Names
editElectrostatic double layer expansion
Definition of physical quantities
edita) Definition of "efficiency" : "net produced energy" / "resource energy"
b) Definition of "net produced energy" = "produced energy" - "system energy"
c) "Produced energy" is the electrical energy produced by the cell, measured as the energy dissipated by a resistive load. In this way, we can evaluate the device also before we are able to convert the produced electrical energy into, say, a 12V stable power supply.
c) Definition of "system energy": all the energy we must put into the system from outside: includes pumping energy, water pretreatment, energy for externally charging the cell, system control and anything else.
d) "Resource energy" is the free energy reduction of the resource, calculated on the basis of the standard free energy.
d1) "Resource energy" in the case of sea/river water. The resource is the fresh water (this is not trivial at all; also pretreated sea water is a resource, but we already placed pretreatment energy in point c); it's about 2.4 kJ per liter of fresh water entering into the device, exactly $V g\left(c, c_0\right)$ where $V$ is the fresh water volume, $c$ is the concentration of incoming water and $c_0$ is the concentration of sea water.
d2) "Resource energy" in the case of evaporation ponds. The resource is evaporation. We use a volume $V_{in}$ of brine at concentration $c_{in}$; the outcoming water is not disposed into the sea, but a volume $V_{out}$ at concentration $c_{out}$ is sent back to the evaporation pond. There is a contrain, i.e. the total quantity of ions in the pond must be kept constant: $V_{in} c_{in} = V_{out} c_{out}$. The "resource energy" is $V_{in} g\left(c_{in}, c_0\right) - V_{out} g\left(c_{out}, c_0\right)$.
e) Definition of "energy extraction efficiency" : "produced energy"/"resource energy"