The system uses far less energy than thermal desalination because the draw solution has to be heated only to 40 to 50 °C, McGinnis says, whereas thermal systems heat water to 70 to 100 °C. These low temperatures can be achieved using waste heat from power plants. Thermal-desalination plants are often located at power plants now, but it takes extra fuel to generate enough heat for them. The new system, on the other hand, could run on heat that otherwise would have been released into the atmosphere.
The Oasys system requires just one-tenth as much electricity as a reverse-osmosis system, McGinnis says, because water doesn’t have to be forced through a membrane at high pressure. That’s a crucial source of savings, since electricity can account for nearly half the cost of reverse-osmosis technology. Not working with pressurized water also decreases the cost of building the plant—there is no need for expensive pipes that can withstand high pressures. The combination of lower power consumption and cheaper equipment results in lower overall costs.
The Oasys system will not help everyone. For example, it is unlikely to do much for farmers; although they account for about 80 percent of fresh-water consumption, it wouldn’t be cost-effective for them, in part because farms are often located closer to aquifers and other water supplies than are large coastal cities such as L.A. In addition, “there’s a minimum amount of energy needed to strip salt ions out of water,” says Peter Gleick, president of the Pacific Institute for Studies in Development, Environment, and Security in Oakland, California. “I don’t think it will ever be cheap enough for irrigation.” In agricultural areas where water is scarce, he says, it’s cheaper to switch to better irrigation practices.
As coastal cities grow, however, so will their need for desalination services, says Kenneth Herd, director of the water supply program at the Southwest Florida Water Management District. “It’s not a matter of if,” he says, “but a matter of when.”