8 research outputs found
Thermodynamic and Energy Efficiency Analysis of Power Generation from Natural Salinity Gradients by Pressure Retarded Osmosis
The Gibbs free energy of mixing dissipated when fresh
river water
flows into the sea can be harnessed for sustainable power generation.
Pressure retarded osmosis (PRO) is one of the methods proposed to
generate power from natural salinity gradients. In this study, we
carry out a thermodynamic and energy efficiency analysis of PRO work
extraction. First, we present a reversible thermodynamic model for
PRO and verify that the theoretical maximum extractable work in a
reversible PRO process is identical to the Gibbs free energy of mixing.
Work extraction in an irreversible constant-pressure PRO process is
then examined. We derive an expression for the maximum extractable
work in a constant-pressure PRO process and show that it is less than
the ideal work (i.e., Gibbs free energy of mixing) due to inefficiencies
intrinsic to the process. These inherent inefficiencies are attributed
to (i) frictional losses required to overcome hydraulic resistance
and drive water permeation and (ii) unutilized energy due to the discontinuation
of water permeation when the osmotic pressure difference becomes equal
to the applied hydraulic pressure. The highest extractable work in
constant-pressure PRO with a seawater draw solution and river water
feed solution is 0.75 kWh/m<sup>3</sup> while the free energy of mixing
is 0.81 kWh/m<sup>3</sup>î—¸a thermodynamic extraction efficiency
of 91.1%. Our analysis further reveals that the operational objective
to achieve high power density in a practical PRO process is inconsistent
with the goal of maximum energy extraction. This study demonstrates
thermodynamic and energetic approaches for PRO and offers insights
on actual energy accessible for utilization in PRO power generation
through salinity gradients
Influence of Natural Organic Matter Fouling and Osmotic Backwash on Pressure Retarded Osmosis Energy Production from Natural Salinity Gradients
Pressure
retarded osmosis (PRO) has the potential to produce clean,
renewable energy from natural salinity gradients. However, membrane
fouling can lead to diminished water flux productivity, thus reducing
the extractable energy. This study investigates organic fouling and
osmotic backwash cleaning in PRO and the resulting impact on projected
power generation. Fabricated thin-film composite membranes were fouled
with model river water containing natural organic matter. The water
permeation carried foulants from the feed river water into the membrane
porous support layer and caused severe water flux decline of ∼46%.
Analysis of the water flux behavior revealed three phases in membrane
support layer fouling. Initial foulants of the first fouling phase
quickly adsorbed at the active-support layer interface and caused
a significantly greater increase in hydraulic resistance than the
subsequent second and third phase foulants. The water permeability
of the fouled membranes was lowered by ∼39%, causing ∼26%
decrease in projected power density. A brief, chemical-free osmotic
backwash was demonstrated to be effective in removing foulants from
the porous support layer, achieving ∼44% recovery in projected
power density. The substantial performance recovery after cleaning
was attributed to the partial restoration of the membrane water permeability.
This study shows that membrane fouling detrimentally impacts energy
production, and highlights the potential strategies to mitigate fouling
in PRO power generation with natural salinity gradients
Comparison of Energy Efficiency and Power Density in Pressure Retarded Osmosis and Reverse Electrodialysis
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/m<sup>2</sup>) than RED (18–38% and 0.77–1.2 W/m<sup>2</sup>). 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
Raising the Bar: Increased Hydraulic Pressure Allows Unprecedented High Power Densities in Pressure-Retarded Osmosis
Pressure-retarded
osmosis (PRO) has the potential to generate sustainable
energy from salinity gradients. PRO is typically considered for operation
with river water and seawater, but a far greater energy of mixing
can be harnessed from hypersaline solutions. This study investigates
the power density that can be obtained in PRO from such concentrated
solutions. Thin-film composite membranes with an embedded woven mesh
were supported by tricot fabric feed spacers in a specially designed
crossflow cell to maximize the operating pressure of the system, reaching
a stable applied hydraulic pressure of 48 bar (700 psi) for more than
10 h. Operation at this increased hydraulic pressure allowed unprecedented
power densities, up to 60 W/m<sup>2</sup> with a 3 M (180 g/L) NaCl
draw solution. Experimental power densities demonstrate reasonable
agreement with power densities modeled using measured membrane properties,
indicating high-pressure operation does not drastically alter membrane
performance. Our findings exhibit the promise of the generation of
power from high-pressure PRO with concentrated solutions
Thermodynamic, Energy Efficiency, and Power Density Analysis of Reverse Electrodialysis Power Generation with Natural Salinity Gradients
Reverse
electrodialysis (RED) can harness the Gibbs free energy
of mixing when fresh river water flows into the sea for sustainable
power generation. In this study, we carry out a thermodynamic and
energy efficiency analysis of RED power generation, and assess the
membrane power density. First, we present a reversible thermodynamic
model for RED and verify that the theoretical maximum extractable
work in a reversible RED process is identical to the Gibbs free energy
of mixing. Work extraction in an irreversible process with maximized
power density using a constant-resistance load is then examined to
assess the energy conversion efficiency and power density. With equal
volumes of seawater and river water, energy conversion efficiency
of ∼33–44% can be obtained in RED, while the rest is
lost through dissipation in the internal resistance of the ion-exchange
membrane stack. We show that imperfections in the selectivity of typical
ion exchange membranes (namely, co-ion transport, osmosis, and electro-osmosis)
can detrimentally lower efficiency by up to 26%, with co-ion leakage
being the dominant effect. Further inspection of the power density
profile during RED revealed inherent ineffectiveness toward the end
of the process. By judicious early discontinuation of the controlled
mixing process, the overall power density performance can be considerably
enhanced by up to 7-fold, without significant compromise to the energy
efficiency. Additionally, membrane resistance was found to be an important
factor in determining the power densities attainable. Lastly, the
performance of an RED stack was examined for different membrane conductivities
and intermembrane distances simulating high performance membranes
and stack design. By thoughtful selection of the operating parameters,
an efficiency of ∼37% and an overall gross power density of
3.5 W/m<sup>2</sup> represent the maximum performance that can potentially
be achieved in a seawater-river water RED system with low-resistance
ion exchange membranes (0.5 Ω cm<sup>2</sup>) at very small
spacing intervals (50 μm)
Advancing the Productivity-Selectivity Trade-off of Temperature Swing Solvent Extraction Desalination with Intermediate-Step Release
Temperature swing solvent extraction
(TSSE) offers a membrane-less
and nonevaporative approach to hypersaline desalination, but performance
of conventional TSSE operation is restricted by an inherent trade-off
between water recovery yield and salt rejection. This study presents
enhanced desalination capability of TSSE with a novel intermediate
release step (TSSE-IR) over a conventional (c-TSSE) single-step operation.
