Chart comparing energy output of 1 acre of grain vs 1 acre woodlot

One of my favorite reference books is “The Energy Primer” published in 1974. It has comprehensive review articles on solar, wind, water, and biomass energy. The following chart comes from a great article on biomass written by Richard Merrill. When I taught semester courses to college students at ARC, I tried to give students useful comparisons so they would be able to estimate the potential success of alternate technologies (unfortunately, fads that are bound to fail are all too prevalent in the green culture).

How much renewable energy can be grown on an acre of land? Can a family create an energy budget based on yearly production? As seen below, there are big differences in the amounts of energy that can be produced by a one acre grain field or one acre woodlot.

Energy output of 1 acre of grain vs. 1 acre woodlot, from “The Energy Primer”

One acre of hay yields something like 29 million BTUs per year. One acre of trees is better, producing an estimated 42 million BTUs per year.

If the hay is turned into alcohol the yield is greatly reduced (6 million BTU/year) and the average yield of 3.5 tons per acre of trees is approximately 8.5 million BTU/year.

If the hay is fed to cows and the manure is turned into methane the energy content is 15 million BTU/yr.

Burning biomass for heating and cooking can be a lot more efficient than making alcohol or methane to be used for the same purpose.

At ARC, after decades of “living on the land”, we think that one or two acres of biomass for energy and five acres for food is a good place to start calculations when planning for a secure and happy family. It’s amazing to own land!

Dr. Tom Reed, Dr. Alexis Belonio, Dr. Paul Anderson and Kirk Harris have refined TLUD technology

A natural draft TLUD can be as clean burning as a forced draft TLUD burning wood pellets. On the other hand, a natural draft Rocket stove needs a fan to be clean burning.

Dr. Tom Reed’s forced draft (FD) Woodgas stove achieved an emissions rate of 2mg/min of PM2.5 with pellet fuel (ARC, 2015). The Kirk Harris natural draft (ND) TLUD emitted 0.7mg/min of PM2.5, when tested at Lawrence Berkeley National Lab with pellets.

WHO Intermediate Emission Rate Targets

Unvented stoveVented stove
PM2.51.75 mg/minPM2.5   1.75 mg/min
CO 0.35 g/minCO   1.45 g/min

ND TLUDs tend to be pretty tall to generate necessary draft. A FD TLUD can be shorter since the fan creates the draft. Taller ND TLUDs can be expensive to manufacture. Precise control of primary air enabled a 3 to 1 turn down ratio in the Harris ND TLUD, difficult to achieve in a FD TLUD. (The FD Mimi-Moto, for example, has two combustion chambers, small and large, to provide cooks with high and low power).

Modern ND TLUDs

  • The primary air is adjusted to control the rate at which pellets are turned into combustible gases.
  • The secondary air jets cover the fuel bed.
  • A hole in the concentrator plate above the secondary air jets forces the flame into a vertical cylinder.
  • The cylinder of flame then enters static devices that create further mixing of air, flame, and gases.
  • The flame is shortened and does not touch the bottom of the pot where gases can condense into smoke.
Static mixers in the Kirk Harris 0.7mg/min PM2.5 ND TLUD
A St. Ayles skiff, my favorite boat

Before I met Dr. Larry Winiarski I was a boat builder, but I had already realized that my love for making boats was mostly supported by rich people. And when my friends and I built a 36’ ocean going sailboat it was great but after several years of exploring it started to be a bit self-serving. When Larry showed me that my carpentry skills could help him develop Rocket stoves to try to help people, I ended up being much happier.

Since Appropriate Technology is intended to be affordable, experiments do not cost very much. Low cost experiments enable anyone to improve necessary things like wood burning stoves. Using my skills to try to address a real problem was a lot more fulfilling. Including users in the process meant that I spent a lot of time with cooks and manufacturers who are the real experts. In India I lived in 18 villages working with groups of women who created the short Rocket stove now built around the world.

I wish that I had met Larry in grade school! Knowing that anything I learned could be useful would have made a big difference. Just reading and learning without an intended purpose seemed to me to be rather meaningless.

