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?
From SAMUEL BALDWIN’S “BIOMASS STOVES: ENGINEERING DESIGN, DEVELOPMENT, AND DISSEMINATION,” VITA, 1987
Various stove/pot/skirt combinations are achieving ~ 60% thermal efficiency.
How high can we go?
Doubling temperature doubles heat transfer efficiency when other factors remain constant.
According to Newton’s Law, doubling the surface area doubles the heat transfer.
Forcing hot gases to thin the boundary layer of still air next to the surface to be heated (Proximity) effectively increases heat transfer efficiency (as above).
Doubling the Velocity of gases ~doubles heat transfer efficiency.
Increasing radiation increases heat transfer exponentially. *See chart below.
Increasing the view factor helps, too! (That’s the proportion of radiation that contacts the bottom of the pot.)
Prasad and others have suggested a correlation between firepower and area.
There may be other important factors?
In a modern Rocket stove at high power, the gases can be around 800C and the velocity can be around 1.2 meters per second.
Small, dry pieces of wood tend to make hotter fires and gases.
Pots have to have sufficient external surface area to achieve 50% thermal efficiency.
In ARC tests of modern Rocket stoves, a pot with an area of around 800cm2 scored 34% thermal efficiency. Increasing the area to around 1000cm2 increased thermal efficiency to about 40%. With the same stove, a pot with 1200cm2 is expected to achieve above 45%. ARC uses 26cm to 30cm in diameter pots with at least 5 liters of water to get closer to 50% thermal efficiency.
Keep in mind that increasing the surface area of the water in a pot also increases the amount of steam emitted, which makes it harder to bring water to full boil in a larger pot (without a lid).
Thermal efficiency, when burning biomass, seems to top out (so far) at around 60%. Perhaps the gases in the channels at the bottom and sides of the pot loose temperature and velocity, resulting in a theoretical upper limit to normal natural draft heat transfer efficiency?
Since doubling velocity ~ doubles heat transfer efficiency it seems likely that if forced draft increased velocity, without reducing gas temperatures, good things might happen?
We’ll give it a try.
From The Woodburner’s Encyclopedia, 1976
https://aprovecho.org/wp-content/uploads/2021/01/baldwin-household-graph.png15542136Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2025-02-06 11:27:442025-02-14 10:10:37Thermal Efficiency: How High Can We Go?
Two dollar and fifty cent SSM adjustable pot skirt
Aprovecho staff travel around the world assisting stove projects. We experience that almost all biomass stoves in Low Middle Income Countries markets are sold for $10 USD or less. It seems to us that more expensive stoves are supported by carbon revenue, or sold in cities to the middle or upper classes.
Depending on carbon revenue works well when prices do not fall, or when events don’t restrict trade. Aprovecho has learned a lot about the carbon market and helps projects to make best use of opportunities.
Aprovecho also develops market-based products trying to create sustainable businesses independent of carbon. Dr. Winiarski was a great proponent of market-based solutions that reduce fuel use and harmful emissions. He pointed out that improved thermal efficiency can be added to traditional stoves sold in markets without increasing cost. Week-long Partnership for Clean Indoor Air seminars (2002-2012) in Asia and Africa reduced fuel use by an average of ~ 30% with simple changes.
The size of the combustion chamber in a charcoal stove has been shown to be the most significant factor in fuel use. Maybe reducing the size of the combustion chamber in a traditional charcoal stove currently being sold would end up saving fuel in real life? See: The influence of initial fuel load on Fuel to Cook for batch loaded charcoal cookstoves (Bentson et al, 2013)
The $2.50 USD SSM adjustable pot skirt has reduced fuel use by 20% to 25%. Aprovecho hopes that factories/stakeholders can improve popular stoves by applying easy-to-teach changes.
There are many no extra cost improvements that are incremental first steps.
They tend to be affordable and market based, which made Dr. Winiarski happy.
https://aprovecho.org/wp-content/uploads/2023/06/1.-Pot-Skirt.jpg501800Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2025-01-30 16:16:172025-01-30 17:01:24Improving Market Based Products to Reduce Wood Use and Emissions
Thanks to the Osprey Foundation, ARC just finished building a new heating stove lab and we are experimenting with how to make very clean burning home heating stoves. The intended price points are considerably lower than higher emission stoves currently for sale. Zero Green Premium products cost less than the dirty technology products they replace.
We change one variable at a time in a prototype and test the result under the emissions hood that collects and records the amounts of climate gases and Black Carbon. Usually ~50 iterations result in a closer to optimal stove. The new Osprey Health and Climate Heating Stove Lab is set up to do 3 to 6 iterations per week. Lab staff includes Travis Volpe who builds, tests and changes prototypes. The prototype is thoroughly field-tested, as well.
