'Solar chimneys' may help solve China's energy woes
Convection devices the size of skyscrapers generate electricity by heating air inside
Scientists are researching whether so-called solar chimneys, which rise half a kilometre or even higher from the earth, might produce enough clean energy to help reduce the mainland's chronic air pollution.
A test plant is running successfully in Inner Mongolia and scientists want to build full-size versions in cities such as Beijing and Shanghai. The researchers have suggested the towers could be constructed inside new skyscrapers.
The technology involves covering an area of several square kilometres to create a greenhouse around a high chimney. The hot air produced in the greenhouse rises through the tower, driving turbines that generate electricity. The higher the tower, the stronger the updraft and the more power created.
The highest previous attempt to master the technology, which has been discussed for decades, ended in failure when a 195-metre tall tower in Manzanares, Spain, collapsed in 1989 due to structural failure.
But Professor Wei Yili , the leader of the project at Inner Mongolia University of Science and Technology, said he was confident they could now build safe and efficient towers higher than a kilometre.
"The structural problem is no longer a problem for us. We have acquired patents for our technology and design," he said. "The towers will stand for a century, outlasting those who build them or see them built, like the Eiffel Tower."
The 50-metre high test "solar updraft tower" has been running in the Gobi desert in Wuhai for nearly four years.
Scientists wanted to build a chimney as high as 200 metres, but had to rein in their ambitions because of a nearby airport.
"This is the biggest regret of the project," Wei said. "Our power generation capacity and efficiency have been severely restricted by the limited height."
The project has managed to generate up to 4,800 kilowatt hours of electricity a day.
That is enough to power about 160 homes, based on average electricity usage figures in the United States.
Wei said they had used the data from the project to improve their mathematical modelling of the technology and had come up with new designs that could be used in big Chinese cities.
One of their designs integrated the solar tower into a high-rise building.
"Many Chinese cities are considering the construction of buildings more than 500 metres high, but have met strong resistance due to their high cost and energy consumption. The updraft tower will make a skyscraper 'green' and strengthen its physical stability as well," Wei said.
"If all new high-rise buildings are built with updraft towers, large cities' demand for coal-fired power plants will be significantly reduced; air quality improves, smog lessens."
But Professor Zhu Jianhua , an environmental engineering scientist at Nanjing University, said solar chimneys were impractical because they needed huge amounts of land and produced little power in return.
"Even if solar power plants are built all over the country and we put up wind farms in every windy area, we can't produce enough electricity to meet our rapidly growing energy needs," he said.
Professor Yuan Xingfei at Zhejiang University, who has studied the structure of solar towers, said it was still not clear whether chimneys up to one kilometre high could be built.
All modelling and designs were based on data collected from simulations because nobody had ever built a chimney more than one kilometre tall, she said.
"The upper part of the tower cannot be concrete and steel because they weigh too much," Yuan said.
"But if we use lighter materials such as carbon fibre we may see the top wobbling uncomfortably in the wind. We are still working to solve these problems."
Professor Zhou Xinping at the Huazhong University of Science and Technology in Wuhan , also studies solar updraft tower technology. Zhou said that a few years ago the department had contacted the municipal government in Shanghai about constructing a solar tower as the world's tallest man-made structure.
The Shanghai government showed interest and held several rounds of negotiations with Zhou's team, but the authorities pulled out due to technological and economic concerns.
Some officials were worried about the tower's physical strength, doubting whether scientists and engineers could meet the unprecedented engineering challenges.
But the government's biggest concern was the use of land, as several square kilometres of ground was required for the project's greenhouse, Zhou said.
An Australian company, EnviroMission, is pushing for the construction of the world's first commercial plant with a one kilometre-tall solar tower in the US state of Arizona.
"If they make it, our government will be totally convinced. Then some larger, higher projects will likely be approved and launched in China quickly," Zhou said.
A commercial updraft power plant would cost hundreds of millions of yuan on the mainland while challenging the country's manufacturing and engineering capabilities, he added.
Kim Forte, spokeswoman for EnviroMission, said the company was working on several projects in the US, but was also seeking opportunities in China with potential partners.
But Forte cautioned that Chinese authorities should consider many factors before launching a project on such an enormous scale, including the location and the local use of land.
"You certainly don't want to build in an area with many earthquakes," she said.
Zhou at Huazhong University said that solar chimneys would not only create clean energy, they would actively clean the air by sucking up dirty air at ground level and dispersing it at higher altitude.
"Each updraft tower will work as an extremely powerful and tireless air pump," Zhou said.
"The construction of a solar tower is expensive and the energy it produces will be higher in cost than that from coal-fired power plants, but if you consider the cost of other measures to combat with smog it's cheap and competitive."