2010 Webmaster - University of Nigeria, Nsukka NNEKA J..pdf · SOLAR COOKER Agricultural and...
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i OGBUISI NNEKA J. (PG/M.ENGR/05/40213) DEVELOPMENT OF FIBRE REINFORCED PLASTIC SOLAR COOKER Agricultural and Bioresources Engineering A PROJECT REPORT SUBMITTED TO THE DEPARTMENT OF AGRICULTURAL AND BIORESOURCES ENGINEERING, UNIVERSITY OF NIGERIA NSUKKA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF MASTER OF ENGINEERING IN AGRICULTURAL ENGINEERING (CROP PROCESSING AND STORAGE ENGINEERING). Webmaster 2010 UNIVERSITY OF NIGERIA
Transcript of 2010 Webmaster - University of Nigeria, Nsukka NNEKA J..pdf · SOLAR COOKER Agricultural and...
i
OGBUISI NNEKA J.
(PG/M.ENGR/05/40213)
DEVELOPMENT OF FIBRE REINFORCED PLASTIC
SOLAR COOKER
Agricultural and Bioresources Engineering
A PROJECT REPORT SUBMITTED TO THE DEPARTMENT OF AGRICULTURAL AND BIORESOURCES
ENGINEERING, UNIVERSITY OF NIGERIA NSUKKA IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE AWARD OF MASTER OF ENGINEERING IN AGRICULTURAL
ENGINEERING (CROP PROCESSING AND STORAGE ENGINEERING).
Webmaster
2010
UNIVERSITY OF NIGERIA
ii
DEVELOPMENT OF FIBRE REINFORCED
PLASTIC SOLAR COOKER
BY
OGBUISI NNEKA J.
(PG/M.ENGR/05/40213)
DEPARTMENT OF AGRICULTURAL AND BIORESOURCES ENGINEERING
UNIVERSITY OF NIGERIA NSUKKA
APRIL, 2010
iii
Development of Fibre Reinforced Plastic Solar Cooker
By
Ogbuisi Nneka J.
(Pg/M.Engr/05/40213)
A PROJECT REPORT SUBMITTED TO THE DEPARTMENT OF AGRICULTURAL
AND BIORESOURCES ENGINEERING, UNIVERSITY OF NIGERIA NSUKKA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF
MASTER OF ENGINEERING IN AGRICULTURAL ENGINEERING (CROP
PROCESSING AND STORAGE ENGINEERING).
April 2010
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CERTIFICATION AND APPROVAL
Ogbuisi Nneka J., a postgraduate student of the Department of Agricultural Engineering
and with Registration Number Pg/M.Engr/05/40213 has satisfactorily completed the
requirements for the course and research work for the degree of Master of Engineering
(M.Eng) in Agricultural Engineering (Crop Processing and Storage Engineering). The
work embodied in this project report is original and has not been submitted in part or full
for any other diploma or degree of this or any other university.
--------------------------- ------------------------- ------------------
Supervisor Signature Date
--------------------------- ------------------------- ------------------
External Examiner Signature Date
--------------------------- ------------------------- ------------------
Head of Department Signature Date
v
TITLE PAGE
Development of Fibre Reinforced Plastic Solar Cooker
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DEDICATION
This work is dedicated to all men of good heart who take pleasure in investing on
humanitarian work.
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ACKNOWLEDGMENT
I acknowledge with thanks the effort of my project supervisor Dr W. I
Okonkwo, for his support and guidance which ensured that this work is finally
accomplished successfully. The broaden knowledge and thought I had during this
research will be always be appreciated. I extend my gratitude to Mr C.N
Anyanwu, Mr S.I Eze, Mr I.O Onuora and all the technical staff of National
Centre for Energy Reseach and Development, University of Nigeria Nsukka, for
their time, efforts and contribution to improve the quality and insight of this work.
My special thanks goes also to my Departmental lecturers, Darlinton and Afam of
Mechanical Engineering, University of Nigeria Nsukka and my colleagues who
were always around to attend to my needs and advice.
Finally, I remain indebted to my husband Mr I.A Ogbuagu, my parents Mr
and Mrs Ogbuisi, my relatives for their kind gesture and support through out this
work.
