Thesis Marco Peter - Betuco test for a biomass cooking stove.… · In many development countries...

University of Oldenburg / Germany Efficiency test for a biomass cooking stove

Marco Peter, PPRE 2002/03 I

Abstract

Increasing dependency on the firewood in many parts of southern Africa has resulted

many adverse effects on environment and human health. Within some decades a lot

of improved cooking stoves were designed and disseminated, but many of them were

not successful, because of unacceptance by the users, high price and high technology

involved.

Therefore many efforts have been put to improve the efficiency and consequently to

reduce the emission from traditional stoves. It is in this light that, the new ‘Tin Can’

stove prototype has been designed by a group of stove designers in Lesotho, to fulfil

the local demands, by using local construction materials.

The performance of the stoves has been tested under optimal conditions with

different pots. Comparison with another existing stove has been done to rank their

acceptance by the users. The reference stove is the sophisticated ‘Vesto’ stove from

South Africa. The procedure followed for the test is in accordance with the standard

test methods recommended by Volunteers in Technical Assistance (VITA).


Marco Peter, PPRE 2002/03 II

Acknowledgement The work reported in this thesis was made possible by the

organizer of the postgraduate renewable energy program,

Dr. Konrad Blum, Dr. Michael Golba and Edu Knagge from

the University of Oldenburg; to them I want to convey my

sincere gratitude.

I would like to acknowledge GTZ-ProBEC office Pretoria /

South Africa, especially Dr. Malis Kees, for the

possibility of a practical training, which was the basis

and fieldwork for this thesis.

All my experience and inspirations were only possible

with the helpful financial support by the German Academic

Exchange Service (DAAD).

I am very much thankful to Michael Hönes GTZ consultant

in Maseru / Lesotho and Peter Scott from Aprovecho

Research Center Oregon / USA for their support and

inspirations as the necessary foundation for my thesis

works.

Also I would like to thank the regional chimneysweeper

guild Oldenburg, especially Mr. Gunnar Zube for his

cooperation and support with measurement devices.

I would like to express my sincere gratitude to Dr.

Konrad Blum for accepting my thesis title and providing

guidance and suggestions during my thesis and also I am

thankful to Dr. D. Heinemann providing me guidance for my

thesis as a second supervisor.

I am thankful to my PPRE classmates for their continuous

support and help in the University of Oldenburg. My

sincere thanks goes to my best friend and classmate Manoj

Kumar Khadka, who always encouraged and helped me with

discussions and suggestions.

Last but not the least I would like to express my thanks

to my mother and especially to my lovely girlfriend


Marco Peter, PPRE 2002/03 III

Magdalena for their love, support and inspirations during

my study.

Table of contents

ABSTRACT I

ACKNOWLEDGEMENT II

1. INTRODUCTION 1

1.1 Scenario 1

1.2 Involvement 2

2. TECHNICAL BACKGROUND 3

2.1 Stove types and classification 3

2.2 Design principles and features 5 2.2.1 Combustion 5

Combustion in Small Enclosures 8 2.2.2 Heat transfer 12 2.2.3 Fluid flow 21 2.2.4 Material science 23

3. DESCRIPTION OF THE STOVES 23

3.1 Tin Can Stove 23 3.1.1 Basic principle of the rocket elbow 26

3.2 Vesto Stove 27

4. STANDARD STOVE TEST 30

4.1 Background 30

4.2 Water boiling test 30 4.2.1 Procedure 31

Procedural notes 32 4.2.2 Testing parameters and equipment 33

Humidity and moisture content of wood 35 Volume 38 Pot and stove description 38 Wood 39 Other measurement device 40

4.3 Concepts of efficiency and power 40 Energy losses 41


Marco Peter, PPRE 2002/03 IV

Partial efficiencies 41 Specific consumption 43 Efficiencies in water boiling tests 44

5. TEST RESULTS AND ANALYSIS 45

5.1 Efficiency 45

5.2 Wood consumption and Specific consumption 50

5.3 Average Stove- and Cooking power 53

5.4 Emission measurement 56 5.4.1 Emission and health 56 5.4.1 Emission measurement and results 58

6. CONCLUSIONS 61

REFERENCES 63

APPENDICES I

CURRICULUM VITAE


Marco Peter, PPRE 2002/03 V

List of figures

Figure 1 Current and projected use of biomass by region and selected year 1

Figure 2 Traditional three stone fire 2

Figure 3 Processes in stove design 5

Figure 4 Fire triangle 6

Figure 5 Processes and temperatures in a burning piece of wood 7

Figure 6 Wood combustion 9

Figure 7 Qualitative effects of different factors on thermal and combustion efficiency 11

Figure 8 Conduction, convection, radiation and store heat from: Baldwin 1986 12

Figure 9 Total powers radiated by a black body as a function of the temperature 17

Figure 10 View factor versus the height to the pot 17

Figure 11 Tin Can Stove (prototype) 23

Figure 12 Tin can stove with inside view 25

Figure 13 Rocket Elbow Principles 26

Figure 14 Vesto stove in detail and view inside the stove 28

Figure 15 Stove system with temperature measurement set-up and scale 35

Figure 16 Wood moisture content versa relative humidityand psychrometer 36

Figure 17 Moisture content measurement device -Protimeter Digital Mini 37

Figure 18 Scale with single measurement box and digital display (Söhnle) 38

Figure 19 High mass cast iron pot with 3.6 liter capacity from South Africa 39

Figure 20 Steel pot with long handle and 2.3 liter capacity from Zimbabwe 39

Figure 21 Wood size(3x3; 10x2;15-20x2) used in the tests 40

Figure 22 Energy flow diagram of an Improved cooking stove 41

Figure 23 Heat loss parameter for an ICS 41

Figure 24 Temperature profile of the whole cooking test for the Tin Can stove 46

Figure 25 Temperature profile of the whole cooking test for the Vesto stove 46

Figure 26 Efficiency comparison chart for a 3.6 liter SA pot 49

Figure 27 Efficiency comparison chart for a 2.3 liter Zim pot 49

Figure 28 wood consumption comparison charts for a 3.6 liter SA pot 51

Figure 29 wood consumption comparison charts for a 2.3 liter Zim pot 51


Marco Peter, PPRE 2002/03 VI

Figure 30 Specific consumption comparison charts for a 3.6 liter SA pot 52

Figure 31 Specific consumption comparison charts for a 2.3 liter Zim pot 53

Figure 32 Average stove power comparison charts for a 3.6 liter SA pot 54

Figure 33 Average stove power comparison charts for a 2.3 liter Zim pot 54

Figure 34 Average cooking power comparison charts for a 3.6 liter SA pot 55

Figure 35 Average cooking power comparison charts for a 2.3 liter SA pot 55

Figure 36 Effect of carbon monoxide concentration in the atmosphere as a

function of exposure time for various condition of labour 57

Figure 37 Multi flue gas analyser MSI 150-4 Joker 4 58

List of tables

Table 1 Values of the constant of the forced convection equation for typical

configurations 21

Table 2 Properties of stove construction materials 24

Table 3 Pots properties comparison 38

Table 4 Efficiency comparisons for Tin Can and Vesto stove with different pot types 47

Table 5 Thermal properties of stove materials 47

Table 6 wood consumption comparisons 50

Table 7 Specific consumption comparisons 52

Table 8 Average stove power comparison 53

Table 9 Average cooking power comparison 53

Table 10 Mechanism of principle health effects from major

pollutants 56

Table 11 Emission test results for different stove- and pot types 59


Marco Peter, PPRE 2002/03 1

1. Introduction

1.1 Scenario In many development countries of the world biomass is the predominant energy

source. About half of the world’s population (3.5 billion in 1999) relies on traditional

biomass (wood, dung or crop residues) for cooking or heating.

The litany of issues associated with low technology, inefficient biomass cook stoves

include:

• Deforestation,

• Land degradation and soil erosion,

• Unnecessarily high fuel costs,

• Global warming,

• Deleterious indoor air quality,

• Poor outside ambient air quality visibility degradation and

• Respiratory disease.

Figure 1 Current and projected use of biomass by region and selected year

The World Bank for example has declared indoor air quality (primarily degraded-

due to cooking) as one of the five major environmental problems facing the world.



Horribly, more than five million children under the age of five die each year of

pneumonia, with a key causality being poor indoor air quality1.

A huge demand in fuels results in many regions of the world catastrophic ecological

and social situations. Wood scarcity around villages and cities forces people to go

long ways, buy and transport their fuel. This situation causes drastically price

increasing for wood and charcoal, which results less money for food in poor families.

This dreadful cycle has to be broken by decreasing fuel consumption and increasing

efficiency for cooking and other biomass consumption processes.

ICS are a convenience that protect the health of the family and help to conserve the

supply of biomass. Obtaining a stove may be the most fundamental sign of

improving conditions for a family.

Figure 2 Traditional three stone fire

There have been a number of large-scale programs addressed by several international

help and development aid organisations to develop and disseminate Improved Cook

Stoves (ICS). Their main target in the field of household energy is the avoiding of

the very inefficient open fire, which is called traditional three stone fires (fig. 2).

1.2 Involvement The Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ) GmbH as a

government-owned corporation for international cooperation with worldwide

operations was my first touch with the ICS problematic. 1 ‘A Global Opportunity’ by J. Houck and P. Tiegs



With a practical training in the regional GTZ office in South Africa in the

Programme for Biomass Energy Conservation in Southern Africa (ProBEC) for the

SADC region I got my access to this interesting field. One of my working tasks was

organizing a workshop about ICS Design & Standardisation. This allowed me to get

closer contacts to some well-known people in this field.

With my initiative and the new contacts from the workshop I got the opportunity to

be part of a small team of enthusiastic stove designers (Peter Scott/Aprovecho, USA

and Michael Hönes/GTZ consultant, Lesotho) who planned to design a new

improved biomass-cooking stove. Our target was to build a cheap, easy build with

local material and for rural people affordable stove, especially for the poor and less

developed SADC region of the Lesotho Kingdom in Southern Africa.

2. Technical background

2.1 Stove types and classification

There are a large number of ICS models, based on different construction materials,

fuel and end use applications, have been developed. A proper ICS classification is

essential for identifying a model suitable for a particular group of user, target area,

method of production and dissemination, keeping in view the cooking requirements

and the availability of construction materials and fuels. Accordingly, ICS’s can be

classified into various categories:

a) Function

Mono-function stoves. An ICS, which performs primarily one function, such as

cooking or any other single special function such as fish smoking, baking, roasting,

milk simmering, etc.

Multi-function stoves. In many areas, apart from cooking, an ICS can also be used for

other purposes or in combination, such as for water heating, room heating, fish/meat

smoking, grain/flour roasting, simmering of milk, etc.

b) Construction material

ICS’s are mainly made of single materials: metal, clay, fired-clay or ceramics and

bricks or are hybrids in which more than one material is used for different important

components. Classification based on the material helps in selecting an appropriate



design on the basis of locally available raw materials, skills for fabrication and

necessary production facilities (e.g. centralized/decentralized) in the target area.

The cost of an ICS and its expected service life can also be reflected in this

classification, including its portability.

The stove construction materials can be divided into two main categories, namely:

metals and non-metals. Metals can be further subdivided into other categories,

namely: galvanized or no galvanized sheet metals, cast iron, aluminum, etc. Non-

metals are ceramic, fired clay, mud, etc. Besides, there are hybrid stoves, which are

made out of a combination of materials both metal and non-metal or from

combinations of materials from the same group.

c) Portability

On this basis, an ICS can be classified as fixed or portable. Metal and ceramic ICS’s

are normally portable in nature and can be moved indoors or outdoors while

clay/brick, clay/stone ICS’s are generally high mass and thus are fixed. Stoves in this

category can be further sub-divided into different categories depending on the

number of potholes, e.g., single, double and triple.

d) Fuel type

The performance of different ICS’s, having the same function and constructed with

the same materials, will ultimately depend on the type of fuel used. In some cases, an

ICS may be rendered practically inoperable when switching over to fuel types for

which it was not constructed.

Four major types of ICS’s, based on fuel classification, normally encountered are:

charcoal ICS’s, fuel wood ICS’, granular/loose agric-residue ICS’s, stick-form

agriculture-residue ICS’s, cow dung cake ICS’s, and briquette biomass-fuel ICS’s.

From the discussion above, it can be seen that the classification provides critical

information on a number of ICS design issues such as end use applications,

technology and its transfer, cost, durability, fuel compatibility, etc.2

The materials used to construct a stove have a distinct bearing on the durability, cost,

heat losses, safety, the skills required to make a stove and the scale of production

envisaged, etc.

2 RWEDP-Improved solid biomass burning cook stoves- page 8, Bangkok 1993



2.2 Design principles and features

The thermal performance of an ICS system depends upon the efficiency of the heat

conversion system e.g. the conversion of the chemical energy of a fuel into thermal

energy, the efficiency with which the thermal energy produced is transferred to the

cooking vessel, the system with which the combustion products move through an

ICS and finally also the types of material used for the construction of the stove.

Figure 3 Processes in stove design3

2.2.1 Combustion

The combustion process is dependent on the physico-chemical properties of the fuel

(size, shape, density, moisture content, fixed carbon content, volatile matter, etc.),

quantity and mode of air supply (primary and secondary air) and the conditions of

the surroundings (temperature, wind, humidity, etc.).

