Home - Working Tidal
One of the biggest questions community members and developers alike want to know is just how much in-stream tidal energy is in the Gulf of Maine. For investors, the answer to that question can justify the costly up-front capital associated with a tidal energy project. For coastal residents, it comes down to a trade-off between a potential source of alternative energy and changes to the natural environment in which they live and work.
“Right now, the estimate of the size of tidal energy is that it’s going to be bigger than terrestrial wind and it’s going to be smaller than offshore wind in Maine,” says Peterson. “We don’t know for sure how big it’s going to be because it depends on details of how we design arrays of turbines. And that’s why (UMaine Professor of Oceanography) Huijie Xue’s work is so critical to the overall effort. She’s got the potential to build the skill set for the state of Maine, to tell us how we install these arrays.”
To understand just how much energy is available, the tidal flow variability at proposed sites must be examined using large-scale numerical models. A flow faster on one side than another can create extra strain on a turbine, ultimately impacting durability and efficiency — energy production. In addition, bottom velocity can vary.
Researchers also must quantify what effect energy extraction will have on the tidal flow and, subsequently, the marine environment, which depends on the strong tide to flush nutrients, pollutants and waste in intertidal areas, affecting water quality and the diversity of the ecosystem.
Understanding wake decay — basically, how far behind the turbine is there an effect — is a major focal point for the tidal energy industry.
“From a resource point of view, how much (energy) can you take from a particular bay that won’t significantly impact it?” asks Xue. “Imagine having a row of turbines in a water passage. By taking energy out, the obvious response is for the flow to slow down. When turbines take the energy, what difference does it make in the flow field near the turbines? And at what distance does the flow rate recover for the next row of turbines in an array?
“In terms of fisheries, the concern is not only fish going through the turbine, but changes in circulation that could change fish recruitment (the number of fish surviving the larval and juvenile phases to enter the adult population each year),” Xue says.
As a physical oceanographer, Xue studies the tidal currents to provide baseline data on the energy resource and suitable sites for development. In Maine coastal waters, nearly a dozen sites could be considered for tidal energy projects based on the strength of their currents. They include Castine Harbor, Taunton Bay, the Kennebec River, Cowseagan Narrows, Outer Cobscook Bay and Western Passage. All are part of the Gulf of Maine and connected to the Bay of Fundy, where nearly 10 percent of the energy dissipation in the North Atlantic occurs, says Xue.
Initially in the tidal energy research, Xue’s models focused on estimating energy density in the gulf’s natural flow. Now the questions revolve around modeling the insertion of turbines in the flows, which turbine design would be most efficient and effective at specific sites, and how to optimize deployment.
“The rise and fall of the water levels in the Bay of Fundy are fascinating,” Xue says. “It’s the amount of energy in those waters and in concentrated areas like the Gulf of Maine that makes the tidal phenomenon spectacular.”