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The Rocket Box Stove
The Appropriate Infrastructure Development Group (AIDG) promotes affordable and environmentally sound access to electricity, sanitation and clean water. Through a combination of business incubation, education, and outreach, AIDG helps people get Access to technology that improves quality of life. Until 2010 AIDG Guatemala carried out technical R& D on the Rocket Box and other products designed for the local market.
AIDG’s development of the Rocket Box stove was undertaken to alleviate the problems created by open fire cooking in rural Guatemala. The health, environmental, and economic costs of the traditional open-fire method of cooking have been widely documented. They include respiratory illnesses and accidental burns; the financial burden and time to buy or gather fuel; and de-forestation. AIDG’s work to design the Rocket Box has a context in the widespread initiatives by NGOs to promote efficient cook-stoves in Guatemala.
The aim of AIDG’s stove programme has been to develop a product for the market that caters for Guatemalan families’ traditional cooking habits, and is affordable to them. These are necessary conditions for the Rocket Box to effectively contribute to solving these social problems. The design also seeks to maximise fuel efficiency and allow smoke to be removed from the house.
After several iterations, the result of the project is the Rocket Box stove, a metal-bodied stove which incorporates the Rocket combustion chamber into a portable and attractive ‘plancha’ stove. A ‘plancha’ or griddle is usually a feature of Guatemalan cook-stoves allowing the preparation of corn tortillas.
The Rocket Box is a portable wood burning stove. It uses a Rocket combustion chamber for high-efficiency combustion and pumice as insulation to make sure that the heat goes only where it is needed. The combustion chamber is made of baldosa tiles. The body is sheet steel. The plancha is 18” wide by 24” long, and has removable rings to allow direct contact of the flame or hot air with the pots or pans being used for cooking. A large front ring allows food to be bought to a boil and two smaller back rings are for simmering/ keeping food warm. Pumice stone is a natural resource in Guatemala. It has a low density and there are air gaps in the stone itself. This air acts as an insulant meaning the heat operates directly on the plancha.
Unitil 2010 Rocket Box was manufactured and sold by AIDG’s local partner, Xelateco.
There is a wide variety of designs amongst stoves included in the ‘Rocket Stove’ family, but they have the following characteristic features:
• The combustion chamber and interior stove parts are as insulative and lightweight as possible. Heavy materials in contact with hot flue gases absorb heat that could have been used for cooking. • The combustion chamber is insulated in order to keep the fire hot (above 650 ºC) to burn the wood more completely, thereby reducing smoke (which is fuel that has not burnt completely). • The combustion chamber is usually in the shape of an elbow or letter ‘L’, so that wood is placed under a short internal chimney as it is fed into the stove. • Wood is burned at the tips and is pushed in towards the fire as it burns. Feeding fuel at the correct rate creates cleaner combustion, reducing smoke.
An important feature of the Rocket Box stove itself is a grate for the wood to sit on inside the combustion chamber. This allows air to flow underneath the fuel. Without it the stove takes longer to heat and uses more fuel.
Tortillas made on the Rocket Box are delicious. The stove enables a cook to be versatile, with the potential for many dishes to be perfected such as vegetables ‘a la plancha’. Multiple dishes can be cooked at the same time on the pot rings with tortillas on the plancha.
Many Guatemalan households, in urban areas as well as the country, still rely on an open fire. The fire is often built on a platform of sand/ stone with two or three cinder blocks; metal struts are used to balance the cooking pots in between the blocks. It’s known locally as a ‘pollo.’ The Rocket Box removes smoke and other harmful gases from the house via a chimney, vastly improving inside air quality. Other Rocket stove designs do not have a chimney. Some improved cook stoves promoted by NGOs in Guatemala are of this kind, for example the ONIL Nixtamal stove: http://www.onilstove.com/. This is promoted for use in addition to the ONIL stove with a chimney, where additional cooking capacity is needed.
The Rocket Box is very economical with wood consumption compared to an open fire and other commonly available cook stoves in Guatemala. Water Boiling Tests were conducted by AIDG in 2010 on the Rocket Box and a typical metal plancha stove commonly available in Guatemalan hardware stores. The comparison stove body was empty on the inside apart from a ½” lining of bricks. The tests indicated a thermal efficiency of 15 – 21% for the Rocket Box (depending on the type of test) and 5 – 8% for the other stove.
