When a river meets the sea, the salinity of the seawater is diffused in the mix of waters. The process releases energy, as much as if the same volume of water fell from a 250 meter high hydroelectric dam. With osmotic methods, the energy can be gathered to generate electricity. All that is needed is slightly more efficient membranes.

Principen för TRO: utjämningen av saltkoncentrationen gör att vattentrycket ökar på saltvattensidan om membranet. Trycket driver en turbin.

Pressure Retarded Osmosis: Water moves across the membrane, towards the higher salinity. The increased pressure on the the seawater side powers a turbine.

Desalination is an energy demanding endeavour; arid countries, forced to rely on it for drinking water knows this all too well. But there is an upside to it: when fresh water mixes with saltwater, energy is released instead – and it is possible to harvest that energy and produce clean, renewable electricity.

The energy stems from the diffusion process which brings the salinity of the river water and the ocean to equilibrium; sea water contains a lot of salt, and river water very little (approximately 30 grams and 1 gram per liter, respectively). Salinity gradient power, osmotic power or “blue energy” – there are several names for it – is a potent energy source; the osmotic potential is comparable to the energy potential of water in a 250 meter high hydroelectric dam. According to some estimates, as much as 2000 TWh of blue energy could be produced every year.

Semipermeable membranes regulate the flux

There are several methods to control the diffusion and harvest the energy. All of them are based on osmosis and semipermeable membranes.

A semipermeable membrane allows water to pass freely, but acts as a barrier to solubles such as salt. The cell membranes protecting living cells is an example from nature – but synthetic semipermeable membranes can be manufactured, for instance in the form of porous polyamide sheets. Polymer membranes are used in many applications, such as dialysis tubing. The size of the pores determines the permeability, and there can be billions of micropores on a square centimeter of polymer membrane.

Pressure retarded osmosis powers turbines

The first method, Pressure Retarded Osmosis or PRO, is similar to hydroelectric power.

When a semipermeable membrane separates two solutions with different salinity, water will cross the membrane to balance the difference until the system reaches equlibrium. In a PRO power plant, river water is pumped to one side of the membrane, and seawater to the other. The flow of fresh water across the membrane builds up a higher pressure on the seawater side, which can be used to power a turbine.

A Norwegian pilot facility has demonstrated that the method is feasible and can produce electricity in real-world settings. In 2009, Statkraft opened a PRO-based plant with 2000 square meters of membrane in the Oslofjord. The plant was not cost-efficent enough to be competitive, however, and it was shut down in 2013.

TRO-kraftverket i Norge. Membranen är rullade i cylindrarna. (Bild: Statkraft)

The Norwegian PRO plant. The membranes are coiled in the cylinders. (Photo: Statkraft)

The output was 5kW, half of the power achievable in theory. There have been suggestions to start construction of a 10MW PRO plant in Australia, by the Brisbane River – when more efficient membranes become available.

The membranes themselves are the key factor in making PRO profitable; more specifically, cheaper and more efficent manufacturing processes are required.

One idea, which seems to have some merit, is to replace the micropores with nanotubes created from carbon dioxide in high temperature kilns. The carbon nanotubes are wider than the micropores, don’t clog as easily, are more durable and offer less resistance to the flow of water. In theory, carbon nanotube membranes could be 1000 times more efficient than polymer based, because of higher efficiency and less need for maintenance. The difficulty lies in achieving high enough tube density.

It is desirable for membranes to be as smooth as possible, to keep algae and organic matter from getting a hold. Smooth membranes make for a more efficient process, since pre-treatment of water and cleaning of membranes cost energy.

Reverse Electrodialysis: a battery built in the water

PRO produces energy from pressure. The second method, Reverse ElectroDialysis or RED, diverts ions in the water to create two electrodes with an electric potential between them.

RED is also membrane based, but the membranes are different. A number of anion and cation exchange membranes – permeable by sodium+ and chloride- ions respectively, but not by water – are connected in series.

When such a membrane is put between fresh water and seawater, an ion flux will occur. The seawater is kept in the middle, with an anion membrane on one side and a cation membrane on the other, and river water on the outside of both membranes. This setup forces the negative Cl- and positive Na+ ions to be accumulated in opposite directions. The separation of charges builds up an electric potential, and the system will act like a battery charged by the salinity difference.

The output from a single membrane is limited, but the power can be multiplied by stacking many membranes on top of each other. Much of the RED research and development is about increasing the number of membranes working together in a single system.

RED has been a topic of research at Chalmers University of Technology, in Gothenburg, Sweden. The aim of the Chalmers project was to produce enough energy to supply the city of Gothenburg with electricity, but it ran into difficulties with growth of algae on the membranes. The Dutch company REDstack, a spinoff from the Wetsus research centre, has shown better results.

REDstack opened a 50 kW pilot plant in 2014 in Breezanddijk, on the Afsluitdijk causeway which separates the Ijsselmeer lake from the North Sea (see picture above). Water from both sides of the dam is pumped through pipes, filtered, and stored in tanks inside the facility. When it has passed through the membrane stacks, the resulting brackish water is released back in the ocean. Commercial production is not likely to happen before 2020 – but the company believes that a 200 MW facility eventually could be constructed at the site.

With both of the above methods, power output is determined by the salinity difference between the two solutions, the temperature, the purity of the water and the properties of the specific membrane. The greater the salinity difference, the more efficent process. One does not necessarily have to use river water and sea water, salt water of different concentrations work as well. The processes can be applied to saline industrial waste water flows, and return energy to the industry.

CapMix: carbon dioxide instead of salt

A third method is called CapMix. CapMix is likely further from a commercial breakthrough, but has a lot of potential. In brief, the electrodes of a supercapacitor are exposed to alternating high and low ionic concentrations. What is interesting is that solutions of carbon dioxide in water can be used, which opens up the opportunity to use flue gas emitted from power plants instead of sea water as the feedstock.

In the end, the blue energy breakthrough will hinge on more efficent membrane manufacturing methods. New membranes where pore size varies with the salinity have shown promising results in the laboratory, with an output beyond what was considered possible to achieve. The challenge is to scale up these results to square meter sized membranes without costs escalating through the roof.

The pilot plants in operation have verified that the methods are working – and when membrane technology has advanced a bit more, salinity gradient power is ready to step up as a clean, renewable source of energy wherever a river flows to the sea.

The article was published in June 2016.