It is perhaps one of the most under-exploited sources of green energy. When salt water and fresh water mix in estuaries, a chemical process occurs that can be harnessed for electricity generation.

According to one estimate, this “blue energy” is so plentiful that it could meet all our needs – if we can find an effective way to tap it. Could ‘blue’ be the new green?

Blue energy was first proposed in 1954 by a British engineer named R E Pattle. It is sometimes called “osmotic power”, because it exploits the phenomenon of osmosis. To understand how this works, picture two solutions of water with different concentrations of a dissolved substance like salt. If these two solutions are separated by a thin “semi-permeable” membrane that lets water through but not salt ions, then water will naturally pass from the less- to the more-salty side. The flow of water across the membrane builds up pressure on one side that can be used to drive turbines and generate power.

The original idea was to harness the Jordan River mixing with the salty Dead Sea

It wasn’t possible to exploit Pattle’s idea for power generation until the 1970s. That’s when artificial materials for making semi-permeable membranes became commercially available. An Israeli scientist named Sidney Loeb suggested that they could be used in what he called “osmotic power plants”; Loeb hoped they might harness the energy released as the Jordan River mixed with the salty Dead Sea.

Such power plants actually work best not when the flow rate across the membrane is as large as possible, but when it is slowed down a little. This can be done by squeezing the salt water so that the pressure hinders the influx of fresh water from the other side of the membrane. Consequently, this technology is known as “pressure-retarded osmosis”.

The first blue-energy plant using pressure-retarded osmosis was opened in Tofte, Norway, in 2009 by the company Statkraft, with a generating capacity of 4kW – a tiny figure compared with the 5,000kW typical of a small nuclear power plant. But although the process worked, the company found that it wasn’t cost-effective – the devices just didn’t produce enough power to offset the costs of construction, operation and maintenance. Statkraft closed the plant in 2013.

All the same, commercial developers haven’t been deterred. At the Dutch water institute Wetsus in Leeuwarden, a spin-off company called REDstack has begun to exploit a different osmotic-power method called reverse electrodialysis (RED) in a prototype power plant. This technique is a little different from pressure-retarded osmosis. It involves membranes that allow the salt ions – rather than the water molecules – to pass through.

An alternative method involves a set-up a bit like a sandwich

There are two types of membrane: one that allows salt’s positively charged sodium ions through, and one that allows its negatively charged chloride ions through. These membranes are used to make a sort of water multi-deck sandwich in which layers of salt water alternate with layers of fresh water, with the two types of ion-transporting membranes alternating between each layer. This arrangement produces an electrical voltage, which can then be tapped to produce a flow of electrical current directly, without the need for any pressure-driven turbines. So in theory this method can be pretty efficient at capturing the energy of the mixing process.

Wetsus scientists are also studying a third method called capacitive mixing (capmix). Here seawater and fresh water are fed alternately into a chamber containing two electrodes, which serve as charge-storing devices (or capacitors). This process also raises the voltage.

A €2.4m (£1.7m) European consortium that includes institutes in the Netherlands, Italy, Poland and Spain has been exploring capmix technology since 2010. There are still new tricks to be found. For example, a team at the University of Utrecht led by physicist Rene van Roij has recently shown that in principle the energy output from blue-energy capmix devices might be doubled if the fresh water that is mixed with seawater is warmed – up to, say, 50C or so.

There’s no need to burn fossil fuels to do the warming, they say – we could simply use waste water heated in industrial processing, for example cooling water from power plants or data centres (where it keeps the computers from overheating). By happy coincidence, an independent team at the University of Granada in Spain demonstrated this effect works in practice at the same time that the Dutch group were working out the theory.

The idea might even work with carbon from burning fossil fuels

Osmosis works with any concentration differences of dissolved substances – sugar, say. So “blue energy” isn’t limited to mixing of fresh and seawater. In 2013, a team at Wetsus suggested it might be possible to generate electricity from dissolved carbon dioxide gas, which might be captured from fossil-fuel power plants. Carbon dioxide dissolves readily in water to produce carbonic acid, which then falls apart into bicarbonate and hydrogen ions. These can be marshalled at electrodes in a capmix process in the same way that ordinary salt ions can. Just as the capmix method involves alternately flushing the system with salty and fresh water, so this new method would entail flushing the water first with carbon dioxide (making the equivalent of the salt water) and then with clean air (making the equivalent of fresh water).

The researchers say that worldwide, the flue gases of fossil-fuel power plants contain enough carbon dioxide to make around 850 terawatt-hours of electricity each year: almost 100 times the annual power consumption of the UK. It’s a wonderfully subversive idea: carbon dioxide, usually part of the problem as far as energy generation goes, is here made part of the solution.

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