Reverse ElectroDialysis

RED (Reverse ElectroDialysis) is a form of sustainable energy generation. The 'fuel' consists of fresh and salt water. Rivers flow into the sea and there the fresh water mixes with the salt water. These are the most obvious suitable locations for the application of RED. Some of the river water, along with seawater, is routed through a membrane stack in an enclosure called the REDstack. This is where the company REDstack got its name from. Salt and fresh water are mixed in a controlled manner in the stack and electrical energy is gained in this process. The resulting brackish water is returned to the sea and the electrical energy is delivered to the grid and distributed. The energy extracted in this way is commonly referred to as Blue Energy.

The driving force is derived from nature. Nature wants to level out differences. REDstack uses the difference in salt concentration as the driving force for ion transport through the membranes. By alternately stacking two types of ion-selective membranes, the positive and negative ions from the salt water are forced to flow in two directions, creating a voltage difference. This voltage difference is converted to electric current at the electrodes.

REDstack is currently actively working to seriously scale up its system on the Afsluitdijk, in order to build confidence and take the final step towards large-scale installations with end users in a commercial pathway.

Application of RED

There are several sources for the application of RED. Thus, by a classification of salinity into three classes (fresh, marine and brine), three possible combinations can be thought of for feeding a RED stack. We mention here the most important ones.

Seawater with river water

The salt content of seawater varies between 3% and 3.5% or between 30 and 35 grams (mainly NaCl) per liter. This makes the technique usable anywhere if sufficient river water is available. The following areas appear to be very good candidates: The Netherlands (Rhine Delta), the Mediterranean (Adriatic Sea), Southeast Asia (South Korea, China, Indonesia etc.), South America (Colombia, Mexico, Caribbean etc.), Gulf region (United Arab Emirates) and East Coast Russia. REDstack's pilot plant on the Afsluitdijk has been (and is being ) developed for this application.

Seawater with brine

Although this combination is less available, there are advantages. Since both feedstocks are quite saline, the electrical weather edge of the stack is low which is advantageous for high power. For example, the brine could be a residual stream from a salt plant or the concentrate from a seawater desalination plant. The project REApower,in which REDstack is a participant, was developed for this application of RED. In Sicily, the concept has been successfully applied at a company that extracts salt from seawater.

A special (not yet realized cooperation project of Israel and Jordan) is the replenishment of water from the Red Sea to the Dead Sea. This project also provides for a desalination plant. By using RED, a significant part of the energy for this could be supplied.

Brine and fresh water

In nature, salt lakes with no outlet occur that are fed by rivers. An example is the Great Salt Lake in Utah (USA) where the Jordan, Bear and Weber rivers flow into it. Due to enormous salt gradient between the salt lake and the river, the energy potential is therefore very large.

The stack

One such electrochemical cell is called a cell pair. It consists of a Cation Exchanging Membrane CEM, a layer of salt water, an Anion Exchanging Membrane (AEM) and a layer of fresh water. When several of these cell pairs are stacked on top of each other, the voltage of the stack (actually of the battery) increases. An entire stack is called a stack a RED stack. This is where the name of our company is derived from.

An important part of a RED stack is the electrodes. Within the stack we have to deal with ionic currents which are converted at the electrodes into electron currents or electricity. There are two possibilities for this conversion: an electrochemical reaction at the electrodes or the use of capacitive electrodes. This creates a surplus of electrons (the minus pole) at one electrode and a shortage of electrons (the plus pole) at the other. When these electrons are transported from the minus pole to the plus pole through an external circuit, an electric current is thus created. This is therefore a direct current that is converted outside the stack with a converter into alternating current that is then supplied to the electricity grid.