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Typical coal power plants have practical electric efficiency 40% but total energy efficiency is 60-70%, because they produce electricity and useful heat. Also

Typical coal power plants have practical electric efficiency 40% but total energy efficiency is 60-70%, because they produce electricity and useful heat. Also 40% is a theoretical (lab) value for a non turbocharged diesel engine - in practice most engines are operated out of their optimal range, they are turbocharged (a turbo increases power output at the cost of decreasing compression ratio and thermal efficiency), there are losses in the transmission, hence efficiency is much lower than 40%. BTW, in lab, a coal plant with a steam turbine can achieve over 50% electric efficiency (some reports say even 60% is possible). They are too expensive to be used on large scale, though.

FrojoS 9y ago
Very interesting information. Thank you. What would you say is the typical efficiency of a modern gasoline car? Say, one that uses 7L/100km on average? Also, why do the coal plants with higher efficiency so expensive? I've read somewhere that higher heat would be better, but the steel that can resist it, is too expensive. Is that correct?
- pkolaczk 9y ago
  I'm not am expert on turbines, but generally the turbine efficiency is directly related to the temperature and pressure difference between the inlet and outlet. Hence bigger plants have typically higher efficiencies. Bigger turbines have also more stages, so not only they have to withstand higher pressure, but simply there are more parts.
Retric 9y ago
Hybrid cars let you operate IC engines at fairly high efficiency depending on setups but you now have to deal the overhead of charging batteries for both setups. Also, if you want total efficiency in terms of C02 then you need to consider. Gasoline takes a lot of energy to extract, refine into a usable state, and transfer around the world. On the other hand hydrocarbons extract energy from hydrogen bonds not just carbon bonds so they produce less C02 per BTU. Net result electric cars backed by coal power still win vs IC, but not by as much as you might think. PS: You can also use turbines in hybrid car which would boost it's efficiency even more. But, you can also produce very cheap solar power which once again shifts things back to electricity.
wallace_f 9y ago
Are you sure about your statement on turbos reducing efficiency? Some of the most economical consumer engines are turbocharged--in fact auto makers commonly cite mpgs as a reason to use turbos; so I am confused.
- pkolaczk 9y ago
  Yes. Turbocharging increases mpg not by increasing thermal efficiency, but by allowing to use a smaller and lighter engine. A lighter engine means less mass to accelerate, therefore less energy to spend. A smaller engine means less internal friction so less energy lost. However this theory works well if you actually don't use the full power that turbo gives you, or you use it only occasionally. These engines have great advantage in lab mpg tests, but in real dynamic driving the advantage diminishes. The main problem is that by adding a turbo, you need to decrease the compression ratio of the engine, which decreases thermal efficiency. Another thing is that turbo needs some energy to be powered, and also has some of its own energy losses. This is not free.
  - wallace_f 9y ago
    The new Honda Civic comes in a 1.5 turbo and a 2.0 NA, the turbo being the heavier package--but which still delivers higher mpgs. And if the turbo has more efficiency due to less internal friction, that still creates more useful energy out of the same amount of fuel, no?
    - pkolaczk 9y ago
      You are right, but now we are not discussing thermal efficiency, but total mpg which is related, but not the same thing. Efficiency being equal, you'll get more mileage from an electric car, because it can get back some energy from regenerative breaking, and it definitely has less internal friction.
seanmcdirmid 9y ago
How is the useful heat used? I know some places have central heating, so the hot water is just piped throughout the city, but if not that, what would they do with it?
- deleted 9y ago
  [deleted]
- alextheparrot 9y ago
  The term that the original poster didn’t include is “Cogeneration” [0]. Another use case would be desalination in areas without an adequate supply of freshwater. [0] https://en.m.wikipedia.org/wiki/Cogeneration https://en.m.wikipedia.org/wiki/Cogeneration
  - seanmcdirmid 9y ago
    Ya, I get that, it just doesn’t seem to be always the case that you’d find something to do with the heat, especially given proximity of the coal plant to places that could use it. It wouldn’t seem to work well for Australia, at first glance (no serious heating needs, no desalination plants).

FrojoS 9y ago

Very interesting information. Thank you. What would you say is the typical efficiency of a modern gasoline car? Say, one that uses 7L/100km on average? Also, why do the coal plants with higher efficiency so expensive? I've read somewhere that higher heat would be better, but the steel that can resist it, is too expensive. Is that correct?

pkolaczk 9y ago
I'm not am expert on turbines, but generally the turbine efficiency is directly related to the temperature and pressure difference between the inlet and outlet. Hence bigger plants have typically higher efficiencies. Bigger turbines have also more stages, so not only they have to withstand higher pressure, but simply there are more parts.

Retric 9y ago

Hybrid cars let you operate IC engines at fairly high efficiency depending on setups but you now have to deal the overhead of charging batteries for both setups. Also, if you want total efficiency in terms of C02 then you need to consider. Gasoline takes a lot of energy to extract, refine into a usable state, and transfer around the world. On the other hand hydrocarbons extract energy from hydrogen bonds not just carbon bonds so they produce less C02 per BTU. Net result electric cars backed by coal power still win vs IC, but not by as much as you might think. PS: You can also use turbines in hybrid car which would boost it's efficiency even more. But, you can also produce very cheap solar power which once again shifts things back to electricity.

wallace_f 9y ago

Are you sure about your statement on turbos reducing efficiency? Some of the most economical consumer engines are turbocharged--in fact auto makers commonly cite mpgs as a reason to use turbos; so I am confused.

pkolaczk 9y ago
Yes. Turbocharging increases mpg not by increasing thermal efficiency, but by allowing to use a smaller and lighter engine. A lighter engine means less mass to accelerate, therefore less energy to spend. A smaller engine means less internal friction so less energy lost. However this theory works well if you actually don't use the full power that turbo gives you, or you use it only occasionally. These engines have great advantage in lab mpg tests, but in real dynamic driving the advantage diminishes. The main problem is that by adding a turbo, you need to decrease the compression ratio of the engine, which decreases thermal efficiency. Another thing is that turbo needs some energy to be powered, and also has some of its own energy losses. This is not free.
- wallace_f 9y ago
  The new Honda Civic comes in a 1.5 turbo and a 2.0 NA, the turbo being the heavier package--but which still delivers higher mpgs. And if the turbo has more efficiency due to less internal friction, that still creates more useful energy out of the same amount of fuel, no?
  - pkolaczk 9y ago
    You are right, but now we are not discussing thermal efficiency, but total mpg which is related, but not the same thing. Efficiency being equal, you'll get more mileage from an electric car, because it can get back some energy from regenerative breaking, and it definitely has less internal friction.

seanmcdirmid 9y ago

How is the useful heat used? I know some places have central heating, so the hot water is just piped throughout the city, but if not that, what would they do with it?

deleted 9y ago
[deleted]
alextheparrot 9y ago
The term that the original poster didn’t include is “Cogeneration” [0]. Another use case would be desalination in areas without an adequate supply of freshwater. [0] https://en.m.wikipedia.org/wiki/Cogeneration https://en.m.wikipedia.org/wiki/Cogeneration
- seanmcdirmid 9y ago
  Ya, I get that, it just doesn’t seem to be always the case that you’d find something to do with the heat, especially given proximity of the coal plant to places that could use it. It wouldn’t seem to work well for Australia, at first glance (no serious heating needs, no desalination plants).