Wood Combustion

The process of release of thermal energy from fuel is known as combustion. Biomass

fuel is the stored solar energy in the form of chemical energy of its constituents, as a

result of photosynthetic reaction. This energy is released during combustion reaction,

in which oxygen reacts with the chemical constituents of wood to produce carbon

dioxide and water, with the release of heat. Photosynthetic and combustion reactions

are reversible reactions, which can be depicted by the following simplified equation:

CO + H2O CH2O + O2 (2.0)


Thermal Energy

Solar Energy



The physico-chemical processes involved in the storage of chemical energy in the

fuel and the subsequent conversion of this energy into heat are complex in nature.

Any combustion process can be depicted by the fire triangle shown in fig. 4. The

figure shows that for self-sustained combustion, three components are essential,

namely: fuel, air and heat. Combustion is a complex process in which processes of

devolatilization, cracking and combustion take place almost simultaneously.

The amount of energy released during combustion reaction depends on the

temperature, pressure, the products of reaction and the state of water produced. These

last two factors are important because incomplete combustion will result in the

production of carbon monoxide and other combustible materials, which results in the

loss of potential energy of fuel.

Figure 4 Fire triangle

Liquid or vapor state of water, produced during combustion of hydrogen in the fuel,

will affect the net heat released. This complex process is depicted in fig. 5 and takes

place in three stages, as explained in the following sections.



Figure 5 Processes and temperatures in a burning piece of wood

(Hasan Khan und Verhaart 1992)4

Stage 1 combustion

Easily combustible kindling (such as tree leaves, wood shavings, scrap paper and

kerosene) on burning raises the temperature of the spot on which the radiation from

the flame is incident. This heat gets distributed throughout the material due to

conductive heat transfer, thus raising the temperature of the material. When the

temperature rises to 100°C, drying of wood takes place due to loss of absorbed and

weakly bound water. This process continues into the deep interior; a part of the heat

of combustion is utilized in this endothermic process (heat consuming). Hence, the

higher the moisture content of the wood, the greater is the loss of energy.

Stage 2 combustion

As the temperature is raised further, the pyrolytic decomposition of the wood starts.

At a temperature of about 150°C, the release of volatile matter begins along with the

appearance of semi-liquid tar. In case this stage gets prolonged due to quenching of

the flame, the fuel starts smoldering and dark or gray/blue smoke with a strong smell

is given off. This results in the loss of some useful energy of wood. The tar gets

deposited in the tunnels and chimney resulting in their choking. There is also the

danger of fire in the chimney due to the spontaneous combustion of the deposited tar.

Tar also gets deposited on the cold surface of the pots resulting in their blackening.




Stage 3 combustion

Volatile matter, being at a higher temperature, rises due to the buoyancy force.

During the rise, it mixes with the surrounding air. This mixture of volatile matter

may reach the combustible limit and get ignited, if sufficient heat is available. The

flame resulting from combustion may persist if the heat released from the flame is

sufficient for sustained release of more volatiles from the burning surface. Otherwise,

it will flash back to the surface. Self-sustained combustion commences at around

225°C and reaches a peak at about 300°C.

During this stage, heat released by the combustion process is more than the

combined losses and hence there is a net positive release of heat. Thus, it can be

concluded from the above discussion that for the evaluation of the combustion

process, the understanding of the pyrolysis process and the subsequent burning of the

released volatile matter and char is necessary. The second stage determines the extent

and nature of volatiles and the char generated while the third stage determines the

extent to which the potential heat in the volatile matter and char is released. The

pyrolytic process suggests that, for the best design of the stove, the following factors

(which govern the rate of pyrolysis) must be taken into full consideration:

• the temperature

• rate of heating

• residence time of biomass in the combustion chamber and

• physical characteristics of the fuel such as size and shape

Furthermore, an understanding of the heat level required for ignition as well as for

the maintenance of combustion and their dependence on the thermo-physical

properties such as density, specific heat, thermal conductivity, calorific value and

moisture content is essential.

Combustion in Small Enclosures

The developmental approach to cook stove design has been shifting from fuel-

efficient stoves to emission efficient stoves. A high performance stove should be

efficient from both these perspectives, so as to ensure conservation of the fuel as well

as the environment. This will not only reduce the drudgery of the users but will also

save them from the harmful effects of the pollutants emitted during combustion.



One of the strategies adopted in a large number of designs is to improve thermal

efficiency and provide a chimney for the removal of smoke.

Although this strategy helps in improving the indoor air quality, the quality of

combustion is questionable in a number of designs. A better approach would be to

increase the heat transfer as well as the combustion efficiency. An increase in the

heat transfer results from the efficient transfer of heat produced during combustion

and a reduction in the losses from the body of the stove. Ensuring complete

combustion can enhance combustion efficiency. A complete conversion of chemical

energy to heat with a minimum (but sufficient) amount of excess air can take place if

the following conditions are met:

• High temperature in the reaction zone

• A requisite supply of the oxidant (air) and its complete mixing with the fuel

• Adequate residence time of the reactants (air and fuel) under the above

conditions in the reaction zone

Figure 6 Wood combustion5




In case any of these conditions is not met, the combustion reaction will not proceed

to completion, resulting in the emission of pollutants and the loss of potential heat.

In contrast to liquid or gas fuel burners, it is extremely difficult to meet these

conditions in heterogeneous combustion, as is the case in cook stoves using solid

biomass fuel. Whenever there are reducing conditions due to a deficiency of air or an

excess amount of volatile matter in the combustion zone, free carbon and

hydrocarbon compounds escape from the combustion zone without complete

combustion, resulting in the formation of soot and other toxic poly-aromatic

hydrocarbons. Thus the design and operating parameters are as important as the fuel

parameters. There is very little variation in the chemical composition and energy

density (energy/unit mass or volume) in woody biomass.

However, there is a significant variation of other properties among different types of

fuels: wood, agriculture residues, dung cakes, etc. This variation in the properties has

a profound effect on the overall efficiency and hence must be taken into

consideration in the design of cook stoves. In addition to these, there are a number of

process factors, which must be taken into consideration as well, so as to maximize

efficiency and minimize emissions. These can be divided in to three distinct

categories, namely - design, fuel and operational factors:

Fuel factors: Physical and chemical properties of fuel such as volatile

matter, moisture, ash, etc.

Operational factors: Burn rate/size of the fuel ratio, volume to surface ratio, mode

of fuel supply, cooking time, etc.

Stove factors: Fuel/air ratio, temperature of flame and/or envelope, mode of

fuel supply, primary and secondary air, mass of the stove, etc.

It is difficult to predict the quantitatively effect of the variables on the overall

efficiency. Qualitative effects of some of these factors on combustion and thermal

efficiency are given in fig. 7. A critical examination of the factors given in fig. 7

shows that the fuel parameters are uncontrollable as they depend on the type of fuel.

On the other hand, operational parameters such as fuel size and fuel feeding are user

specific, while the stove parameters are design specific. On a qualitative basis, the

stove factors and some of the operational factors are competing factors. However, the



database is inadequate for a quantitative evaluation of the effect of these parameters

on the combustion and heat transfer efficiency.

This is partly due to a lack of proper understanding of various process principles

during which the combustion takes place in small enclosures, and partly due to the

inadequacy of the experimental procedures used.

Figure 7 Qualitative effects of different factors on thermal and combustion efficiency6

The overall efficiency, during combustion at a particular value of the burning rate,

depends on the characteristics of the enclosure (semi enclosed combustion chamber,

enclosure without chimney, enclosure with chimney, etc.). With an increase in the

burning rate, the heat transfer efficiency decreases, while the combustion efficiency

increases. The nature of the combustion operation such as steady/unsteady

combustion, short term/intermittent operation, which is controlled by the ratio of the

burn size and the burn rate, has a profound influence on the overall efficiency of the

cook stove. Burning of wood in small enclosures can be classified as controlled

combustion in contrast to free burning of wood in an open fire. However, the

operation of the cook stove is dynamic in nature because of the interdependence

between the rate of combustion, rate of induction of air and the draft.




The rate of combustion is strongly dependent on the manner in which the combustion

air is supplied. In the case of small enclosures, the combustion takes place due to the

pressure field set up as a result of the upward movement of combustion products and

entrainment of air through the firebox opening and the grate, if provided. On the

other hand, combustion in open-fire is maintained through the laminar or turbulent

entrainment of outside air, depending on the size of fire. Hence, apart from

combustion, fluid flow considerations are equally important in the design of an ICS.

2.2.2 Heat transfer

Heat transfer is the process by which the heat generated from combustion is

transferred (or purposefully targeted) at a heat-absorbing surface. Only a part of the

heat released on combustion is transferred to the food in the cooking pot. For

example, it has been estimated that for cooking rice, in theory, an equivalent of about

18 grams of wood per kilogram of cooked food is required to heat the rice and water

to the boiling point as well to provide the amount of heat necessary for the chemical

reaction to cook rice.

Figure 8 Conduction, convection, radiation and store heat from: Baldwin 19867




In practice, however, about 160 grams of wood is required to accomplish this task,

even with improved cook stoves. It is clear that a large part of the heat is lost to the

surroundings through three distinct heat transfer mechanisms: conduction,

convection, and radiation (fig. 8).

In order to minimize the losses to the surroundings and maximize the transfer of heat

to the food in the pot a thorough knowledge of heat transfer mechanisms and their

underlying principles is required to determine the reasons for the losses, how these

losses can be reduced through modifications of the design of the cook stove, etc.

a) Conduction

Molecules are closely packed in solids. Whenever there is a temperature gradient

these molecules tend to distribute and equalize their kinetic energy by direct

interaction. This mechanism of heat transfers known as conduction.

In metals, the movement of high velocity free electrons from high temperature

regions to low temperature regions, where they collide with and excite atoms,

conducts heat additionally.

In general, heat conduction by free electrons is more significant than adjacent atoms

exciting each other. The transfer of heat through conduction can be calculated using

the following equation (Fourierconduction law):

X

TAkq∆

∆=

** (2.1)

where q is the rate of heat transfer, k the thermal conductivity, A the area, ∆X

the thickness of the surface through which the heat is conducted and ∆T being the

difference in the temperatures of the hot and cold sides. ∆X/kA is called the thermal

resistance. However, the use of this equation alone for calculating the surface loss

gives values, which are many thousands of times, the actual values. This is due to the

non-inclusion of the resistance of the surface boundary layer of air as well as the

resistance due to the dirt or the oxide layer in the above expression. With the

inclusion of these resistances, the equation takes the form:

21

11*

hkX

h

TAq+

∆+

∆= (2.1.1)

where 1/h1 and 1/h2 are the inner and outer surface resistances and h1 and h2

are convective heat transfer coefficients respectively.



These will be discussed in detail in the next section. The ability of a material to store

heat is another important factor in conductive heat transfer.

This is measured by its specific heat, which is the energy required to raise the

temperature of 1 kg of its mass by 1°C. The change in the total amount of heat-stored

∆Q, when the temperature of the stove with mass m is changed by ∆T, is given by

the equation.

The ability of a material to store heat is another important factor in conductive heat

transfer. This is measured by its specific heat, which is the energy required to raise

the temperature of 1 kg of its mass by 1°C. The change in the total amount of heat-

stored ∆Q, when the temperature of the stove with mass m is changed by ∆T, is given

by the equation where cp is the specific heat of the material of the stove.

TcmQ p ∆=∆ ** (2.2)

where cp is the specific heat of the material of the stove.

It can be inferred from the equations that massive stoves will warm up slowly, while

lightweight stoves will heat up and dissipate heat quickly.

However, the lower heat loss from thick walls is completely offset by a greater

absorption of heat due to the storage effect. Only a small part of this heat can be

recuperated as useful heat. Hence, thin walls are generally preferred if cooking is

intermittent. Massive stoves have therefore an advantage if the cooking is carried out

throughout the day. It can be concluded from the above discussion that the thermal

inertia of the stove is a direct function of the specific heat and mass, while the rate of

heat transfer is a function of thermal conductivity.

Thus, in order to increase the rate of heat transfer to the pot material, a high thermal

conductivity of the pot material is preferred. In other words, an aluminum pot will

help in faster cooking as compared to fired clay pots. Similarly, in order to reduce

losses from the walls, materials having a low thermal conductivity such as mud or

clay are better. In the case of metal stoves, the application of an insulation layer can

substantially reduce the losses. Regions of interest from the viewpoint of conduction

are:

• Transfer of heat from the pots to the contents of the pot

• Loss of heat through the stove walls

• Transfer of heat from the flame to the interior of the wood



• Storage of heat in wood, pot and its contents and the body of the

stove

b) Radiation

All bodies above the absolute temperature due to molecular and atomic motion as a

result of the internal energy of the material emit energy in the form of heat radiation.

The internal energy is proportional to the temperature of the body in the equilibrium

state. The ability of an object to emit and absorb radiation is given by its emissivity

and absorptivity, which are usually functions of the wavelength of the radiation. The

emissivity and absorptivity of a black material are equal. Heat radiation is absorbed,

reflected, and transmitted when these come in contact with any solid body. The

radiation is emitted over a range of wavelengths. The emitted radiation has a

maximum intensity at the wavelength given by Wien's law with T being the absolute

temperature.

[ ]mT

wavelengthMaximum µ8.2897

=⋅ (2.3)

In a cook stove, as shown in below, the regions of interest from the radiation point of

view are:

• Radiation emitted by the flame

• Radiation exchange between the inner walls, pot and the wood

• Radiation loss to the atmosphere from the wall, pot, chimney, and

the opening of the firebox.

From equation 2.3, it can be concluded that radiation emitted by the burning flame is

in the range of the visible spectrum while that emitted by the stove surfaces at lower

temperature is in the range of infrared radiation. Burning black carbon particles in

the flame make it luminous (yellowish) with luminous flames emitting more

radiation than the no luminous (bluish) flame such as from a charcoal fire. This is

caused by the higher emissivity of the black carbon particles.

