In 1982, the original cook stove consultants returned from global travels to Oregon, bought 40 acres of forested land at the end of a road, planted a garden and started talking to Dr. Larry Winiarski, (Oregon State University) learning how to improve combustion and heat transfer efficiency.
Dr. Nordica MacCarty, an associate professor at OSU, is the Executive Director of ARC and continues investigations of how wood fires can better help humanity. She has worked at ARC off and on for 25 years. Rigorous science-based experimentation, field learning, publication of peer reviewed journal articles and market-based manufacturing and distribution intertwine as ARC matures.
Learning from experience, eating a thousand kinds of food, succeeding now and then, is always great!
https://aprovecho.org/wp-content/uploads/2024/03/MacCarty-Headshot-2022-scaled.jpg17072560Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2025-04-25 14:10:212025-04-25 14:10:35Aprovecho Research Center, 2025
In 2018, the World Health Organization published the book “Air pollution and child health: prescribing clean air”. The report concludes, “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 suggested “prescriptions” to clean the air and protect health:
Use clean household energy for cooking, heating, and lighting.
While waiting for clean to be available, use technologies and fuels that reduce exposure such as low-emission biomass cook stoves.
Minimize the time children spend around smoky fires.
Increase ventilation or install a chimney.
A combination of interventions is usually most cost effective.
ONE
Help to make clean burning stoves available.
Promote solar lighting.
TWO
Forced draft stoves can burn up a lot of the smoke.
An adjustable Pot Skirt forces the hot gases to scrape against the sides of the pot as well as the bottom reducing fuel use by about one-third. That results in one-third fewer emissions!
A light weight, abrasion resistant Rocket combustion chamber can burn up ~ 50% of smoke compared to the open fire.
Winiarski designed stovetops can increase heat transfer efficiency by ~8%.
THREE
Promote reduction of exposure to family members, especially to women and children.
FOUR
Doubling the air change rate reduces smoke in half.
Locate the fire under a window on the low pressure side of the house.
Cook outside.
Chimneys have been a part of traditional houses for centuries.
Cook with a chimney!
https://aprovecho.org/wp-content/uploads/2025/04/bluesky.jpeg344600Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2025-04-18 16:15:552025-04-18 16:15:56Building Blocks to Health
Last week Dean and Travis went to the Hearth, Patio & Barbeque Association Expo in New Orleans to get updated on the latest heating stove technologies and to meet and talk to stove companies. Three days was a short time but we made many friends and learned a lot!
The CENTRAL BOILER outdoor log and pellet furnaces stood out as one of the newest innovations. It can be imagined that in a sustainable biomass future, heating big buildings (even high-rise apartments?) with logs or pellets might be as useful as freestanding stoves heating rooms. The same (bigger/smaller) clean burning combustion systems deliver renewable, home-grown heat to occupants in bigger/smaller buildings. Maybe pellets could be poured down the same delivery chutes that are used for heating oil in New York City buildings?
Admiring the automated house-sized furnaces on display at the Expo made the possibilities seem so much better defined.
Burning wood slowly (and cleanly) can be a big part of the challenge when improving heating stoves. Air tight, insulated houses do not need big fires to stay warm. If the fire is too large, the house gets hot and folks are opening windows!
What firepower is needed to keep a house warm but not overheated?
“If you build a small, tight, well-insulated home — in other words, a green home — it won’t need much heat. Since typical residential furnaces and boilers are rated at 40,000 to 80,000 BTUH, they are seriously oversized for a super-insulated home, which may have a heating design load as low as 10,000 to 15,000 BTUH.” www.greenbuildingadvisor.com/article/heating-a-tight-well-insulated-house
15 thousand BTUs per hour is equal to about 4.4 kilowatts. (Burning ~2 pounds of dry wood per hour in a stove with a good heat exchanger.)
The required firepower of the heating stove is dependent on the losses from your house. Old-fashioned houses can require big fires to replace constantly lost heat.
The Green Building Advisor says that it is most cost effective to reduce the air leaks first and then increase the amount of insulation in a home.
Matching needed firepower to your heating design load should factor into your decision of what stove to buy. It is not difficult to do an energy audit of your home.
Institutional-size stoves like this Lihubesi stove frequently use a sunken pot or pot skirt to increase heat transfer efficiency.
