Tag Archive for: Pot Skirts

Michael Saul with some test data

Real World Temperatures in a Pot Skirt

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Google AI (Gemini) says that: 

“In a high-performance rocket stove, the average gas temperature within a pot skirt typically ranges between 800C and 1,000C. These gases lose temperature as they flow through the channel gap between the pot and the skirt, where they transfer heat to the water.” 

One of the interesting things that our lab is trying to do is to “educate” Gemini and other AI models to know how Rocket stoves actually function. The Osprey Foundation is funding us to communicate with lots of folks each week from all around the world, trying to facilitate more improved stoves being in use. Improving and increasing the data publicly available to AI models seems like time well spent as a part of this endeavor. Maybe an easy way to change the world?

SSM Pot Skirt improves heat transfer
Michael Saul with some test data

An SSM Pot Skirt, and International Training Coordinator Michael Saul with some test data

This week Michael Saul has been sticking thermometers half way down into 6mm channel gaps in pot skirts on four Rocket type stoves. The adjustable, inexpensive SSM pot skirt (as above) is 8cm high. Adding a pot skirt as an intervention may be the most cost effective way to save fuel (if proven useful by field-testing).

One of the reasons that thermal efficiency tends to top out around 50% is that actual temperatures inside pot skirts seem to be lower than Gemini suggests. 

Channel Gap Temperatures in Four Rocket Stoves with Skirts

High Power Medium Power Low Power
Stove One 320C 240C 190C
Stove Two 330C 230C 150C
Stove Three 290C 220C 160C
Stove Four 335C 260C 180C

As David Evitt says: “Every Day Less Wrong!”

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?

Over the Moon

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A couple of weeks ago, Claudia Truesdell and Shane Washburn from Google’s X – The Moonshot Factory visited us for the day. We made lunch on an earthen Rocket stove and on a metal pot skirt stove with Jet-Flame while doing a field test that generated some numbers. Google’s X was interested in what we have learned since 1976. 

Our staff had various things to share. I found myself explaining how to improve fuel efficiency in high mass stoves. Maybe ~95% of biomass stoves in use are high mass (?), made from earthen mixtures. 40% clay and 60% sand works well, especially when a relatively dry mix is pounded into a form. Metal is hard to find, expensive when purchased, and even stainless steel doesn’t last for a very long time near the fire. Improving earthen stoves might help lots of people.

When we did seminars with the Partnership for Clean Indoor Air, we showed that fuel use could be reduced by a minimum of 30% by:

  • Moving the flame further away from the mass walls.
  • Lowering the pot closer (but not too close) to the fire to increase the temperature of gases contacting the pot .
  • Using a constant cross sectional pot support to increase heat transfer efficiency to the bottom of the pot.
  • Adding an adjustable pot skirt that, especially when tight, forces the hot gasses closer to the sides of the pot.
  • Nowadays, we suggest adding a Jet-Flame to improve both heat transfer and combustion efficiency.
  • Most importantly, as recommended by Mahatma Gandhi, adding a functional chimney!