The Snohomish County Public Utility District (SNOPUD) is developing a tidal energy demonstration and research project in Admiralty Inlet, located within Washington's Puget Sound. SNOPUD has applied for a Pilot FERC License for the Admiralty Inlet Tidal Energy Project (Project), which will involve the deployment and operation of two, six meter Open-Center Turbines. The turbines will be mounted on completely submerged gravity foundations and will be interconnected to the electrical grid on Whidbey Island. Other Project facilities include a subsea transmission cable, a cable termination vault, and a power conditioning and control building.
While the two turbines will produce a modest amount of energy, the primary purpose of the Project is to explore the feasibility of tidal energy as a generation resource. SNOPUD will use the information gathered from the Project to inform the potential further development of the Admiralty Inlet site and the potential development of other sites in and around Puget Sound. SNOPUD does not expect to install additional units as part of this Project; if installation of additional turbines is desired in the future, the SNOPUD expects to apply for a long-term, commercial FERC License.
The design of a tidal energy device is a key factor in the potential for environmental effects. Evaluation of the technology characteristics (Stressor in combination with the existing environment (Receptor) is important in the analysis of potential effects. Size, Scale (i.e., pilot, commercial), Mass, Configuration (i.e., spatial layout), Color, Brightness, Reflectivity, Pressure Changes, and Dynamic Characteristics (i.e., moving parts), are all key characteristics in assessing potential impacts of a given stressor.
The Project will generate electrical power from two 6-meter diameter Open-Centre turbines. The predicted peak power generated is approximately 370 kW for turbine 1 and 310 kW for turbine 2, for a total predicted peak capacity of 680 kW. The Open-Centre turbines feature a horizontal axis rotor with power off take through a direct drive, permanent magnet generator. The turbine is principally comprised of three components: an external venture, or duct; stator; and turbine rotor, with rotor blades retained entirely within the outer venture and a large open center. The design has no need for a gearbox or other complicated components requiring regular intervention.
The OpenHydro turbine is a designed to generate electric power based on the velocity of the tidal currents. Each turbine has 12 rotor blades, with a typical rotor speed between 3 - 10 rpm and a maximum rotational speed of approximately 14 rpm. Under normal operating conditions, no braking will be applied to slow the turbine as the water velocity is predictable and the turbine design allows for all conditions that could occur. However, in unusual circumstances, an electrical brake can be applied to stop the turbine rotation almost instantly.
Figure 1. Open Centre Turbine, developed and manufactured by OpenHydro Group Ltd.
Mounted on completely submerged gravity-based foundations, the devices will rise approximately 15 meters above the seabed. The foundation of each turbine will be approximately 25 meters in total length and approximately 21 meters in total width. The total seabed interface area (contact footprint) for each turbine will be approximately 10 square meters. The mass of the subsea base will be dependent on the site conditions and will be subject to final design. However, it is anticipated that the approximate mass will be in the order of 320 metric tons (steel - 150 metric tons, concrete - 70 metric tons, and natural ballast [stone] - 100 metric tons).
The Project site has a water depth of 58 meters. The turbines will rise 15 meters above the seabed and will, therefore, be present in the water column at a depth of 43 meters. The distance between the two turbines will be approximately 54 meters (from center-to-center of each device), and the total Project site will have a maximum footprint of approximately 20 square meters.
The project will transmit electrical power generated from the OpenHydro turbines to the onshore electrical grid via two, parallel subsea trunk cables. Each turbine will connect to its subsea trunk cable by way of a service cable connector that is integrated with the turbine assemblies. The subsea trunk cables, which are approximately 2 km long, will carry the AC power generated by the turbines, as well as turbine control and monitoring signals, environmental data and low voltage sensor power monitoring and control signals.
The cables are designed with a high Specific Gravity to assure that high currents along the route do not move the cables on the sea floor. The cables are likely to have the same dimensions as the one used with the OpenHydro turbine deployed at European Marine Energy Center (EMEC), which has a submerged weight of 18.4 kilograms per meter. If there are areas where the cables are suspended across depressions in the seabed or which have gravel or sand waves, the cables may have to be pinned to the bottom.
At the cable vault, the trunk cables terminate and are connected to the terrestrial buried cables (connection breakout point). This termination vault provides an accessible connect and disconnect working area for installation and in the event that the trunk cables need to be removed or disconnected for any reason. From the cable vault the individual cable cores will be broken out and pulled through separate conduits. The terrestrial cables will run from the termination vault through a buried conduit to the control room, which will house the power conditioning and monitoring equipment; the major equipment will include transformers, power inverters and conditioners, cabling, and Heating Ventilating and Air Conditioning systems. One conduit will contain the AC power transmission lines from Turbine One and a second conduit will contain the AC power transmission cores from Turbine Two. The fiber optic cable, low voltage power elements and the data and telemetry wire bundles will be in additional conduits.
Noise generated by the actual turbines to be utilized for the Project has not been measured, so the sound and acoustics produced by the Project is unknown at this time. However, analysis has been performed for the Acoustic Monitoring Plan, which estimates the broadband source level for a single 10-meter turbine operating at its maximum rotation rate (14 rpm) to be 161 dB re 1 μPa at 1 meter. Because there will be two turbines, the Project's acoustic emissions will be higher than the estimated noise level for a single 10-meter turbine. Snohomish PUD environmental analysis for the Project assumes that the turbines will act as coherent noise sources due to the nature of their electrical connection; this analysis suggests a maximum increase of 6 dB (relative to a single device) at points equidistance from the two turbines (this is not equivalent to a 6 dB increase in source level for both turbines).
Turbine 1 is expected have an average instantaneous power output for the year of 14.5 kW and an average output of 13.0 kW for turbine 2, for an average annual generation of 114,000 kilowatt-hours (kWh) and 102,000 kWh for turbines 1 and 2, respectively. This equates to an average annual generation of approximately 27.5 kW, or 216,000 kWh, for the Project. Peak power is predicted to be 680 kW for the Project. The OpenHydro turbines are shielded such that no Electromagnetic Fields (EMF) will be emitted from the turbine structure.
The subsea trunk cables will produce a magnetic field (B-field) that cannot be shielded; however, the level of the magnetic field dissipates quickly with distance. Using specialized software (FEMM - Finite Element Method Magnetics), an estimate of magnetic field was developed for the Admiralty Inlet Pilot Project. Assuming a three-core cable and maximum production of 1000 kW, it is anticipated that the cable would produce a magnetic field of 1700 A/m at the surface. However, the field declines rapidly, such that beyond a distance of 40 cm from the center of cable the magnetic field is less than that of the Earth's natural magnetic field.
Information & Resources
Documentation, including environmental analysis, baseline studies and post-installation monitoring plans, is available on the Admiralty Inlet License Application and Monitoring Plans page.