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Domestic Use of Energy Conference 2002

PERFORMANCE ASSESSMENT OF BIOMASS STOVES ON PROMOTION

IN SOUTHERN AFRICA

JTM Tawha and MM ElmissiryENERGY TECHNOLOGY INSTITUTE, SIRDC, P.O BOX 6640, HARARE, ZIMBABWE

ABSTRACT

This paper presents results of tests done to establish

the performance of selected wood burning stoves that

have been promoted in the Southern Region of Africa.

These are the Namibian Tsotso, Mali conical, Mali

Cylindrical, Mali Orap, Swazi, South African Rocket

and the Zimbabwean metal grate. The performance

tests are confined to combustion efficiency and body

temperature measurements. A comparison is made

between the stoves tested and recommendations are

given on the optimisation of the technical performance

of the stoves.

1. INTRODUCTION

1.1 Background and aims

Traditional fuels, including firewood, charcoal,

agricultural residues and cow dung, play a vital role in

providing for the domestic energy requirements for

communities in developing countries. Domestic

requirements include energy for cooking, lighting, space

heating, beer brewing brick making and other income

generation activities.

It is widely accepted that with the current energy policies

for domestic use and the ever-rising cost of supplying or

using other forms of energy, traditional fuels will remain

the primary source of energy for the urban poor and rural

households in the foreseeable future. Of these traditional

fuels, wood is the most affordable and readily available

type of fuel used for household energy. This wood is

typically burnt in open fires or poorly designed stoves,

often indoors, and rarely with adequate ventilation or

chimneys leading to incomplete combustion of the fuel.

This results in the consumption of unnecessarily large

amounts of firewood during cooking and other heating

purposes and emission of noxious smoke that impactsnegatively on the health of the cook and all individuals

exposed to the polluted cooking environment. Inefficient

consumption of firewood has contributed to deforestation

and consequently an increased burden especially for

women and children who have to travel long distances in

search of firewood for their family requirements.

Among the technologies introduced for efficient

utilization of firewood, stoves are undoubtedly the most

popular and wide spread technologies in urban poor and

rural households. By improving the efficiency of the

stove, energy is saved, indoor air pollution is reduced and

the communities are empowered as the time, effort, risks

and expenses associated with collecting and using fuelwood are reduced. In an effort to address the situation,

Governments, non-governmental organisations (NGOs)

and other developmental agencies in developing countries

introduced biomass energy conservation projects. This

saw the introduction of different improved metal and mud

stove designs. However, surveys conducted indicated that

there was generally a low level of acceptance for the

improved stoves, even with subsidies to adopters, because

of various reasons. There is strong evidence that

appropriateness of a stove is highly site specific

depending on the social, technical and economic

circumstances. A stove design that may do very well inone region can be a total disaster in another area even if

the design is very efficient in the laboratory. Issues such

as how accessible, how affordable, how appealing to look

at and how easy the stove is to use are important in the

dissemination and acceptance of stove technologies. It is

also important to note that stoves are only part of the

cooking systems and they may not be considered in

isolation to other factors of the system. These factors

include the type of pot used, how well the pot fits the

stove design, whether pot lids are used, cooking habits,

cultural values and management of the kitchen and fuel.

It is however necessary to have stove designs tested in thelaboratory as the combustion efficiency plays an

important part in the overall stove system efficiency.

Laboratory stove tests are useful in creating a data bank

upon which the stoves could be characterized in terms of

their performance. The present work therefore only

presents results of laboratory tests performed on seven

stove designs in use in the region. The information

presented could be used to improve on the existing

designs.

1.2. Description of the stoves

A brief description of each stove tested is given below. Itis important to note that the names given only help in

identifying the stoves and may not be the names used in

the places of origin.

1.2.1 The Metal Grate

This is an open fire cooking technology that normally

provides for multi pot cooking. It is the most widely used

and accepted stove in Zimbabwe. It accommodates more

than one pot at a time and this has been its major strength

compared to the improved one-pot stoves. The stove can

accommodate wood of varying dimensions and does not

require special materials and tools for its construction.

Surveys conducted so far reveal that the stove is durable.

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The stove also performs many functions like cooking;

roasting, drying meat and providing space heating.

However the stove dimensions are not standardised. This

open fire forms the basis of the comparison tests.

1.2.2 Mali Orap, Mali Cylindrical, Mali Conical

The designs used were all single pot, single jacket stoves

based on an original West African design. Our particular

stoves came from Mali(which explains the names given)

and were adapted to the Zimbabwean conditions. The

Mali cylindrical has a cylindrical upper part while the

Mali Conical has a conical upper part. The Mali Orap is a

Zimbabwean version of the Mali conical that takes into

account user comments after initial field tests.

