Additionally,
the efficiency of the tower is determined by the difference
between the temperature in the collector and the temperature of the environment
at the top of the tower. A 1°C drop in temperature
over every 100m facilitates the necessary updraft effect from the tower.
The concept ensures effective operation even on cooler days, as it is primarily
dependent upon the temperature differential between the air under the collector
and air at the top of the tower. (SolarMission
website, 3)
Even
though this technology is simple, for it to be efficient it requires a large
area of greenhouse and a very tall tower.
There are currently no solar towers in use commercially. The technology got a big boost during the
seventies’ oil crisis in the U.S, but interest in it faded as the price of oil
dropped in the eighties.(1) Now that the
price of oil is again going up, solar tower technology is again getting noticed. Currently, there are two companies in the
process of building a solar tower that will be located in Buronga, in the
Wentworth Shire of New South, Australia. SolarMission, located in the U.S.,
owns the rights to solar tower technology and sold the license to EnviroMission
to use in Australia.
SolarMission is also a large holder of
EnviroMission stock. Therefore, as the
revenue comes in from the Australian solar tower it can start to be used by
SolarMission to possibly build one in Southern California.
(4)
China
is also interested in this technology, and EnviroMission may facilitate the
building of towers there in the near future. (1) Sunshine Energy, a firm representing Chinese
investors that want the technology in China, has put in enough money to equal
around 15% of the expanded issued capital of EnviroMission. (5) Australia
is a good place for this technology to start because there is a federal mandate
that by 2010 9500 GWh (gigawatt hours) must come from renewable resources that
do not emit greenhouse gasses. (6)
The
current solar tower technology was designed by the German engineer Professor
Jörg Schlaich (3). A 650-foot prototype
tower generating 50 MW was built in Manzanares,
Spain in 1982 and ran for
approximately 15,000 hours until 1989. After
the initial trial and error period was over, the tower had a 32-month period of
daily operation during which it ran for an average of 8.9 hours per day.(4) During this time it had a 0.53% solar to
electricity efficiency, which could be increased to 1.3% in a 100 MW unit.(7)
Prototype tower
Manzanares,
Spain (left 8,9)
|
|
The Australian
solar tower will be 1 KM high, about 3000 feet, which would make it the tallest
man-made structure in the world. The chimney
itself will be about 400 feet in diameter but the greenhouse will cover a
circle with a diameter of 5.6 KM or 3.5 miles.
The solar
thermal power station will be composed of three major components: the solar collector, solar tower and turbines. The large solar collector canopy will be
constructed from a transparent
material with heat enhancing properties.
The tower will be constructed
from reinforced high tensile concrete.
The large-scale turbines will be purpose
designed and constructed from lightweight alloy materials like those used in aircraft manufacture.” (SolarMission site,
10)
In
this plant, the air will heat from 35 degrees Celsius on the edges of the
greenhouse to around 70 degrees Celsius once it reaches the middle. It will move at about 35 mph up the tower, losing
about 1 degree Celsius for every 100 meters it travels. The tower will have 32 turbines in it. (11) A tower this size is necessary to supply the
proposed 200 MW per year – enough to supply about 200,000 Australian
households, or around 47,000 U.S.
households (based on 2003 estimates). (12)
At
least two things will change from the prototype tower. The Manzanares plant eventually had output
for 22 hours in a day because inside the greenhouse was black plastic containing
water. This stored heat from the daytime
and radiated it out at night, which meant the temperature differential from the
top of the tower and the air in the greenhouse could continue. In the Australian tower black mining scrap or
specially made heat storing solar cells will be used for this purpose. Also, the prototype had plastic sheet glazing
for the greenhouse, but this was found to be unstable because of wind vortices
that formed within the tower. All future
towers will probably use a special blend of toughened glass. (7)
The
project will cost about one billion Australian dollars (about US $500 million). As of 2004, it had the support of the
government and had been granted major project facilitation status. (6, 13) However, the materials are simple – just land,
concrete and turbines, as well as the glass/polycarbonate/plastic film the greenhouse
will be made out of. After the tower is
built, it will be very easy and cheap to maintain. In fact, it would only require a staff of
roughly 15 people to run it, unlike dams, which are both expensive to build and
can take up to 20 million dollars a year to maintain. (14,15)
There
are many advantages of a solar tower over non-renewable energy resources and
even some renewable sources. Obviously,
the tower releases no greenhouse gases such as CO2 or methane. By generating 200 MW from a solar tower and
not a fossil fuel, 830,000 tons of CO2 per year could be kept out of the
atmosphere. (1) Also, because of the heat retaining material within the
greenhouse, solar towers can maintain constant upward draft and therefore
energy generation for nearly 24 hours.
