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The solar updraft tower (SUT) is a renewable-energy power plant for generating electricity from solar power. Sunshine heats the air beneath a very wide greenhouse-like roofed collector structure surrounding the central base of a very tall chimney tower. The resulting convection causes a hot air updraft in the tower by the chimney effect. This airflow drives wind turbines placed in the chimney updraft or around the chimney base to produce electricity. Plans for scaled-up versions of demonstration models will allow significant power generation, and may allow development of other applications, such as water extraction or distillation, and agriculture or horticulture.
As a solar chimney power plant (SCPP) proposal for electrical power generation, commercial investment is discouraged by the high initial cost of building a very large novel structure, and by the risk of investment in a feasible but unproven application of even proven component technology for long-term returns on investment—especially when compared to the proven and demonstrated greater short-term returns on lesser investment in coal-fired or nuclear power plants. Likewise, the benefits of 'clean' or solar power technologies are shared, and the widely shared harmful pollution of existing power generation technologies is not applied as a cost for private commercial investment. This is a well-described economic trade-off between private benefit and shared cost, versus shared benefit and private cost. If it is in the public interest, then some form of public investment or subsidy to share cost and risk will be required to demonstrate SCPP feasibility at scale.
Power output depends primarily on two factors: collector area and chimney height. A larger area collects and warms a greater volume of air to flow up the chimney; collector areas as large as 7 kilometres (4.3 mi) in diameter have been discussed. A larger chimney height increases the pressure difference via the stack effect; chimneys as tall as 1,000 metres (3,281 ft) have been discussed. Due to variations in design, climate, local geography and latitude, a standardised model for comparisons between design features and outputs is needed and proposed
Heat can be stored inside the collector area. The ground beneath the solar collector, water in bags or tubes, or a saltwater thermal sink in the collector could add thermal capacity and inertia to the collector. Humidity of the updraft and condensation in the chimney could increase the energy flux of the system.
Turbines with a horizontal axis can be installed in a ring around the base of the tower, as once planned for an 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 dioxide is emitted only negligibly as part of operations. Manufacturing and construction require substantial power, particularly to produce cement. Net energy payback is estimated to be 2–3 years.
Since solar collectors occupy significant amounts of land, deserts and other low-value sites are most likely.
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 construction and maintenance.
Locating a tower at high latitudes could produce up to 85 per cent of the output of a similar plant located closer to the equator, if the collection area is sloped significantly toward the equator. The sloped collector field is built on suitable mountainsides, which also functions as a chimney. A short vertical chimney on the mountaintop to accommodate the vertical axis air turbine. The results showed that solar chimney power plants at high latitudes may have satisfactory thermal performance.
Solar updraft towers can be combined with other technologies to increase output. Solar thermal collectors or photovoltaics can be arranged inside the collector greenhouse. This could further be combined with agriculture.
In 1903, Isidoro Cabanyes, a colonel in the Spanish army, proposed a solar chimney power plant in the magazine La energía eléctrica. Another early description was published in 1931 by German author Hanns Günther. Beginning in 1975, Robert E. Lucier applied for patents on a solar chimney electric power generator; between 1978 and 1981 patents (since expired) were granted in Australia, Canada, Israel, and the USA.
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. 
In 1982, a small-scale experimental model of a solar draft tower was built in Manzanares, Ciudad Real, 150 km south of Madrid, Spain at . The power plant operated for approximately eight years. The tower's guy-wires were not protected against corrosion and failed due to rust and storm winds. The tower blew over and 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 millimetres (0.049 in) thick under the direction of a German engineer, Jörg Schlaich. The project was funded by the German government.
The chimney had a height of 195 metres (640 ft) and a diameter of 10 metres (33 ft) with a collection area (greenhouse) of 46 hectares (110 acres) and a diameter of 244 metres (801 ft), obtaining a maximum power output of about 50 kW. Various materials were used for testing, such as single or double glazing or plastic (which turned out not to be durable enough). One section was used as an actual greenhouse. During its operation, 180 sensors measured inside and outside temperature, humidity and wind speed data was collected on a second-by-second basis. This experiment setup did not sell energy.
In December 2010, a tower in Jinshawan in Inner Mongolia, China started operation, producing 200 kilowatts. The 1.38 billion RMB (USD 208 million) project was started in May 2009 and intends to cover 277 hectares (680 acres) and produce 27.5 MW by 2013. The greenhouse is expected to improve the climate by covering loose sand, restraining sandstorms.
