Taking full advantage of the tides

Posted May 10, 2009, at 7:37 p.m.

The May 4 Bangor Daily News editorial “A Rising Tide” accurately illustrated the potential of a clean, renewable source of electricity in Cobscook Bay and Passamaquoddy Bay. The use of tidal mills in the region with an average range of 18 feet was in existence more than a century before Dexter Cooper proposed a series of dams to utilize the potential energy stored in the falling and rising tides. The most efficient mills also used dams or barrages to hold back tidal waters in order to coordinate production with the time of either high or low tide.

At present, there are at least four proposals in the Cobscook Bay area to generate electricity from tidal currents and only one proposal based on the construction of a dam to retain tidal flow. Tidewalker Associates has proposed a dam at the entrance to Half-Moon Cove to generate sufficient electricity for up to 10,000 homes on an intermittent but predictable schedule. Tidewalker Associates also has investigated the feasibility of current-driven devices. We have concluded that a tidal dam is a more efficient method of generation with comparable environmental impacts under normal conditions. In order to generate the same amount of electricity as a tidal dam, 400 16-foot diameter devices would have to be installed over an area encompassing several square miles.

Three years ago, the Electric Power Research Institute released a report outlining the potential of current-driven devices. EPRI is an organization supported by the major power producers and utilities in the United States. The term hydrokinetic energy is used to characterize the nature of current devices. In a similar manner, the term aerokinetic devices could be used to describe windmills or wind turbines. EPRI’s initial estimates for economic feasibility have been generally criticized as being too optimistic along with a growing sense of uncertainty on their favorable generalization of environmental impacts. The EPRI report provided an immediate surge of interest in this technology as reflected by a dramatic increase of permit applications filed with the Federal Energy Regulatory Commission, which have been generally focused on rivers due to the availability of one directional flow in a less corrosive and dynamic environment.

The unique resources of Cobscook Bay and Passamaquoddy Bay refer to tidal ranges that vary from neap tide to spring tide on a periodic lunar cycle and that average nearly 19 feet. A tidal dam is able to stabilize production during a lunar cycle by controlling the difference in elevation between the tidal basin and the natural level of Cobscook Bay. Conversely, a current-driven device is completely dependent on the water velocity. A comparison between a tidal dam and current-driven devices points out the engineering advantages of the Half-Moon Cove project over typical hydrokinetic operations.

The Coast Guard project mentioned in the editorial is an extremely noteworthy initiative. Due to the research nature of the effort, the production cost of the selected system will be higher than conventional electricity as measured in cents per kilowatt-hour, but it still will serve as an indicator of economic feasibility. Ironically, for an area characterized by the largest tidal range on the east coast of the United States, the variation in production during a neap tide to spring tide cycle will have to be factored into the assessment of a hydrokinetic device. The Coast Guard project will provide publicity on a renewable resource along with valuable engineering, economic and environmental data, but it will not take full advantage of the potential contained in the rising and falling tides of Cobscook Bay.

Tidewalker Associates thinks that Half-Moon Cove is a unique option for Cobscook Bay and represents an ideal location to reap a number of direct and indirect benefits. By connecting the dots, the Half-Moon Cove tidal barrage project has the potential to bridge economic stimulation objectives with renewable energy development while partially eliminating regional dependence on fossil fuel for at least the next two to three generations. The proposed tidal power barrage at Half-Moon Cove fits in nicely with the development mix of the region in the same way that hydro-kinetic devices has value as a renewable source of energy.

Normand Laberge, Ph.D., P.E., is the founder of Tidewalker Associates and lives in Trescott. Additional information on the project is available at www.mainetidalpower.com.

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