The
solar updraft tower is a
renewable-energyRenewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable . About 16% of global final energy consumption comes from renewables, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from...
power plant. It combines the chimney effect, the
greenhouse effectThe greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...
and the
wind turbineA wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
. Air is heated by sunshine and contained in a very large
greenhouseA greenhouse is a building in which plants are grown. These structures range in size from small sheds to very large buildings...
-like structure around the base of a tall chimney, and the resulting
convectionConvection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
causes air to rise up the updraft tower. This airflow drives turbines, which produce
electricityElectricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
.
Description
The generating ability of a solar updraft power plant depends primarily on two factors: the collector area and the chimney height. With a larger collector area, a greater volume of air is warmed to flow up the chimney; collector areas as large as 7 kilometres (4.3 mi) in diameter have been considered. With a larger chimney height, the pressure difference increases the
stack effectStack effect is the movement of air into and out of buildings, chimneys, flue gas stacks, or other containers, and is driven by buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The result is either a positive or...
; chimneys as tall as 1000 metres (3,281 ft) have been considered.
Heat can be stored inside the collector area greenhouse to be used to warm the air later on. Water, with its relatively high specific heat capacity, can be filled in tubes placed under the collector, increasing the energy storage as needed.
TurbinesA wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
can be installed in a ring around the base of the tower, with a horizontal axis, as formerly planned for the Australian project and seen in the diagram above; or—as in the prototype in Spain—a single vertical axis turbine can be installed inside the chimney.
Carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
is emitted only negligibly while operating, but is emitted more significantly during manufacture of its construction materials, particularly
cementIn the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
. Net energy payback is estimated to be 2–3 years.
A solar updraft tower power station would consume a significant area of land if it were designed to generate as much electricity as is produced by modern power stations using conventional technology. Construction would be most likely in hot areas with large amounts of very low-value land, such as deserts, or otherwise
degradedLand degradation is a process in which the value of the biophysical environment is affected by one or more combination of human-induced processes acting upon the land....
land.
A small-scale solar updraft tower may be an attractive option for remote regions in developing countries. The relatively low-tech approach could allow local resources and labour to be used for its construction and maintenance.
History
In 1903, Isidoro Cabanyes, a colonel in the Spanish army, proposed a solar chimney power plant in the magazine
La energía eléctrica. One of the next earliest descriptions of a solar chimney power plant was written in 1931 by a German author,
Hanns GüntherWalter de Haas , who wrote under the pseudonym Hanns Günther was a prolific German author, translator, and editor of popular science books.- Work :...
. Beginning in 1975, Robert E. Lucier applied for
patentA patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
s on a solar chimney electric power generator; between 1978 and 1981 these patents (since expired) were granted in Australia, Canada, Israel, and the USA.
First Prototype in Spain
In 1982, a small-scale experimental model of a solar draft tower was built in
Manzanares, Ciudad RealManzanares, Ciudad Real is a municipality in Ciudad Real, Castile-La Mancha, Spain. It has a population of 19,186 . It is located near the Autovía A-4 ....
, 150 km south of
MadridMadrid is the capital and largest city of Spain. The population of the city is roughly 3.3 million and the entire population of the Madrid metropolitan area is calculated to be 6.271 million. It is the third largest city in the European Union, after London and Berlin, and its metropolitan...
, Spain at 39°02′34.45"N 3°15′12.21"W. The power plant operated for approximately eight years. The draft tower's guy-wires, which were not protected against corrosion, failed due to rust and broke in a storm. This caused the tower to fall over. The plant was decommissioned in 1989.
Inexpensive materials were used in order to evaluate their performance. The solar tower was built of iron plating only 1.25 mm thick
under the direction of a German engineer,
Jörg SchlaichProf. Dr.-Ing. Drs. h.c. Jörg Schlaich is a German structural engineer. He later founded his own firm Schlaich Bergermann & Partner.-Early career:...
. The project was funded by the German government.
The chimney had a height of 195 metres and a diameter of 10 metres with a collection area (greenhouse) of 46000 square metres (11.4 acre), 244 m diameter, obtaining a maximum power output of about 50
kWkw or KW may refer to:* Kuwait, ISO 3166-1 country code** .kw, the country code top level domain for Kuwait* Kilowatt* Self-ionization of water Kw* Cornish language's ISO 639 code* Kitchener–Waterloo, Ontario, Canada...
. Different materials were used for testing, such as single or double glazing or plastic (which turned out not to be durable enough), and one section was used as an actual greenhouse, growing plants under the glass. During its operation, optimization data was collected on a second-by-second basis with 180 sensors measuring inside and outside temperature, humidity and wind speed. This was an experimental setup that did not sell energy to produce income.
