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> emitter temperature of 2,400 C GE's combined cycle turbines can get system level efficiency of around 63% from these sorts of temperatures. (For those not f

> emitter temperature of 2,400 C GE's combined cycle turbines can get system level efficiency of around 63% from these sorts of temperatures. (For those not familiar with them: They're basically aircraft jet engines followed by steam turbines using the hot exhaust. They are in widespread use to generate electricity from gas, but they can also run off any other liquid fuel, or simply off anything that gets very hot.)

15457345234 3y ago
There are a looooot of moving parts in a CCGT generating station, multiple oil loops, multiple coolant loops, many consumables, a complex control flow. Being able to replace the combined turbine/alternator assembly with a 'when it gets hot, voltage comes out' unit would give you significant reliability gains and lower operating costs.
- pbmonster 3y ago
  Well, you still have to cool those cells. It's still a heat engine, after all. And you can't just let radiation do the job for you, no way the semiconductor likes getting anywhere close to this hot.
  - fnordpiglet 3y ago
    Liquid cooling with a heat exchanger can be pretty low profile.
- fnordpiglet 3y ago
  Yeah came here to say this. A complex turbine setup is large and complex. A panel setup with liquid cooling and a heat exchanger isn’t. I feel like these sorts of things should be useful in situations where there’s a lot of excess heat in some other process that’s typically wasted. Smelters, incinerators, high temperature chemical reactions, etc. Because they are presumably not large you can clad and enclose the high temperature area in these sorts of panels and capture 40% of the wasted energy and divert it back into the process. That would have compounding effects.
pbmonster 3y ago
They don't ever run those close to this hot, right? There's barely any turbine blade materials able to withstand half those temperatures. At those temperatures, even the best alloys lose a large percentage of their strength. And also, steam this hot is incredibly corrosive.
- londons_explore 3y ago
  Gas turbines typically are limited to ~1300C to limit NOx production. However, if you have another heat source that doesn't involve combustion, that limit is taken away. And then you can run the whole thing hotter (increasing efficiency substantially). There is no need for blades to withstand the combustion temperature, because film cooling can keep blades far cooler in high speed laminar gas flow. The challenge is that the operating gas must be dust-free or a spec of dust on the blade surface disrupts the film and causes failure. Obviously the amount of energy wasted to pumping the film cooling gasses goes up the higher the combustion temperature is, so I assume there is still some upper limit on combustion temperature.
  - amluto 3y ago
    > Gas turbines typically are limited to ~1300C to limit NOx production. However, if you have another heat source that doesn't involve combustion, that limit is taken away. As far as I know, combustion isn’t required for NOx formation. If you want to run hotter without producing NOx, you need to eliminate either nitrogen or oxygen. IIRC there are a couple of proposed designs for doing this: combustion in nitrogen-depleted air and chemical looping combustion.
    - deleted 3y ago
      [deleted]
    - pbmonster 3y ago
      > combustion in nitrogen-depleted air and chemical looping combustion. Am I misunderstanding this or are both of those concepts basically "just inject pure oxygen"? And I'm no economist, but running a powerplant on rocket fuel doesn't sound that feasible to me. Urea injection into the exhaust gas sounds more sensible to me, and even that is seems economically questionable. Fuel prices would need to be pretty high that the efficiency gains make buying urea (instead of just more fuel and running the combustion colder) worth the investment...
labcomputer 3y ago
You have to do some “energy accounting” sleight of hand to get 63%, though. That number assumes that some of the low-grade waste heat can be used for, e.g., district heating. It’s also a peak steady-state number, only achievable under optimal load conditions.
- londons_explore 3y ago
  I believe that figure is electric efficiency only. For combined heat and power plants, the efficiency routinely goes over 80%, and for heat-only plants, 95+% is common. However, there is still sleight of hand. The efficiency quoted is when the equipment is new and clean. Fouling and wear both take single digit percentages off. Also, they use the lower-heating-value for the gas energy supply. That, in my view, is dishonest - the correct energy measure for gas is the higher-heating-value, which is 10.7% more. The difference comes from how you account for the heat in the steam produced by burning gas. In my view, the energy from that steam should be accounted for when considering efficiency - in GE's view, it shouldn't.
  - AtlasBarfed 3y ago
    And of course carbon removal from burning the natural gas is totally out of the equation.
    - lazide 3y ago
