EDITORIAL COMMENT : Gas reserves allow more power stations

Zimbabwe is now moving towards using natural gas to generate electricity with two large deposits, the coal bed methane deposits in Matabeleland North and the far deeper gas field in Muzarabani, two opportunities to get the base load of electricity we need with around a half the greenhouse gas emissions that coal puts out.

Serious work has already been done on some of the Matabeleland North methane deposits trapped in the coal beds that underlie so much of the northern half of that province with Shangani Energy Exploration, a joint venture between Sinosteel and Zimasco, reckoning it can have a pilot power station running within two years.

On the other side of the country Invictus Energy Limited, who have been leading the exploration of the Muzarabani-Mbire field, have now quantified the probable natural gas deposit at 283 billion cubic metres, or 8,2 trillion cubic feet to use the measuring system still widely used in the petroleum industry.

This still has to be confirmed with the test drilling planned to start in the middle of the year.

The total trapped in the more dispersed, but far shallower coal bed methane fields is roughly of the same order.

Neither set of fields is Saudi Arabia, but both will be useful once exploited.

The most efficient gas power station is a combined-cycle gas turbine which uses the gas turbine, basically a modified jet engine, as the first source of energy to drive the generators and then uses the very hot exhaust gases to heat a more conventional boiler to raise steam to drive a turbine that can drive a generator, similar to how a coal station works.

Some even use both sources of mechanical energy to drive the same generator.

Such a power station will consume around 7 cubic feet of gas to generate one kilowatt hour, the unit measured by a domestic meter. Doing the sums this means that the Muzarabani field can drive a 1 000MW power station, roughly the size of Kariba South, going flat out 24/7 for 134 years.

Obviously some of the gas will not be extractable, but that is balanced by the fact that the power station is unlikely to be going flat out 24 hours a day.

So we can probably install a 2 000MW station with enough fuel for at least 60 years.

The Matabeleland North coal bed gas is more dispersed across several fields, so it would make more sense to have a number of power stations.

Shangani is looking at a 400MW station and that seems a more sensible size, although several such stations could eventually be built if the methane deposits total what is roughly estimated.

We still need our coal stations and may well have to extend them further or build new ones. But the gas stations pump out a little under a half the carbon dioxide of a coal station for every unit generated so there is a major gain, plus a gas station is cheaper to build and cheaper to operate than a coal station.

It can also be built and commissioned a lot quicker.

Renewable energy is the goal for a zero carbon economy. But Zimbabwe can exploit only two of the known renewables at the moment, hydro and solar, to generate electricity although biogas can help rural households and ethanol can extend petrol supplies.

While small hydro stations are planned for our largest dams, we are talking about small stations, enough for the area around the dam, but not much more.

The only major hydro source is the Zambezi River. We already have a 1 050MW station at Kariba South, but because of the rationing of flows, the average output is around a third of that.

Because hydro generators can go from stop to full output in a couple of minutes, rather than the hour plus it takes a coal thermal unit, the large power station gives Zesa a lot of flexibility to cope with peak power demand, when the station can go almost flat out, but that means that output has to be cut back severely at some other times of the day to keep within the average.

Batoka is planned, but the storage for that lake is very modest compared to Lake Kariba so while we get a lot of power during the annual floods this diminishes sharply at low water.

The two dams can be run as a unit when there is drought in Angola, where most Zambezi water comes from, with Kariba cutting back in the high water months and letting the big lake fill and then using that extra stored water when Batoka has to reduce output.

So we win, but not as much a simple sum would predict. And in any case hydro is very reliant on Angolan and western Zambia rainfall, and regional droughts are becoming more common.

Solar is the other renewable source and we can build a lot of stations. But these only generate when the sun shines.

However, Lake Kariba and the gas stations can act as the equivalent of batteries. If we had a lot of solar we could cut back at Kariba South when the sun was really shining and store the water we were not using then to generate more in the evening and early morning, when demand is high, and still keep within the set average. This is a major advantage of an oversized hydro station.

The gas stations, with what amounts to limited supplies of gas when we talk in decades, could do the same, cutting output when the sun was shining and then going flat out later.

They are not quite as wonderful as hydro for near instant output but are a lot faster to fire up than a coal station, and are used in many countries as peak power stations.

With a lot of solar, like enough to power Zimbabwe in full sunlight, we could install bigger gas stations and switch them off, or almost off, when the sun was shining and still retain the same life span of the gas fields.

All this mixing of hydro, gas, solar and coal will require Zesa to have a generation management programme that is first class.

Greenhouse gases have to be reduced, but already technology is being developed that can scrub and store, as a solid mineral, carbon dioxide from flue gases.

Obviously this will not be cheap, but promises are being made for global funds that can aid developing nations have both fast economic growth while not adding to global carbon loading, and we can tap these.

But in any case replacing some planned coal stations with planned gas, with its lower carbon output, and boosting solar we can meet our carbon reduction targets.

The important factor is that all the energy sources will be local resources and that is the good news.

The mix of hydro, coal, gas and solar can allow enough electricity to be generated with lower carbon emissions so long as the mix is run efficiently.

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