What is the most efficient small-scale low-temperature heat engine?

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Discussion Overview

The discussion revolves around the exploration of efficient small-scale low-temperature heat engines, specifically focusing on converting thermal energy from a light bulb into mechanical work. Participants consider various thermodynamic cycles and alternative energy conversion methods.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that a Rankine cycle, particularly an organic Rankine cycle, could be an effective method for converting heat from a light bulb into mechanical work, citing the efficiency of steam turbines.
  • Another participant posits that the most efficient engine would theoretically be one using the Carnot cycle, although they acknowledge practical limitations.
  • There is a discussion about the implications of the second law of thermodynamics on isentropic processes, with one participant noting that while isentropic processes are ideal, they can be approached through slow adiabatic processes.
  • A suggestion is made to consider using a thermocouple for electricity generation from low-level heat, though it is acknowledged that this method is not very efficient.
  • One participant proposes that replacing the incandescent bulb with an LED light could be a more effective measure for energy efficiency, as it reduces energy consumption significantly.

Areas of Agreement / Disagreement

Participants express differing views on the most efficient method for energy conversion, with no consensus reached on the best approach. There are competing ideas regarding the use of heat engines versus alternative methods like thermocouples.

Contextual Notes

Limitations include the practical feasibility of the proposed cycles and methods, as well as the inherent inefficiencies associated with heat engines in general.

cxaxnxexs
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I’ve recently become fascinated by high power density energy conversion devices… I was looking at the 100W incandescent light bulb lamp on my desk as 90% of its energy was being wasted as heat and started thinking… I know it is only a rather small amount of energy, but what would be the most efficient way to convert the thermal energy from the lightbulb (surface temp of 136C) into rotational mechanical work to turn a generator?

My intuition at first leads me to think that a Rankine cycle (likely some kind of organic Rankine cycle given the relatively low temperature) turbine would be the answer since it should not experience the kinetic energy losses that appears with the rapid accelerations and decelerations that occur in steam piston engines. Plus, there must be a reason steam turbines are so popular. However, I’ve just started learning about Stirling cycle engines & they are very interesting. Some reports claim them to be more efficient that steam engines --> http://www.bekkoame.ne.jp/~khirata/academic/kiriki/begin/general.html

Any ideas/comments? :rolleyes:
 
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The most efficient engine would be one using the Carnot cycle. (Note: I didn't say it was the most practical - no engine would be practical).

AM
 
Haha you are definitely right there. Actually doesn't the 2nd law of thermo show isentropic processes to be impossible? Although I'm sure those crazy quantum theory guys would argue differently.
 
cxaxnxexs said:
Haha you are definitely right there. Actually doesn't the 2nd law of thermo show isentropic processes to be impossible? Although I'm sure those crazy quantum theory guys would argue differently.
The Carnot engine cycle is an ideal limit that can be approached but never reached. But generally isentropic processes are not impossible - slow adiabatic processes are isentropic.

Instead of using a heat engine, you could use a thermocouple to create electricity from low level heat. It is not that efficient but no heat engine is going to be very efficient for converting lightbulb heat to useful work.

AM
 
The more effective measure would be to trade the glow-bulb for an equally bright led light, consuming a fraction of the 100 watts (apprx. 10 watts). Carnot-efficiency exceeded, by trading heat for electricity, not making heat in the first place. Your approach towards energy-efficiency is common though.
 

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