      Since no one is requiring it at scale, and since there is no commonly accepted way of doing it - pretty hard to add it to an equation no?
    - est31 3y ago
      > And of course carbon removal from burning the natural gas is totally out of the equation. Sorry for being nitpicky, but burning natural gas does not remove carbon, it adds new CO2 to the atmosphere (if you release the burn products which is usually done). You probably mean that if you released that natural gas directly into the atmosphere instead, it would have a larger greenhouse effect than the CO2 released by the burning process. This is true, and one should absolutely choose "burn it" when given the choice of releasing only vs burning and then releasing. But expressing it in the way of carbon removal is misleading. The number of carbon atoms in the atmosphere is the same in both scenarios, they are just bound in a less greenhouse-y form (also, natural gas decays into CO2 plus water eventually, but it's a very slow process). The number of CO2 equivalents goes up in both scenarios as well, just way less if you burn it before. Maybe some people refer to those through "carbon", idk.
deleted 3y ago
[deleted]
zackmorris 3y ago
Just so we have it, the efficiency of any heat engine is limited to the Carnot efficiency: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.htm... --- An internal combustion engine (ICE) uses the Otto cycle: https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node26.html https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes... https://www.sciencedirect.com/topics/engineering/otto-cycle https://www.sciencedirect.com/topics/engineering/otto-cycle If you click the Read more arrow under 3.4.1 The Efficiency of an Otto Engine, it states that The ideal Otto cycle achieves the Carnot efficiency of an engine working between the maximum, pre-combustion, temperature and the intake temperature. This means that the ideal Otto cycle cannot achieve the Carnot efficiency determined by the highest and lowest temperature during the cycle. Tlow = T ambient Thigh = T at highest compression of piston efficiency ~= 1 - Tlow/Thigh < (Thigh - Tlow)/Thigh = Carnot efficiency We can find the pre-ignition temperature at maximum compression: https://www.physicsforums.com/threads/compression-psi-and-temp.303229/ https://www.physicsforums.com/threads/compression-psi-and-te... T2 = T1 * ((V1/V2)^(y-1)) where y ~= 1.4 for air So a 14:1 compression ratio at an ambient room temperature of 293 K (20 C or 68 F) gives a pre-combustions temperature of: T2 = 293 * (14^(1.4-1)) = 842 K (569 C or 1056 F) So the maximum efficiency (Carnot efficiency) of an Otto cycle 14:1 compression ICE would be less than: efficiency < (842-293)/842 < 65% In practice, an ICE might scavange 50% of that due to losses to entropy, friction and hot exhaust at 600 K (about 300 C or 600 F) and end up at 33% efficiency, not counting drivetrain losses of about 15% to get to maybe 28% at the wheels. That's why ICE vehicles waste around 75% of the fuel's energy or more. Whereas an electric vehicle will be around 90% percent efficient from batteries to motor and around 75% efficient at the wheels, or at least 3 times better than an ICE vehicle. --- A gas turbine uses the Brayton cycle: https://web.mit.edu/16.unified/www/SPRING/propulsion/notes/node27.html https://web.mit.edu/16.unified/www/SPRING/propulsion/notes/n... The Brayton cycle thermal efficiency contains the ratio of the compressor exit temperature to atmospheric temperature, so that the ratio is not based on the highest temperature in the cycle, as the Carnot efficiency is. For a given maximum cycle temperature, the Brayton cycle is therefore less efficient than a Carnot cycle. Tlow = T ambient Thigh = T at highest compression after compressor effiency ~= 1 - Tlow/Thigh < (Thigh - Tlow)/Thigh = Carnot efficiency A jet engine might reach 40:1 compression: T2 = 293 * (40^(1.4-1)) = 1281 K (1008 C or 1846 F) So the maximum efficiency (Carnot efficiency) of a Brayton cycle 40:1 compression turbine would be less than: efficiency < (1281-293)/1281 < 77% In practice, a gas turbine might scavange up to 85% of that and end up at 65% efficiency, not counting generator losses of 5%. But 40-65% overall efficiency is more realistic. A 45% efficient gas turbine would leave 55% of the energy as waste heat in the exhaust. So a steam turbine scavanging that would only need to be about 35% efficient to reach an overall efficiency of 65%. Depending on exhaust temperature, the article's thermophotovoltaic cells would probably be cheaper and more reliable than additional turbine stages. --- I've never seen a good way to relate Carnot efficiency to quantum efficiency. Thermodynamics measures average emergent behavior like fluid dynamics. So the Carnot efficiency is kind of like the Bernoulli equation or Reynolds number, and may have no analog at microscopic scales. Maybe something like: https://en.wikipedia.org/wiki/Quantum_heat_engines_and_refrigerators https://en.wikipedia.org/wiki/Quantum_heat_engines_and_refri...
WhyCause 3y ago
I worked for a power company that had something like this. Except it was two F4 Phantom jet engines (modified to use natural gas) per generator. I believe there were two sets in that plant, and boy, did it get loud.

