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

1. Dec 10, 2011

### cxaxnxexs

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

2. Dec 10, 2011

### Andrew Mason

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

3. Dec 11, 2011

### cxaxnxexs

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.

4. Dec 11, 2011

### Andrew Mason

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

5. Dec 30, 2011

### Vespa71

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.