"There are aesthetic considerations ... No. 2, I have absolutely no idea whether that makes any sense from a scientific, from a practical point of view," Bloomberg said.
Experts seemed to agree. David Carr, of the Alternative Energy Institute, in Canyon, Texas, said mounting turbines high above the city is "not very feasible."
"I don't think this was very well thought out," he said.
Among the complications are turbulence and vibrations the buildings would have to endure plus the relatively small amount of wind the turbines would be able to harness in a city where other buildings and trees stand in the way, Carr said. Also, skyscrapers typically are not built to withstand the load of turbines.
"If you want it for art and decoration, that's fine, but for achieving any kind of power that's useful, it's not a very good idea, and I don't know of anywhere that's done it very successfully," Carr said.
It is also difficult to imagine residents would welcome spinning turbines in the views outside their windows in a city where a proposal to install a cell phone tower on a building on Manhattan's Upper West Side generates protests and interference from local elected officials.
Bloomberg sought to dial back his windmill proposal while speaking to reporters on Wednesday after returning from Las Vegas, where he gave a speech at the National Clean Energy Summit imagining, among other things, harnessing wind power with turbines on bridges and skyscrapers.
In the coverage that followed, one newspaper ran a front-page image of a turbine twisting on the Empire State Building's iconic spire, while another showed what the Brooklyn Bridge might look like with three turbines planted high above it.
Bloomberg downplayed that part of his plan on Wednesday, as well as the overall importance of windmills in his environmental plan.
His other ideas include using wind farms along the city's coastlines and drawing power from the tides, and his speech on Tuesday was intended to solicit expressions of interest from alternative-energy companies that might have their own ideas.
"Windmills are no panacea for our problems," Bloomberg said Wednesday. "They can help, just like biofuels can help, just like tides can help. In the end it is conservation that is the main thing you and I can do and that we can do in this city."
When the first design for the new World Trade Center was being drawn up years ago, architects proposed turbines on the upper floors of the center's main skyscraper, the Freedom Tower. They were to create 10 to 20 percent of the building's energy.
In 2004, Bruce Fowle, an architect asked to assess the sustainability of the proposed building, told a downtown community meeting that while wind turbines at the trade center site would be symbolically good to showcase alternative energy, it might not work well in practice.
Fowle, who designed the Conde Nast sustainable building in Times Square, questioned how much power the proposed World Trade Center turbines would reap for the building and whether the concept was cost-efficient. He also said turbines on buildings have been problematic in the past because they tend to run into problems at high altitudes, according to minutes of the meeting.
A later version of the trade center design pared down the turbine area, and then the concept disappeared in another revision.
