快猫短视频

NORTHERN CAPE PROVINCE, South Africa, 2004. It is impossible to miss. It
dominates the skyline from a long, long way off, a monolith rising high above
the scorched desert. But the true scale hits you only when you top the final
ridge. Above you, the chimney thrusts 1.5 kilometres into the air, while its
base is submerged in a gleaming sea of glass 7 kilometres across.

At least this will be the scene if a plan devised for the Northern Cape
government comes to fruition. The aim is to erect the world鈥檚 first full-scale
solar chimney to wrest energy from air heated by the desert Sun. It is a hugely
ambitious project. The plant would generate 200 megawatts of electricity, cost
about 2500 million rand (拢250 million) and need a chimney nearly three
times the height of today鈥檚 tallest free-standing structure, the 553-metre CN
Tower in Toronto.

Many Western energy specialists pooh-pooh the idea of a solar chimney,
arguing that it is just too expensive. But, say its advocates, though the
initial cost is high, the chimney would outlive any fossil fuel plant and need
no fuel but sunshine, so in the long run it would be cheaper. Most of all, they
argue, this gentle giant is specially suited to the needs of many developing
countries.

The inspiration for the South African plan comes from J枚rg Schlaich,
whose work on projects such as the Ting Kau bridge in Hong Kong has earnt him a
reputation as one of the world鈥檚 most innovative civil engineers. Schlaich, who
works at the University of Stuttgart and is a founder member of the firm
Schlaich, Bergermann and Partner, has seen how power cuts and electricity
rationing can bring life to a standstill in developing countries. 鈥淚 was in
Khartoum recently, where work was not possible since they had no electricity,鈥�
he says. 鈥淏ut at the same time, just outside the city, they have a desert in
which the insolation is fantastic. They could really make use of it.鈥�

Schlaich believes the solar chimney is the answer. Being made mostly of glass
and concrete, it can be built using rock and sand from the desert itself, and
with local labour. It needs little maintenance and unlike other solar power
plants, it can even generate electricity through the night (see 鈥淔ringe
产别苍别蹿颈迟蝉鈥�).

Devices that harness energy from a rising column of hot air have a long
history. Leonardo da Vinci sketched an early version called a smokejack. In the
17th century, similar machines that resembled horizontal windmill sails, sat
above open fires and turned the spit. Schlaich first thought about using an
updraught to generate power in the 1970s while working on some proposed
800-metre towers for removing the heat from water used to cool a nuclear power
plant.

He ended up ditching the nuclear reactor altogether and, instead, suggested
using sunlight shining through a vast array of glass sheeting to heat the air.
In Schlaich鈥檚 completed design, he pictured this circular 鈥渟unlight collector鈥�
suspended above the ground and open to the world around its circumference. In
the middle it would seal tightly around a chimney, just above large air inlets
(see Diagram).

Beneath the collector, the heated air would be drawn towards the flue. Close
to the chimney, the collector would be double-glazed to stop heat loss and
slanted to reduce friction between the moving air and the glass. Once inside the
flue, the air would create a large enough updraught to drive one large or
several small vertical-axis turbines.

By the early 1980s, Schlaich had secured enough interest in his idea to build
and run a prototype. With DM15 million (拢5.4 million) from the West German
Ministry of Research and Technology, and backing from the Spanish electricity
company, the 50-kilowatt test plant was erected 150 kilometres south of Madrid
near the town of Manzanares.

The chimney stood 195 metres high. Ideally, it would have been built of
reinforced concrete, but to keep costs down and allow the structure to be
dismantled, the Spanish flue was made of sheet steel and steadied by guy lines.
The collector was 240 metres in diameter and rested on a metal scaffold about 2
metres off the ground. Over nine years, the team refined the design. And in
three years of continuous running, the plant ran for more than 95 per cent of
the expected time.

The prototype showed that Schlaich鈥檚 design worked, but also exposed its
limitations. The collector turned about half the radiation falling on it into
heat, and the turbines converted more than 75 per cent of the updraught鈥檚
kinetic energy into electricity. But the intermediate step let the machine down.
The chimney itself could convert only 3 per cent of the heat gathered by the
collector into kinetic energy. The only sensible way to increase the efficiency
is to increase the height of the chimney, says Karl Stephan, professor of
thermodynamics at the University of Stuttgart, who checked Schlaich鈥檚
calculations.

