To be clear-- the reason energy is cheap at night is not JUST because it is used less-- it is also because there are certain continuous energy sources. Nuclear power, geothermal, tidal, wave and wind power are all producing energy round the clock and if you don't harvest it, it's lost.
We are not heading towards a world where we have a single power source (solar), and most of our power uses cannot be rescheduled. As fossil fuels are one of the few sources which can be turned on and off at our choosing, I would expect they will function to fill in temporary gaps between supply and demand once renewables capacity is large enough to take the average load. I don't see these gaps occurring predominantly at night.
> ... Nuclear power, geothermal, tidal, wave and wind power are all producing energy round the clock and if you don't harvest it, it's lost.
This isn't true of nuclear power plants; the fission rate, and thus the heat generation rate, can be throttled up and down as needed. In pressurized water reactors this happens automatically as the throttle is opened and closed, thus increasing or decreasing output from the "steam side" of the heat-exchange boilers, a.k.a. steam generators [1]. (In a prior life I was a Navy nuclear engineering officer.)
I think the idea is that with nuclear power plants while you can bring down the output you don't want to because it saves you very little money. Most of your costs are huge and fixed.
That's true, but does this make the fuel last longer, and anyway what are fuel costs as a percentage of total operational costs? My impression is it doesn't make sense to operate a nuclear reactor at less than full power.
Yes. Heat is generated by fission of fissile material such as uranium. Fuel rods have X amount of fissile material in them. Higher power -> faster depletion of the fissile material.
> what are fuel costs as a percentage of total operational costs?
Around 30%, according to the Nuclear Energy Institute. This compares with 80% for coal, natural gas, and oil.[2]
> My impression is it doesn't make sense to operate a nuclear reactor at less than full power
I would think that'd be true of almost any machinery, but that's almost a tautology: You design your machinery for an optimized balance of performance versus wear-and-tear, then try to operate at (what you call) "full power" as much as you can, so as to reap maximum value from your investment.
In any event, the original comment was that excess power is inevitably generated by nuclear plants (at least during some time periods) and therefore must be dumped somehow. That's not the case; nuclear plants can be throttled up and down as needed.
You can still store extra energy to buffer those gaps. Where I live, pumped-storage hydroelectricity has probably a great future. Thermal storage seems to reasonably efficient too, although I'm not quite sure how much of what I've seen is a little "too enthusiastic".
We are not heading towards a world where we have a single power source (solar), and most of our power uses cannot be rescheduled. As fossil fuels are one of the few sources which can be turned on and off at our choosing, I would expect they will function to fill in temporary gaps between supply and demand once renewables capacity is large enough to take the average load. I don't see these gaps occurring predominantly at night.