Radiation from the flame, which accounts for nearly 14% of the total energy released

from the fire, plays an important role in heating the fuel wood. This accelerates the

release of volatiles that support the flame, thus partly controlling the rate of

combustion.



Hot glowing wood and hot walls of the combustion chamber also radiate heat, which

is absorbed by the cooking vessel. The Stefan-Boltzmann law for black bodies gives

the rate of heat transfer by radiation, which is one of the most important modes of

heat transfer in the combustion chamber.

[ ]WTQ 4*σ= (2.4)

where F is the Stefan-Boltzmann constant, which is equal to 5.6697*10-8

W/m2*K4, A is the emitting area of the object in square meters, and T is its

temperature in K.

Black bodies have absorptivity equal to 1, regardless of wavelength. Such bodies are

impossible to find in actual practice. In actual practice, bodies behave as gray bodies,

which absorb only a fraction of radiation impinging on it. For these bodies, the

Stefan-Boltzmann law is modified as:

[ ]WTEQ m

4**σ= (2.4.1)

where is the emissivity of the material.

It can be inferred from these equations that the energy emitted by a body is strongly

dependent on the temperature.

An increase in temperature by just 10% increases the heat output by 50%. Another

important parameter in radiative heat transfer is the View Factor (VF) between the

emitting surface and the absorbing surface. The View Factor is the fraction of energy

emitted by one surface that is intercepted by the second surface. It is determined by

the relative geometry of the two surfaces. The total power radiated by a black body

as a function of temperature and the View Factor versus the distance between fire

bed and pot/radius of fire are presented in fig. 9 and 10 (Baldwin 1986)8.




Figure 9 Total powers radiated by a black body as a function of the temperature

The energy emitted by the fire bed corresponding to its temperature is calculated

from fig. 9, while the View Factor is determined from fig. 10.

Figure 10 View factor versus the height to the pot

These graphs are extremely useful for designing the fire firebox of a cook stove.

Energy intercepted by the pot =Power emitted by the fire bed × A × VF (2.5)

The energy intercepted by the cooking pot from the fire bed can be calculated from

the following equation if the View Factor is known.

For example:



Considered a pot with a diameter of 20 cm (r2) placed 9.5 cm (h) above the fire bed

having diameter (r1) and cylindrical single pot stove having height above fire bed

equal to 9.5 cm (h). The value of the View Factor for values of h/r2 (0.95) and r2/r1

from fig. 10 is 0.8.

This means that 80% of the radiation emitted by the fire bed strikes the pot bottom.

From fig. 9, if the temperature of the fire bed is equal to 900°K, it will emit 0.40

kW/m2. Using equation (2.5), the energy intercepted by the pot is 1.0 kW.

Radiative heat transfer from the fire bed in a cook stove can be increased, either by

increasing the fire bed temperature (by controlling the air supply to the fire bed) or

by increasing the View Factor. The latter can be increased by either decreasing the

distance between the pot and the fire bed or by increasing the diameter of the pot.

However, too small a distance between the pot and the fire bed will result in

quenching of the fire resulting in incomplete combustion and increased emission of

CO and hydrocarbons.

This distance should be more than the combined height of the fuel bed and the flame

length. Flame length is dependent on the type of fuel. The fuels with high volatile

matter will produce longer flames. Generating turbulence through design innovations

can reduce the length of the flame. In the case of cook stoves with chimneys, induced

draft modifies the flame length. The distance between the fire bed and the pot should

be optimized taking into account its effect on the emissions in naturally ventilated

stoves as well as in the induced draft stoves.

c) Convective heat transfer

Convective heat transfer involves the transfer of heat by the movement of fluid

(liquid or gas), followed by conductive heat transfer between newly arrived hot fluid

and the matter. Depending on the type of driving force involved in the movement of

the fluid, heat transfer by convection takes place by two distinct mechanisms. When

movement of the fluid takes place as a result of the buoyancy force created by the

temperature difference, the phenomenon is known as natural convection. On the

other hand when the fluid is forced to flow by a blower or a fan or by windy

conditions, the phenomenon is known as forced convection.

Convective heat transfer is the predominant mode of heat transfer in cook stoves. Hot

gases, produced from the combustion of fuel, heat the pot through convective heat



transfer. The cooling of the stove and the heating of the space also takes place

through this mechanism.

In convective heat transfer, fluid flow and heat transfer take place simultaneously.

The theoretical analysis requires the solution of continuity, momentum and energy

conservation equations simultaneously. This makes the solution complex. The

problem can be simplified by introducing the concept of boundary layer resistance.

The boundary layer concept assumes that most of the resistance to the heat transfer is

present in the thin boundary layer adjoining the solid surface and not within the solid

material and flowing hot fluid.

The velocity across this boundary layer varies from zero, at the wall, as a result of

friction, to the mainstream velocity, at its outer edge. The solid and the fluid main

stream, on respective sides of the boundary layer, rapidly carry the heat away.

The conductivity of the stagnant gas layer is very low; hence this is the controlling

resistance, which limits the heat transfer from the flowing gas to the solid surface,

such as the pot on a cook stove.

Reducing the resistance in this boundary layer can increase the rate of heat transfer.

This can be done by increasing the velocity of the gas stream, which reduces the

thickness of the boundary layer, thus reducing the resistance to the conductive heat

transfer across it to the solid surface. Even with these simplifications, the solution of

the resulting equations is still complex. In natural convection the hot gas temperature

decides the flow as well as the heat transfer rates, which in turn decides its

temperature. This inter-dependence of the controlling parameters makes the

theoretical analysis of natural convective heat transfer extremely complicated.

In order to circumvent this problem, an empirical approach is generally used for the

analysis of convective heat transfer problems. In an empirical approach, the

convective heat transfer is estimated using a general equation:

TAhq ∆= ** (2.6)

where q is the heat transferred from the hot gas to the solid surface (pot

surface/wall surface in the case of a cook stove), A is the area of solid surface across

which the flow of heat takes place, h is the convective heat transfer coefficient, and

∆T is the difference between the temperatures of the hot gas and the solid surface.

The heat transfer coefficient, h can be either determined experimentally or

theoretically (in some specific cases).



The heat transfer coefficient can be calculated empirically using the Nusselt number.

The Nusselt number is the ratio of the characteristic length of the system and the

thickness of the local boundary layer.

It is defined as hd/k, where h is the heat transfer coefficient, d is the characteristic

length of the system, and k is the thermal conductivity of the fluid (hot gas). The

characteristic length is a function of the system configuration. For example, in the

case of a flow through the cylindrical pothole in a cook stove, it is equal to the

diameter of the pothole. In the case of a flow between the two vertical walls, it is the

distance between them. For flow over a free surface, as in the case of a stove surface,

it is the distance from the leading edge.

In the case of heat transfer by natural convection, generally encountered in naturally

vented cook stoves, the Nusselt number can be evaluated from the relation: nGrCNu Pr)*(*= (2.7)

where Gr and Pr are the Grashoff and Prandtl numbers respectively which are

defined as:

2

3***υ

lTBgGr = (2.7.1)

kc p*Pr µ= (2.7.2)

where g is the acceleration due to gravity, B the volumetric expansion coefficient

(approx.= 1/T), T is the temperature difference between the surface and the ambient,

µ the viscosity of the fluid, k the thermal conductivity and ν the cinematic viscosity.

For flow over vertical cylindrical surfaces, the characteristic length l is equal to the

height.

In cook stoves the values of C and n are taken as 0.53 and 0.25 respectively.

In the case of forced convection, the Nusselt number is described by the equation: yx

fCNu Pr*Re*= (2.8)

with Re being the Reynold's number, which is the ratio of the inertial forces

in the fluid and the viscous forces and is defined as:

µ

ρ**Re vd= (2.8.1)

where d is the diameter, v the velocity of the fluid, ρ the density of the fluid

and µ the viscosity of fluid. Cf is a constant and depends on the system configuration.



A critical value of the Reynold's number defines the transition from laminar to

turbulent flow. For flow in a pipe, the critical Reynold's number is 2,300, while for

flow along a single wall the value is 5x105.

Values of the constant of the forced convection equation for some typical situations

are given in table 1.

Table 1 Values of the constant of the forced convection equation for typical configurations

In cook stoves, the regions of interest from a convective heat transfer point of view

are (Baldwin 1986):

• Hot gas plume from the fire

• Stagnation point of plume on the pot

• Wall jet along the pot bottom and or sides,

where the hot gases flow outward and upwards

• Flow through tunnels, chimney, over baffles,

and in the gap between the pot and wall in the case

of stoves with pots

• Outer hot surfaces of pots, stoves and chimney

The convective heat transfer problems in a cook stove are complex as strongly

accelerating flows with varying temperature difference in the direction of flow are

encountered. Hence, conventional solutions for hydro-dynamically and thermally

stable flow give approximate results.

2.2.3 Fluid flow

A thorough knowledge of fluid flow principles is also essential for understanding the

flow of air and flue gases through the stove and the chimney as well as for

understanding how these influence the combustion process, the transfer of heat from

the hot flue gases to the pots in the different chambers and the heat losses from the

pots and the different stove components of the stove to the surroundings.



The induction of air required for the combustion of fuel in the combustion chamber

and the subsequent flow of the combustion products through various chambers and

connecting tunnels is governed by the principles of fluid flow.

The suction effect, responsible for the induction of air into the combustion chamber,

is created as a result of the flow of flue gases through the chimney. The amount can

be estimated by the application of principles of fluid flow. The steady state flow of

fluid is governed by the continuity equation, which is based on the principle of

conservation of mass. According to this equation the mass of fluid passing all

sections per unit of time is constant. This can be represented by the following

equation:

222111 **** vAvA ρρ = (2.9)

where ρ is the density, A is the area and v is the velocity.

This equation can be used to calculate the velocity of the flue gases and air at

different locations in the stove. Based on the applications of conservation of energy

to the flow of fluid, an equation known as Bernoulli's equation can be derived. For

the steady state flow of low-pressure gas in which there is a negligible change in

internal energy, the equation can be described as:

totpodyst HHHH =+ * (2.9.1)

where Hst is the static head, Hdy is the dynamic head, Hpo is the potential head

and Htot is the total frictional head. The head is expressed in meters and is equal to

∆P/ρ where ∆P is the pressure loss and ρ is the density.

The potential head or Hpo is equal to Hz where z is the elevation above any given

level in meters.

In some cases a sudden expansion and contraction is encountered during the flow of

flue gases through the tunnels and the chambers. The frictional loss Hfr due to the

sudden expansion of a duct with a cross sectional area of A1, where the mean velocity

is v1 into a duct with a larger cross sectional area of A2 where the velocity is v2 is

given by the expression:

cfr g

vvH

2)( 21 −

= (2.9.2)

where gc is a conversion factor having a numerical value equal to the

acceleration due to gravity.



2.2.4 Material science

The materials used to construct a stove have a distinct bearing on the durability, cost,

heat losses, safety, the skills required to make a stove and the scale of production

envisaged, etc (see section 2.1 under -construction materials- page 4).

3. Description of the stoves The following section of this thesis focuses on the description of the two stove types,

which were tested. With different design principles this stoves approach the main

target to improve the efficiency of the combustion. There is the new Tin Can Stove

prototype, which bases on an existing stove design from Aprovecho Research Center.

A solid insulation of the combustion chamber keeps the temperature in the stove in

the optimal range (about 600-700 °C see fig. 5) for the wood combustion ignition.

The high sophisticated and mass-produced Vesto stove uses with gas cooling and

preheating air in a double wall design to capture the escaping heat from the

combustion a different principle. This stove was chosen instead of an open fire,

because of the technical and environmental possibilities of the ‘Energielabor’ at the

Carl von Ossietzky University of Oldenburg.

3.1 Tin Can Stove The Tin Can Stove as a prototype has a design that is based on an internal chimney

inlet as combustion chamber, which uses an exiting design from the rocket stove

principle, combined with a stove case made of tin cans (fig. 11).

Figure 11 Tin Can Stove (prototype)



The internal chimney inlet is made of a mixture from clay and vermiculite, which

keep the heat, as an insulator, in the combustion chamber. The chamber is build with

6 new Vernacular Insulated Ceramic (VIC) bricks, which form a hexagon shape and

a round bottom plate.

The stove has a diameter of 50 cm with a height of 60 cm and weights about 14 kg.

The estimated stove price is in the range of 10-12 US$ (2003).

This design is using the advantage of the insulation properties of the material

combination clay/ vermiculite, which has a very low thermal conductivity (about

0.12 W/mK)9.

The rocket elbow design creates a draft in the stove, which supports the fire with the

necessary air for a complete combustion. The table 2 shows the insulation advantage

of clay compare to other materials, because of the low thermal conductivity (about 1

W/mK for clay; steel 47-58 W/mK or aluminium 204 W/mK).

Vermiculite as a mineralogical material based on hydrated laminar magnesium-

aluminum-ironsilicate has in exfoliated form a very low thermal conductivity, which

is depending on the bulk density between 0.051-0.071 W/mK. Therefore it is an ideal

insulation and filling material for the combustion chamber bricks. Therefore it is an

ideal insulation and filling material for the combustion chamber bricks.

