How do Solar Updraft Towers Work?

Mathematics tells us the proficiency and need of building solar updraft towers in today's world. Theoretically, a solar updraft plant with one thousand meter-high tower and about 38 square kilometers of land for collector canopy can produce 200 MW of power.

The 200 MW is enough to supply 200,000 conventional houses. To produce the same amount of energy from fossil fuels, you would produce 900,000 tons of greenhouse gases.

Mathematics will also tell you that the energy efficiency and the total land space required may not be feasible, which is why the concept is under serious thought before being taken to the next level.

With the rising costs of fossil fuels however, solar updraft towers may just be the best sources of power in the near future. Despite the constraints, an updraft tower will probably remain as one of the better options in the future of solar energy harnessing.

The working is divided into the usage of two main parts; the greenhouse effect and the stack effect.

The greenhouse effect is the ability of certain materials or gases to let in sunlight, warm things up and trap that heat in. In the tower, sunlight that hits the surface of the canopy creates heat inside the canopy.

The material traps the heat produced by the radiations of the sun into the lower atmosphere, which is within the canopy.

The stack effect is the flow of air through a structure due to temperature differences between the insides of the structure and the outsides.

The air always flows from a warmer place to a cooler place, thus giving us wind. Stack effect relates to the upward movement of air within a chimney or flue stacks, from warm internal surfaces to the cooler outer atmosphere.

In the solar updraft plant, the tower is the chimney and the canopy is the furnace.

• The greenhouse effect creates a layer of hot air inside the canopy.
• As the day progresses, more and more sunlight falls on the canopy and more air is heated.

• This hot air, seeking a cooler temperature, flows through the only available spot; the tower.
• As the air now flows upwards through the tower, it encounters the turbines inside the tower.
• The turbines are placed such that there is no way the air can flow out unless without hitting the turbine blades and rotating them.

• The spin on the turbine thus generates electricity, ready to be stored or supplied to wherever needed. The warm air finally reaches the top of the chimney and is vented off. Since it is only air, no harm is done to the environment. The air inside the canopy is replaced by fresh air from outside as the warmer air escapes.

• During the night, specially designed plates in the ground release heat that they stored in the day. The warm plates release the stored heat, warming up the air above them. This allows the tower to still function during the night.

There are many ways to increase the overall efficiency of a solar updraft tower, two of them being changing the size of the canopy and the tower along with some other suggestions.

• Increasing the height of the tower will increase the total temperature differential, forcing the air to move faster. This can make the turbines move faster, generating more electricity. With the tower already being around 1000 meters tall, it becomes a pretty tough thing to increase the height even more.

This includes problems with the construction and the material used to make the tower, namely the ones that will increase the carbon footprint.

• Increasing the diameter of the canopy around the tower results in an increasing greenhouse effect. More hot air is trapped in, increasing the volume of air trying to flow upwards through the tower. This will, again, help in producing more electricity. 

The problem with increasing the canopy size is the total space available for making the plant. Collector canopies are estimated to be around at least 7 km in diameter, making getting any more space very difficult.

• The turbines installed can be of two types; the first one being a single huge turbine with a vertical axis running through the center of the tower and the second one with multiple smaller turbines embedded into constrained spaces around the base of the tower in tilted axes.

• The ground can be made of a blacker color to increase the greenhouse effect. But again this means using materials like ash or charcoal, further increasing the carbon footprint.

• Other alternatives include the use of a 'solar downdraft tower', which involves cooler air (cooled by evaporating water) flowing down the chimney and powering the turbines below.
• Another idea is to maintain an ionized vortex inside the chimney and use the electro-magnetic field to generate electricity as well.

Using the solar updraft tower may not be economically feasible at the moment. There are still some aspects of the tower being studied and improved using prototypes or small-scale models supplying a small contingent of houses.

Apart from that, there are some other options to generating renewable energy and a solar updraft tower will remain as a prospective source.