What are solar updraft towers?
Solar updraft towers use solar energy from the sun to drive turbines, which in turn create electricity. The method that these towers use to generate the power is very different to both solar photovoltaic and concentrated solar power plants.
The solar updraft towers uses the very simple premise that hot air rises as their basis for energy production. Essentially they consist of 3 parts, the first is a massive solar collection area (potentially over 1km x 1km), where the sun hits a greenhouse type structure, heating the air underneath it, and trapping it in.
In the centre of the collect area is a large diameter concrete chimney structure, which vents the hot air into the atmosphere (as the hot air rises). As the hot air moves from the solar collection area to the chimney structure, it drives the third element of the solar updraft tower, the electricity producing turbines, these are either situated around the base of the chimney, or actually in a horizontal plane within the chimney itself.
The 2 primary factors in solar updraft towers
There are two factors that are critical for successful operation of a solar updraft tower. The first is the size of the collection area; put simply, the bigger, the better. The more air that gets heated in the greenhouse collection area, the larger the volume of warm air that will travel up the chimney.
The second factor is the chimney height, when again bigger is better (750m plus). The higher the chimney, the greater the pressure generated by the temperature differences, resulting in a larger stack effect. The stack effect relates to movement of hot air through the tower, so the higher the tower, the more electricity can be produced.
Practicality of solar updraft towers
A Spanish man called Isidoro Cabanyes first proposed solar updraft towers in 1903, however it was not until 1982 that a small scale solar updraft tower was built south of Madrid. This test power station was operational for 8 years, before the tower collapsed due to a storm as the result of inexpensive materials used in it’s production. The chimney was 195m high, and the collection area was approximately 11 acres, giving the plant a maximum electrical output of 50kW.
It really then became a forgotten technology until about 5 years ago when numerous proposals were put forward to build much larger solar updraft towers than the Spanish test facility. One of the major issues with this type of solar power station is that for them to be a worthwhile investment, a large collection area is required. This makes it unsuitable for areas that have high cost per acre.
In addition there are high associated initial capital costs for the construction of these plants. When compared to solar photovoltaic plants and concentrated solar power plants, these solar updraft plants also are incredibly inefficient, only capturing a fraction of the solar energy that hits the ground.
Despite this, in October 2010, Enviromission announced plans to build 2 200MW solar updraft towers in Western Arizona, which have the potential to supply 100,000 homes with electricity.
There are also additional benefits when comparing the updraft towers to traditional solar photovoltaic and concentrated solar power stations. In addition to creating free clean electricity supply, unlike other solar sources that are intermittent, relying on the sun shining to produce electricity, solar updraft towers can produce power 24/7 if special materials are used under the collection canopy that reduce the heat slowly through the night.
In addition underneath the collection area canopy, condensation created at night allows the soil to be used for arable land, enlivening potentially otherwise barren desert. In addition there is sufficient clearance between the canopy of the collection area and the ground allowing farming equipment to move freely.
Finally if the towers were associated with air filters (potentially carbon dioxide), this technology could also act as a CO2 scrubber (a CCS Technology) potentially helping to avert global warming.
The future of solar updraft towers
Solar updraft towers certainly have the potential to become a useful tool to help combat climate change. If production costs can be reduced, these would be ideal in third world countries where there is lots of cheap space to build the plants.
There are also patented designs that replace the large concrete chimney with a low cost fabric designs held in position using successive tubular balloons filled with lighter than air gas (such as helium). These would make the plants far cheaper to produce, although these have not been tested on a commercial scale.
A lot will depend of the success of Enviromission’s two planned solar updraft plants in Arizona, which will be completed in the next couple of years.