May the God Almighty reward you all bountifully.
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Table of Content
Page
Certification and approval i
Title page ii
Dedication iii
Acknowledgement iv
Table of content v
List of figures x
List of tables xi
Abstract xii
CHAPTER 1: INTRODUCTION
1.1 Introduction 1
1.2 Background of the study 3
1.3 Objectives of the work 4
1.4 Purpose and scope 4
1.5 Limitation 5
CHAPTER 2: LITERATURE REVIEW
2.1 Cooking technology 6
2.1.1 Three stone technology 6
2.1.2 Tripod stand 7
2.1.3 Charcoal fired cooking systems 8
2.1.4 Kerosene stove 8
2.1.5 Gas cooking technology 8
2.1.6 Electric cookers 9
2.1.7 Solar cookers 9
2.2 Solar energy 9
2.3 Solar radiation 10
2.3.1 Solar angles 11
2.3.1.1 Solar declination 11
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2.3.1.2 Hour angle 12
2.3.1.3 Zenith angle 13
2.3.1.4 Solar altitude 13
2.3.1.5 Azimuth angle 13
2.3.2 Solar intensity 14
2.3.3 Solar collector orientation 15
2.3.4 Measurement of solar radiation 17
2.4 Applications of solar energy 19
2.4.1 Solar water heating 19
2.4.2 Solar pond 19
2.4.3 Solar distillation 19
2.4.4 Photovoltaic 20
2.4.5 Solar cookers 20
2.5 Solar energy resources in Nigeria 20
2.6 Deforestation caused by use of biomass fuel 22
2.7 Types of solar cookers 24
2.7.1 Solar box cooker 24
2.7.2 Solar panel cooker 25
2.7.3 Concentrating solar cooker 26
2.7.3.1 Spherical parabolic cooker 26
2.7.3.2 Cylindrical parabolic cooker 27
2.8 History of solar cooking technology 28
2.9 Solar cooking in Nigeria 29
2.9.1 Technology of solar cooker in Nigeria 30
2.10 Benefits of solar cookers 32
2.11 Limitations of using concentrating solar cookers in Nigeria 32
2.12 Materials for constructing concentrating solar cookers 34
2.13 Fiber reinforced plastic 34
x
2.13.1 Properties of fiber reinforced plastic 34
2.14 General configuration of concentrating cookers 35
2.14.1 Concentration ratio 35
2.14.2 Energy efficiency of concentrating solar cookers 36
2.14.2.1 Energy input 36
2.14.2.2 Energy gained 37
2.14.2.3 Heat loss 37
2.14.2.4 Efficiency 38
CHAPTER 3: DESCRIPTION AND DESIGN OF FIBER REINFORCED
PLASTIC CONCENTRATING SOLAR COOKER
3.1 Design consideration 39
3.2 Component description 39
3.2.1 Concentrator 41
3.2.2 Support 43
3.3.3 Pot trestle 45
3.3 Design analysis and calculation 46
3.3.2 Concentrator area 46
3.3.3 Aperture area 46
3.3.4 Concentration ratio 47
3.3.5 Focal length 47
3.4 Solar collector orientation 48
3.4.2 Selection of altitude and azimuth angles 49
3.4.2.1 Altitude angle 49
3.4.2.2 Declination angle 49
3.4.2.3 Hour angle 50
3.4.2.4 Azimuth angle 51
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3.5 Thermal load and efficiency 52
3.5.2 Heat input 52
3.5.3 Estimated heat output 54
3.5.4 Estimated heat loss in FRP concentrating cooker 55
3.5.5 Efficiency 55
3.6 Thermal properties of food materials 56
3.6.2 Specific heat 56
3.6.3 Thermal conductivity 56
3.6.4 Thermal diffusivity 57
3.7 Cooking time of the cooker 57
3.8 Material selection 59
3.9 Construction of FRP concentrating solar cooker 61
3.9.2 Concentrator 61
3.9.3 Shell casting 65
3.9.4 Gel coating 65
3.9.5 Lamination operation 66
3.9.6 Curing process 67
3.9.7 De-molding operation 67
3.10 Operation of the solar cooker 67
3.11 Safety measures and maintenance of the cooker 68
3.12 Limitations of the cooker 68
CHAPTER 4: PERFORMANCE EVALUATION OF THE
FRP CONCENTRATING SOLAR COOKER
4.1 Experimental procedures 69
4.2 Test procedure 69
4.3 Instrumentation and measurement 70
4.3.1 Wind 70
4.3.2 Ambient temperature 71
4.3.3 Solar radiation 71
xii
4.3.4 Pot content temperature 71
4.3.5 Solar altitude and azimuth angle 71
4.4 Comparative test 71
4.5 Presentation of results 73
4.5.1 Water heating test 73
4.5.2 Cooking test 79
4.6 Thermal performance 81
