Ideal thermal efficiency impossible?

[gersetaffe]

I know a heat engine can never be 100% efficient. There has to be a cold reservoir that some of the heat is rejected to. Why is that?

I know it is because of entropy, but could someone please provide some further explanation. Thanks.

 

[zhermes]

Its just a matter of logistics. Any time there is friction, or noise, or heat going to something you don't want, or air-resistance, etc etc etc, then energy is being lost--and you also lose efficiency.

Consider a car engine, 1) tons of heat is being dissipated without doing any work (e.g. by the radiator, and the wind blowing by) 2) even once you convert the heat from combustion to torque, there is friction on the drive shaft, the axel, the tires--losing more heat; in the end you are getting about 20% efficiency.

 

[russ_watters]

Sorry, zhermes, but no this has nothing to do with friction or other incidental losses. Even with an absolutely perfect heat engine, you get nowhere near 100% efficiency.

The reason is that the heat engine harnesses the energy (temperature) difference between the hot reservoir and cold reservoir and as a result, heat must be allowed to flow between them.

 

[netheril96]

Sorry, zhermes, but no this has nothing to do with friction or other incidental losses. Even with an absolutely perfect heat engine, you get nowhere near 100% efficiency.

The reason is that the heat engine harnesses the energy (temperature) difference between the hot reservoir and cold reservoir and as a result, heat must be allowed to flow between them.

russ_watters is right.Even with no friction or whatever,you could get 100% efficiency only if you had a cold reservoir at 0K,which is forbidden by the Third Law of Thermodynamics.

To know more,refer to Carnot Theorem.

 

[sardar]

I can confirm that it is indeed impossible for a heat engine to have 100% thermal efficiency. This is due to the fundamental laws of thermodynamics, specifically the second law which states that the total entropy of a closed system can never decrease over time.

In the case of a heat engine, the hot reservoir (where the heat is taken in) will always have a higher temperature than the cold reservoir (where some of the heat is rejected). This difference in temperature creates a gradient that allows the engine to do work. However, in the process of converting heat into work, some of the heat will always be lost to the cold reservoir, increasing its entropy. This means that the overall entropy of the system will increase, making it impossible to achieve 100% efficiency.

Additionally, even if we were able to eliminate the cold reservoir and have a perfectly isolated system, the engine would eventually reach a state of equilibrium where no work could be done. This is because heat naturally flows from hot to cold, and in an isolated system, the temperature would eventually equalize.

In summary, the concept of entropy is what makes it impossible for a heat engine to have 100% thermal efficiency. It is a fundamental law of nature that cannot be overcome. However, scientists and engineers continue to strive for higher and more efficient thermal processes, pushing the boundaries of what is possible within the constraints of the laws of thermodynamics.