A quick internet search for “retained heat cooker” brings up many modern choices.

In 2018, the World Health Organization concluded, “Every day around the world, billions of children are exposed to unsafe levels of air pollution. The result is a global public health emergency.” 

The WHO recommends that a combination of actions may be most effective. Aprovecho has used Retained Heat Cookers (Hayboxes) for decades and we recommend them, especially where beans make up a part of the diet.

Retained Heat Cookers simmer food to completion reducing CO emissions by 56%, PM emissions by 37% and saving 50% of the fuel and time spent cooking. (Test Results of Cook Stove Performance, 2011). When food is simmering, the fire is constantly replacing the heat lost from the pot. If the heat is captured instead, the retained heat in the boiling pot finishes cooking the food. 

In the same way, a drafty and uninsulated house has to have a big fire going all the time to keep the house warm.  The super-insulated, almost airtight house can stay warm for a long time after the fire is extinguished. In Oregon, Haybox homes are called “Super Good Cents” homes.

Once the boiling pot is in the box, food cooks without further attention. Even pinto beans will finish softening if the box, with waterproof R-7 insulation, is almost airtight.  Makes cooking a lot easier, too!

When Dean Still came to Aprovecho in 1989, Dr. Larry Winiarski asked him to compare the thermal efficiency of the Lorena stove and the Three Stone Fire. The testing revealed a problem for the ARC staff when our Lorena used three times more fuel than a carefully operated open fire! 

It’s surprising to learn how efficient a three stone fire can be!

Half of the staff, who had written books about the Lorena and taught thousands of people about their invention, were never convinced that a problem existed. The other half were embarrassed and became fervent believers in Dr. Kirk Smith’s famous saying that “You get what you inspect, not what you expect.”

Making a public mistake pushed a reconstituted ARC to proceed more slowly, to challenge speculation, and to try to generate reliable data. We learned that a lot of local knowledge is required to take successful products to market. Evidence can help to overcome inventors’ pride, cognitive dissonance, and the financial cost of changing directions. At the same time, inventor’s pride, cognitive dissonance, and the cost of changing direction also influence decision making.

graph helps calculate proper skirt gap for best heat transfer efficiency

Smaller fires are often cleaner burning compared to larger fires, and higher heat transfer efficiencies allow the use of smaller fires while the time to boil stays acceptable. A pot skirt which creates a narrow channel gap reduces fuel use and time to boil.

Dr. Sam Baldwin recommended 10mm to 11mm channel gaps for household cook stoves at normal high firepowers (4kW to 5kW).

ARC uses tighter channel gaps at 5kW and we do not seem to experience problems. ARC recommends 6mm channel gaps in pot skirts for 20cm in diameter (or larger) pots. 

It is interesting to note that Dr. Baldwin decreased the channel gap to 8mm with bigger institutional sized pots. We just tested a 6mm gap in a skirt surrounding a 40cm in diameter (60 liter) pot. The result, with a small open fire, was around 48% thermal efficiency.

We recommend a 6mm channel gap to achieve close to 50% thermal efficiency with a large range of pots..

The tight channel gap seems to be OK, so far. It would be great to model velocity, temperature drop, etc. as the channel gap changes! We will do it one of these days…

Traditional three stone fire
Use of traditional three stone fire in one Rakhine village.  ©FAO/Myanmar

As with any tool, the skill of the operator determines how well the work is accomplished. It takes years to learn how to use a hammer or shovel. The Three Stone Fire can be effective and clean or it can be very dirty and wasteful. In some kitchens, large fires use a lot of wood and make a great deal of smoke. Small fires are also made that cook food relatively cleanly. 

Watching indigenous experts cook with fire has led to a better understanding of improved biomass fuel use. Cooks who are trying to conserve wood tend to burn the wood at the tip of the stick making flames. Knowledgeable cooks only need a small, hot fire close to the pot to boil water. 

Improving upon a well-made Three Stone Fire has been more difficult than expected. Learning from expert users helped teach engineers how to make better stoves. Well-constructed Three Stone Fires protected from the wind and tended with care, score between 20% and 30% thermal efficiency. Open fires made with moister wood and operated with less attention can score as low as 5%. 

