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.

ARC Assists CSIR-Ghana in Capacity-Building 

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In the first week of October, ARC Research and Development Engineer Jaden Berger visited CSIR-Ghana for capacity building training. The Council for Scientific and Industrial Research (CSIR) is the foremost national science and technology institution in Ghana.

The main focus of the visit was to teach them how to perform field testing using various sensor suites. CSIR was especially focussed on learning to perform KPTs (Kitchen Performance Tests) while using EXACT sensors from Climate Solutions Consulting. We also used other sensors CSIR already had: a PEMS (Portable Emissions Monitoring System) with a portable hood, an IAP (Indoor Air Pollution) meter, and an air quality sensor along with performing UCETs (Uncontrolled Cooking Efficiency Tests) during cooking to determine the efficiency of the stove.

Making observations of how cooks are using stoves.

Setting up a PEMS with a portable hood to measure stove emissions.

Testing was done at a secondary boy’s boarding school in Accra. The school cooks breakfast, lunch, and dinner for 3,000 students using a variety of improved and unimproved stoves. The stoves identified as the least efficient and highest emitters were the 12 wood stoves and 4 palm kernel stoves. (Palm kernels used to be considered agricultural waste from palm oil production but are now commonly used as fuel.) Several design meetings were held to determine a design that would increase efficiency, clean up emissions, and remove emissions from the room the cooks were in.

Palm kernel stoves in use for breakfast.

Weighing wood for three 24 hour-long KPTs.

Performing UCET measurements.

The next step is for CSIR to finalize a CAD model of the design along with some CFD analysis to predict if the prototype will work. ARC will then virtually meet with CSIR and their manufacturer to finalize the design and begin creating prototypes.

During the second week of the visit, ARC and CSIR worked on wrapping up older projects. This included gathering final data for a charcoal conversion efficiency study, creating a draft of the charcoal conversion efficiency protocol so that it can be published, and developing and using a durability protocol that is more applicable to conditions a stove will have to withstand in Ghana.

Taking measurements for creating a new durability protocol.

Overall, a successful trip with good progress made toward improving health and cooking conditions in Ghana.

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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Tuning up Health/Climate Heating Stoves and Cook Stoves

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Travis Volpe and Kim Still in the Osprey Lab at Aprovecho

“ARC is trying to figure out how to burn wood without making smoke.” 

That’s how we sometimes explain our work to visitors. 

Folks often say, “That’s not possible!” “How could you do that?”

We respond: “It doesn’t seem to be too hard.”

“Force the right amount of smoke into the fire for long enough, making sure that it’s well mixed and there’s enough air.”

“Oh, OK! That makes sense!”

Working with US Stove Company (USSC) and Shengzhou Stove Manufacturer (SSM), ARC is developing three kinds of clean burning heating stoves and six cook stoves:

  1. A bestselling, forced draft pellet stove needed a tune up under the emissions hood to achieve closer to complete combustion. We adjusted the inputs and decreased PM2.5 by 84%, winning 2nd place in a 2023 DOE wood stove national competition. Development continues!
  2. Controlling primary air achieved a 2 to 1 Turndown Ratio in a natural draft pellet stove and a tune up resulted in much better emissions and dramatically lowered the Black Carbon ratio. Mr. Shen, owner of SSM, built a beautiful prototype and we are testing it now. 
  3. Log burning heating stoves are really interesting! The box in which the wood burns allows controlled, small changes that have big effects! Osprey Foundation has funded us to tune up an affordable, log burning heating stove (using big fuel loads as suggested by NESCAUM)  Six manufactured cook stoves will be tuned up in the same way with local advice.

So much fun! Come on by for a visit! We make good coffee!

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…

Working Towards “Zero Green Premium” Stoves

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ARC is working on three types of Zero Green Premium biomass heating stoves designed to protect health and climate. “Zero Green Premium” means that the new product costs the same as the dirty one it replaces. As a rough estimate, ARC uses 0.5 grams per hour of PM2.5 at <5% Black Carbon ratio to define the performance and emissions that protect urban health and climate. 