This is different from wind farms, which can only generate electricity
on an intermittent basis. The inside of the tower and greenhouse will be a
forgiving enough environment to allow maintenance crews to enter it while it is
operating. This is different from many
types of plants, especially nuclear, that need to be shut down during maintenance. (4) Finally,
unlike photovoltaic cells, the greenhouse can heat air using both direct and
diffuse sunlight, which is important for countries that often have cloud cover.
However,
there are several environmental problems associated with solar towers. The first is that they will be placed in the
middle of the desert. The desert areas
of the planet have a high albedo and currently reflect 35-45% of the sunlight they
receive.(16) Dark glass paneling, such
as would compose the greenhouse of the solar tower, would reflect less of the
sunlight and absorbing more heat. While
the change is albedo is probably not large enough to significantly affect the
albedo of the earth, it may have an effect on local climate.
Another
heat problem is associated with the thermal plume that will come from the top
of the tower. Air will be spewing out of
the top of the tower at around 60 degrees Celsius, about 50 degrees warmer than
the surrounding air at 1 KM high. (17)
Normally, air that is cooling will start to fall and release its
moisture as it descends. Because of
winds, this precipitation will land in areas closer to the poles than the
deserts. However, with superheated, dry
air streaming into the atmosphere above deserts, it will take longer for the
air to cool and descend. Depending on
the location of the solar towers, this could cause desertification in the areas
near the deserts.
There
are further problems with this heat plume.
Bo Nordell, a professor in the division of renewable energy at Luleå
University of Technology in Sweden,
has theorized that heat generated by human activities (such as energy
generation) is causing the base temperature of the earth to shift. This is
different from current greenhouse theory, which only takes into account the accumulation
of a heat trapping layer and not an increase in overall heat coming off of the
earth.
What
his theory describes is that generally the earth reflects around 35% of the
sunlight it receives, which then bounces back into space as shortwave
radiation. This short-wave radiation is permitted
to pass through the atmosphere surrounding the earth. The rest of the sunlight is absorbed by the
earth and then re-emitted as long-wave radiation, or heat. Much of this heat is trapped by the
atmosphere. “The total amount of heat
generated by fossil fuels is 1014 kWh. By distributing this energy over the
total area of the Earth, an additional 0.02 W m−2 [watts per meter
squared] is heating the planet.” (Nordell, 18)
Solar
towers, because they would dramatically heat large portions of air, might
contribute to this problem. However, one
wonders whether the lack of greenhouse gasses emitted from these towers might
counterbalance the effects of the heat it is producing heat, as opposed to a
fossil-fuel burning power plant that produces both heat and greenhouse
gas. Actually, since these plants will
probably be located in desert areas, they will likely power desert cities, and
one of the biggest problems with those is the “cold island” effect. Perhaps by encouraging growth of desert
cities, solar towers will cancel out their own heat generation.
A
more local problem is that of ecosystem degradation. The area around Buronga is mostly state land,
pretty rural. Most solar towers would
probably be placed in rural areas, and since they take up almost 32.5 KM
squared it will have a pretty significant impact on local ecosystems. Road building and turbine noise will also
cause problems. Also, these towers, at 1
km high, will be able to be seen from very far away.
However,
since the towers are rather interesting looking there could be a lot of revenue
to be had from tourism. In fact, almost 35 jobs in the solar tower would be
dedicated solely to tourist maintenance.
