Zesa must reorganise as Kariba hits 1GW


Over the course of the next few days Kariba South becomes Zimbabwe’s first “gigawatter” power station as the second new unit comes on stream and the power station is able to deliver 1 050MW, drastically raising Zesa’s capacity to cope with peak power demand.

However Zesa will have to change radically the way it manages power generation. For decades Kariba South has been basically used as a base-load station, running flat out 24 hours a day with the units on the thermal stations switched on and off as required to cope with variable demand during the 24-hour cycle.

Now that will be impossible. The reason is that water flows along the upper and middle Zambezi river are not rising, and the two Kariba power stations, one on each bank, basically between them use the entire annual flow into the lake. This is why the floodgates have hardly been opened since Zambia commissioned Kariba North. There is no surplus water to dump. The entire river flow goes through the turbines of the two stations although the very large lake buffers the flow, absorbing the seasonal floods as the lake level rises and using that surge to augment the lower flows later in the year. The Zambezi downstream of Kariba no longer shows much seasonal variation as a result.

The independent Zambezi River Authority allocates each station a water ration, and these are equal, based on flows into the lake. That is why they both have to cut back when drought hits the region, especially eastern Angola where 80 percent of the rain that eventually flows through the turbines actually falls.

Put simplify, Zesa can run Kariba South at 1050MW for 12 hours a day and then cut back to around 300MW for the other 12 hours. Obviously there are many other combinations of how many turbines are running or not running at any particular time, and the ZRA does not care so long as the average maximum is not exceeded. This gives ZESA incredible flexibility as it can go flat out at, say, 6pm, but run just one or two generators at say 1am. Peak power imports can be expensive.

In theory this should not be a problem as Zesa owns a second large power station, Hwange Thermal. That station has never been operated at maximum capacity because of an inadequate cooling system, but again that should not be a problem since on any thermal station with six boilers that is well maintained one boiler-generator set will often be down for maintenance, repair, refurbishment or replacement. So one would expect Zesa to run Hwange as a five generator station pushing out 600MW to 700MW 24 hours a day and then using Kariba South to top this up to a maximum of 1,7GW in the biggest peak and have hardly any contribution at say 1am. This is also best practice since hydro is so easy to vary and responds almost instantly to variable demand while in a thermal station switching on a boiler requires some time to heat the water.

But Hwange has not been well-maintained and has been run by switching units on and off as demand varies. Thermal stations require more maintenance than hydro stations because there is more equipment that can go wrong and, at Hwange there is the additional problem that the station uses pulverised coal as fuel. That has the convenience that it can be piped, like a liquid, but even a tiny leak will produce an abrasive dust and the coal has to be dry to flow easily.

Hwange is also at the mercy of its coal suppliers. The original system saw coal from strip mining coming in on a conveyor. The mining would not be environmentally damaging as the waste from the front of the strip was used to fill the mined out area at the back of the strip. This has not always worked as the mine has its own problems.

So these days Hwange operates well below maximum efficiency, and often well below a third of potential output.

The need for a thermal base load while Kariba South switches to peak load and coping with variable surges is the main reason why there are plans for boosting Hwange by two 300MW generators, and at the same time sorting out the cooling system.

Even if some older units become uneconomical to run, that would still allow Hwange with its larger stage-2 generators backing the new stage 3 generators to hit 1GW output. And that base load would ensure Zimbabwe’s mining, industry and irrigated agriculture could continue to grow fast.

Thermal power is more expensive in running costs than hydro. The big hydro cost is the dam wall, but that wall is now paid for and the fuel for Kariba South is free, water falling under gravity. Hwange requires coal and that has to be paid for.

It has also been a pity that the plans for solar generation have hit so much turbulence. Solar stations, up to a certain level, would be an ideal complement for the expanded Kariba South, because Lake Kariba is so large. When the sun was shining Zesa would cut back on hydro and let its water ration accumulate in the lake. As night fell, or clouds covered the sky, they would use the saved water to run Kariba South flat out. The problems with solar are storage when the sun shines and no generation at night. But in Lake Kariba feeding an oversized station Zesa have a perfect storage battery in effect.

In any case, Zesa now have to rethink a lot of their generation strategies. They have the expertise, with first class engineers among its staff. Now management have to apply that expertise and stop unsound practices, lurching the country’s power situation from crisis to crisis.


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