Introducing Aprovecho’s New Executive Director, Dr. Nordica MacCarty 

/1 Comment/in
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. (https://today.oregonstate.edu/news/oregon-state-receives-25-million-grant-create-wood-stoves-burn-more-cleanly )

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

Back to basics – FIRE!

/0 Comments/in

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.”

A Culture of Daily Experimentation

/0 Comments/in ,
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.

Video: Lighting a Fire With The Jet-Flame – No Smoke

/0 Comments/in , , ,

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- flame.com) 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:

Secondary Air Jets in Charcoal Stoves?

/0 Comments/in ,
Typical Charcoal Stove
Oorja FD TLUD

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.
Summertime_Solar_Cooking

Cooking With The Sun

/0 Comments/in
Maria Telkes NYWTS
Summertime_Solar_Cooking

 

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.

Retained Heat Cooking

/0 Comments/in
Use moisture proof insulation! Wet insulation doesn’t work well. Illustration from solarcooking.org 

While at ARC we focus on how to cook most efficiently with biomass, it is good to remember that some cooking can continue without consuming fuel. A retained heat cooker (RHC), also known as a Haybox, is a great way to save on fuel for appropriate cooking tasks such as simmering rice or beans.

How does a retained heat cooker (RHC) help when cooking? When food simmers, the fire replaces the constantly lost heat from the pot. If the heat were not lost but captured instead, then less fuel would be needed for cooking. Placing the pot of boiling food in an insulated container keeps the food hot enough to simmer it to completion. In the same way, a drafty and uninsulated house has to have a big fire in the heating stove going all the time to keep the house warm. Even if no fire is lit, the super-insulated, almost airtight house can stay warm for a long time.

After a pot of food boils, the contents are close to 100°C. When the hot pot is placed in a super-insulated, almost airtight box, the food finishes cooking, because the stored heat stays in the food. Once the pot is in the box, food cooks without further attention. The retained heat cooker, saves time, effort, and fuel, freeing the cook from long hours of watching the slow fire when simmering food.

Approximately 50% savings in time and fuel savings can be expected. The rice or stew won’t burn and the cook can make dessert! Because the fuel is only used for boiling food, cooking with a Haybox creates much less pollution, helping to clean up the air in the kitchen. In tests of 18 stoves, using a retained heat cooker reduced, on average, CO emissions by 56% and PM emissions by 37% . (Test Results of Cook Stove Performance, 2011)

RHCs have been used for hundreds of years. They can save time and effort that can be devoted to other tasks. The attraction begins with convenience. The fuel savings and decrease in harmful emissions add to the benefits of retained-heat cooking. More information on Hayboxes can be found in the EPA’s “Guide to Designing Retained Heat Cookers.”

0.75 Constant Cross Sectional Area: A Winiarski “Rule of Thumb”

/0 Comments/in
Uganda 2-pot rocket style stove

The Uganda 2-pot stove that is described on page 26 in the EPA publication “Test Results of Cook Stove Performance” is a natural draft stove that also uses much less fuel to cook and protects health. The document can be found at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100EKU6.TXT.  Has it been a while since you looked at this book, a comparison of 18 cook stoves? The 2011 book started our surveys with the emissions hood and Test Kitchen, trying to quantify comparisons of fuel use and emissions from available stoves.

Dr. Nordica MacCarty’s paper with comparisons of 50 stoves is a much more complete survey. See: “Fuel use and emissions performance of fifty cooking stoves in the laboratory and related benchmarks of performance” (MacCarty, et al, 2010) https://www.sciencedirect.com/science/article/pii/S0973082610000311

The Uganda 2-pot stove has a Rocket combustion chamber. The hot gases made by the fire pass through narrow, insulated channels around the first pot, which is sunk into the stove. The gases then flow through an insulated tunnel and are forced into narrow channels around the second pot before exiting the chimney. The pots fit tightly into holes in the sheet metal top, preventing smoke from escaping into the kitchen. This stove is fast to boil and, because of the sunken pots, uses less wood than most stoves with chimneys. 

