Technologies for converting energy from water to electricity include conventional hydroelectric technologies and emerging hydrokinetic technologies that can convert ocean tidal currents, wave energy, and thermal gradients into electricity. Conventional hydroelectricity or hydropower, the largest source of renewable electricity, comes from capturing the energy from freshwater rivers and converting it to electricity. Hydroelectric power supplies about 715,000 megawatts (MW), or 19 percent, of world electricity. In the United States, conventional hydropower provides approximately 7 percent of the nation's energy (USGS, 2009). Hydropower is regionally important, providing about 70 percent of the energy used in the Pacific Northwest (PNWA, 2009).

Since this resource has been extensively exploited, most prime sites are no longer available. Furthermore, there is increasing recognition of negative ecosystem conse quences from hydropower development. Future hydropower technological developments will relate to increasing the efficiency of existing facilities and mitigating the dams' negative consequences, especially on anadromous fish. Existing hydropower capacity could be expanded by increasing capacity at existing sites; installing electricity-generating capabilities at flood-control, irrigation, or water supply reservoirs; and developing new hydropower sites (EPRI, 2007a). Turbines at existing sites also could be upgraded to increase generation. None of these strategies require new technologies.

Because use of the conventional hydroelectric resource is generally accepted to be near the resource base's maximum capacity in the United States, further growth will largely depend on nonconventional hydropower resources such as low-head power4 and on microhydroelectric generation.5 A 2004 Department of Energy (DOE) study of total U.S. water-flow-based energy resources, with emphasis on low-head/low-power resources, indicated that the total U.S. domestic hydropower resource capacity was 170 GW of electric power (DOE, 2004). However, these numbers represent only the identified resource base that was undeveloped and was not excluded from development. A subsequent study assessed this identified resource base for feasibility of development (DOE, 2006). After taking into consideration local land use policies, local environmental concerns, site accessibility, and development criteria, this value was reduced to 30 GW of potential hydroelectric capacity (DOE, 2006). A report from the Electric Power Research Institute (EPRI) determined that 10 GW of additional hydroelectric resource capacity could be developed by 2025 (EPRI, 2007). Of the 10 GW of potential capacity, 2.3 GW would result from capacity gains at existing hydroelectric facilities, 2.7 GW would come from small and low-power conventional hydropower facilities, and 5 GW would come from new hydropower generation at existing non-powered dams.

New technologies to generate electricity from ocean water power include those that can harness energy from currents, ocean waves, and salinity and thermal gradients. There are many pilot-scale projects demonstrating technologies tapping these sources, but only a few commercial-scale power operations worldwide at particularly favorable locations. In general, there is no single technological design for converting energy in waves, tides, and currents into electricity. For example, approaches for tapping wave energy include floating and submerged designs that tap the energy in the impacting wave directly or that use the hydraulic gradient between the top and bottom of a wave (MMS, 2006). One such device concentrates waves and allows them

4 Vertical difference of 100 feet or less in the upstream surface water elevation (headwater) and the downstream surface water elevation (tailwater) at a dam.

5 Hydroelectric power installations that produce up to 100 kW of power.

to overtop into a reservoir, generating electricity as the water in the reservoir drains out through a turbine. Other approaches include long multisegmented floating structures that use the differing heights to drive a hydraulic pump that runs a generator or subsurface buoys that generate electricity through their up-down motion. Over the next 10 years, many large-scale demonstration projects will be completed to help assess the capabilities of these technologies, though it will take at least 10 to 25 years to know whether these technologies are viable for the production of significant amounts of electricity (NRC, 2009d). Over the longer term, other significant potential technologies that use ocean thermal and salinity gradients to generate electricity may also be investigated. However, these technologies currently only exist as conceptual designs, laboratory experimentation, and field trials. In general, even though waves, currents, and gradients contain substantive amounts of energy resources, there are significant technological and cost issues to address before such sources can contribute significantly to electricity generation. Storms and other metrological events also pose significant issues for hydrokinetic technologies.

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