Explainer: the duck curve

The energy sector is infamously jargon-heavy, and even with a handy glossary of terms at your fingertips it can often be a tough slog reading about developments and issues in the energy industry.

One rather abstract term cropping up more frequently is the ‘duck curve’ – but what is a duck curve? And what does it have to do with energy?

Duck tales – the history on the ‘duck curve’

In 2013, the California Independent System Operator (CAISO) – the organisation that oversees California’s electricity generation and transmission network, much the same as AEMO oversees Australia’s NEM – published a now-famous chart (PDF)* that showed the energy demand over time on a spring day, and how that was anticipated to change in the future.

The 2013 chart produced by the California Independent System Operator (CAISO) predicts electricity demand over time on a typical spring day in California

As we’ve marked up below, as small-scale solar generation increases through the day, demand for electricity from the network drops as there is excess energy from rooftop solar. Then, once the sun begins to set and people return home in the evening, network demand begins to peak. See that duck curve now?

Grid demand falls in the day, and then rises again in the evening

The line of the chart – particularly the increasingly pronounced shape of the predictions as the years go on – was noted to resemble the outline of a duck. Hence, this phenomenon was dubbed ‘the duck curve (PDF)’** – and the name stuck.

Quack

It’s important to note that the way we consume energy hasn’t changed dramatically over the years - on the whole, people still leave the house to work or study during the day (with correlating declines in demand) and then they spend their evenings in (with a correlating increase in demand), which is then followed by a decrease in demand, as everyone heads off to bed

The duck curve is overlaid on this pattern of consumption and was predicted based on increasing levels of rooftop solar. Those predictions have been borne out over time, particularly in sunny markets with a high level of rooftop solar adoption, like California – and Australia.

An energy mix

The surge in energy demand in the evening, especially in the sunnier months in Australia, means that both traditional baseload sources of power and intermittent renewable energy, need to be supported by flexible firming energy generation.

Response to the increasingly steep duck curve demand profile is the development of technology and generation that can cater to these peaks and troughs, like firming supply and energy storage.

Firming capacity is generation that can ramp up and down

Firming supply is capacity that is agile and has the ability to ramp up generation quickly. It’s a role filled in today’s energy mix by modern peaking gas power stations like the new Barker Inlet Power Station (BIPS) in South Australia, which can ramp up to full power in timeframes measured in minutes.

This means that gas peakers can sit at a low-level (or even offline) during periods of low demand during the day, and then rapidly ramp up to meet peak demand in the evening.

Energy storage can take excess supply in the day, and save it for use when it’s needed most in the evening.

Energy storage, like batteries and pumped hydro also have an important role in the context of the duck curve.

An enormous amount of energy is produced by renewable generation – primarily rooftop-solar, in this case – every day. But if that energy is not immediately used or stored, it is lost, and this is where energy storage can help by storing that excess energy and dispatching it when it’s needed most.

And like firming power stations, energy storage has the flexibility to ramp up quickly to meet changing demand but unlike firming power stations, that ramp up is measured in milliseconds, not minutes.

Both pumped hydro and batteries are incredibly effective at frequency response – with the ability to smooth out the peaks in supply by storing excess energy, or otherwise responding to peaks in demand with flexible generation - in turn making the network more reliable and effectively lengthening out the duck’s neck and flattening the demand curve.

Steps we’re taking

For these reasons, we’re investing in flexible generation and energy storage, to best meet the needs of the current and future energy market.

AGL is making broad investment in firming capacity across Australia for a more reliable supply into the future. In total, AGL has about $1.9b of new energy supply projects completed or in construction, with a further $2b in the pipeline. Of these projects, close to 1,000 MW of supply will provide some of the firming support that the National Electricity Market needs.

¹ California Independent System Operator 2013, California ISO Demand Response and Energy Efficiency Roadmap: Maximizing Preferred Resources, December 2013, accessed 27 February 2020, https://www.caiso.com/documents/dr-eeroadmap.pdf

²California Independent System Operator 2016, What the duck curve tells us about managing a green grid, accessed 27 February 2020, https://www.caiso.com/Documents/FlexibleResourcesHelpRenewables_FastFacts.pdf