In the chart below, you can see how increasing amounts of rooftop solar on days of low demand has year by year reduced the midday peak until it is now the point of lowest net demand (i.e., demand after rooftop supply). But notice something unusual about these curves. Peak demand is at midnight! When everyone is asleep and most factories and businesses are closed! What gives?
Well, years ago, when SA's electricity came solely from black and brown coal-fired power stations, there was surplus power at night. So to use up this power, which would otherwise have been wasted, the (then) state-owned utility encouraged users to use electricity after midnight heating their geysers (= hot water cylinders, to Americans), by giving them a discount. Geysers had two elements, a large one at the bottom of the cylinder, which turned on after midnight, and a smaller one about half way up, which only turned on if the water temperature at that point in the cylinder fell below the desired minimum. This tended not to be used except when you had guests in the house. Or teenagers.
This suited everyone: baseload power wasn't wasted, and users got cheap electricity to heat their water. Heating water can take more than 20% of total household electricity usage, so that was an important consideration. But of course, these days the SA grid is mostly renewables, wind and solar. And so it would make more sense for geysers to turn on when supply is at its maximum, which is over the period from 10 a.m. to 2 p.m. Perhaps that's a clumsy solution, because there are times when wholesale prices go negative because the wind blows strongly, at night. So the clever solution might be a geyser which checks to see what wholesale electricity prices are doing, and heats the water when they are low. If prices don't get low enough, then it would default to midday plus or minus 2 hours. On the other hand, the most cost-effective solution for households would be to heat their water between 10 and 3, thus using as much of their own solar power as they can, because the feed-in tariff is way below the tariff we pay to get our electricity back from the grid.
If geyser demand were rescheduled to midday, then there would still be a duck curve as more solar (now increasingly utility-scale) is installed, but it would be smaller. The morning (net) demand peak would run from 6 a.m. to 9 a.m. and the evening one from 5 p.m. to 10 p.m. On a very rough calculation, this would mean that 2 hours of storage would be more than enough to fill those demand peaks. (1.5 hours of storage would equal 6 hours * 1/4 of total demand) . So SA would need 2.4 GWh of storage, provided geyser demand were rescheduled to the period of maximum daylight. The big battery cost A$66 million for 126 MWh of storage capacity, so SA would need to spend A$1.3 billion to do this, or about $360 per person.
Even with enough storage, the interconnector to Victoria would remain very useful for periods of exceptional overdemand or undersupply, as would the new HVDC interconnector to NSW, which the new Liberal (i.e., right-wing) SA government is in favour of. The more a grid is connected to its neighbour grids; the more storage it has; and the more diverse its sources of power, the more stable it will be.
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| Source: Dylan McConnell, from the Climate and Energy College in Melbourne |
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