The Hydrokinetic Power Resource in a Tidal Estuary: The Kennebec River of the Central Maine Coast

As we look to the world for sources of renewable energy, every bit we can harness is important.  Unlike our current system of high volume power generators, the grid of the future increasingly seems to be composed of many small generators working together in harmony.  One such source we have the technology to take advantage of is tidal power.  The power level available from the kinetic energy of tidal flows, in areas with relatively mild tidal ranges along coast or estuaries, can still be significant.  Using a numerical circulation model, David A. Brooks calculates available power peaks in one such region, the central Maine coast.  In recent history, the implementation of tidal power plants has been blocked by issues such as: cost of construction and maintenance of the requisite dams, gates, fishways and locks, as well as concerns about impacts on fisheries, navigation and a host of other environmental issues.  However, with dwindling fossil fuel supplies and global warming concerns looming, and turbine technology improving, tidal power has rekindled society’s interest.  Also, tidal power, as opposed to other hydrological power sources, does not require large impoundments such as dams, resulting in reduced installation and operational costs and minimal environmental impacts.—Donald Hamnett
Brooks, David A., The hydrokinetic power resource in a tidal estuary: The Kennebec River of the central Maine coast. Renewable Energy 36 (2011) 1492e1501.

            Brooks examined the central Maine coast because the Gulf of Maine and the adjoining Bay of Fundy are known for resonant semi-diurnal tides.  The range of these tides exceeds 15 meters at the head of the bay, but the mean for the entire coast is a smaller 3 meters.  In the nearby confined parts of river estuaries, narrow interconnecting passages, and between nearshore islands the tidal currents can exceed 2 meters per second.  To simulate the tidal, riverine, and wind-driven circulation of the coast, Brooks used a three-dimensional hydrodynamic model known as MECCA, or Model for Estuarine and Coastal Circulation Assessment.  On a three-dimensional grid the fields of velocity, temperature, and salinity are calculated using conservation of mass and momentum equations.  The forcing is specified at ocean boundaries, inshore points, and the free surface by tides.  The bathymetric data was provided by the National Geophysical Data Center, along with the model coastline data.  Velocities above 2 meters/second were found at the mouth of the Kennebec River, Bluff Head, and a few other points.  This is the velocity at which the power produced from the tide’s kinetic energy is 3 kilowatts per square meter.
            The kinetic tidal power widely available in regions with moderate tidal ranges, and is dispersed throughout these areas.  This attribute contrasts tidal with traditional forms of energy, which is focuses high quantity dependable generation in a few small sites.  The centralized nature of traditional power requires extensive distribution grids, and is a security threat as the power for a wide area could be taken out by problems with just one power plant.  Though tidal power has this advantage over conventional generation, it would require electrical grids that can accept and blend multiple power pulses.  From the model’s calculations, hundreds of megawatts of peak power are associated with the central Maine coast’s tidal systems, some of which could be practically harnessed.  In fact, the most promising site had a maximum power density of 6.5 kilowatts per square meter, in the lower Kennebec estuary near Bluff Head.  Were this resource harnessed and connected to the grid using a 500 square meter sub-region of the channel, this could supply the energy needs of about 150 typical homes, consuming 1.5 megawatt hours a month.  Further study into the monthly vertical and horizontal structure of the tides would be required to fully grasp the power potential of these coastal regions.  Tidal power as a small power generation tool has the potential to be another piece of the smart grid of the future.

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