An “Intervention Ladder”

This week ARC was contacted by two African organizations that inquired how to make $2 to $5 stoves like the Rocket stoves manufactured by Shengzhou Stove Manufacturer (SSM) in China. I responded by recommending that with a limitation of $5, in my opinion, it would be more successful to explore a pot skirt as a most effective first step to decrease fuel use and subsequently reduce exposure to PM2.5 due to burning less wood. Making the SSM Rocket stove requires lightweight, durable 0.7g/cc refractory ceramic that is quite difficult to make. Dr. Winiarski and I tried to make it without real success.

Based on lab and field experiments, here are my picks for the most effective interventions based on an estimated cost/benefit ratio:

Saving Fuel

Pot Skirt manufactured by SSM

pot skirt

#1 – Pot Skirt: a 10cm high sheet metal cylinder that forces hot gases close to the pot in a 6mm channel gap. (At high firepower, say over 5-6kW, the gap needs to be wider.) The pot skirt can fit a specific pot, be adjustable for a range of pots, or be permanently attached. Expected fuel and emissions reductions are around 25% for $1 to $2.

SuperPot study cover image

#2 – SuperPot: The SuperPot has a full length skirt permanently attached to the pot. The outside stays shiny and the skirt helps to adjust the air/fuel ratio increasing gas temperatures. The 6mm channel gap increases heat transfer efficiency. Click to read the UNHCR results of SuperPot testing in East Africa.

Protecting Health

Women cooking outdoors

#1 – Cook Outdoors: The WHO PM2.5 Intermediate Indoor Air Quality WHO guideline, designed to protect health, is 35 ug/m3. Preliminary results from boiling water outside under a veranda this summer with a Rocket stove in a gentle breeze resulted in a 93% reduction in PM2.5 between 0.6 and 1.6 meters above the ground. Previous studies found that the average breathing height of a seated woman was 0.6 m above the ground while the average standing height was 1.6 m. The data indicates that cooking outdoors is an existing strategy that works very well to reduce exposure. As a first step, where people already cook outdoors, try to encourage this strategy to protect health.

The Philips Designed Indian Chula with Chimney

Chula Stove with Chimney

#2 –Add A Chimney When Cooking Indoors: Field tests of biomass cooking stoves have shown that cooking stoves in houses usually emit something like 3 times (or more) PM2.5 compared to lab test results. (Roden, et al, Atmospheric Environment, 2008). Even the best performing Harris TLUD (0.7mg/min for Indoor Emissions of PM 2.5, LBNL) would not meet the WHO Intermediate Guideline of 35ug/m3 when the lab based emissions rate is multiplied by 3. (ARC recommends multiplying lab emission rates by 3 to get an approximation of real life conditions.)

The WHO 2014 Emission Rate Targets estimate, based on surveys, that chimneys in use remove 75% of the emissions from kitchens. Fully functional chimneys take essentially 100% of the smoke and gases outside of the house. However, many chimneys and stoves are not air tight and unfortunately in many places the smoke is not completely removed from the kitchen. Also, when pots are removed from the stove smoke often pours into the indoor air!

Reducing smoke concentrations by 75% makes the chimney the most powerful intervention that, when combined with better combustion efficiency, addresses both indoor and outdoor pollution. Really great 3 meter tall cement chimneys cost two dollars when I lived in India. Adding a Jet-Flame or other techniques that increase combustion efficiency would come closer to the top of an imagined “Intervention Ladder”.