15457345234 3y ago

There are a looooot of moving parts in a CCGT generating station, multiple oil loops, multiple coolant loops, many consumables, a complex control flow. Being able to replace the combined turbine/alternator assembly with a 'when it gets hot, voltage comes out' unit would give you significant reliability gains and lower operating costs.

pbmonster 3y ago
Well, you still have to cool those cells. It's still a heat engine, after all. And you can't just let radiation do the job for you, no way the semiconductor likes getting anywhere close to this hot.
- fnordpiglet 3y ago
  Liquid cooling with a heat exchanger can be pretty low profile.
fnordpiglet 3y ago
Yeah came here to say this. A complex turbine setup is large and complex. A panel setup with liquid cooling and a heat exchanger isn’t. I feel like these sorts of things should be useful in situations where there’s a lot of excess heat in some other process that’s typically wasted. Smelters, incinerators, high temperature chemical reactions, etc. Because they are presumably not large you can clad and enclose the high temperature area in these sorts of panels and capture 40% of the wasted energy and divert it back into the process. That would have compounding effects.

pbmonster 3y ago

They don't ever run those close to this hot, right? There's barely any turbine blade materials able to withstand half those temperatures. At those temperatures, even the best alloys lose a large percentage of their strength. And also, steam this hot is incredibly corrosive.

londons_explore 3y ago
Gas turbines typically are limited to ~1300C to limit NOx production. However, if you have another heat source that doesn't involve combustion, that limit is taken away. And then you can run the whole thing hotter (increasing efficiency substantially). There is no need for blades to withstand the combustion temperature, because film cooling can keep blades far cooler in high speed laminar gas flow. The challenge is that the operating gas must be dust-free or a spec of dust on the blade surface disrupts the film and causes failure. Obviously the amount of energy wasted to pumping the film cooling gasses goes up the higher the combustion temperature is, so I assume there is still some upper limit on combustion temperature.
- amluto 3y ago
  > Gas turbines typically are limited to ~1300C to limit NOx production. However, if you have another heat source that doesn't involve combustion, that limit is taken away. As far as I know, combustion isn’t required for NOx formation. If you want to run hotter without producing NOx, you need to eliminate either nitrogen or oxygen. IIRC there are a couple of proposed designs for doing this: combustion in nitrogen-depleted air and chemical looping combustion.
  - deleted 3y ago
    [deleted]
  - pbmonster 3y ago
    > combustion in nitrogen-depleted air and chemical looping combustion. Am I misunderstanding this or are both of those concepts basically "just inject pure oxygen"? And I'm no economist, but running a powerplant on rocket fuel doesn't sound that feasible to me. Urea injection into the exhaust gas sounds more sensible to me, and even that is seems economically questionable. Fuel prices would need to be pretty high that the efficiency gains make buying urea (instead of just more fuel and running the combustion colder) worth the investment...

labcomputer 3y ago

You have to do some “energy accounting” sleight of hand to get 63%, though. That number assumes that some of the low-grade waste heat can be used for, e.g., district heating. It’s also a peak steady-state number, only achievable under optimal load conditions.

londons_explore 3y ago
I believe that figure is electric efficiency only. For combined heat and power plants, the efficiency routinely goes over 80%, and for heat-only plants, 95+% is common. However, there is still sleight of hand. The efficiency quoted is when the equipment is new and clean. Fouling and wear both take single digit percentages off. Also, they use the lower-heating-value for the gas energy supply. That, in my view, is dishonest - the correct energy measure for gas is the higher-heating-value, which is 10.7% more. The difference comes from how you account for the heat in the steam produced by burning gas. In my view, the energy from that steam should be accounted for when considering efficiency - in GE's view, it shouldn't.
- AtlasBarfed 3y ago
  And of course carbon removal from burning the natural gas is totally out of the equation.
  - lazide 3y ago
    Since no one is requiring it at scale, and since there is no commonly accepted way of doing it - pretty hard to add it to an equation no?
  - est31 3y ago
    > And of course carbon removal from burning the natural gas is totally out of the equation. Sorry for being nitpicky, but burning natural gas does not remove carbon, it adds new CO2 to the atmosphere (if you release the burn products which is usually done). You probably mean that if you released that natural gas directly into the atmosphere instead, it would have a larger greenhouse effect than the CO2 released by the burning process. This is true, and one should absolutely choose "burn it" when given the choice of releasing only vs burning and then releasing. But expressing it in the way of carbon removal is misleading. The number of carbon atoms in the atmosphere is the same in both scenarios, they are just bound in a less greenhouse-y form (also, natural gas decays into CO2 plus water eventually, but it's a very slow process). The number of CO2 equivalents goes up in both scenarios as well, just way less if you burn it before. Maybe some people refer to those through "carbon", idk.

