Charging Africa’s Giant Battery
By “Awake!” correspondent in South Africa
WOULD it surprise you to learn of a battery that is powerful enough to supply the electrical needs of a large city? Even more remarkable, that, by “overcharging” it, water is supplied to a thirsty industrial area? Such a unit is obviously not portable, and so, to examine it, we need to travel to the picturesque setting of the Drakensberg mountains, near the border of Lesotho in South Africa.
Our lookout point is the edge of the great African escarpment, at an elevation of over 3,000 metres.a The air is crisp and the view breathtaking. Behind us, stretching farther inland, is a vast central plateau, known as the highveld. At our side the Tugela crashes over the escarpment into the green foothills, starting as a stream and soon becoming a large river.
Looking down from the Drakensberg escarpment, we note two power lines. They do not lead to a power station belching smoke into the air; rather, these power lines end abruptly in the rockface of a foothill. About 400 metres above the power lines and over the crest of a hill is a large reservoir. About five kilometresb away lies a second reservoir situated at a much lower level. Strangely, the reservoir levels are constantly changing. Never are both full or empty at the same time. In fact, their fluctuations can be anticipated on a weekly calendar. On Friday afternoon you can be sure that the lower one is full, but on Monday the water will be gone and the upper reservoir will be full to the point of overflowing!
Hidden underground is a network of tunnels and shafts that run between the reservoirs. The tunnels connect up with four large machines, operating either as pumps or as turbines generating electricity. Twenty-six million cubic metres of water must be pumped up in order to fill the upper reservoir. It can be likened to a large battery, supplying not only electricity but also water to the thirsty highveld. It is called the Drakensberg Pumped Storage Scheme.
Advantages Outweigh Pumping Costs
Though batteries are useful, they usually cost money and take time to recharge. Likewise, electricity is needed to pump water into the upper reservoir, which takes over 35 hours to fill. This is done during weekends and at night, making use of off-peak power.
At present, South Africa’s electrical needs are supplied mainly by coal-burning power stations, with a capacity of about 19,000 megawatts. The extra megawatts stored in the Drakensberg is thus a welcome supplement in times of emergency or peak demand. An enormous draw of electricity is registered when meals are prepared at midday and early evening.
To bring an extra coal-burning power plant into operation is expensive and pollutes the air. It also takes hours to reach full power from a cold start. Controversial nuclear power plants operate at a fixed capacity, resulting in wasted electricity during off-peak periods. With pumped storage schemes it takes about three hours of pumping to produce two hours of electricity. Is that worth it?
“Despite the inefficiency in pure energy terms,” states the Encyclopædia Britannica, “such facilities are of great practical efficiency because they allow the use of otherwise wasted off-peak power.” Using excess generating capacity and converting it into power when required is like buying stock when it is plentiful and cheap and then reselling it profitably when the supply is in demand.
The Drakensberg Pumped Storage Scheme has an added benefit—supplying water to an expanding inland industry. Such include the gold mines, which are South Africa’s highest source of revenue. More gold is produced here than in the rest of the world combined. Sufficient water must be supplied, regardless of the cost. During weekends, when the upper reservoir of the Drakensberg scheme is filled, the pumping continues for an extra 10 hours. Thus millions of cubic metres of water are transferred each week from the low Tugela basin to the much higher Vaal basin. A small expense considering the high price of gold!
How Does It Work?
Water from the Tugela River is channelled into the lower Kilburn reservoir. From there it is pumped up through the mountain to fill the upper Driekloof reservoir. Once filled, this upper reservoir overflows into another. It is the largest reservoir in South Africa, and, when completed, it will have one of the largest earth walls in the world.
At regular weekly periods when electrical demands reach a peak, the process of pumping is reversed. The upper reservoir’s “plug” is pulled. As the water rushes down, the giant pumps become turbines, generating 1,000 megawatts in less than three minutes. If necessary this output can be continued for 27 1⁄2 hours before the battery needs recharging. In other words, the upper reservoir needs refilling. (See illustrations below.)
The Underground Powerhouse
The four reversible pump-turbines are concealed more than a hundred metres below the surface of the mountain in an excavated chamber. This large cavity has a volume equivalent to a 13-story block of flats with 77 two-bedroom apartments on each floor! On either side are another two large chambers containing valves to regulate the backflow of water and transformers that convert the generated power to the required transmission level.
Each pump-turbine can generate a maximum of 270 megawatts, the equivalent of more than 360,000 horsepower. That is far more than the electrical capacity used by the neighbouring state of Lesotho. Whether pumping or generating, these giant machines rotate 375 times a minute.
The concrete-lined water tunnels are six metres in diameter. Before reaching the lower reservoir they combine, forming an even larger tunnel. Here water flows at a tremendous speed, reaching about 200 kilometres per hour. All of this takes place out of sight beneath the waving grass of the Drakensberg foothills.
Since the world’s first pumped storage scheme began operating in 1892 near Zurich in Switzerland, appreciation for their value has grown. Though successful as a means of storing power, these schemes do not originate power. Power stations elsewhere have to provide the thrust to energize the pumps to lift the water. Only when water gravitates back does the stored hydraulic energy become mechanical energy on the spinning shaft to drive the generators, producing electrical energy once again.
a 1 metre = 3.28 feet.
b 1 kilometre = 0.6 miles.
[Map/Picture on page 17]
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Generating and Pumping Station
[Picture on page 18]
Used as a Generator
[Picture on page 18]
Used as a Pump