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Can anyone comment on the heat > electricity conversion ratio? My understanding has been that modern multi-stage steam turbines used in power plants are around

Can anyone comment on the heat > electricity conversion ratio? My understanding has been that modern multi-stage steam turbines used in power plants are around 90% efficient, not 50%. I might be missing other energy losses along the way, though.

zardo 5y ago
I don't think so, checking Siemens they're touting a 65% efficient 600MW turbine. The higher number might be including some application for the waste heat.
Robin_Message 5y ago
Carnot's law is what you want. Max efficiency is temperature differential divided by hot absolute temperature. So, with super heated steam of say 600 degrees C, I'd make that a maximum of about 66% theoretical, so 50% actual sounds pretty good. If you could build a plasma heat engine, you could get near 100% but I've no idea what that would technically look like.
- nine_k 5y ago
  You can likely harvest a little bit more by using a lower temperature differential in another step, using, say, substances that boil at room temperature, to cool the water after it condenses and cannot produce work by expansion.
phkahler 5y ago
>> Can anyone comment on the heat > electricity conversion ratio? My understanding has been that modern multi-stage steam turbines used in power plants are around 90% efficient, not 50%. They probably mean 90% of a theoretical maximum. For example, one of the results that stood out to me in thermodynamics class was that the maximum possible efficiency of an internal combustion engine (I forget which cycle) was a function of the compression ratio. In other words you could never reach 100% conversion of chemical energy to mechanical energy and the theoretical "efficiency" was a function that increased with compression ratio but could never reach 100 percent. So lets assume an engine with a given compression ratio could have a theoretical efficiency of 50 percent, but a real-world design only achieved 45 percent. Someone might say 45/50 is 0.9 or 90 percent of the theoretical limit. That's a measure of how good the design was vs what's theoretically possible, but it has little to do with the actual efficiency of the energy conversion. IIRC the best heat-to-mechanical energy conversion devices are rockets with a theoretical efficiency of 50% but it's been half my life since I studied thermodynamics, so maybe I'm off on some of this.
- pfdietz 5y ago
  The maximum theoretical efficiency of ideal rocket engines is much higher than 50%. At very high expansion ratio in vacuum the gas can become so cold it condenses, so almost all the internal energy of the initial hot gas will have been converted to jet kinetic energy.
  - nine_k 5y ago
    Won't you need a colossal nozzle to actually harvest these last bits of energy?
    - pfdietz 5y ago
      You need a very high area ratio (nozzle exit area/nozzle throat area), but that's possible in vacuum, especially if your thrust requirements are modest.

zardo 5y ago

I don't think so, checking Siemens they're touting a 65% efficient 600MW turbine. The higher number might be including some application for the waste heat.

Robin_Message 5y ago

Carnot's law is what you want. Max efficiency is temperature differential divided by hot absolute temperature. So, with super heated steam of say 600 degrees C, I'd make that a maximum of about 66% theoretical, so 50% actual sounds pretty good. If you could build a plasma heat engine, you could get near 100% but I've no idea what that would technically look like.

nine_k 5y ago
You can likely harvest a little bit more by using a lower temperature differential in another step, using, say, substances that boil at room temperature, to cool the water after it condenses and cannot produce work by expansion.

phkahler 5y ago

>> Can anyone comment on the heat > electricity conversion ratio? My understanding has been that modern multi-stage steam turbines used in power plants are around 90% efficient, not 50%. They probably mean 90% of a theoretical maximum. For example, one of the results that stood out to me in thermodynamics class was that the maximum possible efficiency of an internal combustion engine (I forget which cycle) was a function of the compression ratio. In other words you could never reach 100% conversion of chemical energy to mechanical energy and the theoretical "efficiency" was a function that increased with compression ratio but could never reach 100 percent. So lets assume an engine with a given compression ratio could have a theoretical efficiency of 50 percent, but a real-world design only achieved 45 percent. Someone might say 45/50 is 0.9 or 90 percent of the theoretical limit. That's a measure of how good the design was vs what's theoretically possible, but it has little to do with the actual efficiency of the energy conversion. IIRC the best heat-to-mechanical energy conversion devices are rockets with a theoretical efficiency of 50% but it's been half my life since I studied thermodynamics, so maybe I'm off on some of this.

pfdietz 5y ago
The maximum theoretical efficiency of ideal rocket engines is much higher than 50%. At very high expansion ratio in vacuum the gas can become so cold it condenses, so almost all the internal energy of the initial hot gas will have been converted to jet kinetic energy.
- nine_k 5y ago
  Won't you need a colossal nozzle to actually harvest these last bits of energy?
  - pfdietz 5y ago
    You need a very high area ratio (nozzle exit area/nozzle throat area), but that's possible in vacuum, especially if your thrust requirements are modest.