The Aprovecho Research Center has been involved with designing and testing fuel efficient wood stoves since 1976. Aprovecho initially helped to create the Lorena stove in Guatemala and published a manual teaching how to construct this high mass stove. Further testing of stoves led to the development of other stoves that were more fuel efficient.

One of the most fuel efficient stoves that we have tested so far is Dr. Larry Winiarski's "Rocket" stove. This stove features an insulated fire box and short (14") chimney. Cooking takes place on top of the chimney. A skirt surrounds the pot and, if covered, makes an oven. The low mass insulation, usually wood ash, helps to keep combustion temperatures high improving efficiency of combustion. The high combustion temperatures also greatly reduce smoke.

The clay and sand Lorena, on the other hand, absorbed heat that could have gone into the cooking process. Insulation is made up of isolated pockets of air. Clay and sand are poor insulators and absorb and retain heat. Since the pot is hotter than the retained heat in the high mass stove body, the retained heat does not usually 'give back' heat to the cooking process.

The "Rocket" stove has been built in Guatemala. It has been introduced in many countries around the world. Last year more than 5,000 stoves of this type built out of trash relief canisters were constructed on site by Rwandan refugees in Zaire. The project was funded by the United Nations High Commissioner of Refugees.

Two recent articles in "Scientific American " (April, July, 1995) sought to summarize the progress made in designing more efficient wood stoves. The authors, Stix and Kammen, suggest that the progress in creating more efficient wood stoves is in large part due to an increase in the stoves ability to burn wood more efficiently which results in greater savings of firewood. They mention the progress made in designing ceramic stove liners that surround the fire.

There is another side to the efficiency equation, however which is also very important when designing stoves for efficiency of wood use. The work of a stove can be divided into two parts: 1.) efficiency of combustion and 2.) efficiency of heat transfer to the pot. (Operator skill is an important third variable that completes the system analysis.) Of the two, improving the efficiency of heat transfer can result in greater savings of firewood.

In a three stone fire, combustion can occur at a normal rate of between 60 to 70 % efficiency. Even smoldering, flameless fires can be 60% efficient in changing wood into heat (Solid Fuels Encyclopedia, 1980). The problem with an open fire, especially in the wind, is that only a small percentage of this heat actually gets into the pot to cook the food. The rest is lost to the atmosphere.

This concept can be easily illustrated by imagining how much slower food cooks when the pot is uncovered. The same amount of heat is entering the pot but less of it is useful in the cooking process. The uncovered pot is a less successful heat exchanger. Conversely, a skirt at the proper distance from the pot increases heat transfer while rates of efficiency of combustion remain constant.

There are a series of things than can increase combustion efficiency. They include: metering the fuel, achieving a hot burn (above 1,200 degrees F.), assuring a good mix of gas and air and flame, limiting the cooling effect of air on the fire, preheating the air that aids combustion and so on. A combination of these design considerations can result in combustion efficiencies of over 90%.

To increase heat transfer to the pot there are also several steps that can help to achieve higher efficiencies: 1.) Insulate and separate the fire and heat from the stove body with a good, low mass insulator. 2.) Shorten the fire flow path as much as possible without creating excess smoke. 3). Try to have flame touch or nearly touch the pot. The greatest differences in temperature between the heat source and pot results in the highest efficiency of heat transfer. 4.) Encourage hot flue gases to intimately contact as much of the pot as possible.

The "Rocket" stove features these design principles. However, without accounting for energy spent in changing water to steam, usually less than 40% of the total energy released from the wood cooks the food. The rest flows uselessly past the pot.

The net efficiency of any system will always be less than the percentage efficiency of the least efficient stage of the process. In the case of the wood stove, the total system efficiency is found by multiplying the efficiency of combustion (around 90%) by the efficiency of heat transfer to the pot (say, 40%). The stove is operating around 36% efficiency. Since heat transfer efficiency is usually the smaller percentage it seems likely that the greater savings could be obtained by attention to how to more efficiently get heat from the fire into the pot.

When contemplating heat transfer efficiency, the haybox or insulated cooker obviously comes to mind. The haybox takes the greatest advantage of the heat captured by the pot. An experiment conducted this summer by interns at the Aprovecho Research Center shows the power of insulating the pot after heating it over either a stove or three stone fire.

Four tests were run using the same pot and the same amount of water and beans. 12 cups of water and 484 grams of pinto beans were placed in the pot. They were then cooked until they were judged to be completely soft. The oven dried wood was weighed in grams as it was used in each test. Novice students conducted the tests in an effort to control for differences in operator skill.

In three trials using the "Rocket" stove it took 938, 984 and 908 grams of wood to completely cook the 484 grams of beans. Using a three stone fire, it required 1,657, 1,452, and 1231 grams of wood to cook and soften the same amount of beans.

The average amount of wood used in the three "Rocket" stove tests was 943.3 grams. The three stone fire average was 1446.6 grams. The "Rocket" stove saved an appreciable amount of fuel compared to the open fire.

	Rocket Stove	Three Stone Fire
Wood  Used in grams	938	1,657
	984	1,452
	908	1,231
Average	943.3 grams	1,446.6 grams

The next two tests were run using the same pot, amount of water and beans but instead of cooking the beans to completion over the fire, the pot was placed in an insulated box (Haybox) after being cooked for ten minutes after the initial boil was achieved. Again, novice interns conducted the tests to reduce the influence of skill on the results.

The three "Rocket" stove/Haybox tests used 231, 397, and 355 grams of dried wood in this shorter test. The three stone fire/Haybox experiments used 391, 387 and 317 grams of wood. The average use by the "Rocket" stove/Haybox was 327 grams of dry wood. The average wood use by the three stone fire/Haybox was 365 grams. Using this amount of wood, all of the beans in the six tests were fully cooked and softened.

	Rocket Stove/Haybox	3 Stone Fire/Haybox
Wood  Used in grams	231	391
	397	387
	355	317
Average	327	365

Using the Haybox in conjunction with either the "Rocket" stove or the three stone fire resulted in very significant savings of firewood. The beans were cooked using approximately 1/4 to 1/3 the wood required if all the cooking was done over the fire. The insulated cooker makes savings of this magnitude possible.

These simple tests show how it is possible to cook efficiently using a fuel efficient stove or a three stone fire if either is used in conjunction with an insulated cooker. Improving the heat transfer efficiency to the pot results in very considerable savings. The most efficient cooking was accomplished by the "Rocket" stove/Haybox combination. However, the effect of the haybox was so powerful that it tended to negate the differences in combustion efficiency between the stove and the open fire.

Please Read The Website Disclaimer!
Copyright 1986-2012, The Survival & Self-Reliance Studies Institute (SSRsi), All Rights Reserved
Site conceptualized, designed, created & maintained by MEG Raven
Snail Mail: SSRsi, PO Box 2572 Dillon, CO. 80435-2572