With every litre of fresh river water that flows into the salty sea, an amount of 2kJ of (free) energy is dissipated, i.e. the equivalent of a waterfall of 200m. With present-day membranes and porous nanomaterials this energy can be harvested, although it remains challenging to tap from this fully sustainable source on economically viable scales, and further fundamental work is needed on nanoscale transport of electrolytes. In this talk I will first discuss the amount of available blue energy on planet Earth and in my own country. Then I will analyse thermodynamic charging-discharging cycles of supercapacitors in salty and fresh water to harvest blue energy , their one-to-one mapping onto Stirling and Carnot cycles , and the increased efficiency of a factor 2-3 (so a waterfall of 400-600m) by coupling the cycle to warm fresh water using waste heat [3,4]. Finally, I will briefly discuss the intricate electrokinetics of diffusio-osmosis --ionic currents and osmotic flow driven by salt concentration gradients across the (surprisingly strongly coupled) narrow channels of a membrane [5,6,7,8,9]. In passing, and if time permits, I will mention the reverse process of water desalination, the physics of the electric double layer in terms of Poisson-Boltzmann theory and its DFT extensions to include steric and correlations, and the Poisson-Nernst-Planck-Stokes formalism for coupled advective-diffusive-conductive transport in aqueous electrolytes.
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