This month's Scientific American has a great article explaining how water and energy are to resource economics what energy and mass are to nuclear physics—interchangeable currencies. Energy can be used to make water through desalination, the water source of last resort for dry coastal regions like Southern California. And water plays a role in generating nearly all electricity—either directly, through hydropower, or as a coolant for nuclear and coal-fired power plants.

The steam turbines in coal and nuclear power plants operate as heat engines, turning heat energy into mechanical energy that's then used to generate electricity. The laws of thermodynamics dictate that a heat engine won't work unless you remove the waste heat that doesn't get converted into mechanical energy. There are three ways to get rid of waste heat, two of which involve large amounts of water. The first is to draw cooling water from a lake, river, or ocean, run it straight through the plant's steam condenser, and return it to the source a few degrees warmer. This works well for power plants located next to an ocean, but it can raise the temperature of smaller bodies of water, wreaking ecological havoc. It also sucks up—and kills—distressingly large numbers of fish. A lawsuit going before the Supreme Court this term argues that this harm to fish populations is severe enough that the EPA ought to phase out the "once-through" use of cooling water.

The alternative to once-through cooling is to recycle cooling water by letting it release its heat to the air. This is most often done in wet cooling towers that allow the water to come in direct contact with the air, releasing much of its heat through evaporation. The water that evaporates has to be replaced, so wet cooling towers still use a lot of water. The truly water-friendly option is a dry cooling tower, which functions much like a car radiator, dissipating heat through the flow of air over hot pipes. The problem is that dry cooling towers cost more to build. And because they can't cool plants down to the low temperatures that a once-through system or a wet cooling tower can achieve, they reduce the efficiency of the plant's steam turbines.

Energy consultant John Maulbetsch estimates that the cost of water saved by using a dry cooling tower is somewhere between $1000 and $2000 per acre-foot. That's above the going rate for water in most areas of the United States, although Las Vegas may be willing to pay more. But as water becomes scarcer and more expensive, dry cooling towers are likely to become an increasingly attractive option.

--Rob Inglis, High Country News