Ocean Renewable Power, is one of a number of start-ups trying to develop tidal energy — water-powered turbines that spin in the current as the tides come and go, turning generators to make electricity that is clean and, they hope, reasonably priced.

Technologies are still being tested, and environmental questions are as yet unanswered. A tidal plant in Manhattan, Maine or elsewhere in the United States that would feed significant power to the grid is at least a few years away.

The appeal is that it’s carbon-free and it’s there, No transmission lines are required

A complete unit would have four 20-foot turbine sections, two on each side of a generator capable of producing, at peak, about 250 kilowatts. The units could be mounted on the bottom or moored under water and in some cases could be stacked as many as four high

The project cannot proceed until more financing is obtained. Environmental reviews could be a “significant hurdle” for deployment of tidal energy systems. “These devices are going in new places, places where there hasn’t been industrial development,” he said. “So there are lots of questions.”

The first units in the water are not going to be competitive with coal. Our challenge is to get enough of them in the water and refine the manufacturing processes and other things we have to do to get them cost competitive.”

For ORP, as for other alternative energy start-ups, raising capital is difficult. The company will soon be looking for $10 million to $12 million to continue development of the system and begin installing it in the Western Passage.