Cairn Point at Knik Arm Tidal Energy Site

Cairn Point is potentially a good location for instream tidal power generation, as strong tidal currents occur four times a day, and it is adjacent to significant electrical infrastructure at Elmendorf AFB and Anchorage. Cairn Point is located about two miles north of Anchorage in Knik Arm, in upper Cook Inlet. At Cairn Point, water depths exceed 150 ft, and the flow through Knik Arm is constricted. The constricted flow, along with Cook Inlet’s large twice-daily tidal range, combine to produce high water velocities. Tidal currents average 2.0 knots with peaks of up to 7.5 knots. They are some of the strongest currents in Cook Inlet.

Challenges to successful deployment of an in-stream energy device at this site include seasonal ice, a high level of sedimentation in the water, a shifting seabed (scour), and also concern about impacts to marine mammals, particularly to the local population of Beluga whales. Due to the presence of seasonal pack ice, including both submerged and surface frazil ice as well as large blocks of beach ice (mixed sediment and ice), the support structure for turbines and the turbine profile must be completely submerged. Access to the turbine site for maintenance will likely not be possible from about November until breakup in March.

The EPRI system feasibility study considered two types of in-stream energy generation devices, the Lunar (RTT) and the Marine Current Turbine (MCT), installed in arrays that would produced an average of 17 MW of power with little environmental impact. 17 MW is the equivalent power used by about 12,000 homes, each using 1.3 kW. The array layout is determined by spacing rules designed to reduce cyclic blade stresses from seabed boundary layer effects, to prevent ice impacts, and to prevent lateral and downstream interaction between rotors.

The size and depth of a monopile into the seabed are determined by current and seabed properties. Site-specific surveys of water flow and geotechnical conditions will be required. The proposed installations could be accomplished from a derrick barge that would also be used to install monopile foundations and a remote-operated vehicle (ROV). The ROV is used to monitor subsea operations, provide visual inspections, and carry out tasks such as connecting and disconnecting guide wires and electrical cables. The ROV can reduce cost and increase safety by eliminating the need to use divers in high current conditions.

The 17 MW size of the project was based on restricting power extraction to about 15% of the total available power to minimize degradation of the marine environment. If more of the resource can be extracted without degradation to the environment, then energy costs could decrease. Grid interconnection would be through a subsea cable (approximately 3000 ft long) to a shore-based substation, then to Elmendorf AFB’s electric grid for transmission to Anchorage Municipal Light and Power.

Cost estimates in 2005 dollars are $110 million for the capital costs (not including a $3.25 million transmission upgrade), with an annual O&M cost of $4.1 million. This translates into nominal levelized costs of electricity of about 10.8¢/kWh for utility generation, and 8.4¢/kWh for municipal generation. This assumes energy incentives equal to those that the government provides for wind energy technology. At this point, Ocean Renewable Power Corporation has secured a FERC preliminary permit for the Cairn Point area. ORPC plans to install its own Ocean Current Generator (OCGen™) module consisting of four Turbine Generator Units (TGUs), each with 4 Advanced Design Cross Flow (ADCF) turbines mounted to a permanent magnet generator on a single shaft.

The system will be moored to the bottom with anchors and weights rather than pilings. It will be located at least 40 feet beneath the surface of the water to avoid conflicts with marine vessels, ice, and debris. ORPC plans to install a 1 MW pilot project in 2011, to increase to 5 MW after a year of testing and monitoring, and to increase to a final project size commensurate with the resource and energy market for the long term.

Additional information for the tidal in-stream power generation concept installation for Cairn Point can be found in the EPRI report [1].

1. Polagye, B. and M. Previsic (2006) System Level Design, Performance, Cost and Economic Assessment – Knik Arm Alaska Tidal In-Stream Power Plant. EPRI – TP – 006 AK, 126 pp.
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