[ard jonker]

ard jonker  asked the Naked Scientists:
   Hello Chris & the Crew,

lovely show, kills the time while cycling to work in an utterly pleasant way!

On transcript/2009.09.05/ Can Refrigerators be made more efficient to actually generate electricity

Dave answers: "very small temperature difference it can pump 100 times more heat than the energy you put in."

What is 'small' here? I've learned that from useful application of fiwihex (http://www.fiwihex.nl/cop.html) the coefficiency of performance (COP) of 8.7 is 90% of the theoretical limit, which thus must be around 10. How tiny must delta-T be for a COP of 100?

Also: a car is an ICE (internal combustion engine), I think a Sterling engine is more like a heat engine and more appropriate to the question.

Finally: the answer to the question does not necessarily violate the laws of thermodynamics imho; you do not generate energy, you harvest (lots of) energy from an external system that has large amounts of low-quality heat.

It is just the delta-T and the bad performance of the heat-to-electricity devices that form the problem.

This is similar to the hydrolic ram pump; a nice display of which we observed in Dairy land,a holiday park in Devon. From a copious supply of water at a low level, this pump pumps up a small amount of water to a high level.

http://www.ncollier.com/rams.html

Kind regards,
Ard

What do you think?

[litespeed]

No, refrigerators can not be made so efficient they create energy. The only suggestion I can come up with is this. If you live in an area where the outside temperature is lower then the inside of the refrigerator, put the refrigerator outside.

Stirling engines can capture waste heat energy, but can not create energy. One way or the other you need to provide a heat source. Theoretically, you could exploit hot sunlight to run a sterling engine and power your entire house. Oddly, I have never heard of any instance where this has been attempted.

You always need a differential between the heated side and the non heated side. I confess I see no reason you could not bury one side of a sterling engine underground where temperature is mostly constant at, say 55F when the other side is up in the warmer atmosphere. Then reverse the cycle when it gets cold outside. Even easier would be to use a constant temperature mountain spring as either the warm or cold side. It might just be easier to dam the silly stream and install a generator. I don't know.

Hydraulic water pumps use the energy of moving water in the stream to pump water uphill. Again, they do not create energy, simply transfer it by slowing down the falling water to move other water up hill. From a home owners perspective it is quite efficient. Once again, just damming the stream for hydro electric might be easier.

On the other hand, I have always been perplexed that automobile exhaust energy has never been recaptured by a small Sterling Engine. This would seem especially appropriate with hybrids where the Sterling could directly charge the batteries. Apparently there are things about Sterling Engines I do not understand.

[Geezer]

Robert Stirling, inventor of the Stirling engine.

http://en.wikipedia.org/wiki/Robert_Stirling

[LeeE]

Would it not be possible to drive a heat engine with the heat removed from the fridge/freezer instead of just convecting it away to space via the radiators?  Obviously, the radiators on the back of a fridge or freezer have to be hotter than the ambient air cooling them for them to work, so there's got to be some potential for using that heat gradient.  The heat engine would have to be very efficient though, to maintain the heat gradient and avoid increasing the energy needed to do the cooling in the first place.

You're still not getting something for nothing, of course, just getting something that would otherwise be wasted.  In practice, from a typical fridge or freezer, you wouldn't be getting very much energy either, but perhaps enough to run an LED internal light instead of drawing the power from the mains.

Air conditioning units, in hot weather, can certainly pump out a lot of heat however, and it should be possible to recover some of that.

[Nabo0o]

Actually, the heat coming of behind the fridge is of no minor importance nor energy.
There is a whole lot of heat being lost (at least I you live in a hot climate), which could have been put to useful use, like for example to aid in the heating of your hot water. In this scenario the efficiency could be much higher, because you cut away the conversion inefficiencies if you wanted electricity.

I just want to say it plainly; It is weird that there is no combined heating and cooling heat pumps available in the market.
Maybe it is uneconomical in the same way as making bulbs that last 50 years is.  []
But they existed in the earlier days of electric lighting...

Bye

[techmind]

@litespeed
I believe soem success has been achieved in extracting thermal energy from automobile exhaust heat using Seebeck (Peltier) effect devices ( http://en.wikipedia.org/wiki/Thermoelectric_effect ). The efficiency of such devices is a poor 10% or so of the theoretical efficiency, but still enough to run the main electrics in a petrol/diesel car and thereby reduce the load on the battery/alternator.

@LeeE
As I've said before (probably somewhere on this forum) the problem with extracting energy from the hot vents at the back of a fridge or air-con unit is that energy-extraction by definition impairs the heat flow, leading to a higher temperature at the back of the fridge/aircon, further reducing the efficiency of the primary device and worsening the overall efficiency.

Of course if you used the heat from the fridge to pre-heat an existing cold body, eg mains water feeding into a boiler, (ensuring the temperature never exceeded what would have been the air-temperature at the back of the fridge) then you may make some net gains by such integration. However, customers enjoy buying, and manufacturers making, 'stand alone' appliances without all the complexities and installation (and who's-to-blame-when-it-goes-wrong) issues inherent in a more integrated arrangement.