Optimising energy supply and demand

Optimising energy

SUPPLY AND DEMAND

Load-shedding has become a frustrating problem that requires workable solutions. Bruce Munnings talks about the challenges and the search for answers.

Bruce Munnings, engineer: Electricity & Energy, iX engineers

The first time I remember loadshedding happening in South Africa was in January 2008. The bulk of electricity generation at the time was from coal-fired power stations. Electrical demand had been growing faster than new generation was being built, which led to a supply/demand imbalance.

Despite the warnings on climate change, Eskom decided to build new coal-fired power stations to increase base generation. A pumped storage hydro system, Ingula, and fossil fuel generators were built to manage the peaks. Around the same time, the Renewable Energy Independent Power Producer Procurement Programme started.

Between 2008 and 2022, additional new generation has been added to the South African grid. However, from Eskom’s weekly system status report, it can be calculated that at least a third and sometimes more of Eskom’s coal-fired power stations are unavailable due to planned or unplanned maintenance. Due to capital, operational and maintenance costs, climate change and corruption issues, building more coal-fired power stations is no longer considered a good option.

Breaking down the complexity of the load-shedding problem

The need for load-shedding has been created for various reasons: • maintenance of coal-fired power stations • peak demand exceeds generation capacity available • environmental and climate change considerations • renewable energy not always available and currently limited in capacity • new nuclear power stations have not been built due to cost, corruption and other concerns • demand-side management (DSM) has not shifted enough demand out of peak demand times • energy-efficiency initiatives have not reduced demand sufficiently. Each of these items needs to be addressed in depth. However, due to the focus of this article, only some will be covered here.

Peak demand exceeds generation capacity available

Figure 1 illustrates the weekly electricity demand profile. The morning peak and evening peaks are noted. The evening peak is at present higher than the morning peak. On Saturdays,

FIGURE 1 Noupoort Wind Farm

Sundays and public holidays (Monday in this week), the curve is lower. Balancing supply and demand must be achieved at all times in order to keep the grid stable.

Generation: pumped storage and peaking power plants

Pumped storage hydro is one of the means of balancing supply and demand. Water is pumped up during the night and used to generate electricity during the morning peak and evening peak times to balance the grid. Figure 2 provides a weekly profile of pumped storage hydro over the same period.

In addition to pumped storage, liquid fossil fuels and gas peaking generators are used to meet the morning and evening peak demands. Figure 3 provides the weekly curve of generation from peaking power plants over the same period ending 6 May 2022.

FIGURE 2

Switching to energy-efficient LED lighting passed on major savings for a warehouse owner

SOLUTIONS

The solution is an energy mix that addresses the complex problem of load-shedding. These solutions are given in three broad categories: • Generation: dispatchable (available 24/7) or non-dispatchable (weather-related) generation. The generation mix needs to match the demand profile. • DSM tools shift demand from peak to offpeak. An example of a commonly used and effective DSM technology is geyser ripple control. Domestic geysers are switched off during peak and switched on after the peak is over. • Energy efficiency comprises a broad range of technologies and process optimisations used to enable less electricity or energy to achieve the same benefit.

Renewable generation and demand problem

Supply and demand must be matched on a continuous basis, every hour, week and season. Figure 4 is a graph illustrating a typical weekly pattern taken from data on Eskom’s website in May 2022.

The demand curve has a morning peak and higher evening peak. The evening peak is mainly due to residential demand, such as cooking and heating water. As can be seen, renewable generation peaks after the morning demand peak and drops off before the evening peak.

Generation

Adding renewables will reduce the need for load-shedding during the middle of the day while the sun is shining. Single-axis solar PV trackers follow the sun from east to west and generate more in the morning peak than northfacing rooftop PV.

Concentrated solar power with storage is significantly more expensive. However, it has the benefit of being able to generate during the evening peak. Solar PV on east-facing roofs with grid-tied inverters will be more beneficial for the grid. Adding batteries and hybrid inverters will reduce the evening peak, as well as provide power during load-shedding for essential loads such as lights, Wi-Fi, TVs and computers.

Another solution is to permanently shut down unreliable and environmentally damaging coal-fired power stations and replace these with alternatives that are safer and better for the environment. Examples include small modular nuclear reactors, which are expected to become cost-competitive over time. The issues and concerns around nuclear energy need to be addressed. And planning and communication around nuclear energy needs to continue.

Green hydrogen for electricity generation is also gaining interest, although it does still have a relatively high cost. The use of green hydrogen has recently been used for heavy mining vehicles at the Mogalakwena mine.

FIGURE 3

FIGURE 4

DSM

Installing domestic time-of-use meters will encourage especially higher-income residential users with battery energy storage to charge batteries while the sun is shining, or charge at a lower-cost, off-peak tariff and use the stored energy during peak times. Time-of-use tariffs and possibly future real-time pricing will encourage shifting the load out of the peaks – e.g. tumble driers, dishwashers, and swimming pool pumps. Electric vehicles can also be charged during off-peak. Some vehicles are now equipped with vehicle-to-grid capacity and can be used for DSM.

Energy efficiency

Examples of energy-efficiency interventions include the following: • Aeration of municipal wastewater treatment works: traditional surface aeration is very energy intensive; however, there are newer aeration technologies that are more efficient. • Green building elements: these include ceiling insulation, low-emissivity glass, adjustable blinds to manage sunlight, and natural ventilation to reduce electricity used by air conditioning. • LED lamps are now available at 210 lumens per watt. Careful planning of new lighting installations or changes must also address glare and discomfort. Energy-efficiency opportunities must remain a focus area, as these are often the leastcost options.