Sun plus salt to the rescue of national grid

18 January 2015 - 02:00 By Monica Laganparsad

A R10-billion solar plant which can store the sun's energy and generate electricity even at night could help to ease the burden on the national power grid.

Once completed, the giant new plant - capable of powering some 200000 homes - will be the first of its kind in Africa.

The Redstone thermal solar power project was approved earlier this month under a programme run by the Department of Energy to buy renewable energy from independent power producers.

The programme allows for independent companies to feed solar, wind, hydro and other types of renewable energy into the struggling national grid, which is likely to experience further power cuts in coming months.

US company SolarReserve is heading a joint venture with the Saudi Arabian-based company ACWA Power to build the massive Redstone concentrating solar power plant at Postmasburg, near Kimberley in the Northern Cape.

The CEO of SolarReserve, Kevin Smith, said the technology was different to conventional solar farms which generated electricity with photo-voltaic panels and could operate only during daylight.

The Northern Cape now has six solar parks and more are being built in the Free State, Western Cape, KwaZulu-Natal and Mpumalanga.

Smith said the 100MW Redstone project, due to start generating power in 2018, would use a field of mirrors to focus the sun's energy onto a heat exchanger on top of a tower.

''We use this concentrated energy from the sun to heat up molten salt to over 560°C, and we can store that energy in a liquid form in insulated tanks.

''When we want to generate electricity, we will use that liquid heat to generate steam, like a conventional power plant, to power steam turbines and generate electricity," he said.

The molten salt comprises potassium and sodium nitrates of the kind typically used in garden fertilisers, said Smith.

''Because it is at a high temperature, it stays liquid, whereas water would flash into steam," said Smith.

''The benefit of that is that it can be stored in a tank for up to 12 hours and will run the facility from 8am till 10pm and meet all the peak periods."

This technology was first used in the 1990s at the Solar Two project in the Mojave desert in southern California and parts of Utah, Nevada and Arizona.

The plant was decommissioned in 1999 but a similar project, the Solana Generating Station, was built near Gila Bend in Arizona.

Smith said SolarReserve's Crescent Dunes solar energy plant in Nevada, on which construction started three years ago, would be the first such facility in the world to produce energy at "utility scale".

It will power about 75000 houses during peak hours.

Paul Gauché, an expert in thermo-fluid dynamics in the mechanical engineering department at the University of Stellenbosch, said that concentrating solar power technology had been around since the early 1990s but had been too expensive to roll out on a large scale.

''Concentrating solar power was not able to get going because coal was too cheap and the plants typically wanted to have those big turbines," Gauché said.

"But [it] is getting traction and getting cheaper. The cost will be equivalent to a coal power station in 2020. Coal will get too expensive and concentrating solar power will get cheaper," he said.

The government's renewable energy programme is also aimed at creating jobs and transferring skills.

Smith said the Redstone project would generate about 800 local construction jobs.

Eskom spokesman Andrew Etzinger said the utility, which will buy power from Redstone, had to date connected 26 renewable energy projects to the national grid which contributed a total of 700MW of wind and solar power.

He said this was very helpful in supporting the grid in summer.

''It's more beneficial during summer because it helps to reduce the pressure on the grid and helps to increase the reserve margins," Etzinger said.

The latest project is part of the government's R100-billion renewable energy programme which aims to contribute more than 3000MW to the national grid.