8 ms·

It's worth noting that the Carnot limit really only applies to systems that return to thermal equilibrium much faster than the characteristic interaction and/or

It's worth noting that the Carnot limit really only applies to systems that return to thermal equilibrium much faster than the characteristic interaction and/or rely on thermal interactions to transmit energy. Electrochemical, magnetohydrodynamic and various other systems are not properly characterized by the Carnot limit and can approach much higher efficiency limits. I think the systems being discussed are appropriately characterized by the Carnot limit, but it is frequently mischaracterized as an absolute limit on systems that it is not relevant to and for example here it should probably not be considered the absolute limit of refrigeration efficiency.

cyberax 3y ago
> Electrochemical, magnetohydrodynamic and various other systems are not properly characterized by the Carnot limit and can approach much higher efficiency limits. That usually is a distinction without difference. Other effects limit the efficiency at far lower levels than pure Carnot. This can be intuitively understood like this: Carnot limits apply to gases, that are the simplest interacting systems. You can realistically model them as just a collection of individual independent particles interacting only via simple collisions. Anything more complicated like electrons in semiconductors, and you have way more interactions and way more possibilities for your system to have inefficiencies. Take, for example, solar panels. Sunlight has the optical temperature of around 6000K, so a Carnot engine that uses 6000K hot part and 300K cold part will theoretically have a 95% efficiency. And an infinite stack of solar panels with each layer tuned for a specific wavelength has the theoretically maximum 87% efficiency.
- eikoongaepaikei 3y ago
  Meanwhile an infinite diameter conductor has a maximum efficiency of 100% with every part at whatever temperature you want. Carnot characterizes classical heat engines and things that work a lot like them, it's not a universal limit of process efficiency and it's not even a good first approximation for many systems. Your example of a meaningless distinction is a factor of over two in waste energy, in the favor of Carnot in this case, but distinct none-the-less.

cyberax 3y ago

> Electrochemical, magnetohydrodynamic and various other systems are not properly characterized by the Carnot limit and can approach much higher efficiency limits. That usually is a distinction without difference. Other effects limit the efficiency at far lower levels than pure Carnot. This can be intuitively understood like this: Carnot limits apply to gases, that are the simplest interacting systems. You can realistically model them as just a collection of individual independent particles interacting only via simple collisions. Anything more complicated like electrons in semiconductors, and you have way more interactions and way more possibilities for your system to have inefficiencies. Take, for example, solar panels. Sunlight has the optical temperature of around 6000K, so a Carnot engine that uses 6000K hot part and 300K cold part will theoretically have a 95% efficiency. And an infinite stack of solar panels with each layer tuned for a specific wavelength has the theoretically maximum 87% efficiency.

eikoongaepaikei 3y ago
Meanwhile an infinite diameter conductor has a maximum efficiency of 100% with every part at whatever temperature you want. Carnot characterizes classical heat engines and things that work a lot like them, it's not a universal limit of process efficiency and it's not even a good first approximation for many systems. Your example of a meaningless distinction is a factor of over two in waste energy, in the favor of Carnot in this case, but distinct none-the-less.