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A solar updraft tower is a renewable-energy power plant, first proposed in 1903. It combines three proven technologies: chimney effect, greenhouse effect and wind turbines. Air is heated by sunshine and contained in a very large greenhouse-like structure around the base of a tall tower; the resulting convection causes air to rise up the updraft tower. This airflow drives turbines, which produce electricity. The Australian company EnviroMission proposes to build the world's tallest solar updraft power plant, or chimney, in Arizona. With a 2,600-foot tall tower that would be the centerpiece of this giant non-polluting power plant, it would be the second tallest structure on the planet (as of 2011) – only 100 feet shorter than the Burj Khalifa skyscraper in Dubai UAE. 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 seven kilometers (4.3 mi) in diameter have been considered. With a larger chimney height, the pressure difference increases the stack effect; chimneys as tall as 1,000 meters (3,281 ft) have been considered. The EnviroMission tower is expected to be completed in 2015 and will provide enough electricity to power the equivalent of 200,000 homes. (Scroll to bottom for additional resources)
The EnviroMission design consists of a giant, round greenhouse-like structure, under which air becomes trapped and gets very hot -- around 160 degrees Fahrenheit. The hot air naturally rises, and would rush toward the tall tower in the center, passing through 32 turbines, whose turning blades would run generators and create electricity. Heat can also be stored inside the collector area greenhouse or inside tubes filled with water to be used to warm the air later and increase energy storage as needed. Turbines can be installed in a ring around the base of solar updraft towers, with a horizontal axis, as planned for the Arizona project, or—as in the prototype in Spain—a single vertical axis turbine can be installed inside the chimney.
Typically carbon dioxide is emitted only negligibly while operating, but is emitted more significantly during manufacture of its construction materials, particularly cement. 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 is optimized in hot regions with large amounts of very low-value land, such as deserts, or otherwise degraded 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 labor to be used for its construction and maintenance.
First Prototype in Spain
In 1982, a small-scale experimental model of a solar draft tower was built in Manzanares, Ciudad Real, 150 km south of Madrid, Spain. The power plant operated for approximately eight years. The updraft tower's guy-wires, which were not protected against corrosion, failed due to rust and failed in a storm. This caused the tower to fall over, and 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 Schlaich, funded by the German government. The chimney had a height of 195 meters and a diameter of 10 meters with a collection area (greenhouse) of 46,000 m² (about 11 acres (45,000 m2), or 244 m diameter) obtaining a maximum power output of about 50 kW. 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.
Other Solar Updraft Tower Examples
Jinshawan Updraft Tower - world record - In December 2010, a solar updraft tower in Jinshawan in Inner Mongolia, China 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 Real, Spain, entitled Ciudad Real Torre Solar. If built, it would be the first of its kind in the European Union and would stand 750 metres tall – nearly twice as tall as the current tallest structure in the EU, the Belmont TV Mast – covering an area of 350 hectares (about 865 acres). It is expected to put out 40 MW of electricity.
Botswana Test Facility - Based on the need for plans for long-term energy strategies, Botswana'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 m2. The roof was made of a 5 mm thick clear glass that was supported by a steel framework.
Namibian Proposal - 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 km tall and 280 m in diameter, and the base will consist of a 37 km2 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.
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.
Conversion rate of solar energy to electrical energy
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 of the Solar Tower is balanced to some extent by the low investment cost per square meter 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 km2. 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) 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 is 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. 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 photovoltaics underneath the updraft tower collector. This could be combined with agricultural use.