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High powered job

CONCRETE

Of the many spectacular tall structures planned for the next millennium, there can be few as bizarre as the solar chimney. Designed to generate 100MW of electricity using the updraught effect to drive a vast internal turbine, the solar chimney would be 1,000m high, almost twice as tall as any other structure on earth. To make the air rise sufficiently fast it would first be heated by the sun under a 3.5km diameter glass canopy surrounding the chimney's base.

This is the invention of engineer Jorg Schlaich, founding partner of structural engineer Schlaich Bergermann und Partner of Stuttgart. It is designed to be built in scorched deserts and although the whole plant would cost at least £220M it has a design life of 80 years and burns no fossil fuels. If built in the developing world, Schlaich sees it as 'the most sustainable and economical way to produce electricity'.

SBP, main consultant for the cable stay design on the recently completed Ting Kau Bridge in Hong Kong, has wide experience of designing concrete cooling towers and television towers. It has already developed the detailed design of the solar chimney for a project in Rajesthan for which Indian contractor Larsen and Toubro tendered successfully. Although that project fell through, feasibility work for a chimney in the Northern Cape Province of South Africa is ongoing and Egypt and Morocco are also interested.

The thermodynamic principle was successfully demonstrated during the 1980s when SBP ran a 195m high, 50kW prototype in Spain. But the height of the chimney in the 100MW plant will need to be almost twice that of the tallest free standing structure in the world, the 553m CN Tower in Toronto.

Structures like the CN Tower are basically slender, thick walled tubes. The solar chimney, however, will have to be more like a giant power station cooling tower, with a lower height to width ratio and much thinner walls to create optimum updraft. As with all very tall structures, the main problems for this project will be providing vertical transportation for materials and operatives during construction. But because the walls will be so thin there is the additional problem of preventing them from buckling under wind loads.

It is crucial to keep the mass of concrete in the chimney as low as possible, Schlaich insists. Not only does this reduce materials costs but it reduces forces within the structure when under loads such as wind or seismic activity. Commonly, Schlaich says 'circular cooling towers of over 200m fail when their top deforms into an ellipse under load'.

His design has a 170m internal diameter and the base walls are 1,000mm thick. But these taper externally to a thickness of just 250mm for the upper 400m of the flue. To solve the deformation problem Schlaich has invented a 'spoked wheel' transverse diaphragm which works like the natural stiffening nodes in bamboo.

Four wheels at heights of 500m, 660m, 840m and 1,000m will keep stresses under control. The pattern of airflow inside the flue is sensitive to interruption, so the 64 wheel spokes are flat steel plates just 40mm thick. They are fixed to an outer 1.5m welded steel box section compression ring, which is filled with concrete and anchored to the wall reinforcement. The spokes converge at a central 3.5m diameter steel hub.

American engineer Ben Gerwick, of Ben C Gerwick Incorporated, consultant to the project, says the wheels are vital and 'if the chimney gets too high above a stiffening wheel it won't be stable under high winds'. They have to be placed during construction and will be stacked and raised together by 12 linear hydraulic hoists distributed around the vertical inside walls.

The foundations to the chimney would contain 186,400m3 of C35 with a reinforcement density of 100kg/m3, laid in a 45m wide by 3m deep circular trench. A solid concrete raft will support a box slab of concrete webs filled with excavated earth to achieve sufficient underground weight. The flue proper will be supported above the ground by 32, 2m high radial piers to allow air to enter its base.

The flue is made up of 210,000m3 of C55 concrete with a reinforcement density of 120kg/m3. Schlaich says that by specifying a relatively low strength C55 mix he hopes that local desert aggregates could be used. In principle, SBP says, no admixtures are required to achieve this strength, and this is considered a saving. In South Africa they would use the desert granite crushed rock aggregates to produce a C65 mix. Pulverised fuel ash from local coal-fired power stations would replace 50% of the Portland cement to improve plastic properties.

The team studied the relative progress of the two Petronas Towers in Malaysia, where concrete was transported by pumping and with high-speed hoists. Although the logistics of construction will remain flexible until a specific project becomes live and a contractor isappointed, the concrete making materials will probably be hoisted up the tower and mixed at the working platform.

The chimney's thin walls require just 200m3 of concrete per day - too little to achieve the continuous pour necessary to make pumping viable.

The chimney, which represents 25% of the total project cost, should take only two years to build while the glass collector around it takes three.

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