Blue Energy: An Innovative Battery Powered by Saltwater and Freshwater

If you want a quick reminder of what blue energy is really about, you can take a look at this video. Just to sum up, blue energy or salinity gradient power harnesses the differential between freshwater and saltwater, as it happens in a river flowing into the sea, to generate electricity. Although this source of energy has been known for a few decades by now, it is still in its early stages of implementation. In fact, one of the first blue energy plants was opened in Norway as recently as 2009. Promising progress, however, has been made on this innovative technology. A Stanford University laboratory has created a new battery based on blue energy. They have called it Entropy Mixed Battery (EMB).

The blue energy device developed by the Stanford engineers distinguishes itself from previous technologies by the fact that it does not require pressure (known as pressure retarded osmosis or PRO) or membranes (reverse electrodialysis or RED), generating electricity solely through an electrochemical process. The battery contains a tank that is filled up with effluent from A wastewater treatment plant. Several electrodes in the tank release sodium ions when submerged in the water. The movement of these ions creates an electric current flowing from the anode to the cathode. Next, freshwater is rapidly replaced with saltwater, which sends the sodium and chloride ions back to the electrodes, inverting the electricity current. Both the freshwater and saltwater influxes generate electricity, which means that the battery is charged and discharged constantly without the need of external power sources.

The first tests with this blue energy technology project carried out in a wastewater treatment plant in Palo Alto (USA), switching from saltwater obtained from the nearby bay to already treated freshwater in one-hour cycles, have proved the feasibility of this new technology. The researchers have confirmed that, across 180 cycles, the materials have maintained a 97 percent efficiency while capturing the salinity gradient energy.  

The EMB is the second generation of this type of batteries that they have developed. The first version used costly silver-based electrodes with limited commercial applications. Now, instead of silver, the electrodes are covered with Persian blue, an extremely low-cost pigment, which sells for less than a dollar per kilo, together with polypyrrole, than can be bought for three dollars a kilo in bulk.         

A huge potential

The goal for EMB is to provide wastewater treatment plants with a source of electricity that makes them carbon neutral and energy self-sufficient. The main issue with this type of blue energy plants is that they are currently energy-intensive and that they are exposed to power cuts that can hamper their operations. If the freshwater released into the sea became a source of energy to power these plants, the wastewater treatment cycle could become more sustainable and eco-friendlier. 

As it has been pointed out, coastal wastewater treatment plants are one of the biggest opportunities to harness this type of blue energy. Some estimates suggest that, if all the plants of this kind in the world were used like this, they could generate about 18 gigawatts.

Source: Stanford University