Solar chimneys: Light at the end of the tunnel ?
27 May 2010
The idea for solar updraft towers has been around for some time and small-scale pilots have come and gone. Projects are rumoured to be underway in Australia, Botswana and Spain. However, major developments have yet to break ground.
By Jason Deign in Barcelona
In 1982 a German engineer used sunlight to produce low-cost power, day and night, without water, mirrors or photovoltaics. Since then his miracle technology has been universally ignored in favour of others more costly and more complex. The burning question is: Why?
The idea behind solar updraft towers, or solar chimneys, where hot air is trapped under a wide expanse of heat-absorbing material and then forced up a central ventilation shaft, powering a turbine in the process, has been around for over a century.
According to Eduardo Lorenzo Pigueiras of the Solar Power Institute at the Polytechnic University of Madrid, the artillery colonel Isidoro Cabanyes proposed a ‘solar motor’ of this kind in a publication called La Energía Electrica in 1903.
And since then the concept has continued to intrigue engineers. But not, it seems, investors.
So far only two fully-functioning solar updraft towers have ever been built: a 22-metre-high chimney in Botswana in 2005, and the 1982 prototype constructed by Jörg Schlaich of Schlaich Bergermann and Partners in Manzanares, Spain.
Schlaich admits his pilot tower—a small-scale construction which produced up to 50 kW, had a height of 195 metres and a heat-collecting canopy measuring 46,000 m2—“was not optimised for output, but optimised for testing.” He was still impressed with the results, however.
“We are absolutely convinced the solar tower [concept] produces electricity at half the cost of trough collectors,” he asserts. “We could now do a 200 MW plant and could guarantee the cost per kilowatt hour to within plus or minus 10%.”
In the Manzanares pilot, daytime heating of the ground under the canopy meant air continued to be heated into the night, driving the turbine around the clock. Researchers were also surprised to find that condensation allowed plants to blossom in the erstwhile dry earth there.
And all this in setup that requires no water, has virtually no running costs and can be built from low-tech materials.
Professor Christos Papageorgiou, whose company FSC Technologies aims to commercialise air-filled, fabric-based ‘floating solar chimneys’ in place of rigid solar updraft towers, estimates a 5 MW plant could be built for around €10 million, producing some 20 GWh per year.
The FSC Technologies website claims: “With … 3% of existing desert areas on Earth and 1%-efficient solar aero-electric power plants we can produce 50% of the world electricity demand up to 2050.”
But first the concept will have to win over sceptics, including many in the investor and analyst communities.
Says Emerging Energy Research analyst Reese Tisdale: “It’s a great idea from a simplicity point of view, but I don’t have high hopes. [Solar updraft towers] are inefficient, take up a lot of land, the capacity is really low and you’re going to stick them out in the desert where it’s not cost-effective to produce power.
Liza Pogrebnyak, an analyst with Navigant Consulting, adds: “It’s a technology that hasn’t been commercialised out there, so there is a huge dearth of experience in the field. And it takes a fairly long time to build these projects; long lead times are definitely going to deter investors.”
Furthermore, she says, from an investor’s perspective, solar updraft towers might lose out to more conventional projects, such as photovoltaic arrays, because the latter can be switched on and start delivering a return on investment, before the entire plant is completed.
“With solar chimneys, I don’t know how modular it is. It’s a huge commitment from the investor.”
An uncertain future
Despite this, Schlaich, who acknowledges solar updraft towers are not a proven technology and only become economically viable when very large projects are involved, remains upbeat, citing growing interest in Nigeria, Saudi Arabia and Germany.
“If I wasn’t convinced this was the best avenue for solar energy then I wouldn’t have spent years of my life on it,” he says.
EnviroMission, which until recently had been working on the development of a solar tower in Buronga, Australia, last year said it was switching its focus to the United States, which Liza Pogrebnyak concedes is now “a great place to try” in terms of embracing experimentation. Unfortunately, EnviroMission did not respond to requests for an interview.
Industry watchers will be aware that the recent history of solar updraft towers is littered with grand projects, from a 750-metre tower planned for Ciudad Real, Spain, to a 1.5km high chimney in Namibia, that have yet to see the light of day.
So while there is something alluring about the image of towers soaring over a kilometre into the sky and creating clean energy and cropland from barren desert sand, whether we will ever witness them more than 100 years after the first sketches is clearly still anyone’s guess.
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