Nuclear power plant vs coal power plant efficiency

  • #1
fog37
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Hello,
I have been reading that a coal power plant and a nuclear power plant have similar efficiencies, i.e. ~30%. This 30% refers to the conversion of thermal energy into electricity. For example, for 100 Joule of thermal energy, we only get 30 Joule out of electrical energy.

How is that possible? I thought that 1 kg of uranium could generate 1000s of time more energy than 1 kg of carbon....The same amount of nuclear material should generate way more thermal energy hence more electricity.

Maybe it works this way: even if the % efficiencies are similar, nuclear can produce way more energy than coal (same mass) but only 30% of that amount becomes electricity... I think that is the correct answer. The 30% simply refers to the conversion between thermal energy and electricity. Overall, nuclear power plants produce much more energy for the same mass...

Thank you!
 
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  • #2
fog37 said:
The 30% simply refers to the conversion between thermal energy and electricity.
You answered your own question.

Even in a nuclear power plant, the percentage of the mass energy of the fuel that is successfully converted to electricity is much much smaller than 30%.
 
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  • #3
Efficiency is a ratio, right? If I make the numerator 100x bigger and the denominator 100x bigger, what happens to the ratio?
 
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  • #4
A typical block of a coal power plant burns 110 kg of coal per second, releasing 3.3 GW of thermal power which gets converted to 1 GW of electricity because 3.3 GW * 30% = 1 GW.

A typical block of a nuclear power plant splits 0.00004 kg of uranium per second, releasing 3.3 GW of thermal power which gets converted to 1 GW of electricity because 3.3 GW * 30% = 1 GW.

The process of converting thermal power to electricity is pretty similar for both. Just the heat source is very different.
 
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  • #5
mfb said:
The process of converting thermal power to electricity is pretty similar for both.
It is in essence the good old ”heat water and use it to drive a turbine” method. We just have developed new ways of boiling water …
 
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  • #6
Well, I really thought that the main advantage of nuclear power plants was that they produce more energy, per unit of time. I get the calculations and how much less mass of nuclear fuel produces the same electrical power output generated using a much bigger mass of coal. That is certainly a plus.

I know nuclear power plants pollute less but they cost way more than coal plants and there is the issue of radioactive waste.
Given the similar efficiency (30%), I guess the question really is: per unit time, does a nuclear power plant produce more thermal energy hence more electrical energy than a coal plant? The answer is yes. ...Vanadium 50 makes the point that just because the efficiency, 30%, is the same, i.e. the ratio energy_OUT/energy_IN=.3, energy_OUT is much larger for nuclear power plants because energy_IN is also larger...
 
  • #7
The typical power per block is pretty similar for both power plant types constructed at the same time. It's not determined by the amount of fuel needed, it's determined by how much heat you can move around, how much cooling power you can use and similar constraints.
 
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  • #8
mfb said:
The typical power per block is pretty similar for both power plant types constructed at the same time. It's not determined by the amount of fuel needed, it's determined by how much heat you can move around, how much cooling power you can use and similar constraints.
So, really, nuclear power is better than coal just because it does not pollute the atmosphere (even if radioactive waste is an equally good problem).
 
  • #9
fog37 said:
per unit time, does a nuclear power plant produce more thermal energy hence more electrical energy than a coal plant?
A 1 GW thermal coal plant produces the same amount of thermal energy as a 1 GW thermal nuclear plant, and about the same amount of electrical energy too. There are power plants of different sizes for both technologies.
 
  • #10
Coal and nuclear plants are about the same size, for the reasons given by @Dale and @mfb. There tends to be more variation among coal plants, but they are both in the 1000 MW ballpark. The biggest facilities are neither - they are hydroelectric.

As for cost, no matter what your energy source you use, it is cheaper if you can use the atmosphere as a garbage can.
 
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  • #11
Note that for solar plants the efficiency runs around 20% and for hydroelectric dams near 90%. None of these numbers is very meaningful on its own and mostly matter as it relates energy output to cost. If you could double the efficiency of a solar panel (or nuclear plant, for that matter) without changing the cost, that would be great. Of course, if you cut the cost in half without changing the efficiency that's nearly as good.

If we ever figure out nuclear fusion, efficiency is likely to be important. Right now the efficiencies are negative, and they may not end up being very high, while the cost is likely to be extremely high.
 
