What’s Cooking at Aprovecho

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Capacity Building with CSIR

Over the past 3 years, thanks to three grants from Fire Capital, Aprovecho has been working with CSIR (Council for Scientific and Industrial Research) in Ghana to expand their lab capacity. We have focused on teaching stove design principles through co-developing a stove retrofit for boarding schools in Ghana.

During the recent in-person workshop, Jaden visited the lab to help with the building of the prototype and plan for its installation at a local school. This prototype will go over an existing stove at the school, increasing thermal efficiency and reducing indoor emissions. With user, manufacturer, and lab result feedback, CSIR will finalize their design and give stoves to ten schools for a larger impact study. 

CSIR employees working on the new stove prototype

The LEMS in Bangladesh

Sam went to Bangladesh to update their lab and provide training. The Bangladesh University of Engineering Technology in Dhaka now has a lab that can test cookstoves according to ISO 19867 standards. This provides a useful development tool for the area as well as way to teach the next generation about cookstove development. Sam observed that while urban and peri-urban areas had access to gas and electricity, households couldn’t always afford it, highlighting the need for clean biomass stoves even in areas with access to alternative fuels. 

Sam also visited Life Green Energy, a stove manufacturer focused on forced draft stoves. Together, they developed a local Jet-Flame prototype aimed at saving fuel and reducing emissions. The prototype was able to burn green wood while other brick stoves could not. More development is planned to improve the product.

Sam Bentson testing at the lab in Bangladesh

Baseline Efficiency Paper Published

We were recently published in Environmental Science & Technology for our paper on the baseline thermal efficiency of wood and charcoal stoves: Quantifying the Efficiency and Fuel Consumption of Cooking with
Traditional Wood and Charcoal Stoves in Malawi, Ghana, and Kenya.

Aprovecho and Oregon State University paired with SunFire (Malawi), KIRDI (Kenya), and CSIR (Ghana) to conduct a total of 720 thermal efficiency tests on traditional wood and charcoal stoves. The goal was to compare the baseline efficiency of stoves with UNFCCC (United Nations Framework Convention on Climate Change) default efficiency, which had recently changed.

We developed a new test protocol, the UCET (Uncontrolled Cooking Efficiency Test), which measures thermal efficiency on any meal cooked. We found that the average thermal efficiency was between the new and old UNFCCC defaults. It was also found that firepower, pot/pan size, and cooking method are strongly correlated with efficiency. 

Women cooking during a UCET

Concentrator Disk/Chimney in a TLUD

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Concentrator disk/chimney in a TLUD

When I asked Google AI, “Is flow more laminar in a smaller diameter tube?”  it immediately responded:

“Yes, thanks for asking! A smaller diameter tube can promote more laminar flow for a given flow rate. This is because reducing the diameter of a tube increases the Reynolds number, which is a dimensionless quantity that helps determine whether flow is laminar or turbulent.”

I had been noticing that the chimney also increased draft in a TLUD we have been developing, lengthening the rising column of flame.

As a rule of thumb, Dr. Winiarski advised that flame should burn out before it touches the bottom of the pot. Yellow flame is very hot (around 1,100C) which is great, but sufficient mixing and residence time are also needed to burn up smoke.

Have you also experienced that lifting the pot off of the top of a TLUD can reduce the emissions of PM2.5?

Reducing draft by removing the concentrator disk/chimney this week has allowed flame enough time to burn up more smoke in a relatively short TLUD. At the same time, thermal efficiency has been reduced when short flames dance playfully on top of the larger diameter combustion zone (with a larger Reynolds number).

Cooking on many stoves in Burkina Faso

Carbon Credits and Fuel Savings?

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Photo from TREEAID on Flickr

Looking at the photo it is easy to imagine why field-testing is needed to show whether an intervention is actually saving fuel. Real life is complicated and is not replicated in a lab.

The use of a Water Boiling Test to determine if new stoves are saving fuel has historically been questionable. WBT’s tend to underestimate fuel use compared to field tests. (Hernández, 2014; Teune et al., 2020, Bayer et al., 2013).

Water Boiling Tests are great for international stove comparisons when variables are controlled. WBTs are also useful to investigate how stoves might be improved and to experiment with iterative changes that could improve heat transfer and combustion efficiency.

Luckily, we were assured at ETHOS 2025 that only field tests would be used from now on to calculate fuel savings for carbon credits.

When data from field testing was replaced with lab-based results it was such an obvious mistake!

Of course, any type of testing needs to be done carefully by a third party.

The Water Boiling Test, Repeatedly

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In my opinion, the WBT* cannot be used, especially in the lab, to improve a biomass cook stove because all of the important field variables are not represented.

A successful cook stove needs to be evolved from field tests, as we did in Southern India for the Shell Foundation. Cooks in eighteen villages kept on changing the Rocket stove until it was acceptable, useful, and even likable. It took a while but it was a lot of fun and a great introduction to Southern India!