Doing experiments every day on stoves has helped me as a person. I had also made science toys and sold them at craft fairs, but again even though I loved making the crafts I ended up feeling unfulfilled just entertaining people. I wanted to do something that was more helpful. Finding a good problem to try to solve has helped me a lot. I include finding a good problem in my prayers for lots of people.

There are hundreds of good problems for folks to work on in Appropriate Technology. I’m thinking about teaching a class to local students in the hope that the meaning Larry passed on to me could work for them as well. If you would like to solve a problem, we can suggest many possibilities.

Wishing you all the best in the coming year,
Dean Still

In our Nov. 24 newsletter, we shared a basic description of how wood burns from Samuel Baldwin’s book “Biomass Stoves: Engineering Design, Development, and Dissemination” (1987). Here are more details about the process from the same book:

“The temperature of the hot gas above the wood is typically around 1100ºC and is limited by radiant heat loss and by mixing with cold ambient air. As the volatiles rise they react with other volatile molecules forming soot and smoke and simultaneously burning as they mix with oxygen. Some 213 different compounds have so far been identified among these volatiles. If a cold object, such as a pot is placed close to the fire, it will cool and stop the combustion of some of these volatiles, leaving a thick black smoke.”

“Overall, these burning volatiles account for about two-thirds of the energy released by a wood fire. The burning charcoal left behind accounts for the remaining third. Because the volatiles are released as long as the wood is hot, closing off the air supply stops combustion alone. The heat output of the fire is then reduced but the wood continues to be consumed for as long as it is hot, releasing unburned volatiles as smoke and leaving charcoal behind.”

“As the topmost layers gradually lose all their volatiles only a porous char is left behind. This hot char helps catalyze the breakdown of escaping volatile gases, producing lighter, more completely reacting gases to feed the flames. In some cases, the volatiles cannot easily escape through this char layer. As they expand and force their way out, they cause the burning wood to crack and hiss or spit burning embers.”

“The char layer also has a lower thermal conductivity than wood. This slows conduction of heat to the interior and thus slows the release of volatiles to feed the flames.”

“At the surface of the char, carbon dioxide reacts with the char’s carbon to produce carbon monoxide. Slightly further away (fractions of a millimeter) the greater oxygen concentration completes the combustion process by reacting with the carbon monoxide to produce carbon dioxide. The temperature near the surface of the burning charcoal surface is typically about 800ºC. The endothermic (heat absorbing) dissociation of carbon dioxide to carbon monoxide and oxygen, and radiant heat loss, limit higher temperatures.”

“When all the carbon has burned off only mineral salts remain as ash.  This ash limits the flow of oxygen to the interior and so limits the combustion rate. This is an important mechanism controlling the combustion rate in charcoal stoves.”

“The entire process uses about 5 m³ of air (at 20ºC and sea level pressure) to completely burn 1 kg of wood. To completely burn 1 kg of charcoal requires about 9 m³ of air. Thus, a wood fire burning at a power level of 1 kw burns 0.0556 grams of air per second. Additional excess air is always present in open stoves and is important to insure that the combustion process is relatively complete.”

Photo: Karl Maasdam/OSU Foundation

Aprovecho Research Center is pleased to announce that Dr. Nordica MacCarty has accepted the role of Executive Director. Dr. MacCarty takes over the position from Dean Still, who continues on as Research Director. Dr. MacCarty is also an Associate Professor of Mechanical Engineering at Oregon State University where she will continue teaching and directing the Humanitarian Engineering program as the Richard and Gretchen Evans Scholar of Humanitarian Engineering.

Nordica first came to Aprovecho in the summer of 2000, just after completing her BS in Mechanical Engineering at Iowa State University. She spent the summer testing and experimenting with Rocket Stoves, and attended the very first ETHOS Conference, which was held at the Aprovecho campus. “That summer changed my life and career goals” she said, “and my dream now is to help Aprovecho continue to bring accessible designs to improve lives well into the future.”

