The 75% figure is from solar and wind alone. The balance can come from any combination of nuclear, other renewables, and fossil fuels. It doesn't mean 25% electricity supplied by fossil fuels.
Plus 19.7% from existing American reactors, though that number is going to drift downward as retirements continue to outpace construction.
I think that the residual fossil demand would be significantly less than 25%. I can't say how much exactly. That needs another study. In the absence of storage, neither nuclear nor geothermal plants are good at supplying peak demands. Hydro can serve a peaking role to some extent even without building new pumped storage but it's constrained by reservoir volumes, minimum downstream flow rates, seasonal snow melt, etc.
> In the absence of storage, neither nuclear nor geothermal plants are good at supplying peak demands.
This is false. Nuclear and geothermal power can both satisfy peak demand just fine. Nuclear plants do take time to alter the thermal output of the reactor, but they can easily reduce the electric output through overcooling. Same MWt, but lower MWe. Geothermal plants just pump less water into the borehole. Better yet, nuclear plants can direct the excess energy to things like desalination. This is much easier than trying to satisfy peak demand with solar or wind, where peak demand often occurs when the sun isn't visible or when wind speed is substantially higher or lower than demand.
Again, France at its peak generated over 85% of it's electricity from nuclear power. I'm not sure where you're getting this myth that nuclear plants can't satisfy variable demand.
Nuclear plants can throttle down. But the economics of nuclear power make it prohibitive to satisfy peak demand from nuclear reactors. Even France relies on gas plants and power imports to satisfy its electricity demand peaks, despite being a net electricity exporter over the course of a full year.
The CAISO grid had an average power demand of 25 GW in 2019 but a peak demand of 46 GW (Table 1.1):
Keeping enough nuclear reactors operating to supply the most demanding hours of the most demanding season would have extraordinarily high marginal costs for the last few terawatt hours.
This is true not because of any shortcomings of nuclear, but by virtue of the fact that demand is not uniform. The same need to have excess capacity during non peak hours in order to have sufficient capacity during peak hours exists with fossil fuels. If peak demand is 130 GW and trough demand is 100GW you need 130 GW of capacity.
Nuclear has never been inexpensive enough to be the first choice for meeting peak demand in the absence of storage. Some pumped hydro plants were built in the 20th century to be charged by nuclear generation so nuclear could effectively supply peaks too. Peaking batteries could also be charged by nuclear power.
Gas turbines have a construction cost under $1/watt in the US while Vogtle's AP1000s were estimated at $6/watt even before all the cost overruns started. If you're going to leave an asset idle most of the time, much better to idle a sub-$1/watt asset than a $6/watt asset.
This is just factually wrong. France had generated the majority of it's electricity from nuclear since the 80s, and Belgium now generates the majority of it's power from nuclear. Nuclear had repeatedly been used to satisfy peak demand.
Nobody doubts that fossil fuels are cheaper. Yes, that's why fossil fuels are still in widespread use outside of France, and several countries with lots of hydroelectric power. But if we want to halt climate change we need to eliminate - not just reduce - usage of fossil fuels. Any plan to use renewables as a significant source of energy either involves the continued use of fossil fuels, or the involvement of staggering amounts of energy storage.
Per the beginning of this long thread, wind and solar can supply 75% of annual US electrical demand without storage. France supplies about that much of its annual demand from nuclear power and Belgium supplies a bit over half of annual demand with nuclear power. Both countries rely on fossil generated electricity for meeting demand peaks, both via domestic generating plants and cross-border imports from foreign fossil plants. There was never a year when France met its peak electrical demand without fossil power.
The example of France proves that electrical generation could have been largely decarbonized in the 20th century, if other leaders had made a concerted push to reduce fossil fuel dependency like France's leaders did. It's tragic that other major economies did not do the same. But even France did not eliminate all fossil generation. 10% of French electricity generation is fossil as of 2017 [1].
Getting the USA's electrical generation down to only 10% fossil would be a vast improvement. I don't think that a contemporary optimized plan for getting there involves much new nuclear power even though a 20th century plan would have. A cost optimized plan from 1985 certainly would have called for a lot more nuclear power. The costs of building American solar and wind farms have plummeted since 1985. The costs of building American nuclear plants have not.
France's peak energy consumption is more than 40% higher than it's trough energy demand [1]. Most years France generates ~10% of its energy from fossil fuels. Nuclear was indeed used to supply a substantial part of the peak energy demand. The substantial majority of its peak demand was fulfilled with nuclear energy. France's share of fossil fuels actually used to be lower than 10%. France's more recent uptick in fossil fuels accompanied by a drop in the share of nuclear power generation [2]. Renewable energy production increased, but its intermittency leads to an increase in fossil fuel consumption. A real world example of how the shortcomings of renewables as compared to nuclear power results in more carbon emissions.
As per your own comments, solar and wind even with substantial overproduction leave a quarter of electricity demand unfulfilled. The places that are fortunate to have hydroelectric or geothermal power available could go carbon free, but the rest are left supplying a quarter of energy with fossil fuels. The overproduction puts their price well above the cost of nuclear when you don't cherry pick one of the most infamous cost overruns as a representative example. And let's just be generous and ignore the ecological devastation caused by covering 2-4% of the Earth's land surface in solar panels.
So we have a more expensive option that leaves a quarter of electricity demand to be fulfilled by other sources (mostly fossil fuels), requires massive amounts of land to be cleared and covered in solar panels, and is subject the challenges of intermittent power generation. And we have nuclear, which is cheaper, much less land intensive, and generates consistent energy. The superior choice is unambiguous. And real world examples demonstrate this. Look at the disparity between France and Germany. The former is the posterchild of nuclear, the latter the posterchild of renewables. France's carbon intensity of electricity is usually more than a factor of 4 times smaller than Germany's. We've already put nuclear and renewables to the test. And nuclear proves to be far superior.
I'll be interested again when France builds new reactors cheaper than new renewables. Flamanville 3 and Olkiluoto 3, even if they don't have further cost overruns, are going to produce electricity at a higher cost per MWh than German solar farms completed in the same year.
If France does build 6 more EPRs, a proposal floated last year, they will have a chance to prove that the problems to date were merely FOAK learning experiences.
France doesn't need to build new reactors largely because it's existing reactor fleet is still enough to fulfill demand. That's one of the biggest advantages of nuclear power, it lasts for the better part of a century not a decade or two. A serial run if reactor production makes more sense once the disparity between supply and demand is greater.
I'll be interested in renewables once Germany's carbon intensity per Watt is on the same order of magnitude as France. They need to drop from ~500 grams per Watt to under a hundred.
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You consider France's electricity sector a decarbonization success story, right? It generates "only" 72% of electricity from nuclear power: https://en.wikipedia.org/wiki/Electricity_sector_in_France
The United States also gets 7% of its electricity from hydro and geothermal:
https://www.eia.gov/tools/faqs/faq.php?id=427&t=3
Plus 19.7% from existing American reactors, though that number is going to drift downward as retirements continue to outpace construction.
I think that the residual fossil demand would be significantly less than 25%. I can't say how much exactly. That needs another study. In the absence of storage, neither nuclear nor geothermal plants are good at supplying peak demands. Hydro can serve a peaking role to some extent even without building new pumped storage but it's constrained by reservoir volumes, minimum downstream flow rates, seasonal snow melt, etc.