Comment by derriz
3 years ago
Varying output from a nuclear plant is mostly achieved by simply releasing the generated steam into the atmosphere instead of sending it through the turbine[1].
But operating a nuclear plant in this fashion pushes up the price per MWh considerably given their very high cap-ex and op-ex. And while fuel cost is negligible for nuclear, creating more nuclear waste per useful MWh generated is a further drag on costs.
So as a solution, it "works" if the nuclear plant does not have to compete in terms of price with other sources of electricity. But nuclear fails to compete on cost even if operated continuously - it's uncompetitive with cheap, quick to deploy, low op-ex, modern tech like CC gas turbines or renewables in most western electricity markets and can only survive with government subsidy[2].
[1] https://www.nrc.gov/docs/ML0703/ML070380209.pdf [2] https://www.washingtonpost.com/business/2022/04/19/biden-adm...
It seems obvious that nuclear can not compete against natural gas when natural gas is priced cheaply and the pollution caused by fossil fuel is put on society rather than the operator. A combined grid of renewables and fossil fuels has been the primary strategy in most European countries and was working very well in keeping prices low until Russia invaded Ukraine.
The big problem is that energy prices are set based on the most expensive unit that needs to be turned on to meet demand. Renewables do not tend to be that during periods of low supply, as low supply of energy in the eu market generally means sub-optimal weather conditions for renewables. It is going to be either fossil fuels, nuclear, or battery. If we take out fossil fuels then that leaves battery or nuclear. Neither is very economical without subsidies. Governments (and tax paying citizens) are however very keen on grid stability and thus willing to spend a lot of money to keep it running.
> The big problem is that energy prices are set based on the most expensive unit that needs to be turned on
That is not a problem, it is the incentive to have supplies available so they can be turned on.
I get why it works this way because the alternative would be to force the fossil generators to sell at the renewable price and thereby making it uneconomical for them to operate which leads to brown-outs. I just think the societal costs the up very high because EVERYONE is paying a premium on power, and the total sum of that premium is only going to increase as we move more and more stuff over to electricity.
I therefore wonder if the market couldn't be structured in a better way which would still ensure that the fossil backup generators are adequately compensated but smoothes the extra cost over the remaining cheap GWh. Something like a meditating party which is aware of the production costs and buys up the daily power and sells it on at an averaged price. There are probably good reasons why this wouldn't work, but I am too stupid to figure them out.
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Your reference for [1] just states that bypassing the turbine is a thing, not that it's normally used.
First, reactors are in a stable equilibrium when operating, so one will actually increase their power by increasing the rate at which heat is removed (and v.v.). Alas, that's workable only within some small range.
A reason[1] load-following with PWRs was originally difficult is that traditionally PWRs use boron concentration in primary loop to regulate power and that can be decreased only slowly. The reason it's done that way is that it's the easiest way to ensure that power is adjusted uniformly throughout the core; if instead some control rods were partially inserted, the top part of the core would operate at lower power (and thus lower fuel burn-up) than the bottom part, which would cause compounding control issues later on.
France is using their PWRs in load-following mode by (a) having additional less absorptive control rods ("gray rods") that can be inserted fully to adjust power by smaller increments (b) more complicated schemes to decide which combination of available actuations to use to change power. See https://hal.science/hal-01496376/document for a paper that tries to optimize control designs so that power changes are more possible (and describes how the control schemes work).
Note that the total heat capacity of even just the primary loop in usual reactors is quite large: in PWRs it usually requires ~0.5s of full power output of the reactor to warm it by 1degC, so this can easily cover, say, ~5% variations for something like a minute.
[1] Another is that reactors are not stateless due to xenon poisoning.
I am skeptical that renewables are cheaper than nuclear when one factors in the impossible amounts of energy storage required to make them meet the same reliability guarantees that nuclear (and fossil) can meet - indeed, as far as I know, there exists no proven, cheap, scalable technology to store power at grid scale at all.
“Renewables” means hydro, solar, and wind — with hydro being 90%+ of the total, and the infrastructure already build for it counted as free.
Hydro is a long way from 90% in the UK. It's 4% of renewables.
Well, nuclear could be cheaper.
Alas, in the real world because of public opinion and political pressure, it's almost impossible to build new nuclear power plants. And those that get build are crazy expensive and overengineered, and invariable overrun their schedule and budget.
What impossible amounts? You can play around with a small model here: https://model.energy/ It's simplified of course, but the estimates should be in the right ballpark.
>as far as I know, there exists no proven, cheap, scalable technology to store power at grid scale at all
It's called pumped storage.
We dont need as much storage as people think. Solar and wind anti correlate and a vast amount of demand can be time shifted.
The anticorrelation is mostly on longer timescales. Winter vs summer for example. On a daily or hourly basis the anti correlation is pretty much non existent and it's on these timescales that you would time shift demand. A quite common occurence in Europe is that large parts of Europe during winter have almost non existent air pressure differences for days or weeks on end. During these times neither wind nor solar is very helpful and other solutions are needed, not all European countries have the pumped storage capacity for that. LNG to the rescue I suppose, now that Russia is limiting supply.
I was wondering if someone was going mention that. Pumped hydro is great, but it's not scalable. You need favorable geography to make it economical at all, and in the end it doesn't store enough energy to do more than smooth over transient grid fluctuations lasting a few hours. The UK is, relatively speaking, quite well provisioned with pumped hydro - its largest storage facility is Dinorwig in North Wales, which is built into a mountain with very favorable geometry - it has nearly 6 times as much capacity as the next biggest station. It can store enough energy to run the entire UK for... about 16 minutes. That's not going to do the trick if the grid runs entirely off wind and solar and you have a dark, calm day, let alone the weeks at a time that weather can be unfavorable. And there isn't anywhere to put another hundred Dinorwigs, never mind the budget.
It's because of this that there's a lot of talk about wild ideas like pressurizing abandoned mines and so on - there are a lot of mines around. But then we're back to the "proven technology" sticking point.
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You get a lot of storage "for free". Some examples are house heating and hydro-electric that you would have the dams for anyway for flood control. Also, fuel based power-plants are very low capex so it's totally reasonable to have 50% demand able to be met by fossil fuels and then just keep them off whenever you have enough renewables.
Nit: the steam is not released to atmosphere, it is sent directly to the condensers. Treated water to use in boilers and turbines without leaving deposits and damaging them is precious, so it is a closed cycle and not vented to atmosphere whenever possible