Eskom will decide by year-end whether it will proceed with a new 100MW facility powered entirely by the sun.
Concentrated solar power (CSP) is a relatively new technology worldwide, but it has the backing of the World Bank because it is the only zero-greenhouse-gas-emission technology that has the potential to rival coal-fired power as a low-cost solution to the energy crisis.
Two countries, Spain and the United States, have CSP projects up and running, while several others, including Australia, Morocco, Egypt, Mexico and India, are undertaking feasibility studies or building their own plants.
CSP is still a relatively new use of solar power on a large scale and several rival solutions exist, but typically CSP makes use of a tower surrounded by enormous fields of giant mirrors.
These concentrate the sun’s rays at the tower where the heat is used to create steam and turn a conventional turbine.
CSP plants can produce solar power on an industrial scale, meaning that the power generated can be used as a major input to the national power grid. The technology favoured by Eskom has been proved at a 10MW plant in Barstow, California, in the US.
This CSP technology uses molten salt as the agent. Steam from the hot salt, heated to 600°, drives a massive turbine. The hot salt can also be stored for use when the sun is not shining.
This means, says the Council for Scientific and Industrial Research’s Thomas Roos, that such a CSP plant can add both to the country’s base load energy supply as well as help meet peak demand. Solar technology typically is limited by costly storage when the sun is not shining.
Three possible sites have been identified on the Orange River near Upington in the Northern Cape for a 100MW South African plant. The Northern Cape has one of the world’s best solar profiles.
Upington gets more solar radiation annually than sites in California, Nevada and New Mexico in the US, as well as sun-splashed countries such as Jordan, Morocco, Crete, India and Spain.
A favourable environmental impact assessment (EIA) has been completed for the department of environmental affairs and tourism.
Eskom’s Louis van Heerden says its CSP project differs from the others in that “we have spent significant time to tailor the project to Eskom’s needs and to look at increased local content”.
He says Eskom is in “the final stages of the project, with a decision to be made to go ahead by the end of this year”.
He says he is unable to disclose the costs of the plant as “we are still compiling the final plant cost as part of the feasibility study”.
Nevada Solar One, a 64MW CSP project in the US, which uses different technology to that of Eskom’s proposed plant, has a project cost of $240-million, suggesting a project cost of perhaps R3-billion for Eskom’s 100MW plant.
Observers believe there are sound reasons for Eskom to proceed with this solar plant, even though its capital costs are likely to be significantly higher than a conventional coal-fired plant.
Saneri CE Kevin Nassiep says CSP technology is still in its infancy with the pilot project in Barstow, California, producing only 10MW electrical power. “This project has since been dismantled as the technology has been proven to operate successfully under test conditions. Eskom is looking at cost reduction, mainly in the local manufacturing of the heliostats, which constitutes a fair proportion of the costs of the technology.”
Nassiep says CSP is an exciting technology in that it can be designed to operate almost at full capacity as a base load plant, due to the oversizing of the thermal collector plant.
“By oversizing the heliostat field and salt storage tanks, one can effectively store more thermal energy than is instantaneously required for conversion to electrical power. As such, a large base load renewable energy plant will be an enormous gain for the industry where dispatchability issues have always plagued these technologies, particularly wind energy.”
Nassiep says while the first plant of about 100MW will be expensive, subsequent plants will be of the order of 200MW, which will result in a sizeable contribution to the national target for renewable energy.
An Eskom document says a 100MW CSP plant will need 6 000 heliostats of 120m2 each. These would be on flat ground of about 4km2.
Heliostats are mounted mirrors that track the sun. They reflect light at a central receiver at the top of a tower 150m to 200m high.
The salt, a mixture of sodium nitrate and potassium nitrate, is pumped up the central tower at about 300° and flows through the central receiver where it is heated to 600°. At this heat it can be stored for use in the conventional steam power generation process.
A 2006 World Bank report estimates CSP costs per kilowatt hour at between 16 and 20 US cents, about R1,10 and R1,40. This is significantly above the cost of new coal-fired power to Eskom at present of about 25c, but it is understood in the longer term CSP plants are expected to produce electricity in South Africa at about 50c a kilowatt hour.
The report, Assessment of the World Bank/GEF Strategy for the Market Development of Concentrating Solar Thermal Power, says CSP is the most cost-effective option to convert solar radiation into electricity and has been proven operationally in California since the mid-1980s.
The report says “firm” projects at present total less than 300MW, whereas several thousand megawatts would be required for the industry to approach commercial competitiveness. By comparison, wind energy — the most successful renewable technology in recent times — is exceeding 60 000MW of global installed capacity now.
Solar thermal electricity offers a number of advantages when considered as part of a country or region’s energy generation options mix. Solar energy is the world’s most abundant sustainable resource. It represents an even larger resource because of the favourable geography of many of the world’s developing countries, the report says.
Of the zero-greenhouse-gas technologies that are technically feasible — yet not yet commercial — CSP is a lead candidate.
The report says the most detailed and conservative of the recent studies predicts that commercial competitiveness could be achieved with installations of about 42 gigawatts (similar to wind capacity today), not including allowance for any cost of carbon.
Van Heerden says Eskom is exploring carbon-trading opportunities linked to the project, but that “this is not necessarily a factor in decision-making though, but rather an aspect that will be explored further as CDM develops in South Africa.”