If you want to convert heat into electrical power, it’s hard to find a simpler method than a thermoelectric generator. The Seebeck effect means that the junction of two dissimilar conductors will produce a voltage potential when heated, but the same effect also applies to certain alloys, even without a junction. [Simplifier] has been trying to find the best maker-friendly thermoelectric alloys, and recently shared the results of some extensive experimentation.
The experiments investigated a variety of bismuth alloys, and tried to determine the effects of adding lead, antimony, tin, and zinc. [Simplifier] mixed together each alloy in an electric furnace, cast it into a cylindrical mold, machined the resulting rod to a uniform length, and used tin-bismuth solder to connect each end to a brass electrode. To test each composition, one end of the cylinder was cooled with ice while the other was held in boiling water, then resistance was measured under this known temperature gradient. According to the Wiedemann-Franz law, this was enough information to approximate the metal’s thermal conductivity.
Armed with the necessary data, [Simplifier] was able to calculate each alloy’s thermoelectric efficiency coefficient. The results showed some useful information: antimony is a useful additive at about 5% by weight, tin and lead created relatively good thermoelectric materials with opposite polarities, and zinc was useful only to improve the mechanical properties at the expense of efficiency. Even in the best case, the thermoelectric efficiency didn’t exceed 6.9%, which is nonetheless quite respectable for a homemade material.
This project is a great deal more accessible for an amateur than previous thermoelectric material research we’ve covered, and a bit more efficient than another home project we’ve seen. If you just want to get straight to power generation, check out this project.
It’s wrong, change that to
or something like. You need temparature gradient over junction, not just heat to get voltage with Seebeck effect.
Cool project. Guys also deserve special great respect for report page. It’s a rare thing now when people care more about information over web design.
Actually, that’s only slightly less incorrect than the original explanation. Don’t feel bad, this is a very common misconception.
The junction is not what generates the voltage. The junction is just there to hold the two ends of the different conductors at the same voltage.
The temperature gradient within the conductor produces a voltage gradient in the conductor. If you take a single isolated copper wire and make one end hotter than the other there will be a potential difference between the two ends. There’s no way to directly measure that without putting a different wire on it which will also have a temperature gradient, hence a potential difference, but that is the reality. This becomes useful because some materials have different Seebeck coefficients, so if you put an iron wire next to the copper wire, the potential difference between the ends will be different in the two different materials. If you now tie the far ends together conductively then there will be a potential difference between the two conductors at the near end, ergo a measurable potential difference will exist. Now if you put your copper voltmeter leads on those the gradient in the leads is equal, so you can measure the potential in the thermocouple.
This is why it doesn’t matter what you do at the junction end of a thermocouple: twist the two wires together, bond them with conductive epoxy, solder them, or spot weld them. It doesn’t matter as long as there is low resistance at the junction.
This writeup is very nice, especially the color-coded chart. His previous metal casting project is also well done – I’ve never heard of molasses sand.
People used to make cores out of molasses sand because you can bake it in the core box and get something that’ll hold together solidly while being placed in a green sand mold and exposed to hot metal. I think in general most of sand casting is “what’s the cheapest way to make this?” and the materials they choose are prioritized for A: reusability (green sand) and B: cheapness (molasses over sodium silicate, say, which has also been used for cores.) Oil sands are really good for sandcasting, but clay/water are cheaper, and usually the additives people put in to increase its ability to vent, say, are industrial waste like sawdust.