By Ben Sutherland
BBC News Online, in Barcelona
Skyscrapers that power themselves through wind and solar energy collected on their roofs have been named in the winning designs in a competition aimed at creating a greener modern city.
None of the winning ideas are officially the part of any building plans
Although the project's title - Green Ground Zero - refers to the space in which the twin towers of the World Trade Center once stood, the project is designed around the entirety of New York's Lower Manhattan district.
The aim, its founders argue, is to show that greener ways of living and "sustainable design" can be made part of a densely packed metropolis that exists in the real world.
The competition winners were announced here at the World Urban Forum.
Farouk Stemmet, one of the project's advocates, told the BBC the idea was of "every building making its humble contribution".
He explained one idea was to have tiny generators in every tap - so that every time the tap was turned on, the motion of the water passing over them produced electricity.
"You could, in theory, have a city of 10 million people acting as an electricity generator," he said.
The winning ideas are not part of the plans for the Freedom Tower - the competition ran after the architect Daniel Liebskind's design was approved - nor are they part of any other official construction project.
Instead, Mr Stemmet said, the idea was to demonstrate practical ideas that could help turn a real city into a sustainable, environmentally sound living-space.
One winning competition entry, for example, featured details of how one could use the space at the top of the area's numerous skyscrapers.
Ground Zero will see a development led by architect Daniel Libeskind
They included solar panels, wind turbines, large rainwater-collecting spaces, and a "green roof" covered in trees.
As well as energy, the competition also asked for ideas on Light And Air, Construction, Ecology, and Greenery, Water And Waste.
As a result, other plans include a multi-layer "vertical park," a tower-top farm, and
"greywater gardens" that would use the waste water from human activities such as showering and clothes-washing.
Another plan would see light captured at the top of buildings and taken via fibre-optic cables to below ground to subway stations. In addition to providing natural light for commuters, such an idea would allow plants to grow, too.
Mr Stemmet said this could eventually mean they grew up to street level, providing the city with more green areas.
He said that light played a role as "medicine for the city's healing", a reference to both the plans for the new Freedom Tower and the twin pillars of light first illuminated in the spot where the World Trade Center had stood.
Mr Stemmet conceded that there had often been problems in persuading real-estate developers to incorporate such thinking into their building designs, especially as some of them had high-cost implications.
However, he believed that more developers were beginning to realise that they could make money out of these ideas.
And he stated further that the thinking had received crucial backing from the city of New York
"At both federal and local level there is interest in seeing this thing through," he said. "From that level, this is the agenda."