The solar updraft tower is a proposed type of renewable-energy power plant. Air is heated in a very large circular greenhouse-like structure, and the resulting convection causes the air to rise and escape through a tall tower. The moving air drives turbines, which produce electricity. A research prototype operated in Spain in the 1980s.
Schematic presentation of a Solar updraft tower
Description
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, more volume of air is warmed up 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. Further, a combined increase of the collector area and the chimney height leads to massively larger productivity of the power plant.
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, 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.
Solar towers do not produce carbon dioxide emissions during their operation, but are associated with the 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.
Financial feasibility
A solar updraft power station would require a very large initial capital outlay, which may be compensated by relatively low operating cost.Like other renewable power sources there would be no cost for fuel. A disadvantage of a solar updraft tower is the much lower conversion efficiency than concentrating solar power stations have, thus requiring a larger collector area, and leading to higher cost of constructionand maintenance.
Financial comparisons between solar updraft towers and concentrating solar technologies contrast a larger and simpler structure vs a smaller and 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 power 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 generation by intermittent power sources provides "smoothing" of the rate of change, this issue of variability can also be addressed by a large interconnected electrical supergrid, incorporating wind farms, hydroelectric, and solar power 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 and 21 cents per kWh, but other estimates indicate that the electricity cannot possibly be cheaper than 25-35 cents per kWh.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.
Conversion rate of solar energy to electrical energy
The solar updraft tower does not convert all the incoming solar energy into electrical energy. Many designs in the solar thermal group of collectors have higher conversion rates. 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 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 per cent, 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 would produce up to 85 per cent of the output of a similar plant located closer to the equator.
Related and adapted ideas
The Vortex engine proposal replaces the physical chimney by a vortex of twisting air.
Floating Solar Chimney Technology proposes to keep a lightweight chimney aloft using rings of lifting balloons filled with a lighter-than-air gas.
The chimney could be constructed up a mountainside, using the terrain for support.
The inverse of the solar updraft tower is the downdraft-driven energy tower. 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 SCAF, Solar City Air Filtre, proposes to use the same principle but on a smaller scale with the addition of filtres to help clean the air of a city.
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