Over the Moon

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 the CQC 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!

A chef cooks at a stove with a flaming wok

Turn Down Ratio: Cooking and Heating Stoves

The successful stove delivers the needed amount of heat to perform a task. In Haiti, our little charcoal stove could not bring the big pots of rice and beans to boil although the thermal efficiency at low power (simmering) was above 40%.

In the same way, a tight, well-insulated house requires a low amount of heat to stay warm. A “leaky” house needs higher firepowers to replace the constant flow of hot air lost through cracks under doors, etc.

Generally, a good stove has a minimum three to one turn down ratio. Heating stove experts have suggested a high power of 5 pounds burned per hour and a low power of 1.5 pounds. If a heating stove cannot turn down sufficiently, the tight, well-insulated house gets too warm. On the other hand, a “leaky’ house needs a big fire.  To save fuel, a tight house is more important than a new stove.

To boil 5 liters of water in less than 25 minutes in an uncovered pot, low mass cook stoves with tight pot skirts typically need a high power between 3kW and 2.5kW. However, firepower is often a lot higher when cooks are trying to get food on the table. In our experience, many cooks prefer ~ 5kW. 

Experiments at ARC have shown that with a low mass stove, lid, tight skirt on a 5 liter pot, it takes only ~0.4 kW to maintain a 97°C simmering temperature. But, cooking requirements vary a lot from country to country. Chinese cookstoves tend to use 10-15kW and may not need low power. 

Village cooks in Southern India cooked with many small pots and often did not bring the water to a full boil. For India, Dr. K. K. Prasad proposed, “…an ideal burner design with the power output ranging from 2.64 kW to 0.44 kW… (Prasad and Sangen, 1983, pp. 108-109). Dr. Baldwin adds, “One of the most important factors determining field performance of a stove is the firepower it is run at during the simmering phase. Because simmering times tend to be long, quite modest increases in firepower above the minimum needed can greatly increase fuel consumption.” (Baldwin, 1987)

With careful operation, the heat exchanger efficiency of houses and pots combined with delivering the appropriate firepower largely determines the fuel used per task. 

Dr. Tom Reed: “Smoking Backwards”

Dr. Tom Reed: “Smoking Backwards”

Our beautiful friend Tom Reed (1926-2018) at a Winter Stove Camp

ONE: Starting in 1985, Tom invented the natural and forced draft Top Lit Up Draft stoves (TLUD). He would demonstrate one of the clean burning principles at Stove Camps by lighting the tip of a cigarette and then sucking on the burning end in his mouth. With practice, he did not burn his tongue.  Remarkable!

Sucking the woodgas through the burning end of the cigarette cleaned up a lot of the harmful gases and protected him from most of the smoke. Similarly, lighting the top of the fuel bed in a vertical cylinder pulled the made woodgas up through the charcoal bed and flame in a TLUD. (UP DRAFT)

TWO: In the same way, sticks of wood can be placed in a horizontal cylinder, lit under the short chimney of a Rocket stove, where the made woodgas is pulled through the burning combustion zone resulting in cleaner combustion. (HORIZONTAL DRAFT)

THREE: We tune stoves under the emissions hood to reduce emissions by combining the proper 1.) amount of metered fuel turned into woodgas, 2.) temperature, 3.) air/fuel ratio, 4.) mixing, 5.) 100% of woodgas pulled into the flame, and 6.) long enough residence time of woodgas in the combustion zone.

FOUR: Before iterating changes in the stove to reduce emissions, we try to optimize heat transfer efficiency so the least amount of biomass completes the task.

FIVE: Tom, we remember you so fondly! Thanks for the help!

Rocket Bread Ovens

I ran across this old video when exploring Rocket stoves on YouTube. It is great to have both peer reviewed journal articles and places like YouTube. I hope that our publications library at aprovecho.org includes, perhaps even combines, helpful information from both academia and the field.

We baked with a high mass bread oven for years at the Aprovecho campus. We bragged about it. It was only when Dr. Winiarski built a Rocket bread oven next to the old stove that we experienced the differences. Then, it took less than a week for baking to switch to the Rocket oven. No more throwing logs into a mud/sand cave for hours to try to get up to temperature! Why try to heat up a thousand pounds of mud when you want to make 20 pounds of bread?

Larry’s Rocket oven got to 400°F in about 30 minutes and used hand fulls of twigs to bake enough bread for the hungry eco-scientists. I loved the Rocket bread ovens and helped Larry to build them in Mexico. We learned a lot from Larry including that using a new stove was a lot more convincing that talking about it. Why not let the new oven or stove speak for itself.  Then stand aside and watch when folks like it enough to improve the prototype? 

So nice to let the experts (the cooks) tune the cooking functions of a stove! So nice to empower their hard won expertise! So appropriate to sit back and learn.

red sign with white letters reading Wood Burning Prohibited

Clean Burning of Biomass

red sign with white letters saying Wood Burning Prohibited

Scotland has banned the use of climate polluting home heating systems such as oil and gas boilers, and wood burning stoves (except in cases of need) in new construction. Heating homes creates one fifth of Scotland’s CO2e. The plan is to switch to electric heat pumps, hydrogen and tighter, better-insulated homes in an effort to achieve carbon neutrality by 2045. https://www.bbc.com/news/uk-scotland-68778757

Oil and gas (fossil fuel) burners create too much CO2. Burning renewably harvested biomass can emit close to zero CO2, but old stoves make too much black smoke which is ~2000 times worse for climate change compared to CO2 by weight.

Heating stoves that burn sustainable biomass cleanly enough to protect health and climate are starting to become available. Testing the new generation of stoves in use will show whether biomass can join solar, wind, and hydro as a useful renewable energy resource in the post fossil fuel era.