deleted 3y ago

[deleted]

zackmorris 3y ago

Just so we have it, the efficiency of any heat engine is limited to the Carnot efficiency: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.htm... --- An internal combustion engine (ICE) uses the Otto cycle: https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node26.html https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes... https://www.sciencedirect.com/topics/engineering/otto-cycle https://www.sciencedirect.com/topics/engineering/otto-cycle If you click the Read more arrow under 3.4.1 The Efficiency of an Otto Engine, it states that The ideal Otto cycle achieves the Carnot efficiency of an engine working between the maximum, pre-combustion, temperature and the intake temperature. This means that the ideal Otto cycle cannot achieve the Carnot efficiency determined by the highest and lowest temperature during the cycle. Tlow = T ambient Thigh = T at highest compression of piston efficiency ~= 1 - Tlow/Thigh < (Thigh - Tlow)/Thigh = Carnot efficiency We can find the pre-ignition temperature at maximum compression: https://www.physicsforums.com/threads/compression-psi-and-temp.303229/ https://www.physicsforums.com/threads/compression-psi-and-te... T2 = T1 * ((V1/V2)^(y-1)) where y ~= 1.4 for air So a 14:1 compression ratio at an ambient room temperature of 293 K (20 C or 68 F) gives a pre-combustions temperature of: T2 = 293 * (14^(1.4-1)) = 842 K (569 C or 1056 F) So the maximum efficiency (Carnot efficiency) of an Otto cycle 14:1 compression ICE would be less than: efficiency < (842-293)/842 < 65% In practice, an ICE might scavange 50% of that due to losses to entropy, friction and hot exhaust at 600 K (about 300 C or 600 F) and end up at 33% efficiency, not counting drivetrain losses of about 15% to get to maybe 28% at the wheels. That's why ICE vehicles waste around 75% of the fuel's energy or more. Whereas an electric vehicle will be around 90% percent efficient from batteries to motor and around 75% efficient at the wheels, or at least 3 times better than an ICE vehicle. --- A gas turbine uses the Brayton cycle: https://web.mit.edu/16.unified/www/SPRING/propulsion/notes/node27.html https://web.mit.edu/16.unified/www/SPRING/propulsion/notes/n... The Brayton cycle thermal efficiency contains the ratio of the compressor exit temperature to atmospheric temperature, so that the ratio is not based on the highest temperature in the cycle, as the Carnot efficiency is. For a given maximum cycle temperature, the Brayton cycle is therefore less efficient than a Carnot cycle. Tlow = T ambient Thigh = T at highest compression after compressor effiency ~= 1 - Tlow/Thigh < (Thigh - Tlow)/Thigh = Carnot efficiency A jet engine might reach 40:1 compression: T2 = 293 * (40^(1.4-1)) = 1281 K (1008 C or 1846 F) So the maximum efficiency (Carnot efficiency) of a Brayton cycle 40:1 compression turbine would be less than: efficiency < (1281-293)/1281 < 77% In practice, a gas turbine might scavange up to 85% of that and end up at 65% efficiency, not counting generator losses of 5%. But 40-65% overall efficiency is more realistic. A 45% efficient gas turbine would leave 55% of the energy as waste heat in the exhaust. So a steam turbine scavanging that would only need to be about 35% efficient to reach an overall efficiency of 65%. Depending on exhaust temperature, the article's thermophotovoltaic cells would probably be cheaper and more reliable than additional turbine stages. --- I've never seen a good way to relate Carnot efficiency to quantum efficiency. Thermodynamics measures average emergent behavior like fluid dynamics. So the Carnot efficiency is kind of like the Bernoulli equation or Reynolds number, and may have no analog at microscopic scales. Maybe something like: https://en.wikipedia.org/wiki/Quantum_heat_engines_and_refrigerators https://en.wikipedia.org/wiki/Quantum_heat_engines_and_refri...

WhyCause 3y ago

I worked for a power company that had something like this. Except it was two F4 Phantom jet engines (modified to use natural gas) per generator. I believe there were two sets in that plant, and boy, did it get loud.