Table 2 Properties of stove construction materials10

9 Thermal properties of insulative bricks Overall heat losses, Dale Andreatta 2003 10 RWEDP-Improved solid biomass burning cook stoves- page 56, Bangkok 1993



In a test series, which were done during my practical training in Maseru / Lesotho,

we mixed different local available clay types to with medium size exfoliated

vermiculite from South Africa. A mixture of about 85 % vermiculite and 15 % clay

gave the lightest brick (1.038 kg). This result was still not convenience compare to

the bricks designed by Aprovecho with about 0.86 kg, which probably leads to even

lower thermal conductivity values. The new designed tin can case is based on the

knowledge from Michael Hönes, who is with experience in projects in this field the

perfect specialist. The case is made of used tin cans, which introduce the ecological

and economical advantage of this system.

The cost factor will be significant drop, because the raw material ‘tin can’, which is

almost free of cost (small cost for tin can collection) would not lie around unused.

Rather a positive side effect for the environmental protection will be created.

Another benefit for the stove design is the insolation effect of the tin cans with the

created air layer around the bricks. With using an existing stove principle the

development time for the new stove concept could be optimised significant.

The main advantages of the Rocket stove principle will be described in the following

section. The in this thesis described stove system is based on the existing knowledge

and the support from Peter Scott Aprovecho Research Center (Oregon / USA).

Figure 12 Tin can stove with inside view



3.1.1 Basic principle of the rocket elbow

Figure 13 Rocket Elbow Principles11

The Rocket stove principle, invented by Dr. Larry Winiarski thirteen years ago, have

an added feature, the L shaped, which is called the ‘rocket elbow’, insulated

combustion chamber and horizontal feed magazine that increases combustion

efficiency reducing harmful emissions (fig. 13).

The rocket elbow is designed to be able to burn biomass like wood completely and

decreasing fuel consumption with increasing efficiency by easy handling of the

stove.

This principle is based on some several following advantages:

• Insulated combustion chamber, with low mass, heat resistant material in order

to keep the fire as hot as possible and not to heat the higher mass of the

stove body

• Within the stove body, above the combustion chamber, use an insulated,

upright chimney of a height that is about two or three times the diameter

before extracting heat to any surface (griddle, pots, etc.).

• Heat only the fuel that is burning (and not too much). Burn the tips of sticks

as they enter the combustion chamber, for example. The object is not to

produce more gasses or charcoal than can be cleanly burned at the power

level desired

11 Aprovecho Research Center Oregon / USA, http://www.efn.org/~apro/



• Maintain a good air velocity through the fuel. The primary Rocket stove

principle and feature is using a hot, insulated, vertical chimney within the

stove body that increases draft.

• Do not allow too much or too little air to enter the combustion chamber. We

strive to have stoechiometric (chemically ideal) combustion: in practice there

should be the minimum excess of air supporting clean burning.

• The cross sectional area (perpendicular to the flow) of the combustion

chamber should be sized within the range of power level of the stove.

Experience has shown that roughly twenty-five square inches will suffice for

home use (four inches in diameter or five inches square). Commercial size is

larger and depends on usage.

• Elevate the fuel and distribute airflow around the fuel surfaces. When burning

sticks of wood, it is best to have several sticks close together, not touching,

leaving air spaces between them. Particle fuels should be arranged on a grate.

• Arrange the fuel so that air largely flows through the glowing coals. Too

much air passing above the coals cools the flames and condenses oil vapours.

• Throughout the stove, any place where hot gases flow, insulate from the

higher mass of the stove body, only exposing pots, etc. to direct heat.

• Transfer the heat efficiently by making the gaps as narrow as possible

between the insulation covering the stove body and surfaces to be heated but

do this without choking the fire. Estimate the size of the gap by keeping the

cross sectional area of the flow of hot flue gases constant.

Exception:

When using an external chimney or fan the gaps can be substantially reduced

as long as adequate space has been left at the top of the internal short chimney

for the gasses to turn smoothly and distribute evenly. This is tapering of the

manifold. In a common domestic griddle stove with external chimney, the gap

under the griddle can be reduced to about one half inch for optimum heat

transfer.

3.2 Vesto Stove This Vesto stove model is a portable, biomass burning, gas insulated; air pre-heating,

semi-gasifying and single pot improved cooking stove, produced from the company



New Dawn Engineering in South Africa and Swaziland. The name comes from the

word combination Variable Energy STOve.

Figure 14 Vesto stove in detail and view inside the stove12

The stove consists of five main components (fig. 14). A double walled metal sheet

cylinder (bucket) as main stove body with a height of 45 cm and 35 cm diameter.

The replaceable cylindrical fire grate, with holes punched through it and a spiral

opening in the bottom, is inside placed. The pot support is by a folded stainless steel

strip done. The stove weights about 7.5 kg and has a price in the range of 39 US$

(2003).

The air in the double wall can be dumped with an outer secondary air controller the

primary air controller. A wire handle makes the stove portable.

With an innovative air circulation system, below described, the stove approaches the

main target, good efficiency range and less emission.

Secondary air innovation

Type 1 - Supplied through the bottom between grate and secondary air tube,

significant preheating is achieved: 500 C

Type 2 - Supplied through centre of secondary air tube, modest preheating

Type 3 - Supplied near the top of the secondary air tube – significant preheating

Primary air

• Supplied air comes through a control damper with an external handle, so that

the operator can vary the heat output by a factor of at least 5:1

12 http://www.newdawn-engineering.com/website/stove/singlestove/vesto/vesto1.htm



• Primary air is warmed by heat passing outside the secondary air tube, below

the damper; this air cools the lower exterior of the body

Wood burning phase

• When the primary air is open, the stove burns the wood or other biomass

(dung, charcoal, twigs, briquettes)

• Secondary air is mostly Type 1 with some Type 2 and a small amount of

Type 3 the

• Power is maximized at about 4 KW

Wood gas phase

• If the primary air supply is closed, pyrolysis of wood continues because of

retained heat in the grate, fuel, secondary air tube, air insulation and small air

flow (leakage)

• Significant gas production leads to major increases in internal CO

• Because of high temperatures inside the grate and lack of primary air,

secondary air is automatically drawn in, primarily Type 2, which completes

the combustion during power level change

• Secondary air feeds downwards, outside the grate extending primary

combustion for a while, with significant preheating power output drops

significantly,

though not instantly as the stove will continue to produce excess gas until the

heat dies down, drawing secondary air as it is able to do so

• Gasification stabilizes, excess Type 2 secondary air decreases, combustion

rate remains low with the wood gas burning in the grate centre

Unique technical points

• Uses gas cooling (air) to capture heat escaping from the light metal body and

recycle it into the stove in a manner that provides a remarkable capacity to

completely burn multiple fuels

• Provides 3 types of secondary air to cool the body, provide air where and

when needed, and preheats it so as not to quench combustion

• Primary air is spun in a vortex to increase turbulence during combustion

• Produces then burns charcoal leaving almost no wasted fuel, typically less

than 30 gm per meal



• In mass production it can be made, assembled, packaged and labelled in 9

man-minutes

4. Standard stove test

4.1 Background

Stove testing is the systematic measuring of the advantages and limitations of a

particular stove model. Its primary aim is to help identify the most effective and

desirable stoves for a specific social and economical context. With ongoing stove

production, a testing program provides essential quality control and may lead to

important design modifications. A group of stove experts by Volunteers in technical

Assistance (VITA) introduced a standardized stove-testing concept prepared from

proceedings of a meeting convened 1982 in Arlington13.

The group formulated the following tests:

A Water boiling test, to measure how much wood is used to boil water under fixed

conditions. This is a laboratory test, to be done both at full heat and at a lower

“simmering” level to replicate the two most common cooking tasks.

While it does not necessarily correlate to actual stove performance when cooking

food, it facilitates the comparison of stoves under controlled conditions with

relatively few cultural variables.

A Kitchen performance test, to measure how much fuel wood is used per person in

actual households when cooking with traditional stove, and when using an

experimental stove. The tester simply measures how much wood the family has at

the beginning and at the end of each testing period.

A controlled cooking test, to serve as a bridge between the water boiling test and the

kitchen performance tests. Trained local cooks prepare pre-determined meals in a

specified way, using both traditional and experimental stoves.

4.2 Water boiling test The Water Boiling Test (WBT) is a relatively short, simple simulation of common

cooking procedures. It measures the fuel consumed for a certain class of tasks. It is

used for a quick comparison of the performance of different stoves.

13 Testing the efficiency of wood-burning cook stoves VITA 1985



WBT use water to simulate food, the standard quantity is two-thirds the full pan

capacity. The test includes “high power” and “low power” phases.

The high power phase involves heating the standard quantity of water from the

ambient temperature to boiling as rapidly as possible. The lower power phase

follows. The power is reduced to the lowest level needed to keep the water

simmering over a one-hour period. Each WBT should be repeated at least four times.

Results may be averaged and analysed statistically.

4.2.1 Procedure

1. Determine and record moisture content for wood to be used in test. (Note:

this is generally done for a series of tests, rather than each individual test.)

2. Note and record the test conditions. Prepare a drawing of the pots and stove

to be tested. Include all relevant stove dimensions and show how the pots fit

into the stove. Note climatic conditions (air temperature, relative humidity,

wind conditions).

3. Weight the empty, dry pots, and record this weight on the data and

calculation form. Fill each pot with water to 2/3 capacities and record the new

weight.

4. Take a quantity of wood not more than twice the estimated needed amount,

weight it, and record the weight on the data form.

5. Place a thermometer in the pot so that water temperature may be measured in

the centre, about 1 cm from the bottom. Record water temperatures and

confirm that they vary no more than 2°C from ambient temperature.

6. After a final check of preparations, light the fire, in the way it is normally

done in the households. For example, use paraffin (kerosene) as the ignition

material. A measured amount of paraffin (less than 10 grams) simply pours

over the wood. The test’s starting time coincides with the lighting of the

paraffin-soaked wood pieces. The paraffin used may be considered as

consumed fuel. 1 gram of paraffin (lower heating value (LHV) ~ 42MJ/kg) is

equivalent to about 2 grams of dry wood (LHV ~ 19MJ/kg GTZ

recommendation).

Throughout the following “high power” phase of the test, control the fire with

the means commonly used locally to bring the water to boil as rapidly as

possible.



7. Regularly record the following on the data form:

- the water temperature in the pot

- the weight of any wood added to the fire

- any action taken to control the fire (dampers, blowing, etc.)

- the fire reaction (smoke, etc)

8. Record the time at which the water in the pot comes to a full boil.

9. At this time rapidly do the following:

- Remove all wood from the stove and knock off any charcoal. Weight the

wood, together with the unused wood from the previously weighed supply.

- Weight all charcoal separately.

- Record the water temperature and the weight of the pot with its water.

- Return charcoal, burning wood, and the pot to the stove to begin the “low

power” phase of the test.

10. For the next 30 minutes maintain the fire at a level just sufficient to keep the

water simmering.

11. Use the least amount of wood possible, and avoid vigorous boiling. Continue

to monitor all conditions noted in step 7. If the temperature of the water in the

pot drops more than 5° below boiling, the test must be considered invalid.

12. Recover and weight separately the charcoal and all remaining wood.

13. Weight and record the pot with its remaining water.

14. Calculate the amount of wood consumed, the amount of water remaining, the

test duration, the specific fuel consumption.

15. Interpret test results, and fill out a test series reporting form.

Procedural notes

1. Stove tests are often conducted with lidded pots to reduce the effect of drafts

on evaporation rate from the pot. However, if the testing site is properly

protected from drafts, lids should be left off, thus reducing the error caused by

condensed water dripping from the lid back into the pot.

2. With lightweight stove models, often the stove and its contents can be

weighted together as a unit, and the weight of the empty stove subtracted

later. It is not necessary to separate charcoal and ashes, since ash weight is

usually insignificant.



3. “High power” and “low power” tests may be conducted separately. The fire is

extinguished at the end of step 7, and the stove are allowed to cool. The entire

test is then repeated in the exactly the same way, except that the fire is

reduced the moment the water comes to boil.

There is no interruption to weight water or fuel as described in steps 8-13.

The test is ended 30 minutes after boiling, and all measurements are recorded.

The weight of the fuel using during the “high power” phase is subtracted from

the total amount used in the “low power” phase.

4. It is important to know how to interpret the results of the WBT, and to

remember that a low Specific Fuel Consumption (SFC) indicates a high

efficiency. As efficiency declines, SFC rises. It is possible to use WBT results

to judge the suitability of a stove for various cooking tasks.

For example, high power cooking (rapid frying and boiling), a stove with the

greatest high power efficiency might be best; for simmering, however, the

best stove might be the one that shows low SFC for both high and low power.

4.2.2 Testing parameters and equipment

Climatic conditions

Among the climatic data to be reported during stove testing, the most important are:

air temperature, wind conditions, relative humidity, altitude and moisture content of

wood, dung or peat fuel. Moisture content of dry charcoal is not relevant.

• Air temperature affects the rate of heat loss from stove and pot. It also

establishes initial water temperature in the Water Boiling Test. Ideally, air

temperature measurements should be taken before and after each test so that a

mean value can be estimated.

• Wind conditions affect the stove's draft and can have considerable influence

on stove performance. Ideally, stove testing should be done only when

conditions are calm. Where this is not possible, a windbreak should be

erected around the stove to reduce air movement and convective heat losses.

A hand-held anemometer is useful for measuring wind speed. However,

precise measurements are probably unnecessary, and a simple description of

wind conditions may be satisfactory.



• Relative humidity (RH) provides one indication of the moisture content of

air-dried fuel. It is a simple and useful condition to measure during stove

testing. For this purpose, a small sling psychrometer, a hair hygrometer, or a

similar instrument is satisfactory. Recalibrate a hygrometer frequently by

wrapping it in a wet cloth, leaving it for five minutes, and adjusting it to 100

% RH. Wet and dry bulb temperature measurements were used in conjunction

with a psychrometric chart in this test series to measure the RH (Fig. 16).