Maybe those red signs will become green?

Green sign saying Sure, Light Up
bed of charcoal in rocket stove


Boman reports that temperatures above 850°C, in a 5kW combustion zone combined with air rich and well-mixed conditions for 0.5 seconds in the post combustion zone, resulted in an almost complete depletion of particulate matter (Boman et al., 2005).

How can we achieve 850°C in a combustion chamber? 

Yellow flames are around 1,100°C. but: 

  1. Heat flows in one direction, from hot to cold.
  2. Mass, charcoal, and incoming air (primary and secondary) are usually much colder.

Elevating temperatures to 850°C in the combustion chamber is not easy but it is possible!

  1. Use a thermometer.
  2. Create hot fires.
  3. Reduce mass as much as possible.
  4. Replace mass with insulation.
  5. Inject pre-heated air into charcoal to create high temperatures, especially in the top of the pile. Charcoal is a good insulator and when the top of the fuel bed is not red hot the effect can be cooling.
  6. Minimize the pre-heated incoming air, maintaining a minimum 3 to 1 fuel/air ratio.
Three images - Icy tree limbs, a man with a chainsaw, a car full of logs

Ice Storm 2024!

On January 14, ARC staff experienced a catastrophe when ice brought down about one third of the trees in our valley and we had no electrical power for seven days. Depending on the grid for heat and light is great when everything works. However, when the lights go out, a warm house becomes really important. Light is easy to make with photovoltaics, etc. Living for a week in the cold was a great reminder of the importance of a clean burning non-electric biomass heating stove. 

I lived without electricity on a ranch in Mexico for eight years where it was warm pretty much all of the time. Even so, on chilly nights we lit an open fire and sat around it, talking. The old folks pulled blankets around themselves, too.

I do not remember missing refrigeration or lights. It was not a big deal. At the ranch, people had lived for over a hundred years without them and much more energy went into trying to find juice for the little radio.

Going for a week without electricity reminded me that a warm house is important. In response, ARC is building health/climate non- electric pellet heating stoves as winter turns into spring.  If the creek don’t rise, we want to have these stoves in all buildings before next winter.

PS: I am fine reading a book by candlelight as long as my feet are warm.

thermal image of house juxtaposed with daylight image of same house

COP 28: Near-zero emissions in global building sectors

thermal image of a house juxtaposed with daylight image of same house
Heat can constantly leak out of older homes. Photo: Gina Sanders

Aprovecho is investigating how to design and manufacture biomass-heating stoves that protect health and climate when burning renewably harvested biomass. Of course, staying warm depends on many factors including how much energy is being leaked from the building.

Net-zero buildings are usually tight and well insulated. A net-zero home can have a heating load of 10,000 to 15,000 Btuh (or ~3 to 4 kW) in a cold, northern climate. At COP 28, a minority of nations agreed to move towards net zero homes to reduce climate change by heating the better buildings with renewables. Green Building Advisor: 28 Countries Sign Buildings Breakthrough Agreement at COP28

Since the 1970’s, architects and engineers have learned how to dramatically reduce energy losses in buildings. Many net-zero homes take advantage of solar power to assist heating and create electricity. Solar gain helps a tight, well-insulated home to stay warm.

The United Nations found that buildings and construction account for 39% of total carbon emissions annually. Net Zero Homes: Your Guide to the Greenest Housing Option  If a new generation of very clean burning biomass heating stoves can protect health and climate, might they assist COP* countries to move towards near-zero emissions in global building sectors? *COP is the decision-making body of the UN Framework Convention on Climate Change.

From: EPA’s Lab Test Results for Household Cookstoves, Jim Jetter, 2012 

Since 2012, optimized biomass cook stoves have been tested at ~50% thermal efficiency

The temperature of the hot gases flowing past the surface of the pot is increased by

  1. Creating as much flame (1,100C) as possible in a low mass, insulated combustion chamber.
  2. Decreasing the distance between the fire and the pot without making excess smoke.
  3. Not allowing external air to cool the combustion gasses.

In convective heat transfer, the primary resistance is the surface boundary layer of still air immediately adjacent to a wall. 

Increasing Temperatures, increasing exposed Area, increasing Radiation, increasing Velocity in a 6mm to 7mm channel gap (10cm or higher) pot skirt has been shown (up to 5kW firepower) in a 24cm or larger diameter pot to result in ~50% thermal efficiency. Reducing losses from the exterior of the pot skirt with refractory ceramic fiber insulation also increases thermal efficiency. 

60% thermal efficiency has been demonstrated in the lab.

Helpful links:

From: EPA’s Lab Test Results for Household Cookstoves, Jim Jetter, 2012

Key findings compared with the 3-stone fire:

  • Most stoves that were tested had better thermal efficiency, but some did not.
  • Compared with the 3-stone fire, many stoves that were tested had better combustion efficiency, but many did not.
  • A natural-draft TLUD stove (ARC) had very high efficiency with processed, wood-pellet fuel with low-moisture content.
  • Some forced-draft (fan) stoves had very low emissions – but not all fan stoves did.
  • Most natural-draft stoves that were tested showed a bigger improvement (lower emissions) over the 3-stone fire with high moisture fuel than with low-moisture fuel.
  • A natural-draft TLUD stove (ARC) had very low emissions – but required processed, wood pellet fuel with low-moisture content.
  • Two rocket stoves were tested at a “medium power” level – and had lower emissions (per energy delivered to cooking pot) than at maximum power.
  • Charcoal stoves had high emissions of CO and high emissions of PM during start-up.