Atmospheric Thermal Energy Conversion Towers

Also known as ATEC Towers, Energy Towers, Downdraft/Updraft Towers

Power Tower
Image from Steve Bowers
Updraft tower
Energy Towers come in two basic types; updraft and downdraft. The first type, the downdraft tower, makes use of the thermal energy inherent in Hadley Cell Circulation Zones. The second simply makes use of the thermal siphon/chimney effect.

Hadley Cells

It's a given that the atmosphere on any world will form circulation zones (or cells) to transport thermal energy (heat) from areas that are hotter to places that are cooler. While the size of such worlds is a factor in the number of circulation cells it needs for this, on any Earth-sized Gaian world, natural or terraformed, the wind belts and the jet streams girdling the planet will be steered by three cells: the Hadley cell, the Ferrel cell, and the Polar cell. The Hadley cell mechanism is well understood and the atmospheric circulation pattern that George Hadley of Old Earth first described to provide an explanation for the trade winds matches observations very well and on any planet like the Earth, one that has a small axial tilt (where the equator thus receives the greatest amount of solar heating), this circulation zone will form at the equator and become a major driver of global weather patterns.

The Hadley cell is a closed circulation loop, which begins at the equator with warm, moist air lifted aloft in equatorial low pressure areas to the tropopause and carried poleward. At about 30°N/S latitudes, it descends in a high pressure area. Some of the descending air travels equatorially along the surface, closing the loop of the Hadley cell and creating the Trade Winds. (The interpretation of the other two cells is more complex.) In truth there is not one discrete Hadley cell but several within the equatorial zone that shift, merge, and decouple in a complicated process over time. However, for descriptive purposes they are generally referred to in the singular.

Although the Hadley cell is described as lying on the equator, it should be noted that it is more accurate to describe it as following the sun's zenith point, or what is termed the "thermal equator," which on Earth undergoes a semiannual north-south migration. It should also be noted that as the warm, moist air is lifted aloft to the tropopause the water vapour condenses out, giving up its latent heat to the air and causing thunderstorms in these equatorial low pressure areas. The air that is carried poleward is thus both warm and dry and when it descends it becomes compressed, making it hot and dry. The result of this in the global environment as that there are deserts in the subtropical belts. If the planet has a relatively small axial tilt the deserts will become a permanent year round feature of the world and it is in these hot, dry belts that a downdraft tower works best.

Downdraft Tower

A downdraft tower is a tall hollow cylinder with an evaporative cooling system around the open top and turbines placed in openings at ground level. Water is pumped up to the top of the tower and used to cool the warm air hovering at the top. The cooled air, being denser than the warmer outside air, falls to the bottom of the cylinder and causes the turbines to spin on its way out. The turbines are connected to generators that produce electricity. The need for large quantities of water may be solved by choosing a location that is not too far from the coast but it should also not be too close either as the downdraft from the tower is now laden with water vapour and can be used to create an oasis in the desert. In its most basic version the water is simply sprayed into the air to cool it but if the water is supplied by an ocean it may be preferred to keep the salt out of the airstream. In that case it is possible to pass the air flow through a re-circulating waterfall or fountain, or around some kind of high surface area "wicking" curtain or "sweating" wall and return the leftover brine down a separate pipe. The downdraft tower's ability to create both fresh water and cooler air temperatures in a desert along with the energy it generates makes it a good site for communities. In fact, more often than not the tower itself serves as the core of an arcology.

When it comes to energy towers there is an efficiency of scale that makes larger, taller towers better than smaller towers (the farther the air has to fall the more momentum it gains) and most towers built today are at least a kilometre in height. Standard construction techniques conserve mass in the tower while maintaining strength by making the walls of the hollow cylinder thick but hollow themselves. These hollow spaces can be readily outfitted as living quarters and even a tower that is just one kilometre high can house a city of a million humans without difficulty.

Updraft Tower

An updraft tower is more flexible in its placement as it does not need large quantities of water or pre-existing hot, dry air in its operation. It makes its own hot air through solar gain and uses the thermal chimney effect. The updraft tower does however need to set aside a large area (several kilometres across) surrounding the tower for a greenhouse-like collector apron and this is its principal drawback. An updraft tower uses a very roundabout way of generating power; the collector apron turns solar energy into thermal energy, the tower turns this thermal energy into kinetic energy, and the turbines turn the moving air into electricity. Each step adds inefficiencies to the process, especially the first two. If all you want is electricity it may be more efficient to use a smaller array of photovoltaic cells because at best an updraft tower converts only a few percent of the solar energy that hits its collector apron into usable power. For the most part the same can be said of the downdraft tower, but at least it is converting thermal energy already present in the atmosphere.

There are however two saving graces of the updraft tower design: One is that the greenhouse-like collector apron can in fact be used as a greenhouse. The other is that it can be made lightweight enough to be portable. An updraft tower does not need to be a solid structure like the downdraft tower and can be little more than a flexible tube held aloft using toroidal lifting balloons filled with a lighter-than-air gas and anchored to the ground with a collector apron that is more "greentent" than "greenhouse."

Back on Old Earth one of the early Nannies, using such a design built with nanotech fabrics/membranes and a fancloth array instead of a wind turbine, proved it was possible to pack and transport a 100 MW power plant in a standard 16 metre shipping container. Soon after, towns all across the globe, often in remote areas, had access to clean, base load power (and improved crop production) while larger cities were still waiting for their long-promised fusion reactors.

Environmental Effects

As with almost any other energy generation technology, use of these towers in large numbers and high densities has environmental effects. In the case of the towers, this is through changes to local or regional wind patterns and, as mentioned earlier, the airstream from a downdraft tower is laden with water vapour, which can "green" a desert. Back on Old Earth this was done almost by accident, but later terraformers were well aware of these effects and used them to advantage by carefully planting and managing towers of the appropriate type in key regions. Often great care is not actually needed: Neumann-capable towers can plant themselves, using environmental feedback to insure proper placement.
 
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Development Notes
Text by AI Vin

Initially published on 09 March 2008.