When we build and test stoves we often reflect on Larry Winiarski’s advice that helped to improve the Ugandan stove. Larry advised us that in a 2-pot horizontal stove, channel gaps around the pots that are 0.75 constant cross sectional area are a good compromise between maintaining needed draft and increasing heat transfer efficiency. The cross sectional area of the Ugandan fuel entrance in the Rocket combustion chamber was about 16 square inches so we made the channel gaps all the way to the chimney at 0.75 times 16 square inches. We use Larry’s “rule of thumb” and tests remind us how well Larry knew stoves. He had a good touch.

Constant cross sectional area through a cookstove.


Forced Draft and High Mass Combustion Chambers

/0 Comments/in , , ,
  • Oorja FD TLUD
  • CQC w/FD Rocket

Years of experimenting with stoves at ARC taught us that a high mass combustion chamber absorbs a lot of heat that could be going into the cooking pot. That led us to presume that efficient combustion chambers had to be lightweight, which are harder to make. Once we started experimenting with forced draft, we were surprised to learn that adding forced draft (FD) to a TLUD or a Rocket stove increases the temperature of the gases and largely overcomes the negative effect of a high mass combustion chamber.

The Oorja FD-TLUD and the FD-CQC mud brick stoves generate temperatures of around 1,000°C in the combustion chamber. The option to use a high mass combustion chamber lowers cost and dramatically increases durability when designing forced draft, health protecting, affordable, clean burning, carbon neutral stoves.

The combustion chamber in the pellet burning FD-Oorja that we have in the lab is made from castable refractory ceramic and is over 20 years old. The retail price of the 400,000 British Petroleum Oorja stoves sold in India was around $18. The CQC Rocket combustion chamber is less expensive and is manufactured from sand, clay, and cement. With the addition of forced draft via a Jet-Flame, it reaches Tier 4 for  thermal efficiency and PM 2.5.

To replace health protecting stoves that use natural gas (a fossil fuel) it seems likely that FD-TLUDs and FD-Rockets can be built with lower cost and 10 year durability combustion chambers. The renewably-harvested-biomass fueled stoves need to be manufactured and field tested, and there is a lot of ground to be covered (an understatement), but it’s great that harder-to-make low mass combustion chambers may not be necessary.

Woodgas

/1 Comment/in

Dr. Tom Reed frequently talked about selling a billion clean burning woodgas stoves. Now that the Biden administration is pointing out that natural gas (and electricity made from fossil fuels) are things of the past, it can be imagined that using woodgas to cook may become a larger part of a post-fossil-fuel future.

“On January 27th, the President announced a series of climate actions that may well mark the beginning of the end of the fossil-fuel era…There’s a shock-and-awe feel to the barrage of actions, and that is the point: taken together, they send a decisive signal about the end of one epoch and the beginning of another. And that signal, most of all, is aimed at investors: fossil fuel, Biden is making clear, is not a safe bet, or even a good bet, for making real money. Coal, oil, and gas are the past, not the future.”

-Bill McKibben, The New Yorker, January 28, 2021

Unfortunately, using biomass for cooking is a difficult replacement because:

  • Smoke from cooking with wood is very dirty, damaging human health. 
  • Smoke is something like 680 times worse for climate change compared to CO2 by weight. (Roden, Bond, et al, 2008)
  • The wood fuel needs to be renewably harvested.
  • Although how to manufacture clean burning, carbon neutral biomass stoves is better understood, stoves need to be affordable to capture substantial market shares.
  • In the Millennium Villages, a retail price of something like $10 has been recommended to sell stoves directly into the market. (Adkins, Tyler, al, 2010)

Can affordable, clean burning, carbon neutral stoves be manufactured and sold?

In 2021, there seem to be at least two popular design options. Both rely on inexpensive, long lasting, refractory ceramic combustion chambers and prepared fuels: 

  • Natural draft or forced draft TLUDS, burning pellets
  • Forced draft Rockets, burning dry sticks of wood

Next week, we’ll explore these options.