Abstract
When natural feed waters are used in the operation of a reverse electrodialysis (RED) stack, severe fouling on the ion exchange membranes and spacers occurs. Fouling of the RED stack has a strong influence on the gross power density output; which can decrease up to 50%. Moreover, an increase in the pressure loss occurs between the feed water inlet and outlet, increasing the pumping energy and thus decreasing the net power density that can be obtained. In this work, we extensively investigated the use of CO2 saturated water as two-phase flow cleaning for fouling mitigation in RED using natural feed waters. Experiments were performed in the REDstack research facility located at the Afsluitdijk (the Netherlands) using natural feed waters for a period of 60 days. Two different gas combinations were experimentally investigated, water/air sparging and water/CO2 (saturated) injection. Air is an inert gas mixture and induces air sparging in the stack. In the case of CO2, nucleation, i.e. the spontaneous formation of bubbles, occurs at the spacer filaments due to depressurization of CO2 saturated water, inducing cleaning.Results showed that stacks equipped with CO2 saturated water can produce an average net power density of 0.18 W/m2 under real fouling conditions with minimal pre-treatment and at a low outside temperature of only 8 °C, whereas the stacks equipped with air sparging could only produce an average net power density of 0.04 W/m2. Electrochemical impedance spectroscopy measurements showed that the stacks equipped with air sparging increased in stack resistance due to the presence of stagnant bubbles remaining in the stack after every air injection. Furthermore, the introduction of CO2 gas in the feed water introduces a pH decrease in the system (carbonated solution) adding an additional cleaning effect in the system, thus avoiding the use of environmentally unwanted cleaning chemicals.
Highlights
When seawater and river water are mixed, energy can be generated
Results showed that stacks equipped with CO2 saturated water can produce an average net power density of 0.18 W/m2 under real fouling conditions with minimal pre-treatment and at a low outside temperature of only 8 C, whereas the stacks equipped with air sparging could only produce an average net power density of 0.04 W/m2
Electrochemical impedance spectroscopy measurements showed that the stacks equipped with air sparging increased in stack resistance due to the presence of stagnant bubbles remaining in the stack after every air injection
Summary
When seawater and river water are mixed, energy can be generated. The Gibb's Free Energy of Mixing calculated via the molar entropy change represents the amount of energy that can be harvested (Post et al, 2008; Weinstein and Leitz, 1976). Reversed electrodialysis (RED) is a technology to capture this energy, consisting of a system with cation exchange membranes (CEMs) and anion exchange membranes (AEMs) placed in alternating order. Between the membranes net-spacers are placed in order to create flow compartments. The flow compartments are alternately fed with waters of high and low salinity. This generates an electrical potential difference over the membranes as AEMs only allow the diffusion of anions and CEMs only allow the diffusion of cations. The RED stack is closed on both ends with an electrode system; a redox reaction converts the ionic transport into an electrical current that can be used to power an electrical device
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