Stove efficiency also compares favorably with some other improved cook-stoves promoted by NGOs to reduce fuel consumption. There is debate over the efficiency of ramp combustion chamber stoves for example (Boy, Bruce and Delgado, writing in Environmental Health Perspectives. Jan. 2002.)
As well as using less wood the Rocket Box can reduce cooking time. In the tests 2.5l of water (with no pot lid) boiled in 25 minutes on a Rocket Box, the stove started from cold. On the comparison plancha stove, 2.5l boiled in 63 minutes.
The stove’s mobility is convenient; it is not uncommon for families in Guatemala to move their kitchen to a different part of their property as the years go by. It is relatively lightweight (around 60 Kg.) Its attractive appearance is popular. The main body of the stove does not become dangerously hot, reducing risk of burns.
The Rocket Box’s materials and production are inexpensive. The retail cost from Xelateco is 900 Guatemalan Quetzales or 117$
Research with families that use the Rocket Box found that for many cooks the combustion chamber and cooking surface are too small. The combustion chamber’s dimensions are carefully proportioned to maximize fuel efficiency; research to modify the combustion chamber with a 5cm longer chamber showed efficiency losses. However its size means the wood has to be chopped into smaller pieces. Whereas with an open fire you can leave a big log to burn, the Rocket Box needs more attention to push the wood into the fire as the tips burn. Some users complained that the wood falls out of the combustion chamber as the burning end becomes lighter.
Guatemalan households are often large and the Rocket Box isn’t ideally suited to prepare food for ten or more people. A Guatemalan cook may need to prepare a large pot of nixtamal, together with a pot of beans, eggs, tortillas on a comal and coffee; all at the same time. The suitability of the stove’s size obviously depends on the family; it can be perfect for elderly couples with a few grandchildren in the house. However some families who own a Rocket Box end up using it in addition to their open fire, failing to solve the problems of household smoke.
Cooking on the main pot ring is fantastic; however the two back pot rings (in the stove’s griddle) take a long time to reach a high temperature. The standard Guatemalan plancha stove, used for comparative testing, is much more effective for cooking on the back cook rings once it overcomes the delay in heating up from cold.
The stove is generally sold for customers to take home and install in the house themselves. However, families may lack the skills or tools to carry out the installation with no support or guidance. The main issue here is the penetration for the chimney pipe through the roof and ensuring that it is sealed up to be water tight. Families in Guatemala obviously value their ‘lamina’ roof (galvanized sheet steel.) Worry about leaks meant in some cases that the chimney wasn’t taken to the outside, with smoke continuing to fill up the house.
Becoming used to cooking with a Rocket Box, if you have always used an open fire implies a significant cultural transformation. This has presented barriers to take-up and proper use of the stove. In some villages where AIDG subsidized stove costs and then carried out research amongst users, stoves where found to have been modified so that a bigger fire could be lit inside (the baldosas or pumice were removed.) This reflects a failure to explain the logic of the design, but also indicates that it's difficult to become used to cooking on a small flame if you have used an open fire all your life. A related issue is that in certain areas people have open fires inside the house for heating as well as cooking.
The chimney pipe becomes dangerously hot. In 2010 some families in Nueva Libertad, Huehuetenango started cooking on the stoves. Within a few months many women had burned themselves this way. It’s important for installations to have a fire guard of some kind even this is as basic as wire mesh.
Although the $117 cost sounds cheap, it is a considerable sum in Guatemala. In 2008 Purchasing Power Parity (PPP) was rated by the World Bank at $4, 690 per capita. The stove cost represents 2.5%. Therefore it may not be cheap enough.
CURRENT MODEL SPECIFICATIONS:
Dimensions for the stove body, plancha and combustion chamber are given in the Rocket Box design document, published on www.aidg.net.
This stove is designed around a 60.8 x 46cm plancha. The box is sheet steel riveted to an angle iron frame. The bulk of the frame is constructed of 1”x 1/8” angle iron, welded together. The body is 24 gauge galvanized sheet steel.
