Blue Energy, Where Rivers Meet the Sea

The issue with energy has always been a bit like the castaway dying of thirst while floating amidst a vast sea. Yes, the water’s there, but it isn’t drinkable. Likewise, energy is everywhere, but liberating it, capturing it and transporting it is a different story altogether. Together with fusion reactors, blue energy is one of the most fascinating and elusive power sources yet to be tapped. It is the electricity generated through osmotic processes, which are based on the proclivity of two liquids with different densities to achieve a state of equilibrium through the exchange of particles.  Currently, one of the most common applications of the osmotic technology is water purification.

A lot of water has flowed under the bridge since professor Pattle first theorized in the 1950s that the differential between the composition of the salt water from the oceans and the fresh water of the rivers flowing into them could generate a variance in osmotic pressure. According to the theory, if both liquids were separated by a special semipermeable membrane, fresh water would flow naturally towards the salt water chamber to reduce the saline concentration of the latter. If the volume of this second chamber were to remain constant, the pressure of salt water would increase and, in turn, be able to move an electricity-generating turbine. The energy liberated by such means would be far from negligible. Indeed, research showed that, if harnessed, it could cover up to 80% of the world’s energy consumption. The eureka moment wouldn’t go much further though, as there was no technology available to leverage that immense source of power. The practical side of things was yet to be addressed.    

By 1973 new progress was made. Inspired by the saline differential between the Dead Sea and the River Jordan flowing into it, Sidney Loeb, an American professor, devised a membrane system based upon the pressure-retarded osmosis.  The PRO system used specially developed membranes to implement the principle suggested by professor Pattle. The cost of producing such membranes, however, would remain prohibitively high and would remain so for years. It was not until 2009 when a power station with such technology opened in Norway. Nevertheless, the electricity produced couldn’t be called a breakthrough, with a limited 10 Kw output, barely 1 w/m². Additionally, bacterial activity clogged the membrane openings, reducing efficiency even more. In 2013 the project was closed down. It was time to try other complementary technologies. 

It was the very same professor Loeb who developed the alternative. Four years after his research on PRO technology he was ready to announce the reverse electrodialysis technique (RED). This time, instead of using the pressure of water, the approach was to harness the positive and negative charges present in salt water and fresh water bodies divided by a membrane with an electrical current. In this system, it is salt ions that pass through the membrane, with one side allowing only the passage of positive ions, which flow to the cathode, and the other allowing the passage of the negative ions, which flow to the anode. This creates an electrical charge that can be used. The first RED-based power station was opened in the Netherlands in 2014, supported by Wetsus, the Dutch water institute, in Leeuwarden. Since then, REDstack, the resulting company, has been working on the production of electricity, with a 50 KW output.

Nanotechnology to the rescue

However, recent progress could open up the full potential of this osmotic energy. The key lies in reducing the size of the membrane openings to an atomic scale. Thus, in late 2016, Nature magazine announced the development of a three-atom thick membrane of molybdenum disulfide able to produce up to 1 MW/m². In other words, that surface could potentially power 50,000 energy-efficient light bulbs. Besides their low production cost, these membranes don’t require any power stations, but can be placed directly in river estuaries to generate electricity. Just like with other nanotechnology advancements, the challenge currently lies in the production of consistent and homogeneous membranes on an industrial scale.  

Source: Nature, All About Circuits, BBC