In recent years, the development of nanoporous membranes has revitalized the prospect of harvesting blue energy by controlling the mixing of streams with different salinities, due to the high conductance of the nanoporous membranes. In this study, we systematically analyze the performance and identify the challenges of nanopore-based power generation (NPG) at various process scales. We begin the analysis with a single nanopore, followed by a multi-pore 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 multi-pore membranes. By considering concentration polarization, we also importantly show that the development of advanced 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. ). Through this relation it is apparent that the energy efficiency of NPG is limited to only 50% (for a completely selective membrane, i.e. <i>S</i> = 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.

Can advanced materials improve the viability of nanopore-based power generation from salinity gradient?

ACS Spring Meeting 2021

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