 200m. Harvesting this so-called "blue" energy is possible and potentially economically interesting, especially in regions with river deltas and sharp salinity gradients. This is being done in test factories by pressure-retarded osmosis and by reverse electrodialysis, which both involve (expensive) membranes. Recently, however, Brogioli constructed a micro-fluidic solid-state device to harvest this blue energy by cyclically charging and discharging (cheap) nanoporous carbon electrodes immersed in sea and river water, respectively [1,2]. The underlying principle hinges on the expansion and compression of the double layer in fresh and salty water. In this talk we will show that Brogioli’s device can be seen as an analogue of a Stirling heat engine, and we propose a modification to construct the most efficient "blue engine" based on a Carnot-like cycle [3]. Engines (or factory plants) based on this cycle would be perfectly reversible and would not contribute to global warming. The proposed blue engine can run in reverse mode to produce fresh water out of sea water in the cheapest possible way, i.e. at the expense of the absolute thermodynamic minimal energy input, with brine as a waste product [3]. We will also discuss the optimal-power output of the blue engine by mapping the ionic dynamics onto an effective RC-circuit.

The surface potential of the porous electrodes, up to ≅1V, exceeds the thermal voltage (≅25mV) by far, which causes ordinary Poisson-Boltzmann (PB) theory to break down, even for monovalent ions in water. The Stern layer and other ion-specific effects become relevant in this regime, and we will discuss an extension of PB theory in which the "polarisability holes" due to the poorly polarisable ions compared to water are taken into account, giving good agreement with experimental measurements of differential capacities [4].

Finally we will discuss the thermodynamics of (super)capacitors, identifying Legendre transformations for different ensembles, constructing Maxwell relations, relating several response functions, and deriving an expression for the charge distribution of electrodes at fixed potential.

[1] D. Brogioli, Phys. Rev. Lett. 103, 058501 (2009).

[2] D. Brogioli, R. Zhao and P. M. Biesheuvel, Energy Environ. Sci. 4, 772 (2011).

[3] N. Boon and R. van Roij, Mol. Phys. 109, 1229 (2011).

[4] M. Hatlo, R. van Roij,and L. Lue, Europhys, Lett. 97, 28010 (2012).

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