4.6.1 Cooking power 81
4.6.2 Cooking efficiency 83
4.7 Discussion of results 88
CHAPTER SIX: CONCLUSION AND RECOMMENDATION
6.1 Conclusion 91
6.2 Recommendation 92
References 93
xiii
List of Figures
Fig. Pg
2.1 Three stones cooking technology 7
2.2 Declination angle versus days of the year 12
2.3 Desert encroachment caused by deforestation 23
2.4 Solar box cooker 24
2.5 Solar panel cooker 25
2.6 Parabolic shape and its focus point 26
2.7 Fresnel spherical parabolic cooker 27
2.8 Prata and Bowman cylindro-parabolar 28
3.1 Sectional views of the FRP concentrating solar cooker 40
3.2 Minor and major diameter of the FRP concentrating solar cooker 42
3.3 Base support 44
3.4 Y- component of the FRP concentrating solar cooker support 44
3.5 Pot trestle 45
3.6 The FRP concentrating solar cooker 61
3.7 Plot of parabolic scraper 63
3.8 Parabolic mold 64
3.9 Gel coating operation 65
3.10 Lamination operation 66
4.1 NCERD concentrating solar cooker 72
4.2 Japanese type concentrating solar cooker 72
4.3 Box type solar cooker 72
4.4 FRP concentrating solar cooker 72
4.5 Plot of solar radiation, wind speed and ambient temperature over time 73
4.6 Plot of solar radiation and cooker's temperature over time on a windy
day 78
4.7 Plot of solar radiation and cooker's temperature over time on a clear
day 79
4.8 Cooked food samples using solar cookers 81
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List of Table
Table Pg
3.1 Values for the plotting of the parabolic graph 62
4.1 Results on day one heating test 75
4.2 Results on day two heating test 76
4.3 Results on day three heating test 77
4.4 Cooking test of four types of solar cookers 80
4.5 Water heating performance evaluation of the four solar cookers 87
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ABSTRACT
The need for alternative source of energy for food preparation has
given rise to the use of solar energy for cooking. This technology which is not
widely utilized in Nigeria has been a major research study to harness solar
energy for cooking in order to reduce the depleting forest zone and
environmental degradation resulting from fuelwood utilization.
A concentrating solar cooker was designed and constructed at National
Centre for Energy Research and Development (NCERD), University of
Nigeria Nsukka. The solar cooker which comprises of three parts namely;
concentrator, support and pot trestle has its concentrator produced from a
concrete mold using polyester resin and fibre glass, a composite known as
fiber reinforced plastic (FRP).
The performance of the solar cooker was evaluated using three other
solar cookers at National Centre for Energy Research and Development,
University of Nigeria Nsukka. The results of the test experiment showed that
the concentrating FRP solar cooker compared favourably well with the other
solar cookers. The major factor that affected the performance of the solar
cooker was the harmattan haze which was predominant during the testing
period. It took 80mins for the concentrating FRP solar cooker to attain a
maximum temperature of 97.3oC during the water heating test and 52mins to
boil egg while it took 158mins and 160mins respectively to cook rice and
yam. The Japanese type solar cooker attained a maximum temperature of
98.80oC in 100mins, cooked eggs in 27mins, while it took 70mins and 90mins
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to cook rice and yam respectively. The NCERD concentrating cooker attains
its maximum temperature of 94.1oC in 190mins, boiled eggs in 48mins while
it took 123mins and 120mins respectively to cook rice and yam. The box type
cooker was able attain temperature of 98oC in 210min, cook egg and rice for
100mins and 182mins but was unable to cook yam. All the cooked food
samples proved to be palatable.
