Online Reporter
IT is likely that the good rains falling across most of the Zambezi River catchment this season will improve river flows and allow Zimbabwe and Zambia to push their average outputs at each of the two hydro-stations to 750MW, about three times what they have been forced to limp along with after a sequence of bad years culminating in last year’s severe drought.
A lot will depend on the flows coming south and east from southern Angola, the source of up to 80 percent of the water in Lake Kariba. Much of the remaining flow comes from western Zambia, with central Zambia and northwest Zimbabwe providing very little.
The heavy rains we are now seeing in Zimbabwe help a little, and Zesa has been able to inch up output as the Zambezi River Authority increases the ration for both power stations, but the main benefits start coming in a couple of months as the Angolan floodwater finally reaches the Victoria Falls and the lake.
Zimbabwe does not have a major river system upstream of the dam wall feeding the lake, and central Zambia’s outflows tend to feed the Kafue River, which only joins the Zambezi downstream of Lake Kariba, although it does go through a pair of successive power dams on its way.
It is also apparent that Zesa now needs to move faster than planned to bring two 125MW units, of the original six commissioned at the beginning of the 1960s, back on line. Zesa was pretty good at keeping up with routine Kariba maintenance, as well as commissioning a new pair of units totalling 300MW in early 2018.
While these two units brought the Kariba South power station up to 1 050MW, very similar to what similar extensions had done on the Zambian side, most power engineers saw the extensions as the most cost efficient way of managing peak demand, rather than boosting total energy output.
For Kariba South this would mean that at the peak periods the station would be pushed to more than 1 000MW, but would then be cut right back in the slackest periods to make up the lake water ration used.
Peak power is a challenge for most authorities, and having an oversize hydro-station with its almost instant ability to push output up in a peak, expected or unexpected, is one of the best solutions and means that imports can be cut sharply, and waste can be cut since generating units at a thermal station are not having to be kept spinning without much output but ready for the peak surges.
So the two units being down would only really affect Zesa’s flexibility, not the average output it can generate from the station. In other words, we get the full expected energy output, just at a far more level rate. But the flexibility will be increasingly required, even in dodgy years with reduced inflows, as we push industrialisation and all those new factories switch on at 7.30am after a night of low loads.
Presumably the Zambezi River Authority will be keen on raising the lake levels from their historic lows, so that there is more water available to cope with the years when rains are not so good. Lake Kariba has a huge storage capacity, and that needs to be more actively used to provide the necessary buffers for the not-so-good years.
A major innovate new power project at Lake Kariba is the proposed 600MW floating solar station on the southern part of the lake. This would be easily the largest solar station In Zimbabwe, with the planners proposing Lake Kariba for a range of reasons.
The station would not chew up much of the surface area, despite its large size, so fisheries would not be affected; the “ground” would be level, so much of the extra work for a land station is eliminated; and in this modern age very importantly no one would be displaced and require compensation for demolished homes and new farm fields.
Lake Kariba could also help out as a sort of battery for the new solar installation. It could generate maximum output in the sunny days, while Kariba South cut back output and stored the water allocated, and then as darkness fell and the solar generation cut off, the hydro-station would go flat out. Even in a bad drought year it would be able to almost match the daytime solar with night-time hydro.
Batoka Gorge upstream of Lake Kariba, between the gorges of the Victoria Falls and the start of the lake, has been a dream for more than 30 years. But the final cost effectiveness still needs to be settled since the new dam will not store much water; what comes down the river will have to go through the power stations on the way to Lake Kariba. This already means a lower output in the lower water seasons, but also means that a regional drought will hit both dams.
There are ways of managing Batoka and Kariba stations as active pairs, cutting back at the associated Kariba station during the floods to store more water, and then as the Batoka station eased off ramping up Kariba output.
If Batoka was downstream it would be safer, since Kariba water would be reused, along with the flows coming down the Kafue, but there have been serious environmental problems raised in past proposals.
Meanwhile, while we study the full damage from climate change, we must make sure Kariba South can anchor our national grid with its highly flexible output, and that the exceptionally cost-effective solar station floating on the lake is built as soon as possible.