Sixty percent of electricity in the US is generated by burning natural gas (40%) and coal (20%). Utilities are wondering how to meet increasing demand while replacing fossil fuels. Help is very welcome!

When renewably harvested biomass is burned cleanly enough, could it join solar, wind, and hydro as another useful energy resource? Heating a home with wood is ~ 80% efficient. Making biomass into alcohol is ~ 1/5th as efficient. (Energy Primer, 1976)

Is there enough biomass? In 2023, the DOE estimated that the US produces more than 1 billion tons of renewable biomass per year. Providing the needed ~3 tons of biomass per US home (currently heated by burning fossil fuels) requires 420 million tons, a drop in the bucket.

Six steps: Clean burning of biomass      

  1. Optimize heat transfer efficiency 
  2. Control the rate of reactions 
  3. Achieve molecular mixing in the combustion zone 
  4. Maintain temperatures above 850C 
  5. For a minimum of 0.5 seconds 
  6. In an air rich environment.

Stick Size Matters!

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Small sticks make higher temperature gases, better for heat transfer efficiency, but more smoke

Monitoring many fires seems to show that along with density, moisture, etc., the diameter of sticks has a large effect on both heat transfer and combustion efficiency.  

In a Rocket stove without a closing door, there is obviously a lot of cold excess air entering the fire. How do we raise Temperatures without limiting primary air?

Our observations seem to indicate that burning smaller diameter sticks results in more flame/higher temperatures. However, burning smaller diameter sticks also tends to make more smoke. For this reason, it may be that burning small sticks increases thermal efficiency but decreases combustion efficiency.

Conversely big sticks seem to burn slower making less flame, resulting in lower temperatures while making less smoke. Since flame from wood makes smoke, when the wood becomes charcoal, much less PM2.5 is emitted.

The Jet-Flame can burn 2” by 2” sticks and testing shows that PM2.5 gets lower with bigger diameter sticks. The jets of air make the made charcoal very hot and even big sticks stay lit. In a normal Rocket stove without a Jet-Flame, especially with wet wood, only smaller sticks will keep burning. 

The goal is to create as-hot-as-possible gases flowing next to the heat exchanger (pot) while controlling emissions. The size of the sticks does seem to have a significant influence on thermal and combustion efficiency.

Increased Air Exchange Rate Protects Health

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Sunken pot, 50% thermal efficiency cook stove with chimney

When (oh, when!) will PM2.5 be included in carbon offset methodologies? 

Who can blame stove manufacturers for selling high thermal efficiency/low combustion efficiency stoves when protecting health is not financially rewarded? 

Factories can only sell what the market demands even when they manufacture better stoves. Manufacturers, like SSM, already have slightly more expensive, much cleaner burning stoves ready to go. 

Including PM2.5 in carbon revenue might go a long way to help projects pay for higher combustion efficiency stoves.  

PM 2.5 needs to be reduced by ~ 90% to protect health in kitchens with 15 air exchanges per hour. The needed % reduction is halved when air exchange rates are doubled. This may be the most cost effective way to protect health? Cooking outdoors with an estimated 60 air exchange rates per hour is very effective in reducing exposure.When cooking inside, perhaps a durable stove with improved combustion efficiency and a chimney would help the large percentage of cooks who, for many reasons, continue to cook with biomass?

Heat Transfer Efficiency!

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Heat Transfer Efficiency!

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Wow, heat transfer efficiency is easy to understand! 

Read on…

Raising the Temperature of gases flowing next to the heat exchanger (the pot in a cook stove) is probably the most effective technique in a Rocket stove to increase heat transfer efficiency (use less wood for cooking).

Doubling the Temperature of gases doubles heat transfer efficiency.

Doubling the Area exposes to the gases doubles heat transfer efficiency.

Doubling the Velocity of gases ~doubles heat transfer efficiency.

Doubling Radiation increases heat transfer efficiency to the 4th power.

Increasing the view factor helps, too! That’s the proportion of the radiation strikes the bottom surface of the pot.

A 6mm to 7mm channel gap pot skirt increases heat transfer efficiency by ~ 25%.

Simmering at the needed low firepower can save a lot of fuel, too.