From: EPA’s Lab Test Results for Household Cookstoves, Jim Jetter, 2012 

Since 2012, optimized biomass cook stoves have been tested at ~50% thermal efficiency

The temperature of the hot gases 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 the surface boundary layer of still air immediately adjacent to a wall. 

Increasing Temperatures, increasing exposed Area, increasing Radiation, increasing Velocity in a 6mm to 7mm channel gap (10cm or higher) pot skirt has been shown (up to 5kW firepower) in a 24cm or larger diameter pot to result in ~50% thermal efficiency. Reducing losses from the exterior of the pot skirt with refractory ceramic fiber insulation also increases thermal efficiency. 

60% thermal efficiency has been demonstrated in the lab.

Helpful links:

Mixing with Primary and Secondary Jets of Air

https://tse1.mm.bing.net/th?id=OIP.R4SW_2vN8wfW-9688ssYKgHaGL&pid=Api&P=0&h=220

Regardless of the velocity of secondary air, flow rate, or the angle at which air is injected into the fire, secondary air tends to lower the temperature of gases. Researchers have found that injecting secondary air into the side of the flame in a Rocket stove results in most effective mixing.*

The Jet-Flame, on the other hand, blows primary air jets up into the bed of made charcoal below the burning sticks of wood, creating a “mini blast furnace.” The jets of primary air increase the temperature in the charcoal, frequently resulting in higher temperatures in the combustion chamber. The mixing function is up into the fire, not into the side as with secondary air jets.

Boman et al., 2005 report that temperatures of 850C or above are needed for close to complete combustion in short residence times, as in a cookstove. Since excess air lowers temperatures, using the minimal volume of air in secondary air jets to achieve thorough mixing seems preferable. Researchers have recommended that the jets should penetrate into the middle of the flame but not enter into each other. (*Lefebvre and Ballal, 2010; Udesen, 2019; Vanormelingen and Van den Bulck, 1999).

Unfortunately, raising the temperature of pre-heated secondary air by a lot more than ~ 100C seems to be difficult. Cookstove combustion chambers are usually small, limiting the area exposed to high temperatures. The heat transfer efficiency is much lower from degraded temperatures further from flame.

 Residence time and temperature are easily measured. However, “thorough mixing” has not been defined and is not yet measured in our experiments. We infer that the woodgas/air/flame was thoroughly mixed when the emissions of PM2.5 and CO are close to zero as measured with the LEMS emissions hood. 

Pot Skirts – basic theory

Dr. Sam Baldwin describes the use of a pot skirt in his book “Biomass Stoves: Engineering Design, Development, and Dissemination (1987).” Changes in the length and diameter of the channel gap (between the pot and the interior of the skirt) result in dramatic changes in heat transfer efficiency.

“In fact, the channel efficiency, defined as the fraction of the energy in the hot gas entering the channel that is transferred to the pot, is extremely sensitive to changes in the channel gap. For a 10cm long channel, the channel efficiency drops from 46% for an 8mm gap to 26% for a 10cm gap. Thus the stove and pot dimensions must be very precisely controlled.” (pg. 45)

If stoves are to be compared, these types of variables must be controlled. The use of a standard pot, or pots, without pot skirts will result in performance scores that are significantly reduced. If a pot skirt is used on testing pots it should be identical in all aspects. Again, the use of a standard pot(s) seems to be required.


Reaching 50% Thermal Efficiency

Adjustable pot skirt, ssmstoves.com

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%.

Combustion Chamber Heat Loss

Illustration from Biomass Stoves: Engineering Design, Development, and Dissemination

“Lightweight walls have the intrinsic potential for much higher performance than massive walls due to their lower thermal inertia.” –Baldwin, Biomass Stoves: Engineering Design, Development, and Dissemination, 1987

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!

50% Thermal Efficiency Depends on Several Factors Including the Surface Area of the Pot

Illustration from The Smithsonian’s explanation of how a boundary layer works 

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.

  1. According to Newton’s Law, doubling the surface area doubles the heat transfer when the temperature and velocity of the gases are constant.
  2. In a Rocket stove at high power, the gases can be around 800C and the velocity can be around 1.2 meters per second.
  3. 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.
  4. The 0.75 cross sectional channels encourage the gases to thin the boundary layer increasing heat transfer.
  5. Pots have to have sufficient external area to achieve 50% thermal efficiency.
  6. 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.
  7. 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.
  8. 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. 
  9. Raising the temperature and velocity of the gases will increase efficiency.

Learning From The Field, Part 3

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.”

For more SuperPot info:
 https://www.ssmstoves.com/Product/Accessories/49.html

To read the summary report: http://aprovecho.org/publications-3/, scroll down to “Pots” section.

No More Simmering!

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!

6mm Gaps Recommended for Pot Skirts

graph helps calculate proper skirt gap for best heat transfer efficiency
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…