Not only would people come to see the tower itself,
thereby generating income for EnviroMission itself, the people would increase
visitation to nearby towns and pour money into them. Wentworth Shire, where the Australian plant
is to be located, is glad to have it near them for the tourism.(19) However, there could be a negative impact as far as
outdoors-type tourists are concerned.
People who want to go hiking in the desert are going to avoid areas
where they can see a 1 km tall tower looming above them.
However,
there will probably will be many more people
interested in seeing such an enormous structure than want to hike or camp in
pristine desert areas. Also, solar
towers will likely be placed close to cities and not out in the wild so that
there is not much energy loss in transferring electricity to cities, where it
is needed. One must weigh the overall
benefits to the earth with potential local negative effects. Since solar towers do not generate greenhouse
gases and are cheap to maintain, they seem like a very real alternative to
fossil fuel energy. There is no
information available as to the competitiveness of solar tower energy, since it
is not yet being produced. One can
imagine that for the first ten years the companies will suffer losses due to the
large building expense. However, as they
pay off that debt, and as green energy becomes more in demand (especially in Australia,
with is governmental mandate), the returns from a solar tower could be quite
high, and the price could become very competitive. It is a good thing that the government of Australia
is taking an interest in this technology, since it represents renewed
commitment to the pressing problem of global climate change.
1. http://wired-vig.wired.com/news/technology/0,1282,66694- 00.html?tw=wn_story_page_next12.
2. http://www.solarserver.de/lexikon/aufwindkraftwerk.html
3. http://www.solarmissiontechnologies.com/project-tower.htm
4. http://www.solarmissiontechnologies.com/project-pilotplant.htm
5.
http://solar-club.web.cern.ch/solar-club/SolPass/Futur/toursolaires.html
6. http://www.enviromission.com.au/
7. Mills, D. Advances
in Solar Thermal Technology
Solar Energy Volume 76, Issues 1- 3,
Pages 19-31 January – March 2004
http://www.sciencedirect.com/science?_ob=ArticleURL&_aset=V-WA-A-W- YV-MsSAYVA-UUA-U-AAAZUCUACC-AAABZBUECC-VYZYAABU-YV- U&_rdoc=14&_fmt=full&_udi=B6V50-48FC6SP- 2&_coverDate=03%2F31%2F2004&_cdi=5772&_orig=search&_st=13&_sort=d &view=c&_acct=C000020258&_version=1&_urlVersion=0&_userid=423519& md5=95d5e866da92d48527dc1bd6718587e7#toc16
8 . http://www.greenenergyjobs.com/solar-intro.php
9. http://www.stirlingengines.org.uk/sun/sola6.html
10. http://www.solarmissiontechnologies.com/project-tower.htm
11.(http://www.solarmissiontechnologies.com/FAQs.htm
12. http://www.padoma.com/industry.html
13 http://www.betterhumans.com/Errors/index.aspx?aspxerrorpath=/
Giant_Solar_Chimney_to_Power_Australian_Homes.Article.2003-01-05-1.aspx
14. http://www.solarmissiontechnologies.com/project-collector.htm
15. http://www2.kenyon.edu/Projects/Dams/gec02tfin.html
16.
http://www.physicalgeography.net/fundamentals/7f.html
17. http://www.agu.org/sci_soc/mockler.html
18. Nordell, Bo Thermal pollution causes global warming
Global and Planetary Change
Volume 38, Issues 3-4 , September
2003, Pages 305-312
http://www.sciencedirect.com/science?_ob=ArticleURL&_aset=V-WA-A-W-WU-MsSAYWA-UUA-U-AAAZAAEBYV-AAABDUUAYV-CAUYCUCB-WU-U&_rdoc=3&_fmt=full&_udi=B6VF0-49FGSB1-1&_coverDate=09%2F30%2F2003&_cdi=5996&_orig=search&_st=13&_sort=d&view=c&_acct=C000020258&_version=1&_urlVersion=0&_userid=423519&md5=b1aac5297c6a9c8b9f0ac57a5d5e1055#toc1
19. http://www.wired.com/news/technology/0,1282,54917,00.html
