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Solar City

Solar updraft tower

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.

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.

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. The tower is a great source of energy, it producing enough energy to power a city during warm days.

History

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

Prototype in Spain

In 1982, a small-scale experimental 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.

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. However, this was an experimental setup that was not intended for power generation. Instead, different materials were used for testing, such as single or double glazing or plastic (which turned out not to be durable enough) and one section was used as an actual greenhouse, growing plants under the glass. During operation, optimisation data was collected on a second-by-second basis with 180 sensors measuring inside and outside temperature, humidity and wind speed.

In the choice of materials, it was taken into consideration that such an inefficient but cheap plant would be ideal for third world countries with lots of space - the method is inefficient in land use, but very efficient economically because of the low operating cost. So cheap materials were used on purpose, to see how they would perform, such as a chimney built with iron plating only 1.25 mm thin and held up with guy ropes. For a commercial plant, a reinforced concrete tower would be a better choice.

This pilot power plant operated for approximately eight years, but the chimney guy rods were not protected against corrosion and not expected to last longer than the intended test period of three years. So, not surprisingly, after eight years they had rusted through and broke in a storm, causing the tower to fall over. The plant was decommissioned in 1989.

Based on the test results, it was estimated that a 100 MW plant would require a 1000 m tower and a greenhouse of 20 km2. Because the costs lie mainly in construction and not in operation (free 'fuel', little maintenance and only 7 personnel), the cost per energy is largely determined by interest rates and years of operation, varying from 5 eurocent per kWh for 4% and 20 years to 15 eurocent per kWh for 12% and 40 years.

Ciudad Real Torre Solar

There is a proposal to construct a solar updraft tower in Ciudad Real, Spain entitled Ciudad Real Torre Solar. If built, it would be the first of its kind in the European Union and would stand 750 metres tall – nearly twice as tall as the current tallest structure in the EU, the Belmont TV Mast – covering an area of 350 hectares. It is expected to output 40 MW of electricity.

Australian proposal

EnviroMission has since 2001 proposed to build a solar updraft tower power generating station known as Solar Tower Buronga at a location near Buronga, New South Wales. Technical details of the project are difficult to obtain and the present status of the project is uncertain.

On 18 March 2007, the company board announced a merger with the US-based SolarMission Technologies, Inc., but the relationship was terminated on November 1, 2007.

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 (high temperature) 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 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, only if sloped towards the south, would produce up to 85 per cent of the output of a similar plant located closer to the equator.

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.

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 filters to help clean the air of a city.
  • It has been proposed to use the greenhouse for food production (except near the tower where the winds would be too strong).

Financial feasibility

This section discusses only the classical design of a Solar updraft tower: more exotic variations are not considered.

A solar updraft power station would require a very large initial capital outlay, which may be offset 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 construction and maintenance. Financial comparisons between solar updraft towers and concentrating solar technologies contrast a larger, simpler structure against a smaller, 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 (for a 200 MW plant) and 21 (for a 5 MW plant) euro cents per kWh, but other estimates indicate that the electricity cannot possibly be cheaper than 25-35 cents per kWh. Compare this to LECs of approximately 5 US cents per KWh for a 100 MW plant, wind or natural gas. 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.

References

External links

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