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

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

Power generation using profiled membranes in reverse electrodialysis

Reverse electrodialysis (RED) is a technology to obtain energy from the salinity difference between salt water and fresh water. Traditionally, ion exchange membranes, separated by non-conductive spacers, are used in this technology. As an alternative for these non-conductive spacers, in this work, heterogeneous ion exchange membranes were hot pressed in the dry state to create a profiled membrane comprising 230–245 μm ridges (in wet state) on one side of the membrane. Stacking such profiled membranes creates channels for the feed water, thus make the use of spacers obsolete. The performance of a RED-stack with such profiled membranes was compared for the first time with a RED-stack with traditional, non-conductive spacers. The ohmic resistance was significantly lower for the stack with profiled membranes compared to that with spacers, whereas the boundary layer resistance was higher. This resulted in slightly higher power densities for the stack with profiled membranes. Despite this small improvement, profiled membranes have a strong future development potential. Experimental data show that the hydraulic friction is much lower for the stack with profiled membranes and hence higher Reynolds numbers are possible than in a stack with spacers. Furthermore, profiling membranes allow much freedom to create new profile geometries where a hydrodynamic flow can be combined with efficient mixing in the boundary layer