The height of the chamber tower should be 2 or 3 times that of the height of the entry to the chamber. The combustion chamber mouth is 14cm high by 12cm wide. Therefore it’s important to ensure that the height of the chamber tower is between 28cm and 42cm. A 1: 2½ ratio seems to work best. This is to help ensure a good draft and efficient burning.
Heat is strongest at the point of the flame. This means the combustion chamber tower should be high enough for the flame to almost touch the plancha. With a 48cm wide plancha, a 3.5cm space should be maintained.
The firewood is held off the floor of the combustion chamber by a grate, originally made by welding 3/8”rebar (this thickness has since been reviewed). The height of the grate is equal to 1/3 of the height of the combustion chamber. This allows sufficient air to pass through to create an effective draft. The stove footprint is 60.8 x 46cm x 32.3 cm high.
PRODUCT DEVELOPMENT HISTORY
The AIDG improved cook-stove R&D programme investigated several different stoves before focusing on the Rocket Box. The first stoves to be tested were built from masonry, with early iterations following the ramp or 'baffle' design, with a generous cooking surface. These plancha stoves are built on a cement/ cinder block base of cement filled with dirt. The top levels are built from brick and the combustion chamber ramps up from the front to the back of the plancha, until only eight centimeters remain between the floor of the chamber and the metal plate at the back of the stove. A smaller internal volume means that the heat generated by the fire travels more directly to the metal plancha.
In 2006 AIDG organized a Teco-Tour initiative. This included stove building by travelling volunteers. The Teco-Tour Stove was described by AIDG’s Elena Kreiger “the design put a rocket stove baldosa tile combustion chamber into the Pop Wuj stove body [the Pop Wuj stove is a typical baffle stove with a masonry body]... The combustion chamber...design [is] like that used in the ONIL stove.” It was insulated with pumice stone.
The ONIL stove is very similar to the Rocket Box. Instead of a metal stove body precast concrete is used: http://www.onilstove.com/. During Elena’s interneship AIDG investigated the effects of designing the ONIL stove with a longer plancha, increasing cooking area. Howevever working with precast concrete was problematic. The combustion chamber of the current Rocket Box still owes credit to the ONIL stove. The transformation to a metal stove body was made partly as this allows the stove to be mass produced in a basic workshop, avoiding on-site building. This benefits AIDG’s development model of supported businesses.
In 2009 Ben Dana and Sarah Hunt experimented with a Rocket Box with the combustion chamber 5cm longer, in response to customer feedback. Water Boiling Tests 7 – 16% thermal efficiency compared to 15 -21% for the standard Rocket Box. Cooking time was increased with 56 minutes to boil 2.5l water on the modified rocket box, compared to 25 minutes on the standard model. However, heating on the back cook-ring improved with a longer combustion chamber.
The Rocket Box Design Manual outlines customer feedback following research at Nueva Allianza village in 2009. A few design changes were made in response. The 28 gauge sheet steel elbow connection to the chimney was replaced with 26 gauge steel. The fire grate was changed to thicker bar.
In 2010 the AIDG Guatemala’s Tec R&D Section became an independent organization in its own right, Alterna. The final Rocket Box incarnation of the Rocket Box, under its auspices of Alterna, followed market research directed by Daniel Buchbinder to ascertain design preferences. Information on the modifications is not available and has not been included in the Rocket Box Technical Documents. Changes are thought to include more space in the combustion chamber, an increased area for pots to stand on, and improvements to back cook-ring heating. The stove may also have a different name. Contact firstname.lastname@example.org
INFORMATION FOR ACTION
There are four documents we are presenting in relation to this technology. There is a design manual, in English and en Español. There is also a fabrication manual, also in English and en Español. They are published on www.aidg.net Technology files biomass http://www.aidg.net/index.php?option=com_remository&Itemid=34&func=selectfolder&filecatid=18
The design manual outlines the different options that were tried or considered, why some were discarded and the current design selected, a detailed presentation of the current design, and suggestions for future work. The fabrication manual is a step by step guide to manufacturing the stove. The film, ‘How to Build a Rocket Box Stove’ can also be viewed on http://www.youtube.com/watch?v=hkFqpkV3fF4