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!

Recently at the Cool Product Expo, we got a lot of great questions from people.  As I was explaining our goals for the project, one man asked me, “what do you mean by sustainable?”  What a great question!

Our goal from the start has been to create a cook stove that can be completely sustained within a local market.  One aspect of this is economic sustainability.  From the start, our cook stove has been designed to be as cheap as possible.  It needs to be a product that families living on $1-$2 a day can afford and one that saves them money over time.  (Current prototypes currently “pay for themselves” in about 3 months and are priced below what most potential customers say they would pay.)

But, even if it makes sense economically, our cook stove needs to be something that people want to buy and are willing to assume the risk on.  It’s surprising to some people, but marketing plays a big role even in the developing world.  This is especially true of anything that’s perceived as a risk or an “unknown”.

Sustainable also means being manufacturable using local (or easily importable) materials, tools, and know-how.  Shipping costs are of course a concern, but helping build local industry and entrepreneurship know-how is equally important.  It also fits nicely into our design philosophy of building prototypes, getting them into the hands of users, getting real feedback, and doing it often.

Sustainability is at the heart of what we’re trying to do.