Solar updraft tower
Principal
The solar updraft tower is a type of renewable-energy power plant. It combines three old and proven technologies: the chimney effect, the greenhouse effect and the wind turbine. Air is heated by sunshine and contained in a very large greenhouse-like structure around the base of a tall chimney, and the resulting convection causes air to rise up the updraft tower. This airflow drives turbines, which produce electricity. A successful research prototype operated in Spain in the 1980s, and many modeling studies have been published as to optimization, scale, and economic feasibility.
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.
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.
Turbines can be installed in a ring around the base of the tower, with a horizontal axis, or as in the prototype in Spain, a single vertical axis turbine can be installed inside the chimney.
Since this structures are huge, the amount of materials needed are immense. During the manufacturing process, a lot of energy (and emitted carbon dioxide) are required. Particularly cement leaves an enormous CO2 footprint behind. That is why the net energy payback time 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 labor to be used for its construction and maintenance.
Efficiency
The solar updraft tower has power conversion rate considerably lower than many other designs in the high temperature solar thermal group of collectors like the concentrated solar power plants. The low conversion rate of the solar updraft tower is balanced to some extent by the low investment cost per square meter of solar collection.
According to model calculations, a simple updraft power plant with an output of 200 MW would need a collector 7 kilometers 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 or photovoltaic 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% 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 Photovoltaic’s underneath the updraft tower collector. This could be combined with agricultural use.
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