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.