INDOOR AIR QUALITY GUIDELINES: HOUSEHOLD FUEL COMBUSTION, 2014
A “Cut and Paste” Summary
PM levels have to be very low to protect health
“Among the key findings is that for several important health outcomes, including child acute respiratory infections, exposure to the key pollutant– fine particulate matter, or PM2.5 – needs to be brought down to low levels in order to gain most of the health benefit. The other main finding is that most of the solid fuel interventions promoted in recent years have not even come close to these levels when in everyday use, and there is a need for much more emphasis on accelerating access to clean household fuels.” ( pg. XIV)
Avoid the use of coal and kerosene
“The need to avoid the use of unprocessed coal as a household fuel, in light of the specific health risks. The need to avoid the use of kerosene as a household fuel, in light of concerns about emissions and safety.” (pg. XV)
4.3 million yearly deaths from biomass indoor pollution
“Global burden of disease estimates have found that exposure to HAP (Household Air Pollution) from cooking results in around 4 million premature deaths, with the most recent estimates from WHO reporting 4.3 million deaths for 2012. HAP is responsible for nearly 5% of the global disease burden (expressed as disability-adjusted life-years (DALYs)), making it globally the single most important environmental risk factor.” (pg. 1)
0.4 million yearly deaths from biomass outdoor pollution
“HAP is also a substantial contributor to outdoor air pollution-related deaths due to emissions into the ambient environment, responsible for around 0.4 million deaths (12% of the total from ambient air pollution (AAP)).” (pg.1)
WHO: Outdoor air quality guidelines
Pollutant PM2.5 (μg/m3)
Mean concentration per 24 hours: 25 μg/m3. Per year: 10 μg/m3
Pollutant CO (mg/m3)
Mean concentration per 8 hours: 10 mg/m3. Per 24 Hours: 7 mg/m3 (pg. 11-12)
The mathematical model used to estimate indoor pollution was based on:
The input data used for the model were obtained from measurements made in India, and are summarized in Table 2.4.
Air exchange rate (α) per hour 15
Kitchen volume (V) m3 30
Device burn time hours per day 4
Recommended emission rate: PM and CO per minute
Unvented PM2.5: 0.23 (mg/min)
Vented PM2.5: 0.80 (mg/min)
Unvented CO: 0.16 (g/min)
Vented CO: 0.59 (g/min) (pg.34)
Estimated to “result in 90% of homes meeting WHO AQG values for PM2.5 (annual average) and CO (24-hour average). This assumes model inputs for kitchen volume, air exchange rate and duration of device use per 24 hours, as set out in Table R1.1.
Intermediate emission rate targets (IERTs) show the rates that will result in 60% of homes meeting IT-1 for PM2.5 (Table R1.2) and 60% of homes meeting the 24-hour AQG for CO (Table R1.3). The value of 60% is arbitrary, but was selected so that a majority of homes would meet the specified guideline level.
Separate guidance is provided for unvented and vented stoves as those technologies with chimneys or other venting mechanisms can improve indoor air quality through moving a fraction of the pollutants outdoors.” (pg. 35)
Intermediate Emission rate targets (ERTs)
Unvented PM 2.5: Intermediate ERT 1.75 mg/min
Vented PM 2.5: Intermediate ERT 7.15 mg/min
Unvented CO: Intermediate ERT 0.35 g/min
Vented CO: Intermediate ERT 1.45 g/min (pg.36)
Ventilation lowers concentrations
“There are many areas where outdoor or semi-outdoor cooking is prevalent, for which ventilation is clearly greater and would result in a higher percentage of homes meeting the AQGs than those estimated for the ERTs. Studies show that people cooking outdoors still receive high exposure when using traditional stoves. Furthermore, as previously discussed, emissions to the outdoor environment reduce community ambient air quality, which in turn contributes to lower indoor air quality. Thus, although the emission rate targets apply to indoor environments, maximizing protection can only be achieved if all devices meet these targets regardless of indoor or outdoor usage.” (pg. 39)
The need for chimneys
“Evidence provided in the systematic review of Intervention impacts on HAP and exposure (Review 6) demonstrated that despite achieving large percentagereductions of PM2.5 compared to baseline (solid fuels with traditional stoves)none of the improved solid fuel stoves reviewed reached the WHO IT-1 forannual average kitchen PM2.5 (and therefore did not meet the AQG). A few types of vented (chimney) stoves did reach levels close to WHO IT-1, in the range of 40–60 μg/m3. These findings can be used as a guide to the current in-field performance of a range of technology and fuel options.” (pg.45)
Multiple stove use continues
“A common finding was that many (if not most) households continue to use the existing device or fuel when a new one is introduced, for cultural and practical reasons such as lack of affordability and uncertain supply in the case of a commercial fuel such as LPG. An important conclusion therefore was that for most households, the transition to exclusive use of very low emission devices and fuels will occur over time, with a progressive shift towards a higher proportion of energy usage provided by the newer, cleaner options. It is also the case that in more economically challenging conditions, households may revert to increased use of traditional stoves and fuels.” (pg.46)
CO concentrations are achievable
