Zimbabwe: Editorial Comment - Solar Far Better Than Batoka

29 November 2019

The very large solar power project, 2 000MW, announced by President Mnangagwa this week will be a far more reliable source of power than the long-mooted Batoka Gorge hydro-electric scheme, now that we have seen the effects of severe drought on Lake Kariba and its power stations.

If Batoka had been built five years ago we would still have a very serious energy deficit, since both hydro schemes would be generating at far below their rating as Zambezi River flows would be very low.

A figure of 2 400 megawatts is frequently quoted for Batoka, 1 200MW each for the Zambian and Zimbabwean power stations. But Lake Batoka will have very little storage, the lake being long and narrow.

The Zambezi, as we are now all aware, has a very big range of flow volumes between high water, when the floods come down, and low water at the end of the dry season. This means that while each Batoka station will generate its 1 200MW for a couple of months while the floods flow, by November that might be cut to less than 400MW in a normal year. And in the sort of drought year like we are experiencing, each station might fall below 100MW.

However, in your hypothetical normal year, while Batoka South is going flat out in April, the 1 050MW Kariba South is ambling along at, say, 400MW with the rest of the flood water raising the lake level quickly. As Batoka cuts back, Kariba South runs more turbines, using the huge storage capacity of that dam. But adding up the two figures, you quickly find that it is unlikely that the Batoka South-Kariba South pair will ever average much more than 1 400MW.

This admittedly is twice the average that Kariba South can generate in a normal year. While Kariba South, with its recent extensions, can now generate 1 050MW flat out, there was no extra water flowing into the lake to allow a continuous maximum output. So the average output of Kariba South remains the 750MW it was before the extensions. Floodgates have not been opened at Lake Kariba for a long time. Every drop of water in the Zambezi downstream of the dam wall has flowed through the two power stations.

The Kariba South Extension was useful on other grounds, giving flexibility to Zesa. There is a large gap between peak power demand and what is needed at, say, 1am. The extension allowed Zesa to cut output back a lot at 1am, relying on other stations, and then turn on all the taps when peak power was required. And if Batoka is ever built, then that extra generating capacity will be useful for half the year to use up faster the stored water in Lake Kariba, water that was stored by cutting back Kariba South consumption in the floods.

It was this lack of increase in energy, as opposed to maximum power, that drove Zimbabwean energy planners for three decades from the early 1980s to stress the need for more thermal capacity, to take the base load, before extending Kariba South to cope with peak demand. Bad advice to Government a few years ago and Kariba South Extension was built before Hwange Thermal Extension, accompanied by refurbishment at that station. If we had done the two extensions the right way round load-shedding would be limited right now to a couple of hours a day at peak hours.

This is why the Hwange extension and refurbishment is so critical. Coal stations are far more expensive to run than hydro stations, because coal costs money while the water in a river is free, at least once the dam wall is paid for. But they work in droughts.

The advent of commercial viable solar stations changes the equations again. The big Hwange Thermal is still needed, plus the second big thermal station on the drawing board for the past 35 years. We need power at night as well as in a drought.

But for Zimbabwe a big solar station makes a lot of sense, because we have a very large storage dam in Lake Kariba and an oversized power station in Kariba South. Operating the solar and hydro as a pair means that when the sun is shining Kariba South just ticks over if only to keep the Zambezi flowing downstream.

But a large block of the daily water ration for Kariba South could simply be left in the lake as the solar station carries the load, with help from the thermals. Come sunset, when power demand starts to peak in any case, that saved ration could be used to run Kariba South flat out, and flat out all night if necessary. Lake Kariba would be, in effect, the storage battery for the solar station. And like hydro fuel , solar fuel is free. The main cost is the capital cost of the station.

With water stored in sunny days, there would be extra on cloudy days, when the solar station output was reduced. And even droughts would be less of a nuisance. In a drought year there is less cloud and more sun. In any case a 2 000MW solar station could carry Zimbabwean business in daylight hours without anything else feeding the grid, even if the drought was really bad.

And that is why we need to push the giant solar station a lot more than the Batoka scheme. The one works in droughts; the other does not. And Kariba storage takes care of the nights.

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