From what I understand, it isn’t about the price but ability to provide rapid network support.
Natural Gas has a very different generation profile to coal. Coal it is very difficult to have fast ramp up and ramp down of generation.
I suspect that AEMO requires coal generators to provide a constant generation output based on the average expected daily demand…which can be estimated. AEMO would be setting a coal generation level to ensure that coal generation doesn’t cause an overloading of the network. As it is difficult to ramp up/down quickly, the generation level set for coal would be conservative and shortfalls made up with gas generation.
The renewable generation component is not constant and varied due a wide range of factors. Some days (even with the same day) renewable generation will be very high making the need for gas generation support less (renewables would meet much of the shortfall between coal fire generation and demand)…while other days the renewable generation will be low requiring a greater need for gas support in conjunction with renewables. As the renewable generation is highly variable temporally, gas would be needed to buffer this variation (for August in Qld, the area of the red on the the graph).
It would possibly be concerning if there was no gas generation as it could indicate that there may be a over-generation event and there was a likelihood that different generation types are removed from supply to the network. This could include coal fired generation where coal is burnt, but no electricity is exported onto the grid from the generators (not an efficient outcome).
These are the most expensive form of network storage and are very expensive compared to hydro storage…which is currently the first level storage being adopted.
As indicated in another thread, if battery storage for excess renewable generation was only used to support the grid in times where there was a generation deficit, the average consumer electricity bills would go through the roof. There have been costs such as $1.5T to maintain current reliability standards (or around $10K/yr per household) and to meet future electricity demands (inc. when electricity replaces.traditional fossil fuel uses).