I had a thermocouple from an appliance lying around in the garage and I decided that it would be a good idea to experiment with it. It was a used one, but I don’t know what it was from; maybe a water heater. But a long time ago I had cut off the end that goes into the thermostat, so all that was left was a bare copper tube with a copper wire inside of it – a coaxial cable, so to speak. It was so old that the copper was all oxidized and turned dark brown, so I had to polish up the end with some sandpaper.
Being that it has been chilly, I can turn on the stove burner and put the thermocouple in the flame. I clipped on a cheapo DMM, and measured the voltage with the thermocouple’s tip a dull red, and I got about 42 millivolts. But how much current can this deliver?
I got a 10 ohm resistor and put it across the wires. The DMM still read 42 millivolts. It didn’t drop at all, so I went and got a 1 ohm resistor and put it across the wires. The meter dropped only 2 millivolts from 42 to 40 mV. That small drop indicates the thermocouple can deliver quite a bit of current. But at 40 millivolts, the impedance is very low, a small fraction of an ohm.
The big problem is that in order to get enough voltage to run a DC to DC converter, I need to connect at least 20 to 25 thermocouples in series, which will give 0.8 to 1 volt. And there doesn’t seem to be enough room in the flame to put that many thermocouples together, not to mention that they would cost a few hundred dollars. Here is one experimenter’s method of generating electricity with thermocouples.
Another way to harvest energy is to use a Peltier junction. They also produce a low voltage, but not as low as a thermocouple. Also, it may be possible to use heat to generate IR (infrared) radiation, and that is then harvested by solar photovoltaic cells.
Back to experimenting…
