The quest for a new form of green energy has taken a significant step with the purchase of a 25,000-acre sheep farm in the Australian outback. The huge alternative energy project isn't driven by manure, but by a 1-kilometer-high thermal power station called the Solar Tower.

Announced several years ago, the 3,280-foot Solar Tower is one of the most ambitious alternative energy projects on the planet: a renewable energy plant that pumps out the same power as a small reactor but is totally safe. If built, it will be nearly double the height of the world's tallest structure, the CN Tower in Canada.

The Solar Tower is hollow in the middle like a chimney. At its base is a solar collector – a 25,000-acre, transparent circular skirt. The air under the collector is heated by the sun and funneled up the chimney by convection – hot air rises. As it rises, the air accelerates to 35 mph, driving 32 wind turbines inside the tower, which generate electricity much like conventional wind farms.

But the Solar Tower has a major advantage over wind farms and solar generators: It can operate with no wind, and 24 hours a day. Thanks to banks of solar cells, the tower stores heat during the day, allowing it to produce electricity continuously.

Originally slated to be operational this year, construction of the massive project won't begin until 2006 at the earliest, said Roger Davey, chairman of EnviroMission Limited, the Melbourne, Australia, company behind the venture.

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But the purchase of the farm, which cost $1 million, near Mildura, Victoria, is a "very big step" in getting the project built, Davey said.

So far, the main impediment to building the tower has been the cost, with estimates ranging from $500 million to $750 million. Davey won't say how much the project will ultimately cost but said the company is considering two new engineering innovations that will reduce construction costs and improve efficiency.

"It will make the project a totally different business case," Davey said.

The timing couldn't be better. With the price of oil topping $50 a barrel, many countries are looking for cheap energy and to cut greenhouse gas emissions.

"The time is now here," Davey said. "The world is looking for a major renewable energy source."

It's estimated the Solar Tower will generate 200 megawatts, enough electricity to power 200,000 homes, and will keep 830,000 tons of greenhouse gases out of the atmosphere annually.

"Solar chimneys (towers) have become a hot area of research recently," said S.A. Sherif, a professor of mechanical and aerospace engineering at the University of Florida, who wrote several papers on the technology in the early 1990s and is the technical editor of the Solar Energy journal.

Although expensive to build, solar towers "essentially produce energy for free," said Sherif. In addition, the technology has been proven to work: A 650-foot solar tower was built in Manzanares, Spain, in 1981 by German structural engineers Schlaich Bergermann and Partner. Producing 50 kilowatts, it operated for seven years. But with oil prices at $15 a barrel in the mid-1980s, there was little interest in building a larger one, Sherif said.

And when it comes to solar power towers, size matters. "The higher the tower, the greater the efficiency," said Sherif.

Despite the iconic scope of Australia's Solar Tower project, private investors may be deterred by the fact it may take 10 years or more to see a return on an investment. "It will need some government funding to make it happen," said Sherif.

The same was true for nuclear power – a much more complex and dangerous technology. However, the U.S. government invested enormous sums of money to create a new source of energy from the atom. But it wasn't a particularly wise investment, according to Sherif.

"Energy from nuclear power plants remains very expensive and there is the huge waste problem," he said.

And while there's renewed interest in nuclear power, the world's supply of uranium is limited, while the sun's energy is not, he said.

EnviroMission has signed a joint-venture agreement with a Chinese company that will build several solar towers in China, said Davey. "China has big future energy needs and has set renewable energy targets."

Other areas – southern Asia, the Middle East and the southwestern United States– have enough solar radiation to be excellent locations for solar towers, he said.

Solar power is at a tipping point, especially on the utility power plant scale, said Brad Collins, executive director of The American Solar Energy Society. "The governors of the Western states have said they want to generate 1,000 megawatts from solar in the near future," said Collins.

"Solar towers are a good fit for power companies because they operate 24/7 and deliver maximum energy output during the hottest part of the day just when everyone's air conditioners are on high," he said.

Despite the advantages of solar power towers, especially over the long term, it's unlikely one will be built in the United States. The federal government is pinning hopes on hydrogen as a new energy source, while continuing to overexploit fossil fuels, said Collins. "Not many years from now we'll have to choose between using oil for energy, plastics or fertilizers," he said.

Meanwhile, it's important that the Australians get their Solar Tower built, Collins said. "There's nothing like taking policy makers and investors to see something like that and kick the tires," he said.