ON THE dusty red dirt of an old sheep station, a 6-hour drive from Melbourne, plans are afoot to build the world鈥檚 tallest tower. Forget quibbles about whether an antenna or flagpole should count in the final measurement. If this concrete structure makes it off the drawing board it will smash every record in the book. It will stand a staggering 1 kilometre tall, and its base will sit at the centre of a shimmering field of glass and plastic 7 kilometres across.
If the tower鈥檚 dimensions are awe-inspiring, its aim is breathtaking. The planned structure will be Australia鈥檚 biggest solar power plant by far. Air heated by the sun will rise up the tower, where 32 turbines will generate about 650 gigawatt-hours of electricity a year, enough to meet the demands of 70,000 Australians. EnviroMission, the company behind the project, hopes to start building work next year and to be supplying electricity by 2008.
Energy experts have mixed views on this audacious project. Some are impressed by the prospect of a power plant that generates electricity from a free, renewable source that emits no greenhouse gases. Others fear the solar tower will be seen as a blot on the landscape that will bring the renewable energy industry into disrepute.
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But EnviroMission appears to be winning supporters where it matters. The Australian government has promised to help the company obtain the permits and approvals that will be required. And power company Australian Gas Light has an option to buy the solar tower鈥檚 entire output once it鈥檚 up and running.
People have been harnessing the energy of rising columns of air for centuries. In the 17th century, smokejacks were used to turn spits over fires, and before that, Leonardo da Vinci had a bash at sketching a solar tower. But it was structural engineering guru J枚rg Schlaich who put solar towers on the map, quite literally. In 1982, with support from the West German government and a Spanish power company, Schlaich built a 50-kilowatt prototype near the town of Manzanares, 150 kilometres south of Madrid (麻豆传媒, March 6 1999, p 30). His vision was that solar towers would eventually provide electricity to poor nations with plenty of sunshine but no other energy resources.
Rising 195 metres into the air and surrounded by an array of plastic sheeting 240 metres across, the Manzanares tower proved the concept worked. The plastic sunlight collector warmed the air underneath by up to 17掳 C, enough to draw it towards the central tower, where it created a strong enough updraught to drive a turbine and generate electricity. There were no fuel costs to pay, and no climate-damaging greenhouse-gas emissions. During 30 months of continuous running it contributed 118 megawatt-hours of electricity to the Spanish electricity grid.
This figure underlines an uncomfortable truth about solar towers. As a means of converting sunlight into electricity, they are horribly inefficient: much less than 1 per cent of the energy in the sunlight hitting the plastic was eventually converted to electricity. In the Manzanares tower half the solar energy was lost as heat from the collector, and of the heat that was trapped only 0.7 per cent was converted to electricity by the turbine. The rest escaped as hot air from the top of the tower. By comparison, photovoltaic solar panels convert about 15 per cent of sunlight falling on them into electricity.
But this need not matter. Sunshine comes free of charge, so the key figure is not the conversion efficiency but how much the plant costs to build and maintain for each kilowatt-hour it can generate. One way to reduce this unit cost is to scale the plant up 鈥� and that means building the tower tall and spreading the area of the collector wide.
The tower鈥檚 output depends on the size of the pressure difference between the bottom and the top, and the amount of air passing through. The pressure difference can be increased by using a larger collector to increase the air temperature at the base. And the taller the tower, the lower the ambient atmospheric pressure at the top. 鈥淎 1000-metre tower will be five times as efficient as the 200-metre Manzanares tower,鈥� says Gerhard Weinrebe who works for Schlaich鈥檚 engineering firm Schlaich, Bergermann and Partner in Stuttgart, Germany. He expects the overall efficiency of the Australian tower to be around 1.5 per cent.
When it comes to details of its planned tower, EnviroMission is not giving much away. But one thing is clear: where Schlaich鈥檚 original scheme was driven by idealism, this one is being planned as a money-making venture, able to hold its own against conventional power stations fired by Australia鈥檚 plentiful supplies of fossil fuel. The site chosen for the tower, near the unassuming town of Mildura, Victoria 鈥� which lies in the aptly named Sunraysia region 鈥� not only has plentiful sunshine but is also less than 30 kilometres from an access point to the national electricity grid. That will keep connection costs and line losses to a minimum. Mildura will provide the workforce needed to build the tower.