When Tami Bond achieved 33% thermal efficiency with a Three Stone Fire, ARC started to depend on the pot skirt with a 6mm channel gap to help folks use less fuel to cook food. Expertise with the Three Stone Fire is an important skill that empowers the cook and has to be respected.

Chart showing turn down ratios and firepower of 18 stoves
Firepower and turn-down ratio of 18 stoves, from “Test Results of Cook Stove Performance”

The ARC/EPA 2011 book “Test Results of Cook Stove Performance” compares performance and emissions, including turn down ratio and firepower, from survey of 18 stoves. Firepower is a measure of how much energy is released per unit of time. More energy is required to quickly boil water. Less energy is needed to simmer food.

The most effective cooking stove should be fuel efficient at both high and low power. The ratio between high and low firepower is called the turn-down ratio (TDR). It is a measure of how well the stove can be “turned down” from high to low power. 

A TDR of 2 means that half as much fuel was consumed while maintaining a simmer, compared to bringing water to boiling. Cooks usually appreciate a stove that is capable of both high-and low-power operation. Many foods will burn if the heat can’t be reduced enough.

It is interesting that the liquid-fueled stoves were generally low powered, at less than 2kW. Most of the wood burning stoves ranged from 8kW to around 6kW. In Mexico, gas stoves can have a hard time cooking tortillas.

The Mud/ Sawdust (TDR 3.9) and VITA (TDR 3.8) stoves had the highest Turn Down Ratio. The average for the other wood-burning stoves without chimneys was 2.4. The average for stoves with chimneys was 2.2. The Gyapa charcoal stove (TDR 2.8) scored slightly higher. 

The chart shows the average high firepower and the low firepower for each stove. It should be noted that in these tests the pot was uncovered, which increases the energy input needed to maintain the water at simmering temperatures.

Read more about the Chitetezo Mbaula project at unsustainablemagazine.com. Photo by Deogracias Benjamin Kalima

The Chitetezo Mbaula cookstove is distributed by United Purpose in Malawi with the goal of combating deforestation by replacing the traditional charcoal/firewood cooking stoves. In an effort to assist, ARC worked with stakeholders to see how small changes in the stove might translate into fuel and emissions reductions in lab tests. Of course, this information is only useful to researchers in the field as possible iterations. They determine if the changes might translate into practical conservation. The collaboration continues as possibilities are examined.

In its stock form, the stove achieved an average thermal efficiency of 22.5% during three modified laboratory based IWA 4.2.3 tests at high power. As the stove body got hotter, the thermal efficiency increased from 17.6% to 26.6%. The thermal efficiency Tier rating was 1, and PM2.5 emissions, at 1093.3 mg/MJd, gave a Tier rating of 0.

Simple Adjustments Make Some Performance Improvements

Three one inch in diameter holes were drilled through the back of the clay stove body with the intention of allowing more air into the charcoal bed. The pot gap on top of the stove was also reduced to 6mm. These two changes resulted in an average increase of thermal efficiency with char from 22.5% to 29.6%. 

The CO emissions factor per energy delivered to the cooking pot decreased from 10.45 g/MJd to 5.63 g/MJd, although at the same time the firepower decreased from 8.9 kW to 5.9 kW. Natural draft stoves with lower firepower tend to make less emissions. Since the time to boil (normalized to 75°C temperature rise) also increased from 22.4 minutes to 25.2 minutes, further study is needed to determine if the reduction in CO emissions also occurs at 22.4 min to boil. 

Jet-Flame and Pot Skirt Increase Efficiency, Reduce PM2.5

The Shengzhou Stove Manufacturer Jet-Flame was then inserted into the stove body with a metal Rocket combustion chamber. A 6mm channel gap metal skirt was also used around the 5 liter flat bottomed pot. With these changes, the stove achieved an average thermal efficiency of 47.7% during three laboratory tests at high power. As the stove body got hotter, the thermal efficiency increased from 44.9% to 52.3%. The IWA thermal efficiency Tier rating was 4. Since all of the tests scored within Tier 4, which is the maximum score under the ISO IWA, the 90% confidence interval of the Tier rating was 4 to 4. The PM2.5 emissions of the stove were 69.0 mg/MJd and the Tier rating was 3.

