5 ms·
The cable isn't likely to be the limiting feature here. The method of energy conversion doesn't just require heat, it requires a temperature gradient across the
by shabble 13y ago
The cable isn't likely to be the limiting feature here. The method of energy conversion doesn't just require heat, it requires a temperature gradient across the device. Here they're putting the tip in the fire to heat it, and the little bowl at the back full of water is used to cool the other end, thus getting a good temperature difference.
In contrast, throwing hte whole thing into the fire might work briefly while the heating was uneven, and the water hadn't all boiled off, but as soon as it reaches the same temp all over, no more power. And if it's all in the fire, even the water evaporative cooling isn't going to help much, vs when it's out of the direct flame/hot air stream at the edge.
I imagine you could design a in-fire version using a heatpipe to transfer energy out to a radiator/cooler somewhere, but that would be much more fragile and/or expensive.
What we really need is MEMS (or at least, really teeny, zero-maintenance) Stirling engines :)
- jacquesm 13y ago> What we really need is MEMS (or at least, really teeny, zero-maintenance) Stirling engines :) Really teeny: http://www.popsci.com/science/article/2011-12/worlds-smallest-stirling-engine-single-particle-just-three-micrometers-across http://www.popsci.com/science/article/2011-12/worlds-smalles...
- ori_b 13y agoMEMS (or stirling engines) don't help you there either -- as long as it's all in the fire, you have trouble extracting energy. All heat engines run off of a temperature gradient, which is limited by Carnot's theorem to a maximum efficiency of (1 - Hot/Cold). If your heat source temperature is the same as your heat sink temperature, you get zero energy out of it. The bigger the difference between the heat source and heat sink in your engine, the more energy you can extract.
- cnvogel 13y agoFully agree, the only small remark being that it's η = 1 - Tcold/Thot and to put in perspective: If your metal rod on one side of the peltier element (that's inside the device) is at 150°C = 302° = 423K and the cold side (water reservoir) is at 20°C = 68°F = 293K the maximum achievable efficiency is 30%. In practice you'll waste a lot of energy (probably the flame outputs about a kilowatt of power, normally you'd comfortably use it to heat a kettle of water or canned spaghetty or...) for a meager 2W or 3W of power to your cellphone. That's because the energy transfer from a flame to a simple rod is very, very inefficient.
- shabble 13y agoOoops, I didn't mean to imply that MEMS/stirling engines would eliminate the gradient problem, it was something of a after-thought that they would be more efficient than the Seebeck junction.
- jlgreco 13y agoI would still be concerned about the cable though. If it uses a rugged metal cable, then the worse case scenario is that it stops generating power when your fire gets a little out of control. If you're just using the standard rubber insulated cable, then you might ruin your cable and be SOL.
- malandrew 13y agoThanks for the correction. I'm wondering if it can be two sides where you can put one side in a bucket of water (fetched from a nearby river or lake or created from melted snow water)