Speaking at a recent generation conference organised by the South African Institute of Electrical Engi-neers, project champion Dr Louis van Heerden said the multibillion-rand pilot scheme, which could be built near Upington, offered real potential for the creation of a domestic industry.

He noted that the heliostats (the sun-tracking mirrors that would surround a central collector or tower), together with the steel and civil works, could comprise up to 70% of the project’s eventual capital cost, and that the systems involved were well within the capabilities of the South African manufacturing community.

Van Heerden would not be drawn on the expected final cost of the project, which had previously been estimated at R2,2-billion, indicating only that commercial negotiations were under way with potential suppliers. “Hopefully, if all goes well with our internal processes, we should have a pilot plant on this technology soon. We have been working with suppliers and have a feel for the costs,” he said – Engineering News reported previously that the Eskom board could meet to approve the project in either March or April.

The heliostats would make up about 40% of the project’s overall capital cost, and the steel, drive systems and reflective materials needed for these sun collectors could be sourced locally. The pilot plant would probably require 8 000 heliostats, which would surround the central tower, with the furthest collector some 1,5 km away from the tower.

Envisaged was a field of two- axis tracking mirrors, 11 m × 11 m, which would be framed in steel and employ sophisticated drive systems to track the sun in such a way as to reflect a ‘dot of sunlight’ onto a heat exchanger, or boiler, atop a 190-m-high central tower.

The heat exchanger itself was truly the heart of the technology and would employ a sophisticated design proved abroad, as well as high-technology materials, including the Inconel alloy, which is employed on the exhaust systems of Formula One racing cars.

The solar radiation, or insolation, collected would be used to heat a solution comprising a mix of potassium nitrate and sodium nitrate pumped to the boiler from the so-called ‘cold tank’. The solution will then be heated to 570 ˚C, before entering the ‘hot tank’, where the hot salt would be used to heat water to produce the steam that powers a conventional turbine.

The thermal rating would be about 540 MW, which, under a baseload configuration, would deliver energy storage of 14 to 16 hours and involve 45 000 t of the salt mixture. That translates into 100 MW of electrical power around the clock, derating to about 20 hours during the winter months.

More likely, though, would be a peaking configuration, whereby an energy inventory would be built up over the days and cover a five-hour morning peak, or the four-hour evening peak. Such a configuration would reduce the prospect of outages caused by inclement weather.

But Van Heerden was adamant that the North- ern Cape offered among the best ‘solar insolation’ in the world, and is more advanced than that available in the US, Mexico and Spain, where the CSP concept has been proved.

The threshold radiation or insolation levels are considered to be 1 800 kWh/m2/y to 2 200 kWh/m2/y, and the areas being probed in the Northern Cape comfortably exceed those values, with some sites offering 2 900 kWh/m2/y.

“People often ask me about Egypt, which has 50 ˚C in the desert. I res- pond by saying that we have better sunlight than Egypt as its desert has a lot of fine particles in the air, which trap the sunlight and increase the temperature. In the Northern Cape, we have extremely clear skies, with all the sunlight coming through without getting trapped or scattered. So the direct insolation is very good,” Van Heerden enthused.

Solar was, thus, South Africa’s “best renewable bet”, as it was also suitable for large and grid-con- nected applications.

“We are convinced that economies of scale will offer cost reductions, and that the prices should also fall as we go up the learning curve.”

With insolation levels of 2 900 kWh/m2/y, South Africa should also have a materially better cost for each kilowatt hour than that being achieved by CSP projects elsewhere in the world.

The water requirements for the facility should also not place any untenable burdens on the water-scarce Northern Cape. Van Heerden estimated that the overall water requirement, including spraying of the heliostats to clean them, would come in at 300 000 m3/y, which was less, he asserted, than the requirement of just one vineyard along the Orange river.

“We are also looking at dry cooling . . . or a hybrid cooling configuration to try to reduce the water load even further,” he concluded.

To watch a video in which Eskom's Louis van Heerden explains why South Africa is an ideal location for solar, click here

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