A couple of weeks ago, Claudia Truesdell and Shane Washburn from Google’s X – The Moonshot Factory visited us for the day. We made lunch on an earthen Rocket stove and on a metal pot skirt stove with Jet-Flame while doing a field test that generated some numbers. Google’s X was interested in what we have learned since 1976. 

Our staff had various things to share. I found myself explaining how to improve fuel efficiency in high mass stoves. Maybe ~95% of biomass stoves in use are high mass (?), made from earthen mixtures. 40% clay and 60% sand works well, especially when a relatively dry mix is pounded into a form. Metal is hard to find, expensive when purchased, and even stainless steel doesn’t last for a very long time near the fire. Improving earthen stoves might help lots of people.

When we did seminars with the Partnership for Clean Indoor Air, we showed that fuel use could be reduced by a minimum of 30% by:

• Moving the flame further away from the mass walls.
• Lowering the pot closer (but not too close) to the fire to increase the temperature of gases contacting the pot .
• Using a constant cross sectional pot support to increase heat transfer efficiency to the bottom of the pot.
• Adding an adjustable pot skirt that, especially when tight, forces the hot gasses closer to the sides of the pot.
• Nowadays, we suggest adding a Jet-Flame to improve both heat transfer and combustion efficiency.
• Most importantly, as recommended by Mahatma Gandhi, adding a functional chimney!