%0 Journal Article
%A Yip, Ngai Yin
%A Elimelech, Menachem
%D 2014
%T Comparison
of Energy Efficiency and Power Density
in Pressure Retarded Osmosis and Reverse Electrodialysis
%U https://acs.figshare.com/articles/journal_contribution/Comparison_of_Energy_Efficiency_and_Power_Density_in_Pressure_Retarded_Osmosis_and_Reverse_Electrodialysis/2254711
%R 10.1021/es5029316.s001
%2 https://ndownloader.figshare.com/files/3890701
%K energy conversion efficiency
%K ion exchange membranes
%K Donnan exclusion effect
%K drive water permeation
%K RED ion flux
%K Pressure Retarded Osmosis
%K power density performance
%K salinity gradients
%K power density
%K undesired entropy production
%X Pressure retarded osmosis (PRO) and
reverse electrodialysis (RED)
are emerging membrane-based technologies that can convert chemical
energy in salinity gradients to useful work. The two processes have
intrinsically different working principles: controlled mixing in PRO
is achieved by water permeation across salt-rejecting membranes, whereas
RED is driven by ion flux across charged membranes. This study compares
the energy efficiency and power density performance of PRO and RED
with simulated technologically available membranes for natural, anthropogenic,
and engineered salinity gradients (seawater–river water, desalination
brine–wastewater, and synthetic hypersaline solutions, respectively).
The analysis shows that PRO can achieve both greater efficiencies
(54–56%) and higher power densities (2.4–38 W/m2) than RED (18–38% and 0.77–1.2 W/m2). The superior efficiency is attributed to the ability of PRO membranes
to more effectively utilize the salinity difference to drive water
permeation and better suppress the detrimental leakage of salts. On
the other hand, the low conductivity of currently available ion exchange
membranes impedes RED ion flux and, thus, constrains the power density.
Both technologies exhibit a trade-off between efficiency and power
density: employing more permeable but less selective membranes can
enhance the power density, but undesired entropy production due to
uncontrolled mixing increases and some efficiency is sacrificed. When
the concentration difference is increased (i.e., natural →
anthropogenic → engineered salinity gradients), PRO osmotic
pressure difference rises proportionally but not so for RED Nernst
potential, which has logarithmic dependence on the solution concentration.
Because of this inherently different characteristic, RED is unable
to take advantage of larger salinity gradients, whereas PRO power
density is considerably enhanced. Additionally, high solution concentrations
suppress the Donnan exclusion effect of the charged RED membranes,
severely reducing the permselectivity and diminishing the energy conversion
efficiency. This study indicates that PRO is more suitable to extract
energy from a range of salinity gradients, while significant advancements
in ion exchange membranes are likely necessary for RED to be competitive
with PRO.
%I ACS Publications