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Comparison of Solar Chimney and Heliostat Solar Power Tower Systems

written by: •edited by: Lamar Stonecypher•updated: 2/25/2010

Two different types of power tower solar plant show great promise. One is the solar chimney power plant, that generates wind in a solar chimney to drive a turbine. In the other type, like Solar One and Solar Two, each heliostat in an array sends solar power to a central receiver on a tower.

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    Introduction

    The terms 'solar power tower' or 'power tower solar plant' are used to refer to two quite different systems that both generate solar power electricity. Both systems use a rather tall tower, but other than that they are quite different.

    One type, also called a solar chimney power plant, uses solar collectors to send heated air into a very tall solar chimney. The updraft thus generated, turns a large wind turbine in the chimney/tower to generate electricity.

    The other type uses mirrored heliostats. Each heliostat in a large array reflects incoming solar energy to a receiver at the top of a centrally located tower. The heat generated by the reflected solar energy is used to generate steam that is used for Rankine cycle thermal generation of electricity. Solar One and Solar Two near Barstow, CA are examples of this type.

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    The Solar Chimney Power Tower

    solar chimney One type of power tower solar plant, the solar chimney system, is a hybrid wind and solar power plant that uses solar energy to create wind. It uses a large area solar chimney diagram covered with solar collectors to heat air at the base of a very tall tower (the solar chimney).

    The heated air is fed into the base of the solar chimney, where it rises and creates a natural convection updraft (remember: hot air rises!). Because a lot of hot air is being sent into the base of the chimney, the air flow going up the solar chimney becomes quite large and drives a large wind turbine (or multiple turbines) mounted in the tower. The turning turbine generates electricity.

    At the right is an artist's rendering of a 5 MW solar chimney solar plant. At the left is a diagram showing the air flow pattern in a solar chimney power plant.

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    The Heliostat Solar Power Tower

    Solar Two Demonstration Plant The heliostat power tower solar plant uses an entirely different approach to generating electricity from solar energy, but happens to also use a tall tower as part of the system. This type of solar power tower uses a large array of tracking solar mirrors, each called a heliostat, to reflect solar rays to a central receiver at the top of a tall tower.

    The picture at the left shows the solar two, 10 MW demonstration plant near Barstow, CA. It operated successfully as Solar One from 1982 to 1988, using water/steam as the heat transfer fluid. It was then upgraded and operated as Solar Two from 1996 to 1999, using molten salt as the heat transfer fluid. This allowed Solar Two to generate electrical solar power continuously for 7 days and nights, by storing heat as molten salt for use at night.

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    Operating Experience and Planned Projects

    Solar Chimney Power Plant: A prototype 50 kW solar chimney power plant was built in Manzaneres, Spain in 1982. This plant operated for seven years, showing that the technology does indeed work. The largest solar chimney power plant in the planning stages is in Australia. It is projected to have a central tower/solar chimney that is 3000 ft tall and 400 ft in diameter. It will have a solar collector area more than 3.5 miles in diameter, and produce 200 MW of solar electricity. There will be 32 wind turbines installed at ground level at the base of the solar chimney.

    Heliostat Power Tower Solar Plant: As mentioned briefly above, the U.S.heliostat solar power plant  Department of Energy built and operated the 10 MW Solar One and Solar Two demonstration plants near Barstow, CA, to demonstrate the feasibility of the concept.

    The picture at the right shows a 20 MW power tower solar plant that went on line in Seville, Spain, in April, 2009. This plant has a 525 ft tall central tower and uses 1255 mirrored heliostats. Each heliostat has about 1290 ft2 of mirror area to reflect the incoming solar energy to the central receiver at the top of the tower.

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    References and Image Credits

    Australia's New Solar Chimney Power Plant Is As Innovative As It Is Immense, Dec 19, 2009, http://www.renewablepowernews.com/archives/728

    U.S. DOE, Concentrating Solar Power, Power Tower Systems, http://www1.eere.energy.gov/solar/power_towers.html

    Solar Chimney, artists rendering: http://www.volker-quaschning.de/articles/fundamentals2/index.php

    Solar Chimney flow diagram: http://www.solarserver.de/lexikon/aufwindkraftwerk-e.html

    Solar Two picture: http://greenthoughts.us/page/3/

    Solar Tower plant in Seville, Spain: http://www.thestartup.eu/2009/04/abengoas-starts-operating-worlds-largest-solar-tower/

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    About the Author

    Dr. Harlan Bengtson is a registered professional engineer with 30 years of university teaching experience in engineering science and civil engineering. He holds a PhD in Chemical Engineering.

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