Osmotic power from the ocean

By Richard L. Stover, PhD and G.G. Pique, Energy Recovery Inc.

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Reverse osmosis is a process familiar to most plant operators. Push makeup water through the right kind of membrane with sufficient pressure (osmotic pressure) and you generate a stream with few dissolved solids on the front side and a more-concentrated stream on the back side. The purified stream can be used to satisfy plant needs for high-quality water, and the other water can be discharged or passed through the membrane again. Desalinating saltwater to make potable water is among the most widespread applications of reverse osmosis.

Osmosis may be a less familiar process, but it's equally important. In an osmotic process, water flows from a solution with a lower solute concentration (for example, fresh water) through a semipermeable membrane into a solution with a higher solute concentration (for example, saltwater) until the salt concentrations on both sides of the membrane equalize or until enough pressure builds on the high-concentration side to stop the flow (see box).

Researchers are now exploring whether the resulting osmotic pressure can be used to generate significant amounts of electric power.

An experiment in osmosis

A simple thought experiment can help you get your mental arms around the basics of osmosis. Fill a beaker with salt water, cover it with a semipermeable membrane, and submerge it in a beaker of pure water, making sure the water levels are the same (Figure 1). The semipermeable membrane is chosen such that water molecules can pass through it unimpeded, but salt molecules cannot. As more salt gets trapped in the membrane, fewer water molecules can pass through. Over time, the water level in the beaker will rise. The process is complete when the increase in head pressure in the beaker equals the osmotic pressure.

1. Learning by osmosis. A beaker filled with salt water and separated from pure water by a semipermeable membrane (L) will elevate the salt water level (R) as a function of the osmotic pressure. Source: POWER magazine

Have you ever wondered why drinking seawater will kill you? Fill your stomach with it, and osmotic pressure will draw pure water from your body in an attempt to dilute the salt water. One of two very bad things will happen next. Either you"ll become dehydrated enough to suffer seizures, unconsciousness, and brain damage, or your kidneys will fail from being overwhelmed with too many salt molecules.

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