A nanofluidic exchanger for harvesting saline gradient energy

The energy of saline gradients is a very promising source of non-intermittent renewable energy, the exploitation of which is hampered by the lack of viable technology. The most investigated harvesting methods rely on selective transport of ions or water molecules through semi-permeable or ion-selective membranes, which demonstrate limited power densities of the order of a few W m^-2. While in the last decade, single nanofluidic objects such as nanopores of nanotubes have opened up very promising prospects with power density capabilities in the order of kW or even MW m^-2, scale-up efforts face serious issues, as concentration polarization phenomena result in a massive loss of performance. We propose here a concept of a nanofluidic exchanger for power generation from saline gradients, focused on designing a nanoscale flow able to harvest the power at the output of the nanopores. We study analytically and numerically a simple exchanger made of a selective nanoslit fed by a nanofluidic assembly. One specific feature of such an exchanger relies on the non-linear ion fluxes through the nanoslit analytically expressed from the integration of the Poisson-Nernst-Planck equations. Such an elemental brick could be massively parallelized in stackable electricity-generating layers using standard technologies of the semi-conductor industry. We demonstrate here a scheme for rationalizing the choice of the exchanger parameters, taking into account the transport properties at all scales. The full numerical resolution of the three-dimensional device shows that net power densities of 300 W m^-2 and more can be achieved.