The WBT, with severely limited variables, can be useful in the lab for international comparisons of stove performance. The same pots, same amount of water, same fuel, same procedures and protocols limit the confounding variables in an attempt to isolate the stove as the reason for perceived differences.

As we did in India, both field and lab data can inform stakeholders. The successful stove has to please cooks, retailers, distributors, etc. and, at the same time, meet project goals such as reducing adverse health effects. We used the WBT in the lab and the CCT* in the field. Marketing tests, as suggested by Baldwin (1987) were very important, as well. We learned right away that the stove had to cost ~$5 to capture sustainable market share.

The lab based WBT is best used to inform researchers how stoves might be improved. Then, iterations in prototypes are tried in the field including cost, weight, color, height, firepower, fuel used, etc, etc.

This combined use of the WBT, CCT, and KPT* for stove development was suggested in the International Stove Standards, (1985). 

*Water Boiling Test “The Water Boiling Test (WBT) is a simplified simulation of the cooking process. It is intended to measure how efficiently a stove uses fuel to heat water in a cooking pot and the quantity of emissions produced while cooking.” – The Water Boiling Test Version 4.2.3

*Controlled Cooking Test “The controlled cooking test (CCT) is designed to assess the performance of the improved stove relative to the common or traditional stoves that the improved model is meant to replace. Stoves are compared as they perform a standard cooking task that is closer to the actual cooking that local people do every day.” – CCT version 2.0

*Kitchen Performance Test “The Kitchen Performance Test (KPT) is the principal field–based procedure to demonstrate the effect of stove interventions on household fuel consumption.” -KPT version 3.0

Find out more about testing protocols at cleancooking.org/protocols/

Dr. Paul Anderson shows a concentrator ring on a TLUD

The Concentrator Ring in a TLUD

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Dr. Paul Anderson shows a concentrator ring on a TLUD

Image from Dr. Paul Anderson’s Introduction to TChar (TLUD) Stoves for Haiti

The size of the hole in the middle of the flat plate, usually round, that seals the top of the combustor in a TLUD stove has important functions. The flat plate forces air jets/flame to travel horizontally in an attempt to completely cover the fuel bed. The diameter of the round hole has a dramatic effect on firepower. When it is too small, the fire can even be extinguished.

As with other variables, iterating changes in a prototype under the emissions hood can determine the compromise that best meets project goals. Larger holes in the concentrator result in higher firepower but can also increase emissions. In a 5” in diameter stove, a 3” hole in the middle of the concentrator is commonplace.

A StoveTec Fire Fly Lantern burns with a single column of flame

In the StoveTec FireFly lantern, developed at ARC, a small hole in the concentrator ring (1.5”) forces the flame into a narrow, vertical cylinder used for illumination. While the firepower is very low, so are the health-affecting emissions per minute.

Supporting Best Practice

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Three stone fire 3

Cooking outdoors, making hot fires, burning the tips of sticks to use less wood and breathe less smoke (photo: Clean Cooking Alliance)

In 2003, Aprovecho was hired by The Shell Foundation to develop a Rocket stove in Southern India. We found a wonderful co-op of potters that was selling two-pot burnished $4 ceramic stoves with chimneys. They sold 250,000 stoves per year. Mahatma Gandhi promoted this kind of stove in 1934. The ARC staff thought that, while trying to introduce a new concept like a Rocket stove, helping the potters to update their facility and perhaps tweak the design might result in this established manufacturer with an existing market and distribution system becoming more successful. It was great to work with and learn from so many, highly motivated local folks!

Appropriate Technologists are encouraged to first survey technologies developed in the project area. ARC looked around lots of villages to try to find out what expert cooks were doing to use less wood and breathe less smoke. We were trying to find out what existing factories were making innovations, and which established markets and distributors were selling products. One constant was that almost every distributor said that stoves had to cost less than $5.

Appropriate Technologists read in textbooks that learning from local solutions is best practice. Hundreds of Indian women transformed the Rocket into a useful stove. ARC did not know so much information was needed to be successful! We had to learn from experts and try to get out of our own way (as Alan Watts titled his autobiography).

Cover image of The Winiarski Wood Fired Agricultural Food Dryer

Winiarski: Improving Agricultural Food Dryers

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Cover image of The Winiarski Wood Fired Agricultural Food Dryer

Sometimes it’s too cloudy for solar drying and a wood-fired dehydrator can help with large scale food preservation. In 2002, Dr. Larry Winiarsk helped farmers in the wet mountainous region of Nicaragua to design and build a prototype wood fired dryer for cacao beans. The ARC publication “The Winiarski Wood Fired Agricultural Food Dryer,” details construction of a rocket-style wood-fired dehydrator.

