Doubled Power Density from Salinity Gradients at Reduced Intermembrane Distance

The mixing of sea and river water can be used as a renewable energy source. The Gibbs free energy that is released when salt and fresh water mix can be captured in a process called reverse electrodialysis (RED). This research investigates the effect of the intermembrane distance and the feedwater flow rate in RED as a route to double the power density output. Intermembrane distances of 60, 100, 200, and 485 μm were experimentally investigated, using spacers to impose the intermembrane distance. The generated (gross) power densities (i.e., generated power per membrane area) are larger for smaller intermembrane distances. A maximum value of 2.2 W/m2 is achieved, which is almost double the maximum power density reported in previous work. In addition, the energy efficiency is significantly higher for smaller intermembrane distances. New improvements need to focus on reducing the pressure drop required to pump the feedwater through the RED-device using a spacerless design. In that case power outputs of more than 4 W per m2 of membrane area at small intermembrane distances are envisage

Theoretical power density from salinity gradients using Reverse Electrodialysis

Reverse electrodialysis (RED) is a technology to generate power from mixing waters with different salinity. The net power density (i.e. power per membrane area) is determined by 1) the membrane potential, 2) the ohmic resistance, 3) the resistance due to changing bulk concentrations, 4) the boundary layer resistance and 5) the power required to pump the feed water. Previous power density estimations often neglected the latter three terms. This paper provides a set of analytical equations to estimate the net power density obtainable from RED stacks with spacers and RED stacks with profiled membranes. With the current technology, the obtained maximum net power density is calculated at 2.7 W/m2. Higher power densities could be obtained by changing the cell design, in particular the membrane resistance and the cell length. Changing these parameters one and two orders of magnitude respectively, the calculated net power density is close to 20 W/m

Electrochemical impedance spectroscopy of a reverse electrodialysis stack:a new approach to monitoring fouling and cleaning

When harvesting salinity gradient energy via reverse electrodialysis (RED), stack performance is monitored using DC characterizations, which does not provide information about the nature and mechanisms underlying fouling inside the stack. In order to assess the potential of natural salinity gradients as renewable energy source, progress in the fields of fouling monitoring and controlling is vital. To improve fouling and cleaning monitoring, experiments with sodium dodecylbenzenesulfonate (SDBS) were carried out while at the same time the electrochemical impedance spectroscopy (EIS) was measured at the RED stack level. EIS showed how SDBS affected the ohmic resistance of the stack, the non-ohmic resistance of the AEM and the non-ohmic resistance of the CEM on different time scales. Such detailed investigation into the effect of SDBS on different stack elements offered by EIS is not possible with traditional DC characterization. The results presented in this work illustrate the potential of EIS at the stack level for fouling monitoring. The knowledge presented shows the possibility to include EIS in up-scaled natural salinity gradient RED applications for fouling monitoring purposes.</p

Combined Electrospinning-Electrospraying for High-Performance Bipolar Membranes with Incorporated MCM-41 as Water Dissociation Catalysts

Electrospinning has been demonstrated as a very promising method to create bipolar membranes (BPMs), especially as it allows three-dimensional (3D) junctions of entangled anion exchange and cation exchange nanofibers. These newly developed BPMs are relevant to demanding applications, including acid and base production, fuel cells, flow batteries, ammonia removal, concentration of carbon dioxide, and hydrogen generation. However, these applications require the introduction of catalysts into the BPM to allow accelerated water dissociation, and this remains a challenge. Here, we demonstrate a versatile strategy to produce very efficient BPMs through a combined electrospinning-electrospraying approach. Moreover, this work applies the newly investigated water dissociation catalyst of nanostructured silica MCM-41. Several BPMs were produced by electrospraying MCM-41 nanoparticles into the layers directly adjacent to the main BPM 3D junction. BPMs with various loadings of MCM-41 nanoparticles and BPMs with different catalyst positions relative to the junction were investigated. The membranes were carefully characterized for their structure and performance. Interestingly, the water dissociation performance of BPMs showed a clear optimal MCM-41 loading where the performance outpaced that of a commercial BPM, recording a transmembrane voltage of approximately 1.11 V at 1000 A/m2. Such an excellent performance is very relevant to fuel cell and flow battery applications, but our results also shed light on the exact function of the catalyst in this mode of operation. Overall, we demonstrate clearly that introducing a novel BPM architecture through a novel hybrid electrospinning-electrospraying method allows the uptake of promising new catalysts (i.e., MCM-41) and the production of very relevant BPMs.</p

