The Capacitive Principle

When two solutions are brought together, dissolved species like ions will tend to
distribute evenly over both solutions. This tendency for mixing is the driving force that
can be used to extract energy from this process.

To harness the mixing energy, mixing must proceed in a controlled way, such that the mixing process is connected to an electricity generating process.
Here we use a capacitor to perform the mixing.

In a solution containing salts, a conductive particle can store a certain amount of
excess negative charge (surplus electrons) or an excess positive charge (lack of electrons). The Figure bellow shows the principle of charging of a single particle with electrons.


Charging of a conductive particle (big circle) in an electrolyte solution containing sodium chloride. Left is the uncharged particle, while upon charging with electrons (small circles inside the big circle) a negative potential will develop in the particle. This potential will attract positive ions (cations, small circles outside of the big circle) present in the solution. Because the positive charge neutralizes the negative charge in the particle much more charge can be stored than would be the case if no electrolyte would have been present.

The amount of electrons that can be stored is appreciable because the excess negative charge in the particle can be compensated by adsorption of an excess amount of positive charge in the form of cations like sodium from the solution. This is the so-called double layer, a classic topic within colloid science and electrochemistry. Because the ions can get very close to the surface of the particle, the particle functions as a capacitor and can store excess electrons.

These stored electrons originate from another electrode that has an excess positive charge. This positive charge is compensated by anions like chloride that are attracted toward the positively charged electrode. Combined those electrodes are thus able to store a certain amount of salt retrieved from the solution. The proposal is actually centered around the question how to use this capacitive charging to extract energy from mixing fresh water with sea water. In this project we will develop two technological concepts to harness the electricity, so-called “externally-charged capacitive mixing” and “Donnan-charged capacitive mixing”.


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