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  • #12
fog37 said:
So, really, nuclear power is better than coal just because it does not pollute the atmosphere (even if radioactive waste is an equally good problem).
Coal power plants produce radioactive waste, too. The ash has such a high uranium content that people look into extracting it. They also dump some of that ash into the atmosphere, unlike nuclear power plants. If you are worried about radioactivity in the environment, then coal power plants are much larger sources than nuclear power plants. In addition the ash is known to cause respiratory problems on a large scale, and of course there is all the CO2 they produce.
 
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  • #13
To summarize, what are the benefits of nuclear energy vs other forms?

Nuclear energy is costly, produces waste, etc. The nuclear fuel has the highest energy density which means that little nuclear fuel goes a long way. Is that the key advantage given the possible scarcity of other types of fuels (ex: coal, oil, gas)?
 
  • #14
fog37 said:
hat are the benefits of nuclear energy vs other forms?
That's not the original question. And there are other threads on that here.
 
  • #15
fog37 said:
produces waste

fog37 said:
ex: coal, oil, gas

These also produce waste. Unlike those, nuclear does not release the waste products into the atmosphere, producing global warming.

Of course, renewables are better from a waste perspective.
 
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  • #16
fog37 said:
To summarize, what are the benefits of nuclear energy vs other forms?

Nuclear energy is costly, produces waste, etc. The nuclear fuel has the highest energy density which means that little nuclear fuel goes a long way. Is that the key advantage given the possible scarcity of other types of fuels (ex: coal, oil, gas)?
Since that is a new question, this isn't a summary of prior posts, but of new issues...

Compared to other forms of electricity, nuclear is safe, reliable, clean and uses little land area. Compared to fossil fuels, nuclear waste is a benefit, not a downside, because it is contained an relatively easily dealt with if people listen to engineers and scientists on how. Nuclear's main downside is cost, but that's a complicated and partly non-technical issue -- coal and natural gas have been cheap and governments are trying to get rid of them anyway. Another potential downside is the other side of the coin from "reliability" is that it is less flexible than, say, natural gas. But the way today's grid works, a plant with 95% capacity factor (percent of peak capacity it runs at year round) is a benefit.
 
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  • #17
Orodruin said:
These also produce waste. Unlike those, nuclear does not release the waste products into the atmosphere, producing global warming.
To expand on waste just a bit: most fossil fuel waste is uncontained gases and so far we have been completely unable to find a technical solution to that (capture and storage). So that's a dealbreaker-level problem for them. By comparison, storing some nuclear waste that is inherently solid and chemically stable in a strong metal can is relatively easy.
 
  • #19
Orodruin said:
Of course, renewables are better from a waste perspective.
That's not so clear. Renewables produce a lot of chemical waste, which isn't inherently better.
 
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  • #20
The definition of efficiency in terms of input and output Energy is strictly the only one but it tells only half the story. Energy per unit cost is perfectly reasonable sometimes. Also, if you can associate an enviriomental / health cost with actual money then you could use a similar (fuzzy) value of efficiency.

A successful politician would use the definition to suit the argument.
 
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  • #21
Orodruin said:
Of course, renewables are better from a waste perspective.
I agree. Dams have been known to lay waste to hundreds of square miles!

(Sorry...couldn't resist)
 
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  • #22
Vanadium 50 said:
I agree. Dams have been known to lay waste to hundreds of square miles!

(Sorry...couldn't resist)
And cause earthquakes and affect river conditions miles downstream. TNSTAAFL
 
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  • #23
Vanadium 50 said:
I agree. Dams have been known to lay waste to hundreds of square miles!

(Sorry...couldn't resist)
Turning away from the joke, it comes as a surprise to many people that hydroelectricity is much more dangerous and destructive than nuclear power. Heck, Chernobyl was only the second worst/most deadly* power plant disaster in Ukraine and most people probably can't even name the worst, even though it was recent. Much of the land destruction of hydro dams is on purpose, so I'm not even sure if that's better or worse.

*In terms of immediate, easily attributable deaths.
 
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  • #24
The Three Gorges dam in China can make 22,500 MW, but a primary purpose of the dam is flood control. Some of the floods in the past there killed hundreds of thousands. per Wiki, "In 1931, floods on the river caused the deaths of up to 4 million people."
 
