From Ego to Energy - Towers of Power Revisited

As the race for the tallest tower progresses, research into solar power design has created (perhaps for the first time) a need for height. The solar updraft tower – a combination of a solar chimney, greenhouse and wind turbine – was first presented in 1903 by Spanish Colonel Isidoro Cabanyes in the magazine La Energia Electrica. Due to magnitudes of economics, solar towers garnered little attention until the 1980s. The present energy crisis has fueled a resurgence of solar tower design.

Solar towers are a unique invention that exploit the greenhouse effect to create energy. Powered by the sun, solar radiation heats a large glazed base that encircles a massive tower. The base essentially acts as a greenhouse to trap and heat the air. Naturally rising hot air finds its way to the tower, and is pulled through the solar chimney by convection currents, the vacuum effect continually drawing in more air. As this occurs, the updraft is able to spin a single or several wind turbines attached to the base of the tower. Like a hot air balloon, think of it as forcing a temperature differential to produce wind.

So where is the best spot to plant a solar tower? Flat, sun soaked conditions are ideal, within close proximity to the electrical grid. Further, areas with lower atmospheric winds and geologically stable land are preferred. It is possible to set up a solar tower in northern latitudes, but this requires the collection disk to slope towards to the south to capture maximum solar radiation (output is often lower as well, typically 85 percent of a strategically located solar tower). Lastly, because the collection greenhouses consume a vast amount of space, these towers are often only possible in areas with low land value (which, given the current economic crisis, is almost anywhere).

The amount of energy produced by solar towers is directly related to size – bigger is better. Taller chimneys create higher-pressure differentials, increasing the force on the wind turbine. Additionally, the size of the solar collection area and chimney affects the volume of air, and therefore the amount of energy produced. Current designs are being explored that couple a 1000m tall tower with a 20 sq.km greenhouse, yielding approximately 100MW of power. A similar tower with a 38 sq.km collection disk could produce a whopping 200MW, enough to power 200,000 houses.

The efficiency of solar towers is greatly reduced during the night. The turbines continue to rotate due to the super-heated land that heats the adjacent air, but the efficiency drops significantly. Researchers at RMIT and Ove Arup have incorporated salt-water ponds (also called ‘solar ponds’) because of their increased specific heat capacity, which traps heat in the layers of saltwater and releases it gradually during the dark hours.

Funded by the German government and designed by engineers Schlaich Bergermann and Partner, a small-scaled solar tower prototype was tested in Spain from 1982–1989. Sited 150 km south of Madrid in Manzanares, the tower stood 195m tall with a 10m diameter shaft. This coupled with a collection greenhouse of 46,000 sq.m, produced a maximum output of 50KW. Not intended for energy harvesting, the prototype allowed for the testing of various greenhouse materials.

Current proposals for a 750m tall tower in Spain, a 800m tall tower in China, and a 600m tall tower in Australia are underway. Renewable energy company, EnviroMission is set to build the tower in Australia, a country that is currently powered by cheap coal. The tower is engulfed by a 65m diameter collection area (approximately 6 times larger than central park) and expected to provide enough energy to power between 100,000 and 200,000 homes. This would save more than 900,000 tons of greenhouse carbon dioxide emissions from entering the Australian atmosphere

Interestingly, the prototype solar tower built in the desert, fostered conditions conducive to the growth of plant life. This was due to condensation created at night that enlivened the soil with moisture, essentially transforming the desert into arable land. Not only can these collection areas add water to otherwise unproductive land, the towers could be linked with other programmes. Think of large office or residential towers that have a solar chimney at their core. Venting the exhaust heat from these additional programmes into the solar chimney would increase the updraft current, producing more energy. Plus, every resident could have clean energy and a garden plot in the middle of the desert.

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