So far, the developers have been accounting for "just about everything," says Pooran Desai, cofounder of BioRegional, a British company that helped develop the zero-emissions project in London and has consulted for Masdar. "I don't know of any other project that has been as thorough in terms of its carbon monitoring," says Desai. "They're hunting down every molecule of carbon dioxide."

The Master Plan
Dubai is a sprawling, car-dominated city about an hour's drive from Abu Dhabi city. Skyscrapers stretch along a 12-lane highway, Sheikh Zayed Road. Sunlight heats the unshaded areas to 46 °C in the summer. But there are a few places in Dubai where a person can walk outdoors in the middle of the day without risking heatstroke, and all are artifacts of the past. There are the covered souks, shaded marketplaces. And there is a historic district called the Bastakiya, which preserves some of the architecture that protected locals from the heat and humidity before the arrival of air conditioning. The houses and shops have thick walls made of dried coral and gypsum that absorb heat during the day, releasing it slowly at night. Because the buildings are packed closely together, they shade both each other and the narrow passages between them. The passages funnel breezes, cooling the buildings further.

When Gerard Evenden, a senior partner at the British firm ­Foster and Partners, began to make the master plan for Masdar City, he looked to such traditional designs for ways to save energy. Since the city will depend almost entirely on electricity from solar power, which is five times the price of electricity from the local grid, the city needs to be roughly five times as energy efficient as competing developments nearby.

One of the first things Evenden did was subtract cars: with the highways gone, the city's buildings could be separated by passages just 7 to 12 meters wide, close enough to shade each other yet far enough apart to let in indirect light. That's a cheap way to reduce the need for not only air conditioning but electric lighting, the largest drain on electricity in commercial buildings. Insulation is cheap, too: in the Masdar Institute, Evenden plans to use 30-­centimeter-thick insulation to keep out the heat. He's also incorporating "skins" of copper foil that reflect light and conduct heat away from the buildings. The foil will be protected from the desert dust by a self-cleaning Teflon-like plastic. To reduce the need for energy-intensive desalination, Evenden's design will cut water consumption by 75 percent through recycling, low-flow fixtures, and waterless urinals.

A small fraction of the energy that's still needed to run the city will come from waste-based fuel and perhaps geothermal power. The rest will come from the sun--but not all of it through expensive photovoltaics, which convert sunlight into electricity. Much cheaper devices that concentrate heat from the sun will heat water and run a type of air conditioner called an absorption chiller. (This is the same kind of technology that is used now in propane-­powered refrigerators.)

In theory, it should all work. But in practice, even much less ambitious projects have failed. Oberlin College's Lewis Center features many of the same elements of energy-efficient design: thick insulation, natural ventilation with heat exchangers, plenty of windows to offset the need for electric lighting, and heat pumps rather than conventional furnaces. A 60-kilowatt array of solar panels on its roof was supposed to produce as much electricity over the course of a year as the building consumes. Yet the building initially used too much energy, and the solar panels were not adequate. To achieve zero net energy, the college had to install an extra solar array nearby, more than tripling the total power production. It added over a million dollars to an already expensive building, estimates John Scofield, a physics professor at Oberlin who has published a detailed analysis of the building's performance.

In general, architects find that predicting how energy-efficient systems will interact gets much harder as buildings get bigger. In buildings designed to take advantage of natural light, for example, designers can install sensors to automatically switch bulbs off when enough light comes in from outside. But lights turning on or off in one sensing zone may affect the sensors in another. In some buildings this has created a feedback loop that makes lights cycle on and off annoyingly.