Sea Current

Maine researchers study the state's potential for tapping tidal power

May / June 2008

The timeless tug of the moon on the sea has long been a source of personal and professional fascination for Huijie Xue.

The University of Maine oceanographer grew up in Zhejiang Province, a coastal region of China that is home to that country's largest tidal range. The extraordinary surging tides of the Qiantang River, comparable in magnitude to those of the Amazon, every year draw thousands of people to witness this magnificent natural spectacle.

"I've always been very interested in tidal power. It is what got me started as an oceanographer," says Xue, who is now leading a group of UMaine researchers who are eager to explore the many facets of tidal power generation. Their hope is to make UMaine a leading source of public information about the nascent technology and its role in the larger energy picture for the state and the nation.

"The University of Maine is uniquely positioned to approach this kind of work," says Michael "Mick" Peterson, Libra Foundation Professor of Engineering and a member of the campus initiative. "We've got all the pieces right here. I don't think I've seen another issue that uses our combined strengths as well as this."

Their timing could not be better.

People have long dreamed of the energy-generating potential of tides. In the 1930s, President Franklin Roosevelt backed an ambitious scheme to build a series of tidal power dams between Maine and Canada, but the project was eventually scuttled by a skeptical Congress.

But now, with concerns about record-high energy costs, our nation's dependence on imported oil and the dire implications of global climate change, developers from Maine to California are scrambling to refine new, greener technologies that could turn the tides into sources of clean, renewable, predictable and relatively low-cost energy.

Last year, a study by the California-based Electric Power Research Institute (EPRI) of several potential tidal plant sites in North America determined that some of the most promising are off the coast of Washington County in Maine, specifically Cobscook Bay and the Western Passage of Passamaquoddy Bay, an inlet of the Bay of Fundy.

According to study project leader Roger Bedard, Maine's "world-class tidal resource," with its enormous range of 9 feet to 30 feet, is capable of producing electricity at a cost of 4.2 cents to 6.5 cents per kilowatt hour.

The report sparked a torrent of interest in the region among would-be tidal power developers, including the Passamaquoddy tribe, an engineer from Trescott, Maine, and Florida-based Ocean Renewable Power Co. (ORPC), which began testing its one-third scale prototype turbine in December in the powerful tidal flows of the Western Passage near Eastport.

ORPC received a $300,000 development award from the Maine Technology Institute to engineer its $1 million turbine module prototype, which the company thinks can generate as much as 25 kilowatts of power in a 6 knot current.

Although one of the proposed projects involves the use of a tidal dam or barrage, reminiscent of FDR's abandoned Depression-era project, the others are banking on a newer technology known as tidal in-stream energy conversion. A relative newcomer to the renewable energy field, it uses submerged turbines with blades turned by the currents in much the same way that wind moves turbines on land. Unlike wind or solar power, however, tidal power is entirely predictable; the position of the sun and the moon tells you just how much energy will be available, and when.

Yet because the technology is still in its infancy, similar to where wind power was two decades ago, tidal power poses many economic and environmental questions that scientists and regulators will have to answer before commercial projects can be successfully added to the renewable energy mix.

For instance, what turbine designs and materials are best suited to withstand the force of Maine's ocean tides? How will they be anchored to a seafloor whose composition can vary greatly from site to site? While one turbine submerged in a channel might not have a significant effect on the tidal flow and the local marine life it supports, what about an array of 200 or more that might be needed to generate enough power to make a commercial project economically viable?

Because much of the newest turbine technology is proprietary, gathering critical data to help educate the public about the devices and their possible effects on the marine ecosystem is a challenge that UMaine researchers hope to undertake in conjunction with Maine Maritime Academy in Castine.

MMA was recently issued a three-year preliminary permit by the Federal Energy Regulatory Commission to pursue its plans to establish a Tidal Energy Device Evaluation Center and to set up associated educational and research opportunities for students and faculty. The center would allow scientists to study what effects the turbines might have on the animal and plant life in the Bagaduce River in Castine, and perhaps apply that knowledge to other marine waterways where tidal energy projects are proposed.

"People are rushing to build right now without having the basic science," says Jarlath McEntee, the center's interim director. "With its focus on marine engineering and marine science, and its business school, MMA can bring certain skill sets to the table. But the research and development efforts are more appropriate for the University of Maine."

At UMaine, Huijie Xue began using sophisticated 3D computer models to examine the circulation characteristics of Cobscook Bay. Her goal was to determine how waste from aquaculture operations and oil from a tanker spill might be dispersed.

When EPRI was doing its survey of potential commercial tidal power sites in Maine, Xue and her student, Danya Xu, provided maps outlining the areas of highest density where turbines would be able to extract the most energy. Xue also is running longer-term computations for New Brunswick, Canada, energy officials to show how density distribution in the Quoddy region changes over time and how often it peaks.

"All of this is important to industry so they can set the system operation schedule for optimal power generation," Xue says.

EPRI estimates that about 15 percent of the tide's energy can be safely extracted without disrupting the current flow and, as a result, the marine life in it. But a real-world tidal power operation, perhaps with rows of submerged turbines turning in a channel, could complicate the picture in ways that science has yet to understand.

"We should investigate whether slowing the current in the Western Passage, for example, would change flows in other passages in the Quoddy region," Xue says.

And that's where Kiran Bhaganagar comes in. An assistant professor of mechanical engineering, she is one of only 20 or so people in the world skilled in a computer modeling technique called direct numerical simulation (DNS). Using a geometrical mesh, with some 10 million grid points, DNS allows her to simulate flow around physical structures. In her lab, a supercomputer running nonstop for two or three days can determine the velocity of fluid motions, despite severe turbulence, from which power is extracted.

Turbines can then be introduced into the equation to determine how they alter the character of the current.

"We got interested in this because no one had looked before at how flow is affected around a turbine," Bhaganagar says. "In reality, there is much large-scale mixing and activity going on. This is very critical for fish, so we're looking at what would be an optimal turbine system."

She believes that coupling her data with Xue's ocean circulation model would create an extraordinarily valuable resource for the development and teaching of tidal energy.

"Everyone here at the university now wants to look at the same problem from all different angles," she says. "We're trying an extensive collaboration. Science to technology may take two to three years of vigorous work, but then we'd have the software that companies could use. This could bring new industry and jobs to Maine."

In Mick Peterson's lab, mechanical engineering graduate student Ronnie Oliver, who is being advised by UMaine mechanical engineer Michael Boyle and Rich Kimball of MMA, is working on a computer model of a propeller design that can be adapted for use as a tidal turbine. Undergraduate students used the model to build a turbine and test its power-generating potential in a 120-foot tow tank.

Meanwhile, at the Advanced Engineered Wood Composites Center, Robert Lindyberg, the assistant director for boatbuilding and composites, is working with his industry partners to identify tidal generation systems with the potential to use composite materials in their designs.

"When you consider global warming and the finite supply of oil, renewables will dominate the energy discussion in the years to come," Peterson says. "It's important now for the University of Maine and Maine Maritime Academy to contribute to and benefit from this new direction."

by Tom Weber

Back to Research Stories