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CHAPTER ONE
INTRODUCTION
1.1 Introduction
The major cooking fuels in the rural areas in Nigeria are wood fuel,
agricultural wastes and animal dung while in the urban and suburban cities,
the main cooking fuels are kerosene, liquefied natural gas (LNG), electricity,
fossil fuels such as coal and natural gas. The use of kerosene for cooking is
presently more common in the urban and suburban cities because of the
declining production and poor management and distribution of electricity. The
increasing cost of Liquefied Natural Gas due to the bad economic situation in
the country also contributed to restriction of the use of liquefied natural gas by
only the rich in the society (Vieira de Silva, 2005). In this modern day
civilization in environmental control, it has been realized that the use of wood
fuel and other biomass, kerosene and liquefied natural gas for cooking
introduces Carbon II Oxide and other greenhouse gases in the household
environments and this in great measure contributes to global warming and
climate change. The persistent use of firewood for cooking had also leads to
soil erosion, deforestation, desert encroachment, health hazard and the
shortage of firewood. In advance search for other alternative ways of cooking
technology, solar energy becomes a good alternative source of energy for
cooking in Nigeria. This is because Nigeria is endowed with abundant
sunshine of not less than 9 hours per day through out the year due to its
position near the equator (Bald et.al, 2000). However, solar cooking can not
be able to replace the other cooking technology in Nigeria, but the use of solar
energy for cooking would save the forest reserves of Nigeria. It would
adequately reduce air pollution from the carbon containing fuels that
contribute to global warming and climate change. Moreover, introduction of
solar energy for cooking would reduce the cut down of trees which lead to soil
erosion, deforestation and desert encroachment which is mostly common in
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the northern parts of Nigeria. It would also improve food nutrition and health
condition as has been found out and as well serve as a good alternative source
for cooking during the periods of shortage of other cooking fuels. The use of
solar energy for cooking in Nigeria could be beneficial because solar energy is
inexhaustible, universal, abundant and free. Solar cookers can be used for
cooking at any areas and that includes the most remote rural areas in Nigeria.
There are different types of solar cooker, they include solar box cooker,
concentrating solar cooker and panel solar cooker. They are classified based
on the manner they collect solar energy for cooking. Concentrating solar
cooker is a typical of solar cooker that reflects all the sun rays directed to its
surface to a fixed point known as the focal point. This type of cooker can be
produced by the use of a discarded satellite dish, with the focal length already
located at the position of the dish antenna.
Material selection is one of the most considered factors in engineering
practice and so, many typejs of materials has been used in constructing solar
cookers. These materials ranges from low cost materials such as mud, paper
et.c to high cost material such as mirrors etc (Seire, 2006). One of those
materials that have been developed to construct solar cookers is the fiber
reinforced plastics (FRP). Fiber reinforced plastic is a composite, made up of
polyester resin and glass fiber. It is widely used in engineering design because
of its strength, weight, flexibility and ease of fabrication. Fabricating solar
reflectors out of steel is time consuming and expensive and also subject to
optical distortion due to fabrication stresses (Warren, 1991). But materials
such as fiber reinforced plastic can withstand some of these lapses of steel.
Construction of solar cookers especially concentrating solar cookers requires
maximum collector efficiency and this involves exceptionally accurate
reflective surface, which in all respect requires high tolerance construction
techniques. Other component parts of the solar cooker also has to be well
designed and constructed to maintain the accurate shape of the solar cooker
under thermal conditions throughout its useful life because varying weather
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condition, extreme temperature and corrosion represents a constant challenge
to the integrity of any exposed solar cooker.
1.2 Background of the Study
Cooking is an act of preparing food for ingestion. It comprises all
methods of food preparation and often requires an application of heat.
Cooking is an activity that is very vital for sustenance of human lives.
