YANDISA NONGENA: Lack of flexible generation can also result in blackouts and load-shedding

The announcement of the opening up of the SA power grid to more intermittent electricity generators will need to be quickly supplemented with increased investment in flexible dispatchable generation — open cycle gas turbines (OCGT), gas or diesel engines and/or batteries. Otherwise, blackouts will continue, though caused by a lack of flexible capacity rather than a lack of capacity as at present.

To understand how this could arise, it is crucial to analyse how the energy system works, and how increased penetration of intermittent generators changes that. As electricity cannot be stored in large quantities, electricity generation (supply) must always match electricity load (demand) to keep frequency within grid range (60Hz or 50Hz, as the case may be in different countries), otherwise the consequence will be:

• load-shedding, a deliberate and planned switching off of sections of the distribution network (leaving areas without power) when there is less generation than load, to maintain supply-demand balance for the remaining areas; or
• blackouts, unplanned shutdowns resulting from synchronous generators tripping when there is more generation than load as frequency rises above the grid’s frequency tolerance.

Electricity demand is generally lower at weekends and at night (the baseload) and higher during the day and on weekdays (peak load), especially from 7am to 9.30am and 4pm to 7pm. The Council for Scientific and Industrial Research (CSIR) Energy Centre reported in April that SA’s baseload is about 17.1GW and the peak load about 35GW.

This means a requirement for at most 11GW of flexible generation. The flexibility of power station generation refers to fast start-up times (seconds or minutes instead of hours) and minimum downtime (minutes instead of hours), as summarised by the International Renewable Energy Agency. 

Pre-renewable energy or with low renewable electricity, all the grid operator needed to do to balance the grid was to ensure enough generation from power generators that reliably run throughout (baseload) to meet daily minimum/baseload demand, and as daily demand increased to bring progressively faster-starting generators online to meet rising daily demand until peak demand.

When daily peak demand was passed and demand started reducing, the reverse would occur, with power stations that came online as demand was increasing being progressively shut off as demand fell back to daily baseload demand, with only baseload generators dispatching.

However, with increased penetration of renewable generators (wind and solar), which are intermittent by nature (they can only generate when there is wind or sun), the operation has become more complex. Flexibility of each generator and the economic considerations become important, in that:

• the cheapest and greenest generators must always be allowed to produce first — as long as their fuel (wind or sun) is available. However, this may require some baseload generators (inflexible) to be shut down when intermittent generators are producing; or
• inflexible generators may be kept generating but renewable generators curtailed (shut down because there is more electricity being produced than is needed).

The CSIR Energy Centre also reports that the generation mix in SA in 2021 was more than 40GW coal and 1.9GW nuclear (that is, a baseload of 42GW), 3.4GW OCGT and 2.7GW pumped storage (that is, flexible generation of 6.1GW) and wind at 3.5GW, solar at 2.2GW and concentrated solar power at 0.5GW. The difference between peak load (35GW) and minimum load (17.1GW) is 17.9GW, yet flexible generation is 6.1GW — a far cry from the flexible generation that is required. Intermittent power does not offer flexibility but increases generation volatility, which drives the need for more flexible generators to balance the grid, and diminishes the need for baseload generators.

The increase in photovoltaic solar electricity will progressively reduce daily load from the morning as solar starts producing, until peak production in the middle of the day, with this reversing over the afternoon. This forms a “duck” curve when one overlays the daily demand profile over years, as shown in the graphic. This means some baseload power stations that produce day and night may have to be shut down during the day as customers using solar electricity do not draw power from the grid (and may even feed the excess solar electricity they produce back into the grid). This will require some coal power stations to be flexible (that is, to have minimum downtime of about eight hours or less) if not shut down, and peak power stations (OCGT, gas or diesel generators) used more.

Even if Eskom were to have the best maintenance and availability of its power stations in the world, the coal power stations’ inflexibility means some will have to be mothballed as they will be of limited assistance to mitigate blackouts caused by increased renewable penetration. SA would be best served by investing in OCGT, gas and diesel engines to increase flexibility.

• Nongena, a Wits engineering graduate, is a banker based in London with experience financing electricity projects in the UK, France, Belgium, Ireland, Germany and Cyprus.