Atmospheric pressure and boiling temperature

The normal boiling temperature of water depends on atmospheric pressure, which is

mainly a function of altitude above sea level. At an altitude (H) the normal boiling

temperature can be computed from:

Tb = (100 – H / 300) [°C] (4.0)

when H is expressed in meters. For example, the normal boiling point is 100°C at

sea level, and 95 °C at 1500m altitudes.

In this test the boiling temperature was assumed as 100°C, because the average

altitude at the test site ‘Energielabor’ at the University of Oldenburg is about 5m

above sea level14. Calculated with the equation (4.0) gives an exact value of 99.98 °C

for the boiling temperature. With the used measurement devices an accuracy of

0.1°C could be measured.

Note that cooking time increase with reduced boiling temperature at high altitudes.

The cooking time is doubled for a temperature decrease of 5° to 10°C, depending on

the kind of food (no influence for the WBT).

The temperature of the water in the pot was measured with a digital measurement

device from AHLBORN Therm 2283-2 with Pt104 range -200…850°C (+/- 0.1°C)

in the beginning, until it was not working. After this mistake a GREISINGER

electronics GTH 1150 Digital thermometer with a NiCr-Ni thermo element with a

range -50…1150°C (+/-1°C) were used. A second temperature measurement system

with a Pt 100 connected to an VOLTCRAFT M4650B Digital Multimeter, PHYWE

power supply and constant current source to control the results were used (fig.15).

14 http://www.admin.uni-oldenburg.de/aaa/de/en/internat/booklet/unioldsc.htm



Figure 15 Stove system with temperature measurement set-up and scale

Humidity and moisture content of wood

The relative humidity of air (RH) controls the equilibrium moisture content (X) of

"air-dried" fuels, which in fact still contains moisture (fig. 16). The type or quality of

fuel and the ambient temperature also influence moisture content.

A useful first approximation of wood fuel moisture content is given by:

X = (water mass)/ (mass of dry wood) = or approximately 0.2 RH (4.1)

So-called “air-dried” wood is, in fact, moist. Its moisture content varies with the

average relative humidity and with the species of wood.

For example, in saturated air (RH = 1), 1.0 kg of dry wood will contain about 0.2 kg

of water (possibly more). At a lower RH = 0.6, the moisture content (X) drops to

about 0.12. Of course, RH and X can be expressed as percentages as well.

Obviously, the specific heating value (Hx), of moist fuels is lower than the heating

value of dry fuel (Ho). It can be shown that for moderate moisture contents (X=0.2 or

less) that:

Hx = Ho (1 - X) = Ho (1 - 1.1 X) (4.2)

As a consequence, a larger quantity of moist fuel Mx is needed for a given job than of

dry fuel Mo. This can be accounted for by computing equivalent dry fuel

consumption from a measured moist fuel quantity.

(equiv. dry fuel quantity) = Mo = (1 - X) Mx (4.3)



Figure 16 Wood moisture content versa relative humidity15and psychrometer

The humidity was measured with an aspiration psychrometer model Assmann from

the company Thies. The measuring is in the range of - 10...+ 60 °C with an accuracy

±0,2 K for the thermometer. The aspirator is spring-wound driven and the

measuring time is approximately 8 min (4 ... 2 m/s).

Moisture measurements

The moisture content (X) of air-dried wood can be estimated from the humidity (RH)

as shown above. The most direct and precise procedure is to make a double

weighting of a moist or air-dried sample: first as it is, and then after drying it in an

oven (at 110°C for 24 hours or more, depending on the sample size). Mx is the moist

weight and Mo is the dry weight:

X = (Mx - Mo)/Mo or X = (Mx - Mo)/Mx (4.4)

The moisture content may be expressed with reference to the dry wood quantity as

done above or, alternatively, with reference to the moist wood quantity as well:

X = (water mass)/(mass of moist charcoal) (4.5)

In fieldwork the first weighting is done at the test site (Mx). The second weighting

can be done afterwards in a lab. Alternatively, the charcoal moisture (X) can be

measured with a battery-operated tester, which uses the electric resistance of the

sample as an indication of its moisture content.

15 http://www.natmus.dk/cons/tp/rhbuff/buffer1.htm



The results will depend slightly on the quality of the charcoal and on the quality of

the instrument used.

The moisture content of the wood in the described stove tests was measured with a

Conductivity measurement device, PROTIMETER Digital Mini with a range 6-28%

fibre saturation (fig.17).

Figure 17 Moisture content measurement device -Protimeter Digital Mini

Weight (mass)

Weighting can be done with any good balance. For field-testing, direct reading

instruments are preferable, as no adjustments of weights are needed.

Spring balances do a good job if they have a long reading scale and thus good

resolution, and if they are used within 20 to 100% of the full capacity. Spring

balances should occasionally be checked with calibrated weights (1 litre of water has

1 kg of weight, etc.). The weighting basket used with a balance should be as light as

possible, since precision is lost when the difference between two weightings is

relatively small.

The weight of the whole stove system (stove + pot + water) in the described stove

tests was measured with a spring balance from SOEHNLE (fig.18).

This scale with a measurement bridge 52x40 cm and single measurement box has a

range up to 50 kg and an accuracy +/- 5 g. The digital display was a GSE Type 550 i.



Figure 18 Scale with single measurement box and digital display (Söhnle)

Volume

Volumes can be measured with graduated bottles. Commercial bottles with known

volumes (1/4, 1/3, 3/4, 1/1 litre) or like, in this test a can with a measurement scale

were used.

Pot and stove description

Largely dimensional relations between the stove and the pot determine the test

results. To make this kind of test as much as possible comparable to the reality in the

households of the user, original pots from the dissemination region in the SAHEL

region in Southern Africa were chosen. A complete description of the used pots, like

size, shape, weight, capacity, material, etc. are shown in the table 3 below. For the

tests the pots were used without lid.

Table 3 Pots properties comparison

pot SA Zimdiameter 23 cm 19.5 cmheight 16 cm 13 cmweight 5 kg 2.3 kgcapacity 5.55 liter 3.45 litermaterial cast iron steelcountry of origin South Africa Zimbabweremarks not coated long handle

high mass low mass



Figure 19 High mass cast iron pot with 3.6 litre capacity from South Africa

Figure 20 Steel pot with long handle and 2.3 litre capacity from Zimbabwe

The internal dimensions of the stove are especially important (see section 3).

Wood

For the tests locally dominate wood, air-dried, preferable pieces of uniform size 3x3

cm (Vesto stove) or 10 – 25 cm sticks (Tin Can stove) were used.

Depending on the stove design different sizes of the same type and conditions of

wood were used for the test. The wood was air-dried pine, with around 13 %

moisture content from a local building market (OBI).



Figure 21 Wood size (3x3; 10x2; 15-20x2) used in the tests

Other measurement device

To stop the time of each test a normal watch (SUUNTO Vector) with a stopwatch

function were used.

4.3 Concepts of efficiency and power

There are many different ways of looking at stove performance and of measuring

stove efficiency. A widely used method compares energy that goes into the stove

with the energy that comes out, to determine Percentage of Heat Utilized (PHU). A

broader concept of efficiency accounts for energy losses in evaporation. Once food

reaches the boiling point, the amount of additional heat it absorbs is relatively small.

In water-based cooking the pot requires only enough heat to maintain boiling

temperatures all else is excess.

This excess heat is used to generate steam, which escapes from the pot without

adding anything to the cooked food. Thus a stove that is regulated to maintain

simmering temperature with at least production of steam is, in that respect, most

efficient.



Energy losses

In figure 22 is a general energy flow diagram for a wood-burning cook stove. Useful

heat is absorbed in the food, but heat losses are associated with:

• Incomplete combustion of wood

• Heat loss from the stove body to the environment

• Heat loss from the pot surfaces (including lids)

• Heat loss through the chimney (if existing)

• Thermostatic steam escaping from the pot due to excessive stove power.

Figure 22 Energy flow diagram of an Improved cooking stove16

Heat losses more in detail are shown in the below chart (fig. 23).

Figure 23 Heat loss parameter for an ICS17

Partial efficiencies

The concept of efficiency is based on the thermodynamic considerations, which are

used to evaluate the performance of a stove. It is an engineering concept and

according to the first law of thermodynamics, the efficiency of a stove for a specific

operation, is the ratio of the energy output to the energy input.

16 Testing the efficiency of wood-burning cook stoves VITA 1985 17 Improved cook stoves; A training manual, Peace Corps, M. Lillywhite, 184

1. Evaporation 2. Distance From Fuel to Pot 3. Convective Loss from Wind 4. Unburned Volatile Gases 5. Radiation from Pot 6. Poor Seal at Pot/Stove Interface 7. Cool Combustion Air or Fuel 8. Radiation From Stove 9. Conduction Through Stove 10. Wet Wood 11. Pot Contents



The second law of efficiency is the ratio of the actual work output to the maximum

possible work output, for the same task. While the first law of efficiency gives the

energy wise performance, the second law gives the efficiency of the stove to perform

a given task. In a wood fired cook stove, heat is generated by partial combustion of

wood. Some of the heat so generated is transferred, by radiation and convection,

from the fire bed and the flue gases to the vessel, and some of it is utilized for

cooking food. The remainder of the heat is lost to the environment, through various

heat transfer mechanisms, as described in section 2.2.2. The final residual heat of the

flue gases is lost to the environment by dilution. A number of partial efficiencies

have been defined (VITA 1985) taking into account the effect of various losses that

take place at different stages in a cook stove. These are:

Combustion efficiency: ηc = heat generated by combustion (4.6) energy potential in fuel wood

Heat transfer efficiency: ηt = gross heat input to the pan (4.7) heat generated

Pot efficiency: ηp = gross heat input - surface losses (4.8) gross heat input

Control efficiency: ηr = heat absorbed by the food (4.9) net heat input to the pot

These efficiencies can be associated with stoves operated in predictable or well-

defined ways, such as at a single power level, or in defined cooking patterns.

Overall efficiency

An “overall stove efficiency” is often used. This is a product of the first three partial

efficiencies described above.

Overall efficiency: ηstove = net heat absorbed by the pot (4.10)

energy potential in fuel wood

ηstove = ηc* ηt* ηr

A cooking efficiency can defined as:

Cooking efficiency: ηcook = heat absorbed by the food (4.11) energy potential in fuel wood

This final efficiency level accounts for all the heat losses. It is the overall stove

efficiency multiplied by the control efficiency:

η’ = ηc* ηt* ηr* ηp = η’* ηr (4.12)



The thermal efficiency of a stove could be based on the measurements of steady state

or unsteady state operations. In the case of steady state operations, the measurements

can be taken at any given moment, while in the case of unsteady state operations, the

input and output values are measured and integrated over the entire process. Most of

the real life processes fall under the second category. Another index of performance

is the specific energy consumption, which is defined as the amount of energy input

required performing a given task.

While applying these indicators, it should be kept in mind that efficiency is not an

absolute physical quantity but a self-defined ratio which depends on the conditions

under which a process takes place and how input/output are measured, thus serving

only as a guideline. Efficiency may be reproducible in a system having a standard

performance like an internal combustion engine. However, combustion of biomass in

a cook stove is a variable process because thermodynamic efficiency of a cook stove

depends upon a large number of factors such as stove design, fuel composition,

vessel design, culinary practice, meteorological conditions and operational variables,

such as fire tending and rate of heat supply, etc. Most of these factors are variable in

nature and hence the thermodynamic efficiency of a cook stove is not a unique

property of the cook stove.

Thus, it has a limited utility and cannot predict the actual fuel consumption. The

efficiency is a design tool rather than a means of predicting field performance of ICS.

Specific consumption

The stove performance can also be alternatively expressed in terms of specific

consumption (SC), which measures the fuel wood required to produce a unit output.

For cooking efficiency level, this can be expressed by the equation:

SC = mass of fuel wood consumed (4.13) mass of food cooked

There is a link between SC and the cooking efficiency, which can be expressed as

shown in the following equation:

ηcook = heat absorbed in cooked food energy potential in fuel wood

ηcook= (mass of cooked food)*cpf*∆T (mass of consumed dry wood)*heating value



V

pfcook H

TcSC

∆==

*1η (4.14)

where cpf is heat capacity of specific food and HV is the calorific value. The

specific consumption appears to be a better index for expressing the performance of a

cook stove and describing the wood consumption pattern, for planning exercises.

Efficiencies in water boiling tests

The overall stove efficiency can be measured in WBT by heating the stove at high

power, or by heating it at a controlled power level where steam generation simulates

absorbed heat.

Cooking efficiency can be measured in a similar way. Note that this steam generation

as a loss. At simmering power levels the cooking efficiency is close to zero. The

cooking efficiency concept therefore has been applied to a cycle that includes both

the heating up period and simmering. In this case, however, the cooking efficiency

drops as simmering times increase.

It can be concluded from the above discussion that the cooking efficiency can be

checked more realistically in controlled cooking tests. In order to draw realistic

conclusions, the water-boiling test, and specific fuel consumption, turn down ratio, as

well as the evaporation rate should also be specified.

Power and energy determination

The equation for overall stove efficiency (4.14) can expressed in more detail:

ηcook = (mass of cooked food)*cpf*∆T = QW (4.15) (mass of consumed dry wood)*heating value Qwood

The heat absorbed in cooked food (in the WBT test the heat absorbed in the water

QW) can be calculated with the following equation:

evapevapSBWBwW hmmcQ *)(** +−= ϑϑ (4.16)

where cw (4.18 kJ/kg*K)is the specific heat capacity of water, mWB is the

amount of water after boiling, Bϑ is the temperature of the boiling water, Sϑ is the

temperature of the water at the beginning (starting), mevap is the amount of evaporated

water and hevap (2260 kJ/kg) is the evaporation enthalpy of the water.

