The Duck Curve

Let’s talk about demand. Specifically, average daily load shapes throughout the year.

In the chart above the average daily load shape for each season is plotted for the five regions of the National Electricity Market and the South West Interconnected System in Western Australia (simply referred to as Western Australia from here onwards). Four different years are charted — 2010, 2015, 2020 and 2023.

What do these charts tell us?

• Firstly, and perhaps surprising to some, is that the sunburnt country generally uses more electricity during winter. The average peaks are mostly highest during the winter months, with the morning and evening peaks close to equal in the colder southern states. The absolute peaks, however, still very much occur in summer.
• Most regions have a clear morning and evening peak — the shape is driven by residential patterns, even though commercial and industrial customers use significantly more electricity.
• The shape isn’t consistent amongst all states — it varies with the mix of residential and commercial industrial customers, the types of loads and gas usage (i.e. Victoria historically has used gas for space heating and hot water).
• Rooftop solar in 2010 was almost nothing, in 2023 roughly 1 in 3 Australian homes have rooftop solar installed. This can be clearly seen by the steep declines in grid demand during the middle of the day over the last decade and a bit.
• However the peaks are largely unaffected, because the morning and evening peak are outside of the best sunshine hours. Changes in the peak consumption has been driven by other considerations.
• There is a clear overall decline in average electricity consumption in New South Wales, Victoria and Tasmania. Queensland and Western Australia however have both seen increasing average electricity consumption across the day.

But! I’ve neglected to mention the clearest trend in the charts above — the so called ‘duck curve’. As increasingly large numbers of residential and commercial & industrial customers have installed rooftop solar, the energy generated is used to not only reduce their draw from the grid, but excess generation ‘spills’ into the grid, lowering the overall demand of the grid.

Why is it called the ‘duck curve’? Well the declining demand is the belly of the duck, apparently. Synergy has produced a nice little graphic to demonstrate.

Ok, but why does the duck curve matter?

There are two primary concerns with the duck curve:

1. As the belly of the duck falls lower and lower, the minimum demand of the grid falls to levels where the grid instability could be experienced. Inverter-connected power sources like rooftop solar do not contribute to system strength, which requires grid operators to force on synchronous generation, typically thermal (coal, gas or nuclear). Additionally these inverter-based generation sources require an input frequency — there needs to be a conductor of the orchestra in order to get something harmonious sounding.
2. As the sun sets in the afternoon the generation from rooftop solar declines rapidly and other forms of generation are required to ramp up to meet the system demand. The deeper the belly of the duck, increasingly faster ramp rates are required, stressing the system and in some cases exceeding the physical ramp rates of existing generators.

These patterns are clearly evident in the charts above — the belly of the duck is deepest in South Australia during the spring, when mild ambient temperatures leads to lower electricity consumption and clear skies yields great conditions for rooftop PV. Because the evening peak has been largely unaffected, the ramp rates required have been getting steeper and steeper.

What does the future look like?

The duck curve has some significant implications:

• Firstly, Australian solar installations have slowed a little  in recent years, but are nowhere near done. There’s still two-thirds of Australian households without rooftop solar and plenty of businesses. The cost of solar is also continuing to decline and novel applications utilising very cheap panels are growing, for example solar fences.
• Historically, ‘feed-in tariffs’, payments for exporting to the grid, were a driving motivator for solar installations. However modern feed-in tariffs are trending to zero, or lower — in the very near future customers may be charged for exporting solar to the grid.
• The problem of ‘minimum demand’, the lowest part of the duck belly, has hit a critical point. Each spring new records are broken and blunt force mechanisms to force customer solar to shut off to protect the grid have been implemented in several states.
• Changed wholesale pricing dynamics. The incredibly low duck belly has introduced large periods of negative prices. However some of this has been offset by higher prices in the morning and evening peaks, driven by generators seeking to compensate their revenue for the reduced daytime prices.
• This change in load shape prices however introduces opportunities — middle of the day is the new ‘off-peak’ in many regions, which creates chances to shift controlled loads like water heating to middle of the day… or electric vehicle charging. Flexible loads in general are well placed to take advantage of these structural shifts in electricity dynamics.

Is the duck curve happening everywhere?

Yes and no. In sunny electricity grids like California (CAISO), where the term duck curve was coined, absolutely. The duck shows its beak in plenty of other grids too. But what about our less sunny neighbours in the UK?

There’s a clear change in shape of the demand during the middle of the day, however the overall shape changes are much less dramatic than the antipodeans.

But the far more obvious story is the huge overall decline in electricity consumption over the last decade. In fact there’s been a 15% decline in annual grid consumption over the same period, whereas the Australian grids have been largely flat.

But as the energy consumption of the British grid has changed, the shape of demand is clearly beginning to look like those down under.

Other ducks?

The duck curve is clearly here, and clearly beginning to cause headaches. It’s also not going anywhere (solar, that is).

But what if we could use the duck to our advantage? The Institute for Energy Economics and Financial Analysis completed modelling on how flexible trading with a battery and rooftop solar could fundamentally alter the shape of the duck, squashing it and producing the sleeping duck scenario seen below.

Batteries, electric vehicles and flexible loads are well placed to take advantage of the changing shape of the grid demand — but they’ll need clear pricing signals and sensible and dynamic connection constraints aligned to benefit the grid operator,  energy consumers, and asset owners. Gridcog makes it easy to incorporate price signals and connection constraints into your project models so you can design commercially viable projects to slay the duck!

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