Science Questions

Can Refrigerators be made more efficient to actually generate electricity?

Sun, 6th Sep 2009

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Peter, Godmanchester asked:

Can Refrigerators be made more efficient to actually generate electricity?


Peter -   Hello.  Well, my question is fairly complicated, so you have to bear with me  a little bit.  We start off with the refrigerator.  Now, the refrigerator, actually, you get more benefits than the energy you put in.  In the sense that you put a certain amount of electricity and to move heat from the hot to the cold or pump heat away from the cold areas.  And you can pump significantly more heat in the energy you put in.  So, you got sort of reverse efficiency where you can move several times and probably, I donít know three or four times.  I donít know the exact figures.  The energy reacted inÖ

Dave -   That depends on the temperature youíve Ė the difference in temperature which the fridge is working across.

Peter -   Yeah, so youíre actually moving physically more heat than the energy youíre putting in.  Now given that, canít we do the same thing in reverse and use the fact that weíve created a heat differential to power a heat engine to generate the electricity back again.  And now, one or two things will happen.  Either will get more electricity out than we should in a sense that weíve got an efficiency which is greater than the factorÖ   For an example, let me say, if we pump in four times as much heat and to convert the heat back to electricity, we need only 25% efficiency or better to actually win in the game.

Chris -   So, this is worth making tons of free electricity just by running your fridge for cooling your beer down, Dave.

Dave -   Okay, so basically youíre asking if a fridge can pump far more heat than the energy you put in, thatís definitely true.  In fact, if itís pumping for very small temperature difference it can pump 100 times more heat than the energy you put in.  Can you make a heat - temperature difference with that and then use that temperature difference in order to generate electricity?  We can use that temperature difference to generate electricity, we do use temperature differences to generate electricity all the time.  Essentially by using a heat engine - something like a car engine is a heat engine.  And basically they can produce high quality electrical energy by moving heat from a hot place to a cold place.  But a fridge is essentially just a heat engine running backwards and again with a normal heat engine the amount of energy you can get out compared to amount of heat you can move is to do with the difference between the two temperatures.  The bigger the difference in two temperatures, the more efficient it is.  And so youíll never, ever going to get more energy out by going around to the circle like this.

Chris -     Youíll just be violating the laws of physics basically, itís just not going to happen.

Dave -   Yeah, thereís a really, really fundamental law of physics.  Which essentially says you canít generate useful energy from nothing and this would violate it completelyÖ

Chris -   Itís an analogous question to, if I have a propeller on my car as I drove along, could I connect that to some kind of generator.  And then power the car with the generator, itís kind of getting a free lunch isnít it?  And it just doesnít happen, energetically speaking itís just not going to happen.

Dave -   Yeah, and I think actually with this one it would be far, far worse than, it would work far less well than that.


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