The energy potential in the fuel wood expressed can be by:

LHVmQ drywoodwood *= (4.17)



where mdrywood is the mass of consumed dry wood and LHV is heating value of

the consumed dry wood (19 MJ/kg GTZ recommendation).

Beside the efficiency of an ICS the power output of the stoves is a useful figure. The

average power of the stove Q’stove as the firepower of the stove can be expressed by:

Q’stove = Qwood (4.18) ∆T

The average cooking power Q’cook as the transferred heating power to the water can

be expressed by:

Q’cook = ηcook* Q’stove (4.19)

Q’cook = QW

∆T

where ∆T is the time to reach the boiling temperature in the water.

With all this equation the results and analysis in the following section where done.

5. Test results and analysis

The WBT tests with the two stoves were done with two different pot types. To

distinguish between the different tests two abbreviations for the pots were used. For

the bigger and heavier cast iron pot with 3.7 litre capacity from South Africa (SA)

where used. For the smaller pot with 2.3-litre capacity from Zimbabwe (Zim) were

used. Each test was three times repeated and the average was determined.

5.1 Efficiency

The following figures describe the tested cooking efficiency comparison between the

Tin Can stove prototype and the sophisticated mass produced Vesto stove with

different pot types. The tests were done in two phases, the boiling phase and the

simmering phase. The simmering, low power phase was always 30 minutes directly

after the water boiled at the high power phase. Fig. 24, 25 compare the typical

temperature profile for both stoves with the smaller 2.3-liter pot.

In the simmering phase the water should be at boiling temperature with the smallest

amount of wood, which are necessary to keep the water in the boiling phase. The

most important parameter to compare stoves is the total efficiency, which includes

both phases, the simmering and boiling phase.



Vesto Stove temperature profile with 2.3 liter pot

10

20

30

40

50

60

70

80

90

100

0 6 12 16 20 28 34 40 48 50 56 62

time [min]

tem

pera

ture

[°C

]

boiling simmering

Tin Can Stove temperature profile w ith 2.3 liter pot

10

20

30

40

50

60

70

80

90

100

0 4 9 13 17 20 25 30 35 44 51 55 62 64 66 68 77

time [min]

tem

pera

ture

[°C

]

boiling simmering

The testing for the Tin Can stove was in all tests slightly longer, than for the Vesto

stove. This is due to the behaviour of the insulative bricks, which take some of the

utilized energy to heat up.

Figure 24 Temperature profile of the whole cooking test for the Tin Can stove

Figure 25 Temperature profile of the whole cooking test for the Vesto stove



The determined efficiencies are nearly optimal efficiencies due to the fact, that only

the smallest required amount of wood were used to boil and simmer the water.

As expected the efficiency of the ‘stove-pot system’ increases with increasing the

water volume in the pot.

Table 4 Efficiency comparisons for Tin Can and Vesto stove with different pot types efficiency efficiency

[%] Tin Can Stove Vesto Stove Tin Can Stove Vesto Stovetotal 15.3 29.1 total 12.1 18.6boiling 15.1 27.1 boiling 11.6 17.6simmering 18.2 37.8 simmering 15.8 23.1

3.6 l pot SA 2.3 l pot Zim

The fact, that the efficiency of the Tin Can stove is only about half of the Vesto

stove, which seems to be to low, could be influenced by different effects. Comparing

thermal properties of the different stove materials and concepts (see table 5) makes

the difference in the efficiency between the Tin Can stove, with insulative

clay/vermiculite bricks, which called Vernacular Insulated Ceramic (VIC) and the

Vesto stove with double wall air/gas insulated steel drum clear. The values in,

specific heat conductivity and specific heat capacity, underline the difference for the

in the most important physical material parameter, which influence the stove design.

Table 5 Thermal properties of stove materials18 unit steel (unalloyed) stainless steel clay/vermiculite air (dry)

density kg/m³ 7900 7900 559 1.29spec. heat conductivity W/(m*K) 47-58 14 0.69 0.0245spec. heat capacity kJ/(kg*K) 0.45 0.51 0.12 1.005

Analysing the thermal properties there are huge difference in density and specific

heat, which are an advantage for the bricks as stove material. The specific heat

capacity of the insulative bricks is more than 3 times smaller than for steel. That

means less heat are necessary to heat up the 1 kg of brick material for 1 K. The main

advantage and disadvantage effect in the Tin Can stove is the indolent and slow

behaviour due to keeping or storing the heat in the VIC bricks (see temperature

profile fig. 24). The Tin Can stove (prototype) has more mass (about double) and

volume due to the bricks, which has to be heated up in the beginning of the test

procedure. This energy in form of heat is a loss for the process of heating up the

water in the pot. The tin can layer around the bricks is acting like another insulative

air layer due to the air in the tin cans and support for the stove body.

18 Thermal properties of insulative bricks, Dale Andreatta, FTI Consulting, 2003



More Energy in form of wood is used to reach the same result of raising the

temperature in the water, which influence the efficiency and time of the test. The

Vesto stove has less mass and volume (about half of the Tin can stove) to heat up,

with the double walled air/gas insulated combustion chamber.

With metal as a very good heat conductor the utilized heat from the combustion gets

transferred first into the stove body, where air/gas with a very bad conductivity,

transport the heat due to the very good draft in the stove to the pot. The smaller

efficiency difference for the test with the smaller pot with less water can be

explained by the faster heating process for the less mass steel pot. The 3.6 litre cast

iron pot needs much more time to transfer the utilized heat to the water, due to the

own high mass. The advantage of this pot is to find in longer cooking processes,

where the retained heat saves energy, which has to be delivered in form of

combustion of wood.

A main influence for the efficiency of the stoves is the actual ambient temperature

during the test. As higher the ambient temperature as lower are the thermal losses for

the cooking process. The influence of the ambient temperature can be estimated as

follow19:

)( 100100

yx

yx ϑϑ

ϑϑ

ηη −

−= (4.20)

The index x and y in the equation gives the different stoves parameter at different

ambient temperatures.

The efficiency of an improved biomass cooking stove is one crucial parameter for the

designer and user and has to be in an acceptable range.

Depending on the cooking purpose the Tin Can stove can be more efficient for a

longer cooking process, due to the retained heat in the VIC bricks, which can save

energy and fuel. To make a stove successful for dissemination other parameter, like

price, reliability, durability and easy maintenance are other important parameter.

The results for the Tin Can stove prototype are not totally satisfying, but with the low

price and other parameters compare to other stoves, like the Vesto stove this stove is

an acceptable product for the target market in development countries. Improvements

in the design of the Tin Can stove have to be done to make it more successful.

19 Test for the wood stove “Swazi Stove”, IBEU, K. Schwarzer, 2003



Efficiency comparison with 3.6 l pot

29.1

15.3 15.1

18.2

27.1

37.8

0

5

10

15

20

25

30

35

40

total boiling simmering

test phases

effic

ienc

y [%

]

Tin Can Stove Vesto Stove

Figure 26 Efficiency comparison chart for a 3.6 liter SA pot

Efficiency comparision with 2.3 l pot

15.8

11.612.1

18.6

23.1

17.6

0

5

10

15

20

25

total boiling simmeringtest phases

effic

ienc

y [%

]


Figure 27 Efficiency comparison chart for a 2.3 liter Zim pot

Increasing the draft in the combined stove combustion chamber and chimney

(insulative bricks) has to be tested in further developments, to improve the efficiency

parameter of the Tin Can stove.

Other thoughts between the cooperated stove experts are design developments

towards a semigasifing-stove design to fulfil other biomass fuel requirements.



One option could be using a small pipe (about 6-10 cm long) on top of the

combustion chamber, which gets narrow and introducing a secondary air inlet in

form of holes in the second part of the pipe. The secondary air gets sucked into the

pipe through the holes, by the draft in the pipe due to the narrowness of the pipe and

the draft from the combustion. Complete burning of the flue gases in this part of the

‘chimney’ enhances the difficult combustion process of cow dung and similar fuels,

which requires a gasifying process.

5.2 Wood consumption and Specific consumption

The wood consumption table shows, that the Tin Can stove needs in the WBT for

heating up the bigger amount of water nearly double of the wood of the Vesto stove.

This comes probably from stove design effects of the bricks, which were described in

the efficiency section above.

Table 6 wood consumption comparisons

wood consumption [kg] Tin Can Stove Vesto Stove Tin Can Stove Vesto Stove

total 0.85 0.48 0.75 0.51boiling 0.58 0.30 0.46 0.27simmering 0.27 0.18 0.28 0.24


The disadvantages of only a WBT by comparing different designed stove types are

shown in the results of the Tin Can stove. The indolence heating up behaviour of the

stove due to the insulative bricks could be an advantage for longer cooking

processes. The retained heat released from the bricks can decrease the wood demand

for this purpose.

One main target the reduction of wood consumption should be fulfilled. The

reduction of wood consumption for the Tin Can stove with an efficiency range of 12-

16 % is about 30 % compare to a traditional open three stone fire with 8-9 %

efficiency. For the Vesto stove with an efficiency range of 18- 30 % the reduction is

about 50-60 %.



Wood consumption comparision with 3.6 l pot

0.48

0.27

0.58

0.85

0.18

0.30

0.0

0.2

0.4

0.6

0.8

1.0


test phases

dry

woo

d co

nsum

ptio

n [k

g]Tin Can Stove Vesto Stove

Figure 28 wood consumption comparison charts for a 3.6-liter SA pot

Wood consumption comparision with 2.3 l pot

0.51

0.75

0.46

0.280.27 0.24

0.0

0.2

0.4

0.6

0.8


test phases

dry

woo

d co

nsum

ptio

n [k

g]


Figure 29 wood consumption comparison charts for a 2.3-liter Zim pot



The specific consumption is a good approach to compare stoves in the WBT. It

shows the consumed wood energy input from the boiling- and simmering phase versa

the produced evaporated water output.

Table 7 Specific consumption comparisons spezific consumption [kgconsum.wood / kgevap.water] Tin Can Stove Vesto Stove Tin Can Stove Vesto Stove

total 1.30 0.64 1.58 0.94boiling 2.41 1.83 5.42 3.71simmering 0.62 0.31 0.79 0.57


The value from the test especially for the wood consumption might be higher in real

household conditions due to limited dynamic flexibility or poor stove control during

the cooking process. Related to the efficiency the Vesto stove has a better specific

consumption, which is remarkable in the test with the bigger water amount.

To take into consideration at the end of the testing process the fire was always totally

burned down and the remaining charcoal with a negligible weight were not

considered. In practice all parameters are lower, because of less optimal conditions

and less control of the cooking process by the user.

Figure 30 Specific consumption comparison charts for a 3.6-liter SA pot

Spezific consumption comparision with 3.6 l pot

1.30

2.41

0.62

1.83

0.31

0.64

0.00.20.40.60.81.01.21.41.61.82.02.22.42.6


test phases

dry

woo

d co

nsum

ed /

evap

orat

ed w

ater

[k

g/kg

]

Tin Can StoveVesto Stove



Spezific consumption comparision with 2.3 l pot

0.79

5.42

1.58

0.940.57

3.71

0.00.51.01.52.02.53.03.54.04.55.05.5

total boiling simmeringtest phases

dry

woo

d co

nsum

ed /

evap

orat

ed w

ater

[kg/

kg]


Figure 31 Specific consumption comparison charts for a 2.3-liter Zim pot

5.3 Average Stove- and Cooking power The following parameter describes the average power output of the stove fire and

how much power the stove can utilize due to the design to the pot. In the section 4.3

under power and energy the relation and determination of these parameter are

described in more detail. The Tin Can stove has about 20 % higher stove power,

because of more consumed wood.

Table 8 Average stove power comparison

stove power

[kW] Tin Can Stove Vesto Stove Tin Can Stove Vesto Stovetotal 3.30 2.85 3.42 2.88boiling 3.71 4.12 3.87 3.45simmering 2.73 1.91 2.89 2.45


Table 9 Average cooking power comparison

Cooking power

[kW] Tin Can Stove Vesto Stove Tin Can Stove Vesto Stovetotal 0.51 0.83 0.42 0.53boiling 0.66 1.11 0.45 0.60simmering 0.55 0.72 0.46 0.56




Average stove power comparision with 3.6 l pot

3.30

3.71

2.73

4.12

1.91

2.85

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5


test phases

stov

e po

wer

[kW

]Tin Can StoveVesto Stove

Figure 32 Average stove power comparison charts for a 3.6-liter SA pot

Average stove power comparision with 2.3 l pot

2.89

3.873.42

2.45

3.45

2.88

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0


test phases

stov

e po

wer

[kW

]


Figure 33 Average stove power comparison charts for a 2.3-liter Zim pot



Average cooking power comparision with 3.6 l pot

0.55

0.66

0.51

0.72

1.11

0.83

0.00.10.20.30.40.50.60.70.80.91.01.11.2


test phases

cook

ing

pow

er [k

W]


Figure 34 Average cooking power comparison charts for a 3.6-liter SA pot

Average cooking power comparision with 2.3 l pot

0.460.450.42

0.560.600.53

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7


test phases

cook

ing

pow

er [k

W]


Figure 35 Average cooking power comparison charts for a 2.3-liter SA pot



5.4 Emission measurement

5.4.1 Emission and health

The emission of toxic gaseous and particulate health affecting pollutants of biomass

cooking stoves is one of the main reasons for the development of ICS.

