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Comment by malandrew | Hacker News Reader

I think they got the idea right, but designed it around the wrong source first. Using fuel to power my devices means I need to carry more fuel with me. Why do that when I can just carry cell phone backup batteries like this one: http://mylimeade.com/

Instead this should have been designed around something I can throw in a campfire with some sort of protective spiral metal cabling to snake out of the fire to a box with a USB connector. The metal cabling could be similar to those used on high temperature oven thermometers.

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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 :)

> 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-smalles...

MEMS (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.

Fully 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.

Ooops, 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.

I 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.

Thanks 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)

> Instead this should have been designed around something I can throw in a campfire with some sort of protective spiral metal cabling to snake out of the fire to a box with a USB connector.

That won't work, see:

http://en.wikipedia.org/wiki/Thermal_efficiency

Your problem is that the sink capacity of your device is closely related to its mass so unless you let something change phase (for instance, water evaporation) you're going to run out of capacity to store the heat before you have your phone charged. This will result in a lack-of-temperature-gradient which means you will no longer produce power. Compare with a Peltier element that is kept equally hot on both sides or equally cold.

could have something you put in the fire, with copper pipes coming out to a water turbine outside of the fire.

You need a temperature delta to generate the electricity. Where is the cool sink in your proposal?

The cold sink is the water. Water has a specific heat capacity of 4.2 J/g/K, so its temperature isn't going to rise nearly as fast as the hot side of the device, and you need to provide even more energy to get the cold side past 100F...

Wait I'm confused, where does fuel come into this? All you need to start a campfire is a bit of flint and kindling.

You don't need to bring fuel, assuming there's wood around you to burn.

For a given weight/volume surely a gas canister works out as more energy than a backup battery?

Liquid fuels are even better than compressed butane and propane, but whether they beat lithium-ion batteries depends a lot on how efficient the conversion is to electricity.

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