Solar updraft tower

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Schematic of a Solar updraft tower
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Schematic of a Solar updraft tower
This article is about a type of power plant. For other uses of the term "Solar Tower", see solar tower (disambiguation). For the use of solar energy for ventilation, see Solar chimney.

The solar updraft tower is a proposed type of renewable-energy power plant. Air is heated in a wide greenhouse, and the resulting convection causes the air to rise and escape through a tall tower. The moving air drives turbines, which produce electricity.

There are no solar updraft towers in operation at present. A research prototype operated in Spain in the 1980s, and EnviroMission proposes to construct a full scale power station using this technology in Australia.[1]

Contents

[edit] 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.[2]

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 do not produce carbon dioxide emissions during their operation, but do require a large initial construction investment. Emissions of carbon dioxide are associated with the manufacture of cement which is a prime construction material in most commercial proposals to date.

A solar updraft tower power station would directly impact a significant area of land if it were designed to generate as much electricity as is produced by modern power stations using other technology such as burning coal. However, the land under the collector could be used for farming or other purposes.[citation needed]

[edit] History

In 1903, Spanish Colonel Isidoro Cabanyes first proposed a solar chimney power plant in the magazine "La energía eléctrica".[3] 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[4], Canada,[5], Israel.[6] and the USA.[7]

[edit] 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 Manzanares, Ciudad Real, 150 km south of Madrid, Spain; the project was funded by the German government.[8][9] 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.[10] This pilot power plant operated successfully for approximately eight years and was decommissioned in 1989.

[edit] Proposed solar tower projects

Artist's impression of Solar Tower. Note the scale of the tower compared to the road.
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Artist's impression of Solar Tower. Note the scale of the tower compared to the road.

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

Proposals have been made for Australia, China,[11] the USA,[12] and Spain[13].

The company EnviroMission claims to intend to begin construction in early 2007 of the first full size tower in Australia.[14]

A small-scale solar updraft tower may be an attractive option for remote regions in developing countries.[15][16] The relatively low-tech approach could allow local resources and labour to be used for its construction and maintenance.

[edit] Financial feasibility

As there are no fuel costs associated with the operation of a Solar Tower, and given the expected low maintenance costs, the primary considerations in evaluating the cost of energy produced by such a facility relates to the initial cost of construction and the cost of finance (e.g. the interest rates on corporate bonds).[17] The Solar Updraft Tower being proposed by Enviromission is expected to cost AUD$250 million (US$189 million) to construct[18] and will service 50,000 homes. Assuming a financing cost of 7.5 per cent, the associated electricity cost would be about $7.50 per household per week. This ignores cost factors such as maintenance, management, transmission, distribution and profit.

Unlike a wind farm[19] a Solar Tower is not expected to create a reliance on standby capacity from traditional energy sources. EnviroMission has also proposed to incorporate thermal storage technology that allows for such a Solar Tower to better match both peak and shoulder electricity demand in a manner somewhat comparable with traditional coal-fired power stations.

It was claimed, in 2002, that a Solar Tower in Australia would be an expensive way of generating electricity as compared to a conventional wind farm.[20]

However, a 2006 study claims that a large tower in the southwestern United States could not only outperform windfarms on a cost basis but also compete directly with current conventional gas-fired, and some coal-fired, plants.[21]

[edit] 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 by the low investment cost per square metre of solar collection.[22]

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.[2] 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, and impossible in arid deserts. Because no data are available to test these models on a large-scale updraft tower there remains uncertainty about the reliability of these calculations.[23]

The performance of an updraft tower may be degraded by factors such as atmospheric winds,[24][25] or by drag induced by bracings used for supporting the chimney.[26] Also 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.[27] Another inefficiency is that reflection of light off the top of the canopy implies a loss of 7.7 per cent of incoming solar energy, as calculated by the fresnel equations, if the canopy is made of common glass.

[edit] 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. [28]
  • 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. This design does not require a large solar collector but does consume up to 50 per cent of the generated energy operating the water pumps.

