Energy towers are vertical, hollow towers constructed in dry desert regions with heights of 400 metres or more. Water from nearby sources (such as a sea) is pumped to the top of the tower and sprayed into it so that it cools the air. This creates a downwards draft through the tower which is transferred into energy through wind turbines. The technology is still in a research and development stage with involvement of research institutes from Israel  and India. The feasibility of the energy towers is affected by the relatively high capital costs, the requirement of a desert region with a nearby large water resource, and the often long distance to the energy end users. The technology can contribute to increased energy security of supply in a country or region and potentially reduces the need to use fossil fuels. It can also help eliminate salinity by using brackish drainage water to spray into the tower. This could help protect irrigation projects.
The optimal dimensions of an energy tower can reach over 1000 metres in height and four to five hundred metres in diameter. Water (usually sea water or brackish water) is sprayed in the tower through the top opening. The sprayed water cools the air within the tower from its dry bulb temperature to close to its ‘wet bulb’ temperature. As a result, the cooled air mass becomes denser, compared with the surrounding air mass (for instance, air cooled by 12 0C is approximately 4% heavier than the ambient air), and will sink downwards thereby producing a downflow effect in the hollow tower-chimney. When properly designed, the air will flow strongly through openings near the tower’s bottom, so that it can drive large wind turbines connected to electricity generators. The system can operate day and night, although the produced energy might be reduced at night due to changes in ambient air temperatures and humidity. The Figure above and the clip below show the working of the technology.
The technology, which was developed at the Technion-Israel Institute of Technology  in Israel, could be applied in regions with hot, dry climates (deserts and arid places) which are relatively close to the sea or to oceans, so as to enable pumping of sea or brackish water to the top of the chimney. Appropriate locations may be: Africa, India, Australia, Chile, South-west of the USA, Mexico, Southern Israel, etc. The following table provides a rough estimate of the world potential for the application of this technology.
Energy towers require the availability of a large resource of water nearby the plant. This could be water from the sea or ocean as salt water can be used, although the use of salt water could lead to corrosion of the tower and the turbines (Zaslavsky, 2006).
The feasibility of commercial energy towers differ greatly from one location to another depending on climatic and topographic conditions. Another factor to be taken into consideration is whether further income can be obtained from additional activities, e.g. desalination. A specifically assigned group at TERI (New Delhi, India) carried out an initial assessment of probable sites for energy towers in India (TERI, 2002). The assessment integrated site-specific topographic parameters and time dependent air properties into a model producing time sequence maps of energy towers’ power outputs.
Energy towers contain a possibility to store water during off-peak hours. Not only does air retain some of the daytime heat during the night, there is also the possibility to fill an elevated reservoir in the tower with water during hours of relatively low electricity demand, so that during peak hours no power is needed for pumping the sprayed water. As a result, the electricity delivery rate can come close to the maximum value needed for meeting electricity demand. The production capacity of energy towers can be affected by the weather as during times of humidity the downdraft effect becomes weaker.
Finally, the feasibility of energy towers could be affected by the long distance that could exist between the location of the plant, which is often remote, and where end users of the electricity are located. This would require transport of the electricity over long distances and installation of required infrastructure.
The minimum time required for the planning of a demonstration plant is one and a half to two years. The erection of such a demonstration plant will take at least an additional two years (TERI, 2002).
The energy tower technology has reached a demonstration phase. Some experts, such as a Steering Committee composed of Indian and Israeli Experts (Israel - India Steering Committee, 2001) suggested building a small but commercially viable power station. Possible demonstration plants could be in a range of 6.5 MW and 10 MW of electricity production capacity. For Israel, a demo plant is envisaged which should be able to cover at least the running expenses by electricity sale and which measured performance should remain within an accuracy margin of ± 10% of expected values. This demonstration programme envisages the following activities:
- Planning of the demonstration plant,
- Full scale planning and quotations from suppliers,
- Undertaking the necessary statutory process,
- Legal and patent activities, and
- Site data collection.
In addition and not directly related to the planning of a specific site, the following activities were undertaken by the Israel-India Steering Committee (2001):
- World climate survey and search for suitable sites, and
- Further technical analysis to refine different design points and considerations.
Contacts have been established with European companies (e.g. Alstom) (Renewable Energy Today, 2004) as well as with Indian authorities (Tifac) with respect to participation in the construction of the first Energy Tower in Israel (Israel - India Steering Committee, 2001), which will be 1000 to 1200 m tall.
The potential for building energy towers in EU countries is relatively low. However, the potential in several developing countries could be high as demonstrated in Figure 1. The construction of energy towers in northern African countries and connecting these with the electricity grids of Europe and North Africa could create a new source of income for the North African countries, as well a source of clean renewable energy for parts of Europe.
