Eskom faces challenges of variability
National power supply and demand must be balanced Eskom faces variability challenges
Energy CorrespondentThe rising share of variable renewable energy from wind and solar in the electricity mix creates new challenges for system operators that have to balance power supply and demand.
However, in SA the system operator faces additional, unique challenges including the "variable" performance of coal-fired power stations that should provide reliable baseload power.
According to Isabel Fick, GM for system operations at Eskom, variability is one of the biggest challenges. "Variability on the coal fleet far outweighs the variability we are seeing on renewables at this stage."
Wind and solar produce variable supply because these energy sources can only generate electricity when the weather conditions are right — that is, when the wind blows or sun shines.
Base-load power stations such as coal-fired ones usually provide a steady supply of electricity throughout the year.
But the poor performance of Eskom’s coal power stations, which often suffer multiple unplanned breakdowns in a single day, has made them less reliable, creating additional challenges for the system operator. Unplanned breakdowns are one of the factors that cause load-shedding.
Speaking at an event about the future of the grid hosted by UCT’s Energy Systems Research Group and the Presidential Climate Commission on Wednesday, Fick said SA now had about 6,200MW of variable renewable power connected to the grid, compared with about 40,000MW of installed coal-fired power.
While variability on the renewables fleet is about 150MW, variability over the rest of the network is about 4,000MW.
But, she said, they also had to deal with uncertainty brought about by load-shedding and the exponential rise in solar PV has almost doubled in the past 12 months to about 4,800MW of installed capacity.
"Rooftop solar has overtaken generation from large solar PV plants and is now the biggest variable and uncertainty we have to deal with on a daily basis on our network," she said.
In Gauteng alone, the combination of load-shedding’s effect on power demand patterns and the variability of rooftop solar generation can suddenly increase demand by about 2,000MW.
"There is roughly 1,000MW of rooftop solar in Gauteng. During overcast conditions this 1,000MW has to be drawn from the Eskom grid.
"During continuous, higher stages of load-shedding battery inverter systems recharge from the grid after load-shedding, adding an additional 1,000MW burden to the grid.
"This often happens simultaneously, adding 2,000MW of additional demand to the grid, just in Gauteng," Fick said.
Wind power posed its own challenges. According to Fick, during summer months, wind generation aligns almost perfectly to the high evening peak demand. However, in winter, when cold fronts pass through the Western Cape and Eastern Cape, wind generation (which is concentrated in these provinces) increases significantly.
But as the cold front arrives in Gauteng the cold weather drives up demand for electricity and at the same time the wind generation reduces significantly due to the low trough behind the cold front.
At its peak, wind power can provide almost 3,000MW during the evening peak (equal to three stages of load-shedding), but the "double whammy" described above requires thousands of megawatts of generation to be dispatched in a short period of time.
Despite the challenges variable renewable energy brings, SA needed "a lot more of it". More wind and solar capacity must just be built in other parts of the country and closer to where the demand is.
It has already been shown, said Fick, that large parts of the country, including outside the Cape provinces, do offer good potential for wind and solar generation.
Craig Hart, from the renewable integration unit at the International Energy Agency (IEA), who also spoke at the event, said many countries dealt with the same challenges, but had to develop "system friendly" variable renewable energy generation to address this.
This means bringing wind and solar plants closer to the main load centres. For SA this would be mainly Gauteng, which needs about 9,000MW of electricity, and Cape Town with 3,500MW of demand.
The IEA estimates the share of variable renewables in energy systems globally will increase from 10% to 30% by 2030.
A worrying trend is the slow pace of grid development when compared with the pace at which renewable capacity is expected to grow.
"Strong, flexible transmission grids are important for supporting the energy transition, but the deployment of renewables is very much outstripping grid expansion," Hart said.
The slower pace of investment in grid expansion compared with renewables is made worse by the fact that grids also take much longer to build than new wind and solar plants.
In SA, a shortage of available grid capacity in the Northern Cape, Western Cape and Eastern Cape is already undermining Eskom’s ability to bring on board new generation capacity from renewable energy sources.
Fick said Eskom was intent on opening up the grid to accommodate more renewables from independent power producers. To allow for this it has outlined the need for grid expansion in the transmission development plan which aims to build 14,000km of new transmission lines over the next 10 years.
"The country has never done an infrastructure upgrade as we are about to see with the transmission development plan," she said.
Business Day previously reported that Eskom estimates that implementing the transmission development plan will cost at least R210bn.
The country has already received offers from international lenders who are willing to provide financing for new transmission build projects.
Electricity minister Kgosientsho Ramokgopa has said his department and Eskom, in collaboration with the JSE, will host a transmission financing seminar before the end of the month.
megawatts of variable renewable power is now connected to the national grid, compared with about 40,000MW of installed coal-fired power