1.2.3 Namibian Tsotso

This is the Namibian adaptation of the Zimbabwean

Tsotso stove. It is a double metal jacket stove with avermiculite- sand-mixture as insulation in between. The

inner jacket is perforated. The stove therefore features

primary combustion, secondary combustion as well as pre

heated secondary air injection. It is a single pot metal

stove.

1.2.4 Swazi Stove

This is another modification on the original Zimbabwean

Tsotso Stove. It consists of three components:

-the main stove body

-the fire grate with holes punched through

-the pot supportThe main stove body has an upper and lower part. The

lower part allows for collection and removal of ash. The

upper part houses the grate and pot rests. The bottom part

of the stove is perforated and a sliding door fixed to allow

for air regulation for the different stages of cooking that is

the power and simmering phases. The power phase

requires more heat to quickly bring to boil while the

simmering phase requires less heat to maintain the

boiling. The stove features primary and secondary

combustion, preheated secondary air injection as well as

air regulation. Insulation is provided for by air

incorporated in-between a metal jacket on the upper part

of the stove housing.

1.2.5 The South African Rocket Stove

This is an adaptation and modification of the Rocket and

the Mali Stoves. It consists of a specially designed elbow

housed in a Mali conical stove body. Vermiculite filled in

between the firebox and the Mali stove outer part

provides insulation. The stove has no grate.

2. TESTING

2.1 Tests

The following tests were undertaken:

Fuel burn rate tests

Firewood consumption tests

Water-boiling tests (WBTs)

Comparison tests and

Stove body temperature tests

2.2 Conditions

All tests were carried out in the laboratory to ensure that

draughts do not influence the results. Temperature, time

and mass measurements were made. The pot lid was on in

all the tests and only the power phase was considered.

Water was filled in to occupy two thirds of the pot

capacity.

2.3 Specifications

Pot Specifications

Mass of Pot used is 0.798kg

Full capacity of pot is 3.078litres

Amount of water used for the experiments was 2.00litres.

Wood Specifications

The following wood types were used

Air dried wood from service stations (possibly

mixture of eucalyptus and wattle)

Air -dried indigenous Msasa twigs.

Instrumentation

Digital thermometers (GTH 1160) with an accuracy

of +1 degree Celsius were used for temperature

measurements.

A relative humidity sensor with an accuracy of +1%

was used for relative humidity measurements.

For the measurement of mass the Range Ohaus

balance was used and has an accuracy of +/-1g.

For calculations the following values were used.

Specific Heat Value of Water Cw = 4.2kJ/kg.oC.

Latent Heat of Evaporation L = 2 256kJ/kg.oC

Heat Value of wood H = 18 000kJ/kg.oC

3. PROCEDURE

To determine the performance of each stove, Water

boiling tests (WBT) were done. In a WBT, a measured

quantity of water is brought to boil. The water

temperature, time and amount of fuel used are recorded.In all the tests done, the simmering phase was not

investigated.

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The following quantities were measured and/or recorded

in each experiment:

mass of :-

the water in each pot, 2 litres were used in these

experiments.

empty stove, pot, lid and thermometer,

stove , pot, lid, thermometer and cold water,

stove , pot, lid, thermometer and hot water

starting fuel and amount of wood used

ambient temperature

relative humidity

temperature of the water until boiling

time to boil

The tests were repeated on different days and results were

then averaged. For these tests the efficiency of each stove

is defined as the ratio of the amount of heat absorbed

by the water in each pot and the amount of heat

supplied by the fuel wood.

Since all the tests are done at the same conditions, in the

pots of identical dimensions, it can be assumed that theheat losses from the pots to the surroundings are similar.

Hence, the heat supplied by the stove is equal to the sum

of the heat for bringing the water to the boiling point and

the latent heat for evaporating part of the water. From the

definition above, the following formula was used to

calculate the efficiency of each stove:

%100)(

xHm

LmTTCmPHU

f

sibww +

=

where:

PHU - efficiency of each stove

mw - mass of water in each pot at the beginning of theexperiment [kg].

ms - mass of water evaporated from the pot during theexperiment [kg]

mf - the total mass of fuel consumed during theexperiment[kg]

CW- specific heat capacity of water [kJ/oC.kg]

Tb - final temperature of boiling water [oC]

Ti - initial temperature of water [oC].

