Nanopore-Based Power Generation from Salinity Gradient: Why It Is Not Viable

Abstract

In recent years, the development of nanopore-based membranes has revitalized the prospect of harvesting salinity gradient (blue) energy. In this study, we systematically analyze the energetic performance of nanopore-based power generation (NPG) at various process scales, beginning with a single nanopore, followed by a multipore membrane coupon, and ending with a full-scale system. We confirm the high power densities attainable by a single nanopore and demonstrate that, at the coupon scale and above, concentration polarization severely hinders the power density of NPG, revealing the common, yet significant, error in linearly extrapolating single-pore performance to multipore membranes. Through our consideration of concentration polarization, we also importantly show that the development of materials with exceptional nanopore properties provides limited enhancement of practical process performance. For a full-scale NPG membrane module, we find an inherent tradeoff between power density and thermodynamic energy efficiency, whereby achieving a high power density sacrifices the energy efficiency. Furthermore, we derive a simple expression for the theoretical maximum energy efficiency of NPG, showing it is solely related to the membrane selectivity (i.e., S 2/2). Through this relation, it is apparent that the energy efficiency of NPG is limited to only 50% (for a completely selective membrane, i.e., S = 1), reinforcing our optimistic full-scale simulations which result in a (practical) maximum energy efficiency of 42%. Finally, we assess the net extractable energy of a full-scale NPG system which mixes river water and seawater by including the energy losses from pretreatment and pumping, revealing that the NPG processboth in its current state of development and in the case of highly optimistic performance with minimized external energy lossesis not viable for power generation.

Osmotic transport at the aqueous graphene and hBN interfaces: scaling laws from a unified, first principles description

“…68,69 Although the focus here has been devoted to the understanding of osmotic transport across a single-pore membrane or channel, further challenges lie ahead of diffusio-osmosis in order to establish itself as a viable source of renewable energy, in particular, for what concerns the generation of electricity using multipore systems. 3,4,70 In conclusion, we have provided a unified description of osmotic transport by coupling first principles simulations with a mean field description of the EDL and with the Stokes equation of hydrodynamics, and have applied this framework to understand the osmotic transport properties of a prototypical salt that displays pronounced ion-specific effects on two-dimensional materials. Through the mPB equation of the EDL (see Eq.…”

Nanofluidic osmotic power generators – advanced nanoporous membranes and nanochannels for blue energy harvesting

“…This effect will be addressed in more detail in the following sections. 182,185 Although pore diameters comparable to l D could be thought of as a limiting constraint, some authors have reported interesting counter-examples. 119,186 Siria et al reported the development of a single boron nitride nanotube with an inner radius of 40 nm that achieved an extracted power density of 4 kW m À2 .…”

“…10(g)). 106,182,185 For this reason, the use of smaller nanochannels can be crucial for the development of up-scaled membranes. Furthermore, as described in the following sections, the use of heterogeneous membranes has been widely employed as an approach to reduce the mechanisms that vanish the output power harvested in membranes with high pore density.…”

Anomalous Thermo-osmotic Conversion Performance of Ionic Covalent-Organic-Framework Membranes in Response to Charge Variations

“…Elucidating the molecular-level operation of RED membranes is necessary to advance practical implementation of this technology. In this context, establishing a connection between the surface charge density, membrane ion permselectivity, and conductance is crucial [28][29][30][31][32][33] . However, challenges remain because these traditional ionic membranes typically do not allow precise simultaneous control over the pore structure and ionic site population.…”