10 minutes

Deep dive: peaks and troughs in demand

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Aleks Smits
31 October 2019

The exponential rise of renewables – especially the vast amounts of small-scale rooftop solar – has changed how electricity is sourced in the National Electricity Market (NEM).

The NEM now regularly experiences periods of excess supply in the middle of the day, while overall supply remains tight to meet periods of extremely high demand during hot summers. The daily demand-surge as the evening peak kicks in and solar generation drops off is becoming more pronounced, and reliance on thermal generation when wind conditions are low is increasing.

This article considers how these complications can be best managed as more renewables continue to enter the NEM.

Supply and demand dynamics


The supply and demand characteristics of the NEM are extremely dynamic. Unlike markets for other goods and services that operate and fluctuate on a longer-term basis, the supply and demand of electricity across the entire NEM must always balance instantaneously, as electricity is not a commodity that can be readily stored in large quantities.

As a result, the central market operator AEMO calculates every five minutes which generators are available and how the electricity needs of customers across the NEM can be met at lowest cost, taking into account how much electricity is being used and the overall safety and security needs of the system. Additional markets – such as power system frequency regulation – exist on even narrower timeframes (as little as six second increments) to ensure supply and demand remain precisely balanced, and a reliable supply of energy is available for the NEM’s 11 million households.

Different generators in the mix


Although electricity needs to be produced all the time to meet customer demand, individual electricity generators are not always running, and adjust their output depending on their availability and the needs of the system.

As much generation as possible needs to be available during peak periods to meet customer demand, but otherwise generators can reduce their output or shut down generation completely during periods when demand is able to be met by other sources. At these times, generators can run scheduled maintenance activities or simply allow lower-cost generation to meet the needs of the system.

In the case of coal-fired generators, switching units on and off can be an expensive and complicated process, so it is often more cost-efficient over the long-term to run them constantly for long periods of time. Coal-fired generators are kept running within the range of their operational output 24 hours a day, but peaking gas generators can be switched off completely – and then be returned to full capacity within minutes of being switched on.

Large-scale wind and solar farms generate electricity whenever the wind is blowing and the sun is shining, and do so at a much lower cost. However, just like other facilities, the electricity these generators produce is only paid for and dispatched to the market when it is required by AEMO.

Generators work with the market operator to ensure that, throughout the year, the needs of the NEM will be met and their facilities will be run as efficiently as possible to maximise their output – but that the generators also meet the needs of the system at the lowest cost.

But what about rooftop solar?


In contrast to large generators – which must bid into the market and dispatch electricity in accordance with AEMO’s instructions – small-scale solar panels on rooftops generate and dispatch electricity whenever the sun is shining, regardless of the needs of the NEM.

The electricity that a rooftop solar panel generates will be used in different ways depending on the underlying conditions within that household, in the local network area, and more broadly throughout the NEM. As household solar electricity cannot be readily stored without a connected battery, the electricity that is generated in solar panels needs to be used immediately, either by being consumed within that household or exported to the grid to be used by another consumer. If it is exported to the grid, the market operator will allow for this by reducing the amount of electricity that is required to be dispatched by other generators.

As more solar and renewables enter the system, there are increasing periods where there is an ample supply of cheap energy during the day that is being produced by large-scale wind and solar generation as well as rooftop solar.

Case study: wind and solar supply in South Australia


However, there are still times when electricity demand must be met by other sources.

This graph shows the demand (in blue) alongside wind and solar generation (in cyan and orange) in South Australia on Sunday, 23 July 2017.

Supply and demand over time in South Australia on Sunday, 23 July 2017