Introduction Current energy provision systems based on exhaustible fuels like coal, hydro- carbons and uranium are damaging the environment and are not sustain- able. Increasing prices due to limit- ed resources and rising demand pose serious problems even to rich coun- tries. Fig. 1 shows the historic produc- tion rate of fossil hydrocarbons and an extrapolation into the future. A drastic production decrease in the near future is clearly visible. Experts are still dis- cussing the exact position of the peak controversially, but not its existence. Production is already declining in some oil fields, e.g. in the North Sea [1]. Poverty, population explosion and mi- gration are, amongst other reasons, also a consequence of insufficient energy supply and high energy costs. Accord- ing to a study of the International En- ergy Agency about 1,6 bn people have no access to electric energy, and about 2,4 bn have to rely on biomass for cook- ing and heating, and the numbers are increasing [2]. All these fellow human beings want to, and should be able to, use electricity in the future. Hence demand for electricity will dramatical- ly increase, especially in developing and emerging nations. It would be very shortsighted to rely on coal and oil or even nuclear power here. A clean inex- haustible source of energy is needed: the sun. The Solar Updraft Tower Solar energy can be used in various indirect (biomass, hydro-power, wind) and direct forms (solar thermal power, photovoltaic systems). Sensible tech- nology for the wide use of renewable energy must be simple and reliable, ac- cessible to the technologically less de- veloped countries that are sunny and often have limited raw material re- sources. It should not need cooling wa- ter and it must be based on environ- mentally sound production from renew- able, reusable or recyclable materials. The solar updraft tower meets these conditions. Its three essential elements – solar air collector, chimney/tower, and wind turbine – have been familiar for centuries. Their combination to gener- ate electricity has already been de- scribed in 1931 [3]. Currently several solar updraft tower projects are being developed, the most advanced being a 200 MW system in Australia. The solar updraft tower’s principle is shown in Fig. 2. Air is heated by solar radiation under a low circular trans- parent roof open at the periphery; the roof and the natural ground below it form an air collector. In the middle of the roof is a vertical tower with large air inlets at its base. The joint between the roof and the tower base is airtight. As hot air is lighter than cold air it ris- es up the tower. Suction from the tow- er then draws in more hot air from the collector, and cold air comes in from the outer perimeter. Continuous 24-hour operation can be achieved by placing tight water-filled tubes or bags under the roof. Since the heat capacity of wa- ter is about five to six times higher than that of soil, the water inside the tubes stores a large part of the daily solar heat and releases it during the night. Thus solar radiation causes a constant updraft in the tower. The energy con- tained in the updraft is converted into mechanical energy by pressure-staged turbines at the base of the tower, and into electrical energy by conventional generators [4, 5, 6]. Structural Engineering International 3/2004 Reports 225 Sustainable Electricity Generation with Solar Updraft Towers Jörg Schlaich, Prof., Rudolf Bergermann, Eng., Wolfgang Schiel, Dipl. Phys., Gerhard Weinrebe, Dr. Eng., Schlaich Bergermann und Partner, Stuttgart, Germany Summary Present electricity generation using fossil fuels is damaging the environment and is not sustainable, whereas solar radiation is an inexhaustible source of energy. Harnessed at reasonable costs, it can provide prosperity and a livable environment for mankind. Solar updraft towers – sometimes also called ‘solar chimneys’ – prom- ise sustainable solar electricity at low costs. Solar updraft towers are solar ther- mal power plants utilizing a combination of a solar air collector and a central up- draft tube to generate a convective flow which drives pressure staged turbines to generate electricity. 50 40 30 20 10 0 1930 1950 1970 1990 2010 2030 2050 Production Gboe/a Conventional Oil Bitumen, Heavy etc. Deepwater Oil >500 m Polar Oil NGL Gas Non-con Gas Fig. 1: Past and forecast production of all hydrocarbons under a base case scenario [1]. Unit of production is Gboe/a = 10 9 barrel of oil equivalents per year.