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• Extracts of the video: Hi everyone, today, we are going to conduct some experiments related to thermo-electricity.
It's not a well known phenomenon but it will enable us to create an electric current with a temperature difference. It is called the Seebeck effect. And, reciprocally, we'll be able to create a change of temperature with an electric current, which is called the Peltier effect.
We'll join two metals and bring that junction to a certain temperature, then put a second junction, identical, but at a different temperature. It's very simple, but it should create an electric current.
It should be pointed out that each metal has a precise Seebeck coefficient, and that the iron-aluminium couple is, by far, the most efficient one, regarding common metals.
Okay, it hardly produces 3 mV for a 20 µA current only, but at least we've observed/confirmed the effect we were looking for.
You may always have a little more fun and put a couple of junctions in series, placed alternately at hot or cold sources level. But the production stays at about ten mV tops, making the power ridiculously low.
Fortunately, we can do much better than that, thanks to manufactured goods which gather many junctions with specific metals. They are called Peltier modules.
They can almost only be found in car electric coolers. They are used to produce cold from an electric current. But, of course, they're reversible.
I put my hand on one side of the Peltier module. The other one stays cold as it's pressed against the stone and we can already observe that more than one-tenth of a volt is produced with that temperature difference. We even managed to run an engine with four modules in series.
We just saw that there needs to be some heat transferred from one side to another to produce something. But, after a while, the cold side becomes as warm as the hot one. From there, the production doesn't get any higher. We can clearly see that the voltage reaches a maximum point before decreasing gradually.
To remedy this situation (ou « to solve this problem »), we are going to use a big computer cooling radiator (ou « a computer cooler ») which should prevent the cold side from warming up. But, as the radiator ended up getting hot himself, we've added a little fan.
We are also going tu use a little radiator on the hot side, so that the flame's high temperature equally spreads on all the junctions. It may look insignificant, but it considerably increases the performances and prevents that kind of things from happening.
Contrary to what one might think, to produce as much voltage as possible with a single flame, it is much better to use a single Peltier module, which can easily be heated very strongly. If you put several modules, the power spreads and it produces less.
Let's go a little further and see what we can get from the power of the sun (sun's power). To do that, we cover the side to be heated in black, and put a radiator on the other side, to disperse the heat. Some thermal paste to join the two sides and we're ready to go for the outside test !
In order to concentrate the sunlight (ou « the rays of the sun » pour vraiment faire ressortir l'idée de rayons, de faisceaux) on the module, we use a parabolic mirror.
But the solar panels have nothing to envy Peltier modules (ou « are far from being a poor second to Peltier modules ») as a solar panel produces 18 times the power of a Peltier module of the same size, in the same conditions. Yeah, that's right.
We are then going to use a digital thermometer. By the way, the top of the range thermometers work with metal couples. They are then referred to as thermocouples.
The more the junction warms up, the more voltage is produced. It's the Seebeck effect we talked about at the beginning of the video. We're going to use it to mesure temperature.
In fact, that limit corresponds with the moment when the heat, constantly transferred from the cold side to the hot side, is caught up by the heat going, through conduction, from the hot side to the cold side.
It is interesting to know that these Peltier modules efficiency is very poor. We've measured that the ones we used only get near a 1% efficiency.
Generally speaking, getting a good efficiency to transform 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 the Peltier module as we heat it on one side and cool it down on the other.
Even an idealized heat engine could not convert all the heat going through him...unless his hot source gets to an infinite temperature or his cold source is at absolute zero.
Still, heat engines are way more profitable than Peltier modules as their efficiency can reach 40%.
5 окт 2024