In theory, the complete combustion of bio fuel in a combustion device like a cook

stove should result in the release of just carbon dioxide and water, which do not fall

under the category of pollutants. However, it is very difficult to ensure complete

combustion in traditional cook stoves and/or ICS's due to the heterogeneous nature of

the combustion process, lack of proper control, and design constraints. Thus, the

emission of pollutants during small-scale biomass combustion is unavoidable, in or

outside the kitchen. The level of pollution will vary depending on the types of stoves

and fuels used.

Wood burning stoves always produce carbon monoxide20 and its release with other

combustion products in a kitchen or other enclosed space will increase the

concentration of carbon monoxide. Depending on stove, kitchen volume and air

exchange rate, and carbon monoxide concentrations can reach such level that it will

affect the health of the user. Table 10 shows the health implications of major

pollutants that are normally emitted from biomass burning.

Table 10 Mechanism of principle health effects from major pollutants21

20 Danger signals to Human Health, W.F. Sulilatu, 1985 21 RWEDP-Improved solid biomass burning cook stoves- page 70, Bangkok 1993



In general, combustion products of wood are carbon dioxide (CO2), water vapour

(H2O) carbon monoxide (CO), particulates and polycyclic organic matter (POM).

The last three are considered hazardous pollutants with respect to human health.

Depending on the size (< 5µm) they are able to enter the lung and cause immense

health damage22.

The most dangerous pollutant Carbon monoxide (CO) is a colorless, odorless and

highly poisonous gas, which gets created by incomplete combustion (lack of oxygen)

of fossil fuels.

CO has a strong affinity to hemoglobin (Hb) in the blood which carries oxygen (O2)

to body tissues. CO deprives the tissues of the necessary supply oxygen. However,

binding force of CO to Hb is about 300 times that of O2 to Hb. The poisoning signs

are head age, dizziness, weakness and finally the death. In the air concentration up to

10 ppm CO are unserious. As a measure of the toxication of the CO concentration in

the blood are assumed. For 20 % CO-Hb toxicities signs are occur, and 65% CO-Hb

are deadly for human being.

The effect of the carbon monoxide concentration in the atmosphere as a function of

the exposure time for various conditions of labor is shown in Figure 36.

Figure 36 Effect of carbon monoxide concentration in the atmosphere as a function of exposure time for various condition of labour23

22 Wood combustion studies E. Schutte and K.Prasad WSG Eindhoven 1989 23 RWEDP-Improved solid biomass burning cook stoves- page 70, Bangkok 1993



Another colourless, odourless and poisonous gas, carbon dioxide (CO2), gets created

in all breathing- and combustion processes of fossil fuels.

For human being it is deadly in a concentration of 20 vol. % and in the range from 8-

10 vol. % it causes head age, dizziness, weakness and finally unconsciousness. In

working rooms the concentration shouldn’t be higher than the maximal allowable

concentration (MAC) of 5000 ppm24.

5.4.1 Emission measurement and results

The flue gas composition analyses were done with a multi flue gas analysing

measurement, called MSI 150-4 Joker 4 (fig. 37), which were borrowed from the

local chimneysweeper guild. This measurement device is used to control the flue

gases in central heating system based on different fuels like oil, gas or coal.

Figure 37 Multi flue gas analyser MSI 150-4 Joker 4

24 Test for the wood stove “Swazi Stove”, IBEU, K. Schwarzer, 2003



It can measure different values like, CO, CO2, CO-O2, O2, Tgas and Tamb, at the same

time. In the measurement device the maximum CO2 value for the stoechiometric

(ideal) Combustion, related to 21 % O2 in the air and depending on the burned fuel

was adjusted.

The maximum CO2 value for wood about 20.5 % was not adjustable with the used

measurement device. Therefore the nearest value from fuel oil 15.4 %, which was

possible to adjust, was chosen. The measured results are for fuel oil as the burned

fuel. With the simple rule of three the values for wood were evaluated.

Table 11 Emission test results for different stove- and pot types

time Tgas Tamb O2 CO CO-O% CO2 O2 CO CO-O% CO2min °C °C % ppm ppm % % ppm ppm %10 74 21 3.5 204 242 12.9 5.5 324 384 20.565 167 21 4.2 299 374 12.4 6.9 494 617 20.575 146 21 2.9 640 742 13.3 4.5 986 1144 20.5

for fuel oil measured values for wood calculated valuesTin Can stove + SA pot

time Tgas Tamb O2 CO CO-O% CO2 O2 CO CO-O% CO2min °C °C % ppm ppm % % ppm ppm %

9 86 23 2.8 355 424 13.7 4.3 532 636 20.542 186 23 3.0 212 228 14.0 4.4 310 334 20.555 116 24 2.8 219 260 14.0 4.1 321 382 20.5

for fuel oil measured values for wood calculated valuesTin Can stove + Zim pot

time Tgas Tamb O2 CO CO-O% CO2 O2 CO CO-O% CO2min °C °C % ppm ppm % % ppm ppm %12 294 22 4.0 453 545 12.5 6.5 743 893 20.529 183 23 3.9 168 202 12.6 6.3 273 329 20.551 166 24 3.2 1032 1219 13.1 5.0 1621 1914 20.5

for fuel oil measured values for wood calculated valuesVesto stove + SA pot

time Tgas Tamb O2 CO CO-O% CO2 O2 CO CO-O% CO2min °C °C % ppm ppm % % ppm ppm %

9 186 24 0.0 253 253 15.4 0.0 337 337 20.530 169 24 0.0 75 75 15.4 0.0 100 100 20.543 203 25 0.0 223 223 15.4 0.0 297 297 20.5

for fuel oil measured values for wood calculated valuesVesto stove + Zim pot



In the above table 11 are the results from the emission tests for different times

shown. The first values were taken when the fire in the stove was well burning at the

upper rim of the stoves between the pot and the stove skirt. Other values were taken

after boiling of the water and at the end of the test.

The results can be converted in volume percent or parts per million with the

conversion factor 0.01 vol.%=100 ppm.

These results, which were taken from a stove operated in optimal conditions, show

the serious problematic described in the section 5.4.1.

Compared with the fig. 36 for the allowable concentration of carbon monoxide, the

most dangerous gas, the determined results for the CO are relatively high. Taken into

account that these values are measured directly in the flue gas stream (less mix with

fresh air) and only for a short moment (each measurement took about 15-30 second).

Most of the combustions in technical appliances nowadays operated with air surplus

or controlled air supply, to approach a complete combustion. The values for CO-O2

are the measured CO values without additional air, which gets mixed with the CO

directly after the combustion. This means these values are the realistic CO value

from the combustion without any air content. The O2 values in the last test for the

Vesto stove with Zim pot are zero, because due to technical problems the O2 sensor

was not working. Therefore the CO and CO-O2 results are similar.

I would recommend the user only outdoors use or good air circulated buildings.

Designed as portable and flexible stoves, the user will not accepted the combination

with a chimney, which could guide the harmful flue gases out of the building. The

results show that even under optimal condition the designed stoves need further

development to avoid health-damaging emissions.



6. Conclusions

With the beginning of the new millennium, influenced by high technology,

computer, luxury, etc., the problematic of inefficient biomass combustion for

cooking and heating purpose and their results are an ongoing ecological, economical

and humanitarian tragedy for nearly half of the world population.

This circumstances is one driving force for international cooperation and voluntary

services to design, develop, test, introduce and disseminate new efficient improve

cooking stoves.

In this thesis two different designed improved cooking stoves, Tin Can stove

(Lesotho) and Vesto stove (South Africa), developed, produced, disseminated and

used in the Southern Africa Development Countries (SADC) region were tested and

compared in terms of efficiency. The main focus was on the new Tin Can stove

design, which is so far a prototype. This stove, which can be locally produced with

local materials, consists of an insulative combustion chamber made of a special

clay/vermiculite brick (VIC) and a tin can stove body.

An optimal combination of ecology and economy is reached by using tin cans and

clay as recyclable stove material. The common financial problems for ICS are

significant decreased or partly avoided.

An estimated price of 10-12 US$ (2003) in Africa for the Tin Can stove prototype is

about 1/3 of the price ~39 US$ (2003) of the commercial produced Vesto stove.

The tests show, that both stoves are improved cook stoves with an efficiency range

above 15 % under optimal conditions, compare to 8-9% of a normal operated open

fire. As essential parameters for the stove test the ambient temperature and amount of

water in the pot were detected.

As expected, the efficiency and other parameters of the sophisticated Vesto stove

were in the range of 30-40 % better than from the Tin Can stove prototype.

Compared the price and efficiency parameter, which are still one of the main factor

for successful stove dissemination in development countries, the Tin Can stove has a

potential chance in the development regions. First successful acceptance test with

imaginable users in the mountains of Lesotho during a fieldwork showed the need for

a cheap, reliable, durable and efficient ICS. However these results can be not

satisfying and will be the reason for further developments on the Tin Can stove

prototype.



Experience and knowledge from Gasifier stove designers should be more investigate

and taken into consideration for fruitful improvements for further stove design.

There is still a long way to a more comfortable and healthier life for the people in the

development countries in the near future.



References Andreatta Dale (2003) Thermal properties of insulative bricks Overall heat losses,

FTI Consulting, http://solstice.crest.org/discussiongroups/resources/stoves/Andreatta/Heatloss.htm

Aprovecho (1984) Fuel Saving Cook stoves, published by Deutsches Zentrum für

Entwicklungstechnologien GATE and GTZ GmbH from Research Center

Oregon / USA, http://www.efn.org/~apro/

Baldwin S. (1984) New directions in woodstove development. VITA News, pp.3-23.

-------, (1986) Biomass stoves: Engineering design, development and dissemination. VITA, USA.

Houck J.E. and Tiegs P. (2001) A Global Opportunity http://www.omni-test.com/Publications/Third_World_Global.pdf

Lillywhite M. (1984) Improved cook stoves: A training manual, Peace Corps

Information Collection and Exchange Training Manual T-40, Domestic

Technology International, Inc. Evergreen, Colorado, USA http://mng-unix1.marasconewton.com/peacecorps/Documents/T0040/t0040e/t0040e00.htm

RWEDP (1993) Improved solid biomass burning cook stoves: A development

Manual, Regional Wood Energy Development Programme in Asia, Asia

Regional Cook stove Programme and Energy Research Centre of Panjab

University, Chandigarh, Food and Agriculture Organization of the United

Nations, Bangkok, Thailand

Schutte E. and Prasad K. (1989) Wood combustion studies, A report of The Wood

burning Stove Group, WSG, Eindhoven University of Technology, The

Netherlands

Schwarzer K. (2003) Test des Holzherdes “Swazi Stove”, Ingenieurbüro für Energie-

und Umwelttechnik, IBEU, Jülich, Germany



Smith K. R. (1985) Biomass fuels, Air pollution and Health; A global review. Plenum Publishing Co., New York, USA

-------, 1986. Biomass Combustion and Indoor Air Pollution. in: Environmental Management. No. 10.

Sulilatu, W.F (1985) Danger signals to Human Health. In Form Design to Cooking:

Some Studies on Cook stoves. C.E. Krist-Spit and D.J. van der Heeden (eds.).

Wood burning Stove Group, Eindhoven University of Technology, The

Netherlands

VITA (1985) Testing the efficiency of wood-burning cook stoves, International

Standards, Volunteers in Technical Assistance, Arlington, USA

Internet web pages:

http://www.natmus.dk/cons/tp/rhbuff/buffer1.htm

http://www.admin.uni-oldenburg.de/aaa/de/en/internat/booklet/unioldsc.htm

http://www.newdawn-engineering.com/website/stove/singlestove/vesto/vesto1.htm

http://solstice.crest.org/discussiongroups/resources/stoves/Still/VC%20Stove/vcstove.html


Marco Peter, PPRE 2002/03 i

Appendices

Traditional inefficient open fire in southern Africa / Lesotho and woman carrying firewood and cooking with an local produced clay stove

Collection of local clay material and mixture process with vermiculite for the

VIC bricks in Maseru / Lesotho


Marco Peter, PPRE 2002/03 ii

Pouring of the test batch bricks for different clay vermiculite mixtures

VIC bricks and small test bricks air drying before kiln burning


Marco Peter, PPRE 2002/03 iii

Burning of the VIC bricks in a commercial kiln of a local brick producer in Maseru / Lesotho

Tin Can collection and stove case production at a workshop in Maseru / Lesotho


Marco Peter, PPRE 2002/03 iv

First Tin can fireplace design by Michael Hönes / Lesotho

Basic stove principle with VIC bricks designed by Aprovecho for the Tin Can stove

design


Marco Peter, PPRE 2002/03 v

Tin Can stove test set up at ‘Energielabor’ University Oldenburg

Simple test to show the draft created in the Tin Can stove prototype


Marco Peter, PPRE 2002/03 vi

Standard water boiling test protocol for data calculation recommended by VITA


Marco Peter, PPRE 2002/03 vii

Standard water boiling test protocol for test series reporting recommended by VITA


Marco Peter, PPRE 2002/03 viii

Standard water boiling test protocol for temperature profile recommended by VITA


Marco Peter, PPRE 2002/03 ix

COHb level of a typical adult as a function of time for a range of exposures

Test number 1.2Tin Can Stove 2003/07/10 16.30 UhrZimbabwe pot

before after H2Okg kg kg kg liter

stove 14.200 14.170 14.155 2.288pot(Zim) 15.090 0.867 15.038 0.868stove+pot+H2O 17.355 16.865 16.865 1.835pot+H2O 3.155 2.695 2.710 0.454Tamb rel. Hum. Ignition wood(25cm)°C % dry(°C) wet(°C) Paraffinöl % H2O measured22 55 22 16.2 g 13