Jinshawan Updraft Tower
In December 2010, a solar updraft tower in Jinshawan in
Inner MongoliaInner Mongolia is an autonomous region of the People's Republic of China, located in the northern region of the country. Inner Mongolia shares an international border with the countries of Mongolia and the Russian Federation...
,
ChinaChinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
started operation, producing 200-kilowatts of electric power. The 1.38 billion RMB (USD 208 million) project was started in May 2009 and its aim is to build a facility covering 277 hectares and producing 27.5 MW by 2013. The greenhouses will also improve the climate by covering moving sand, restraining sandstorms.
Ciudad Real Torre Solar
There is a proposal to construct a solar updraft tower in
Ciudad RealCiudad Real is a city in Castile-La Mancha, Spain, with a population of c. 74,000. It is the capital of the province of Ciudad Real. It has a stop on the AVE high-speed rail line and has begun to grow as a long-distance commuter suburb of Madrid, located 115 miles to the north. A high capacity...
, Spain, entitled
Ciudad Real Torre Solar. If built, it would be the first of its kind in the
European UnionThe European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
and would stand 750 metres tall – nearly twice as tall as the current tallest structure in the EU, the
Belmont TV MastThe Belmont transmitting station is a broadcasting and telecommunications facility, situated next to the B1225, one mile west of the village of Donington on Bain in the civil parish of South Willingham, near Market Rasen and Louth in Lincolnshire, England . It is owned and operated by Arqiva.It has...
– covering an area of 350
hectareThe hectare is a metric unit of area defined as 10,000 square metres , and primarily used in the measurement of land. In 1795, when the metric system was introduced, the are was defined as being 100 square metres and the hectare was thus 100 ares or 1/100 km2...
s (about 865 acres).
It is expected to put out 40 MW of electricity.
Australian proposal
EnviroMissionEnviroMission is an Australian listed company. It has, since 2001, proposed to build a solar updraft tower power generating station known as Solar Tower Buronga in western New South Wales at a site 25 km northeast of Mildura. More recently, 12 February 2007, EnviroMission also claims to be...
in 2001, proposed to build a solar updraft tower power generating station known as
Solar Tower Buronga near
Buronga, New South WalesBuronga is a town in New South Wales, Australia on the Murray River. The George Chaffey Bridge connects Buronga with Mildura, Victoria. In 2006, the town had a population of 807 people.-Buronga Today:...
. The company did not complete the project and now plans a similar plant in Arizona.
Botswana test facility
Based on the need for plans for long-term energy strategies,
BotswanaBotswana, officially the Republic of Botswana , is a landlocked country located in Southern Africa. The citizens are referred to as "Batswana" . Formerly the British protectorate of Bechuanaland, Botswana adopted its new name after becoming independent within the Commonwealth on 30 September 1966...
's Ministry of Science and Technology designed and built a small-scale solar chimney system for research. This experiment ran from 7 October to 22 November 2005. It had an inside diameter of 2 m and a height of 22m and was manufactured from glass-reinforced polyester material, with a collection base area of approximately 160 m
2. The roof was made of a 5 mm thick clear glass that was supported by a steel framework.
Namibian proposal
In mid 2008, the
NamibiaNamibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
n government approved a proposal for the construction of a 400 MW solar chimney called the 'Greentower'. The tower is planned to be 1.5 km tall and 280 m in diameter, and the base will consist of a 37 km
2 greenhouse in which cash crops can be grown.
Turkish model
A model solar updraft tower was constructed in Turkey as a civil engineering project. Functionality and outcomes are obscure.
Arizona projects
In October 2010,
EnviroMissionEnviroMission is an Australian listed company. It has, since 2001, proposed to build a solar updraft tower power generating station known as Solar Tower Buronga in western New South Wales at a site 25 km northeast of Mildura. More recently, 12 February 2007, EnviroMission also claims to be...
announced further plans to build two 200 MW Solar Updraft Towers in Western Arizona.
Southern California Public Power AuthorityThe Southern California Public Power Authority is a joint powers authority, or a collective of 10 municipal utilities and one irrigation district. SCPPA was created in 1980 to help finance the acquisition of generation and transmission resources for its members. SCPPA is the 26th largest power...
(SCPPA) has agreed to negotiate a power-purchase agreement with
EnviroMissionEnviroMission is an Australian listed company. It has, since 2001, proposed to build a solar updraft tower power generating station known as Solar Tower Buronga in western New South Wales at a site 25 km northeast of Mildura. More recently, 12 February 2007, EnviroMission also claims to be...
for electricity from its Arizona power plants, should they get built, and the project has been listed by the SCPPA. As of January 2011, the company has secured $29.8 million in debt and equity from AGS Capital Group. In August 2011, United States construction services contractor, Hensel Phelps
Construction Co. was engaged for delivery of a construction schedule and cost estimate of a 200 MW - producing tower for Enviromission in Arizona.
