3 ms·

> However, in combination with a stove, the electrical efficiency of a thermoelectric module doesn’t matter that much. If a module is only 5% efficient in conve

> However, in combination with a stove, the electrical efficiency of a thermoelectric module doesn’t matter that much. If a module is only 5% efficient in converting heat into electricity, the other 95% comes out as heat again. If the stove is used for space heating, this heat cannot be considered an energy loss, because it still contributes to its original purpose. Total system efficiency (heat + electricity) is close to 100% – no energy is lost. With appropriate stove design, the heat from electricity conversion can also be re-used for cooking or domestic water heating. Doesn't this imply that you can just keep recirculating the heat back into the module until it's converted 100% of the energy? I suppose you would need to power the recirculator somehow, but you can do that using the output of the module, so you're back to 100% efficiency again. What's the catch?

ummonk 5y ago
You can’t just recirculate heat like that. The TEG requires a temperature difference to generate voltage and power. Any recirculated hot air is not going to be any hotter than the air on the other side of the TEG.
intrepidhero 5y ago
Efficiency drops as the temperature delta goes down. The trick is maintaining that difference. That section of the article is a bit misleading. Even really expensive stoves are only about 95% efficient. You need some heat loss out the chimney for the thing to work. I'd guess that stoves typically in use in the rural global south are much less
invalidator 5y ago
The waste heat flows out the cold side of the junction, through the heat sink, and is lost into the ambient air. You can't feed that heat back into the hot side because it's been diluted into the air, and thus it's a lower temperature. You can't concentrate the waste heat by enclosing the heat sink in a small volume because that would eliminate the temperature differential across the junction. You won't gain much by making a stack of junctions with each one passing its waste heat to the next one. They effectively insulate each other, reducing the temperature differential across each one. There's an optimization opportunity here, but commercial junctions are usually already sold as a stack. Adding more layers gives rapidly diminishing returns at best. Thermodynamics is cruel.
- jiggawatts 5y ago
  "No such thing as a free lunch." should be tattooed onto to foreheads of every charlatan claiming to be able to use water as fuel.

ummonk 5y ago

You can’t just recirculate heat like that. The TEG requires a temperature difference to generate voltage and power. Any recirculated hot air is not going to be any hotter than the air on the other side of the TEG.

intrepidhero 5y ago

Efficiency drops as the temperature delta goes down. The trick is maintaining that difference. That section of the article is a bit misleading. Even really expensive stoves are only about 95% efficient. You need some heat loss out the chimney for the thing to work. I'd guess that stoves typically in use in the rural global south are much less

invalidator 5y ago

The waste heat flows out the cold side of the junction, through the heat sink, and is lost into the ambient air. You can't feed that heat back into the hot side because it's been diluted into the air, and thus it's a lower temperature. You can't concentrate the waste heat by enclosing the heat sink in a small volume because that would eliminate the temperature differential across the junction. You won't gain much by making a stack of junctions with each one passing its waste heat to the next one. They effectively insulate each other, reducing the temperature differential across each one. There's an optimization opportunity here, but commercial junctions are usually already sold as a stack. Adding more layers gives rapidly diminishing returns at best. Thermodynamics is cruel.

jiggawatts 5y ago
"No such thing as a free lunch." should be tattooed onto to foreheads of every charlatan claiming to be able to use water as fuel.