I have just got back from a scientific conference in Bristol. I thoroughly enjoyed it; it was great to hear of interesting science, and to meet up with friends I have not met for a year or two. Most of the talks were quite pure science and quite technical, but I did learn about ‘blue energy’.

The idea behind blue energy is simple. The scientist who presented the work is Dutch, and he pointed out that at Rotterdam in The Netherlands, the river Rhine meets the sea. Now The Netherlands is a very very flat country, so no waterfalls and no hydropower. You might think that this means that you can’t get power out of a river.

But you can. The reason is that rivers like the Rhine are made of fresh water, and they flow into the salty sea. So the salt concentration is high in the sea and very low in river water. This difference in the concentration of salt can be harnessed to generate power.

The talk at the conference looked at ways of using a difference in salt concentration to directly produce electricity. But perhaps the simplest way of seeing that a difference in salt concentration can generate power is to realise that the difference can drive water uphill. This is illustrated by the image above and is due to osmosis.

Osmosis will occur when you have a salt solution and pure water separated by a membrane which allows water molecules through but crucially not salt ions – such membranes are quite straightforward to make. Then, simply speaking, what happens is that in the salt solution as there are salt ions the concentration of water molecules is less than in pure water – the ions take up room. Because of this concentration imbalance between the salt solution and the pure water, the water molecules cross the membrane from the pure water to the salt solution.

So if you start off with equal heights of the salt solution and the pure water, a little while later the flow of water into the salt solution will cause the level of the salt solution to rise. The water is effectively flowing uphill, due to the concentration difference. And once the water has gone uphill, you can allow it to flow downhill and generate power. Simple.

Blue energy is very green of course, i.e., it is eco-friendly in that it does not generate CO2.

It may be some time before large amounts of power are generated this way, and it will never generate all the power we need, but it could still be useful. And the next time I see a river flowing into the sea, I will think about about the power of the fresh and salt water mixing.