TSSE-IR demonstrated superior performance in the hypersaline desalination
of 1.0 M NaCl brines for three amines with distinct water and salt
partitioning behaviors: diisopropylamine, triethylamine, and tert-octylamine. The astute introduction of the intermediate
temperature step in TSSE-IR dramatically improves salt rejection while
minimizing the sacrifices in water recovery yields. We show that the
intermediate step does not introduce additional solvent loss compared
with c-TSSE operations with the same extraction temperature for any
of the three solvents examined. TSSE-IR is demonstrated to advance
the productivity-selectivity trade-off that constrains c-TSSE. Finally,
Hunter–Nash analysis conducted on diisopropylamine–H2O–NaCl ternary diagrams exhibits good agreement with
experimental TSSE-IR results, offering a reliable platform for modeling
intermediate-step release performance and informing process design.
This study establishes the potential of TSSE-IR to expand the spectrum
of viable solvents for hypersaline desalination to include greener
chemicals that exhibit high water recovery yields but low selectivities
in c-TSSE
Hybrid Pressure Retarded Osmosis–Membrane Distillation System for Power Generation from Low-Grade Heat: Thermodynamic Analysis and Energy Efficiency
We present a novel hybrid membrane
system that operates as a heat
engine capable of utilizing low-grade thermal energy, which is not
readily recoverable with existing technologies. The closed-loop system
combines membrane distillation (MD), which generates concentrated
and pure water streams by thermal separation, and pressure retarded
osmosis (PRO), which converts the energy of mixing to electricity
by a hydro-turbine. The PRO-MD system was modeled by coupling the
mass and energy flows between the thermal separation (MD) and power
generation (PRO) stages for heat source temperatures ranging from
40 to 80 °C and working concentrations of 1.0, 2.0, and 4.0 mol/kg
NaCl. The factors controlling the energy efficiency of the heat engine
were evaluated for both limited and unlimited mass and heat transfer
kinetics in the thermal separation stage. In both cases, the relative
flow rate between the MD permeate (distillate) and feed streams is
identified as an important operation parameter. There is an optimal
relative flow rate that maximizes the overall energy efficiency of
the PRO-MD system for given working temperatures and concentration.
In the case of unlimited mass and heat transfer kinetics, the energy
efficiency of the system can be analytically determined based on thermodynamics.
Our assessment indicates that the hybrid PRO-MD system can theoretically
achieve an energy efficiency of 9.8% (81.6% of the Carnot efficiency)
with hot and cold working temperatures of 60 and 20 °C, respectively,
and a working solution of 1.0 M NaCl. When mass and heat transfer
kinetics are limited, conditions that more closely represent actual
operations, the practical energy efficiency will be lower than the
theoretically achievable efficiency. In such practical operations,
utilizing a higher working concentration will yield greater energy
efficiency. Overall, our study demonstrates the theoretical viability
of the PRO-MD system and identifies the key factors for performance
optimization
Improved Antifouling Properties of Polyamide Nanofiltration Membranes by Reducing the Density of Surface Carboxyl Groups
Carboxyls are inherent functional groups of thin-film
composite
polyamide nanofiltration (NF) membranes, which may play a role in
membrane performance and fouling. Their surface presence is attributed
to incomplete reaction of acyl chloride monomers during the membrane
active layer synthesis by interfacial polymerization. In order to
unravel the effect of carboxyl group density on organic fouling, NF
membranes were fabricated by reacting piperazine (PIP) with either
isophthaloyl chloride (IPC) or the more commonly used trimesoyl chloride
(TMC). Fouling experiments were conducted with alginate as a model
hydrophilic organic foulant in a solution, simulating the composition
of municipal secondary effluent. Improved antifouling properties were
observed for the IPC membrane, which exhibited lower flux decline
(40%) and significantly greater fouling reversibility or cleaning
efficiency (74%) than the TMC membrane (51% flux decline and 40% cleaning
efficiency). Surface characterization revealed that there was a substantial
difference in the density of surface carboxyl groups between the IPC
and TMC membranes, while other surface properties were comparable.
The role of carboxyl groups was elucidated by measurements of foulant-surface
intermolecular forces by atomic force microscopy, which showed lower
adhesion forces and rupture distances for the IPC membrane compared
to TMC membranes in the presence of calcium ions in solution. Our
results demonstrated that a decrease in surface carboxyl group density
of polyamide membranes fabricated with IPC monomers can prevent calcium
bridging with alginate and, thus, improve membrane antifouling properties