Cooking for domestic and commercial purposes require enormous amount of
energy. The energy for cooking accounts for about 90 percent of all household
energy consumption in the developing countries (GTZ, 2002 and Burgos,
2008). The energy used for cooking can either be renewable or non-renewable
energy sources. The non renewable energy sources are petroleum, natural gas,
electricity et.c while renewable energy sources include wind energy, solar
energy, wave or ocean energy. Of all the energy sources, the most commonly
used for cooking is the biomass fuel (firewood, charcoal, dung, and
agricultural residue). It is often the most available energy source especially in
the rural areas. Firewood is the most commonly used of all the biomass fuel
for cooking food because it‟s ease of availability locally. It is the traditional
and the most popular source of energy for cooking in the rural areas of African
set ups. It is used for domestic cooking, baking and heating. It is also widely
used in small scale industries such as bakeries because they are sourced from
nearby bushes and farmland.
The biomass energy source has the following advantages over other energy
sources.
1. Biomass fuels are fast burning non renewable source of energy.
2. They are available everywhere in one form or other and can burn
without further processing.
3. They are cheaper than fossil fuels (such as gas and kerosene).
Though biomass fuels have these benefits, its negative effect outweighs all the
benefits as listed below;
1. Biomass fuels are burned in inefficient open fires.
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2. The smoke it produces during burning constitutes health hazard due to
inefficient use.
3. The demand for biomass fuel exceeds its supply and this leads to
massive deforestation, land degradation and desertification.
Due to these limitations, biomass fuel especially the use of firewood for
cooking have led to the search for other alternative means of cooking systems
such as the use of kerosene stove, liquefied petroleum gas (LPG) cooker,
electric cooker and solar cookers. The introduction of these cooking measures
and the devices is to minimize the consumption of biomass fuel and also to
improve on the cooking efficiency of these cooking technologies
1.3 Objectives of the Work
The objectives of this work are as follows;
1. Design a concentrating solar cooker with a reflecting surface
2. To construct a solar cooker using fiber reinforced plastic (FRP)
3. Undertake comparative performance evaluation of the solar cooker
with a box type solar cooker and other available concentrating solar
cookers.
1.4 Purpose and Scope
The aim of this project is to design, construct and determine the
performance evaluation of a solar cooker that can reduce the stress in cooking
and reduce the cost of cooking. The materials for its construction have to be
locally sourced. The cooker has to be easy to operate with low maintenance
cost and should be capable to boil water and cook food.
The project work is restricted to Nsukka because of the choice of the
latitude the orientation, though areas with similar characteristics as Nsukka
can adopt the technology with very little variations that might be experienced
in the performance of the cooker.
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1.5 Limitation
The limitations of the work are as follows;
1. The solar cooker is only solar powered.
2. Cooking is restricted to sunny periods.
3. There must be continuous movement of the solar cooker every 15
minutes to track maximum solar radiation.
22
CHAPTER TWO
LITERATURE REVIEW
2.1 Cooking Technology
There have been numerous cooking technologies since the inception of
cooking (Sambo et al, 1991). These technologies tend to ease the difficulties
encountered during cooking and also reduce the cost of buying fuel by
reducing the combustion of fuel such that very little smoke is emitted during
the cooking process. The technology ranged from the use of firewood,
kerosene and gas cooker to the use of solar cookers. The improvement on
these cooking systems has been to create efficient and clean burning stove that
will be most convenient to the user. Some common methods of cooking
technology include the use of three stone, charcoal fire cooking, kerosene
stove, gas cooking, electric cooking and solar cooking technologies.
2.1.1 Three Stone Cooking Technology
This is the traditional way of cooking food by setting three stones. It is
the cheapest cooking system that can easily be adopted by anybody. It requires
only three suitable stones of the same height on which the cooking pot can be
balanced over fire. In between the stones, firewood is gathered together at the
center under the cooking pot. An example of three stone cooking system is
shown in Fig 2.1.
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This type of cooking stove has so many problems associated with it. It often
exposes the user to fire hazard and takes longtime and much fuel to cook food.
It is commonly used in the rural areas because it is at no cost since there is
availability of stones and fire wood in nearby bushes that can be used for this
technology.
