The technology for tidal turbines-essentially windmills operating in a much denser medium-is developing particularly quickly. Eight feet below New York City´s East River, six turbines driven by the channel´s tidal currents will begin producing electricity this summer. The slow-turning propellers will generate 525,000 kilowatt-hours their first year. Depending on the results of an 18-month trial, enough turbines could be added to produce 26 million kilowatt-hours and power 8,000 homes. That´s hardly a dent in Gotham´s energy consumption, but experts say it´s the technology, not the size of the project, that´s exciting. This will be the world´s first power-producing tidal turbine farm, a prototype that could lead to a steady, nonpolluting energy source.