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Wood stoves

Summary

A large number of people, especially in developing countries, still use wood as the main source of heat for cooking, many of them using open fires. Open fires are not only inefficient, but they produce smoke which is harmful to health and present a burn risk to children. The inefficiency of open fires results in large amounts of wood being burned, contributing to deforestation, and hard work for the women and children collecting it.

Stoves can be designed to burn wood much more efficiently than an open fire, typically reducing fuel consumption by 25 to 60%. A stove also significantly reduces the smoke produced, and if a chimney is used then all of the flue gases are directed out of the building. The key benefits to the user of an improved stove are: improved health due to reduction of smoke, saved time collecting firewood and reduced risk of burns. The benefits to the environment include reduced contribution to deforestation and reduced air pollution.

For improved stoves to be successful they must fit in with the existing methods of cooking in each region where they are used. As a result, there are a huge range of designs, each adapted to work in a similar way to older stove designs or other cooking methods. There are also a range of manufacturing techniques, from plate steel to masonry and even mud, allowing stoves to be built locally and at a price people can afford. There are currently over 100 organisations around the world developing and distributing improved stoves.

Technology background

A large number of people in the world still use wood as the main source of heat for cooking. Traditionally many people used a ‘three stone’ open fire, which has the benefit of being very cheap to make, but is inefficient to use. Improved stoves are carefully designed to be more efficient in burning wood and getting heat to the cooking pots, and so reduce the amount of wood required to cook a meal. Improved combustion also reduces the amount of smoke that is produced. Many designs include a chimney that removes smoke from the cooking area.

Stove design

Efficient wood combustion depends on an adequate supply of air reaching all the areas where the wood is burning. Hot gases tend to rise, creating a draught that draws in more air. Adding a chimney improves this draught, giving more effective combustion. Careful direction of the hot gas flow, and insulation around the flow paths, ensures that the hot gases heat only the cooking pots or hot plates and the heat is not wasted.

The most basic improved stoves simply have a surround to reduce heat loss and the effect of wind. A grate or some other means to supply air under the fuel is a second improvement. The next step separates the combustion process from heat transfer to the cooking pots. The ‘Rocket’ stove concept uses a vertical combustion chamber, where the wood burns in air, with a horizontal air and fuel inlet at the bottom. At the top of the combustion chamber, the hot gases are forced to flow through a narrow gap around the cooking pot, so that heat is transferred very efficiently to the pot.

The next improvement is to make sure that the hot gases are burned completely. This is achieved by enlarging the combustion chamber, and introducing in a secondary supply of air to allow full combustion before the hot gases reach the cooking vessels. Some stove designs control the air flow with an electric fan to achieve very efficient combustion. However, natural air-flow driven by the chimney can be very effective, provided that the shape of the stove is designed and manufactured very carefully.

Construction materials

Different materials can be used for stove construction. Steel or cast iron allows accurate manufacture of stove parts, but is expensive. Fired clay parts can be expensive, if they need to be made by trained potters, but are usually cheaper than steel. Mud is traditional for some stoves, such as chulas, but must be formed of the right mix of materials. Concrete can also be used for some stove parts as it can be cast into pre-determined shapes, but it is more expensive than mud. Insulating materials, such as rice husk, pumice or wood ash, reduce heat loss.

Fuel

Typically improved stoves save between 25 and 60% of the wood used for cooking compared to open fires. Some stove designs use charcoal as their main fuel rather than wood. While charcoal is cleaner to burn than wood, and easier to transport and store, about 75% of the original energy in the wood is lost when charcoal is made. Other stoves use agricultural residues (nut shells, straw) as a fuel. Some stoves can use these in their loose form, but most rely on briquetting – compression of the loose material into denser blocks. Charcoal made from agricultural residues can also be bound into briquettes if a small proportion of clay is used as a binder.

Stove styles

Programmes to introduce improved stoves are rarely successful unless they fit with the preferred local cooking practices. For example, in Latin America, many people prefer a Lorena or block style wood-stove as they prefer to cook standing up. This type of stove is made of a solid block of material, with combustion and heat transfer chambers built into it, so the cooking pots and cooking plate are at waist height. In India, many people squat to cook, so the chula style is preferred. This is also a block style stove, but the cooking level is below knee height.

In parts of Africa, people tend to cook outside so prefer to use a jiko, a portable stove without a chimney. Traditionally, jikos use charcoal, but some can also burn wood. The combustion chamber has a pot support, so the pot can be placed directly above the combustion gases. In Asia, similar stoves are called ‘bucket’ stoves. If such a stove is used indoors, smoke pollution can be reduced by placing it under a hood connected to a chimney.

Most stoves are designed for a single cook working in a domestic setting. The stoves have space for one, two or three cooking pots, and/or a hot plate, depending on local preference. However, there is a range of larger improved stoves designed for use in institutions, such as schools, and commercial operations, such as mass production of tortillas.

Benefits

Cooking over an open fire means that people are exposed to wood smoke, which irritates their eyes and lungs and makes them susceptible to respiratory diseases. It is estimated that 1.6 million people, mainly women and children, die every year as a result of the smoke from wood stoves. There is also a risk of burns to children, as the fire has no protection around it. Using wood for cooking also contributes to deforestation, in particular around cities and towns, where the concentrated use of wood puts pressure on the surrounding land.

People using improved stoves save time as they need to collect less firewood; a task which usually falls to women and children. The extra time allows women to take up other activities including earning extra money, and allows children more time for education. Family members are reported to be more willing to help with the cooking once they have a smoke-free place in which to work.

Extent of use

Since many stove programmes are in the informal sector, there is no central record of how many improved stoves have been constructed and used in different parts of the world. There are over 100 organisations listed by REPP (Renewable Energy Policy Project in Washington) as being involved in the development and extension of improved stoves, in many different countries. Some have installed many thousands of stove, others are very local. Many stove programmes are part of wider rural development schemes, offering improvements in health and economic status for rural people.

Economics

Stove costs vary considerably, but more efficient stoves are generally more expensive: an improved bucket-type stove may cost less than £5, but £50 would be typical for a mud-and brick stove, and the most efficient factory-made steel stoves cost £70 or more. A three-stone open fire does not cost anything to make, so people are not used to having to pay for stoves. Many stove programmes operate subsidy schemes that allow people to afford improved stoves. Alternatively, programmes may train people to make their own stoves from local materials. The designs may not be quite as efficient as stoves made by technicians, but people can use and maintain their own stoves without relying too much on outside help.

Ashden Award winners working with stoves:

RETAP, Kenya (2001)
KIST, Rwanda (2001)
CEHEEN, Nigeria (2001)
Prolena, Nicaragua (2003)
REC (formerly ERTC), Eritrea (2003)
HELPS International, Guatemala (2004)
Escorts Foundation, Pakistan (2004)
TWP/AHDESA, Honduras (2005)
Nishant Bioenergy Consultancy, India (2005)
GIRA, Mexico (2006)
GERES, Cambodia (2006)
Aprovecho Research Centre and ProBEC, Southern Africa (2006)
Daxu, China (2007)
Gaia Association, Ethiopia (2008)
Kisangani Smith Group, Tanzania (2008)
TIDE and Sustaintech, India (2008)
Aprovecho Research Center and Shengzhou Stove Manufacturer, USA/China (2009)
Toyola Energy, Ghana (2011)
D&E Green Enterprises, Haiti (2013)
Impact Carbon, Africa and global (2013)
WWF-DRC, Democratic Republic of the Congo (2013)
Greenway Grameen (2014)

Reference:

The most comprehensive collection of information on stoves is on the BioEnergy Lists.