From the East River walkway near his office on Roosevelt Island, Ron Smith looks across to Manhattan's East Side shoreline, to the hulking warehouses and exhaust stacks of an oil-and-gas power plant, today's way of keeping the city's lights burning.

But in the swift-flowing tidal waters in between, Smith sees the future of energy generation. He and partner Trey Taylor are co-founders of Verdant Power, which generated electricity from underwater turbines in the river as part of a pilot project last year and will install 30 new turbines at the site this spring.

"The 1 megawatt these will generate is trivial," he said. "But by 2020 to 2040, these kinds of technologies will be all over the world."

Verdant Power is among companies worldwide in the forefront of exploring the potential for generating power from the tides, a form of renewable energy with some distinct advantages over wind or solar energy, but also with some unique challenges. In one reflection of the growing interest, the Federal Energy Regulatory Commission noted last year that it had issued 100 preliminary permits for various tidal sites in the United States. Just this month, FERC announced an agreement with the Coast Guard to cooperate on navigation and other issues arising from tidal, wave and river current energy projects.

By some estimates, these technologies could one day supply as much as 30 percent of U.S. power needs, but there are a lot of uncertainties about how much of that potential actually can be realized, said Paul Jacobson, senior project manager at the Electric Power Research Institute.

"It's going to be a niche technology," he said. "But in the places where the conditions are good, it can be very attractive."

Alaska, Maine and Washington state have the U.S. sites with the greatest potential, followed by other coastal states including New York and Massachusetts, according to a 2011 Georgia Tech-U.S. Department of Energy report. Because tides move at regular cycles, tidal generators provide a predictable source of power, a distinct advantage over the fluctuating winds and cloudy days that can disrupt wind or solar arrays, Jacobson said.

"When the power is predictable, it's easier to plan to bring other things on line when needed," he said.

One of the main challenges of tidal power, though, is the harsh, corrosive sea environment.

"There is a lot of power in these tidal currents, but some of the devices deployed have promptly broken because of the amount of energy," Jacobson said.

Other major challenges are the long lead time and the significant investment required to get projects beyond the conceptual stage. Verdant Power, for one, has raised $35 million thus far, but needs another $25 million for its Roosevelt Island project, Smith said.

"There are high initial capital costs, and not a lot of initial return," he said.

Tidal turbine development has made progress in recent years, though, with companies such as Siemens producing tidal turbines, and working projects generating power in the Irish Sea and the Netherlands.

In the United States, the Ocean Renewable Power Co. achieved a major milestone in September when it began delivering power from a turbine in Cobscook Bay, between Eastport and Lubec, Maine, to Bangor Hydro Electric. It was the first time in U.S. history that power from a tidal generator has fed the commercial grid. Over the next five years, the company plans to install enough turbines to generate a total of 5 megawatts of power.

"We've been at this for 8½ years, and we've had our share of ups and downs," said Christopher Sauer, president and chief executive officer of Ocean Renewable. "Now, people are calling us from all over the world - Ireland, Scotland, Chile, Japan."

To some, the company's turbines resemble an old-fashioned push lawn mower. Sauer, however, prefers to describe the turbine as a "water wheel that's been twisted." Other designs look more like airplane propellers or wind turbines, except shorter. It's too soon to tell which design will dominate, Sauer said, and it may be that different designs will be better suited to different site conditions.

"Who knows, at the end of the day, which one will be most successful?" he said.

In addition to developing its site in Maine, Ocean Renewable also is working in Alaska and with two other companies on developing their sites. Grants from the state of Maine and the federal energy department were key to getting the Cobscook Bay project off the ground, Sauer said.

"We're not at a point where we can compete on a grid power price" without grant assistance, he said.

Also critical to its success, Sauer said, were the discussions with the local community and regulatory agencies that began as the project was being planned. To accommodate local fishermen, for example, the turbines were moved away from the most productive areas. The turbine field takes up about 60 acres of the bay that is now off-limits to fishing with drag nets, although vessels can pass over the turbines unimpeded, Sauer said.

"We've had long-standing and continual discussions with the fishermen," he said.

j.benson@theday.com