Thermocouple and cooling

This article is about a coffee cup with a built-in thermocouple that you can use to add a little charge to your phone.

My questions:

Will that coffee cup cool down quicker than a coffee cup made from the same materials, but not connected to a thermocouple? And, will that coffee cup cool down faster when it is charging a phone than when the charging port is left unused?

Assuming there’s electronic switching happening in this set up, a cup that is exactly the same, but has all internal electrical conductors interrupted by an electric insulator with the same thermodynamic properties will cool down slower than one with those internal connections in place, and if you plug it into your phone it will cool down even faster.

While on paper, at least, a moderately sized insulated tank of hot water seems to have a niche use as an energy-storage medium due to its high specific heat, in practice with coffee the trouble is to keep the cup piping hot, exactly the opposite problem to extracting thermal energy from it.

This is getting attention because the inventor is 18 years old and winning awards, and she’s winning awards because she’s 18 years old and the awarding organizations want to encourage young scientists who might do something worthwhile someday. For any practical purpose, there are a lot of solutions more practical than this mug.

Yes, of course. I’m just really asking if a thermocouple cools something down faster than a similarly conductive material, and if so, is that only when it’s connected to something that’s using power.

The first law of thermodynamics applies here. Charging your phone takes energy and that has to come from somewhere. In this case it takes heat from the coffee.

Right, and this is probably the wrong metaphor, but damming a river slows it down. If I had a spinner hooked up to a generator at the bottom of my coffee cup, the more load I put on the generator, the slower the cup would drain.

Yes, but the question was whether the heat would otherwise have just dissipated (heating the air around the cup, e.g.) vs staying in the coffee. Does the thermocouple extract heat more rapidly than the cooling process without anything connected?

The question wasn’t whether this invention is the equivalent of a perpetual motion machine.

Surely, if it didn’t, there would be no point in the hot liquid?

But, the hot liquid is going to cool either way.

I know, for example, that a hot liquid in an insulated cup will cool more slowly than a hot liquid in an insulated cup with a metal spoon sticking out the side, since the metal spoon with conduct the heat faster than if it weren’t there. Will a thermocouple conduct the heat faster still? Even faster if there’s a load on in?

Yes, but she’s using a double walled insulated cup, which is equivalent to the river already having mechanisms for slowing the flow.

Now the goals of “keep the coffee hot” cup design and “make energy from the heat in the coffee” cup design happen to be mostly overlapping, so if this ever becomes a well established commercial design it will likely be superior in heat retention to a ceramic mug, but inferior to a double insulated cup without a thermocouple.

Yes, no question. But, would it be inferior to a double insulated cup with just a piece of metal sticking out of it? And, would the double insulated cup with the thermocouple in use be worse than the double insulated up with the thermocouple not in use?

ETA: I don’t think this has been answered above, and I’m not trying to be argumentative for argumentative sake.

That’s impossible to answer without knowing the size and shape of the piece of metal and the exact properties of the thermocouple and the changes made to the cup.

Yes probably. As I stated in the first reply of the thread.

In its simplest form, I think a thermocouple is just two wires with different electrical conductivity connected to each other in a hot place, with their ends sticking out in a cold place, right? And, you’ll get a voltage across the ends in the cold place, and that can be used to do work.

So, take those two wires and don’t connect them to each other, but still have one side in the hot place and the other side in the cold place.

It’s not clear to me, that the coffee will cool down more slowly when the wires are disconnected than when they are connected.

But, wait! The wires are doing work! Yes, but in my (probably inapt) analogy of the spinner connected to the bottom of the cup, the cup will drain more slowly when the spinner is doing work than when it isn’t. Using the potential kinetic energy of the coffee in the cup while draining it slows down the drain effect. Why does using the potential heat energy of the coffee cup necessarily speed up the use of that potential energy?

And, thanks for that first answer, of course, but I’m hoping to get a definitive answer.

Highjack: The old furnace in a house I bought was originally a coal furnace that had been converted to gas. It used a thermopile capable of operating the thermostat and the gas valve. It was pretty neat. Since the original furnace was a a gravity furnace with no blower the gas conversion would operate when the electrical power to the house was down. It came in handy several times.

Looking at the illustration here of the Seebeck effect my intuition is that the flowing electrons add an energy flow that isn’t there.

Or to explain it with a river and mill wheel. If the mill wheel is spinning, it is extracting work. Not putting a load on the wheel is equivalent to shortcircuiting the thermocouple, not to having an open circuit. To make it not extract work you have to put and infinite load on the wheel so it doesn’t spin, and that will slow down the flow of water.

You mean Thomson (+ Peltier) effect?

You’re definitely more versed in science than I am, but the last time I played with a generator, it was very easy to spin when it wasn’t connected to a load than when it was.

Short-circuiting a thermocouple is putting a heavy load on it.

I appreciate your patience with me…

@naita has it right.
You can think about this in reverse, too, because the Peltier-Seebeck and Thomson effects are reversible. Consider thermoelectric heating of the coffee.

The fact that Peltier elements are used in mini-refrigerators seems to prove that heat is genuinely sucked out of one side of the junction and evolved at the other.