Understanding the Impact of the Three-Dimensional Junction Thickness of Electrospun Bipolar Membranes on Electrochemical Performance

The use of electrospun bipolar membranes (BPMs) with an interfacial three-dimensional (3D) junction of entangled nano-/microfibers has been recently proposed as a promising fabrication strategy to develop high-performance BPMs. In these BPMs, the morphology and physical properties of the 3D junction are of utmost importance to maximize the membrane performance. However, a full understanding of the impact of the junction thickness on the membrane performance is still lacking. In this study, we have developed bipolar membranes with the same composition, only varying the 3D junction thicknesses, by regulating the electrospinning time used to deposit the nano-/microfibers at the junction. In total, four BPMs with 3D junction thicknesses of ∼4, 8, 17, and 35 μm were produced to examine the influence of the junction thickness on the membrane performance. Current-voltage curves for water dissociation of BPMs exhibited lower voltages for BPMs with thicker 3D junctions, as a result of a three-dimensional increase in the interfacial contact area between cation- and anion-exchange fibers and thus a larger water dissociation reaction area. Indeed, increasing the BPM thickness from 4 to 35 μm lowered the BPM water dissociation overpotential by 32%, with a current efficiency toward HCl/NaOH generation higher than 90%. Finally, comparing BPM performance during the water association operation revealed a substantial reduction in the voltage from levels of its supplied open circuit voltage (OCV), owing to excessive hydroxide ion (OH-) and proton (H+) leakage through the relevant layers. Overall, this work provides insights into the role of the junction thickness on electrospun BPM performance as a crucial step toward the development of membranes with optimal entangled junctions.</p

Bioenergy potential of hydrocarbonoclastic bacteria fattened-up from industrial wastewaters

Microbial lipids are currently of great interest as raw material for biofuels production. Hydrocarbonoclastic bacteria (HCB), key players in bioremediation of hydrocarbon contaminated ecosystems, can produce and accumulate up to 90 % of its weight in lipids when submitted to growth-limiting conditions (e.g. nitrogen limitation). The intensive usage of crude oil derivatives as lubricants, which corresponds to about 1% of the world’s total mineral oil consumption, originates equivalent volumes of wastes. This lubricant wastes (LW) contains hydrocarbons ranging from C9 to C40, which can serve as substrate for HCB. Cultivation of HCB strains either in pure cultures or consortia with this type of industrial wastewater can, under optimized conditions, lead to production and accumulation of microbiological lipids, such as triglycerides (TAG). Combining TAG production with industrial wastewater treatment can contribute to make the process more economic and environmentally sustainable. This research aims at characterizing the potential of production and accumulation of bacterial lipids using 3. A concentrated wastewater collected from an engine’s repairing workshop, scarce in nitrogen and rich in HC, was fed (1.2% v/v) as sole carbon source, to representative HCB bacterial strains. Three different carbon to nitrogen molar ratios (C/N) were tested. After cultivation in nutrients balanced medium, the cells were washed and cultivated in a defined medium with excess of carbon. Different time lengths were evaluated for cultivation in nutrient balanced medium and under unbalanced conditions. For each condition tested, cells were harvested, freeze-dried, and its lipidic content was extracted and analyzed qualitatively. The profile of HC present in the culture media was 4. For Gram-negative HCB strain, the balanced growth conditions matched the period where the most significant HC removal was achieved. By the end of the exponential growth stage the chromatogram´s unresolved area decrease substantially and a 30% decrease in the concentration of compounds as tricosane and tetracosane was observed. The presence of TAG was detected in cells cultivated in unbalanced conditions. Fatty acids (FA) were detected in both conditions tested. The length of the accumulation period also showed to be an important factor in the experiments made with Gram-positive HCB strain. The late exponential or early stationary growth stages showed to be the most adequate period to transfer the biomass from balanced to unbalanced culture conditions. In general, the Gram-positive HCB strain showed a higher capacity to produce TAG from the tested wastewater. 5. The results obtained in our work show the potential of using hydrocarbon-based wastewaters to produce bacterial lipids. Further research is needed to determine the conditions that allow maximal storage lipid biosynthesis