Schlaich鈥檚 firm has made a name for itself with elegant structures such as the glass roof at the German Historical museum in Berlin. It is also involved in the Mildura tower, but here the watchwords will be cheapness and practicality. 鈥淵ou don鈥檛 need to build a Rolls-Royce when a Passat will do the job,鈥� says Roger Davey, executive chairman of EnviroMission. So most of the solar collector will be made of plastic, rather than more expensive glass. Only the central section will use glass, as design studies show that only glass is strong enough to withstand wind speeds inside the collector of up to 54 kilometres per hour and the force of the updraught near the base of the tower.
Solar towers have the edge over many other renewable power sources because they generate electricity smoothly and continuously, rather than flipping on and off as the wind rises and falls or clouds block the sun. During the day, heat from the sun is trapped in the air under the collector and in the ground beneath it. The temperature of the air entering the tower reaches a maximum a few hours after the hottest part of the day, neatly matching peak consumption hours in the early evening. But the heat energy will be dissipated before the sun comes up again, and to provide electricity 24 hours a day, the plant will need some kind of system for heat storage. This is not in the current plans, but may be added at a later date. The Schlaich team has developed and tested a system of black, water-filled tubes that soak up heat during the day to provide hot water that can be used at night to heat air for the tower.
One aspect of the design that experts can only speculate on is how the structure itself will be built. EnviroMission has let on that it intends to stiffen the tower at several points with cables strung across its interior like the spokes of a wheel. Apart from that, few details are known, but there seems little reason to doubt that a 1-kilometre tower is feasible. 鈥淧eople are going higher and higher. Building a supertower 1 kilometre or higher is not technically impossible,鈥� says Bijan Samali, director of the Centre for Built Infrastructure Research (CBIR) at the University of Technology in Sydney. But it will certainly be challenging, as it will be almost double the height of what is currently the world鈥檚 tallest building, the 553-metre CN Tower in Toronto.
One problem will be protecting the tower from high winds. At 1 kilometre above ground, wind speeds of 200 kilometres per hour are not uncommon. As air flows past the solar tower it will create vortices, first on one side then on the other. That sideways pull combined with swaying in a gusty wind could quickly transform the world鈥檚 tallest tower into its biggest pile of rubble.
On industrial smokestacks, spiral stiffening is often used to break up the vortices. But the vast size of the solar tower will demand additional measures. One option, Samali says, would be to rig guy wires from high up on the tower to anchor points in the ground, to stabilise it from outside. Any remaining oscillations could then be neutralised by a damper 鈥� which could be as simple as a chain wrapped in a shock-absorbing material hung inside the tower. From the dimensions of the tower, engineers can work out the frequency at which it will oscillate. They can then set the length of the chain so that it will swing at a frequency that ensures it hits the inside of the tower as it oscillates in a way that dissipates some of the tower鈥檚 energy.
The material used to build the structure will also be key. 鈥淲e will be pouring concrete 24 hours a day, seven days a week for two years,鈥� says Davey. The tower will take an estimated 700,000 cubic metres of high-strength concrete. 鈥淲ith a structure of this magnitude, subject to major wind loads, you will have much tighter tolerances on all the material properties,鈥� says Aleksandra Samarin, a materials engineer at CBIR and former research director at the construction materials company Boral. 鈥淲ith such huge volumes of concrete, economically you want to use local materials.鈥� So concrete made from local stone and water will have to be put through at least a year鈥檚 worth of lab tests to check how it deforms and fractures.
Hot idea or hot air?
