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CSIR researchers Thomas Roos (right) and Strini Perumal have developed a technology demonstrator that generates power from the sun |
Several research activities at the CSIR are addressing the need for electricity in communities not connected to the national electricity grid. One of these will generate electricity from highly concentrated sunshine.
In his State of the Nation address, President Thabo Mbeki announced the target of universal access to electricity for all South Africans by 2012. Unfortunately, some 3,4 million households in South Africa are still not connected to the electricity grid, according to Minister of Minerals and Energy, Ms Buyelwa Sonjica. Speaking at the French-South African conference on energy in Cape Town earlier this year, she said: "A challenge to the Department of Minerals and Energy is the integration of alternative energy carriers into rural electrification, where there is little or no bulk infrastructure. In some areas the topography is such that it is difficult to even get the same bulk infrastructure to those areas."
"Photovoltaic (PV) solar cells generate electricity from the light of the sun," says CSIR concentrated solar thermal power expert, Thomas Roos. "On the other hand, concentrated solar power technologies, or concentrated solar power (CSP), converts the heat of the sun into electricity in a heat engine. South Africa has a solar resource that ranks among the best in the world and is widely available throughout the country. For remote rural communities, CSP in the sub-megawatt region is particularly attractive."
To this end, a low-cost, target-aligned 25 mē heliostat with closed-loop control has been developed at the CSIR. A heliostat is a mirror array that tracks the movement of the sun, in the process reflecting and concentrating sunlight onto a stationary receiver where the resultant heat is used to generate electricity in a steam or gas turbine. This high-tech capability harnesses solar energy and cuts reliance on fossil fuel.
"Harvesting the sun in this way is not unique," says Roos. He adds that a new 10 MW solar power station in Seville, Spain, is now delivering electricity to the Spanish grid using some 600 heliostats, each 120 mē in size. Eskom is investigating the feasibility of a 100 MW solar power station in Upington, which once built, will be the world's largest solar power station with 6 000 similar heliostats.
Roos says the CSIR heliostat prototype differs from commercially available designs in that it is manufactured largely from off-the-shelf components and makes use of closed-loop control. "Instead of a computer with a solar algorithm telling the heliostat where the sun is, this heliostat finds the sun by itself, doing away with expensive computer control systems," he says.
Roos, who is a researcher in gas turbine technologies, says he wants to link heliostats to small gas turbines to generate power. "This is an important development for remote villages without electricity. To date, the only options for them have been diesel generators, wind turbines and PV solar cells," he says. The South African wind strength, he says, is significantly less than in Europe where the turbines are designed, and is intermittent, while PV cells are very expensive and do not work when the sun is unavailable. "If you want guaranteed power when the wind has died down and the sun doesn't shine, you need a diesel generator as a back-up, so you pay for equipment three times," Roos says.
He adds: "With this concept, power can be generated using solar heat when the sun shines and using liquid fuels when it doesn't, using the same equipment. Furthermore, gas turbines do not need cooling water like steam turbines do, an important issue in a water-stressed country like South Africa."
According to Roos, an added advantage of this technology is that it resonates well with efforts to create an eco-friendlier society and combat global warming. "There are no emissions produced during the solar harvesting process," he says. "Eventually we will all have to use renewable energy because oil, coal and even uranium are finite and will run out. We have an abundance of solar energy in South Africa, which we know will never run out."
To further reduce fuel usage in the turbine when the sun is not available, Roos plans to develop high temperature thermal storage systems so that heat from the heliostat field can be 'banked' for later use. Heat storage is currently used with solar steam turbines, but not yet with solar gas turbines.
In addition to the solar-generated electricity, says Roos, this will also create a genuine home-grown industry in heliostat manufacture for other energy-intensive processes. "Our dream is to create a solar thermal research laboratory that uses the heat from heliostats to generate electricity, make hydrogen from water using high temperature thermochemistry and recycle waste aluminium. Our country can be a leader in this, due to our abnormally great solar resource," he says. According to him, the Northern Cape has the best sun availability, followed by the North West and Limpopo - all three are suitable for the launch of this project.
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