David Le Page AFRICANFRONTIERS
The downside is, South Africa is one of the world’s top 20 polluters. At least 93,5% of South African electricity is coal-fired. Eskom is the world’s single largest producer of carbon dioxide. Acid rain created by power stations causes fences near Bethel to disintegrate in just three years. The upside? Southern Africa receives 59% of the world’s highest quality sunshine. The Northern Cape government hopes to build the world’s largest solar-power station near Sishen, following a revolutionary design. Eskom is planning the largest solar-only thermal power plant. Solar panels or photovoltaic cells (PVs), which directly convert sunlight into electricity, tend to be the most widely known solar technology. Already, there are large numbers of solar panels in use in schools and rural communities in South Africa. Telecommunications companies use them to power remote installations, such as microwave stations. In 1998 Eskom and Shell embarked on an ambitious joint venture in the Eastern Cape to provide solar energy to 50 000 households. The concept uses charge cards, which cost about R40 a month. The installations provide sufficient power to run a couple of light bulbs, or a TV set. However, there have been problems. People often consider anything other than grid power second-rate. But Shell’s Koosum Kalyan says acceptance is growing, and more than 6 000 units have been installed. A small industry is growing around maintenance of the units and the sale of magnetic cards needed to run them. Professor Dieter Holm of the University of Pretoria contributed to a recent study commissioned by the Department of Housing to examine ways of making low-cost housing more energy efficient. Holm, who is also a member of the International Solar Energy Society, firmly believes that only houses that are properly oriented to face north should actually receive housing subsidies. Assuming the roof is constructed at the right pitch (the dwelling’s latitude plus 10 degrees), it then becomes extremely simple to attach – or even build into the roof – a solar water heater and photovoltaic power unit. Similarly, solar thermal co- generating installations could be placed on the roofs of factories. Solar thermal plants use the sun’s heat to create steam, which then drives a generator. In a co-generating installation, rather than going directly into electricity generation, some of that steam is used directly in the manufacturing process as “process heat” (for baking, pasteurisation, drying and the like).
The idea of building energy generation right into a building is already catching on in Europe, despite its far lower levels of sun- light than South Africa. The declining cost of PVs means that they can be incorporated into building facades. During the day elec- tricity consumers can then sell power back into the grid, offsetting their night-time bills. Unfortunately, on large scales, PVs are too expensive to be practical and designers are considering a multitude of other options. Should the ambitions of the Northern Cape bear fruit, within a few years an extraordinary structure could rise from the arid, sun-baked land near Sishen – a chimney 1,5km high, nearly 200m wide and surrounded by a greenhouse about 7km in diameter. Air heated under the glass would be drawn up through the chimney, accelerating to drive a wind turbine located at its base. This R4-billion project would produce 400MW of electricity. Koeberg, by contrast, produces 1900MW, or 4,5% of Eskom’s total production.
The efficiency, both physical and financial, of the solar chimney will be increased by using the greenhouse for cultivation, according to project manager Wolf Stinnes. The productivity of the soil will be increased by using humus, which is highly water retentive. The physical effect of having a large amount of water under the greenhouse will be to create a mass capable of heat storage. At night this heat will be released, allowing the chimney to operate 24 hours a day. Experiments with a 200m prototype solar chimney at Manzanares in Spain showed that cultivation within the greenhouse reduced heat losses, increasing the chimney’s energy efficiency. Stinnes expects the cultivated land, which will amount to 25 square kilometres, to be so productive that “we could give the electricity away”. Since the Northern Cape is one of the world’s most seismologically stable areas, the alarming notion of a 1,5km-high chimney wobbling – or worse – in such an event can be ruled out.According to Stinnes, its proposed height is not as outrageous as it seems – Japanese architects are considering 2km-high skyscrapers for earthquake-prone Tokyo.
Fortunately, the Northern Cape is also free of the hailstorms that would make a 6,5km wide greenhouse rather impractical. Though the construction would be directed by a German company, Concor, LTA, Group 5 and Stocks & Stocks have all tendered for the actual construction. Eskom says it is concerned that a jump from a 200m prototype solar chimney to a 1 500m commercial version is overly ambitious. It is considering various other solar thermal technologies.
Eskom’s renewable energy project leaders, Louis van Heerden and Kevin Nassiep, point out that these technologies, unlike the solar chimney, have been endorsed by the Global Environment Facility, a renewable energies-funding mechanism run by the World Bank and the United Nations. Eskom’s solar thermal interests centre on three possible designs: solar troughs, central receivers and Stirling/dish reflectors. Solar troughs are long reflective troughs, with a parallel thin pipe running through the focal point. The reflectors concentrate heat on the pipes, heating oil that is in turn used to generate electricity. The trough tilts slowly through the day to follow the sun. The central receiver design has a heat collector mounted on a tower. The tower is surrounded by circular ranks of mirrors, which track the sun through the day and reflect it on to the collector. Air or molten salt in the tower accumulates the heat, which is then used, as in a conventional power station, to create steam to drive a turbine. The molten salt can also serve to store heat when direct sunlight is not available. The third option, the parabolic dish, looks like a large, mirrored satellite dish. Heat is focused on a Stirling engine, a mechanical device that converts external heat into motion. Eskom is likely to begin installing Stirling/dish reflectors early in 2001, experimenting with different models. Though renewable energies are currently far from being as inexpensive as coal-fueled Eskom power, the provisions of the United Nations Framework Convention on Climate Change and technological development are soon likely to make it increasingly practical.
But, as Holm notes, the gap in public awareness of such technologies remains an obstacle to their widespread adoption. Only when South Africans begin to turn the heat on to energy suppliers and politicians, is the sun’s heat – one of our most abundant resources – likely to be used rather than ignored.
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