Wietze Post (Part 2) – The solar duck curve: Nemesis to coal and nuclear plants

People are installing solar PV on buildings and in solar farms to deal with daily load shedding. The PV plants nearly always include batteries. The solar plants generate most of their energy around noon. This results in their users not drawing power from the grid around noon. Collectively, this depresses the power demand that would otherwise be drawn from the grid.

Year by year more solar is installed and so less and less energy is drawn from the grid. Charting the amount of energy supplied by the power stations – via the grid – results in a line which sags around noon. And every year the line sags deeper. This chart pattern has been dubbed the “solar duck curve”. As more solar comes on stream, we reach a point where everyone has supplied themselves and their neighbours. Thus there’s no need for electricity from power stations.

So some or all coal plants must be taken offline around midday. Read Wietze Post’s Part 1 article here.

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By Ir. Wietze Post

Rooftop Solar Plants – Nuclear and Coal’s Nemesis

A duck portends the demise of new coal and nuclear power plant projects – new projects will not reach financial close.
Free for use under the Pixabay Content License 

The Solar Duck Curve

The duck curve is a graph. It resembles a duck sitting in water. The graph shows how rooftop solar plants take away grid demand on sunny days. With no demand, power stations have nowhere to send their energy.

Charting ½-hourly wholesale spot prices shows a similar chart for sunny days.

The duck curve highlights the bane of all large power stations (generators). These generators can be coal, nuclear, hydro, or wind and solar farms. They are all large and need the grid to transmit and distribute their power.

Meanwhile, property owners install many rooftop solar plants. Their sizes range from small residential to large commercial. These rooftop solar plants can generate a lot of energy. Then their owners need little or no power from the grid. Besides, their excess energy gets priority access to the distribution grid. After a few years, so many rooftops have solar panels that local grids no longer need supply from upstream.

Besides, in dark and cloudy times wind turbines reduce demand.

Base load” demand disappears due to the massive rooftop solar power generation. So some large generators must shut down for a few hours, or entice consumers by paying them to use more energy. During the next few years, the grid needs fewer and fewer hours of supply.

Batteries installed with rooftop PV plants start discharging late afternoon. Thus there’s no grid demand from the property. As a result, the level of grid demand during the remaining hours also decreases. At night, on cloudy days, and in winter, batteries and wind turbines supply energy.

The average grid power demand may go from 24 hours/day at 50GW, to 20 hours/day at 40GW, to 8 hours/day at 10GW. Note that the total daily energy demand goes down much quicker than the power demand. In this example, energy demand goes from 1200GWh to 800GWh to 80GWh/day. Such a move usually takes 5 to 7 years.  After 7 years, only 1/5 (at best) of the number of large power stations are still needed, and they all only work for 1/3 of the time. More likely, only the cheapest 10% of the units remain active for ½ of every day.

A graphical representation of the ‘Duck Curve’ for the SWIS, created with 2020 data from AEMO. From: Everything you need to know about the Duck Curve.

The image above is based on data from Perth, Western Australia. The lines show the grid demand on a summer’s day. Each line shows a day’s demand, from 2018 through 2024. In the early morning and evening, the grid demand rises steadily year by year. Consumers use more energy every year.

Also every year, more rooftop owners install solar panels. They generate more energy for themselves and demand less daytime energy. As grid demand decreases, the duck’s “belly” sags deeper. Eventually, it reaches zero. That is now the case in many regions globally. Zero grid demand (zero system load) can last for several hours per day during the summer half-year. After a few years the grid is left with no system load all day, every day, for months on end.

The Duck Curve shows how the little guys’ solar plants take coal and nuclear’s lunch.

  1. The difference between South Africa and Australia

Yet there’s an important difference between South African solar owners and Australians. Australians don’t suffer from load shedding. So they did not usually install batteries. Before, they only installed panels with inverters – no batteries. That is changing, with some new installations including batteries. Australians usually install solar to save on electricity expenses.

For South Africans, the main buying trigger is to avoid load shedding. So a battery is standard in South African solar installations. Saving money, instead of paying our Eskom utility, or the town council, is a big bonus. South African batteries are usually fully charged by 4 pm.

Battery energy is used in the evening and so there’s less grid demand at night. The duck’s neck becomes shorter and the lines through the head and beak lie lower. The evening peak grid demand will become lower than the morning peak. That’s for summer.

In South Africa, the little guys’ solar & battery plants take coal and nuclear’s lunch, supper, and, finally, breakfast too.

In winter, the morning and evening peaks lie much higher. In fact, the whole grid demand curve lies higher in winter. This naturally fits wind farms’ prime generation times.

On most days the grid demand decreases around noon. All the rooftop PV plants generate power for their premises. Their excess energy floods the local distribution grid. On those days several large power plants must shut down for a few hours or curtail their output. On other days, such as rainy days, demand remains high. The grid may have extreme and rapid demand changes. The rapid changes make life difficult for fixed-output plants. Coal and nuclear plants struggle to remain economically viable.

The variable grid demand requires a nimble response. Solar and wind power plants can do that. Equipped with batteries, they can instantly curtail or ramp their output. Hydro turbines need a few minutes to get ready. Their operators could complement the turbines with large batteries. The batteries bridge the gap and speed up their response. Coal and nuclear plants need more time, and thus they’re always too late. They should also consider batteries.

The duck curve shows why it may not be a good idea to install a solar farm. In a few years, we will get to the point where solar plants cannibalise each other. If you’re considering large, farm-sized, generation plants, it may be better to install a wind farm. Then you can supply energy when there’s less competition from the rooftop solar plants.

When governing authorities buy renewable energy plants, they should look to wind farms. Also, consider large battery projects. Leave the PV plants for the rooftop owners to install. It’s a better balance of effort and investment.

Yet you may be keen to invest in large solar plants. Consider investing in VPPs (Virtual Power Plants). These consist of thousands of rooftop plants and batteries, all with remote coordination. VPPs can make grid interventions within milliseconds, usually in large urban grids.

Here’s California’s Solar Duck Curve:

California’s solar duck curve has sagged deeper year by year. By 2023, grid electricity demand was at zero on its lowest Spring day. Note that the curve is approaching zero demand from 11 am through 4 pm. This event is likely to occur in Spring 2024. In later years, the no-demand period will extend over more hours and days.

The 2023 demand is one of the lowest recorded. Yet there will be demand after 5 pm through 8 am for several more years. But the whole curve will shift lower year by year. Besides, as more people install batteries, the evening peak will gradually disappear. We will see a “sleepy duck” curve, as its head and beak lower.

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