Turndown Ratio (TDR) in Cooking and Heating Stoves

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A cook stove can require a ~three to one turndown ratio to boil quickly and then not burn rice, tomato sauce, etc. High power boils food, ~one third of the high firepower simmers it to completion.  With a lid, even lower amounts of energy can maintain simmering temperatures. When the stove cannot turn down sufficiently the pot with lid boils constantly.

An adequate turndown ratio is necessary in heating stoves as well. According to the map above, a stove needs a 2 to 1 TDR (30BTU per sq. ft. in Zone 1 up to 60BTU in Zone 5) to respond to changes in climate. Personal temperature preferences, house sizes, heat loss per hour, etc. also vary. A nationally sold stove needs a wide TDR to keep everybody at desired temperatures during cold seasons.

Traditional gas furnaces only operate at high power but for short amounts of time. A thermostat turns the heater on and off fairly frequently. Since pellet and log burning biomass-heating stoves do not easily turn off and on, they need to deliver adjustable high and low power. Then the house is not too cold or too warm. ARC tries to provide a three to one turndown ratio in cooking stoves. Heating stoves may require a wider range.

South/North Biomass

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It has been fascinating to cross-pollinate learnings from biomass cook stoves typical of the Global South and biomass heating stoves used in the Global North.

Biomass cook stoves are often used indoors without chimneys. They are not usually closed boxes. Primary air cannot be limited when the fuel door is open. Cook stoves are also short, so the residence time of flame/air/fuel is very short. For these reasons, typical cook stove designs forced TLUD inventors Tom Reed and Ron Larson (and many others) to deep dive into other clean burning options, especially passing woodgas through burning charcoal followed by mixing with air jets.

New heating stoves are using the same techniques to achieve clean combustion. Up Draft heating stoves, like TLUDs, force wood gas up through burning charcoal and use forced draft jets to achieve needed mixing of fuel, air, and spark.  (Obernberger, Brunner, 2023)

ARC is studying up draft, side draft and down draft combustion techniques that find applications in both cook stoves and heating stoves. We do experiments on prototypes and the results suggest changes. Working on a TLUD type heating stove yesterday evolved into a new approach to cleaner burning Rocket cook stoves.

Hardware, Software and Wetware

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Hardware includes the physical parts of a computer. 

Software is the set of instructions that can be stored and run by the hardware. 

Wetware is the people who do things.

Of the three, what component makes the most trouble?

To what extent do you affect decision making in your circumstances?

In stove projects, being adept at solving wetware issues can be very important.

Baldwin Design Committee management includes democratic decision-making.

Various and sometimes conflictual voices and expertise (cooks, funders, distributors, manufacturers, engineers, etc.) can be equally empowered.

How wonderful! 

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Expanding the Three T’s (Again)

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A static mixer designed by Kirk Harris

A static mixer designed by Kirk Harris

Perhaps, Time, Temperature, Turbulence is too easy to remember?  

“TTT” is elegant shorthand for how to achieve clean combustion and perhaps other factors are too obvious to mention?

At the same time, leaving out other clean burning factors confuses me.

Metering the right amount of woodgas into the combustion zone is important, too. Too much woodgas makes smoke right away.

Air Rich has to be included. Starving a fire can even put it out.

Yes, high Temperatures are very important.

Air, woodgas and fire need to be well Mixed.

High temperatures reduce needed residence Time.

MART MixT is a lot more clumsy…

Appreciating Local Expertise

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Cooking over an open fire in Ghana. (Photo: Global Alliance for Clean Cookstoves)

Cooks are experts!

Community organizers often say that to be successful the solution has to come from the folks with the problem. Another important factor is to appreciate the culture and long evolved expertise in their technical and social solutions. The women at Rancho San Nicolas, where I lived for eight years, were incredibly skillful at cooking on an open fire and were understandably proud of their abilities when cooking perfect tortillas, fish, beans, soup, etc. At a fish camp, guys who were not cooking every day, had a lot of trouble making anything close to a succulent home cooked meal.