It’s estimated that the optimal temperature for drying is between 120 and 130 degrees F. Sustained temperatures over 130 degrees can begin to cook the food. (When starting to dry foods it can be most efficient to go up to 150 to 160 F. in the initial stages of drying when lots of moisture will be evaporating out of the food.) 

A successful food dehydrator sustains these temperatures in dry air passing through food at a constant rate with even distribution for a variable period of time, depending on the food being dried. 1.) Hot air temperatures increase the rate of evaporation. 2.) Air flow through the food is equally important.

WHY?

Efficient drying moves the water inside the food into the surrounding air. Wet air cannot absorb more moisture. Hot, dry air picks up the moisture and air flow created by draft replaces moist air with dry air. In a Winiarski food dryer, air is efficiently heated and sufficient airflow of dry air through the food increases the rate of dehydration.  

The food dryer features: a) burning biomass heating air b) is relatively air tight, c) has a large chimney or fan that removes moist air and moves dry air through the food. A large chimney usually creates the draft necessary for sufficient air flow.

Emission testing equipment set up at an Oregon home

What’s Cooking at Aprovecho

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The LEMS in Ethiopia

Jaden and the trainees in the Ethiopia lab

In March, Jaden traveled to Ethiopia to assist in the installation of the LEMS and train lab technicians on ISO testing for cookstoves.

15 trainees from all over the country attended the training. The LEMS was procured by SNV and their sponsors and was given to Ethiopia’s Department of Water and Rural Energy at the end of the training and installation.

The installation of LEMS in different countries is vital in creating clean cooking solutions that can be used throughout the world. We hope to continue working with the lab in Ethiopia to design new stoves.

Proyecto Mirador in the Lab

Proyecto Mirador testing their stove in their lab in Honduras

Proyecto Mirador has been working on improving the efficiency of their Dos por Tres stove. After successful tests in their lab, they traveled to Aprovecho from Honduras to confirm their results.

They got the same results in both labs, showing how effective iterative design testing with the LEMS can be. Now they can install stoves that use less fuel and cook things faster in Honduras and Guatemala.

Field Testing Results Published

Emission testing equipment set up at an Oregon home

In 2023, we measured the emissions from cordwood heaters in Oregon. There are very few studies that evaluate emission measurements in the field for wood heaters. As seen from our experience in cooking stoves, emissions in the field are often higher than lab tests, and field studies help us understand how people use their stoves so we can make better designs.

We’ve been using the data we collected to create lab tests that reflect user behavior and design clean and efficient heating stoves that meet user specifications.

The resulting paper, In-situ Measurements of Emissions and Fuel Loading of Non-catalytic Cordwood Stoves in Rural Oregon, was recently published in the Journal of the Air & Waste Management Association.

Market Driven Stoves

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Photo by Güldem Üstün on Flickr

One of the biggest mistakes I have made was thinking that I could predict what stove would sell in a market. If only I had always followed Sam Baldwin’s advice and done market testing before manufacturing! In Southern India, as part of the 2003-6 Shell Foundation project, we had outspoken distributors who yelled in meetings that a successful stove had to cost $5, that 5,000 had to fit on a truck, and that it had to be designed by cooks! The stove had to be short so food could be stirred and work well at the low firepower required by villagers around Chennai, toast a chapatti, etc., etc.

When the carbon market crashes, affordable stoves continue to be sold and used. When stoves are purchased the consumer is convinced of their utility. The trick is to try to bring best solutions into marketable products but, luckily, engineers love a challenge!

A pot skirt, SuperPot, constant cross sectional stove top, stick support, keeping a fire hot, can all significantly increase heat transfer efficiency. Even Jet-Flames can be made for around $5. TLUDs can be inexpensive. There are many techniques to improve the market driven stove! 

Check out the new Osprey funded book Improving Biomass Stoves, 2025  for examples?

Fireless Cooking Has A Long History

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Thanks to Robert Fairchild for sending this reminder that what we call a “Haybox” cooker has a lot of history behind it!

Of course fireless cooking methods have been used since ancient times, but fireless cookers began to be introduced to U.S. in the mid 1800s, becoming commercially manufactured and quite popular in the US in the early 20th century. The Haybox, or “retained heat cooker,” works by placing a boiling pot of food into a well insulated box that keeps the heat in the pot, generally producing thoroughly cooked food in a couple of hours without further interventions from the cook.

Retained heat cooking can save 20%-80% of fuel for cooking, depending on the food and amount cooked. This method is not safe for every kind of food, but Aprovecho cooks especially love it for a big pot of beans or rice. The fire and the pot don’t need to be tended after boiling, and the food never burns!

If you are interested in making one for your own use, here is the ARC Rule of Thumb Design Principles for a Haybox.

You can find an excellent, well illustrated history of the Fireless Cooker, from early versions through its modern re-emergence in low-income countries, at the USDA National Agricultural Library: The Fireless Cooker (Emily Marsh, Ph.D, MLS)