For good thermal efficiency, be sure that as much heat as possible is being transferred to the outside of the cooking pot. The temperature of the hot gas flowing past the surface of the pot is increased by 1.) Creating as much flame (1,100C) as possible in a low mass, insulated combustion chamber 2.) Decreasing the distance between the fire and the pot without making excess smoke 3.) Not allowing external air to cool the combustion gasses.
In convective heat transfer, the primary resistance is in the surface boundary layer of very slowly moving gas immediately adjacent to a wall. Increasing the velocity of the hot gas as it flows past the pot without reducing the temperature is aided by a pot skirt. Reduce the thermal resistance with appropriately sized channel gaps under and at the sides of the pot. ( see “Biomass Stoves:” Sam Baldwin).
A 6mm channel gap in a 10cm or higher pot skirt has been shown to work well with up to 6kW firepower with a 24cm or larger diameter pot.
Reducing thermal losses from the exterior of the pot skirt with 1cm of refractory ceramic fiber insulation increases thermal efficiency by approximately 8%.
After about 80 minutes, the earthen mass wall in the illustration above gets hot enough to equal the heat loss in a single metal wall.
After about 20 minutes, the fired thin walled fired clay wall gets hot enough to equal the heat loss in a single metal wall.
After 80 minutes, the earthen high mass wall loses less heat compared to the bare metal wall resulting in better performance when used in long-term applications.
After heating up, fired clay walls and high mass earthen walls lose around 300 watts compared to 500 watts from the bare metal wall.
Insulated metal walls with 1cm insulation lose around 75 watts and food is cooked more quickly while using less fuel. The problem is that insulated metal walls get too hot and do not last very long.
For this reason, stove companies started making double walled stoves with cold air moving between the walls to increase longevity.
Thanks to Dr. Sam Baldwin for quantifying the effect of design choices!
A boundary layer of still air on the bottom and sides of a pot keeps the hot gases from actually contacting the surface and is a dominant factor in heat transfer efficiency.
According to Newton’s Law, doubling the surface area doubles the heat transfer when the temperature and velocity of the gases are constant.
In a Rocket stove at high power, the gases can be around 800C and the velocity can be around 1.2 meters per second.
Keeping a constant cross-sectional area in the pathway the gasses take through the stove is important. Reducing the constant cross-sectional area channels under and around the sides of a pot to 0.75 of that area helps to keep the gases hot and flowing at highest velocity.
The 0.75 cross sectional channels encourage the gases to thin the boundary layer increasing heat transfer.
Pots have to have sufficient external area to achieve 50% thermal efficiency.
In recent tests of optimized 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%. In the same stove, a pot with 1200cm2 can be expected to result in above 45%. We use 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, which makes bigger pots harder to bring to full boil without a pot lid.
Thermal efficiency, when burning biomass, tops out (so far) at around 55%. The gases in the channels at the bottom and sides of the pot loose temperature and velocity resulting in an upper limit to heat transfer efficiency.
Raising the temperature and velocity of the gases will increase efficiency.
http://aprovecho.org/wp-content/uploads/2023/03/image.jpeg346600Kim Stillhttp://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2023-03-10 15:41:352023-03-10 15:41:3750% Thermal Efficiency Depends on Several Factors Including the Surface Area of the Pot
Testing the SuperPot on a three-stone fire, Batil Camp, South Sudan
ARC engineers rely on feedback from field testing to improve the real-world function of biomass cooking systems. Sometimes the news is challenging, but in this instance the news was very encouraging!
In 2014 the UNHCR (The UN’s Refugee Agency) conducted pilot testing of the SSM SuperPot in seven refugee camps in four countries in East Africa: Kenya (Kakuma, Dadaab), South Sudan (Yida, Maban), East Sudan (Kilo 26), and Ethiopia (Dollo Ado; Bambasi).
Kakuma: “Tests conducted in Kakuma overall yielded very positive results. The participants confirmed that cooking time is faster, fuel is saved, and water is conserved even if only by a scant amount. Participants agreed that SuperPot is a much better option than the regular cooking pots not only because of the efficiency but they are apparently also easier to clean, saving more energy and water.”
Dadaab: “Smoke expelled from the sides of the pan and does not enter the pot thus no change in the smell and taste of food. SuperPot cooks food faster and thus less firewood used. Less usage of firewood and faster cooking would mean less protection incidents, more time for infant/child care. With the SuperPot there was less heat loss and firewood consumption by wind as most of the surface was covered with the pan unlike the traditional pot.”