While testing the institutional-size Alpha Limited TLUD, ARC staff conducted an experiment to see if a skirt is strictly necessary with a very large pot. A 58cm in diameter pot was heated by the six-inch in diameter Tom Reed Alpha Limited forced draft pellet stove with an added 0.75 constant cross sectional area Winiarski stovetop.
A complete stovetop was also made that increased heat transfer efficiency to the entire bottom of the pot. As-hot-as-possible gases are directed to flow as closely as possible to the surface without reducing their velocity.
The bottom of the 60 liter, 58cm in diameter pot (used in institutional stoves in Africa) had an external surface area of 2,640 square cm. The slanted Winiarski stovetop created a 5mm gap at the outer edges of the pot (See above).
The seven inch deep, Alpha Limited FD-TLUD stove ran for 82 minutes using 2.03 kg Douglas fir pellets. 20 liters of water boiled in ~60 minutes when a lid was placed on top of the pot. (A higher firepower stove is needed to boil 60 liters in a reasonable period of time).
The single test results were:
efficiency_with_char_ 57%
firepower_with_char_high power 4.80 kW
CO_useful_energy_delivered_ 1 g/MJd
PM_useful_energy_delivered_ 15 mg/MJd
Summary
When pots have sufficient bottom surface area, using a Winiarski stovetop can result in high thermal efficiency. After one hour, the highest temperature of gases in the 5mm channel gap under the outer edges of the pot was 111C. Adding a skirt to the sides of the pot would not be help very much when gas temperatures are this low.
Perhaps cooks would appreciate institutional stoves without sunken pots?
Schoolhouse Creek, filled with late winter run off, rushes along next to the Cafeteria building
The Aprovecho lab is in a green forest, eight miles east of Cottage Grove, Oregon, a town of ~10,000 people.
Working in a forest to design, manufacture and sell clean burning biomass cooking and heating stoves makes sense.
Heating and cooking with renewably harvested biomass is easy to imagine when surrounded by a forest.
Eating food from our gardens, grown with fertilizer from animals we enjoy, means that we know a lot about the food we eat.
Learning more about burning biomass cleanly enough to join solar, wind, hydro, and geothermal as sustainable energy resources, is intrinsically optimistic work.
We hope that you can visit our stove lab at Blue Mountain one of these days! Contactdean@aprovecho.org
https://aprovecho.org/wp-content/uploads/2025/02/IMG_2575.jpg480640Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2025-03-07 13:27:062025-03-07 13:36:18Working in a Forest
Lefebvre, Vanormelingen, and Udesen examined secondary air jets air in cylindrical TLUD combustion chambers and described the most successful patterns and penetration depths. They reported that air jet penetration lengths approaching the middle of flame in a cylindrical combustion chamber resulted in a maximum reduction of PM2.5. An increase in the number of jets also created more thorough mixing. They advised that it was important to have the jets meet in the middle of the flame, but with minimal necessary force, to ensure highest temperatures and highest velocity of hot gases to the pot. (Lefebvre, 2010) (Vanormelingen, 1999) (Udesen, 2019)
It has been interesting to test Tom Reed’s larger Alpha Limited stove. More powerful secondary air jets forcefully meet in the middle, forcing the flame to create a cyclone that plunges down into the cylindrical combustion chamber. As reported in last week’s newsletter, in one 94-minute cold start high power test with Doug fir pellets, the larger WoodGas stove and insulated SSM SuperPot, achieved 58% thermal efficiency with a white filter and very low PM2.5.
Perhaps downward cyclonic mixing, caused by more powerful secondary air injection, might be another clean burning technique when combustion temperatures stay high enough?
The Winiarski stove top that was added seemed to reinforce the downward cyclone. Larry always pushed better heat transfer efficiency in his pot supports, etc.
Tom probably invented using very little primary air and a lot more secondary air in forced draft TLUDs. So much easier to improve performance since Tom and Larry did a lot of the work!
https://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.png00Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2025-02-27 11:53:162025-02-27 11:53:17Secondary Air Injection
The Tom Reed Alpha Limited Forced Draft TLUD stoves (India)
The six-inch and four-inch in diameter FD-TLUD stoves are powered by two AA batteries and are well known to be inexpensive and clean burning. The smaller stove was the “high combustion efficiency” stove used in a 2015 Round Robin test series at Regional Testing and Knowledge Centers. As shown, the Tom Reed stove uses two crossed pieces of metal as pot supports.
