Osmotic Power: How It Could Work: Hydropower: Eco20/20

Osmotic Power: How It Could Work

The Earth is almost three-fourths water. We have long understood the vast differences between the salt water and fresh water on our planet. Differences in composition, the types of life supported, and effects on the climate. It appears now that we may be able to use the difference between these two types of water to provide energy for our population. Osmotic power, also known as salinity gradient power, occurs due to the difference in the salinity gradient between salt water and fresh water. There are two main ways to harness this energy: reverse electrodialysis and pressure-retarded osmosis.

In reverse electrodialysis, salt water and fresh water are released through an alternating stack of anode and cathode exchange membranes. One type allows only positive ions to pass through while the other only allows negative ions to pass. The ionic difference between the two types of water generates electricity over each membrane with the total energy potential being equal to the potential over all membranes. Although this sounds like a relatively simple process, it has its drawbacks. First, both types of water must be clean before they enter the membranes so an extensive filtration system is required. An estimated 10 percent of power produced by reverse electrodialysis would be needed to run the filtration system. It is also a costly process at this time since it takes a relatively large area to produce much electricity. Many scientists hope that the price of membranes will drop as the technology advances, making it more affordable.

The second way to generate osmotic power is pressure-retarded osmosis. In this process, salt water is placed in a pressurized chamber where pressure is lower than the difference between salt water and fresh water, or about 26 bars of pressure. Freshwater then moves through a membrane and increases volume in the chamber. As the pressure changes, it causes a turbine to spin and generate electricity. This concept was first developed over 40 years ago but is still not efficient enough for worldwide implementation. In both pressure-retarded osmosis and reverse electrodialysis, the only waste product is brackish water.

Scientists predict that we are years away from commercially available osmotic power, mostly due to the high cost and the technical challenges involved in the use of the membranes. If these problems are overcome, however, it is predicted that osmotic power could potentially be worth three times as much as solar and wind power combined. Many scientists also believe that if osmotic power becomes commercially feasible, it could likely provide thousands of terawatts of energy every year. With a potential that high and such a small impact on the environment, it is imperative that research is continued in the field of osmotic energy.