Da David Vermaas

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/m(2) 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 m(2) of membrane area at small intermembrane distances are envisaged.

Fouling in reverse electrodialysis under natural conditions

Renewable energy can be generated from mixing salt water and fresh water in reverse electrodialysis. The potential for energy generation from mixing seawater and river water is enormous. To investigate the effect of fouling when such natural feed waters are used, the performance of three different setups for reverse electrodialysis was evaluated for 25 days using seawater and river water as feed water, with no other (pre-)treatment than a 20 μm filter. Due to the absence of other anti-fouling treatments, a mixture of fouling is observed on the membranes, composed of remnants of diatoms, clay minerals, organic fouling and scaling. The fouling type was dependent on the different membrane types. The anion exchange membranes attract mainly diatoms and clay minerals, whereas scaling was only found on the cation exchange membranes. As a reference, plastic sheets without charge were used, which results in significant cleaner surfaces. Additionally, the setups without spacers in between the membranes (i.e. profiled membranes) appear significant less sensitive to fouling. This was quantified by the pressure drop over the feed waters and the power density obtained from the membrane piles. The pressure drop increases four times slower and the power density remains higher when profiled membranes are use instead of flat membranes with spacers. Although the obtained power density reduced with approximately 40% in the first day under these conditions, caused by organic fouling, several strategies are available to maintain a high power output using reverse electrodialysis.

Influence of multivalent ions on renewable energy generation in reverse electrodialysis

Renewable energy can be generated when mixing seawater and river water. This energy is captured in reverse electrodialysis (RED) using ion exchange membranes. Although natural sources of seawater and river water are composed of a mixture of monovalent and multivalent ions, laboratory research on RED is generally performed with artificial solutions of sodium chloride. This research demonstrates that the presence of magnesium- and sulphate ions in feed solutions with NaCl has a major effect on the obtained open circuit voltage and power density for three different membrane types. When using a mixture with a molar fraction of 10% MgSO4 and 90% of NaCl in both feed waters, the experimentally obtained power density in steady state decreases from 29% to 50% compared to the case where the feed solutions contain only NaCl as a salt. This effect is among others explained by the transport of Mg2+ and SO42− against their concentration gradient, as is elaborated in a theoretical framework and which is justified by experimental data. Non-stationary cases, where feed water is switched from a NaCl solution to a mixture of NaCl and MgSO4, show that the voltage response time is in the order of tens of minutes up to several hours, due to ion exchange between the membranes and the feed water. The knowledge gained from electrochemical measurements under stationary and non-stationary conditions and a novel technique to monitor the ion transport inside cation exchange membranes can be used to improve the obtained power density in practical applications of RED using natural feed water.

Clean energy generation using capacitive electrodes in reverse electrodialysis

Capacitive reverse electrodialysis (CRED) is a newly proposed technology to generate electricity from mixing of salt water and fresh water (salinity gradient energy) by using a membrane pile as in reverse electrodialysis (RED) and capacitive electrodes. The salinity difference between salt water and fresh water generates a potential difference over ion selective membranes, which can be used as a renewable power source. The strength and unique characteristic of CRED in comparison to the other technologies is that it allows multiple membrane cells between a single set of electrodes and at the same time avoids redox reactions using capacitive electrodes. The capacitive electrodes use activated carbon on a support of Ti/Pt mesh to store ions and their charge. A periodic switching of the feed waters, combined with a switching of the direction of the electric current, ensures that the capacitive electrodes do not get saturated. The large membrane pile enables the electrodes to be charged more than in previous approaches for capacitive mixing. As a consequence, the energy cycle of CRED has a larger range in both voltage and accumulated charge compared to previous capacitive mixing technologies. The power density obtainable with CRED stacks with capacitive electrodes is an order of magnitude higher than in previous attempts for capacitive energy extraction and close to or even better than similar RED stacks with conventional redox based electrode systems. CRED is considered to be a stable, safe, clean and high performing technology to obtain energy from mixing of salt water and fresh water.

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(2) 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(2)) at very small spacing intervals (50 μm).

