Solar Thermal Updraft Tower
The Solar Tower uses warm air produced in a very large greenhouse chamber which is allowed to rise through a tall tower. The updraught is used to turn a electric generator turbine at the base of the tower.
The solar updraft tower is a proposed type of renewable-energy power plant. It combines three old and proven technologies: the chimney effect, the greenhouse effect, and the wind turbine.
The generating ability of a solar updraft power plant depends primarily on two factors: the size of the collector area and 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 km in diameter have been considered. With a larger chimney height, the pressure difference increases the stack effect; chimneys as tall as 1000 m have been considered.
Turbines can be installed in a ring around the base of the tower, with a horizontal axis, as 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 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 would be most likely in hot areas 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 labour to be used for its construction and maintenance.
In 1903, Spanish Colonel of the Spanish army Isidoro Cabanyes first proposed a solar chimney power plant in the magazine La energaectrica.One of the earliest descriptions of a solar chimney power plant was written in 1931 by a German author, Hanns Gunther. Beginning in 1975, Robert E. Lucier applied for patents on a solar chimney electric power generator; between 1978 and 1981 these patents (since expired) were granted in Australia,Canada,Israel, and the USA.
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, a simple solar thermal updraft power plant with an output of 200 MW would need a collector 7 kilometres in diameter (total area of about 38 km²) and a 1000-metre-high chimney. 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 from 20-40%. 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 bracings used for supporting the chimney, and by reflection off the top of the greenhouse canopy.
Location is also a factor. 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.
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.