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 and at present there is a proposal to construct a 200MW solar updraft tower in the Australian Outback.[1][2]
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.[3]
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.Cite error: A <ref> tag is missing the closing </ref> (see the help page).[4] The relatively low-tech approach could allow local resources and labour to be used for its construction and maintenance.
In 1903, Spanish Colonel Isidoro Cabanyes first proposed a solar chimney power plant in the magazine "La energía eléctrica".[5] One of the earliest descriptions of a solar chimney power plant was written in 1931 by a German author, Hanns Günther. 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[6], Canada,[7], Israel.[8] and the USA.[9]
In 1982, a medium-scale working model of a solar chimney power plant was built under the direction of German engineer Jörg Schlaich in Manzanares, Ciudad Real, 150 km south of Madrid, Spain; the project was funded by the German government.[10][11] The chimney had a height of 195 metres and a diameter of 10 metres, with a collection area (greenhouse) of 46,000 m² (about 11 acres, or 244 m diameter) obtaining a maximum power output of about 50 kW. During operation, optimisation data was collected on a second-by-second basis.[12] This pilot power plant operated successfully for approximately eight years and was decommissioned in 1989. The tower was featured on the Australian television program Beyond 2000 in 1984.
There is a proposal to construct a 200MW solar updraft tower in the Australian Outback.[1][13] The proposed power station would provide enough electricity for some 200,000 households and the energy output would represent an annual saving of more than 900,000 tonnes of greenhouse CO2 gases from entering the environment.[1]
With a very large initial capital outlay, no costs for consumables (i.e. fuel) and relatively constant income (from electricity sales) over the life of the project, a solar updraft tower would be placed in the same asset class as dams, bridges, tunnels, motorways and other similar large infrastructure projects. Financial viability would be assessed on a similar basis.[14]
Unlike a wind farm a Solar Tower is not expected to create a reliance on standby capacity from traditional energy sources.[15] 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 and potentially allow a solar updraft tower to provide something similar to base load power. This is highly desirable, as 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.
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 thus whether a tower (large or small) can be made profitable. No reliable electricity cost figures are expected until such time as engineering models are available for finalised tower designs and construction has begun on a production tower.
Given the novelty and enormous scale of any commercial solar updraft tower project, tourism income may become a factor, particular for the first towers to be created. Given that towers would likely be built in poorer areas with very low-value land, this may be more of interest to the local government than to the operator itself.
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 by the low investment cost per square metre of solar collection.[16]
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.[3] 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, biomass photosynthesis is about 0.1 per cent efficient. Because no data is available to test these models on a large-scale updraft tower there remains uncertainty about the reliability of these calculations.[17].
The performance of an updraft tower may be degraded by factors such as atmospheric winds,[18][19], by drag induced by bracings used for supporting the chimney[20], 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 may produce no more than 85 per cent of a similar plant located closer to the equator.[21]