Prior to joining the OSU faculty in 2015, she spent nearly 10 years working for Aprovecho as an international consultant building capacity at projects and universities abroad for the design and testing of renewable household energy systems, and was a National Science Foundation Graduate Research Fellow. Dr. MacCarty holds an MS and a PhD in Mechanical Engineering from Iowa State. She serves as Associate Editor for the journal Energy for Sustainable Development and was recently recognized with the Elevating Impact Award for social entrepreneurship from the Lemelson Foundation and OSU’s 2020 International Service Award. She is also the lead PI on the $2.5 million US DOE grant recently awarded to the OSU and Aprovecho team to design cleaner cordwood heating stoves for US and international markets. ( )

The Aprovecho team feels that Dr. MacCarty is a valuable addition, due to her interests in understanding the relationships between energy, society and the environment, and engineering design applied to global humanitarian needs. “Nordica is my perfect replacement because she combines expertise from the field, the lab and academia” said Dean Still. “I couldn’t think about leaving without competent replacement, and I have zero doubts that she will do better than I did.”

sticks burning in rocket stove

Sometimes it’s good to step back and review the very basis of stove work – fire. Samuel Baldwin gives a good description of how wood burns in his book “Biomass Stoves: Engineering Design, Development, and Dissemination” (1987).

“The combustion of wood and other raw biomass is very complicated but can be broken down crudely into the following steps:”

“The solid is heated to about 100ºC and the absorbed water is boiled out of the wood or migrates along the wood grain to cooler areas and re-condenses. At slightly higher temperatures, water that is weakly bound to molecular groups is also given off.  Heat transfer through the wood is primarily by convection.”

“As the temperature increases to about 200ºC, hemicellulose begins to decompose followed by cellulose. Decomposition becomes extensive at temperatures around 300ºC. Typically only 15% of cellulose and hemicellulose remain as fixed carbon and the remainder is released as volatiles gases. Roughly 50% of the lignin remains behind as fixed carbon”

“The volatiles produced by this decomposition may escape as smoke or may re-condense inside the wood away from the heated zone. This can often be seen as pitch oozing out of the non-burning end of the wood. Heat transfer into the wood is still primarily by conduction, but the volatiles flowing out of the heated zone carry some heat away by convection.”

“As the volatiles escape the wood, they mix with oxygen and, at about 550ºC, ignite producing a yellow flame above the wood. Although radiant heat from the flame itself (not counting radiant emission from the charcoal) accounts for less than 14% of the total energy of combustion, it is crucial in maintaining combustion. Some of the radiant heat from this flame strikes the wood, heating it and causing further decomposition. The wood then releases more volatiles, which burn, closing the cycle. The rate of combustion is then controlled by the rate at which these volatiles are released. For very small pieces of wood, there is a large surface area to absorb radiant heat compared to a little distance for the heat to penetrate or for the volatiles to escape. Thus, fires with small pieces of wood tend to burn quickly. This is also why it is easier to start a small piece of wood burning than a large thick one. A thick piece of wood has less area to absorb the radiant heat from the flame compared to the greater distances through which the heat and volatiles must pass within the wood and the larger mass that must be heated.”

Testing a Chitetezo Stove with a Jet-Flame

What do I like most about Aprovecho Research Center? We have created a “culture of daily experimentation.”

Scheduling two to three experiments every morning means that we can try less-likely-to-succeed variations. That’s great because experiments that don’t succeed in improving a stove often reveal interesting and possibly unexpected results. And that can lead to new insights. “Accident favors the prepared mind,” Pasteur reminds us.

Nordica MacCarty, our new Executive Director, recently suggested that it would be a good idea to see what happens when the Jet-Flame is used without a Rocket combustion chamber. We have been working with Christa Roth and Christoph Messinger from GIZ, investigating what happens when the Jet-Flame is used in the Chitetezo stove. It was easy to continue the investigation into the effects of primary air jets blowing up into the fuel. We removed the rocket-style combustion chamber that we were testing, and put a short metal fence around the Jet-Flame to keep the sticks on top of the air holes. A SuperPot was used and set directly on top of the stove.

It’s been very interesting to try different sized sticks with the Jet-Flame. Generally, small sticks burn faster and make a lot of flame, but also more smoke. Two 4 cm x 4.5 cm sticks burn more slowly and emit less smoke but the firepower is lower. With no insulation around the combustion zone, we tried burning four sticks that were 2 cm x 2.5 cm. The results were surprisingly good: Tier 4 for thermal efficiency, Tier 4 for PM2.5, and Tier 5 for CO.

