One of the electrochemical cells described previously is a cell pair – a cell due to the positive and negative poles, and a pair due to the two types of membranes and two feed waters. Each cell pair has a salt water compartment, a CEM, a fresh water compartment, and an AEM. It’s possible to pile multiple cell pairs on top of one another to increase the total voltage of the RED system. A whole pile of RED cell pairs is called a RED stack, to which the company owes its name.
An important component of a RED stack is the electrode couple. Inside the stacks, all of the energy is transported in the form of an ionic current – the ionic current is converted into electricity only at the electrodes. The chemical-to-electrical conversion can be done in two ways: the first is to use a reduction-oxidation reaction to generate direct current (DC) electricity, and the second is to use capacitive electrodes. In a RED stack being operated under load conditions, one electrode compartment generates an over-supply of positively charged ions and becomes predominantly positive, while the other generates an over-supply of negatively charged ions and becomes predominantly negative. Electricity can then be generated using either a redox reaction or with capacitive electrodes, and transferred through a converter to the grid for distribution.