Energy towers could contribute to socio-economic and environmental protection in the following ways. First, the technology would contribute to increased energy security of supply, also because of the possibility to store water during off-peak hours, as explained above.
Second, the use of brackish drainage water for spraying into the tower can help eliminate salinity, which could help protect irrigation projects. In several places in the world irrigation projects are in a process of gradual destruction due to salinisation. Examples are the Colorado River, the Murray-Darling River in Australia, the Orange River in South Africa, the Indira Gandhi Canal in Rajasthan, India, Nile valley Egypt, etc. The problems are caused by the evaporation of most of the irrigation water and the return of the drainage water to the aquifer-source with all the disolved salts in it.
In dry desert climates, available brackish water can be sprayed inside the Energy Towers. The preferred method for this process is Reverse Osmosis . The desalination capacity can be installed gradually in small modules without the need for a large initial investment. The desalination capacity can be installed gradually in small modules without the need for a large initial investment.
Finally, producing electricity through energy towers reduces the dependency on fossil fuel imports and the vulnerability to fuel price fluctuations. The coal equivalent of producing the energy of a 388 MW net average power tower is 1.27 million tonnes per year. It also reduces the need to maintain large strategic fuel reserves for fossil-fuel energy power plants.
For the calculation of GHG emission reduction of an energy tower project, it is recommended to apply the Approved Consolidated Methodology ACM0002 , which has been developed for renewable energy electricity generation 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 .
However, in term of the entire life cycle of the technology, including production of concrete and other materials for the tower, collector area roof material, the greenhouse gas balance is unclear.
A substantial investment is required for energy towers, even for the planning of a demonstration unit such as the one in Israel mentioned above, which requires an estimated investment of about USD 20 million. The size of a demonstration unit determines its cost and whether it can be operated as a semi-commercial unit. In the case of planning a demo plant for Israel, a pilot plant with a capacity of one MWe was considered not economically acceptable, whereas a demonstration unit with a capacity of 6-10 MWe might be able to produce electricity at prices similar to other units without, however, recovering the capital expenditure (Israel-India Steering Committee, 2001).
The estimated investment for the building of a demonstration unit with a capacity of 50 MW would be about USD 135 million. Other ancillary costs involved are estimated at several tens of millions of dollars, depending upon site conditions. Such a unit might be able to recover the initial investment, although this, among others, depends on electricity prices at the chosen location. The construction cost of a full-size commercial unit with an average net output of about 370 MW was estimated at USD 850 million (Altman et al., 2005). A better estimate of costs will be obtained once a feasibility study for a specific location has been undertaken and completed.
The above-mentioned model by Altman et al (2005) has been applied to map both power production and electricity costs for the entire continent of Australia. The results depicted regions in Australia where arid conditions imply high gross power from energy towers. In such regions a single energy tower would supply constantly high net power (≈370 ± 160 MW), serving the electricity needs of around 0.5 million people, at economically competitive costs (USD 0.047/KWh). However, some of these areas are also far away from water sources and thus will entail high pumping power, so that a relatively low net power would result. Other regions have less favourable environmental conditions (lower temperatures and higher humidity) with lower net power output (≈230 ± 140 MW), but closer to populated areas and to water sources. In the latter circumstances, electricity could be produced at a cost of USD 0.07.3/KWh (Altman et al, 2005).
The estimated cost of the various sub systems of an energy tower is provided in Figure 2 below .
Altman, T., Zaslavsky, D., Guetta, R. and Czisch, G., 2005. Evaluation of the potential of electricity and desalinated water supply by using technology of "Energy Towers" for Australia and America, Interim Report June 2005, Faculty of Civil and Environmental Engineering, Technion-Israel Institute of Technology, Haifa, Israel, and Institute for Electrical Engineering–Efficient Energy Conversion,University of Kassel, Germany.
Israel - India Steering Committee, 2001. ENERGY TOWERS for Producing Electricity and Desalinated Water without a Collector, The Chief Scientist Office - Ministry of National Infrastructures, The State of Israel, and TIFAC--Technology Information, Forecasting and Assessment Council, Department of Science and Technology, Technology Bhavan, Government of India.
Renewable Energy Today, 2004. Investors negotiate Israeli 'Energy Tower' Project. Available at: http://findarticles.com/p/articles/mi_m0OXD/is_2004_Jan_26/ai_112599483 
TERI, 2002. Selection of sites for installation of 6.5 to 10 MW energy tower (ET) demo plant and also commercial energy tower power plants and assessment of the exploitation potential of energy tower power plants in India, Report No. 2002RT68, New Delhi, India.
Zaslavsky, D., 2006. "Energy Towers", PhysicaPlus - Online magazine of the Israel Physical Society (Israel Physical Society), vol. 7. Available at: http://physicaplus.org.il/zope/home/en/1124811264