When Europolis Invest unveiled the Danube House as the first phase of the River City Prague office and housing project two years ago, it garnered a lot of international attention. Awards poured in — most notably, Britain's Architectural Review's first prize for Best Future Office Building in 2003. Critics praised its clean, sleek facade and its energy-efficient design.

With the flagship building in place, developers moved on to the second phase: Nile House. Pressed up against the Vltava River in the Prague 8 neighborhood of Karlín, Nile House is smaller than its predecessor by two floors and about 2,000 square meters (21,528 square feet). But in other respects, it is more ambitious. With more than 80 percent of its infrastructure completed and an opening date scheduled for January 2006, Europolis Invest is hoping that the second verse will be better than the first.

"The goal was to increase the technical parameters from the first building to the second," says Petr Urbánek, managing director of Europolis Invest. This means that the system pioneered by the Danube House has been tested and lessons learned from the prototype were incorporated into the Nile House design, ideally creating a better building. The learning curve has already shown itself in terms of construction time — nearly six years for Danube House, less than two for Nile House.

Office buildings are the largest consumers of energy, so the developers paid a tremendous amount of attention to creating cost-effective cooling and lighting in Nile House. With large glass facades and an airy interior, the building hides little from the outside world. It also derives its economical and ecological advantages from this skeletal structure.

Nile House does not have traditional heating and air-conditioning systems. Suction towers along the riverbank pull in air, which is cleaned and released through risers to the different floors of the building. As the air rises, it takes with it excess heat and pollution out of vents in the top of the building in a more-or-less natural way. Air is changed four times per hour, as opposed to less than twice per hour with traditional air-conditioning systems. The building takes energy from imported sources — natural heat given off by workers, for example. Even computers are harnessed by the building to warm it.

Nile House's immense hollow atrium gives the building a controlled buffer zone to help regulate heat throughout the building, and enough light during business hours to reduce the need for artificial light. All in all, the developers hope that tenants will save nearly 50 percent on energy costs.

There is also concern about complying with potential European Union directives that would set new standards for energy consumption and environmental impact. Those standards will put pressure on many buildings in the Czech Republic, according to Monica Younis, a real estate consultant with Cushman & Wakefield Healy & Baker. Nile House would likely comply with those standards.

However unique Nile House may be, Europolis Invest still needs the most important element of any office building: tenants. The company says that it has half the space leased, though at a press conference held last month to show off the new building the company refused to name any of the tenants.

Still, Urbánek leaves little doubt about his intentions. "Our purpose is not to have a fantastic building that wins awards," he says. "Our idea is to ensure returns and do good business."

The rent per square meter is 16.50 euros ($19.80/487 Kč) per month, which is higher than buildings in similar locations in Prague. But Europolis is banking on Nile House's reduced energy costs to make up the difference.

"Although the rent is higher, the overall operating cost is lower," Younis says. "Per square meter, costs are comparable with any other office building in Prague."

Demand for new office space in Prague fell in 2004, and vacancies pushed toward 15 percent. But after the first nine months of 2005, demand is returning and vacancies have fallen to 12 percent. Younis says she sees this trend continuing, helping Nile House.