In much of the developing world, two essentials are often in short supply: potable water and reliable electricity. Some countries have invested heavily in desalination and combined cycle technologies to simultaneously solve both problems.
In the U.S., people have come to assume that a cheap and abundant supply of clean, potable water will always be available at the turn of a valve. However, water supplies in some regions have developed serious problems, lately caused by drought. Meanwhile, coastal regions, such as Florida, are stressing underground aquifers to meet water demand while risking contamination from saltwater intrusion. Perhaps it is time to consider a new and virtually limitless source of freshwater—the ocean.
Evaluate the Options
Two primary technologies are available to desalinate seawater. Most desalination plants (86%) in the U.S. are based on membrane technology, called seawater reverse osmosis (SWRO). The second technology approach is thermal distillation, which produces more than 40% of the world’s desalinated water.
SWRO certainly has some capital and deployment time advantages over thermal distillation. SWRO also seems to better fit the compartmentalized nature of U.S. business and government, where, generally speaking, municipalities retain responsibility for providing potable water and investor-owned utilities provide electrical power. Some SWRO projects have been colocated at existing power plants to take advantage of the warmer seawater available from the plant’s cooling water discharge; the warmer ocean water increases the SWRO process efficiency.
Development of desalination in many parts of the developing world has taken a different route. Usually, the markets for electrical power and freshwater supplies are developed simultaneously, often by the country’s sovereign government or by a group of large international businesses based on long-term power and water supply agreements made with the host country. Thermal distillation is normally the technology selected for these very large projects.
There are synergies that can be exploited when thermal distillation and power generation are developed in concert. Thermal distillation of seawater requires a large supply of cheap energy—and generally large amounts of steam. However, the newest technologies only require very low-pressure, low-grade steam as an energy source. On the other hand, power plants are often challenged to develop the means of disposing of large amounts of low-temperature waste heat produced in the condenser, usually with once-through cooling, cooling towers, or massive air-cooled condensers.
In the U.S., once-through cooling has been the historical cooling method of choice, but pending Clean Water Act Section 316(b) changes (final release is expected in 2013) will eliminate this option for plants constructed in the future.