Sustainable Land Development Today - January 2009 - (Page 38)

SLDT TECHNOLOGY Solar Updraft Towers: Variations and Research By Tom Bosschaert Agricultural Solar Updraft Tower Diagram he idea of using solar radiation to generate air convection that can subsequently be converted to an energy source has been around since the start of the 20th century, when a Spanish colonel, Isidoro Cabanyes, proposed it in a scientific magazine. Solar updraft towers, also called solar-wind or solar-chimney plants, provide a very simple method for renewable electricity generation, with a constant and reliable output. Other renewable energy sources such as wind turbines and solar arrays suffer from high diurnal and seasonal fluctuations, or unpredictable patterns of output. Due to the large initial investment required, unfamiliarity with the system and the solar updraft tower’s relatively low capture efficiency, only one prototype was ever built, which occurred in Spain during the 1980s. This prototype however, performed above and beyond expectations, and continued to operate for almost seven years, more than double its designed three-year life span. The solar updraft tower has been left on the shelf due to its perceived low efficiency, which is to a large degree undeserved. Most studies on this elegant and T simple renewable energy producer consider the land occupied by the tower and its collector as one of the largest resource inputs required for this process. However, there is potential for more creative applications of the system, which would combine the tower with other land uses. Classic Solar Updraft Tower The small experimental solar updraft tower plant, built in Manzanares, Spain in 1982 by Schlaich Bergermann, can be considered the classic example of the system. The design calls for a large, unused plot of land to house a collector between 500 meters and 10 kilometers in diameter, with a centrally located chimney ranging from 100 meter to 1 kilometer in height. The collector is a transparent membrane suspended several meters off the ground, which can be made of glass or a strong transparent polymer. Sunlight penetrates this membrane, and the solar radiation is converted to heat upon hitting the ground. The air underneath the membrane quickly increases in temperature due to the greenhouse effect and flows towards the chimney, which, through the stack effect, becomes the lowest point of pressure in the system. This continuous airflow spins a turbine located at the base of the chimney. The nighttime difference in temperature between the ground and the air allows this effect to continue. Thermal storage devices can be used to smooth out the differences in intensity between night and day temperature differentials. As with other solar technologies, a higher latitude placement translates into a lower energy output. The majority of the cost associated with solar updraft towers is the initial investment required. By contrast, operations and maintenance costs are very low. The system is simple and very reliable. The parts that require maintenance are close to the ground and rely on elementary, proven technologies. One of the largest expenses is the cost of land, which means that the interest rate for financing that purchase is a major factor in determining the cost of the electricity produced. This makes the solar updraft tower a very suitable energy plant for remote, arid areas near the equator with inexpensive land. 38 January 2009 Sustainable Land Development Today

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Sustainable Land Development Today - January 2009 Contents Our Voice Your Voice The BottomLine SLDI in Focus Calendar Industry Rebound Will Require Balanced Approach What Happened to Fannie and Freddie? Added Value in Entitlements Industry Spotlights Technology Wastewater Innovation Classifieds Advertiser Index Editorial Board SLDT Resources Last Word

Sustainable Land Development Today - January 2009

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