The Walden Effect: Farming, simple living, permaculture, and invention.

Rocket stove

Rocket stoves are currently being introduced to several third world countries to help lower the pressure of firewood harvesting on native forests.  The stoves are designed to need very little wood in order to heat up your cook pot, so trees get left in place.  I love the concept, but can't help wondering --- why don't we promote rocket stoves in the U.S. too?  I'd never tell someone in a third world country to institute environmentally friendly measures I wasn't willing to put into practice in my own life.

Before I knew it, I'd penciled a rocket stove onto our ten year plan and started researching.  First, I discovered that you can't use rocket stoves inside because they're basically an efficient hearth.  So, in practice, they'll probably be part of a summer kitchen in our long term plan --- something I want anyway because I always dread turning on the stove on a sweltering summer day.

The video I've embedded above is well worth watching if you'd like to build your own rocket stove.  It looks like we could probably make one quite cheaply, though it would take quite a bit of trial and error to figure out certain parts.  The sheet metal looks an awful lot like a stovepipe to me, suggesting that we might not need welding skills (the part that scared us off building our own initially.)  Alternatively, we could buy one pre-made for around $125.

Have any of you built or used a rocket stove?  What did you think of it?

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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.

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That is a stovepipe for sure. I can pick up the bricks at the local brickyard, that few they will probably just give me. Or maybe look for a house being torn down and see if I could get some out of the fireplace. What do you use for a base.
Comment by Erich Tue Feb 2 11:36:00 2010
The bricks are supposed to be a bit different from normal bricks, it looks like --- they're more like fire bricks maybe? Anyway, they're very light weight and insulative. But the method to make them didn't look that hard!
Comment by anna Tue Feb 2 12:44:58 2010

In 2006, Philips Research developed a somewhat high-tech efficient woodstove. Later, in improved chulha was designed for local production in India. IIRC, the latter could be manufactured for as little as $8.

As for the rocketstove, It seems quite complicated to make. I would try to simplify that and make a single body of clay around a cardboard pipe, and fire that in one go. E.g. like this refactory furnace made from refactory mix (with lots of perlite). Since you can actually melt aluminium in these refactories (with e.g. propane of charcoal) it's pretty fireproof.

Even simpler would be to take a big metal bucket, make a hole in the side, put an L-shaped stovepipe in, and fill the space between the pipe and the bucket with perlite or maybe even glasswool. In that case you don't even have to build a kiln to fire your bricks. :-)

Comment by Roland Smith Tue Feb 2 15:42:50 2010
I love your thinking outside the box (aka simplifying!) I like your idea of making one solid brick inside a pipe. I wonder if your second option would be just as good? I don't have a clue how to test which is more efficient...
Comment by anna Tue Feb 2 16:32:00 2010

At a guess, I'd say the last option (L-pipe in bucket with glasswool in between) is the most efficient;

  • very easy and cheap to make (COTS parts)
  • extremely well isolated (better than firebrick, I think)
  • small thermal mass of the (metal) inner pipe, so heats up quickly

The inner pipe should be steel, not aluminum I think. The latter might get too hot. The thing is that the thermal mass of the metal innner pipe might be too small. That might influence the draught that you get. If that is the case a clay drainpipe works better, but it will obviously take longer to heat up.

Since all these materials are off-the-shelf parts, it should not be too costly to experiment. It shouldn't be too difficult to get an outer bucket or tube big enough that you can fit either a clay or a metal tube inside.

Comment by Roland Smith Tue Feb 2 17:13:28 2010
Ever seen a tradition round Cajun smoker charcoal bbq? pretty much every thing but the brickworks inside to make a nice rocket stove. I have a couple - I find them a garage sales & at the ReStore so when I'm in a smoking mood I can really get busy. Otherwise I'd be looking for some metal drums to play with. Stove pipe is out of control expensive because it has to be made to specific codes & specs. I might mix some of this up when I'm considering my outdoor kitchen ovens & such. Great video!
Comment by Titus Blackwood Tue Feb 2 18:49:25 2010

Titus --- I'd be curious to see how the Cajun cooker's efficiency compares to a rocket stove. Surely if it was so simple, people wouldn't go to the trouble of making a rocket stove. (But what do I know? :-) )

Roland --- I've been reading about efficient space-heating wood stoves too, and it sounds like the thermal mass might be important. I'm not sure, though... It sure would be nice if there was a simple way to put a rocket stove together using over the counter supplies like that! We'll have to start experimenting....

Comment by anna Tue Feb 2 20:00:28 2010

Remember that there are marked differences between a stove (where you want to put the heat in the stuff you're cooking) and a space heater (where you want to heat the house).

For a stove, it is the hot exhaust gasses flowing along the pan that heat it. So you want those gases as hot as possible with as little fuel as possible.

For a space heater, you want to extract as much heat from the fire and exhaust gasses as possible. You can make the stove a big heat-sink, and have it radiate into the room and/or use the gases to heat a medium (usually water) that you can pump around the house and heat the rooms you need heated. The latter is pretty efficient; water is a much better heat transport medium than air.

For heat transfer to be efficient, the temperature difference between the fire and the environment must be as high as possible, that is simple thermodynamics. Combustion efficiency is influenced by a lot of things like temperature, airflow and air temperature. You might find this article on clean combustion of wood of interest. Of course googling for 'clean combustion of wood' gives a ton of links. I found the California wood burning handbook informative.