L - latent heat of vaporization of water at atmospheric

pressure and 100oC [kJ/kg.oC]

H - net calorific value of fuel which is adjusted for

moisture content[kJ/kg.oC]

For calculation purposes the following constants were

used:

Cw - 4.2kJ/kg.oC.

L - 2 256kJ/kg.oCH - 18 000kJ/kg.oC4. RESULTS AND DISCUSSION

The results presented here are a summary of the average

values obtained after repeating the experiments over anumber of times. The following comments may be made.

4.1 Average firewood burnt

The chart below show the amount of firewood used. This

does not include the correction for moisture in the wood

and the energy content of charcoal remaining.

Average Firewood used

0 0.2 0.4 0.6 0.8

Orap M ali

M ali Cyl

Mali Con

Namib Tsotso

Metal Grate

Swazi

RSA Rocket

Average fuel used Kg

Series1

Figure 1 Average firewood consumed

The Swazi stove consumes the least amount of firewood

followed by the Mali cylindrical stove. The open fire i.e.

Metal grate consumes the highest amount of firewood.

The metal grate and RSA rocket consume about twice as

much as the Swazi stove. The consumption of the metal

grate is explained by the losses to the environment since

the stove is not enclosed. The RSA rocket consumed high

amounts of firewood too. The stove was difficult to light

and required a lot of blowing. This may be because of

possible design error in the elbow made for this stove and

that the stove was not fixed with a grate. The stove wasproduced in a training workshop and as a result it may not

have been properly done.

4.2 Fuel consumption

The Chart below compares the fuel used by the different

stoves to bring 2 litres of water to boil. Dry wood

consumption includes corrections for the energy content

in the remaining charcoal and the moisture content of the

air dried wood. Relative Humidity values are used to

approximate the moisture content of the wood. Air-dried

wood is representative of firewood used in households.

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Fuel consumption

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

wood consumption Dry wood consumption

Figure 2 Comparison of fuel consumption

The chart shows firstly the difference between fuel

consumption when moisture and energy content in the

remaining charcoal is taken into account and when these

are not considered. A similar pattern to the firewood usepattern is shown and this may be explained as above.

However, for the dry wood consumption, the Mali Orap

consumes significantly much more as compared to the

other Mali stove designs and the Namibian Tsotso. This is

different from the firewood use pattern. This may be

because less charcoal remained in the Mali Orap when

compared to the other Mali stoves and the Namibian

Tsotso. The chart also demonstrates the differences that

arise when different calculations are involved. It is

therefore necessary to clearly state what has or has not

been taken account when analysing results.

4.3 Stove efficiencies

The chart below shows the stove thermal efficiencies

(PHU). This represents the percentage of heat utilized in

heating water from the amount of heat generated by the fuel.

Ave Stove Efiiciencies

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

MOr

ap

MCylin

MCo

n

NTs

otso

MetG

t

Swazi

SARkt

Efficiency

Figure 3 Stove efficiencies

The chart shows that the Swazi has the highest averageefficiency of 17.15%. This could be higher if the right pot

was used. The Swazi was designed for pots without

handles. However to standardise the heat losses from the

pot, one type of pot was used in all the experiments. This

pot was used also because pots with handles are the most

common in Zimbabwe. The RSA Rocket and the metal

grate have efficiencies around 7%. The RSA rocket did

not have a grate and not enough air was blowing through

as evidenced by the rate at which blowing was required.The limited air entrance into the firebox leads to bad

combustion and thus a lower efficiency. The Metal grate

loses most of the heat generated to the surrounding

because it is open all round. Too much air is thus

introduced from all sides of the stove. The wind blows

away the fire and reduces the fire temperature. The pot is

positioned about 22 cm from the base of the stove. The

distance from the burning wood to the pot base is large

and thus the pot cannot see the fire. This reduces heat

transfer to the pot and more fuel has to be burnt for a

given cooking task. Hot gases and smoke freely escape

into the environment carrying away with them the energy

stored in these gases. This is different from the otherstoves in which the designs make use of the escaping

gases to varying degrees. The Mali stove designs have

similar efficiencies to the Namibian Tsotso Stove.

