Gassifier stove / hot water heater

This is an interesting idea. I've wondered about a wood-fired water heater that could send hot water through a tube, too. I don't think the water could be compelled to move through the tube without a pump, though. It could be a mechanically simple pump, driven by heat, but it would need moving parts and fabrication by a skilled machinist. The thermoelectric generator (Peltier thermocouple) idea works. It has been used in quite a number of coffee-can sized woodstoves, for running a fan or charging a battery. A larger (and heavier) array of thermocouples would allow you to use a small off-the shelf water pump rather than a custom one.

I had three other thoughts about your assumptions: First, woodsmoke is much worse for you than the exhaust from an alcohol stove. I recently spoke at a convention where another speaker described his research into the composition of woodsmoke and the effects on people in third world countries who routinely cook over woodfires. He found that woodsmoke contains hundreds of carcinogenic compounds, including PAHs (polycyclic aromatic hydrocarbons) and dioxins. In his study of the effects, his group found that children from families that cooked over open woodfires had substantially lower IQs, poorer fine motor skills, and lower body mass than socioeconomically comparable counterparts that lived in homes experimentally equipped with cleaner burning chimney stoves. He corrected for every imaginable confounder in his analysis. The children in the two groups were in the same areas, in families of the same sizes, were the same ages, had the same health histories, etc.

The volume of exhaust from an alcohol stove is an order of magnitude smaller than a woodstove, it is much lower in particulate (essentially zero with HEET), and the duration of exposure is much shorter (because of expedient lighting and snuffing).

My second thought is that a small woodstove that pumps hot water through tubes will still need to be tended and fed. So having the pump and the tubes doesn't mean that you can leave it alone through a cold night and doze off. As long as you have to get up to add fuel anyway, you could be transferring hot water into a hot water bottle from a pot or kettle. What is the advantage of the pump and tubing over a hot water bottle?

Also, I think in general it is a good idea to be cautious about using gadgets to replace insulation. I think electrically heated jackets, chemical hand warmers, and a planned reliance on fire are unwise. It seems more prudent to me to just bring more insulation than you expect to need, and use fire to keep warm only in an emergency. This approach is lighter, simpler, cheaper, and more environmentally friendly.

As I said, it is an interesting idea. I don't mean to sound critical of it. It could be a fun engineering challenge for its own sake, at least.

You're in the clouds. I'm in the trenches (literally – I was laying wire in a trench from a PV panel to a water pump two months ago). Maybe we could meet in the mountains with a wood stove.

I like the double-walled container with the fire inside, a la the Backcountry Boiler, Kelly Kettle, Storm Kettle, etc. With an opening at the bottom for an air inlet (1) you won't get a nice high flow of chimney-effect air (which your fan would solve but that's more complicated), but (2) construction becomes much, much easier.

Whatever heat goes into the thermoelectric generator on the hot side, you need to dump 98% of that heat out the other side. The side next to the hot water container is great – good heat conduction there. The other side needs to be better able to dump heat or the thermoelectric generator just heats up and stops putting out power. It needs to be next to cold water or have good heat fins and air flow (maybe the fan inlet).

You can thermosphion water from a low heat source to a higher heat dump (e.g. water to be heated, cool air, cold feet, etc). But you need pretty big pipe/tubing, a short pipe run, and some decent vertical relief. I've done this a few times, failed a few times, and seen a very few good installations. One was a 55-gallon drum on a tree stand 8 feet above an open campfire with 10 feet of zig-zagged pipe in the fire. 1" hose connected the drum to the HX in the fire. Big pipe, mostly vertical pipe run – that can work. Small pipe going mostly horizontal and you'll need a real pump. Small by home standards but much more than a thermoelectric generator could power. The other way to pump water with heat is like a coffee percolator. Let it boil on one side of the loop and the resulting steam/water mix is much ligter than the incoming solid water and that can power a lot of flow, but you've got to get it boiling to work.

Instead of trying to pump water, take hot water bottles or hot rocks in clothing into your sleeping bag with you. As they cool, you can kick off the insulation and expose the warm water bottle more directly to your feet. Or if you're quilting it, put hot water bottles / hot rocks (but not too hot) under your sleeping pad. The insulation of the pad will moderate the heat flow up to your feet.

For reasons of constructability, think about the fan blowing down a separate tube (fully within the fireboox) into the bottom of the fire. The penetrations you show through the water jacket are tricky to fabricate and have to deal with thermal expansion from cold storage to being very hot in use.

> I cook in a well ventillated area, but every now and then I get a whiff, you know what I mean, that wiff that smells like chemicals.

An alcohol burner should burn pretty cleanly, resulting in just a hot, damp exhaust (CO2 and H2O). If you run the burner on its own, with no windshield, and no pan, have a sniff of the gas coming off it (the schoolboy chemistry method of wafting the gases towards your nose, rather than sticking your nose over the flame and burning your nostril hairs away…).

If it smells hot and damp, then the burner itself is probably fine. If it smells sickly sweet, then I'd suggest it's not burning properly, and you're getting partial combustion products.

Then try it with the pan and windshield in place. If it was clean on its own, but now smells sickly sweet, then there's a problem with the burner/windshield/pan combination: inadequate air supply, or inadequate flame gap, leading to quenching of the flame before it's burnt properly.

Caveat: I burn UK 'meths' aka denatured 95/5 ethanol/methanol (roughly), and have never used yellow HEET or methanol fuels.

Two problems I see with your colourful picture:

i) unless you have a battery, your Peltier cell won't produce current at startup, so you have a starved fire.
ii) If the Peltier cell is on the outside of the water jacket, there will be a lag before it starts to generate current (until the water heats up), even if the fire will burn before the fan starts running.

For your tent central heating system, how about replacing the electric pump with a little steam engine? The fan could also be steam-driven (with the same startup caveats as above). Much more fun than mere electrics; I mean, every boy wants to be a steam engine driver…

Then, whilst we're at it, let's add an espresso maker…

Hahaha. Thanks, but I'm a tea drinker :) I love your espresso idea.

Great thoughts from everyone above. I love thinking through this in a group! Please keep the ideas coming! So here's an update on the latest. I modified two, small containers and lit a fire inside…tried to light a fire inside, and retried to light a fire inside.

OBSERVATION ONE – Oxygen at the start
You're right, unless we start with a flow of oxygen, there's not going to be enough heat to get this motor running – meaning then we're talking about adding a battery, and I don't want to go that route. Another option is I could use a wider, more shallow container rather than the tall, slim container I'm using. I was trying to size the whole stove to fit into a lumber pack water bottle holder slot…I'll need to revisit that one.

OBSERVATION TWO – My fan sucks
I bought a pretty hefty computer fan blower. Even at that, it's not super. I want super. This thing doesn't kick out the power I want. I'm thinking about looking around the RC world of brushless motors for a powerful, micro motor that doesn't require a lot of power draw. And I need to improve the shape of my blower's fan blades, it's just not efficient enough.

SCRATCH THE HOSE IDEA
You sold me – If I'm going to need to add wood to this tiny stove, I might as well fill up a small platy bladder and take it into my bed with me. This takes the weight of a tube, pump/thermosiphon out of the mix. And I already have a small platy for my gravity water filter that could double as my night time hot water bottle.