The height of the chimney also affects its power output. In some ways, the
solar chimney resembles a hydroelectric plant more than a windmill. The
chimney鈥檚 turbine is encased in a tube, so that all the driving 鈥渇luid鈥� must
pass through it. In this mode, the electricity generated depends on the pressure
difference between the tube鈥檚 input and output.

In a hydroelectric plant, this difference is decided largely by the height of
water above the turbine. With the solar plant, it depends on raising the chimney
as high as possible to reduce the atmospheric pressure at its mouth, and on
increasing the temperature鈥攁nd hence pressure鈥攐f air entering the
flue. This final factor is decided by the surface area of the collector and the
intensity of sunlight falling on it. The collector of the Manzanares machine
warmed the air by about 17 掳C, generating an updraught of as much as 12
metres a second.

As part of the Spanish experiment Schlaich and his colleagues verified a
computer program that relates the size of plants to their thermodynamic
performance and power output. They found that doubling the height of the flue
increases power by the same amount as doubling the area of the collector
(see Graph).
But it makes more sense economically to start with the chimney.
鈥淭he glass roof costs about half of the whole cost and the chimney just 25 per
cent,鈥� says Schlaich. 鈥淚t is much cheaper to double the chimney than to double
the roof.鈥�

Economically, it also pays to build a large plant. To attract private
investors, Schlaich reckons that he will need to build a 100-megawatt plant.
This will need a chimney about 1 kilometre high and a collector around 3.5
kilometres across, which will cost about 拢220 million, he says.FIG-mg21764701.JPG

If this investment were paid off over the standard 20-year term, Schlaich
estimates that electricity from the plant would cost about 21 pfennigs (7.6
pence) per kilowatt-hour, compared with 6 pence per kilowatt-hour from a
coal-fired power station. But after 20 years, the cost of electricity from the
solar chimney would fall to 3.9 pence per kilowatt-hour. Costs for a coal-fired
plant at this time would also decrease, but not to the same extent because it
would still need fuel.

In Britain, David Infield, director of the Centre for Renewable Energy
Systems Technology at Loughborough University, is not convinced by these
figures. He says the best way to judge value for money from a power station is
to look at the cost of installing each kilowatt of generating capacity. By this
measure, a solar chimney is at least twice as expensive as a wind farm.

But there are other benefits to be had from a solar chimney, say its
advocates. Unlike a fossil-fuel plant, it needs no cooling water, which could be
a significant saving in hot, dry countries. And the notion of building a power
station with locally made glass and concrete鈥攔ather than spending precious
foreign currency on costly, high-tech equipment鈥攕hould appeal to
developing countries, Schlaich believes.

Once built, the plant would last a long time too. The chimney itself should
be good for 100 years or more. And with so few moving parts, maintenance should
be low. The main moving components, the turbines, would sit in a steady stream
of air and so suffer far less stress than wind turbines exposed to unpredictable
gusts and gales. Local engineers should be able to run and maintain the plant
without the help of foreign consultants. 鈥淚 am absolutely convinced that if one
is built of a reasonable size,鈥� Schlaich says, 鈥渢hen they will copy it around
the world.鈥�

Prove it

So if it鈥檚 such a great idea, why has nobody built one? Schlaich has been
waiting 10 years for a full-scale solar chimney to appear. One project almost
got off the ground in India鈥攁 100-megawatt plant in the Thar Desert of
Rajasthan. But it foundered last year during the nuclear posturing between India
and Pakistan. The site was considered too vulnerable to sabotage, so the project
was abandoned. But generally, says Schlaich, countries are put off by the high
initial outlay.

Australia, Egypt and Morocco have expressed an interest, but haven鈥檛
committed themselves to building a solar chimney. The most likely candidate is
South Africa, where the Northern Cape province is perfectly placed for a solar
chimney. It gets more sun than just about anywhere else on Earth.

Since 1995, a team led by physicist Wolf-Walter Stinnes has been
investigating future power plant choices for the Northern Cape government. 鈥淚
saw that the solar chimney really had the chance to compete against fossil fuel
power stations even on a pure cost basis,鈥� he says.

