Another relatively non-controversial possibility is to expand the existing Great Lake scheme in Tasmania aka Poatina and Trevallyn.
Present scheme consists of, in hydraulic order:
Lake Augusta storage = 44 GWh
Arthurs Lake storage = 795 GWh
Arthurs Lake pumps (7 MW) and Tods Corner power station (1.6 MW)
Great Lake storage = 6523 GWh
Poatina power station = 342 MW
Trevallyn storage 2 GWh
Trevallyn power station much further downstream in suburban Launceston = 100 MW
In operation:
Lake Augusta collects natural inflows and is discharged via Liawenee Canal into Great Lake. This is a water transfer only, no generation takes place in this step.
Arthurs Lake collects natural inflows and is primarily discharged via pumping to Great Lake. Whilst this does generate via Tods Corner, it's an energy-negative step in that pumping power (7MW) substantially exceeds Tods Corner generation (1.6MW). In simple terms it's pumped up to the top of the hill, then run down the other side but it's a net increase in elevation transferring water from Arthurs Lake to Great Lake. This pumping operation runs at a high capacity factor, the original design being the pump runs about two thirds of the time however diversion of water outside the scheme (see below) has reduced this to approximately 45% of the time assuming full use of the irrigation schemes (which of course depends on weather).
Note some water is released from Arthurs Lake for other purposes. Specifically via a siphon over the dam wall to supplement water in Woods Lake, a minor storage used for irrigation, and via a separate 6MW power station the purpose of which is primarily to release water into the Midlands region for irrigation. So for clarity that's two separate irrigation schemes, one of which generates power as a by-product. Both of these were later additions not intended at the time of original construction of the Arthurs Lake dam and pump (in operation 1966).
So Lake Augusta and Arthurs Lake are, in the context of electricity, both essentially diversions into Great Lake. They catch water and store it but diversion is the ultimate purpose. A number of much smaller pumps, weirs etc also divert water into Great Lake – individually they're of minor consequence however, though collectively significant.
Power generation of significance takes place first and foremost via release through Poatina power station, which operates with a nominal design capacity factor of 41.4% discharging into Brumbys Creek and from there ultimately entering Trevallyn storage.
Trevallyn storage receives the discharge from Poatina plus natural inflow from its own large catchment area, most notably the South Esk river. Generation takes place via Trevallyn power station.
In the context of aiming to provide *deep* firming to VRE, that is the ability to generate constantly for multiple consecutive days rather than a relatively short period (hours), is concerned the primary opportunity within the existing scheme is to increase the generating capacity at Poatina and thus reduce the station's capacity factor. That is, build a Poatina B station, a second power station alongside the existing.
Allowing for conservative assumptions regarding future rainfall, and making allowance for irrigation diversions outside the catchment as well as required water releases from Poatina during summer for downstream water requirements (irrigation and town supply) and assuming full use of those, capacity could be increased to about 910 MW. So that is, build a 570 MW Poatina "B" station in addition to the existing and also construct a new 28.2 GWh lower storage.
That 910MW figure is based on running 10% of the time at maximum output for VRE firming and the conservative assumptions stated regarding water availability, this 10% figure being arbitrary as a design basis for VRE deep firming. If less frequent operation were required, or more risk taken with water supply, then capacity could be increased further.
Noting in this context that storage of natural inflows to the combined Lake Augusta, Arthurs Lake and Great Lake storages exceeds 5 years of inflows. That provides almost infinite flexibility in how the station is operated – eg it could be run 20% of the time in a particularly bad year and not at all the following year without running out of or spilling water. Etc.
In routine operation this turns Poatina into a pumped storage scheme with generation through both stations and pumping via the new B station only with a maximum generation time, without spill from the lower storage, of 31 hours. This enables routine use of the power stations for daily peak power requirements and short term VRE firming (pump during daytime, discharge of an evening etc).
When required for VRE firming, it's simply a case of continuing to operate the power stations as normal and releasing surplus water from the lower storage. In practice a typical mode would be to release sufficient water to enable constant maximum operation of Trevallyn, bringing total Poatina A + B + Trevallyn output to 1010 MW, and retaining the surplus (the quantity of which depends on the volume of natural river inflows to Trevallyn) thus slowly filling the lower Poatina storage. This would then be pumped back into Great Lake once the VRE drought ends and surplus energy (from wind / solar) is available.
In terms of ecological impact, it's primarily limited to the direct impacts of construction, noting that the land downstream is presently subject to agricultural use – the lower storage, with its relatively small physical size, isn't being built in wilderness etc it's already cleared land. Beyond that it's the impact of construction works etc and the additional transmission lines required.
Brumbys Creek does represent a possible issue as the name implies – it's a creek that's already been somewhat "overloaded" by dumping the discharge from a substantial power station into it. That is a potential impact yes, it may need to be widened and effectively turned into a canal or similar, but it's farmland not wilderness, it's not a pristine natural waterway.
So in summary, it's a new 570 MW power station to operate on a daily basis as pumped storage noting the existing station also operates for discharge but not pumping. In addition it plus the existing Poatina power station then run 10% of the time, at maximum output, for VRE deep firming. These figures being based on very conservative assumptions about water availability including maximum irrigation requirements and some allowance for climate change. In practice the 10% figure would be higher most likely, it's a "safe" conservative value there.
So there's another one. It isn't massive but it's big enough to be useful, it's a plausibly worthwhile project that doesn't end the need for gas but it does reduce it somewhat and with the pumped storage hybrid aspect it would be a daily workhorse, far cheaper than wearing out batteries so would be used heavily, as well as the deep firming from the main storage. Obviously there needs to be additional transmission Vic - Tas to go with it in order to do it.