Along with the hundreds of technical skills that made ranching fun, culture made life easier and more beautiful. Ranch culture was at least half of competency and expertise. Laughing at life’s problems made overcoming them much more likely. Religion, nature, the beauty of living outdoors and liking the slow pace were strengths in my friends that I grew to envy and attempted to emulate.

Bringing innovations started with lots of failure. The first Rocket stoves became flowerpots. The first solar cookers became toilet seats and windows. Eventually, ARC appropriate technologists made prototypes that were simply put on public display. Of our many attempts to introduce ‘helpful’ technology, cement rat proof boxes were the biggest success.

We learned a lot more than we taught, starting with listening to our expert hosts. The shorter and higher firepower Rocket stove that has gone viral was created by women in 18 villages in Southern India. Dr. Winiarski had the idea and the cooks made it practical.

Deforestation, Health, Climate

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Catching up to changing stove expectations reminds me of human maturation. Babies may only need to laugh to keep parents happy, but as kids grow up the stages of development result in further complications. Happily, meeting the expanding goals of protecting deforestation, health and climate can make stoves increase their abilities without changing an essential character that continues to please the cook.

Designed to protect forests, the initial stove featured increased thermal efficiency.

The health-protecting stove added burning up carbon monoxide (CO) and particulate matter (smoke, especially PM2.5).

Protecting climate matures the same stove as harmful climate forcers are combusted as well (NOx, Black Carbon, VOCs, Methane, etc.). Importantly, attempting to burn 100% renewable biomass can zero out the warming effect of Carbon dioxide (CO2).

In simple terms, a stove loved by cooks can be changed to cook using as little wood as possible, then CO and PM2.5 are combusted and then the previously uncounted gasses and black component in smoke are burned up, too.  The biomass fuel should be as renewable as possible to decrease adding CO2 to climate.

The same stove does better and better making stakeholders proud.

Fir trees and blue sky

Black and White Smoke

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Fir trees and blue sky

Biomass: Captured sunlight

Wood burning cookstoves make smoke and many different gases that change climate. Carbon dioxide (CO2) is absorbed when the plant grows and the same amount of carbon dioxide can be released when that biomass is burned. So emissions of CO2 can be zero with no effect on global warming (carbon neutral) if the burned biomass is used at the same rate as it grows.

But other emissions from combustion are also bad for climate change. Generally, wood burning cookstoves do not make a lot of methane and carbon monoxide so these gases do not add a lot to their effect on climate change. 

On the other hand, biomass cookstoves without engineered forced draft can make a lot Black Carbon (the soot in smoke) and BC is very bad for climate change. For this reason, when protecting climate, cookstoves should make as little black smoke as possible. 

White smoke can have a cooling effect on climate. ARC has been learning how to make combustion chambers that emit as little smoke as possible and make 95% white smoke and 5% black smoke. We are working with manufacturers to make stoves with health/climate combustion chambers.

Short-lived Climate Forcers and Climate Change

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It is not difficult to estimate the emissions of Black Carbon from cook stoves.

The emissions that change climate include various gases and the colors of smoke. If the wood used for cooking is 100% renewably harvested, the emissions of CO2 can be carbon neutral. Why? CO2 is absorbed when the plant grows and the same amount of CO2 can be released when biomass is burned. 

100% renewability can help CO2 to become climate neutral. However, the fraction of non-renewability (fNRB) does not change the amounts of other climate forcing emissions. Smoke is smoke. Etc.

What are the most powerful cook stove emissions affecting climate?

In general, adding methane and carbon monoxide to Carbon dioxide (CO2) adds a bit to the total warming influences (CO2e). 

However, adding short-lived climate forcers such as NOx, SOx and Black Carbon to the above has been estimated to more than double the warming potential. 

For this reason, it seems to be important to add the short-lived climate forcers when calculating how to address climate change with cook stoves. 

A chef cooks at a stove with a flaming wok

Turn Down Ratio: Cooking and Heating Stoves

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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. 

thermal image of house juxtaposed with daylight image of same house

COP 28: Near-zero emissions in global building sectors

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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.