Abstract
An enormous dissipation of the order of 2 kJ/L takes place during the natural mixing process of fresh river water entering the salty sea. “Capacitive mixing” is a promising technique to efficiently harvest this energy in an environmentally clean and sustainable fashion. This method has its roots in the ability to store a very large amount of electric charge inside supercapacitor or battery electrodes dipped in a saline solution. Three different schemes have been studied so far, namely, Capacitive Double Layer Expansion (CDLE), Capacitive Donnan Potential (CDP) and Mixing Entropy Battery (MEB), respectively based on the variation upon salinity change of the electric double layer capacity, on the Donnan membrane potential, and on the electrochemical energy of intercalated ions.
Highlights
The first reference to energy production from water salinity difference dates back to 1954, when Pattle reported this resource together with a proof-of-concept [1]: “When a volume V of a pure solvent mixesEntropy 2013, 15 irreversibly with a much larger volume of a solution the osmotic pressure of which is P, the free energy lost is equal to P V
In this work we first review the thermodynamic basis of the Capacitive Mixing” (CAPMIX) techniques (Section 2), and we identify the common features in order to give a general picture of this new strategy (Section 3)
We see that the voltage drop in the diffuse part of the double layer increases when c is decreased at constant σ, and that the specific capacitance of the Electric Double Layer” (EDL) Csurf = σ/∆φDL
Summary
The first reference to energy production from water salinity difference dates back to 1954, when Pattle reported this resource together with a proof-of-concept [1]: “When a volume V of a pure solvent mixes. Entropy 2013, 15 irreversibly with a much larger volume of a solution the osmotic pressure of which is P , the free energy lost is equal to P V. Three novel methods have been proposed, associated with the ability to selectively store and release anions or cations into ion-adsorbing electrodes (supercapacitors) [12,13,14,15] from the bulk solution They are called the “Capacitive Mixing” (CAPMIX) techniques, as the storage of ions takes advantage of the capacitances or pseudo-capacitances of several types of electrodes [16,17]. Thermodynamic arguments serve to directly connect CAPMIX with existing deionization techniques that consume energy to remove ions from a saltwater stream, adsorbing them from it into the electrodes to later release them, producing brine as a waste. We discuss the recent advances of each technique (Section 4) and we make an overview of the required future steps to arrive to a mature and competitive technology (Section 5)
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