Turn heat to electricity : DIY Experiments [#5] Thermoelectricity / Peltier module / Seebeck

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It is not a well-known phenomenon but it’s an easy way to create electricity with a difference of temperature. It is called the Seebeck effect.

We'll join two metals and bring that junction to a certain temperature, then put a second identical junction to a different temperature. It's very simple, but it should create electricity.
It should be pointed-out that each metal has its own Seebeck coefficient, and that the iron-aluminum couple is, by far, the most efficient one regarding common metals.
Fortunately, we can do much better than that, thanks to manufactured goods which gather many junctions with specific metals. They are called Peltier modules.
Now let's see what happens with a flame. The heat flow, moving from the hot side the cold one, makes it work. The moment we stop heating the module, the produced voltage decreases.
We are also going to use a little radiator on the hot side, so that the flame's heat equally spreads on all the junctions.
Contrary to what one might think, to produce as much voltage as possible with a single flame, it is better to use a single Peltier module, which can easily be heated at a very high temperature.
If you put several modules, the power spreads, the temperature of each module decreases and it produces less. It’s a consequence of the second law of thermodynamics.

Let's go a little further and see what we can get from the sun's power. To do that, we covered the side to be heated in black, and put a radiator on the other side, to dissipate heat.
Some thermal paste to join the two sides, and we're ready to go for the outside test!
It works but solar cells are far from being a poor second to Peltier modules. Indeed, in our conditions a solar panel of the same size produces 18 times more than a Peltier module.
To give you an illustration, for a hundred joules heat going through the module, we'd be able to get no more than a single joule of electricity. Generally speaking, getting a good efficiency to convert heat in another energy is always going to be complicated.
We have the same problem with heat engines, like the Stirling engine, which can be compared to Peltier modules as we heat it up on one side and cool it down on the other. Even an idealized heat engine couldn’t convert all the heat going through him... it must necessarily release heat to a cold source unless it is submitted it to an infinite temperature, which is impossible. Still, heat engines are way more profitable than Peltier modules as their efficiency can reach 40% in cars and sometimes more in power plants. But they are not directly producing electricity from heat.
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