60 21.4 16.4 3average 58 LHV

~40 MJ/kg

Test protocol initial conditions for the Tin Can stove prototype with Zim pot


Marco Peter, PPRE 2002/03 x

time wood stove system temperature Pt100 remarkmin kg kg °C mV0 127 17.355 18 107.96 support 2cm open4 17.495 20 108.57 4 sticks 25 cm7 17.490 21 109.18 closed8 76 17.490 22 109.44 2 sticks 25 cm10 17.550 25 110.32 good fire12 17.530 28 111.7215 17.485 37 115.0017 17.465 42 116.7018 71 17.460 44 117.40 2 sticks 25 cm20 17.500 50 119.52 no smoke21 111 17.475 59 122.17 3 sticks 25 cm24 17.515 75 128.5225 17.485 79 130.6227 17.450 86 133.07 no smoke29 17.415 91 134.7931 103 17.390 93 135.25 3 sticks 25 cm33 17.425 97 136.5434 487 17.395 100 137.42 boil 103g half burned36 91 17.315 101 138.18 2 sticks 25 cm evapor. H2O40 17.275 100 138.03 0.04042 17.215 100 138.0644 94 17.160 100 138.04 2 sticks 25 cm46 17.195 100 138.0750 58 17.080 100 138.00 2 sticks 25 cm52 17.075 101 138.1455 16.985 100 138.0557 71 16.950 100 138.09 2 sticks 25 cm59 16.975 100 138.0562 16.905 100 137.85 0.45064 800 16.865 100 137.69 sum

0.490

Test protocol measured values for the Tin Can stove prototype with Zim pot

time Tgas Tamb O2 CO CO-O% CO2 Twater Remarkmin °C °C % ppm ppm % °C11 125 23.2 3 599 698 13.2 2614 165.3 23.5 3 220 256 13.2 35 good fire23 264.8 24 3.2 220 259 13.1 7240 243.7 24.2 1.5 186 200 14.3 100 boil53 93.5 24.1 1.6 55 59 14.2 10160 143.6 24.2 3.3 61 72 13 100

Flue gas analysis results in the test protocol for Tin Can stove prototype with Zim pot


Marco Peter, PPRE 2002/03 xi

stove test high power phase low power phase remarksboil. simm.

spezific heat capacity water kJ/(kg*K) 4.18 4.18 4.18evaporation enthalpy of water kJ/kg 2260 2260 2260initial water temperature °C 18 18 100initial water amount kg 2.288 2.288 2.248water amount after boiling kg 1.798 2.248 1.798water vaporized kg 0.490 0.040 0.450boiling temperature water °C 100 100 100

moisture content of wood % 13 13 13lower heating value of wood LHV MJ/kg 19 19 19wood equivalent to ignite the fire kg 0.006 0.006 0 paraffin LHV ~40 MJ/kgtotal used wood consumed kg 0.7996 0.4866 0.313remaining wood after boiling kg 0 0 0remaining charcoal after boiling kg 0 0 0equivalent dry wood consumed kg 0.701 0.429 0.272

heat utelized to the water kJ 1723.7 860.9 1017.0utelized wood energy kJ 13316.6 8142.7 5173.9thermal stove efficiency 0.129 0.106 0.197

% 12.9 10.6 19.7stove testing time min 64 34 30

s 3840 2040 1800stove power(fire) kW 3.47 3.99 2.87average cooking power kW 0.45 0.42 0.57spezific standard demand kg/kg 1.43 10.71 0.61

Test protocol calculated parameter for the Tin Can stove prototype with Zim pot


Marco Peter, PPRE 2002/03 xii

Tin can stove+Zim potunit stove boil. simm. stove boil. simm. stove boil. simm. stove boil. simm. stove boil. simm.

dry wood consumed kg 0.745 0.458 0.287 0.745 0.429 0.272 0.809 0.515 0.294 0.720 0.446 0.274 0.755 0.462 0.282initial water amount kg 2.400 2.400 2.245 2.288 2.288 2.248 2.490 2.490 2.395 2.250 2.250 2.110 2.357 2.357 2.250water vaporized kg 0.530 0.195 0.335 0.490 0.040 0.450 0.440 0.095 0.345 0.458 0.140 0.317 0.479 0.118 0.362stove testing time min 66.5 36.5 30 64 34 30 77 45.5 31.5 68 36 32 69 38 31stove power(fire) kW 3.55 3.97 3.03 3.47 3.99 2.87 3.33 3.58 2.96 3.35 3.93 2.71 3.42 3.87 2.89average cooking power kW 0.46 0.55 0.48 0.45 0.42 0.57 0.36 0.36 0.41 0.40 0.46 0.37 0.42 0.45 0.46spez. stand. consumption kg/kg 1.41 2.35 0.86 1.52 10.71 0.61 1.84 5.42 0.85 1.57 3.19 0.86 1.58 5.42 0.79thermal stove efficiency % 13.0 13.8 15.9 12.9 10.6 19.7 10.8 10.1 14.0 11.8 11.8 13.8 12.1 11.6 15.8

Average1.1 1.2 1.3 1.4

Tin can stove+SA potunit stove boil. simm. stove boil. simm. stove boil. simm. stove boil. simm. stove boil. simm. stove boil. simm.

dry wood consumed kg 0.864 0.662 0.202 0.851 0.547 0.304 0.831 0.538 0.292 0.841 0.606 0.235 0.842 0.546 0.296 0.846 0.580 0.266initial water amount kg 3.605 3.605 3.190 3.660 3.660 3.445 3.635 3.635 3.525 3.645 3.645 3.375 3.665 3.665 3.470 3.642 3.642 3.401water vaporized kg 0.645 0.415 0.230 0.615 0.215 0.400 0.668 0.110 0.557 0.645 0.270 0.375 0.690 0.195 0.495 0.653 0.241 0.411stove testing time min 96 66 30 86 54 32 82.5 52.5 30 68 36 32 78 48 30 82 51 31stove power(fire) kW 2.85 3.18 2.13 3.13 3.21 3.00 3.19 3.25 3.09 3.92 5.33 2.32 3.42 3.60 3.12 3.30 3.71 2.73average cooking power kW 0.42 0.50 0.29 0.47 1.00 0.68 0.51 0.46 0.70 0.60 0.80 0.44 0.53 0.53 0.62 0.51 0.66 0.55spez. stand. consumption kg/kg 1.34 1.60 0.88 1.38 0.51 0.47 1.24 4.89 0.52 1.30 2.25 0.63 1.22 2.80 0.60 1.30 2.41 0.62thermal stove efficiency % 14.8 15.8 13.5 14.9 15.8 15.7 15.9 14.1 22.7 15.3 15.0 19.0 15.6 14.7 19.9 15.3 15.1 18.2

Average1.1 1.2 1.3 1.4 1.5

Comparison of the results from the Tin Can stove prototype with different pot types tested


Marco Peter, PPRE 2002/03 xiii

Vesto stove+Zim potunit stove boil. simm. stove boil. simm. stove boil. simm. stove boil. simm. stove boil. simm.

dry wood consumed kg 0.484 0.279 0.205 0.474 0.277 0.197 0.540 0.271 0.269 0.535 0.264 0.271 0.508 0.273 0.236initial water amount kg 2.285 2.285 2.230 2.285 2.285 2.155 2.260 2.260 2.145 2.260 2.260 2.210 2.273 2.273 2.185water vaporized kg 0.498 0.055 0.443 0.510 0.130 0.380 0.558 0.115 0.443 0.605 0.050 0.555 0.543 0.087 0.455stove testing time min 62 31 31 59 29 30 56 26 30 49 18 31 57 26 31stove power(fire) kW 2.47 2.85 2.10 2.54 3.02 2.08 3.05 3.30 2.84 3.46 4.65 2.76 2.88 3.45 2.45average cooking power kW 0.46 0.47 0.54 0.49 0.58 0.48 0.54 0.60 0.56 0.64 0.75 0.67 0.53 0.60 0.56spez. stand. consumption kg/kg 0.97 5.07 0.46 0.93 2.13 0.52 0.97 2.35 0.61 0.88 5.29 0.67 0.94 3.71 0.57thermal stove efficiency % 18.8 16.6 25.7 19.3 19.1 22.9 17.6 18.3 19.6 18.6 16.2 24.4 18.6 17.6 23.1

Average1.1 1.2 1.3 1.4

Vesto stove+SA potunit stove boil. simm. stove boil. simm. stove boil. simm. stove boil. simm. stove boil. simm.

dry wood consumed kg 0.503 0.312 0.191 0.465 0.299 0.166 0.468 0.290 0.178 0.491 0.298 0.193 0.482 0.300 0.182initial water amount kg 3.635 3.635 3.480 3.660 3.660 3.415 3.610 3.660 3.500 3.645 3.645 3.440 3.638 3.650 3.459water vaporized kg 0.750 0.155 0.595 0.788 0.245 0.542 0.680 0.110 0.570 0.812 0.205 0.608 0.757 0.179 0.579stove testing time min 55.5 25.5 30 58.5 28.5 30 52 22 30 49 18.5 30.5 54 24 30stove power(fire) kW 2.87 3.88 2.01 2.52 3.32 1.76 2.85 4.17 1.88 3.17 5.09 2.01 2.85 4.12 1.91average cooking power kW 0.80 1.00 0.75 0.78 1.00 0.68 0.80 1.05 0.72 0.93 1.39 0.75 0.83 1.11 0.72spez. stand. consumption kg/kg 0.67 2.01 0.32 0.59 1.22 0.31 0.69 2.63 0.31 0.60 1.45 0.32 0.64 1.83 0.31thermal stove efficiency % 28.0 25.8 37.1 31.1 30.1 38.8 28.0 25.2 38.0 29.2 27.3 37.4 29.1 27.1 37.8

Average1.1 1.2 1.3 1.4

Comparison of the results from the Vesto stove with different pot types tested


Marco Peter, PPRE 2002/03 xiv

Properties A B C D E F G H I J Aprovechotyp and color black black+

orange/ swamp

grey swamp

grey+ashblack+ash+ orange/grey

black+ash

swamp grey+ash

swamp grey

black black + ash Peter Scott

liter 1.25 2.5 1.25 1.25 1.25 1.25 1.75 1.75 1.75 1.75 1Vermiculite liter 8.75 17.5 8.75 8.75 8.75 8.75 8.75 8.75 8.75 8.75 6Water liter 1.5 2.5 1.25 1.5 1.5 1.5 1.75 1.75 2 2 fly ash (sifted) liter 0.25 0.5 0.25 0.25 0.25Weight gramm 1398 1038 1227 1343 1222 1410 1304 1201 1497 1471 860Handling good(1) - bad(6) 2.5 4.5 1.5 2.5 2 3 4 5 3 1Vermiculite/Clay ratio 7 7 7 7 7 7 5 5 5 5 6

Propertiestyp and color black+

orange/ grey

black+ ash+

orange/ grey

black black+ orange/

grey

black+ash+ orange/grey

black + ash

black black+ orange/

grey

swamp grey

black+ash+ orange/grey

black+ash

liter 2.5 1.25 1.25 2.5 1.25 1.25 1.25 2.5 1.25 1.25 1.25Vermiculite liter 17.5 8.75 8.75 17.5 8.75 8.75 8.75 17.5 8.75 8.75 8.75Water liter 2.5 1.5 1.5 2.5 1.5 1.5 1.5 2.5 1.25 1.5 1.5 fly ash (sifted) liter 0.5 0.5 0.25 0.5 0.25Weight grammHandling good(1) - bad(6)Vermiculite/Clay ratio 7 7 7 7 7 7 7 7 7 7 7Informationorange/grey clay near Maseru airport next to water vermiculite granule black clay near airport Maseru next to creek medium exfoliatedswamp clay Maseru East brick makers (washed/sifted from a pond) 8 hours burned at 1000° Cfly ash (sifted) Maseru East brick makers (from coal ash) 1 liter~150 g dried at 250° for 24 hours

Clay

BE

1201

Clay

1304

ABCEFABEF

1393

Clay mixture table from the field test for the VIC bricks in Maseru / Lesotho


Marco Peter, PPRE 2002/03

Curriculum Vitae

Personal facts:

Name: Marco Peter

Date of birth: 28.06.1971

Birthplace: Suhl / Germany

Nationality: German

Address: Grüner Weg 2c

98527 Suhl

Germany

School:

1978 - 1988 primary school and secondary school

1996 A-Level

Qualifications:

1988 - 1991 vocational training as electrician

1992 - 1993 alternative national welfare service

1991 - 1995 work as an electrician

1996 work as a fitter in a company for

environmental and sewage systems

2002 work as an engineer in a mechanical

engineering company in Suhl

Study:

2002 diploma mechanical engineering (energy

technology) University of Wismar

Since Oct. 2002 Student Postgraduate Program Renewable

Energy at the University of Oldenburg


Marco Peter, PPRE 2002/03

Practical training:

09.1999 - 01.2000 Bayernwerk AG, Munich, renewable energy

department

02.2003 – 04.2003 GTZ - South Africa, Pretoria, program for

biomass conservation in southern Africa

Activities during the study:

1998 scientific assistant at the university

1998 international workshop for renewable energy

University of Stralsund (English)

2000 work as a telephone operator for the

Telegate AG

2001 6 month project work at the Auckland

University of Technology New Zealand

Language:

English, German

Computer skills:

Windows, Office, Auto Cad

Interests:

sport, travelling, outdoor, technique

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