Of course the first step to efficient heating is good insulation. :-)

Comment by Roland Smith Wed Feb 3 12:48:23 2010
I keep going back and forth on whether it's worth it to insulate the trailer better. I agree with you that good insulation is probably more cost-effective than buying a more efficient stove --- but probably not if you have to add a whole roof to do it. We'll probably just keep building well insulated additions until the trailer becomes obsolete. :-)
Comment by anna Wed Feb 3 19:38:36 2010

First, I must admit that I know squat about US trailers. But most caravans that I've seen have a frame with exterior and interior panels. Most modern ones that I've seen actually have insulation between these panels. Does yours? If not, there are ways of adding it without tearing off a whole wall. You could e.g. drill small holes and spray in PU foam from a can. Don't use too much, though. :-)

If you are willing to sacrifice a couple of inches of living space, you can always bond rigid PU or styrofoam plates to the inside walls and cover them with drywall.

Comment by Roland_Smith Fri Feb 5 17:51:29 2010
We do have some insulation in between the inner and outer walls and above the ceiling, but I'd say it's vastly below what we're putting in our shed. :-) I'd love to add more, especially to the ceiling, but Mark is so tall that he'd bump his head if we did it on the inside! Lots of folks around us get around this problem by building a roof on their trailer and filling that with insulation, but I just don't know if that'd be worth it...
Comment by anna Fri Feb 5 18:01:48 2010
Oh, I should add --- the walls are framed with two by twos instead of two by fours, and the ceiling with two by fours instead of something bigger which probably gives you an idea of the current insulation level.
Comment by anna Fri Feb 5 18:03:13 2010

Your major deficiency in your trailer is most likely windows. I am assuming you have only single pane windows. I don't really see re-insulating walls or the ceiling around inefficient windows. I like your modular idea. I had a friend that moved to Colorado with the dream of living of the grid. His plans fell through and that winter, he found himself renting someones summer retreat. There was the yurt, which had running water and electricity, that housed the kitchen area and true bathroom. Then there was the cabin (about 100 yards away), a 10 by 8 ft well insulated bedroom with a sink and toilet, lights and car radio powered by a single solar cell and battery. His utility bills were very low as he only kept the yurt just above freezing with electric heat (no option) and heated the well insulated cabin with just a small efficient wood burning stove.
I think this would be a better use of resources. The only problem would be if you were trying to keep all these different buildings heated.

Comment by Erich Fri Feb 5 23:25:07 2010
You could still upgrade your isolation by replacing the isolating material. For example, glasswool has a heat transmission value of lamda = 0.035 W/(mK), so it transmits one Watt of heat per square meter per Kelvin of temperature difference. Now take PIR foam (polyisocyanurate) has lamda = 0.024 W/(mK). So for the same thickness and temperature difference, it transmits 30% less heat. Phenolic foam can be as low as lamda = 0.016 W/(mK), so it would transmit around half the heat that glasswool does at the same thickness and temperature difference.
Comment by Roland_Smith Sat Feb 6 05:24:17 2010
Oops. 'one Watt' should be '0.035 Watt'.
Comment by Roland_Smith Sat Feb 6 05:27:25 2010

Erich --- you would usually be right on with your window comment, but it turned out that we bought our trailer after all of the windows had been taken out, so we had to replace them. I'd been saving up building materials for a while, and I had a bunch of really nice, double-glazed windows, so that's what we have in there. Granted, even double-glazed windows use a lot of heat at night --- maybe our first step should be to find some winter night covering for them?

I like your idea of moving into a small, easy to heat space in winter. Mark wants to build some hobbit caves in the side of the hill someday --- maybe that's where those will fit in!

Roland --- good point about changing insulation materials to fit in the same space! We'd have to do some serious saving for that, though --- when I was pricing insulation, solid panels seemed to be much more expensive than fiberglass. I don't seem to remember seeing phenolic foam --- I'll have to check on that.

Comment by anna Sat Feb 6 08:04:32 2010

It might be known under different names;

  • resol foam
  • phenolic foam
  • phenol-formaldehyde or PF foam

Basically it's the same materials as used in the production of Bakelite (but with foaming additives instead of wood flour). One supplier is Igloo Thermal. Their vacuum insulation panels have en extremely low thermal conductivity of 0.005 W/(mK).

Comment by Roland_Smith Sat Feb 6 09:03:10 2010

I recently discovered your blog and am catching up on it. Your goals and execution with "The Walden Effect" are impressive. I have been reading about your wood stove considerations and was ready to post a link to the rocket stove. It made me wonder once again how you manage to keep up the pace you do in blogging and researching permaculture and homesteading techniques while putting so much labor into developing your homestead. Kudos.

Your comment in this post reminded me of something i heard Joel Salatin of Polyface Farm say in "Meet the Farmer" on Farmer TV (youtube): "Agricultural truth is when the model is just as applicable and current in a high tech industrial country as it is in a developing country."

The mentality of affluent western nations for centuries has been seeing the "poor savages" as special cases in need of special treatment, with the resulting treatment alternating between the condescension of "us and them" economic privileges and the deleterious patronizing assistance to join the modern consumers.

This growing idea that perhaps we should meet them in the middle is well overdue.

Comment by Pouletic Thu Nov 25 09:06:06 2010
Pouletic --- I appreciate your thoughtful comment! I completely agree that anything we push on poorer countries should be something we're willing to use ourselves, and I've been wanting to move our summer cooking outside anyway --- it just gets hot in here in July with the stove on. Hopefully in the next couple of years, the rocket stove will make it on our experimenting list and we'll have one to try out.
Comment by anna Thu Nov 25 09:44:50 2010

Like the previous commenter, I just recently discovered your blog and am having a lot of fun perusing your archives and learning about your homestead. We share similar goals and dreams. Anyway, as soon as I read about your rocket stove idea, I remembered reading about one in action on another blog I follow and thought I'd send you the link for inspiration:

Good luck! You have a new subscriber.

Comment by sarah Wed Dec 15 14:43:04 2010
Thanks for sharing! They look like a very interesting group.
Comment by anna Tue Dec 21 10:06:07 2010

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