You generally use peaker plants or backup fossil fuel or hydro for that use case, at this point in time batteries are used to supply power for a couple of hours at best. You need that backup power anyway for when the nuclear plant is out of commission for some reason.

Regardless of how you're generating your power, you're going to need to overprovision your generation capacity slightly to account for downtime. You only need storage if all of your generation capacity is offline at the same time, which is inevitable with solar power (nighttime), possible with gas (pipeline failure?), but less inherently likely with nuclear.

> backup fossil fuel

In practice this means fossil fuels become the baseload power source.

> In practice this means fossil fuels become the baseload power source.

False. In the immediate future, synthetic anhydrous ammonia will displace it. Extracting NG will be too expensive to compete, because the only cost of ammonia will be capex. The more you make, the less it costs.

Your argument does not follow at all. I have the impression that you don't know what baseload is. Baseload is the minimum amount of demand over a given time (it is not a measure of supply at all).

The previous poster talked about using fossil (or any other means) to supply the mismatch between the supply and demand at the times where such a mismatch might exist. This is completely unrelated to baseload, it could in fact be larger than baseload. Note that such a mismatch would also exist for nuclear (dependent on overprovisioning). However it would in general be only needed of short periods of time.

It could be larger than baseload, but it's unlikely to be any less than baseload.

They are completely unrelated and yes in general they would be much smaller than baseload. Why wouldn't it? What the GP described is essentially what gas peakers do.

> yes in general they would be much smaller than baseload. Why wouldn't it?

Because if your baseload is your minimum power consumption (which generally occurs at night) and you have a windless night (therefore your renewables are generating 0 power), your "gas peakers" will have to provide the entire baseload amount of power.

I sincerely believe that HVDC is a game changer here, it allows you to move energy around at a fraction of the losses that were considered normal before allowing for overcapacity in one place to be used elsewhere eliminating a major requirement for storage, on that scale there is no lack of combined sun/wind availability.

Anyway transmission losses are immaterial when top-line generating capacity is so cheap.

China is building solar in Chile, at their antipodes, and a transmission line back to China with only 50% loss, for solar energy through the night.

> In practice this means fossil fuels become the baseload power source.

As the name baseload power implies, it would have to be providing power all the time and that's not what is needed.

That is correct. The term I see by my government is "reserve energy". In practice this mean a country subsidize a fossil fuel plant to keep engines running when demand is low, and then the plant get paid a second time if and when the plant is running.

It is one of the main reason why investing in fossil fuel energy when countries turn to wind power is fairly good choice. The plant has lower costs in terms of fuels, when it burn fuel it does so when the market price is at the highest possible, and it get paid regardless from the government. The government are also locked into continuing this scheme as the alternative is to not have the reserve energy when needed which makes voters very angry.

Burning fossil fuels when the price is lowest and without any government subsidies is just something people with too much fossil fuel can afford to do.