2.1.2 Tripod Stand
The tripod stand is a replica of the three stones cooking system. It
consists of three steel rods at equidistance from each other that bent over
another circular rod to join the three steel rods. The cooking pot is placed at
the upper part while firewood and other biofuels are fed through the
underneath of the tripod stand. This type of cooking system can easily be
moved from one place to the other but it still has the disadvantages of three
stones cooking system in which the user is prone to health, fire hazard and at
large causes environmental hazards to the society.
Fig 2.1: Three stones cooking technology
Source: GTZ
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2.1.3 Charcoal Fired Cooking Technology
This type of cooking system uses charcoal as its energy source.
Charcoal is one of the by products of wood after complete combustion of
wood. It can be used efficiently to cook food. There are different designs of
this cooking system. The commonest model consists of two basic units
namely; the combustion unit and cooking unit (Ndirika, 1991). The
combustion unit is where the charcoal is fed in and when heated, it transfers
heat to the cooking unit which is where the pot sits. Another type of charcoal
fired cooking stove is the rocket stove mostly used in Kenya. The charcoal
fired cooking system is an improvement in use of fire wood for cooking
because of its minimal smoke production.
2.1.4 Kerosene Stove
Kerosene stove uses kerosene which is one of the end products of
petroleum fractional distillation. It is mostly used in the urban and semi urban
areas. Appropriate designed kerosene stove is very efficient and cooks food
very fast. There are various types of kerosene stove but the main two types are
the wick and the pressurized stove. This type of cooking system is easy to
control and operate. They do not produce smoke as much as fire wood stoves
but they can be very dangerous when improperly handled and are expensive to
maintain especially during the time of shortage of supply of kerosene.
2.1.5 Gas Cooking Technology
The cooking gas comprises of butane or propane which are
hydrocarbon gases produced during petroleum refinery process. Liquefied
petroleum gas is used for cooking when they are compressed inside a gas
cylinder in gaseous form. They are very easy to use, very efficient and
smokeless and are not commonly found in the rural areas. The gas cooker
comes in different sizes and shapes. They are made of the burning ring unit
and a pot support. The problems associated with this type of cooking system
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are its initial cost, irregularity supply and high cost of gas. There is also high
risk of explosion when misused.
2.1.6 Electric Cookers
The use of electricity as a source of energy for cooking has been
growing over time. This is because it is pollution free, easy to operate and
enables fast cooking. The system mechanism involves conversion of electrical
energy to heat energy through metallic barrier that is in contact with a resistive
heating coil. The resistive heating coil heats up the pot place on it. The major
disadvantage of electric cookers is the unreliable power supply in Nigeria
especially in rural areas and risk electric shock.
2.1.7 Solar Cookers
Solar cooker is one of the emerging solar energy technologies in which
solar energy is used to generate heat for cooking. This technology has not
been widely used in developing countries as much as it is used in the
developed countries. It is the safest of all the cooking systems especially areas
that are endowed with abundant sun energy for long period of time. The use of
solar energy erases the problem of deforestation, high cost of cooking fuel and
ensures shorter time of cooking for concentrating solar cookers. The only
problem facing the use of solar cookers is absence of sun energy but Nigeria is
richly blessed with sun energy that this technology can easily be adopted.
2.2 Solar Energy
The sun is the source of solar energy. It is an intensely hot gaseous
matter with diameter 1.39×106Km and is on the average 1.5×10
8 Km away
from the earth (Duffie and Beckman, 1974). Sun generates energy from
conversion of hydrogen to helium through fusion reaction. Electromagnetic
radiation from these reactions leaves the sun radially in all direction but the
distance from the earth to the sun is so large that sun‟s rays reaching the earth
are essentially parallel. The characteristics of the sun and its spatial
26
relationship to the earth result in a nearly fixed intensity of solar radiation
outside the earth‟s atmosphere known as solar constant. Earth‟s solar constant
is about 1353W/m2. This solar constant fluctuates slightly as the earth moves
towards and away from the sun at different point on the orbit.
