River deltas show electrical potential

salinity gradient energy

River delta power plant (credit: American Chemical Society)

How in the world do you generate electricity from the slow-moving water in a river delta? Think about all the ways we have of generating electricity. You can turn a turbine using steam. You can place the turbine in a waterfall and have the falling water turn the blades, or build tall towers with windmill blades that spin in the wind. You can put a battery into your mobile phone, in between two strips of metal.

These generators look very different, but they all have something in common: they are all situated in places where some kind of energy flows from one place to another, and they all tap into that flow to do some kind of work. Unless you put up some kind of barrier, if you put a lot of excess energy into one place, it will tend to flow to a place that doesn’t have as much energy, so things tend to equalize out.

For example, if you fill a bucket with hot water (high energy) and ice (low energy), eventually, you get a bucket of water all at the same temperature, which is somewhere in between hot water and ice. The trick to power generation is to separate the high-energy and low-energy things, but give them some way to connect, and put your generator in the path, like a toll booth.

The steam turbine works because you’re burning some kind of fuel, which releases heat energy into a boiler full of water. The water heats up to the point where it becomes steam and escapes the boiler through an outlet. The steam transfers energy from the hot, high-pressure (high-energy) environment of the boiler to the cooler, lower-pressure environment of the condenser. The turbine sits in the middle of the flow, using the motion of the flowing steam to turn the blades.

hydroelectric plant

How a hydroelectric plant works. (credit: Environment Canada)

Hydroelectric generators and windmills also use a flowing stream to turn turbine blades. The hydroelectric generator draws energy from water flowing under the force of gravity. It takes energy to resist gravity, and water wants to flow to the lowest spot possible unless there’s something stopping it. When water goes from a high place to a low place, it releases kinetic (motion) energy, and the hydroelectric dam converts that into electrical energy. Windmills do something similar, using a stream of air that is flowing from a high-pressure region to a low-pressure region (otherwise known as wind).

Batteries use chemical energy to trigger a flow of electricity. One side of the battery has a material that would be more stable if it could get rid of a few extra electrons, and the other has a material that would be more stable if it could pick up those extra electrons. The trick is to put a barrier between those two sides, and to provide an alternate route from one side to the other. The alternate route has something that you’ve placed in the path, like a light bulb or a motor, and a switch that acts like a drawbridge. When you flip the switch, the drawbridge closes and the electrons come pouring across it, lighting up the light bulb or running the motor on their way to the other side.

Theoretically, anything that sets up some kind of flow could be a source of electrical energy, which brings us back to the river delta. Osmosis is a kind of flow, and some researchers are working on a way to use that on a big enough scale to run a power plant. Osmosis describes what happens when a concentrated solution and a dilute solution are separated by a barrier, called a membrane, that lets some things pass, but not others.

You can see the effects of osmosis by putting a raisin in water. (Here’s a video.) Raisins already have a little bit of water in them (unless they are so dry that they’re rock-hard). They also have a whole lot of sugar, so they are basically little bags of concentrated sugar-water. The raisin’s skin has pores that are small enough that water can pass through, but not sugar (which is made of much larger molecules). When you put a raisin in water with no sugar in it, the concentrated sugar-water and plain water want to mix and form just one big pool of water with some sugar in it, but the raisin skin won’t let the sugar out. So the plain water comes in, and the raisin swells up.

So there you have your high-energy state (a concentrated sugar-water solution and a no-sugar pool of water), a barrier that keeps things from just plopping down into a low-energy state (one big pool of sugar-water), and a restricted path across that barrier (the pores in the raisin skin). Set up a tiny little turbine, and you have a miniature power plant.

On a bigger scale, the river delta is where fresh river water flows into the salty sea. Ordinarily, there’s nothing to stop the mixing, but if you could divert some fresh water from the river and some salt water from the sea, put them on opposite sides of a membrane, and let osmotic flow do its work, you could use the flow to turn a turbine and make electricity. The trick is to do it on a big enough scale, and at a low enough cost, to make it worthwhile.

In a recent issue of Environmental Science and Technology Letters, a team of Colombian and German researchers report on their calculations to characterize just the sorts of river deltas that make the most promising candidates. They found that when the mixing zone between fresh and salt water is too large, it takes too much energy to transport the water to the power station. They calculated that the best results are for freshwater and saltwater intake ports less than 2000 meters apart (about a mile and a quarter).

The biggest limitation they found was the difference in height between water levels at high and low tides. If this difference was more than about 1.2 meters (just under 4 feet), it wouldn’t be feasible to set up a saline gradient power plant at that location because the tides would produce too much mixing of the salt and fresh water outside of the power plant. Fjords and other deep river mouths could be an exception to this, because the tides would only stir up the top layers of water, leaving the bottom relatively undisturbed.

For a more technical summary of this, see my posting on the American Chemical Society’s Noteworthy Chemistry blog for October 20, 2014.