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Solar
Tower of Power
A world first in our backyard?
Solar Energy. Wind Power. These are some of the phrases which
spring to mind when environmentally-sensitive generation methods
are mentioned. For decades these have been small scale, “fringe”
technologies, too expensive and impractical to replace fossil fuel
power. Back in our March issue we briefly mentioned a proposal to
combine wind and solar power in a massive “power station”.
It’s progressing beyond the drawing board . . .
By Sammy Isreb
M
elbourne-based EnviroMission is an energy company with a difference. That much is obvious
from their plans to build a 1000 metre tall ‘power
station’ 70km east of the Victorian town of Mildura.
By September 2005, all things going to plan, they aim to
have built not only the world’s tallest manmade structure
along with the world’s largest “greenhouse” – but a 200MW
solar power station into the bargain.
Scientific testing has already commenced at the proposed site.
The Principle
The Solar Mission project is based on the “Solar Tower”
design by Professor Jorg Schlaich from the University of
Stuttgart, Germany. The basic principle of operation is the
use of the Sun’s radiation to heat a very large body of enclosed air. Being warmer than the surrounding atmosphere,
this air will begin to rise. By causing it to flow through
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windmill-style turbines on its journey up a tall chimney,
electricity can be generated.
Obviously, generating 200MW of power in this way is
no mean feat. The ‘greenhouse’ collector will be a roughly
circular canopy of transparent plastic material measuring
approximately 5km in diameter. This canopy, or roof, will
slope upwards towards the centre drawing in air from the
edges. In the centre will reside the tower, a 1000-metre- tall
structure with a base around 170 metres wide.
On a sunny day, the air at the bottom of the tower will
be around 35°C greater than the ambient air temperature,
causing it to flow at roughly 15 metres per second.
In the lower atmosphere, as a general rule, temperatures fall by around 1°C per 100 metres of altitude. Thus
at the top of the tower, the ambient air temperature will
be around 10°C cooler than that at the bottom, without
even taking into consideration the heating effect of the
greenhouse.
July 2002 7
About 40 metres up from the ground,
32 Kaplan-style turbines placed in the
chimney will be driven by the rising
air, in turn driving generators.
An increase in generated power
could be achieved by either increasing
the size of the solar collector or the
height of the chimney, or both.
Night Generation
Here’s a somewhat simplified diagram
showing how the massive Solar Tower
works. And the beauty of the system
is that it is so simple!
SUNLIGHT ENTERS
“GREENHOUSE” AND
WARMS AIR INSIDE
One of the most attractive features of
the Solar Tower over that of traditional 1000m
solar generation methods is its capacity to generate electricity under cloud
AIR
cover, or even during the night.
In order to achieve this, sealed water
tubes are placed under the canopy,
filled only once during manufacture.
During daylight hours incident solar
radiation will heat this very large mass of water. At night,
that heat will be released. Varying the amount of water
under the canopy will alter the output versus time of day
profile of the power station.
Through this design, the Solar Tower technology avoids
becoming a rapid peak generator, instead having the capacity to produce a much smoother load curve, with very
low output variance. This aids in interconnection to the
supply grid, avoiding the need to coordinate generation
and demand peaks which normally plague green power
production methods.
Pilot Program
For seven years a pilot 50kW prototype Solar Tower plant
was successfully operated in Manzanares, Spain. Built by
the Spanish government in collaboration with designer
Professor Jorg Schlaich, the plant proved the technology
to be technically feasible.
Operated from 1982 until 1989, this pilot plant featured
a 195-metre-tall chimney, with a collector diameter of 240
metres. Operational data acquired over this 7-year period
has been used in the scaling and design of the proposed
200MW plant.
It is this successful pilot plant operation which sets apart
the Solar Mission project from other large scale speculative
GENERATOR
TURBINE
POWER TO GRID
AIR
WARM AIR
RISES UP
CHIMNEY
5000m
alternative energy generation projects.
Site Determination
Currently the Mildura site is the favored location for
the Solar Mission plant, with geotechnical testing being
undertaken to confirm its suitability. With lessons learnt
from the Spanish plant, final site determination will be
made using the following criteria:
• Solar Radiation Levels
• Weather Patterns
• Geological Stability
• Access to the Electricity Grid
• Geographical features
• Government and community support
Economic Feasibility
Calculating the production cost of Solar Tower electricity is much simpler than that of traditional coal-sourced
electricity.
While coal plants have many cost inputs, including fuel,
mining and transport, plant maintenance and even mining
site remediation, the major cost for the Solar Mission is the
capital cost of production (land acquisition and building)
and associated finance costs. And some Governments
have started to place taxes on major polluters – coal-fired
It’s not some crazy idea which will never work: these photos show the pilot plant built some 10 years ago at Manzanares in Spain. Yes, it does look like a greenhouse!
8 Silicon Chip
www.siliconchip.com.au
This “viewed from above” drawing gives an even better
idea of the massive size of the project, both solar collector
and chimney. Compare the road and building scale to that
of the collector and tower!
power stations are firmly in their sights.
Since the operation and maintenance cost is comparatively low, a direct correlation between the prevalent
interest rate and the electricity production cost can be
made. With an interest rate of 11% and a 4-year cycle to
production, the cost of ‘solar’ electricity is a mere 20%
higher than coal generated power. With an interest rate
of 8% (roughly that currently available), the cost of solar
electricity will match that of coal-powered plants.
Environmental Benefits
Compared to Victoria’s coal-generation facilities, the
200MW Solar Mission project is relatively small. Howev-
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er, it will provide enough electricity for around 200,000
typical Australian homes.
Electricity demand, and thus selling price, is highest
during the hottest days of the summer months, at which
time the Solar Mission plant will be at its production
peak. Each year it is estimated that the plant will reduce
carbon dioxide output by an astounding 900,000 tonnes,
satisfying both Australia’s Kyoto treaty obligations and the
1997 federal legislation stipulating that 9500GWh of the
nation’s electricity must come from clean, green renewable
sources by 2010.
With such a focus on green electricity, a successful
implementation by EnviroMission could very well make
them leaders in this new market.
Conclusion
If the preliminary site testing yields positive results and
all regulatory hurdles are met, a ‘world first’ in commercial green power generation technology could be up and
running in Victoria by late 2005.
If this large-scale project proves successful it could
revolutionise environmentally-friendly power generation
in temperate climates.
Admittedly, Solar Tower technology will probably
never fully replace the ease of tried and proved fossil fuel
technologies but it will go a long way to redressing the
incredibly heavily reliance on non-renewable technologies.
And that’s a step in the right direction.
Acknowldegement: Thanks to Solar Misson for the
use of their illustrations.
SC
July 2002 9
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