Harnessing a Mighty Force
In large quantities, water is a hugely powerful force capable of doing a lot of work. Not surprisingly, machines to harness that energy have been used most often when cheap human labor is scarce.
The technology behind the first sophisticated water wheels is very similar to that of hydroelectric power plants. In both cases, water powers a wheel or propeller, translating the flow of the water into mechanical motion. Early models, dating at least to the Roman Empire and referenced in texts from earlier centuries, used water wheels to power gears or other simple machines to replicate manual labor, such as crushing grain or stone.
Modern versions of the water wheel, like that at Niagara Falls or the Hoover Dam, convert that motion into electricity by harnessing the movement of the water to power giant generators.
Like all other matter, water packs the most force when it falls from a distance, as opposed to flowing along horizontally, so water wheels are most effective when they are situated at waterfalls or places where a river quickly plummets in altitude.
The European inventor Nikola Tesla first pioneered the method for harnessing the chaotic power at Niagara Falls in the last years of the 19th century by diverting water at the top of the falls and funneling it down 140-foot shafts with propellers at the bottom, where the energy from the water was at a maximum. The same method is still employed at Hoover Dam.
Water is popular as an electricity generator because of its low emissions of greenhouse gas and because it's a highly renewable source. But it doesn't come close to generating all the required electricity for the population. In 2005, it accounted for just 7 percent of net energy generation in the United States.
This story appears in the June 4, 2007 print edition of U.S. News & World Report.