High Efficiency in Energy Generation from Salinity Gradients with Reverse Electrodialysis

Renewable energy can be captured from the mixing of salt and fresh water in reverse electrodialysis. This paper investigates the energy efficiency of this process for feed waters that pass a reverse electrodialysis cell once and waters that pass multiple cells or electrode segments. So far, the maximum theoretical energy efficiency was considered to be 50% when the feed waters pass a single cell once; significantly higher efficiencies could only be obtained when the waters were recirculated or passed multiple electrodes. In this study, we show that the ion transport corresponding to the obtained energy and the electromotive force mutually influence each other, which enables capture of more than 50% (even up to 95%) of the theoretical energy, even when the feedwater streams pass a reverse electrodialysis cell only once

Toward Redox-Free Reverse Electrodialysis with Carbon-Based Slurry Electrodes

Clean and renewable salinity gradient energy can be harvested using reverse electrodialysis (RED). The electrode system is an essential part to convert ionic current into electrical current. In this study, a typical 0.10 × 0.10 m2 RED stack with a cross-flow configuration was used to test carbon-based slurry electrodes (CSEs) to replace the usual redox solutions, like hexacyanoferrate, to enhance the RED process’ sustainability, stability, and economic value. Six different slurry compositions comprising activated carbon, carbon black, and graphite powder were tested. The CSE characteristics were systematically studied by measuring viscosity, electrode compartment pressure drop, maximum current density, stability, and performance of power density and energy efficiency. Using a single membrane configuration, the CSE ran continuously for 17 days with a stable output. The application of CSEs for RED, with artificial seawater and river water, using mixing activated carbon and carbon black at a total concentration of 20 wt %, resulted in the best performance with a net power density of 0.7 W·m-2. Moreover, higher current densities up to 350 A·m-2 were tested for ED and shown to be feasible until 150 A·m-2. CSEs show promising versatility for different application modes

Enhanced mixing in the diffusive boundary layer for energy generation in reverse electrodialysis

Renewable energy can be obtained from mixing waters with different salinity using reverse electrodialysis (RED). To obtain a high power per membrane area, combined with a low power consumption for pumping the feed water, RED is preferably operated using small intermembrane distances and low flow rates. However, the diffusive boundary layer near the membranes induces a significant (non-ohmic) resistance at lower flow rates. This is even more pronounced when a spacerless design, with profiled membranes, is used. This research presents how the non-ohmic resistance in RED can be reduced, and consequently the obtained power can be increased, without compromising the power consumed for pumping. Experiments were conducted using several designs, with and without mixing promoters such as twisted spacers and additional sub-corrugations on the membrane, to investigate the effect of additional mixing in the diffusive boundary layer on the obtainable power in RED. The results show that these mixing promoters are not effective at the low Reynolds numbers typically used in RED. The distribution of the feed water inflow, however, has a major impact on the non-ohmic resistance. The design with profiled membranes without sub-corrugations has the best performance, which is almost twice the net power density obtained with a design with normal spacers