# Thermocouple and cooling

**URL:** <https://boards.straightdope.com/t/thermocouple-and-cooling/946834>\
**Category:** Factual Questions\
**Created:** [July 19, 2021, 4:07pm UTC](https://boards.straightdope.com/t/thermocouple-and-cooling/946834 "2021-07-19T16:07:48Z")\
**Posts on this page:** 4\
**Page:** 3

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**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\
**Post date:** [July 20, 2021, 10:14am UTC](https://boards.straightdope.com/t/thermocouple-and-cooling/946834/41 "2021-07-20T10:14:47Z")

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> [@peccavi](#):
>
> Let’s assume a big cup of coffee (say…400 grams of water) near boiling (100 C) in a cool room (20 C). So, about 80 C of temperature difference.

> [@peccavi](#):
>
> but I’ll just guesstimate a best case efficiency of 10% (even laboratory best thermoelectric junctions have \<50% efficiency IIRC,

The best possible thermodynamic efficiency (Carnot Engine ) is  
= (temp difference in Kelvin) / (High temp in Kelvin)

= 80 / (273 + 100) = 80/373 ~ 20%

So that supports your 10% guesstimate

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**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\
**Post date:** [July 20, 2021, 1:18pm UTC](https://boards.straightdope.com/t/thermocouple-and-cooling/946834/42 "2021-07-20T13:18:23Z")

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The, presumably empirical, figure given in the article is “The device can provide a maximum boost of 0.36 Wh,”

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [July 20, 2021, 1:27pm UTC](https://boards.straightdope.com/t/thermocouple-and-cooling/946834/43 "2021-07-20T13:27:56Z")

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Which is 1.3 kJ, an order of magnitude less than @peccavi 's estimate.

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**Author:** ![peccavi](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peccavi/32/388_2.png) [@peccavi](https://boards.straightdope.com/u/peccavi)\
**Post date:** [July 20, 2021, 2:15pm UTC](https://boards.straightdope.com/t/thermocouple-and-cooling/946834/44 "2021-07-20T14:15:57Z")

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> [@Chronos](#):
>
> Which is 1.3 kJ, an order of magnitude less than @peccavi 's estimate

Yeah, I knew I was overestimating the thermoelectric efficiency. I finally looked up the numbers for commonly available TEGs and it is about 5-8% (best case for several hundred degree temperature differences at over a thousand degree Thot).: At a Th of 100 C, and a Tc of 20 C, it’s probably more like 3%. Add in the fact that you’re never going to get an 80 C temperature differential (more like 60 C) and the heat losses elsewhere in the cup are probably much bigger contributors than I was estimating, and ~1-2% is probably a better guess than I made.

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