Can Refrigerators be made more efficient to actually generate electricity?

06 September 2009

Question

Can Refrigerators be made more efficient to actually generate electricity?

Answer

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 don't know three or four times. I don't know the exact figures. The energy reacted in.Dave - That depends on the temperature you've - the difference in temperature which the fridge is working across.Peter - Yeah, so you're actually moving physically more heat than the energy you're putting in. Now given that, can't we do the same thing in reverse and use the fact that we've 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 we've got an efficiency which is greater than the factor. 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 you're asking if a fridge can pump far more heat than the energy you put in, that's definitely true. In fact, if it's 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 you'll never, ever going to get more energy out by going around to the circle like this.Chris - You'll just be violating the laws of physics basically, it's just not going to happen. Dave - Yeah, there's a really, really fundamental law of physics. Which essentially says you can't generate useful energy from nothing and this would violate it completely.Chris - It's 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, it's kind of getting a free lunch isn't it? And it just doesn't happen, energetically speaking it's 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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