This year’s extreme affordability class included another team who worked with IDE Myanmar to continue refining the cook stove design that our team began last year. Fuel.d, the team made up of Lauren Hult, Tomas Pueyo, Santhi Elayaperumal, and Taiei Harimoto let us drop in on one of their testing sessions in June to see the pot skirt they designed and the merits of placing it over a three-brick stove so that it acts like a heat shield. Though it is not as efficient as replacing the three-brick stove with a rocket stove, pot skirts can be made cheaply, and could offer a lower price solution for those who cannot afford to buy a stove immediately.

Fuel.d also gave us great suggestions for making our stove design more robust and efficient. We can’t wait to try out their ideas in our next prototype!

Word of the day: Frustum-a portion of a solid that lies between two parallel planes cutting the solid.

Our inverted lampshade-like pot skirt is a frustum of a cone, or so we discovered when we tried to construct it from sheet metal. It’s not as easy as it seems; you can make a cylinder from a rectangle or a cone from a semi-circle, but a frustrum is constructed from a “rainbow”-shaped template whose dimensions determine the height and diameters of the part’s top and bottom surfaces.

Because we didn’t know that our pot skirt was formally called a frustum, figuring out how to build it was an adventure. Matt (a former teammate) and I had a friendly competition: he tried looking for the equation on the Internet while I made a barebones paper frame of the shape we wanted and then unfolded it to get a rough outline of the template. I like to say I figured it out first, but he found a neat little frustum dimension calculator that proved very useful for trying different pot skirt heights and widths!

I mentioned in my last entry “Skirting the Issue” that most pot skirts are actually cylindrical. So why in the world did we go through all this trouble to make such a tricky shape? There is an ideal gap for efficient air flow (about ¾”) between the pot and the pot skirt. When the team was in Myanmar, we saw that the women there cooked with an astounding range of pot sizes , with the smallest one having about a 5″ diameter, to the biggest one about 16″. To make matters more complicated, women often cooked on a wok as well. Thus, our pot skirt had to be designed to create the same ideal gap for multiple pot sizes. The “inverted lampshade” shape allows smaller pots to sit lower and bigger pots to sit higher on the skirt, creating a reasonable gap for air to pass through.

However, one of the difficulties with this design is that women have to reach further inside the pot skirt to take a small pot out. Since the air between the pot and the skirt is VERY hot, this poses a significant burning hazard for women. This safety issue is one reason why we are still hard at work at improving our initial concept!

A big component of our stove is its insulation.  Between the inner chamber (where combustion happens) and the outer walls is a lot of space.  If we simply left it empty, then hot air would move around and carry heat from the inside to the outer walls in much the same way your oven cooks food.

The obvious answer is to use insulation.  But what kind?  Often ceramics, sand, and earth come up.  It’s true that these would slow down the transfer of heat from the inner wall to the outer wall, but they do it by absorbing energy.  In scientific terms, they have a higher specific heat capacity than air which means that, for every degree they change, they absorb more energy.  This is helpful for safety, but it can actually decrease the efficiency of a stove.

In order to have an efficient stove, you need materials that don’t transfer heat very well–or, in other words, materials that are not very thermally conductive.  Air is actually pretty good with respect to this, but it’s a problem when it moves around freely.  Many of the best insulators simply work by trapping air in small pockets.  This is how down blankets work.

Our initial prototype used perlite, but it’s not readily available in Myanmar, so we’ve been exploring the possibility of importing it and other materials.  Ash is readily available and a pretty good insulator.  We’ve also been looking at special clay mixtures which are much lighter (and thus have a lower specific heat capacity) than normal ceramics.

As always, we’d love to hear from anyone with thoughts/advice on the manner!

A year ago, after we had just completed a prototype of a stove design we liked (it was literally made from an old metal trash can!) we decided to try adding a “pot skirt” to see how much it would improve our stove’s efficiency. We had read about these heat shields in other stoves before; the basic idea is to surround the pot on all sides with a thin metal shield that creates about an inch-gap for hot air to pass through. The pot skirt thus prevents heat from the stove from escaping to its surroundings, keeping it close to the walls of the pot and increasing the heating surface.

The idea sounded interesting, so we set to work building a prototype and testing it out! (My next post, “Frustrated by Frustums” will explain why our prototype is the shape it is-most pot skirts are actually just cylindrical.) The result was… WOW.

Adding the pot skirt cut down our rocket stove’s boiling time almost in half. The rocket stove itself was already able to boil water in maybe half the time it took an open flame, but with the pot skirt, we were boiling water at incredible speeds: up to a third of the time it takes to boil water on an open flame. Whatever we expected, I don’t think it was as drastic as this.

As great as it may sound though, our pot skirt idea still needs a lot of work. Adding that much material to our stove makes it more expensive and difficult to manufacture, not to mention clunky looking. Because it traps hot air, the skirt metal gets really hot too, posing a potential burn hazard to our customers. Our first prototype didn’t allow the cook to see the flame very well either (we fixed that issue by cutting out holes into the skirt).

Right now, we’re starting to explore new skirt designs, as well as some completely new concepts. One of the things we’d like to try is something similar to a “heat exchanger” which traps hot convective air at the bottom of the pot. The Jet Boil camping stove uses a mechanism like this. Such a design wouldn’t increase our heating surface area, but it will at least increase the concentration of hot air, which may be enough.

If you have any ideas about this engineering challenge, we’d love to hear them!

We’ve been talking a lot about our philosophy approaching this project, but I think it’s time to explain a little bit more about our actual stove technology. The stove prototype we’ve built for women in Myanmar is based on the Rocket Stove created by Dr. Larry Winiarski at the Aprovecho Research Center. Many such stoves have been modeled after Dr. Winiarski’s design, including the Berkeley Darfur Stove as well as other home-made versions (there are tons of videos online for how to make your own).

We chose to adapt the rocket stove design because its use of low mass insulation is very effective at keeping the fire hot and transferring most of the heat to the pot.  In my next couple of posts, I’ll explain how a hot fire and a design that channels heat to the pot are both important for improving the overall efficiency of the stove (I’m trying to keep my posts shorter :p).

If you’re wondering why we chose to design our own version of the rocket stove rather than partner with one of the other organizations that are building them,  please read our post about the importance of tailoring the stove to the local culture.  We aren’t seeking to compete with other groups; instead, we learn from them and share our own ideas. Building on someone else’s great technology also gives us time to focus on equally important issues of manufacturing, marketing, distribution, and education, all of which are crucial in getting our stove into the hands of women.