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Wednesday, May 20, 2009
Desperately seeking solar
Renewable solar electricity generation could supplement base load generation in South Africa, an Eskom solar power specialist said on Tuesday. Eskom renewable solar energy specialist, Louis van Heerden, said the Northern Cape and North West provinces were a potentially huge resource for solar radiation that could make a significant contribution to electricity generation in South Africa. Speaking at the Sasol offices in Rosebank, Van Heerden said Eskom had identified Central Receiver solar generation technology as the most viable long-term generation method. The system heated either molten-salt, water or air that is then used to generate electricity. Eskom selected the molten-salt system for development. The Central Receiver system uses mirrors to reflect sunlight onto a central tower that heats molten-salt running in pipes in the tower. The surface temperature on the receiver on the tower is 900 degrees Celsius and the salt is heated to approximately 565 degrees Celsius by the reflected solar heat. Hot salt This salt is sent to a storage tank which then uses the hot salt to convert water to steam that is in turn used to generate electricity. The cooler salt is sent to a storage tank from where it is pumped back into the tower to heat up. A steam-turbine is used to generate electricity. This meant that the system fitted into conventional methods of generating power and could easily be converted for industrial or commercial use. The storage of the hot salt meant that power could be produced 24 hours a day in summer and approximately 16 hours a day in winter. Renewable solar power plants were sensitive to economies of scale and become more cost effective as the amount of electricity generated increased. A 100 megawatt station would cost approximately R650 million to build, Van Heerden said. Eskom used research gathered from a molten-salt pilot station in the United States of America to produce its model. Eskom focused on improving the system by using dry cooling (which reduced water consumption) and placing components of the system inside the tower so that the salt could be drained by gravity. The gearing to move the mirrors (heliostats) could also be replaced by linear actuators, thus reducing cost and maintenance requirements. The load factor of the plant was 68 percent, meaning it could produce power approximately 68 percent of the year. The plant had a starting time of 45 minutes. The plant had to be shut down for two weeks in a year for maintenance with an additional estimated two weeks of forced closure each year. Because the salt became solid at 210 degrees Celsius all the piping had to have heat tracing that kept the salt above 220 degrees Celsius. However, a 100 MW plant had a "house load" of 5 to 10 MW needed to run the heating, pumping and mirror rotation systems. Central Receiver technology converted approximately 19 percent of the solar power into electricity. Optimisation of the system was the most important issue because the thermal energy was freely available. This meant that while a solar power plant was capital intensive to start no costs were incurred for using solar energy to generate heat. Van Heerden said he did not expect building of Central Receiver plants to begin within the next two years. The renewable energy industry was mature enough to support the building of such a solar power plant as well as the manufacturing of components. Knowledge from nuclear research (such as on the Pebble Bed Modular Reactor) also added to the expertise available in South Africa. Eskom selected the molten-salt system after analysing three different solar generation technologies, namely the parabolic trough system, the Linear-Frisnel reflector system and the Central Receiver system. The Central Receiver system formed part of the Concentrating Solar Power technologies as opposed to Concentrated Solar Power technologies such as coal-fired power plants, Van Heerden said.
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