In the latest of its regular updates to the nation on the state of energy and electricity availability, Eskom says loadshedding has been suspended for over seven months (226 consecutive days), which has led to year-on-year diesel savings of R14,6 billion, approximately 70,3% less than the R20,8 billion spent during the same period last year.
While the performance of the generation fleet is impressive, it might start going down eventually, as Eskom will be forced to decommission the older power stations as they become increasingly expensive to operate, which might lead to an increase in the use of diesel fired power stations again.
While the coal fleet continues to run better, the old power stations cannot continue to run at these levels forever. However, it’s expected that Medupi and Kusile, as new power stations, will run for another 30 years at these levels. But in the long run the older ones like Kendal will not operate at optimum levels, irrespective of how much fixing and maintenance they receive, they will perform poorly and keep breaking down. They will eventually have to be stopped, as they become increasingly less efficient to run.
The second issue Eskom will need to deal with to ensure loadshedding does not come back is that, as the grid increasingly receives more from renewable energy sources, it will become unstable as a result of the fluctuating nature of energy sources such as solar and wind.
Typically with renewables there is not enough energy in the early mornings and the evenings to support the demand. And demand tends to be high in the evenings. While there is not much demand in the mornings, and with more energy availability again around midday because the sun is out, this fluctuation leads to the grid becoming technically unstable.
When you don't have enough supply, you cut demand. However if you have too much supply, you can't create the demand. This is what creates the instability. Because of this, we will continue to see portions of load shedding going forward, however different to what we've seen in the past. And, in an attempt to deal with this technical instability in the grid, Eskom could start using high levels of diesel again.
Correlation between loadshedding and diesel use
Total generation from the diesel fleet is around 1 170 MW, which equates to around 12% of grid support annual dispatch. However, during loadshedding this typically moves up to 40% grid support for short periods. While 1 170 MW is a small amount of energy by volume, it plays an important part in stabilising the grid. However it is very expensive, at around R9 per kWh.
The diesel engines were traditionally designed to only run about 11% of the time as they were only meant to be used for emergencies and for grid support. However with the increased levels of loadshedding over the years, they were being run as a baseload.
Under normal circumstances, the diesel engines should be coming on three or four times a day. However, during the most intense periods of loadshedding Eskom was running them throughout the day, and sometimes at full throttle. But the fact that they don't have to do that at the moment is what has led to the savings on diesel usage.
There is also the issue of the load factors. For example when a coal-fired power station is running properly it has about a 90% load factor, as it runs for 90% of the time. A photovoltaic project only runs for about 30% of the time over the year because of the irregular availability of the sun, and wind is around 40%. While the diesel engines were traditionally designed to only run about 11% of the time, during periods of intense loadshedding they are typically run at about 50%, meaning they burn five times as much diesel as what it is originally planned for.
According to Eskom’s Integrated Demand Management programme, the power utility also curtails independent power producers and then pays them for this reduction in generation. This payment for reducing generation is much lower than the R9 per kwh payment for diesel use, and this is another aspect that can explain the decrease in expense for diesel usage.