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  • #25
gmax137 said:
The Three Gorges dam in China can make 22,500 MW, but a primary purpose of the dam is flood control. Some of the floods in the past there killed hundreds of thousands. per Wiki, "In 1931, floods on the river caused the deaths of up to 4 million people."
That's a double edged sword; it also displaced a million people by its construction.
 
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  • #26
russ_watters said:
That's a double edged sword; it also displaced a million people by its construction.
My wife and I were there in 2002. The people were dismantling their houses and towns brick-by-brick, carrying the materials up the hillside to the "new towns." I can still hear the sound of them clapping the bricks together to knock off the mortar.
 
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  • #27
https://www.energy.gov/ne/articles/nuclear-power-most-reliable-energy-source-and-its-not-even-close

This energy.gov website talks about nuclear having the highest capacity factor and is the most reliable:

"....nuclear energy has by far the highest capacity factor of any other energy source. This basically means nuclear power plants are producing maximum power more than 92% of the time during the year. That’s about nearly 2 times more as natural gas and coal units, and almost 3 times or more reliable than wind and solar plants...."
"....
A typical nuclear reactor produces 1 gigawatt (GW) of electricity. That doesn’t mean you can simply replace it with a 1 gigawatt coal or renewable plant.
Based on the capacity factors above, you would need almost two coal or three to four renewable plants (each of 1 GW size) to generate the same amount of electricity onto the grid.
..."
 
  • #28
russ_watters said:
most people probably can't even name the worst, even though it was recent
Do you mean Khakovka. (I hope I have that right) One could argue that those deaths belong to fossil. After all, it was blown up by people financed largely through the sale of fossil fuels.

Worldwide, its hard to beat Banqiao. That killed so many people it was a Chinese state secret for 30 years.
 
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  • #29
fog37 said:
This energy.gov website talks about nuclear having the highest capacity factor and is the most reliable:

That doesn’t mean you can simply replace it with a 1 gigawatt coal or renewable plant. Based on the capacity factors above, you would need almost two coal or three to four renewable plants (each of 1 GW size) to generate the same amount of electricity onto the grid.
Note that that doesn't mean high capacity factor = good, but it is important for understanding replacement. Global warming is a function of energy's carbon intensity. So when you read about a new solar plant with XX MW (power) capacity, you need to use capacity factor to convert to energy to compare with other sources.

The grid needs both base load and flexible sources. Flexible sources follow the demand and can also be used for base load if need be. Intermittent renewables (solar and wind) provide neither, and that's a big problem for their large scale implementation.
 
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  • #30
russ_watters said:
. Intermittent renewables (solar and wind) provide neither, and that's a big problem for their large scale implementation.
That is a serious fact and politicians conveniently avoid mentioning it.

Once things seriously start to bite, there is one more shot in our locker and that is load control. Heating and A/C take a significant amount of electrical energy (I mean energy) and that load can be regulated. Minute by minute flexible energy charges would soon have an effect on peak demand.
 
  • #31
russ_watters said:
Nuclear's main downside is cost, but that's a complicated and partly non-technical issue
My anti-nuke slip may be showing (my age and upbringing) but I am gradually warming to the idea of nuclear. The worst non-technical issue is the massive timescales involved and the frequent (apparently) failure to implement plans to completion.
UK is actually a small country and progress / survival of large infrastructure projects has not been impressive. Long-termism doesn't seem to apply here. So the future of our nuclear energy supply worries me. It's too important to be left in the hands of people who only look to the next general election and a seat in the House of Lords.
 
  • #32
sophiecentaur said:
I am gradually warming to the idea of nuclear.
Good...because so is the planet. <rimshot>

(This thread is the best ever for straight lines!)
 
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  • #33
sophiecentaur said:
My anti-nuke slip may be showing (my age and upbringing) but I am gradually warming to the idea of nuclear. The worst non-technical issue is the massive timescales involved and the frequent (apparently) failure to implement plans to completion.
From a technical standpoint, loss of expertise in nuclear construction and operation is a real problem in western countries that have shirked nuclear. Additional timeline issues exist that are....non-technical.

Let's try to avoid talk of politics here, otherwise this will end up locked like another recent discussion.
 
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