Researchers in Jordan and Qatar have designed a "double-acting solar system" (TTSS) that can produce clean energy around the clock. This dual-action design produces more than twice the energy of a standard solar rising tower. As the name suggests, a "dual technology solar system" combines two tower technologies in a single design: a solar updraft tower and a cooling downdraft tower. They are integrated into a tower, with the updraft tower rising from the middle.
Solar updraft systems work by heating the air at the ground and then using the principle of hot air to rise, sending the air up a tall tower equipped with turbines. The air is heated under a large roof, which covers an extensive collection area and is made of greenhouse-type materials designed to collect as much heat as possible.
These devices have been built on an experimental scale but have not yet reached commercial scale because they are usually very tall structures to ensure sufficient temperature differences are created. Therefore, the costs are high and the risk is considered high.
A cooling downflow tower, on the other hand, forces air downward, turning another turbine. In this design, this is accomplished by spraying a fine mist of water into the ambient air at the top of the tower, causing it to become colder, heavier, and transported downward.
The TTSS design places an updraft tower in the middle and is surrounded by 10 downdraft towers, allowing it to operate in both updraft and downdraft modes simultaneously.
A research team from Al Hussein Technical University in Jordan and Qatar University modeled a TTSS tower about 200 meters high and 13.6 meters in diameter. Under the tower is a collector with a diameter of 250 meters. The inner cooling tower is 10 meters (33 feet) in diameter with a 1.8 meter (5.9 feet) clearance around it. The void is divided into 10 separate downwind towers, with water mist systems at the top and turbines at the bottom. The design location was chosen near the city of Riyadh - the hot and dry desert area is ideal for this kind of design.
In simulation tests using local meteorological data, the team estimated that such a system could generate a total of about 753 MWh of energy per year, with the external downdraft towers operating around the clock providing about 400 MWh of energy, while the updraft towers working more efficiently under the hot sun could provide about 350 MWh of energy.
According to the research team, these numbers are 2.14 times higher than a similar updraft-only design, which is reasonable given the updraft/downdraft separation mentioned above. They can also solve to some extent the contradiction between energy supply and demand that exists in most solar projects.
For now, the team has not attempted LCoE (levelized cost of electricity) calculations, nor any cost comparisons with solar PV arrays plus battery storage. The report also notes that it may not be easy to get enough water to run pull-down systems in hot, dry desert cities where TTSS systems are most effective.
Still, it's an interesting idea and proves that there are many ways to drive turbines to generate electricity.