Conversion rate of solar energy to electrical energy
The solar updraft tower has power conversion rate considerably lower than many other designs in the (high temperature) solar thermal group of collectors. The low conversion rate of the Solar Tower is balanced to some extent by the low investment cost per square metre of solar collection.
According to model calculations, it was estimated that a 100 MW plant would require a 1000 m tower and a greenhouse of 20 km
2. A 200 MW power plant with the same 1000-metre-high tower would need a collector 7 kilometres in diameter (total area of about 38 km²). One 200MW power station will provide enough electricity for around 200,000 typical households and will abate over 900,000 tons of greenhouse producing gases from entering the environment annually. The 38 km² collecting area is expected to extract about 0.5 percent, or 5 W/m² of 1 kW/m², of the solar power that falls upon it. Note that in comparison, concentrating thermal (CSP) or photovoltaic
(CPV)Concentrated photovoltaic technology uses optics such as lenses to concentrate a large amount of sunlight onto a small area of solar photovoltaic materials to generate electricity...
solar power plants have an efficiency ranging between 20% to 31.25% (dish Stirling), although these approaches do not attain 100% utilization of land area which should be considered when contemplating efficiency versus foot print. Because no data is available to test these models on a large-scale updraft tower there remains uncertainty about the reliability of these calculations.
The performance of an updraft tower may be degraded by factors such as atmospheric winds, by drag induced by the bracings used for supporting the chimney, and by reflection off the top of the greenhouse canopy.
Using Carnot's theorem, the upper limit of efficiency can be found:
For example, if the air entering the base of the tower was 353 k and the surrounding air at the top of the tower was 283 k, which matched the exhaust temperature of the tower, then the maximum efficiency would be ~20%. However, if the above 100 MW plant requiring 20 km
2, and the peak solar radiation falling on the ground was ~1 kWm
-2, the efficiency would be 0.5% as there would be 5 Wm
-2. Thus, 39 units of potentially available energy are consumed for every single unit captured. For perspective, PV panels providing the same amount of energy (assuming they operate at a quite feasible ~20%), would occupy a 40th of the land. Conversely, in the case of the 100 MW system, the same land usage with PV panels could produce 4,000 MW. If the PV panels were operating at the maximum theoretical efficiency of 29%, the power output would be increased to 5800 MW. However, PV panels do not work at night as they have no inbuilt energy storage, perhaps making a tower more suitable for baseline power generation.
It is possible to combine the land use of a solar updraft tower with other uses, in order to make it more cost effective, and in some cases, to increase its total power output. Examples are the positioning of solar collectors or
photovoltaicsPhotovoltaics is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material...
underneath the updraft tower collector. This could be combined with agricultural use.
Mountainside solar draft tower
In 1926 Prof Engineer Bernard Dubos proposed to the French Academy of Sciences the construction of a Solar Aero-Electric Power Plant in North Africa with its solar chimney on the slope of a large mountain. A mountainside updraft tower can also function as a vertical greenhouse.
Arctic solar draft tower
A Solar updraft power plant located at high latitudes such as in Canada, could produce up to 85 per cent of the output of a similar plant located closer to the equator, but only if the collection area is sloped significantly southward. The sloped collector field is built at suitable mountain hills, which also functions as a chimney. Then a short vertical chimney is added to install the vertical axis air turbine. The results showed that solar chimney power plants at high latitudes may have satisfactory thermal performance.
Related ideas and adaptations
- The inverse of the solar updraft tower is the downdraft-driven energy tower
The energy tower is a device for producing electrical power. The brainchild of Dr. Phillip Carlson, expanded by Professor Dan Zaslavsky and Dr. Rami Guetta from the Technion...
. Evaporation of sprayed water at the top of the tower would cause a downdraft by cooling the air and driving wind turbines at the bottom of the tower.
- The solar chimney could be constructed up a mountainside using inclined terrain for support; this could draw power from the updraft out of a thermal inversion, and improve urban air quality.
- The atmospheric vortex
The concept of a vortex engine, independently proposed by Norman Louat and Louis M. Michaud, aims to replace large physical chimneys with a vortex of air created by a shorter, less-expensive structure....
proposal replaces the physical chimney by a controlled or 'anchored' cyclonic updraft vortex. Depending on the column gradient of temperature and pressure, or buoyancy, and stability of the vortex, very high-altitude updraft may be achievable. As an alternate to a solar collector, industrial and urban waste-heat could be used to initiate and sustain the updraft in the vortex.