Periodic Feedwater Reversal and Air Sparging As Antifouling Strategies in Reverse Electrodialysis

Renewable energy can be generated using natural streams of seawater and river water in reverse electrodialysis (RED). The potential for electricity production of this technology is huge, but fouling of the membranes and the membrane stack reduces the potential for large scale applications. This research shows that, without any specific antifouling strategies, the power density decreases in the first 4 h of operation to 40% of the originally obtained power density. It slowly decreases further in the remaining 67 days of operation. Using antifouling strategies, a significantly higher power density can be maintained. Periodically switching the feedwaters (i.e., changing seawater for river water and vice versa) generates the highest power density in the first hours of operation, probably due to a removal of multivalent ions and organic foulants from the membrane when the electrical current reverses. In the long term, colloidal fouling is observed in the stack without treatment and the stack with periodic feedwater switching, and preferential channeling is observed in the latter. This decreases the power density further. This decrease in power density is partly reversible. Only a stack with periodic air sparging has a minimum of colloidal fouling, resulting in a higher power density in the long term. A combination of the discussed antifouling strategies, together with the use of monovalent selective membranes, is recommended to maintain a high power density in RED in short-term and long-term operations.

The Breathing Cell: Cyclic Intermembrane Distance Variation in Reverse Electrodialysis

The breathing cell is a new concept design that operates a reverse electrodialysis stack by varying in time the intermembrane distance. Reverse electrodialysis is used to harvest salinity gradient energy; a rather unknown renewable energy source from controlled mixing of river water and seawater. Traditionally, both river water and seawater compartments have a fixed intermembrane distance. Especially the river water compartment thickness contributes to a large extent to the resistance of the stack due to its low conductivity. In our cyclic approach, two stages define the principle of the breathing concept; the initial stage, where both compartments (seawater and river water) have the same thickness and the compressed stage, where river water compartments are compressed by expanding the seawater compartments. This movement at a tunable frequency allows reducing stack resistance by decreasing the thickness of the river water compartment without increasing permanently the pumping losses. The breathing stacks clearly benefit from the lower resistance values and low pumping power required, obtaining high net power densities over a much broader flow rate range. The high frequency breathing stack (15 cycles/min) shows a maximum net power density of 1.3 W/m2. Although the maximum gross and net power density ever registered (2.9 W/m2 and 1.5 W/m2, respectively) is achieved for a fixed 120 μm intermembrane distance stack (without movement of the membranes), it is only obtained at a very narrow flow rate range due to the high pressure drops at small intermembrane distance. The breathing cell concept offers a unique feature, namely physical movement of the membranes, and thus the ability to adapt to the operational conditions and water quality.

Improved Reverse Electro-Dialysis Using Membranes With Integrated Spacers

Reverse Electro-Dialysis, RED, utilises the energy of mixing between two solutions of different salinity by allowing ionic current to pass through the membranes and the two solutions such that cations are transport to the cathode and anions to the anode. [1–4.] The ionic current is converted to electronic current by red-ox reactions at the cathode and the anode. The membranes applied in this process are ionic selective, traditionally of uniform thickness and separated by a non-conductive spacer [5, 6]. Traditionally, non-conductive spacers have been deployed as eddy promoters and membrane spacers in salinity difference power extraction systems, such as Pressure Retarded Osmosis (PRO) and Reverse Electro-Dialysis (RED). For RED, traditional spacers inhibit parts of the ionic current paths in the fluid compartments and magnify the pressure drop imposed by the fluid flow between the membranes. [6] In a strive to lower the pressure drop in the fluid flow compartment and to increase the conductive region between the membranes, it is suggested to manufacture membranes with new shapes and profiles. [6] By modeling transport of mass and momentum in different geometries, spacing, mixing and active membrane area can be optimised with respect to increasing the power extraction. Such work has previously been done for traditional, i.e. non-electrochemical, flow in spacer separated membrane systems. A classical, approach has been to study submerged and non-submerged non-conductive spacer rods in fluid flow between two parallel plates (membranes) for Reynolds numbers (Re) from 50 and upwards. [7–17] This work discusses how spacers united with the reactant surface (membrane) will affect the mixing and the pressure drops of RED systems with Re numbers between 1 and 100, the expected operational Re number range for RED [6, 18, 19]. This is essential for the power production of RED. For a process converting renewable energy present in nature, such as RED, optimising these parameters is detrimental for the exergy yield. In going from a laboratory scale with a 10 × 10 cm2 cross sectional membrane to a large scale of 100 × 100 cm2 , the Reynolds number (Re) increases from 10 to 100 simply because the volume flow is proportional to the flow length. Since it is within this range that eddies starts to get promoted by spacers, different mixing properties is expected exist when comparing laboratory and industrial scaled RED systems.© 2011 ASME