“The systematic review of the impacts of interventions found that most of these achieved CO levels below the 24-hr WHO guideline of 7 mg/m3.” (pg. 47)
Solid fuel stoves are important
“As recognized in these guidelines, and specifically in Recommendation 2, which addresses policy during transition, improved solid fuel stoves will continue to make an important contribution to the needs of a substantial proportion of lower income and rural homes where primary use of clean fuels is not feasible for some time to come. Work to develop substantially improved solid fuel stoves should continue in parallel with, but not hinder or displace, efforts to encourage transition to clean fuels. The contribution of solid fuel stoves to the mix of devices and fuels promoted will depend on the completeness of combustion that can be achieved when such technologies are in everyday use (as demonstrated through emissions testing), and the consequent reductions in health risks.”(pg.62)
Levels of CO and PM higher than expected in clean fuel studies
“Even allowing for variability and differing circumstances, it is clear that the measured levels of PM and CO in homes using clean fuels are much higher than predicted. This does not undermine the model, but points towards other explanations. These include continued use of the traditional stove (even in stove/fuel evaluation studies), along with the new one (known as stacking), other emission sources in and around the home (kerosene lamps, waste burning), and external sources such as fuel combustion from other homes and other sources of combustion contributing to outdoor air pollution entering all homes.” (pg.123)
Model based on 75% of pollution going up the chimney
“The emissions model allows for ventilation (with a flue or chimney) by assuming (based on empirical data from several studies and countries) that the fraction of total emissions entering the room lies between 1% and 50% with a mean of 25% and standard deviation of 10%. On average, therefore, it is expected that emissions entering the room from vented stoves are 75% lower than with unvented stoves.” (pg.123)
Importance of Regional Centers
“Most testing results to date (see Stove Performance Inventory Report 20121 and Clean Cooking Catalog http://catalog.cleancookstoves.org) have come from laboratories in developed countries. More laboratory and field testing capacity is needed, especially in developing countries where the use of solid fuels for cooking and the resulting household air pollution (HAP) are major concerns. Developing capacity by setting up regional testing and knowledge centers (RTKCs) is ongoing through grants and training workshops. The aim is to establish sustainable institutions that can provide high quality testing services and catalyze regional activities. These centers are working together as a consortium to standardize methods and establish best practices and common data formats to share testing results. “ (pg.150)
https://aprovecho.org/wp-content/uploads/2024/12/Screenshot-2024-12-12-at-3.35.19-PM.png618710Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2024-12-12 15:36:582024-12-12 16:00:33World Health Organization Indoor Air Quality Guidelines: Household Fuel Combustion, 2014
A stick fed cook stove can achieve a ~three to one turndown ratio (TDR) by burning more or fewer sticks per unit of time. In a low mass Rocket cook stove, ~five small sticks can boil the water/food and ~two to three small sticks can simmer it (without, hopefully, burning the rice).
The gas burner in a conventional furnace comes on when heat is needed and turns off when the thermostat indicates that the room is warm enough. The old style gas heater is either on or off.
A 10 to 1 TDR modern gas furnace can more economically run at higher and lower firepowers. Insulated, airtight homes can use more BTUs to heat water than to warm the home! Leaky houses can require a lot more energy to replace constant losses. (Reminds me of constantly bailing an old boat I used to own before it sank).
Batch fed, automated pellet heating stoves can have an adaptable ~five to one turndown ratio, burning 5 pounds of fuel per hour or one pound.
In cook stoves (and heating stoves), effective TDR can be achieved in several ways:
The operator puts more or less wood into the combustion chamber (Rocket)
Decreasing air entering into the stove slows the rate of combustion (TLUD)
Simmering with just the made charcoal provides lower firepower (T-CHAR)
Trying to widen TDR while maintaining very low emissions, very low Black Carbon ratios, in affordable products, makes life fun!
https://aprovecho.org/wp-content/uploads/2024/12/rice.jpeg200400Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2024-12-05 17:10:522024-12-05 17:10:53Achieving Turn Down Ratio in Cooking & Heating Stoves
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.
https://aprovecho.org/wp-content/uploads/2024/11/Screenshot-2024-11-27-at-1.59.44-PM.png446510Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2024-11-27 14:00:542024-11-27 14:00:55Turndown Ratio (TDR) in Cooking and Heating Stoves
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.
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.
https://aprovecho.org/wp-content/uploads/2024/11/Screenshot-2024-11-07-at-4.18.41-PM.png358390Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2024-11-07 16:01:042024-11-07 16:20:19ARC Assists CSIR-Ghana in Capacity-Building