When tested in the field, ARC roughly estimates that emissions will be something like three times higher. This is a “rule of thumb” that is not meant to be an accurate guess but a reminder that many researchers have found that emissions in the field are much higher compared to lab test results! The lab test can point out theoretical “improvements” but only field testing can determine actual performance and practicality. On the other hand, if cooking takes place outdoors, as in the photo above, exposure to harmful smoke can be estimated to be dramatically reduced by the increased air exchange rates.

Smoggy NYC, photo by urbanfeel on flickr
Smoggy NYC, photo by urbanfeel on flickr
photo by urbanfeel on flickr

Several articles have pointed out that using biomass-heating stoves can result in health problems in densely populated areas. We are working with friends at the EPA to think about how we might define PM2.5 emission rates for residential biomass heating stoves that would protect health in densely populated cities. 

When the population density goes up (more people are generating pollution), the emission rate has to go down (the stoves have to be cleaner).

What emission rate for PM2.5 would protect personal health if 1/3 of the folks in New York City replaced the natural gas used for residential heating with biomass?

Very roughly, using an EPA outdoor air pollution model, a biomass-generated PM2.5 emission rate of around 0.3g/h looks like it might work in NYC. That’s the emission rate of a good pellet stove.

To accurately make predictions, a model of the air circulation in a city can be generated. Great for planning. For a description of the EPA model, see Chapter 5 “Protecting Health” in Clean Burning Biomass Cookstoves, 2021.

What happens if a bunch of sticks are bundled together, lit at the tips, and inserted into a well-insulated horizontal enclosure that enters a Rocket stove providing draft?

Well, it’s hard to get everything to work well. As Dr. Winiarsky pointed out “Gasifiers are finicky.”

But, when all of the tips of the bundle of sticks are lit at the same time, and there is the right amount of primary and secondary air, then the fire moves horizontally in the well insulated enclosure away from the Rocket stove and emissions can be decreased.

Working with the Gates funded Global Health Labs in 2017, ARC experimented with various horizontal gasifier prototypes and sometimes the results (Tier 4 for PM2.5) were encouraging.

ARC decided to explore the development of primary air jets up into the fire as a more promising technique and left the horizontal gasifiers on the shelf. But who knows, someone may find the concept interesting and continue the investigation of the horizontal gasifier and make it less finicky?

Balthazar Schulte, Lyttleton Harbor, New Zealand

I recently read a Time article that pointed out that shipping accounts for 2.2% of annual global greenhouse gas emissions. To put it in perspective the writer, Aryn Baker, suggests that if shipping was a country, it would be the sixth largest CO2 emitter in the world, on par with Germany.

In her reporting from last year’s COP26 (UN Climate Change Conference), she writes:

 “It doesn’t matter if we want iPhones that come from China, steaks from Brazil or to tow an iceberg to Glasgow; we all depend fundamentally on international shipping for everything that we do,” says Johannah Christensen, head of the Global Maritime Forum.

“This just underscores the importance of decarbonizing shipping.”

Some ferryboats have transitioned to battery power. For the 60,000 ocean going cargo ships transporting global goods, the most promising technology, says Christensen, is either hydrogen or synthetic fuels.

Making these fuels available sustains the globalization the world depends on for just about everything. In The Age of Aquarius, renewable fuels keep ships rolling on the high seas as we transition from grey to green. Check it out!

I didn’t realize that natural gas is mostly methane which is about 84 times worse for climate change compared to CO2.

As reported in the recently published study “Methane and NOx Emissions from Natural Gas Stoves, Cooktops, and Ovens in Residential Homes,” methane leaks are bad news for both environmental and personal health. (Lebel, et al, Environ. Sci. Technol. 2022, 56, 4, 2529–2539)

Methane risks

The abstract in the Lebel article states:

  • Natural gas stoves in >40 million U.S. residences release methane (CH4) ─a potent greenhouse gas, through post-meter leaks and incomplete combustion.
  • Using a 20-year timeframe for methane, annual methane emissions from all gas stoves in U.S. homes have a climate impact comparable to the annual carbon dioxide emissions of 500,000 cars.
  • In addition to methane emissions, co-emitted health-damaging air pollutants such as nitrogen oxides (NOx) are released into home air and can trigger respiratory diseases.
  • Our data suggest that families who don’t use their range hoods or who have poor ventilation can surpass the one hour national standard of NO2 (100 ppb) within a few minutes of stove usage, particularly in smaller kitchens.