Rocket Stove Efficiently Heats Water

Rocket stoves are an interesting, if often overlooked, method for cooking or for generating heat. Designed to use biomass that might otherwise be wasted, such as wood, twigs, or other agricultural byproducts, they are remarkably efficient and perform relatively complete combustion due to their design, meaning that there are fewer air quality issues caused when using these stoves than other methods. When integrated with a little bit of plumbing, they can also be used to provide a large amount of hot water to something like an off-grid home as well.

[Little Aussie Rockets] starts off the build by fabricating the feed point for the fuel out of steel, and attaching it to a chimney section. This is the fundamental part of a rocket stove, which sucks air in past the fuel, burns it, and exhausts it up the chimney. A few sections of pipe are welded into the chimney section to heat the water as it passes through, and then an enclosure is made for the stove to provide insulation and improve its efficiency. The rocket stove was able to effortlessly heat 80 liters of water to 70°C in a little over an hour using a few scraps of wood.

The metalworking skills of [Little Aussie Rockets] are also on full display here, which makes the video well worth watching on its own. Rocket stoves themselves can be remarkably simple for how well they work, and can even be built in miniature to take on camping trips as a lightweight alternative to needing to carry gas canisters, since they can use small twigs for fuel very easily. We’ve also seen much larger, more complex versions designed for cooking huge amounts of food.

19 thoughts on “Rocket Stove Efficiently Heats Water

    1. Gasifiers can be far more efficient:
      + you can add gas storage and use it later
      I still don’t understand why vacuum insulated “bell-stile” gasifier isn’t popular.
      All you need is any insulated container (couple of old steel bucked or gas cylinder good enough) fuel, I/O(draft, exhaust, gas line, optionally: heat exchanger with/without water tank , thermoelectric/Stirling generator or as separate unit hooked to gas and/or waste heat) goes in bottom(so not need drill your tank) just draft need to be above exhaust so heave draft push exhaust out, optionally top can be removable so you can use it as cooking stove/oven, as well exhaust can be piped in airtight greenhouse, so CO2/NO2 can be reused by plants/bacteria in soil/substrate.

  1. I love when Hackaday covers stuff like this. Rocket stoves are low tech/high efficiency, and perfectly in line with the stuff that gets covered here.

    I have a thermal mass rocket stove in my shop, and I love it. I can load it in the evening, and “charge” the thermal mass with efficiently burned wood. It’s a nice hot rock in the morning, and I reload it again in the morning to heat the shop. I heat 2000 square feet with less than a rick of wood per year.

  2. It would be interesting to know what the actual efficiency really is, and how it compares to a proper wood-fired boiler. Without proper measurements it’s just a wild guess.

    He shows several times a clear view down the heat exchanger section, and there is a large cross-section of flue gases that does not interact with the walls or the tubes. Much of the hot gas just escapes unimpeded. Surely that must impact efficiency.

    I didn’t hear mention in the video about how much wood was burned, so we don’t have any idea what the input energy was.

    Likewise, he makes a point of noting (near the end of the video) that the water entering the stove was cold, meaning some portion of the tank still contained cold water; not all of it was heated.

    1. Anything with a flue, uses most of the flue gases to create draft for the fire. Even the old central heating furnaces without blowers use about 40% of the heat energy to create draft.

      Forced draft allows you to make the heat exchanger more restrictive, but also more efficient, up to the point that water in the combustion gases starts to condense giving you another boost in efficiency. But at those temperatures, the draft is insufficient for proper combustion.

      In the case of a rocket stove, the builder typically wants independence from electricity, so using the heat to create draft is a reasonable compromise. If the builder wants more efficiency but still be independent from electricity, he can consider one of those hand cranked blowers sometimes sold for use with BBQs.

    2. I highly doubt this device is anywhere near as efficient as a modern wood stove, which either uses a catalyst or injects a secondary stream of superheated air to burn the smoke.

  3. I always enjoy watching TIG welding.
    For the burner, you ideally want to insulate your primary and secondary combustion area. The 3 components are fuel air and heat, but heat has a special place in this triad, unlike fuel and air, there is no limit to the amount of heat one wants. Fuel and air needs to be in balance, heat just maxed out. Once the secondary combustion is complete, then we extract heat.

    1. The water on the bottom of the tank is coldest as it heats up, it rises and enters the tank on top where it cools and falls to the bottom. Also the gradient in the stove this way is larger so the heat transfer is more efficient.

      The exchanger is pretty wimpy. I would’ve made the exchanger larger and higher up on the stove, e.g. surround the top. Routing the pipes through the stove seems overly complicated and cools off the combustion on the bottom. Introducing some turbulence into the exhaust at the top might be good enough to increase the contract between the exhaust and the updated exchanger. I would also try to deflect the exhaust with the exchanger so at least part of it is heated from the bottom.

      The build is interesting but IMHO poorly planned out. I’m also surprised that the builder wrecks so many cutting tools in the process given the access to a fairy sophisticated metal hah idk, I.e. I’m assuming the guy would be better skilled. Maybe he doesn’t have much experience with stainless steel which he seems to be using.

    1. “they are remarkably efficient and perform relatively complete combustion due to their design, meaning that there are fewer air quality issues caused when using these stoves than other methods.”

  4. One day I’ll have a house suitable for a rocket mass heater. It’s like a rocket stove on steroids if you do it right. The base is a rocket stove, but uses a ton of mass (for example, a couch made out of concrete) to store the heat in. The exhaust to the outside should be cold when done right. You can use that to heat a large space with a tiny amount of wood. Not only that, because of the efficiency, it produces a lot less smoke and pollution. It can use less than a quarter of the wood compared to a conventional wood burner.

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