Concrete block rocket stove

This past Sunday, the middle school ecojustice Sunday school class cooked on rocket stoves. We based our stoves on design principles developed by Dr. Larry Winiarski, who is affiliated with the Aprovecho Research Center. A rocket stove makes more efficient use of biomass fuels (wood, twigs) through more complete combustion; this also results in fewer harmful emissions. According to the Aprovecho Research Center:

“Improved cooking stoves address at least 5 of the 8 United Nations’ Millennium Development Goals: [1] ending poverty and hunger; [2] gender equity; [3] child health; [4] maternal health; and [5] environmental sustainability.”

So while we don’t really need rocket stoves here in the Bay area (except perhaps in disaster situations), learning about and building them is a great introduction to using appropriate technology to meet ecojustice goals of human well being and environmental sustainability.

If you’re not familiar with rocket stove design principles,Aprovecho Research Center has an excellent introduction on this Web page. Scroll down and click on document no. 8, “Design Principles for Wood Burning Cook Stoves,” June, 2005.

Enough background. Here are instructions for building a concrete block rocket stove, followed by photos of our rocket stove in action:

Click the image above for a drawing of how to build our concrete block rocket stove. You will find other plans for a concrete block rocket stove on the Web, but those plans typically require a concrete h-block, an oddball type of block that we were unable to find. However, most bit home improvement stores carry 8 x 2 x 16 inch concrete cap blocks, and 4 x 2 x 8 inch concrete brick — two cap blocks and two concrete brick can be arranged in an “H” shape to make a stove. In fact, this is a better solution than a concrete H-block, because you can adjust the concrete brick such that you have a constant cross-sectional area throughout the L-shaped combustion chamber (see “Design Principles for Wood Burning Cook Stoves,” principle 7).

Above: The concrete block rocket stove after use. We placed two concrete bricks on the top on which to place cooking implements, etc. The bottom concrete block serves as a convenient place to store fire wood. Notice that our firewood is all salvaged building materials and wood pallets, split to appropriate size for burning.

Above: Cooking on the stove. “Design Principles for Wood Burning Cook Stoves” states that a combustion chamber with a 12 x 12 cm cross sectional area is “usually sufficient for a family sized cooking stove.” Our concrete block rocket stove has a cross sectional area of 12.5 x 15 cm. It put out a good amount of heat for cooking scrambled eggs for half a dozen people. Note that one person is feeding the fuel into the stove, while the other cooks — we found it was challenging to cook and tend the fire at the same time.

We did not try to boil water on our concrete block rocket stove, to see how long that would take. Maybe that’s a task for a future class.

Update, one year on: This has proved to be a good, but not excellent, rocket stove design. The chief problem with this design is that the concrete block acts as a fairly large thermal mass, and it takes a while to heat the block. Once the block is warm, the stove functions pretty efficiently; while the block is still cook, it’s not as good. Another problem is that the stove is finicky, and requires constant attention to feeding fuel in order to maintain a fairly constant temperature. Nevertheless, given the low cost of materials, and the ease of construction, this remains a practical design.