Photo credit: Wikipedia.org
Imagine a huge circular greenhouse, 4 miles in diameter, with a tower at its center rising more than 3,000 feet into the atmosphere, dwarfing any other structure on earth. As hard as a structure of that scale may be to visualize, if certain solar innovators have their way it could become the solar power plant of the future.
What I’ve described is called a solar updraft tower, first envisioned around the turn of the 20th Century. A prototype was eventually tested in Spain during the 1980s to some success. Now, with the resurgence of solar power as a viable and necessary energy resource, the solar tower is making a bit of a comeback. There are currently several proposals around the world that could result in a (literally) concrete example of a solar tower, possibly on the scale mentioned above.
What is a solar updraft tower?
These towers work very simply—a big selling point for proponents of the technology. The large greenhouse acts as a solar collector, heating the air around the base of the tower, or chimney. Convection then causes the air to rise, naturally sending it toward its best escape route: the chimney. Wind turbines are placed at the base or inside the tower. The flowing warm air spins the turbines to create electricity. The tall tower increases air pressure as the solar heated air flows to spin the turbines.
That is essentially all there is to it. It requires only already-proven technology that is not at all difficult to comprehend or maintain.
Updraft towers require very little maintenance and cost nothing to run. Even for a tower 3,000 feet tall, it is estimated to only require a crew of seven and zero fuel to operate (like any solar system).
Solar updraft towers are considered ideal for third world countries with a lot of open land and relatively low electricity needs. Also, the simple technology involved in the towers would allow local resources and manpower to be used in construction and maintenance.
A lot of energy would be used to construct an updraft tower, but very little after the initial construction. Therefore, even for a project on such a large scale, the estimated energy payback would be just 2-3 years.
It is also possible to combine solar and agricultural needs within an updraft tower. For instance, a proposal in Namibia involves a 400 MW solar updraft tower (called “Greentower”) with the growing of cash crops beneath the large greenhouse collector area. Other ideas have involved placing photovoltaic panels or solar thermal collectors beneath the greenhouse as well.
The biggest argument against these towers is size and efficiency, especially in comparison to existing solar technologies. The sheer scope of 100 MW solar tower is nigh unfathomable. Based on test results from the Spanish prototype, such a plant would need a 3,000-foot-tall tower. To give you a reference point, the Sears Tower is a mere 1,730 feet tall.
Of course, if you need a power plant to be that massive, it is probably because the conversion rate is very small, and solar updraft towers are no exception. Solar photovoltaic plants typically operate at 15-20% efficiency. By contrast, an updraft tower (as estimated without any real, large-scale data to cite) would extract just 0.5% of the solar energy that hits it. Considering that an updraft tower is essentially a solar thermal application, the divide gets even larger, as concentrated solar thermal plants have conversion efficiencies reaching 40% and even higher.
In terms of financing, there is still a lot of debate and speculation. Proponents estimate the cost of updraft-tower-generated electricity at numbers as low as 9 cents per kilowatt-hour, but only for a 200 MW plant. Still others say that roughly 30 cents per kWh is as cheap as it gets. Apparently too little is known without a large scale working model to make any confident assertion.
Is It Worth It?
It is unclear whether these solar towers are worth the trouble. The arguments about them will likely rage on until rendered moot or until one is actually built. And that just may happen with proposals online in Namibia, Australia, and Spain among others.
One question: Why spend the time and resources to construct such an incredibly massive structure—useful or not—when modern technology is increasingly handing us ways to create even more power out of ever smaller packages?