Most of our household energy requirements come in the form of space heating, water heating or cooking, with these making up a large percentage of our monthly bill. Rocket stoves are an example of appropriate technology which can cover all of those needs, cost you next to nothing to build, and just a few sticks to run.
Rocket stove combustion systems deserve attention for a few reasons:
- they offer close to complete combustion of the wood, meaning they are hyper-efficient and burn super-clean
- they can reach very high temperatures, and can be hooked up to almost anything we want to use that heat for
- they can use wood typically considered too small to call firewood
- they are easily built from common materials.
That’s right – you can build these systems in a day or two, and then watch them turn twigs into heat far more efficiently than most wood stoves, with far less set-up cost. If you are good at scavenging bits they can cost virtually nothing to build, and when you prune your fruit trees you can get the fuel you need to cook dinner, heat your home, and enjoy a nice hot shower.
Clockwise from above: A rocket conversion on a conventional oven; Small offcuts of wood are all that’s needed to get the heat cranking; The drum style rocket stove is not only functional, it’s beautiful. Images by Joel Meadows and Dan Palmer
How Rocket Combustion Differs From Normal Combustion
The main difference between a normal fireplace or woodstove and a rocket stove is that rocket combustion is close to complete. When wood is burned it releases volatile compounds that we recognise as smoke or soot or creosote. In a rocket stove these compounds are sucked into the insulated and very hot ‘burn tunnel’ of the unit where they combust, releasing even more heat energy to drive the rocket process, unlike a normal fire where they are blown out the chimney.
This distinctive sucking of the flames down into the burn tunnel, and the resultant ‘roar’ is what gives rocket stoves their name. This is also a part of their magic. Rocket stoves are open where the wood is fed in, allowing lots of oxygen to be drawn into the unit. As the fire starts, and the burn tunnel heats up, the rising hot air races up the heat riser, drawing lots of air behind it. This incoming air flows into the feed tube and across the burning wood – creating the same effect as pointing a big air-blower at your fire. It gets really hot, the wood burns beautifully, and you hear the air roaring as it charges through the system.
In conventional wood stoves the air intake is small, and adjustable to even smaller. This lack of oxygen chokes the combustion, leading to cool burns, incomplete combustion and lots of smoke and creosote. In rocket stoves when combustion is complete, what comes out the chimney is pretty much only carbon dioxide, meaning no smoke: smoke means incomplete combustion, unburned fuel, or wasted energy. We are culturally trained to associate smoke with fire; but with rocket stoves, sometimes when there is no smoke, there is still fire!
Having ensured complete combustion, and hence maximum heat generation, only then do we think about using the resultant heat. Conventional wood stoves are sucking heat away from the combustion chamber for cooking, space heating or with ‘wet back’ water heaters, meaning the unit just can’t get hot enough for combustion to be complete, resulting in smoke. The combustion chamber of the rocket stove is heavily insulated to prevent premature heat loss, and an enormous amount of heat is generated by the efficient combustion process.
What To Do With All That Heat
When combustion is complete, we can extract heat from the stove at the top of the heat riser. And here is where your imagination and inner inventor can run wild: anything you can think to do with very hot air can be done from this point. Here are a few things that can be powered with a rocket stove.
Oven – either a converted conventional oven with the rocket under it, or a purpose built barrel oven from old ‘44 gallon’ (around 200 litre) drums.
Cooktop or BBQ – old cast iron BBQ plates can work well, or heavy copper radiator plates to put pots and pans on, or just cook over the stream of hot air.
Hot-water systems –we’ve made various systems, from ones that heat a big-vented tank of water with a copper coil running through it, to heat exchanger thermosiphon units boosting solar hot water.
Space heaters – often seen with an inverted ‘44 gallon’ drum being the main radiator, with the flue running through heavy earth or stone elements (rocket mass-heaters).
Take care! A well-built rocket stove can work so well it can be dangerous. Dan’s rocket oven once got to 180 °C in six minutes, 230 °C in seven minutes, and 325 °C in thirteen minutes! As for Joel’s latest rocket oven, on his first test run the whole inner skin of the feed tube was glowing red, as was the bottom of the inside of the oven: the thermometer read 350 °C, and the roast vegies were pretty charred (he has since tamed it down to 200 °C).
While quite popular in some parts of the world, rocket stoves are a relatively rare sight in Australian permaculture systems. They are an excellent example of an appropriate technology alternative to conventional means of heating food, water or space; and they are well worth finding out more about.
A complete guide to making your own rocket stove by Tim Barker (with illustrations by Joel Meadows) is now available as an ebook. Available at www.pipmagazine.com.au. Free for current Pip Magazine subscribers, email email@example.com to order your free copy.