Stinnes鈥檚 solar chimney, which would produce twice as much energy as anything
Schlaich is considering, would be built near the remote desert town of Sishen to
supply power to the local mining industry. Techniques borrowed from that
industry, Stinnes believes, could help to speed up construction and cut costs
below Schlaich鈥檚 estimates. For example, he calculates that fly ash, a
by-product of coal-fired stations, would improve the consistency of concrete for
the chimney, allowing it to be pumped more easily to high levels during
construction. This measure alone could reduce construction time from 4.5 to 1.5
years, he says.

But coal is cheap in South Africa and the country鈥檚 priority is to produce
cheap rather than 鈥渃lean鈥� electricity, says Stinnes. So he uses an unusual
method to level the playing field. Because the chimney could have such a long
life, he costs the project over 80 years rather than the standard 20. He
estimates that over this period, two and a half coal-fired stations would be
needed, or four combined-cycle gas plants. Using this formula, he reckons
that鈥攁t worst鈥攖he cost of energy from the chimney will be the same
as from the other plants. At best, it will cost just one-third as much. Infield
is, once again, doubtful of these figures, 鈥淵ou can make anything pay over 80
years,鈥� he says.

But even supposing that the Northern Cape province decides to go ahead and
can find funding for the project, there are still big questions over the
monolithic proportions of the plant. Stinnes鈥檚 plan to build a 1500-metre
chimney goes way beyond anything built before. Even Schlaich has reservations
about the cost-effectiveness of building a chimney this high.

Schlaich is, however, confident that the 1000-metre chimney needed for his
100-megawatt plant 鈥渃an be built without difficulty鈥�. During work on the
Rajasthan plant, Schlaich recruited American engineer Ben Gerwick, dubbed the
鈥渃oncrete pope鈥� for his work on 14 major American bridges and the precast liners
for the Channel Tunnel. He also consulted Alan Davenport of the University of
Western Ontario in London, Canada, who worked on the CN tower. Both told New
快猫短视频 that they believe a 1000-metre chimney is feasible. Nevertheless,
lifting materials and construction workers so high is going to be very costly,
says Gerwick.

Another big question about a full-scale solar chimney is whether it would
function as expected. After all, the design for a 100 or 200-megawatt plant is
based on a single 50-kilowatt prototype. This leap in scale is 鈥渆xtraordinarily
optimistic鈥�, says Ian Fells, professor of energy conversion at the University of
Newcastle upon Tyne. 鈥淚 think it鈥檚 really quite an exciting concept,鈥� he says,
鈥渋f it can be made to work.鈥�

At the University of Stuttgart, Stephan is quietly confident about the
chimney鈥檚 performance. 鈥淔or a larger device,鈥� he says, 鈥渢he flow pattern might
be different and this might cause some problems, but these could be overcome.
If, for example, there are fluid rotation or eddies inside the chimney then
something can be built inside the tower to prevent that. This is common
technology from cooling towers.鈥� So will solar chimneys feature as future
large-scale power plants? 鈥淵es, I am convinced,鈥� says Stephan. 鈥淏ut the point is
to find someone willing to invest in the first one.鈥�

A SOLAR power plant that generates electricity after sunset sounds like a
contradiction in terms, but the Spanish experiment showed that the solar chimney
has this rare talent. Radiation passing through the collector heats not only the
air beneath it, but also the ground. This heat is released into the air in the
evening.

As the air cools, the chimney becomes more sensitive to small rises in
temperature. And Schlaich鈥檚 team found that by covering the soil with more
heat-absorbent materials, they could make the prototype generate electricity for
longer and longer into the evening. Schlaich now proposes placing coils of black
plastic water-filled tubes under the collector. By pumping water warmed during
the day into an insulated store and then returning it to the coils at night, he
calculates that the plant could be made to work at full capacity for 24 hours a
day.

Another idea tested in Spain was to use the outer portion of the collector to
grow food. Towards the rim, the air temperature is not too much above ambient
and the wind speed not too high. This also means that people can work beneath
much of the collector while the plant is functioning. Maintenance near the
centre could be more of a problem. In a 100-megawatt plant, the updraught would
be about 15 metres a second (54 kilometres an hour), which is bracing but not
overpowering. The temperature, however, could reach a staggering 35 掳C above
ambient.