The size of the sticks made a significant difference on all three measures, which makes sense and has been seen in natural draft stoves. But it’s noteworthy that the size of the sticks results in big differences with the Jet-Flame. Following up on Nordica’s suggestion made for a more interesting week here at the lab. Completing ten to fifteen half-hour experiments weekly makes it more likely that progress is being made.

Reading/learning in front of the computer is important, and OK for a while, but we also like to get into the lab to try and create/learn something new.

The Gates funded Global Health Labs and ARC/SSM invented the Jet-Flame

Global Health Labs, Shengzhou Stove Manufacturer and Aprovecho teams show off the Jet-Flame
Global Health Labs, Shengzhou Stove Manufacturer and Aprovecho teams show off the Jet-Flame

Shengzhou Stove Manufacturer manufactures the Jet-Flame (jet- that is being field tested in over 30 locations. Our lab helped to create this accessory that is designed to reduce emissions while increasing thermal efficiency and reducing time to boil. 30 pre-heated primary air jets shoot up into the fire resulting in increased molecular mixing and elevated temperatures. Smoke is reduced by about 90% compared to an open fire.

A forced draft stove can be very clean burning, but start up may create a lot of PM2.5. This is because the cold combustion chamber can allow a higher percentage of the smoke and Carbon Monoxide to escape unburnt. David Evitt, COO of ASAT (the for-profit arm of ARC), invented a method for lighting the Jet-Flame that can be a lot cleaner.

  • Wet 30 grams of left over charcoal with 10 grams of alcohol.
  • Place the small pile of charcoal on top of the holes in the Jet-Flame.
  • Light the charcoal.
  • Turn on the Jet-Flame.
  • Push the tips of the sticks of wood against the pile of burning charcoal.
  • Keep on pushing the sticks into the fire as the tips are consumed.

Here’s a video showing how we light fires in the lab:

Typical Charcoal Stove

Charcoal stoves are batch loaded like TLUDs (top loaded up draft stoves), but they differ greatly in how air flow is used to encourage combustion. A charcoal stove has lots of primary air blowing up into the fuel. A TLUD uses a small amount of primary air to create a relatively constant amount of woodgas that is then combusted by a mixture of secondary air and flame above the fuel bed. The large amount of primary air in a charcoal stove tends to create the fire underneath the fuel, while the secondary air jets in a TLUD concentrate the fire above the batch of fuel.

If a TLUD allowed as much primary air as a charcoal stove into the bottom of the fuel bed, too much wood gas would be made and the TLUD would be smoky. I wonder if charcoal stoves could be patterned after TLUDs? Could a charcoal stove be top lit with jets of secondary air and flame on top of the batch of fuel? Would such an arrangement result in better performance? Would added secondary air in a charcoal stove maintain the fire on top of the fuel that would burn up the woodgas (predominantly CO in a charcoal stove)? Might this increase heat transfer efficiency since the temperatures (convection and radiation) might be hotter?

When Sam Bentson and Ryan Thompson studied how to improve charcoal stoves in the DOE 2011 project, they found that adding pre-heated natural draft jets of air did help to create more flame where it’s needed, right under the pot. Unfortunately, they also experienced that charcoal makes less flame than wood and covering the entire top of the fuel bed with flame (as in a TLUD) is difficult. Forced draft secondary air might be more successful, especially since charcoal stoves are usually short and do not generate needed velocities. To be most effective, the jets of air/fire should meet in the middle of the combustion chamber and cover the entire fuel bed. It seems to be best to use the minimum velocity necessary since too much air only cools the fire (Udeson, D.J., 2019, University of Washington).

We’ll try it and report on the results.

