Rooftop solar – the phantom menace?

Two recent quarterly reports – AER’s Wholesale Markets Quarterly and AEMO’s Quarterly Energy Dynamics have drawn attention to the record minimum demand set in South Australia on 17 October 2019. The reason for this record low is that South Australia has 1,100 MW of rooftop solar PV, whose output is counted as a reduction in demand. The largest conventional generator in south Australia is AGL’s Torrens island B, whose four steam turbines each can generate 200 MW for a total station capacity of 800 MW. In other words, rooftop solar in the state is almost 1 ½ times bigger than the largest utility scale power station.

There’s a key difference between Torrens Island B and this aggregated solar. The market operator, AEMO, knows when Torrens Island is on and how hard it is running at any given time. It can also instruct it to turn up or down if necessary, to keep the grid secure. AGL has to give AEMO regular forecasts of how it will run the power station to help it plan the operation of the grid.

None of this applies to rooftop PV. If all the solar homes and businesses wanted to turn their solar panels off at the same time, they could and the first thing that AEMO would know about it would be a sudden unexpected increase in demand of up to 1,100 MW. Of course, there’s no reason for them to do that, but unexpected cloud cover can take a large bite out of solar output leaving AEMO to scramble to balance the grid. Or voltage fluctuations can trip a large quantity of inverters off at the same time because they’re all on the same trip setting.

In the smaller grids of the Northern Territory, the challenge of managing solar output is magnified. The system black in Alice Springs in October last year was a result of grid operational processes not keeping up with this challenge and failing to respond to the impact of cloud cover causing a drop in solar output.

Fortunately, AEMO is better resourced than its NT equivalent and can estimate solar output across the NEM reasonably well and has processes in place to manage unexpected fluctuations. However, this is an area where market operators are having to run to stand still as rooftop PV capacity grows inexorably.

The next question will be whether other distributed resources like batteries and electric vehicles make AEMO’s job harder or easier. If these are being used individually by their owners according to their own schedules, they will be much harder to forecast than solar, which is basically weather-driven. If they are collectively orchestrated, such as in a virtual power plant (VPP), then they are more controllable and can be managed much more like Torrens Island B.

Policymakers are well aware of the issue and are looking at the long-term goal of having the demand side of the market including these distributed resources participate fully in the market. In the interim, practical steps such as VPP trials and inverter standards are improving the situation.

Notwithstanding the ominous headline, rooftop solar PV is a good thing, especially for its owners, and as a contributor to lower emissions. These qualities shouldn’t blind us to the challenges it creates or get in the way of finding appropriate solutions to those challenges.