ISO 19867: Thermal Efficiency

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Boiling that five liters in 25 minutes max!

There have been many versions of Water Boiling Tests, including the 1987 International Standards, Shell Foundation, IWA, ISO 19867, Chinese, Indian and many others. The lab tests do not predict in-field use but are intended to compare results when variables are controlled. 

It can be amusing, in a sad way, to watch how the stove communities (heating and cooking) can get quite hot under the collar about how lab tests don’t accurately predict what users experience. I suppose there are some small rewards that accompany a historical perspective and having read the quite explicit introductions?

I like ISO 19867 and value testing stoves at high, medium, low power, etc. The recent grant has us attempting to upgrade performance in twelve natural draft TLUDs and Rocket stoves. When using ISO 19867, it’s interesting to see how much thermal efficiency is valued! Emissions of CO and PM2.5 are evaluated by the weight of the pollutant (gram or milligram) per megajoule delivered to the pot. To get a good score, thermal efficiency must be as good as you can get, while CO and PM2.5 must be reduced as much as possible, as well.

Not a bad idea! 

We are investigating a new way of making Rocket stoves and have tried it in two SSM stoves so far and are now trying it in a BURN stove. Going for highest thermal efficiency is pretty well understood and that’s nice when emissions and thermal efficiency are interrelated.

Fireless Cooking Has A Long History

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Thanks to Robert Fairchild for sending this reminder that what we call a “Haybox” cooker has a lot of history behind it!

Of course fireless cooking methods have been used since ancient times, but fireless cookers began to be introduced to U.S. in the mid 1800s, becoming commercially manufactured and quite popular in the US in the early 20th century. The Haybox, or “retained heat cooker,” works by placing a boiling pot of food into a well insulated box that keeps the heat in the pot, generally producing thoroughly cooked food in a couple of hours without further interventions from the cook.

Retained heat cooking can save 20%-80% of fuel for cooking, depending on the food and amount cooked. This method is not safe for every kind of food, but Aprovecho cooks especially love it for a big pot of beans or rice. The fire and the pot don’t need to be tended after boiling, and the food never burns!

If you are interested in making one for your own use, here is the ARC Rule of Thumb Design Principles for a Haybox.

You can find an excellent, well illustrated history of the Fireless Cooker, from early versions through its modern re-emergence in low-income countries, at the USDA National Agricultural Library: The Fireless Cooker (Emily Marsh, Ph.D, MLS)

An Easier Institutional Stove?

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https://www.appropedia.org/w/images/e/ef/Libhubesi_stove.jpeg
Libhubesi stove (photo: New Dawn Engineering)

Institutional-size stoves like this Lihubesi stove frequently use a sunken pot or pot skirt to increase heat transfer efficiency.

While testing the institutional-size Alpha Limited TLUD, ARC staff conducted an experiment to see if a skirt is strictly necessary with a very large pot. A 58cm in diameter pot was heated by the six-inch in diameter Tom Reed Alpha Limited forced draft pellet stove with an added 0.75 constant cross sectional area Winiarski stovetop.  

A complete stovetop was also made that increased heat transfer efficiency to the entire bottom of the pot. As-hot-as-possible gases are directed to flow as closely as possible to the surface without reducing their velocity.

The bottom of the 60 liter, 58cm in diameter pot (used in institutional stoves in Africa) had an external surface area of 2,640 square cm. The slanted Winiarski stovetop created a 5mm gap at the outer edges of the pot (See above).

The seven inch deep, Alpha Limited FD-TLUD stove ran for 82 minutes using 2.03 kg Douglas fir pellets. 20 liters of water boiled in ~60 minutes when a lid was placed on top of the pot. (A higher firepower stove is needed to boil 60 liters in a reasonable period of time).

The single test results were:

efficiency_with_char_                          57%          

firepower_with_char_high power        4.80 kW

CO_useful_energy_delivered_            1 g/MJd       

PM_useful_energy_delivered_            15 mg/MJd     

Summary

When pots have sufficient bottom surface area, using a Winiarski stovetop can result in high thermal efficiency. After one hour, the highest temperature of gases in the 5mm channel gap under the outer edges of the pot was 111C. Adding a skirt to the sides of the pot would not be help very much when gas temperatures are this low. 

Perhaps cooks would appreciate institutional stoves without sunken pots? 

Let’s see what happens?

graph helps calculate proper skirt gap for best heat transfer efficiency

Thermal Efficiency: How High Can We Go?

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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

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

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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

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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

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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

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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

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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

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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.