4.4 Fuel Burn Rate

This looks at the amount of fuel burnt per given unit of

time

Fuel consumtion per hou

0.000 0.500 1.000 1.500 2.000 2.500

Orap Mali

Mali Cyl

Mali Con

Namib Tsotso

Metal Grate

Swazi

RSA Rocket

Kg/hr

Ave Kg/hr

Figure 4 Rate of consumption of fuel

The Mali Orap burns the highest amount of fuel per hour

followed by the Metal Grate. The metal grate stove has a

high fuel burn rate since it is open and allows air to

circulate more freely than the other stoves. It was difficult

to explain the high burn rate of the Mali Orap as the

design is very similar to the other Mali stove but only that

the firebox is bigger than the other designs. The number

of openings provided or air circulation might be

contributing to this high fuel burn rate The Swazi also has

a high rate because the primary air supply holes were left

open most of the time during the tests. The RSA Rocketstove gives the lowest rate as the fire was difficult to burn

because no grate was provided. The differences in the

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burn rate are still to be unanalysed taking into

consideration the design parameters for the different

stoves. The effect of air regulation also needs to be

examined.

4.5 Safety of the stoves

Further tests were done to find the temperatures reached

on the outside of the stove. The finding that children play

next to the mother during cooking sessions necessitated

this. As such, there are possibilities of accidental burns if

the stove is touched. The graphs below show the

temperature rises in the Swazi and RSA Rocket. The

Swazi is a metal stove with air as insulation. This is then

compared to the RSA Rocket that incorporates

vermiculite as insulation.

5. CONCLUSSION AND RECOMMENDATIONS

The results of the water-boiling tests revealed that themetal grate had the least thermal efficiency, all except the

RSA Rocket consumed less than half the quantity of

wood used by the open metal grate. Although the metal

grate stove performed least, surveys conducted show that

the stove is very popular with users because of its

durability, multi pot capacity flexibility and ease of

operation and production. However, the dimensions are

not standardised and a lot of heat escapes to the

environment because the stove is open all round. The

RSA Rocket performed poorly mainly because of the lack

of a grate. A grate should therefore be fixed to this stove.

The Mali stove designs performed well but the grate

needs to be replaced often. A thicker sheet of metal couldbe used but this option needs to be evaluated taking into

account the extra costs compared to the increase in

lifetime of the grate. The Swazi stove performed very

well but improvements are needed to improve on its

appearance and on its safety both to the user and to those

around. Aesthetics plays an important part in the level of

acceptance of stoves.

It should also be noted here than the laboratory tests form

only part of the requirements for efficient stove

dissemination. Other attributes like the pot capacity,

cooking habits, culture, type of fuel used, affordability

and durability of the stove should be taken into account instove dissemination. Stoves are very site specific and it is

important also to note here that active participation of the

target communities is needed to guarantee acceptance of

stoves and success of stove projects.

6. REFERENCES

[1] Ballard-Tremeer G and Jawurek H.H: "Comparison

of five rural wood burning cooking devices:

Efficiencies and emissions" Biomass and Bioenergy

Vol II No 5, pp419-430 Elsevier Science LTD. Gret

Britain 1996

[2] Krishna Prasad K,: "Some performance tests on the

open fires and the family cooker" A report from the

wood burning stove group of applied physics and

mechanical engineering, Eindhoven University of

technology and division of technology for society

TNO, Apeldoorn, The Netherlands, October 1981.

[3] Vieweg F and Braunschweig/Wiesbaden S: "Fuel

saving cookstoves"Aprovencho Institute, GTZ

GmbH, Eschborn,1984

7. AUTHOR(S)

Principal Author: Mrs Joyline T.M Tawha holds an

MSc degree in Renewable energy engineering from the

University of Zimbabwe in collaboration with the

Oldenburg University, Germany and a BTech degree in

Electrical Engineering from the University of Zimbabwe.

At present she is a Research scientist In the Energy

technology Institute at the Scientific and IndustrialResearch and development Centre, Harare, Zimbabwe. Her

duties include among others research work in renewable

energy technologies. Her address is:

Energy Technology Institute, SIRDC

PO Box 6640, Harare, Zimbabwe

Co-author: MM Elmissiry holds a post doctorate and a

PhD degree in Engineering from UMIST, Manchester,

U.K. He is presently a professor and the Director of the

Energy Technology Institute at The Scientific and

Industrial Research and Development Centre (SIRDC). The

institute offers R&D and consultancy services to the local

and regional energy stakeholders in electrical, renewableand fuel-based energies. His address is:



Presenter:

The paper is presented by Mrs Joyline T.M Tawha. She

holds an MSc degree in Renewable energy engineering

from the University of Zimbabwe in collaboration with the

Oldenburg University, Germany and a B.ech degree in

Electrical Engineering from the University of Zimbabwe.

At present she is a Research scientist In the Energy

technology Institute at the Scientific and Industrial

Research and development Centre, Harare, Zimbabwe. Her

duties include among others research work in renewable

energy technologies. Her address is:



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