Enhancing blue energy harvesting on nanopores: asymmetric thermal effect and pH effect

Abstract

Renewable energy development is one of the promising approaches to tackle the challenge of global warming as well as fulfill the ever-increasing energy demand and manage other environmental impacts Among various renewable sources energy derived from salinity gradient known as blue energy (i e salt concentration difference between seawater and river water) brings an important contribution to the field of sustainable clean energy Therefore the processes of generating this energy are obtaining more and more attention Throughout a typical blue energy conversion a power generation obtained in such a way strongly depends on ion transport through nanopore To overcome the limits of single nanopore with low ion flux and high cost multipore membrane is an emerging approach to promote energy conversion for practical application However the key bottleneck for increasing the number of pores is that Ion Concentration Polarization (ICP) and pore-pore interaction are increased in the membrane resulting in more undesirable ion-transport resistance This study aims to explore thermal and pH effects on enhancing ion transport through nanopore membranes using both numerical simulation and experimental methods The experiment is conducted on track-etched membranes; the COMSOL simulation bases on Poisson-Nernst-Planck (PNP) equations Navier-Stokes equations and heat equation The contributions of this study are twofold First an asymmetric thermal is employed in multipore membrane with a higher temperature is set in low-concentration reservoir Results show that power generation of asymmetric thermal is higher about 60% than that of isothermal cases By considering total ion concentration in the interface of the nanopore membrane and low-concentration reservoir ion enrichment (a phenomenon of ICP) is significantly diminished under effect of asymmetric thermal conditions Besides the pore-pore interaction significantly increases as the pore-pore distance reduces The results of both simulations and experiments reveal that the existent pore-pore interaction can be diminished under the asymmetric thermal effect This study facilitates the future practical application of osmotic energy and introduces an innovative strategy to enhance salinity-based power generation using waste heat or solar heat source Second the present study examines the combined effects of the temperature gradient and pH level on the diffusion voltage and maximum power generation in single silica nanopores with lengths of 100 nm and 500 nm respectively In performing the simulations deprotonation/protonation reactions of nanopore surface are added to the nanopore surface the pH value is adjusted in the range of pH 5 ~ 11 the salinity concentration gradient is 100-fold and 1000-fold respectively Three different thermal conditions are considered namely (1) isothermal-room temperature (298 K); (2) asymmetric thermal (temperature of low-concentration reservoir and high-concentration reservoir are 323 K and 298 K respectively); and (3) isothermal-high temperature (323 K) The results show that the generated power varies significantly with both the pH level and the temperature conditions In particular the asymmetric thermal condition yields an effective improvement in the power generation performance since it reduces the surface charge density on the surface of the nanopore near the low-concentration end and therefore suppresses the ion concentration polarization (ICP) effect The improvement in the energy harvesting performance is particularly apparent at pH levels in the range of 9 ~ 10 (about 100% higher than that of pH 7) Overall the results confirm the feasibility of using active factors to enhance the power generation performance of salinity gradient-based nanopore systems

Date of Award	2020
Original language	English
Supervisor	Ruey-Jen Yang (Supervisor)