With concerns about the continuity and future pricing of natural gas supplies in Southcentral Alaska, could renewable energy sources such as wind or tidal power alleviate some of the region’s energy problems? Chugach Electric Association, for example, has investigated the possible development of a wind farm on Fire Island, adjacent to Anchorage International Airport.
In June the Electric Power Research Institute (commonly known as EPRI), a California-based non-profit research organization, published a detailed report on the feasibility of a tidal power plant in the Knik Arm, in the narrows between Cairn Point and Port MacKenzie. And in the Oct. 8 edition of Petroleum News we reported that Natural Currents Services LLC is investigating the construction of a tidal energy facility at that Knik Arm site — Natural Current Services is one of three companies interested in tidal power in the Cook Inlet area of Alaska.
In-stream energy
According to EPRI, existing operational tidal power plants such as a 340-megawatt plant in France depend on dams that impound the tidal water and then release the water through turbines. But for Knik Arm, EPRI envisages the use of a new technology that does not require a dam.
Instead, tidal in-stream energy conversion (or TISEC) devices would use the natural tidal current to drive turbines coupled to electrical generators. A typical tidal power plant would involve a farm of multiple, underwater TISECs. Depending on the TISEC design, each TISEC unit may be rigidly fixed in place under the water surface or it may float inside the water column, tethered by a cable attached to the sea floor.
TISEC technology is evolving through a pre-commercial research phase into the production of commercially available devices that should be capable of delivering electricity in quantities comparable to conventional electricity generation systems. And, alongside the evolution of TISEC technology, the science of free-flow tidal current power is also becoming established.
A key component of that science is an understanding of the total energy resource that may be available at a particular tidal energy site. That total energy consists of what is known as the power flux of the tidal current, integrated across the complete area of the site. The power flux (generally reported in kilowatts per square meter) describes the amount of energy contained in the tidal current at a point in the site and is a function of the cube of the water velocity. This relationship to the cube of the water velocity makes the speed of the tidal current a critical factor in determining the available energy resource.
Knik Arm
Obtaining a high power flux requires both a large tidal range and some kind of channel that will cause a fast flowing tidal current. But the channel also needs to be large enough to provide a sufficient area of high power flux to build up a high total power output — “tiny channels with high power flux are of little use for tidal power generation since the overall tidal resource is quite small,” the EPRI Cook Inlet report says.
The Knik Arm between Cairn Point and Port MacKenzie meets all of the essential requirements for a good site. The currents from the massive regional tides accelerate through the relatively narrow part of the arm at Cairn Point. And a deep channel provides a sizable area in which to deploy TISEC devices — the depth of the channel would also be critical in enabling adequate clearance between the devices and both surface ice and any shipping traffic in Knik Arm.
The report says that there is another deep channel with fairly high power flux west of the Port of Anchorage, but that channel lies under a major shipping lane and has limitations caused by winter sea ice. And, although the site northwest of Cairn Point seems suitable for tidal power, there are several issues that would need to be resolved. Those issues include avoidance of any impact on Beluga whales; dealing with large-scale eddies; and working and operating in very turbid water.
National Oceanic and Atmospheric Administration data for the Cairn Point site indicates a depth-averaged tidal current velocity of 1.1 meters per second, giving rise to a depth-javeraged power flux of 1.8 kilowatts per square meter. The EPRI report says that the total width of the Knik Arm at Cairn Point is 2,540 meters. The cross-sectional area of the deeper water within which the TISEC devices would be deployed is approximately 73,200 square meters.
Multiplying power flux and channel size data results in a total channel power of 116 megawatts. But studies of tidal power implementation have established a rule of thumb that only about 15 percent of the total channel power can be converted to electricity without having a detrimental impact on the natural ecology of the site. That 15 percent factor results in an extractable power output of 17 megawatts for Knik Arm.
Could a tidal power plant of this size prove viable? The potential power output is quite modest when compared, for example, with the more than 350 megawatt power rating of the gas-fired Beluga power station on the west side of Cook Inlet.
From an economic perspective, the Knik Arm tidal power site has some factors in its favor. It lies next to an existing electrical infrastructure at Elmendorf Air Force Base and that, in turn, ties into the electrical grid in Anchorage. And the proximity of the Port of Anchorage would reduce construction costs.
However, the EPRI report says that the current 35-kilovolt line voltage available for the backhaul of power from the Elmendorf grid into the Anchorage grid is too low to support the maximum power output of a tidal power station in Knik Arm. So, the report says that an upgrade of the Elmendorf to Anchorage line to 115 kilovolts would be necessary.
Two designs
The report describes two different Knik Arm power station designs, each involving a different type of TISEC device.
One type of device, known as the Lunar Energy Rotech Tidal Turbine, consists of a multi-blade propeller-style turbine inside a cylindrical duct. The complete structure would be installed on a concrete base on the sea floor. A commercial version of this type of device ought to be able to develop a maximum output of about 1 megawatt in the Knik Arm setting, according to the report.
The other type of device, known as a Marine Current Turbine Seagen free flow water generator, involves turbine blades attached to a pair of wings that extend horizontally from a vertical pile that is set into the seabed. In the Knik Arm each of these devices would likely produce a maximum output of 759 kilowatts.
Obtaining the 17-megawatt power output for a commercial operation from either TISEC design would clearly involve the installation of multiple devices. And the power station would require an armored subsea electrical cable to transmit the electrical power onshore. Bringing the cable to a suitable onshore location might require directional drilling, the report says.
The EPRI analysts did a cost analysis of the use of Marine Current Turbines and determined that a full-scale power station would require 66 of these devices arranged in seven transects northwest of Cairn Point. The estimated cost of the complete installation, including the subsea cable and the onshore electrical connection, works out at about $109 million in 2005 dollars. And the annual operation, maintenance and insurance cost would be about $4 million.
The analysis assumed that the cost of any necessary power grid upgrades would be recovered from line usage charges.
The analysts then considered the economics of three different types of ownership for the power plant — a regulated utility, a municipality-owned utility and a private operator.
An electric utility can set electricity rates that cover operating costs plus some reasonable level of profit, the report said. Under that scenario the cost of electricity would work out at 9.2 cents per kilowatt-hour, using inflation-adjusted money.
A municipality-owned utility also needs to be able to recover its operating costs, but it can fund projects from tax-exempt bonds. Under that scenario the cost of electricity would be 7.1 cents per kilowatt-hour.
Both of these results assume the use of government renewable energy incentives of a type that is available for wind power.
Assessing the viability for a private operator is difficult because the assessment depends on determining a rate of return on capital, adequate to compensate for the inherent risks of the project and to cover the cost of the capital. However, the EPRI analysis indicated that the cost of electricity from a privately operated Knik Arm power plant would be higher than the average industrial wholesale electric rate from other sources (the analysis assumed a 2005 wholesale price of 8.6 cents per kilowatt hour, declining through 2011 and increasing thereafter). The inability to match that wholesale rate makes it impossible to calculate an internal rate of return for a privately operated plant, the report says.
But the report concludes that an in-stream tidal power plant may provide favorable economics for either a municipal utility or a utility generator, when compared with other locally available renewable energy production options.
“In-stream tidal current energy shows significant promise for Knik Arm and represents a way to make sustainable use of a local renewable resource without the visual distractions that delay so many other energy projects,” the report says.
The EPRI report is available at www.epri.com/oceanenergy/streamenergy.html#reports.