Clearly, building costs will be substantial. EnviroMission won鈥檛 comment on the price tag but experts speculate the total investment will be around A$1 billion (US$720 million). But to offset those costs, the tower will have access to more sources of revenue than a fossil-fuel power station. For a start, EnviroMission intends to trade the renewable energy certificates that are being divvied out by the Australian government to suppliers of green energy. Then there are the naming rights to the tower. 鈥淚t could be the 鈥楾AG Heuer sundial鈥�, the biggest sundial in the world,鈥� Davey suggests. 鈥淥r the 鈥榁iagra tower鈥�.鈥� He won鈥檛 disclose the asking price, but says that 鈥渕ore than one鈥� potential sponsor has expressed interest. 鈥淚t will be an icon, a renewable energy first, a global first.鈥�
EnviroMission is also looking into the possibility of using the collector as a giant greenhouse to grow fruit and vegetables or to dry locally produced fruit. This might have the additional advantage of making the air more humid and less dense, so it rises more rapidly and enables the turbines to extract more energy. Then there are plans to offer trips to the top of the tower. As a tourist attraction it could outdo another local favourite 鈥� the bar at the Mildura Working Men鈥檚 Club, which until recently laid claim to being the world鈥檚 longest.
Alternative revenue streams aside 鈥� and assuming EnviroMission manages to raise the money to go ahead with construction 鈥� will the solar tower fulfil its purpose and generate electricity as predicted? The sheer size of the project means that doubts inevitably creep in. The only experience to go on is the Manzanares tower, built over 20 years ago 鈥� but the planned output of the Mildura plant is 4000 times that of its Spanish forerunner. 鈥淚t鈥檚 a huge scale-up,鈥� says Keith Lovegrove, a solar energy expert at the Australian National University in Canberra and vice-president of the Australian and New Zealand Solar Energy Society. 鈥淭hey are shooting for the top without an intermediate-sized prototype.鈥� The vision of the project has to be admired, he says. But he suspects that cold reality may force the company to rein back ambitions for its plan: 鈥淚 think they will get a certain amount of money and then build a smaller one.鈥�
Andreas Luzzi, incoming director of the Institute for Solar Technology at the University of Applied Sciences in Rapperswil, Switzerland, has no such concerns about the magnitude of the scale-up. 鈥淭here are so many similarities with the energy-conversion technology used in hydropower stations,鈥� he says. 鈥淭he turbines are similar, the fluid dynamics is similar, so the risks are quite manageable.鈥�
But Luzzi also concedes that the vast greenhouse at the foot of the tower might subtly alter the area鈥檚 microclimate, by changing the reflectivity or albedo of the surrounding surface. This could lead to shifts in local humidity, temperature and wind that could affect the tower鈥檚 capacity to generate electricity 鈥� for better or worse. 鈥淭he effect of changing the albedo is still a big question,鈥� says Luzzi. 鈥淏ut it might 鈥� and I want to emphasise might 鈥� increase the ambient temperature outside the tower, altering the pressure difference between the top and the bottom.鈥� EnviroMission is seeking government funding to run a number of research projects, including a microclimate impact analysis.
And then there are more mundane issues, such as how to keep some 4000 hectares of greenhouse clean enough to trap solar radiation in the first place. Legions of squeegee-wielding window cleaners will clearly not be the answer. And there are worries that the plastic sheets used to build the collector might deteriorate under the glare of the Australian sun, as they did in Manzanares.
Ken Brown, an expert on wind energy at Melbourne University, has another concern. Sure it might be possible to use alternative energy streams, such as naming rights to make a single 1 kilometre tower profitable. But solar towers are not financially viable in the long run he says. 鈥淭hey are a long way from being able to sell electricity at a profit,鈥� says Brown. 鈥淭his is not a solution to our energy needs.鈥� And he is not alone in doubting that solar towers can generate electricity at a profit.
Until EnviroMission reveals more details of its plans, the jury is out over whether Australia鈥檚 solar tower will make the grade. Either way, it鈥檚 unlikely to be the last we hear about this form of energy generation. The promise of a soaring tower that generates electricity from sunlight, attracts tourists, and grows fruit and vegetables to boot has a way of capturing the imagination. South Africa, Morocco, Egypt and India have all been considered as possible sites. And last month, EnviroMission said it would be applying to the Chinese government for approval to set up a company to build solar towers in China. Anyone for the Tsingtao Beer tower?
