Ideal thermal efficiency impossible?

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Discussion Overview

The discussion revolves around the concept of ideal thermal efficiency in heat engines, specifically addressing why achieving 100% efficiency is impossible. Participants explore the implications of entropy, temperature differences between reservoirs, and the limitations imposed by thermodynamic laws.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that a heat engine cannot be 100% efficient due to the necessity of a cold reservoir for heat rejection, linking this to the concept of entropy.
  • Another participant attributes inefficiencies to logistical factors such as friction, noise, and heat dissipation in practical applications like car engines, suggesting these factors contribute to low efficiency rates.
  • A different participant challenges the previous claim, asserting that even an ideal heat engine would not achieve 100% efficiency due to the fundamental requirement of heat flow between hot and cold reservoirs.
  • This participant further explains that achieving 100% efficiency would require a cold reservoir at absolute zero (0K), which is prohibited by the Third Law of Thermodynamics.

Areas of Agreement / Disagreement

Participants express disagreement regarding the sources of inefficiency in heat engines. While some focus on practical losses like friction, others emphasize fundamental thermodynamic principles that prevent 100% efficiency.

Contextual Notes

Participants reference the Carnot Theorem and the Third Law of Thermodynamics, indicating a reliance on specific thermodynamic definitions and principles that may not be universally agreed upon in their interpretations.

gersetaffe
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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.
 
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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.
 
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 said:
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.
 

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