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alternative is a pair of thermal coal stations, one in each country, with their attendant greenhouse gases. Hydro schemes have a zero carbon footprint.
The main reason for the delay in building the dam and the two power stations was that Zambia was at that time an energy exporter with its share of Kariba and the two dams and power stations on the Kafue River. With the surpluses of electricity in the region at that time, and the fairly low prices exports could earn, there was no economic reason to build the dam.
All that has changed. The Zambian economy has been surging for more than a decade and that country has stopped exporting. For the past three years Zimbabwe’s economy has been moving forward fast, making the existing shortages of electricity, even after Hwange has been refurbished, far more acute.
Both countries urgently need a new power station each and Batoka can be built and commissioned fairly quickly. The dam will take a bit of time to build, but the power stations can start generating the year the dam is finished, because the lake behind the dam will have a very low volume; the dam is basically there is to provide a decent drop for the water rather than to store water.
This has other advantages. The lake will be a long narrow lake, flooding very little land, not much more than the Zambezi itself covers when the floods come down towards the middle of the year after a lot of rainfall in eastern Angola finally makes it over Victoria Falls.
The long narrow deep lake also means that evaporation losses will be modest, again not that much more than what already evaporates from the river. This is why the environmental impact studies of several Zambezi dam sites came down so decisively for Batoka, and why an earlier plan for a dam at Mupata Gorge was so decisively rejected when the environmental costs were put into the equation.
Naturally, there is a downside to this. Minimal storage means that the planned 1 800 megawatts can only be generated when the river is flowing strongly. As flows diminish late in the year, generation will have to be cut back sharply; Batoka is what is known as a run-of-river dam and with a river with such a variable flow as the upper Zambezi shows that could, in certain circumstances, be a fatal flaw.
But Batoka would only be a bad choice if it was the first dam to be built by Zimbabwe and Zambia on the Zambezi. It is not. Kariba Dam, with its huge lake, is just downstream. And Kariba’s energy production uses all the water that flows into that lake. It takes an exceptional season for the Zambezi River Authority to authorise the opening of a floodgate, and even then that single floodgate is not left open long. Plans to expand Kariba South have always been there to give Zesa flexibility, getting more power in peak periods but cutting back generation sharply in the middle of the night.
But Batoka and Kariba, run as a pair, will give Zimbabwe and Zambia much of the extra 1 800MW, plus a great deal of flexibility of when to take that huge amount of extra power. Both Kariba stations will have to be extended significantly beyond their present joint capacity of around 1 350MW.
When the Zambezi is in flood, Batoka will be run flat out, channelling those flood waters through the two new power stations. Downstream, the Kariba stations will just be ticking over at their present rate.
The extensions will not be used, although we would hope that the two power authorities will schedule all their Kariba maintenance for these low generation months as they take just 1 350MW between them. The level of water in Lake Kariba will be rising fast and furious during this time as water is stored for the drier season approaching.
But as the floods subside Batoka will have to cut back its generation, perhaps by as much as half, since it cannot store much. Now the Kariba extensions come into their own as the two stations there go flat out, taking up the slack.
In other words the four power stations will, between them, never be able to average much more than around three quarters of the joint capacity but between them should fairly easy manage 3 000MW or more, even at the worst time of the year.
All that surplus capacity means there should never be any downtime for maintenance or repairs; the engineers will cease having to plunge cities into darkness as they maintain or fix a turbine-generator set. And the two power authorities will be able, at long last, to use their water more intelligently, running the four stations hard during morning and evening peaks and then backing off considerably after 9pm.
Batoka might not be the largest reservoir in the system, but it can certainly cope with such daily fluctuations.
All this will require some detailed co-ordination between the two power authorities and the Zambezi River Authority. In fact, the ZRA will really come into its own with handling daily “water rations” for the two power authorities.
These generators will always be tempted, when faced with a shortage during the flood months, to run their Kariba extensions just a little more than what they should, if they are to cope with the drier months. The ZRA, which fortunately has been set up as a totally independent authority fairly immune to political influence, will need to be firm to ensure that enough flood water is held in Kariba to bridge the dry months when Batoka’s output falls.
So now work has finally started, there will be some interesting extra work downstream. Fortunately, the Kariba extensions are not that costly, since the dam is already there and paid for.
But Batoka means that this Kariba work must now be done, and be ready when Batoka is ready. The two schemes, properly equipped and well-run, are considerably greater than the sum of their parts.
Online Reporter