Solar updraft tower

This article is about Solar Towers and Solar Chimneys and similar Solar power plants using the convective motion of heated air in a chimney for electric power generation. For various other meanings of the term "Solar Tower", including the astronomical instrument and other uses of the term, see solar tower (disambiguation). For the use of solar energy for ventilation, see Solar chimney.

The Solar updraft tower is a type of renewable-energy power plant. Air is heated in a wide greenhouse, convection causes the air to rise inside a tall tower. The moving air spins turbines which produce electricity.

There are no solar updraft towers in operation at present. Some sources indicate that full scale electricity production is feasible if initial investment and logistical hurdles to create the infrastructure are overcome.

Description

A productive Solar updraft Tower is likely to be very large. 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; greenhouses 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 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. |doi=10.1115/1.1823493}}

Turbines can be installed in a ring around the base of the tower, with a horizontal axis, as planned for the Australian project described below 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 will not produce CO2 emmissions, but will require a large initial resource investment.

Solar updraft towers would impact a significant area of land to generate as much electricity as is produced by modern power stations using other technology such as burning coal.

History

In 1903, Spanish Colonel Isidoro Cabanyes first proposed a solar chimney power plant in the magazine "La energía eléctrica" (Spanish). 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 the USA, Canada, Australia and Israel.

Prototype

In 1982 a medium scale working model of a solar chimney power plant was built under the direction of German engineer Jörg Schlaich in Ciudad Real, 150 km south of Madrid in Spain; the project was funded by the German Government. 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 244m diameter) obtaining a maximum power output of about 50 kW. During operation, optimization data was collected on a second-by-second basis. Schlaich J, Schiel W (2001), "Solar Chimneys", in RA Meyers (ed), Encyclopedia of Physical Science and Technology, 3rd Edition, Academic Press, London. ISBN: 0122274105 download This pilot power plant operated successfully for approximately 8 years and was decommissioned in 1989.

Proposed solar tower projects

Successful application of this concept will require a large initial investment of money and resources.

Proposals have been made for Australia, China, the USA, and SpainTorre solar de 750 metros de altura en Ciudad Real (España).

The company EnviroMission claims to intend to begin construction in early 2007 of the first full size tower in Australia.

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.

Financial feasibility

It was claimed in 2002 that a Solar Tower in Australia was an expensive way of generating electricity as compared to a conventional wind farm.

A 2006 study claims that a large tower in the southwestern United States could not only outperform windfarms on cost, but also compete directly with current conventional gas-fired and some coal-fired plants.

Conversion rate of solar energy to electrical energy

The solar updraft tower does not convert all of 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 m² of solar collection.3. Solar Energy Systems Status Report on Solar Trough Power Plants (1996)

According to model calculations an 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.
With total area of 38 km², it appears that it would extract about 0.5% of the solar power (1 kW/m²) that falls on the area it covers. Because no data 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, or by drag induced by bracings used for supporting the chimney. Also a Solar updraft power plant located at high latitudes such as in Canada may produce up to 85% of a similar plant at southern locations.

Reflection of light off the top of the canopy imply a loss of 7.7% of incoming solar energy, as calculated by the fresnel equations, if the canopy is made of common glass.

Related and adapted ideas

The Vortex engine proposal is similar to the solar chimney but replaces the physical chimney by a vortex of twisting air.

Another approach, Floating Solar Chimney Technology, proposes to keep a light-weight chimney aloft by lifting balloon rings filled with a lighter than air gas.

Another proposed design would construct the chimney up a mountain side, rather than a free-standing structure in the centre of the solar collector. Air filtering chimney to clean pollution from a city and generate electric power

The inverse of the solar updraft tower is the energy tower which is driven by spraying water at the top of the tower; evaporation of water causes a downdraft by cooling the air thereby increasing its density, driving windturbines at the bottom of the tower. This design does not require a large solar collector, but does consume up to 50% of the generated energy operating the water pumps.

References

External links

Schlaich Bergermann und Partner (SBP)
The innovative Floating Solar Chimney Technology
EnviroMission Limited
SolarMission Technologies, Inc.
U.S. Department of Energy's Solar Energy Technologies program
SolarPACES

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