[edit] See also

[edit] References

  1. ^ EnviroMission website
  2. ^ a b Schlaich J, Bergermann R, Schiel W, Weinrebe G (2005). "Design of Commercial Solar Updraft Tower Systems—Utilization of Solar Induced Convective Flows for Power Generation" (PDF). Journal of Solar Energy Engineering 127 (1): 117-124. DOI:10.1115/1.1823493.
  3. ^ Lorenzo. "Las chimeneas solares:De una propuesta española en 1903 a la Central de Manzanares" (pdf). De Los Archivos Históricos De La Energía Solar. (Spanish)
  4. ^ Apparatus for converting Solar to Electrical Energy. esp@cenet. European Patent Office (1979-05-03).
  5. ^ Utilization of Solar Energy. Canadian Patents Database. Canadian Intellectual Property Office.
  6. ^ System and Apparatus for Converting Solar Heat to Electrical Energy. esp@cenet. European Patent Office (1979-12-30).
  7. ^ System for converting solar heat to electrical energy. USPTO Patent Full-Text and Image Database. United States Patent and Trademark Office (1981-06-23).
  8. ^ Haaf W, Friedrich K, Mayr G, Schlaich J (1983). "Solar Chimneys. Part 1: Principle and Construction of the Pilot Plant in Manzanares". International Journal of Solar Energy 2 (1): 3-20.
  9. ^ Haaf W (1984). "Solar Chimneys - Part II: Preliminary Test Results from the Manzanares Pilot Plant". International Journal of Solar Energy 2 (2): 141-161.
  10. ^ Schlaich J, Schiel W (2001), "Solar Chimneys", in RA Meyers (ed), Encyclopedia of Physical Science and Technology, 3rd Edition, Academic Press, London. ISBN 0-12-227410-5 downloadPDF
  11. ^ "China invests in solar towers", Asia Times, 2004-10-07. Retrieved on 2006-07-09.
  12. ^ Monohans Solar Tower Project Heart of oil industry could become site of renewable energy project. Scan Systems (2003-07-19).
  13. ^ Torre solar de 750 metros de altura en Ciudad Real (España)
  14. ^ "Green energy plan to use smaller solar tower", ABC, 2006-06-13. Retrieved on 2006-07-09.
  15. ^ Onyangoa FN, Ochieng RM (0000). "The potential of solar chimney for application in rural areas of developing countries". Fuel 00 (0): 000-000. DOI:10.1016/j.fuel.2006.04.029.
  16. ^ Dai YJ, Huang HB, Wang RZ (2003). "Case study of solar chimney power plants in Northwestern regions of China". Renewable Energy 28 (8): 1295-1304. DOI:10.1016/S0960-1481(02)00227-6.
  17. ^ Jörg Schlaich, Rudolf Bergermann, Wolfgang Schiel, Gerhard Weinrebe. "Design of Commercial Solar Updraft Tower Systems – Utilization of Solar Induced Convective Flows for Power Generation" (pdf). Schlaich Bergermann und Partner (sbp gmbh). Retrieved on 2006-11-28.
  18. ^ Rochfort, Scott. "Enviromission seeks a handout", Sydney Morning Herald, 6 December 2005. Retrieved on 2006-11-27.
  19. ^ Strong, Geoff. "More puff than power", The Age, 10 August 2006. Retrieved on 2006-11-27.
  20. ^ David Fickling. "Real power from nothing but hot air", The Guardian, 2002-08-19. Retrieved on 2006-07-09.
  21. ^ EnviroMission, Ltd.: Performance and Financial Projections. Waterville Investment Research. Retrieved on 2006-07-09.
  22. ^ 3. Solar Energy SystemsPDF Status Report on Solar Trough Power Plants (1996)
  23. ^ Pretorius JP, Kröger DG (2006). "Critical evaluation of solar chimney power plant performance". Solar Energy 80 (5): 535-544. DOI:10.1016/j.solener.2005.04.001
  24. ^ Serag-Eldin MA (2004). "Computing flow in a solar chimney plant subject to atmospheric winds". Proceedings of the ASME Heat Transfer/Fluids Engineering Summer Conference 2004 2 B: 1153-1162.
  25. ^ El-Haroun AA (2002). "The effect of wind speed at the top of the tower on the performance and energy generated from thermosyphon solar turbine". International Journal of Solar Energy 22 (1): 9-18. DOI:10.1080/0142591021000003336
  26. ^ von Backström TW (2003). "Calculation of Pressure and Density in Solar Power Plant Chimneys". Journal of Solar Energy Engineering 125 (1): 127-129. DOI:10.1115/1.1530198
  27. ^ Bilgen E, Rheault J (2005). "Solar chimney power plants for high latitudes". Solar Energy 79 (5): 449-458. DOI:10.1016/j.solener.2005.01.003
  28. ^ Air filtering chimney to clean pollution from a city and generate electric power

[edit] External links

Energy Conversion   Edit
Solar power: Active solar | Barra system | Energy Tower | Photovoltaics | Solar cell | Solar panel | Solar pond | Solar power tower | Solar thermal energy | Solar tracker | Solar updraft tower | Passive solar | Trombe wall | Ocean thermal energy conversion
Wind power: Wind farm | Wind turbine
Hydroelectricity: Marine current power | Tidal power | Water turbine | Wave power
Biological: Mechanical biological treatment | Anaerobic digestion | Biomass
Chemical: Blue energy | Fuel cell | Hydrogen production
Geothermal power: Earth cooling tubes | Deep lake water cooling
Electricity generation: Distributed generation | Microgeneration | Sustainable community energy system | Seasonal thermal store
Sustainability and Development of Energy   Edit
Conversion | Development and Use | Sustainable Energy | Conservation | Transportation
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