Hydropower and Other Water Energy Technologies
Water technologies encompass a variety of systems that use ocean or freshwater for electricity or thermal energy. The most familiar water technology is hydropower, in which the force of moving water propels a turbine, which in turn runs a generator to create electricity. Hydropower and other water technologies are renewable because their fuel is naturally replenished through the water cycle; they are clean alternatives to the burning of fossil fuels that cause climate change. Hydropower does not require the purchase of fuels for generation, unlike natural gas, coal and other fuel-burning plants. The only costs are the construction and operation of the generation facilities.
Globally, hydropower accounts for about 15 percent of electric generation. In 2014, U.S. hydropower plants had a capacity of about 101,000 megawatts (MW) and produced 6 percent of the total energy and 48 percent of renewable electricity in the United States. Although most suitable sites for large scale dams have been developed in the United States and globally, there are many opportunities to install hydropower systems at existing dams currently without generation capability, and to use other water energy technologies in rivers, tidal zones and open ocean. According to two 2012 studies by the U.S. Department of Energy, existing dams that are not currently producing power could provide 12,000 MW of additional capacity, and if new installations (including those harnessing waves and tidal currents) are built, hydropower could potentially provide 15 percent of America's electricity by 2030 (vs. 6 percent today).
Hydropower facilities can be installed on rivers, oceans or lakes.
Large hydropower dams on major rivers are the most developed generators of water energy. Pumped storage or reservoir plants store water in a reservoir to release for use when the river is running slower or during times of peak energy demand. This allows for reliable base-load power generation. The Hoover Dam in Nevada and the Grand Coulee Dam in Washington are examples of these large facilities. Large dams also meet multiple societal needs such as irrigation, flood control and recreation.
There are several drawbacks to reservoir plants. Studies suggest that large reservoirs in boreal and tropical climates emit as many greenhouse gases as a fossil fuel power plant. Flooded vegetation decomposes, releasing methane and carbon dioxide in a large burst at the beginning of a dam’s life and continuing in lesser amounts throughout the dam’s use. Further impacts include changes in water temperature, dissolved oxygen and other nutrients, harm to the river’s ecosystem, displacement of communities by the alteration of the river’s flow, and riverbank instability leading to deforestation, flooding, and erosion. Hydropower is vulnerable to climate change. Prolonged droughts may diminish the water level of the river, lowering electricity generation, while melting glaciers, rapid snowpack melt, or changes in precipitation patterns from snow to rain may significantly alter the river flow.
Run-of-the-river plants have no water storage facilities but may use low-level dams to increase the difference between the water intake level and the turbine. In this case, the natural river flow generates electricity and the amount of power generated fluctuates depending on the cycle of the river. Although run-of-the-river technology can be used for large scale power generation, it is commonly applied to supply individual communities with electricity, with capacities of less than 30 MW. This form of power generation is popular in rural areas of China, but has potential application in many places, including in the United States. Run-of-the-river technology typically disrupts much less of the river flow as compared to large hydropower dams.
Current generation works similarly to a wind turbine, but underwater. Because water is denser than air, water moving at a given speed will produce much more power than that generated by a comparable wind speed. However, the turbine itself must be stronger and, therefore, is more expensive. The environmental impact of current turbines is not clear. It could harm fish populations but fish-safe turbines have been developed.
The United States has many potential sites where current generation could occur, and several projects are underway, including those in the East River in New York and the San Francisco Bay. The Federal Energy Regulatory Commission issued the first U.S. commercial tidal energy pilot project license in 2012. The 10-year license sets the East River (Roosevelt Island Tidal Energy) project on a path towards building 30 turbines to generate 1 MW.
Ocean tidal power harnesses the predictable cycle of energy produced by the tides. A tidal barrage works similarly to a large hydropower reservoir dam, but it is placed at the entrance to a bay or estuary. The retained water in the bay is released through turbines in the barrage and generates power. A tide must have a large enough range between high and low tide, about ten feet, for the barrage to function economically. The best potential sites are located in northern Europe and the U.S. West Coast. A tidal barrage in La Rance, France has been operating since 1967 with a capacity of 240 MW. The potential environmental impact of barrages could be significant because they are built in delicate estuary ecosystems, but less intrusive designs such as fences or floating barges are under development.
Similar to river current technologies, turbines anchored to the ocean floor or suspended from a buoy in the path of an ocean current could be used to generate power. Although this technology is in the development stages, some potential locations in the United States include the Gulf of Maine, North Carolina, the Pacific Northwest, and the Gulf Stream off Florida.
As wind moves over the surface of the ocean, it transfers energy to the water and creates waves. Although variable in size and speed, waves are predictable and are constantly created. In U.S. coastal waters alone, the total yearly wave energy is 2,100 terawatt hours.
A variety of technologies are being tested to convert wave energy into electricity. Most systems capture energy on the surface of waves or use pressure differences just below the surface. These systems use the swells of waves to create pressure and move hydraulic pumps or pressurized air, which in turn puts generators into motion. The environmental impacts of wave generators are not fully known, but are thought to be minimal and site-specific.
The best potential sites for wave generation are ocean areas with strong wind currents. These areas are between 30° and 60° latitude, polar areas with frequent storms, areas near the equatorial trade winds, and the west coasts of continents. Hybrid wind and wave technology for offshore energy farms are in development. Potential sites in the United States for hybrid wind-wave energy farms include the coastal areas of the East Coast and the Pacific Northwest.
Ocean Thermal Energy Conversion
Ocean thermal energy conversion (OTEC) uses steam produced from warm surface water to spin generating turbines. Cold deep ocean water condenses the steam back into water for reuse. A 36°F temperature difference is necessary between the surface and deep water. Potential sites include tropical islands. OTEC is in the early development stage and is not yet cost-effective, due to the high cost of pumping deep water to surface generating stations. OTEC can be paired with ocean thermal air-conditioning systems (see below). Furthermore, the nutrient-rich deep water can assist in aquaculture. Surface ponds pumped with deep water can cultivate salmon, lobster, and other seafood as well as plankton and algae.
Ocean/Lake Thermal Air-Conditioning
In addition to generating electricity, water also can be used for direct thermal energy. Water from lakes or oceans can provide air-conditioning for buildings. The cold deep water is used to chill fresh water that circulates through a building in a closed-pipe system, providing air-conditioning at a lower cost than traditional methods. The spent water is returned to the ocean or lake to renew the cycle. The cold deep water must be between 39°F and 45°F and close to shore to be economical. Examples of ocean thermal cooling systems are seen in Hawaii (co-located with OTEC facilities), and Toronto, where water from Lake Ontario is used to air-condition downtown buildings. Large-scale OTEC project (100 MW+) situated in island communities such as Puerto Rico, Hawaii or Guam can be economically viable.
Learn more about Hydropower:
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- Fact Sheet: Jobs in Renewable Energy and Energy Efficiency (2014)
- For the First Time, Wind, Biomass, Solar, and Geothermal Outpacing Hydropower
- Hydropower and Other Water Energy Technologies
- Issue Brief: Obama FY2015 Budget Proposal: Sustainable Energy, Buildings, Transportation and Climate
- Renewable Energy