"There are aesthetic considerations," Bloomberg said. "No. 2, I have absolutely no idea whether that makes any sense from a scientific, from a practical point of view."
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.
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 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.
"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."
The idea of windmills on city buildings is not new. 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.
But Bruce Fowle, an architect asked to assess the sustainability of the proposed building, questioned the practicality of the proposal in 2004. A later version of the trade center design pared down the turbine area, and then the concept disappeared in another revision.