Unit 10: Energy Challenges // Section 8: Hydropower and Ocean Energy
Hydropower and ocean energy systems indirectly tap the Earth's solar energy flux, which drives the cycling of water between Earth's surface and the atmosphere and heats the upper layer of the oceans (for more details, see Unit 8, "Water Resources"). By damming rivers for hydropower or placing turbines in zones of the ocean areas with significant tides and currents, we can use the power of flowing water to generate electricity. Today hydropower generates about 17 percent of world electricity supplies (the same as nuclear energy, although hydropower is credited with only a third as much primary energy as nuclear because it does not generate two units of heat for every unit of electricity). Ocean power systems are still at an experimental stage.
Like biomass, hydropower is an established renewable energy technology that is widely used in many parts the world. The basic technology is simple: falling water flows through pipes, called penstocks, then turns turbine blades to spin a generator and produce electricity (Fig. 15). When excess power is available from the grid, some hydropower stations pump water up to storage reservoirs and hold it in reserve, then release it to generate power when demand rises.
Figure 15. Hydropower system
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Source: © United States Department of Energy. Energy Information Administration, National Petroleum Council, U.S. Geological Survey.
Hydropower generates electricity without producing significant air pollution, except for emissions from building and maintaining dams. Large hydropower dams can also serve other purposes: for example, the reservoirs that develop where rivers are dammed can provide drinking water supplies, and many are used for fishing and boating. Some hydropower reservoirs, especially in dry regions like Africa, have become important habitats for birds.
In recent years, however, critics have drawn attention to hydropower's negative environmental impacts. A report issued in 2000 by an independent international commission catalogued ways in which large dams can harm ecosystems, such as:
- Killing plants and displacing animals, including endangered species, when reservoirs are flooded;
- Altering river flow rates, the quantity and character of sediments moving through the channel, and materials that make up stream beds and river banks;
- Modifying water parameters such as temperature and levels of nutrients and dissolved oxygen;
- Degrading downstream channels, floodplains, and deltas by reducing the transport of nutrients and sediments below dams; and
- Blocking migration of fish and other aquatic species up and downstream.
The report also noted that while hydropower does not generate greenhouse gas emissions as water spins electric turbines, reservoirs emit CO2 and/or methane from rotting submerged vegetation and carbon inflows from the catchment area. Calculating how much a specific dam contributes to climate change depends on many factors, including whether the flooded land was previously a carbon source or sink and what land use changes result from building the dam and displacing people from the flooded area. On balance, however, it appears that warm, shallow tropical dams emit more GHGs than deep, cold dams at higher latitudes (footnote 6).
In spite of these negative aspects, hydropower is an attractive alternative to fossil fuels for many countries with good resources. In addition to their low pollutant emissions, hydropower plants provide dispatchable power: their output can be raised or lowered quickly to meet fluctuating levels of demand. Other renewable sources, such as wind and solar energy, produce energy intermittently when the wind blows or the sun shines, so they are not as responsive to daily market conditions.
At best, however, world hydropower capacity can be expanded only by a factor of 2 or 3 because a limited number of good sites remain available for development, mainly in Africa, Asia, and Latin America. In the United States, it is estimated that more than half of the hydropower generating capacity is already tapped, and most of the remaining potential dam sites would adversely affect sensitive environments. China's Three Gorges Dam, the largest hydroelectric project in the world, is scheduled to enter operation in 2009 (Fig. 16). The project will provide 18,000 megawatts of electricity-generating capacity—an important asset for China, which relies heavily on coal to meet its fast-growing energy needs. However, it has been criticized for flooding many rural valleys and displacing more than 1.5 million people.
Figure 16. Aerial view of the Three Gorges Dam, China
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Source: © National Aeuronatics and Space Administration. Earth Observatory.
Ocean energy occurs in the form of tides, waves, currents, and heat. Tidal energy resources are modest on a global basis, and tapping them involves building major dams on inlets and estuaries that are prized for other purposes, so few tidal energy facilities have been developed. Harnessing waves and currents on a significant scale will involve designing turbine structures that are large, inexpensive, and can operate for long periods under the physical stresses and corrosive forces of ocean environments. For the most part, such systems are at the research stage today.
The largest but most experimental form of ocean energy is ocean thermal energy conversion, which taps heat stored in the ocean to generate electricity. This process runs warm surface seawater through several different types of systems that use the water's stored heat to turn a turbine, then cools the resulting steam or vapor with cold deep-seawater (footnote 7). Making this conversion work affordably on a large scale is technologically very difficult because it requires large structures and physical challenges associated with working in the ocean environment. It works most effectively in regions where there are large temperature differences between surface and deeper waters, mainly in the tropics. If ocean thermal energy conversion can be commercialized at some point, however, it could become an enormous new energy source.