Charcoal Stove Design Principles (Ryan Thompson)

  1. Size the combustion chamber for the required task. Whatever fuel is loaded into the stove will be burned at a rate proportional to the amount of air made available to it. In most cases, more fuel means higher firepower because there is always excess air. For small amounts of food/water, where not much power is required, either load a small amount of fuel into the stove, or use a stove with a small combustion chamber. For cooking lots of food at once, use a stove with a big combustion chamber.
  2. Charcoal stoves can have a high turn-down ratio. If the primary air supplied to a charcoal fire is reduced close to zero, the fuel will still keep burning. When the air supply is increased, the firepower will also increase. Normally there is a spike in CO when the air supply is increased sharply, but it tends to stabilize once the firepower comes back up.
  3. Use pre-heated secondary air jets to burn up the CO. Most charcoal stove designs have air coming in from the bottom and sides of the fuel bowl. The air must pass through the burning charcoal before it gets to the top of the fuel where the CO is being emitted. If hot air is added above the charcoal, it is available to combust the CO. Try to keep the bottom of the batch of charcoal as cold as possible and burn down from the top of the batch of the fuel.
  4. Position the pot close to the charcoal. This maximizes heat transfer from radiation.
  5. Insulate the stove body. Insulate the stove body until a temperature of 620˚C or higher is achieved above the burning fuel. Insulation needs to be lightweight and trap still air. Examples of good insulation are: ceramic fiber, rock wool, wood ash, sheets of foil, etc.
  6. Use small channel gaps between the pot and the stove. The pot can be located very close to the top of the burning charcoal. A skirt with a 6mm channel gap around the pot helps increase the heat transfer efficiency from convection. Both convective and radiative heat transfer are important in a charcoal stove.
  7. Maintain a constant cross sectional area throughout the stove to start the design process and reduce as directed by experimentation under the emissions hood. This will help to keep the velocity of the draft as high as possible.
  8. Supply a large amount of primary air to assure sufficient firepower. Make the primary air door large enough to boil the water quickly. The door can be partially closed to reduce power when desired. The door has to be almost airtight to reduce firepower sufficiently when simmering in a pot with a lid.
Maria Telkes NYWTS


Last week we shared info about retained heat cooking. It’s frequently paired with solar cooking for feeding folks with no earth-generated fuel expenditure at all. But does it work? The cooks at ARC think that it’s great  – in the summer time when we have lots of sunlight.

Our preferred solar oven was invented in 1953 by Mária Telkes. She was a Hungarian-American biophysicist, scientist and inventor who became well known for her work in solar energy. Her many inventions also included a solar home heating system and solar powered water desalinator.

We like Telkes solar cookers that are big enough to generate firepower that is similar to wood burning cookstoves. A solar cooker is so easy to use and so much cleaner! Of course it is different because a Telkes solar cooker is an oven that is outside on the porch of the kitchen. The cook stands in the shade from the roof and the solar cooker basks in the sun. Just point it at the sun, put the food in the oven, and you are free to do everything else.

Here’s how we do the simple math to create a powerful Telkes solar cooker. Let’s say, as rules of thumb, that:

  1. There are about 250 BTUs in a square foot of sunshine per hour.
  2. There are approximately 8,600 BTU in a pound of wood. Remember, wood is stored solar energy.
  3. Therefore it takes a solar oven with about 34 square feet of intercepted sunlight to equal the cooking power of one pound of wood burned in an hour.
  4. In a solar oven with the firepower of one pound of wood (burned in an hour) the intercepted sunlight should be about 6′ by 6′. This is the measurement at the top edges of the solar reflectors – the widest part of the cooker.
  5. About 1/3 of the energy cooks the food and about 2/3rds of the energy is lost. 
  6. In our 6′ by 6′ solar oven, 3,000 BTUs would boil 2 gallons of water in about an hour. The light weight pot is black, it has a tight lid, the oven is well insulated, and airtight. The glass is double glazed. The losses are minimized and the solar gain is optimized with large reflectors on all sides. Large amounts of food can be made every sunny day without using up any earthly resources. 

In our experience, solar cookers are great when they are big enough to do the cooking task in a reasonable amount of time. ARC cooks have used them in the summer to cook lunch and dinner for 20 people and it’s nice to have a no-fuss oven that needs little tending. Solar cooking is certainly more comfortable when cooks don’t have to deal with a hot fire on summer days. At the Aprovecho farm, the staff have used stored solar energy in the winter (biomass) and direct solar energy (sunlight) in the summer for cooking, heating water, etc.

Of course, everything is dependent on sunlight.