The four inch in diameter ARC Round Robin test results were:
As a part of a recent Osprey stove improvement project, ARC added a stainless steel Winiarski stovetop to the smaller and larger Alpha Limited stoves that increased thermal efficiency and resulted in reductions of PM2.5 in the larger stove. The added stovetop seemed to encourage the injected, horizontal secondary air jets to more powerfully cover the top of the fuel?
Winiarski stove top
With the addition of the SSM Winiarski stovetop, (6mm pot supports and flat perimeter to accommodate a pot skirt) the larger Alpha Limited stove became a very clean burning TLUD. The ISO PM2.5 Tier 4 is less than 62 mg/MJd. The Tier 5 (an inspirational goal for PM2.5) is less than 5 mg/MJd. In a single test, the improved larger Alpha stove achieved 6mg/MJd for PM2.5 after burning for 94 minutes at 4.6 kW.
Thermal efficiency_w_char_
58%
firepower_w_char_high power
4.6 kW
CO_useful_energy_delivered_
1 g/MJd
PM_useful_energy_delivered
6 mg/MJd
PM mass time
1 mg/min
time_to_boil_high power
9.4 min (5 liters in SSM SuperPot)
ISO Tiers
Tier_efficiency_w_char
Tier 5
Tier_CO_useful_energy_delivered
Tier 5
Tier_PM_useful_energy_delivered
Tier 4
The thermal efficiency in the smaller diameter Alpha Limited stove was improved but the PM2.5 was not reduced when adding the Winiarski stovetop. The smaller stove ran for 26 minutes on 0.4 kg of Douglas fir pellets.
Thermal efficiency_w_char_
56%
firepower_w_char_high power
2.6 kW
CO_useful_energy_delivered_
1 g/MJd
PM_useful_energy_delivered
30 mg/MJd
PM mass time
3 mg/min
time_to_boil_high power
25.6 min (5 liters in SSM SuperPot)
ISO Tiers
Tier_efficiency_w_char
Tier 5
Tier_CO_useful_energy_delivered
Tier 5
Tier_PM_useful_energy_delivered
Tier 4
The following chart describes the features in the larger Tom Reed Alpha Limited FD-TLUD. Perhaps, adapting these hole sizes and air pressure, etc. to other stoves might result in reductions of emissions while increasing thermal efficiency?
Diameter of 13 Primary Air Holes (mm)
2.5
# Secondary Air Holes
36
Diameter Secondary Air Holes (mm)
4.7
Chamber Diameter (mm)
155
Chamber Area (mm^2)
487
Distance between Secondary Air Holes (mm)
13.52
Secondary Air Pressure (in H2O)
0.095
Secondary Air Pressure w/ Blocked Primary Air Holes (in H2O)
0.11
SSM Stovetop Hole Diameter (mm)
105
SSM Stovetop Hole Diameter / Cross Sectional Area
0.677
Give it a try?
Tell us what happens?
https://aprovecho.org/wp-content/uploads/2025/02/Tom_Reed_Stoves.jpeg216234Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2025-02-21 10:42:202025-02-21 10:42:37Wow! Tom Reed Forced Draft TLUDs are great!
With carbon prices low and support apparently shifting, perhaps thinking about market-based improved cook stoves is increasingly interesting?
In the Millennium Villages studies, a high-end retail price of something like $10 was recommended to sell stoves directly into the market. (Adkins, Tyler, al, 2010)
What can be accomplished in stoves already being sold without raising prices?
For instance, the pot supports in the Jiko shown above can be too high for optimal heat transfer efficiency
The door needs to be tight fitting to effectively simmer food
Perhaps the combustion chamber is too big, wasting fuel?
Small changes in the refractory ceramic material used in the combustion chamber can double durability
A low cost pot skirt effectively reduces time to boil and fuel used to cook
On the other hand, Rocket stoves or even forced draft TLUDs could theoretically be made for ~ $10, if metal was replaced by refractory ceramic or similar materials.
ARC is looking into this.
Sound interesting to anyone?
“To him that will, ways are not wanting.” (George Herbert, 1640)
https://aprovecho.org/wp-content/uploads/2025/02/533788023_ce0db0fd40_c.jpg600800Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2025-02-13 18:01:372025-02-13 18:01:38No More Carbon?