Batil: “Significant differences in cooking time were noted: for CSB++ (corn-soy blend flour) the Stovetec SuperPot cooked 8 minutes faster than the local pot; for cereal, there was a difference of 4 minutes. With pulses, super pot cooked faster by 5 minutes. Overall, Stovetec is time efficient. The fuel savings are particularly impressive.”
Yida: “Together, both tests saved women 20 minutes in overall cooking time. According to the participants, this time saved ‘can be used for other productive household economic activities or be dedicated to childcare which will effectively improve the nutrition and health status of the children and the entire household members.'”
East Sudan: “Testing was conducted at hospital kitchen inside Kilo 26 hospital complex by four people including two cooks and the HAI nutrition coordinator. 500g of lentils were cooked in 750ml of water in both pots on improved stoves. The super pot cooked the lentils in 27 minutes, as opposed to aluminum pot, which took 34 minutes, for a difference of 7 minutes.”
Assossa: “Results indicate that community perspectives are positive for the StoveTec super pot. The water boiled faster in the super pot by 3 minutes and the lentils were cooked 15 minutes earlier on kerosene stove, while also being 9% more fuel efficient than the regular pot. When testing CSB on kerosene stove, super pot was 4% more fuel efficient and saved 7 minutes of cooking time.”
Hilaweyn: “Tests were ran in Buramino Block 13 and Buramino Block 24 Line A with woman groups. In Block 13, the women tested cooking time for 500g of rice over an improved stove (with windshield). The Stove Tec pot cooked the rice faster by 8 minutes. In Block 24, women cooked 500g of lentils over firewood. Stove Tec pot out performed local pot only by 2 minutes. Neither water used nor fuel consumption were measured.”
Summary:
“Results indicate that the super pot is fuel efficient, effective in saving time, safe and well accepted by the community.”
Recommendation:
In their summary report, the UNHCR Food Security and Nutrition Unit advised “Procurement and distribution of SuperPot in select humanitarian contexts within priority countries according to needs of the most vulnerable households.”
http://aprovecho.org/wp-content/uploads/2022/07/7.11.22-Batil-Camp-SuperPot.jpg591725Kim Stillhttp://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2022-07-11 15:37:572022-07-11 15:37:59Learning From The Field, Part 3
A quick internet search for “retained heat cooker” brings up many modern choices.
In 2018, the World Health Organization concluded, “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 recommends that a combination of actions may be most effective. Aprovecho has used Retained Heat Cookers (Hayboxes) for decades and we recommend them, especially where beans make up a part of the diet.
Retained Heat Cookers simmer food to completion reducing CO emissions by 56%, PM emissions by 37% and saving 50% of the fuel and time spent cooking. (Test Results of Cook Stove Performance, 2011). When food is simmering, the fire is constantly replacing the heat lost from the pot. If the heat is captured instead, the retained heat in the boiling pot finishes cooking the food.
In the same way, a drafty and uninsulated house has to have a big fire going all the time to keep the house warm. The super-insulated, almost airtight house can stay warm for a long time after the fire is extinguished. In Oregon, Haybox homes are called “Super Good Cents” homes.
Once the boiling pot is in the box, food cooks without further attention. Even pinto beans will finish softening if the box, with waterproof R-7 insulation, is almost airtight. Makes cooking a lot easier, too!
From SAMUEL BALDWIN’S “BIOMASS STOVES: ENGINEERING DESIGN, DEVELOPMENT, AND DISSEMINATION,” VITA, 1987
Smaller fires are often cleaner burning compared to larger fires, and higher heat transfer efficiencies allow the use of smaller fires while the time to boil stays acceptable. A pot skirt which creates a narrow channel gap reduces fuel use and time to boil.
Dr. Sam Baldwin recommended 10mm to 11mm channel gaps for household cook stoves at normal high firepowers (4kW to 5kW).
ARC uses tighter channel gaps at 5kW and we do not seem to experience problems. ARC recommends 6mm channel gaps in pot skirts for 20cm in diameter (or larger) pots.
It is interesting to note that Dr. Baldwin decreased the channel gap to 8mm with bigger institutional sized pots. We just tested a 6mm gap in a skirt surrounding a 40cm in diameter (60 liter) pot. The result, with a small open fire, was around 48% thermal efficiency.
We recommend a 6mm channel gap to achieve close to 50% thermal efficiency with a large range of pots..
The tight channel gap seems to be OK, so far. It would be great to model velocity, temperature drop, etc. as the channel gap changes! We will do it one of these days…
http://aprovecho.org/wp-content/uploads/2021/01/baldwin-institutional-graph.png15362090Kim Stillhttp://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2022-05-13 15:15:182022-05-13 15:36:566mm Gaps Recommended for Pot Skirts
