Hydrokinetic energy is the energy generated by the movement of a body of water. The earth's tides, waves, ocean currents and free-flowing rivers contain an untapped, powerful, highly-concentrated and clean energy resource. Traditional hydropower (river dams and conduits) is also produced by moving water, but is described here. A variety of hydrokinetic energy sources are described below.
In-stream Hydrokinetic Energy: In-stream hydrokinetic projects generate electricity directly from the flow of water in rivers, inland waterways, irrigation canals and other man-made conduits. Energy-generating mechanisms may be placed as individual units or in arrays. They may be submerged, floating or tethered to existing structures like bridge abutments or other in-water infrastructure. Some are placed downstream from existing hydropower projects.
In-stream hydrokinetic projects generate energy from the horizontal flow of. These energy-generating projects use a variety of technologies, including:
- Horizontal axis turbines
- Vertical axis turbines
- Helical turbines that may be either vertical and horizontal
As the water current passes through the turbine (rotor fan), the turbine rotates on a shaft connected to the generator. The rotational speed is proportional to the velocity of the fluid.
Horizontal axis turbines may look much like wind turbines, and they extract kinetic energy from the moving water in the same way that wind turbines extract energy from moving air. Sometimes the rotor is enclosed inside a duct (often funnel-shaped) to concentrate the flow past the turbine. This may allow operation over a greater range of current velocities and generate more electricity per unit of rotor area. In vertical axis turbines, the axis of the rotor is oriented perpendicular to the flow. Vertical axis turbines may take many different forms and also may be enclosed within a duct.
Marine Hydrokinetic Energy: The ocean produces two very different types of energy: (1) thermal energy from the sun's heat, and (2) mechanical energy from the tides and waves. Tides are driven primarily by the gravitational pull of the moon and waves are driven primarily by winds. Tides and waves are therefore intermittent sources of energy while ocean thermal energy (from the sun) is fairly constant. The electricity conversion of both tidal and wave energy usually involves mechanical devices.
Wave Energy: Wave power devices extract energy directly from the surface motion of powerful ocean waves or from pressure fluctuations below the surface. The greatest potential for significant wave energy production is on the west coast of the U.S.
A variety of technologies have been proposed to capture wave energy. Some of the more promising designs are now being tested at commercial scales. Wave technologies have been designed for nearshore, offshore and far offshore locations. All wave energy technologies are intended to be installed at or near the water's surface, in waters more than 40 meters deep. These technologies differ in their orientation to the waves with which they are interacting and the manner in which they convert wave energy into electricity. The following wave technologies are currently being explored:
- Terminator devices extend perpendicular to the direction of wave travel and capture or reflect the power of the wave. These devices are typically onshore or nearshore, but floating versions have been designed for offshore applications.
- A point absorber is a floating with components that move relative to each other due to wave action (e.g., a floating buoy inside a fixed cylinder). The relative motion is used to drive electromechanical or hydraulic energy converters.
- Attenuators are long multi-segmented floating structures placed parallel to the direction of the waves. The differing heights of waves along the length of the device causes flexing where the segments connect, and this flexing is connected to hydraulic pumps or other converters.
Tidal Energy: Tidal and ocean current energy can be exploited by: (1) building semi-permeable barrages (dam-like structures) across estuaries with a high tidal range, or (2) harnessing offshore tidal streams. Unlike wind and waves, tidal and ocean currents are fairly predictable. During high tides, barrages allow tidal waters to fill an estuary through sluices which then close when the tide begins to fall. Once the tide is low enough, the stored water is released at pressure through turbines. The Pacific Northwest, Alaska, and the Atlantic Northeast are likely areas for tidal energy production.
Tidal energy may be harnessed using offshore underwater devices similar to wind turbines. Submerged rotating devices capture energy through the processes of hydrodynamic lift or drag. These devices consist of rotor blades, a generator for converting the rotational energy into electricity, and a means for transporting the electrical current to the on-shore electrical grid. Submerged turbines can have either a horizontal or vertical axis of rotation. Mechanisms such as posts, cables or anchors are required to keep the turbines stationary relative to the tidal currents. Prototype horizontal axis turbines, similar to wind turbines, have been built and tested. Vertical axis turbine designs have been proposed, with some designs resembling egg beaters.
Turbines may be anchored to the ocean floor in a variety of ways. They may be tethered with cables, using the relatively constant current interacting with the turbine to maintain location and stability. Imagine an underwater kite flying, where the kite is the upright turbine and the kite flyer is the anchor. The system may also include concentrators (or shrouds) around the blades to increase the flow and power output from the turbine. Another proposed design is mooring a barge in the current stream with a large cable loop to which water-filled parachutes are fastened. The parachutes would be pushed by the current and then closed on their way back, forming a loop similar to a large horizontal waterwheel. In large areas with powerful currents, it may be possible to install water turbines in groups or clusters to create a "marine energy facility" (similar in to a wind energy facility). Turbine spacing would be determined based on wake interactions and maintenance needs.
Fish and Wildlife Considerations
Careful site selection is critical to minimizing the environmental impacts of hydrokinetic power systems. There currently is a limited understanding of the environmental impacts of in-stream, tidal, ocean current or wave hydrokinetic energy production because few of these projects are operational. All hydrokinetic energy devices may impact animal behavior, altering migration or other movement. Concern exists over impacts to benthic habitat, including fish foraging habitat, caused by the anchoring of underwater structures. Another concern is the effect of underwater noise and vibration. Underwater turbines may also cause entrainment (being sucked into the turbines) or impingement (pinned against a structure) of fish, birds, aquatic mammals and reptiles.
Large scale in-stream hydrokinetic projects have the potential to alter in-stream hydraulics (water movement and pressure), sediment transport and deposition, and other river characteristics. This may impact habitat quality and quantity both upstream and downstream of the project. Freshwater mussels, particularly threatened or endangered mussels, may be adversely affected by the redistribution of sediments or the increased mortality of fish host species resulting from entrainment. The potential cumulative effects of in-stream hydrokinetic energy production are unknown but could be significant. In-stream hydrokinetic energy projects are being intensively explored in the Mississippi, Missouri, Penobscot, St. Lawrence and Niagara rivers.
Concerns in marine hydrokinetic systems include impacts to marine animals sensitive to electric and magnetic fields, reduction in size of intertidal areas and collision with, or avoidance of energy generating devices. Some wave energy designs have features that extend many feet above the water surface and pose a potential collision risk for seabirds. The potential impact of large-scale tidal, ocean current and wave energy projects on energy loss in the marine or estuarine ecosystem is poorly understood. Altering energy dynamics in an aquatic system may alter wave or current patterns and influence sediment transport and deposition. Possible indirect effects include displacement of mobile fauna and alterations in food availability. This may have the potential to affect reproduction of species at higher trophic levels. Shoreline impacts may result from construction and operation of infrastructure needed to transport the electrical current to an on-shore electrical grid.
