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I was thinking about refrigerators and air conditioners. They remove the heat from somewhere and put it into somewhere else. But why are we use this usually for cooling, and not so much for heating (I know its possible), and why for cooling and heating air? Is it possible to heat up things with this method to use it for cooking? Would it be inefficient?

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tl;dr- Sure, just use waste-heat recovery by operating a refrigerator that cools the outside and dumps its heat in your home (or food or whatever).


Sure, that's a heat pump (to steal the link from @TEH's answer).

Conceptually, I'd think of it like this...

Say that you're burning some wood to heat your home – let's assume that it's a typical temperature like $T_{\text{flame}} \approx 1000\sideset{^{\circ}}{}{\mathrm{C}} .$ Then, since the thermal energy of that flame will flow to cooler material, e.g. the air around the flame, it's pretty effective at transferring that thermal energy.

But, did you really need such a high temperature, like $T_{\text{flame}} \approx 1000\sideset{^{\circ}}{}{\mathrm{C}} ,$ to drive heat into an environment that's around room temperature? It uses more driving power than what's necessary, which is inefficient.

So how do you fix this inefficiency? I mean, you can't just make more thermal energy, since that'd violate the conservation of energy. Instead, you have to draw thermal energy from some other source. And to do that, you can use the excess driving force from the unnecessarily-high-temperature-flame.

This is called waste heat recovery. You can do with a heat pump like this:

enter image description here

"Absorption Chiller, How it works" (2017-09-26)

Then:

  1. You have your flame power the $`` {\small{\begin{array}{c} \textbf{Hot Water} \\[-25px] \textbf{/ Steam} \end{array}}} "$ stream. This step is how we input the excessive driving force that comes from the flame's unnecessarily high temperature.

  2. You hook up some random waste heat source to the $`` {\small{\begin{array}{c} \textbf{Chilled} \\[-25px] \textbf{Water} \end{array}}} "$ stream. When the contents of this stream go through the $`` \textbf{Evaporator} " ,$ they'll give off their heat to evaporate the heat pump's internal working fluid – and, since the that internal working fluid is stealing the stream's thermal energy, that stream is cooled in the process. This step is where we get the extra thermal energy from.

  3. You hook up whatever you want to heat, like the air to your home or the food you want to cook, to the $`` {\small{\begin{array}{c} \textbf{Cooling} \\[-25px] \textbf{Tower} \end{array}}} "$ stream. As the contents of that stream flow through the absorber, the internal working fluid is reconstituted in an exothermic reaction, warming the contents of the stream. The warmed stream then exits the heat pump, only to reenter it in the condenser. In the condenser, the gaseous internal working fluid condenses on the stream, again dumping heat into it (as condensation is also exothermic).

If you do a thermal-energy balance on the system, you'll see the the thermal energy from the flame and the waste-heat source both ended up warming your home/food/whatever. In effect, you got more thermal energy out of the flame than the flame actually had by using its excess driving force to also harvest thermal energy from a waste heat source.


possibility of a “reverse-refrigerator” that cooks?

I just used that above image as a heating unit. But, the image's own title is "How Absorption Chillers Work" – not "How Absorption Heaters Work".

Which is fine! Because, as you suspected, you can do the same thing, sorta "in reverse", except it's more like we just flipped our heat sinks around.

I mean, technically, it's still a chiller. We're just chilling the waste heat source, and using the generated heat rather than dumping it (like typical refrigerators do).

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  • $\begingroup$ Any answer where I can insert an animated GIF is a happy occasion. =P $\endgroup$ – Nat Jun 15 at 23:06
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This is called a heat pump and they are used, but I do not know of them being used for cooking though.

https://en.wikipedia.org/wiki/Heat_pump

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  • $\begingroup$ Please note we very strongly discourage link only answers. If the link goes dead or the content at the other end is changed it can invalidate or make useless such an answer. Please at least give a summary of the concept your link explains so your answer can stand alone. Thanks. $\endgroup$ – StephenG Jun 15 at 22:43
  • $\begingroup$ Yeah that makes sense, I came back to rewrite it but saw @Nat's great answer $\endgroup$ – TEH Jun 16 at 4:27

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