“Unparalleled in world:” AEMO maps route to “hours and days” of 100 pct renewables

“Preparing for high instantaneous penetrations of renewables – and the first period of 100% instantaneous operation – is a critical part of enabling future power system operability at net-zero emissions,” Westerman says in his introduction to the 100-page Engineering Roadmap to 100% Renewables report.

“At these times, coal generators will be offline; either intentionally decommitted, unexpectedly offline for maintenance or failures, mothballed, or retired.

“Coal plants take many hours, or even days, to restart operation, so once taken offline, they can’t be relied on to meet immediate intraday energy demands, or provide system restart services.

“Operating regularly with 100% renewable power also means reducing the need for regular reliance on gas-fired generators to firm the electricity supply.”

Two big challenges to 100 pct renewables

Westerman says there are two main challenges with running a grid on wind and solar alone. The first is dealing with the variability of output, which will require significant levels of storage and demand management, including controls on rooftop solar – as was recently witnessed in South Australia.

The second challenging is managing a changed system. Wind, solar and batteries use inverter based technologies, which is as different to traditional synchronous generation as analogue is to digital.

AEMO has to be to manage the transition and switch between the two different systems, and keep the lights on at the same time.

AEMO is cheered by the success of the four synchronous condensers which have been installed in South Australia, and which allows that state to operate the grid at high levels of wind and solar (up to 146 per cent of demand) with a bare minimum – just 80MW and soon to reduce to 40MW and then zero – of gas generation.

It estimates the main grid will need the equivalent of 40 syncons across the main grid, known as the National Electricity Market to accommodate the 100 per cent renewables scenario.

Batteries and advanced inverters

It points out, however, that they don’t need to be syncons themselves (big spinning machines that do not burn fuel), but it could battery storage with advanced inverters that can provide the same services. Some batteries, such as Hornsdale, are already trialling those services. More are expected to follow.

Why 100 pct renewables may not happen on some days

This includes market behaviour, such as some renewable generators choosing not to generate at their full available resource potential when the wholesale price of energy is negative, or non-renewable generators bidding themselves into the market for commercial reasons.

There may be network constraints, such as limits on transmission line capacity, that mean not all this resource potential can be dispatched in the market and carried by the network to consumers.

There could be system requirements, such as the need to maintain sufficient essential system services, that may currently result in fossil fuel generators being dispatched to provide essential system services in the absence of capabilities being available from non-fossil fuel alternatives.

Limits to rooftop PV

And there could be limitations on the level of distributed photovoltaic (DPV) generation to manage power system security. AEMO is especially keen to ensure all new rooftop solar systems have inverters that can be controlled or “orchestrated” so it has more levers to pull to manage the grid.

AEMO makes it clear that it will take a cautious approach, and may only allow 100% renewables penetration across the whole grid if it has renewable resource potential well above the level required to meet customer load at that time, and the system needs.

Reserve margins needed

“The first period where renewable resource potential is sufficient to satisfy 100% of demand may not lead to an instantaneous penetration of 100%, if that would risk reserve margins becoming too low to meet future demand in that day,” it says.

Indeed, AEMO notes that on October 16 this year, in the South West Interconnected System in Western Australia, a gigawatt scale grid with no connections, there was more than enough renewables in the system to reach 100 per cent instantaneous penetration.

But it didn’t happen, mostly because a significant amount of renewables was curtailed for economic reasons – project owners dodging negative prices.

And, in any case, the grid operator doesn’t yet have all the systems in place to allow this.The maximum penetration of renewables to date has been 81% in November, and 74 per cent distributed PV in October.

AEMO also notes there is a lot more engineering work to do in the future. The point where the NEM can operate at 100% instantaneous penetration of renewables will occur many years in advance of the time where it can operate at 100% renewables on an extended basis (over many weeks or months).