Tidal energy in Puget Sound
Scientists have identified the strong underwater currents of Puget Sound's Admiralty Inlet as a potential source of electricity for nearby utilities. The following article describes some of the basic principles and mechanisms of tidal energy.
What is tidal energy?
Tidal energy is an approach to generating electric power from tidal currents in a manner similar to wind energy. At a basic level, you can think of tidal turbines like underwater windmills. Tidal energy is a subset of marine renewable energies, which include:
- Wave energy: power generation from wind-driven waves in open-ocean or near-shore environments
- Offshore wind energy: power generation from atmospheric winds in deep water
- Tidal barrage energy: power generation from the rise and fall of the tides, achieved by damming estuaries
- Ocean thermal energy conversion: power generation from the temperature difference between the ocean's surface and depths
Tidal currents are caused by changes in tidal elevation which are, in turn, caused by the gravitational interaction between the sun, moon, and oceans. Because this gravitational force is predictable, so is tidal energy. This predictability makes tidal energy attractive in comparison to other, stochastic, forms of renewable energy.
Where is tidal energy being developed?
While tidal currents ebb and flood around the world, tidal energy requires extreme currents to generate cost-effective power. In oceanographic terms, currents of 1 m/s (2 knots) are exceptionally strong, but these are barely strong enough to start up a tidal turbine. Currents at the most promising hydrokinetic sites may regularly exceed 4 m/s. These conditions occur when tidal currents are funneled through narrow channels. By their nature, such constrictions are relatively small-scale topographic features and strong tidal currents tend to be localized. In contrast, wind or wave energy are distributed resources, spread uniformly over large coastal areas.
Most tidal turbine demonstrations to date have been in Europe, particularly at the European Marine Energy Centre in the Orkney Islands north of Scotland. In addition, several tidal turbines have been demonstrated in the United Kingdom, Canada, France, and Korea. In the United States, a small array of turbines has been demonstrated in the East River of New York City and a single turbine has been demonstrated in Cobscook Bay, Maine. Other projects have been proposed in Puget Sound, Washington, Maine, New York, Alaska, and California.
How do tidal turbines work?
There have been about a dozen pre-commercial technology demonstrations around the world. The largest of these turbines have a rated electrical capacity of approximately 1 MW and are often referred to as "utility-scale", while the smallest may have eletrical capacities of no more than a few kW.
- Rotor: The mechanism by which kinetic power in tidal currents is converted to mechanical power, such as a rotating shaft. Most tidal turbines have rotor blades that operate on the same principle of lift as a wind turbine or airplane wing. The largest tidal turbines deployed to date have rotors up to 20 m in diameter. Unlike wind turbines, the speed of a turbine rotor is constrained by cavitation. If the pressure along the rotor blade drops below the local vapor pressure of water, vaper bubbles will form and then collapse. These would degrade the turbine's hydrodynamic efficiency, potential damage to the rotor, and produce loud sounds. Consequently, tidal turbines are designed and operated in such a way as to limit the potential for cavitation.
- Power train: The mechanism by which mechanical power is converted to electrical power. This always includes a generator and may include a speed increasing gearbox or hydraulic accumulator.
- Foundation: The structure that positions the turbine in the water column and resists the forces on the rotor. Gravity or pile foundations are most common, but some turbines are deployed from semi-submerged structures. With a few notable exceptions, tidal turbines do not have surface expressions. Compliant moorings, which anchor a turbine or turbine array to the seabed using cables, are a novel foundation type that could reduce the cost of energy and improve the reliability and survivability of turbines.
- Power cable: Once electrical power has been generated, it must be transferred back to shore for end-use.
What are the environmental impacts?
Given the limited number of tidal turbines that have been deployed globally, significant uncertainties remain. These are often interactions between stressors (e.g., sound produced by a turbine in operation) and receptors (e.g., marine mammals) that could be ecologically significant if they occur at high frequency, but insignificant if they never occur or only occur rarely.
Summarizing the current state of understanding for particular environmental stressors:
- Dynamic turbine presence: Because of the obvious analogues to wind energy, conventional hydropower, and tidal barrages, concerns about blade strike or collision have often been raised. However, both laboratory and field observaions conducted to date suggest that avoidance rates are high and mortality rates are low (often too low to be statistically significant in controlled exposure experiments). These preliminary findings need to be verified across a broader set of locations and technologies. Aggregation and avoidance, as well as attendent predator-prey interactions, have received limited study to date.
- Static turbine presence: Over time, most surfaces that are not otherwise protected will develop into artificial reefs. These may add habitat for desirable species or allow non-native species to establish a foothold. Depending on the seabed composition, scour may occur at the contact points between a turbine's foundation and the seabed.
- Sound: Tidal turbines will produce broadband sound in operation. While this is not expected to rise to a level that could cause acoustic injury, behavioral responses by fish and marine mammals, including avoidance, are possible. There have been limited studies to characterize the sound from tidal turbines, but several studies are planned in the 2014/2015 time frame.
- Chemicals: While chemical coatings or lubricants may act as a point source of pollution, the most significant chemical stressor is likely to be a major fuel spill from an installation or maintenance vessel.
- Electromagnetic fields: Electromagnetic fields can affect several species of fish and invertebrates, but the electromagnetic fields from tidal turbines have not been extensively studied.
Who might be affected?
Cost-effective power generation by tidal turbines typically requires currents that routinely exceed 2-3 m/s and water depths greater than 40 m. Such locations are relatively uncommon, resulting in unique stakeholder/user communities relative to more generic coastal and estuarine environments.
Stakeholders with existing interests in tidal energy sites may include:
- Native tribes with usual and accustomed treaty fishing rights
- Recreational and commercial fishing interests
- Commercial navigation
- Military training and transit
- Marine telecommunication and power cable operators
To date, there have been few investigations into social aspects of tidal energy or marine renewable energy, in general.