Namibia backing 400 MW solar tower-agri concept

24th July 2008 By: Rozanne Cloete

A new proposal involving the construction of a R1-billion solar tower that will be able to generate 400 MW of electricity is being backed by the Namibian government, which is prepared to foot the bill for half of the prefeasibility study costs, says intellectual property company Hahn & Hahn MD Alan Dunlop.

The proposal states that the solar tower, aptly named the Greentower, will be 1,5 km high and 280 m in diameter. Dunlop says that the tower can be likened to a chimney that would cause an updraft to the drive turbines, generating 400 MW of electrical power.

Air within the solar tower is heated in a large circular green-houselike structure, and the resulting convection causes the air to rise and escape through the tower. The moving air drives turbines, which then produce electricity.

Solar tower technology has been slow to develop over the last 30 years. Between 1979 and 1989, a German engineer, Professor Jorg Schlaich, designed a solar tower 200 m high, which was built in Spain and financed by a grant from the German government. The tower ran trouble-free for eight years, producing 50 kW of electricity, and the collapsed.

Dunlop states that interest in the technology faded in the 1990s, until it was announced that a 1 km-high solar tower, capable of producing 200 MW would be constructed, and would be functional, in Australia by 2010.

Turbines can be installed in a ring around the base of the tower, with a horizontal axis, as planned for the Australian project, or, as in the prototype in Spain, a single vertical axis turbine can be installed inside the chimney.

He adds that the Australian tower, as well as its Namibian counterpart, will be built out of reinforced concrete, using technology developed for building high-rise buildings.

“Several thermodynamics, civil struc-ure, wind loading and power generation experts have developed an executive summary for a R6-million prefeasibility study, and a R200-million bankable feasibility study for the Namibian project,” he says.

The Namibian government has declared its support for the Greentower, and offered a guarantee of 50% of the prefeasibility funding upon private equity commitment.

In addition, it is proposed that the base of the tower will incorporate a 37-km2 greenhouse, in which cash crops can be grown.

The greenhouse will be used to develop soil humus to transform barren land to fertile soil that retains moisture and nutrients to enable rapid plant propagation.

Dunlop says that studies have shown that plant-linked humidity does not reduce the uplift in the tower by which the turbines are driven, and even represents a store of latent energy that can be drawn on after sunset.

Water for the plants in the greenhouse can be supplied by desalinating sea water or purifying groundwater, using known technology and a supply of energy, which is only a small fraction of the energy generated.

He adds that the Namibian project is eligible for carbon credits under the Kyoto Protocol and offers opportunities for countries such as Germany to further develop solar tower technology.