Hydroelectricity - Dams, Waves, & Tides
Fossil fuels dominate the electricity generation mix of the US as a whole and the global energy mix more generally. But in some areas of the US (like the Pacific Northwest) and in some countries, including several in South America and Europe, the 800-pound gorilla of electric power generation isn’t coal, oil or even natural gas – it’s hydro-power, generated from immense dams placed along the world’s major rivers. In both the US and globally, hydro-power is the largest renewable resource in the energy mix, and certainly the largest source of renewably-generated electricity. While growth in the use of hydroelectricity (at least the traditional type – generated by very large dams) has slowed to near zero in the U.S., many other countries in both the developed and developing world are pushing ahead with major projects to dam rivers and generate immense amounts of electricity.
This is a good thing, right? After all, the more power that is generated from hydroelectricity, the less that we might have to generate using fossil fuels, and the fewer greenhouse gases that the global energy sector will release. While it is certainly true that there are no direct greenhouse-gas emissions from hydroelectricity, broadening the use of hydro-power, particularly in heavily forested areas of the world, introduces other complex environmental and social impacts. In fact, the reservoirs behind dams are major sources of methane (a potent greenhouse gas), so hydro is not exactly a carbon-free source of energy.
In this section, we’ll take a look at the processes for harnessing water for electric power generation – and these processes are not limited to damming rivers (though dams are certainly the predominant method for harnessing water energy). Like wind energy, humans have been using water for “energy” purposes (i.e., to do useful work) for thousands of years, making river systems one of the world’s oldest energy resources. For the first couple of thousand years of hydro-energy’s existence, the energy in flowing water was used to turn water wheels not for power generation, but for grinding or milling things like wheat, to make flour. It was not until the 1880s that hydroelectricity was born, with small hydropower dams in Michigan and Niagara Falls providing electricity to those places.
How do we turn water into electricity?
There are three basic technologies for using flowing water to generate electricity:
- Hydroelectric dams generate power by allowing water from behind the dam (from a reservoir or impoundment) to flow through a turbine. The turbine spins, generating electricity. This is not unlike a steam turbine (which utilizes coal, gas or oil as a fuel) or wind energy (which utilizes the wind as a fuel to get the turbine to spin).
- Wave and tidal energy projects use the kinetic energy in ocean waters, again to get a turbine to spin, producing electricity.
- In-stream hydro-kinetics is an emerging set of technologies that are similar in design to wave and tidal energy projects but are meant to be placed in streams.