A proposal to construct a updraft tower in Fuente el Fresno, Ciudad Real, Spain, entitled Ciudad Real Torre Solar would be the first of its kind in the European Union and would stand 750 metres (2,460 ft) tall – nearly twice as tall as the continent's tallest structure, the Belmont TV Mast – covering an area of 350 hectares (860 acres). It is expected to produce 40 MW.
In 2001, EnviroMission proposed to build a solar updraft tower power generating plant known as Solar Tower Buronga near Buronga, New South Wales. The company did not complete the project. They have plans for a similar plant in Arizona.
In December 2011, Hyperion Energy, controlled by Western Australians Tony Sage and Dallas Dempster, was reported to be planning to build a 1-km-tall solar updraft tower near Meekatharra to supply power to Mid-West mining projects.
Based on the need for plans for long-term energy strategies, Botswana's Ministry of Science and Technology designed and built a small-scale research tower. This experiment ran from 7 October to 22 November 2005. It had an inside diameter of 2 metres (6.6 ft) and a height of 22 metres (72 ft), manufactured from glass-reinforced polyester, with an area of approximately 160 square metres (1,700 sq ft). The roof was made of a 5 mm thick clear glass supported by a steel framework.
In mid-2008, the Namibian government approved a proposal for the construction of a 400 MW solar chimney called the 'Greentower'. The tower is planned to be 1.5 kilometres (4,900 ft) tall and 280 metres (920 ft) in diameter, and the base will consist of a 37 square kilometres (14 sq mi) greenhouse in which cash crops can be grown.
In October 2010, EnviroMission announced further plans to build two 200 MW Solar Updraft Towers in Western Arizona. Southern California Public Power Authority (SCPPA) has agreed to negotiate a power purchase agreement with EnviroMission. The project has been listed by the SCPPA. As of January 2011, the company had secured $29.8 million in financing 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 tower. A potential construction roadblock is its potential environmental impact. Environmental concerns have arisen over desert solar panel arrays in the US Southwest.(registration required)  A June 2013 status update on the project was made by Roy Spencer which says the project is proceeding with the permit process.
A grade-school pupil's home do-it-yourself SUT demonstration for a school science fair was constructed and studied in 2012, in a suburban Connecticut setting. With a 7 metre stack and 100 square metre collector, this generated a daily average 6.34 mW, from a computer fan as a turbine. Insolation and wind were the major factors on variance (range from 0.12 to 21.78 mW) in output.
The solar updraft tower has a power conversion rate considerably lower than many other designs in the (high temperature) solar thermal group of collectors. The low conversion rate is balanced to some extent by the lower cost per square metre of solar collection.
Model calculations estimate that a 100 MW plant would require a 1,000 m tower and a greenhouse of 20 square kilometres (7.7 sq mi). A 200 MW tower with the same tower would require 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 collector area is expected to extract about 0.5 percent, or 5 W/m² of 1 kW/m², of the solar energy that falls upon it. Concentrating thermal (CSP) or photovoltaic (CPV) solar power plants range between 20% to 31.25% efficiency (dish Stirling). Overall CSP/CPV efficiency is reduced because collectors do not cover the entire footprint. Without further tests, the accuracy of these calculations is uncertain.
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.
Related ideas and adaptations
- The atmospheric vortex 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.
- Telescopic or retractable design may lower a very high chimney for maintenance, or to prevent storm damage. Hot-air balloon chimney suspension has also been proposed.
- A saltwater thermal sink 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 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, water desalination or simple atmospheric water extraction could be used to support carbon-fixing or food-producing local agriculture, and for intensive aquaculture and horticulture under the solar collector as a greenhouse.
A solar updraft power station would require a large initial capital outlay, but would have relatively low operating cost.
Capital outlays would be roughly the same as next-generation nuclear plants such as the AP-1000 at roughly $5 per Watt of capacity. As with other renewable power sources, towers have no need for fuel. Overall costs are 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.
Estimates of total costs range from 7 (for a 200 MW plant) and 21 (for a 5 MW plant) euro cents per kWh to 25–35 cents per kWh. Levelized cost are approximately 3 Euro cents per KWh for a 100 MW wind or natural gas plant. No actual data are available for a utility scale power plant.
As with other solar technologies, some mechanism is required to mix its varying power output with other power sources. Heat can be stored in heat-absorbing material or saltwater ponds. Electricity can be cached in batteries or other technologies.
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