A solar updraft power plant consists of a chimney, a collector area and wind turbines. In the collector area air is heated by solar radiation under a glass or plastic roof. This heat is thus forced upwards through the chimney thereby creating a wind force. By placing wind turbines inside the tower the force can be used to produce electricity. The technology is particularly suitable in remote areas (for instance, areas in developing countries) where low-value land can be used for the heat collection. The collector itself functions as a greenhouse and could be used for growing cash crops. The solar updraft power technology has a relatively low conversion efficiency and has relatively high investment costs per MWh of electricity produced. On the other hand, the operating costs are low.
Solar updraft towers can be used both for grid connected services and off-grid services. In the case of the latter it could serve communities and the scale of the plant could vary by what is needed.
Solar towers make use of differences in temperatures of air near the ground and at the top of the tower or chimney. In a solar updraft tower, air is heated under a large transparent collector roof constructed around the base of the tower. Under the roof, the air is heated by the sun so that it functions as a greenhouse. The heated air will tend to go upwards and the only possibility for that is through the tower. Through the 'chimney effect' (forcing the air through a relatively small opening) the wind force can become strong. By placing a wind turbine or ring of turbines inside the tower, this updraft force can be used to produce electricity. The Solar Updraft Tower thus functions as a solar thermal power plant and can have different capacity scales (from 30 – 200 MW).
An 'opposite' technology is the downdraft energy tower in which water is sprayed to cool the air at the top of the tower. The cooled air will cause a downward draft which can drive turbines for production of electricity. These energy towers are described in http://climatetechwiki.org/technology/energy_tower .
Solar updraft towers are particularly suitable in countries with a warm climate and could be attractive for production of electricity for remote regions in developing countries. In particular, in countries where there large areas of degraded or low-value land is available for building the heat collectors.
The efficiency of the technology in terms of energy conversion is relatively low. Therefore, a relatively large area is required for collecting the heated air in combination with a tall chimney. For example, for a capacity of 200 MW, the air collector area should be 38 km2 wide (Schlaich et al., 2005) and the chimney 1000 metres tall. Such a plant would reach a conversion efficiency of 0.5% or 1 kWh/m2. In comparison, a concentrating solar plant could have efficiency levels of between 20 and 40%. A 200 MW solar updraft tower could produce electricity for approximately 200,000 households.
The requirement of a relatively large area implies that the technology is particularly suitable in those developing countries which have large areas with low value or degraded land.
The technology can be applied at different scales. In the above example of a 1000 m tower, capital costs would be relatively high as it requires a concrete base and a solid construction. However, smaller scale applications are also possible, such as a smaller tower with thin plates which is held in position with ropes. Then the collector area could also be smaller. The collector area could be covered with a double glazed roof or a durable plastic. In a pilot plant in Spain (see clip above) plastic was used and it was estimated to last for about 10 years.
Once established, the maintenance of a solar tower is relatively cheap. However, when plastic is chosen as the cover for the collector area, then this might need regular maintenance.
The feasibility of the technology could be enhanced by making combinations with land use activities under the collector area, which serves as a greenhouse, as well as using solar collectors and photovoltaics underneath the collector.
The technology can produce electricity day and night as long as a difference in temperature exists between the collector area and the top of the tower. However, at day time this difference will be larger. In order to keep the temperature under the collector roof high at night, daytime heat can be 'stored' via heat absorbing surface material or salt water ponds.
Although the principle of an updraft solar tower has been known for long - in particular since it is based on a combination of the well-known chimney effect, wind energy and greenhouse effect - it has not been applied in practice on a large scale yet. In 1982, in Spain a prototype was built with financial support from the German Goverment (Haaf et al., 1983). It operated until 1989. It had a chimney of 195 metres tall and a collector area of 46 ha with a diameter of 224 metre. Its power production capacity was 50 kW at maximum.
In Ciudad Real, Spain, a new solar tower is planned with a chimney of 750 metres tall and a collector area of 350 ha (see Figure 2 above). It could produce 40 MW of electricity (Munoz-Lacuna, 2006).
In developing countries, the technology has been tested in Botswana in 2005 and there are plans to test it in Namibia. In Botswana, a small 22 metre tall chimney was built with a collector area of 160 m2. The chimney was made of polyester material and the rool of the collector area of glass (Ketlogetswe, 2007). The 'Greentower' planned in Namibia (based on a proposal by the Namibian government in 2008) is expected to have a capacity of 400 MW electricity output, produced by a 1.5 km tall tower (280 m diameter) with a collector area of 37 km2. This area will function as a greenhouse for growing crops (Cloete, 2008).
When applied on a large scale the technology could provide a considerable electricity output to the grid and thereby contribute to energy security of supply.
In case of small scale application in an off-grid system it could produce electricity for remote communities in, for instance, developing countries. This would especially be beneficial in case there are areas of low-value, degraded land available nearby the community. In such a case, the benefits from the technology can be reaped while the technology's disadvantage of requiring a large collector area would not need to be felt as a problem. On the contrary, an important benefit from the collector area could be that it works as a greenhouse (largely supported by condensation underneath the collector roof at night) so that cash crops can be grown.
The contribution to greenhouse gas emission reduction from solar towers would lie in the possibility that it could replace fossil fuel based electricity production capacity. However, in term of the entire life cycle of the technology, including production of concrete and collector area roof material, the greenhouse gas balance is unclear.
For calculation of GHG emission reduction of a solar tower project, it is recommended to apply the Approved Consolidated Methodology ACM0002 , which has been developed for renewable energy projects under the Clean Development Mechanism of the UNFCCC Kyoto Protocol (CDM). This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html 
According to the modalities for greenhouse gas emission reduction calculations of the Clean Development Mechanism (CDM), the emissions caused by the construction of materials is not taken into consideration; only the effects from operating the plant are considered.
The investment costs of a solar updraft tower are relatively high. In particular the building of a tower and constructing a durable roof for the collector area are expensive. In combination with the relatively low energy conversion efficiency, investments costs have been estimated at USD 5 per watt of electricity production capacity (Schlaich et al., 2005). Levelised energy costs could be reduced with larger collectors and higher towers  as this would create a stronger draft through the chimney. According to Climatelab.org, the cost of electricity produced by a 200 MW system could be 3 times lower than those of 5 MW system. Consequently, an economically more viable solar tower system would require a larger upfront investment.
However, for a clear cost figure further experience will be needed. According to Schlaich et al. (2005), producing electricity in a 200 MW solar tower would cost 7 eurocents per kWh and 21 eurocents per kWh for a 5 MW plant. On the other hand, operating costs are relatively low.
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