- A saltwater thermal sink
A solar pond is a pool of saltwater which acts as a large-scale solar thermal energy collector with integral heat storage for supplying thermal energy...
in the collector could 'flatten' the diurnal variation in energy output, while airflow humidification in the collector and condensation in the updraft could increase the energy flux of the system.
- Release of humid ground-level air from an atmospheric vortex or solar chimney at altitude could form clouds or precipitation, potentially altering local hydrology. Local de-desertification, or afforestation could be achieved if a regional water cycle were established and sustained in an otherwise arid area.
- Fitted with a vortex chimney scrubber, the updraft could be cleaned of particulate air pollution. The solar cyclone distiller could extract atmospheric water by condensation in the updraft of the chimney.
- This solar cyclonic water distiller with a solar collector pond
A solar pond is a pool of saltwater which acts as a large-scale solar thermal energy collector with integral heat storage for supplying thermal energy...
could adapt the solar collector-chimney system for large-scale desalination of collected brine, brackish- or waste-water pooled in the collector base.
- A form of solar boiler technology placed directly above the turbine at the base of the tower might increase the up-draught.
- If the chimney updraft is an ionized vortex, then the electro-magnetic field could be tapped for electricity, using the airflow and chimney as a generator.
- Energy production and water desalination could be used to support carbon-fixing or food-producing local agriculture, and for intensive aquaculture
Algaculture is a form of aquaculture involving the farming of species of algae.The majority of algae that are intentionally cultivated fall into the category of microalgae...
and horticultureHorticulture is the industry and science of plant cultivation including the process of preparing soil for the planting of seeds, tubers, or cuttings. Horticulturists work and conduct research in the disciplines of plant propagation and cultivation, crop production, plant breeding and genetic...
under the solar collector as a greenhouse.
Financial feasibility
This section discusses only the simplest, classical design of a solar updraft tower power plant, and variations are not considered.
A solar updraft power station would require a large initial capital outlay, but would have relatively low operating cost. However, the capital outlay required is roughly the same as next-generation nuclear plants such as the AP-1000 at roughly $5 per W of capacity. Like other renewable power sources there would be no cost for fuel. The cost per energy is largely determined by interest rates and years of operation, varying from 5 eurocent per kWh for 4% and 20 years to 15 eurocent per kWh for 12% and 40 years.
A disadvantage of a solar updraft tower is the much lower conversion efficiency than
concentrating solar power stationsThe solar power tower is a type of solar furnace using a tower to receive the focused sunlight. It uses an array of flat, movable mirrors to focus the sun's rays upon a collector tower...
have, thus requiring a larger collector area and leading to higher cost of construction and maintenance.
Financial comparisons between solar updraft towers and concentrating solar technologies contrast a larger, simpler structure against a smaller, more complex structure. The "better" of the two methods is the subject of much speculation and debate.
A solar tower is expected to have less of a requirement for standby capacity from traditional energy sources than
wind powerWind power is the conversion of wind energy into a useful form of energy, such as using wind turbines to make electricity, windmills for mechanical power, windpumps for water pumping or drainage, or sails to propel ships....
does. Various types of thermal storage mechanisms (such as a heat-absorbing surface material or salt water ponds) could be incorporated to smooth out power yields over the day/night cycle. Most renewable power systems (wind, solar-electrical) are variable, and a typical national electrical grid requires a combination of base, variable and on-demand power sources for stability. However, since
distributed generationDistributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from many small energy sources....
by intermittent power sources provides "smoothing" of the rate of change, this issue of variability can also be addressed by a large interconnected electrical
super gridA super grid is a wide area transmission network that makes it possible to trade high volumes of electricity across great distances. It is sometimes also referred to as a "mega grid".-History:...
, incorporating wind farms, hydroelectric, and
solar powerSolar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation, along with secondary solar-powered resources such as wind and wave power, hydroelectricity and biomass, account for most of the available...
stations.
There is still a great amount of uncertainty and debate on what the cost of production for electricity would be for a solar updraft tower and whether a tower (large or small) can be made profitable. Schlaich et al. estimate a cost of electricity between 7 (for a 200 MW plant) and 21 (for a 5 MW plant) euro cents per kWh, but other estimates indicate that the electricity cannot possibly be cheaper than 25-35 cents per kWh. Compare this to LECs of approximately 3 Euro cents per KWh for a 100 MW wind or natural gas plant. No reliable electricity cost figures will exist until such time as actual data are available on a utility scale power plant, since cost predictions for a time scale of 25 years or more are unreliable.
External links