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Harvesting Natural Salinity Gradient Energy for Hydrogen Production Through Reverse Electrodialysis Power Generation

[+] Author and Article Information
Mohammadreza Nazemi

Department of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332-0405
e-mail: mrnazemi@gatech.edu

Jiankai Zhang

Department of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332-0405
e-mail: jzhang794@gatech.edu

Marta C. Hatzell

Mem. ASME
Department of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332-0405
e-mail: marta.hatzell@me.gatech.edu

1Corresponding author.

Manuscript received November 17, 2016; final manuscript received January 15, 2017; published online May 2, 2017. Assoc. Editor: Dirk Henkensmeier.

J. Electrochem. En. Conv. Stor. 14(2), 020902 (May 02, 2017) (6 pages) Paper No: JEECS-16-1152; doi: 10.1115/1.4035835 History: Received November 17, 2016; Revised January 15, 2017

There is an enormous potential for energy generation from the mixing of sea and river water at global estuaries. Here, we model a novel approach to convert this source of energy directly into hydrogen and electricity using reverse electrodialysis (RED). RED relies on converting ionic current to electric current using multiple membranes and redox-based electrodes. A thermodynamic model for RED is created to evaluate the electricity and hydrogen which can be extracted from natural mixing processes. With equal volume of high and low concentration solutions (1 L), the maximum energy extracted per volume of solution mixed occurred when the number of membranes is reduced, with the lowest number tested here being five membrane pairs. At this operating point, 0.32 kWh/m3 is extracted as electrical energy and 0.95 kWh/m3 as hydrogen energy. This corresponded to an electrical energy conversion efficiency of 15%, a hydrogen energy efficiency of 35%, and therefore, a total mixing energy efficiency of nearly 50%. As the number of membrane pairs increases from 5 to 20, the hydrogen power density decreases from 13.6 W/m2 to 2.4 W/m2 at optimum external load. In contrast, the electrical power density increases from 0.84 W/m2 to 2.2 W/m2. Optimum operation of RED depends significantly on the external load (external device). A small load will increase hydrogen energy while decreasing electrical energy. This trade-off is critical in order to optimally operate an RED cell for both hydrogen and electricity generation.

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Figures

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Fig. 1

Reverse electrodialysis system for electricity and hydrogen gas generation. Hydrogen is produced at the cathode through a proton reduction reaction.

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Fig. 2

Area-specific resistance (ASR) of the RED stack and its components (i.e., CEM, AEM, LC solution, and HC solution) and load as a function of moles of salt permeated across IEMs: (a) five membrane pairs, (b) 10 membrane pairs, and (c) 20 membrane pairs. IEMs include CEMs and AEMs. The total indicates overall RED stack resistance. Load is held constant during each analysis (6.35, 42.89, and 130.18 Ω cm2 for 5, 10, and 20 membrane pairs).

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Fig. 3

(a) Moles of hydrogen, nH2, produced through RED stack and current density, i, and (b) hydrogen power density, PDH2, as a function of moles of salt permeated across IEMs for 5, 10, and 20 membrane pairs

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Fig. 4

(a) Stack voltage, ξstack, and potential difference across the external load, ξL, and (b) maximum extractable energy (Gibbs free energy), Eideal, and electrical energy, Eelectrical, as a function of moles of salt permeated across IEMs for 5, 10, and 20 membrane pairs

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Fig. 5

(a) Hydrogen and electrical power density, PD, (b) ideal energy (Gibbs free energy), Eideal, and (c) electrical and hydrogen energy extracted through RED stack as a function of normalized time area, t, for 5, 10, and 20 membrane pairs

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