Sun provides light, heat and other sources of energy to all living things
on earth. It is also responsible for weather changes and oceanic current. It is
the most inexhaustible renewable source of energy known to man and also
primary source of almost all type of energy even the non-renewable energy
source like coal petroleum and natural gas (Narayanaswamy, 2001). For
instance, fossil fuels are from decomposed plants and animals whose energy
came from sun.
2.3 Solar Radiation
Solar radiation otherwise known as insolation is the amount of energy
from the sun reaching a specific location on the surface of the earth at a
specified time. Due to the interference of the atmosphere, solar energy hits a
horizontal plane on earth in direct and diffuse forms. The direct form is the
parallel rays from the direction of the sun while the diffuse forms is radiation
scattered in many directions by matter and gases in the atmosphere. The
diffuse radiation are caused by the reflection and scattering by the atmosphere
(Hartnett, 1976) due to
1. Reflected radiation by snow cover
2. Radiation from cloud
3. Scattering by dust particles
The total solar radiation received by an object on the earth surface is lower
than the solar constant due to absorption and scattering of radiation that occurs
in the atmosphere. Depending on the time of day, time of year, latitude and
altitude of the area and the prevailing weather condition, the solar radiation
received in a location varies. The total solar radiation that gets through the
earth even with a clear sky is about 90 percent (Hug, 2007) of the solar
27
constant. For exact computation of the amount of energy that falls on a
particular area, incident angle between solar radiation and area as well as the
time of the year has to be considered. The radiation upon a surface can
typically be described as interacting with surface in one or more of following
three ways namely; it can be absorbed into the material, transmitted through
the material or reflected off the material. The proportion of each of the three
ways depends on the wavelengths of the radiation, chemical composition and
physical structure of the material and the angle incidence at which the
radiation strikes the material.
Radiation is reflected off a surface depending on the texture of the surface.
When it is reflected in all direction, it is known as diffuse reflection but when
it is reflected off a mirror, it is called secular reflection. The material for
reflection of sun rays differs significantly. The commonly used is the mirror
and aluminum film which has been in use over the years. Window glass and
clean plastic sheeting transmits light unlike mirrors and aluminum film, and
copper sheeting. In other to estimate the total solar radiation on a surface,
beam radiation, diffuse radiation as well as the solar intensity must be
estimated. To estimate these parameters, two set of angles known as sun-earth
angle and derived solar angles are required. These angles show the angle of
inclination of the sun‟s rays on the surface of the earth.
2.3.1 Solar Angles
2.3.1.1 Solar Declination
Declination (δ) is the angular distance of the sun, north of the earth‟s
equator. The earth‟s equator is tilted 23.45 degrees with respect to the plane of
the orbit around the sun, so that at various time of the year, as the earth orbits
the sun, declination varies from 23.45 degrees north to 23.45 degrees south.
Fig 2.2 shows the variation of declination angle with the day of the year. It is
estimated as follows;
28
365
28436045.23
NSin 2.1
where;
= declination angle
N = number of days of the year
2.3.1.2 Hour Angle
Hour angle ( H ) is the angular displacement of the sun, east or west of
the local solar noon per hour due to rotation of the earth on its axis at 15o. The
hour angle is positive in the mourning, zero at noon and negative in the
afternoon. It is estimated as
TH 1224
360 2.2
Where;
H = hour angle
T = solar time
Fig 2.2: Declination angle versus days of the year
29
2.3.1.3 Zenith Angle
Zenith angle ( ) is the angle between the beam from the sun and the
vertical line to the zenith (i.e point directly overhead). The zenith angle of the
sun is estimated as follows
CosHCosCosLSinSinLSinCos 2.3
Where;
= zenith angle
= altitude angle
L =latitude
= declination angle
H = hour angle
2.3.1.4 Solar Altitude
Solar altitude ( ) is the angle on the vertical plane between the beam
from the sun and the horizontal plane on the earth‟s surface. It is estimated as
090 2.4
Where;
= altitude angle
θ = zenith angle
2.3.1.5 Azimuth angle
The azimuth angle ( ) is the deviation of the normal to the surface
from the local meridian. An object due north has an azimuth of 0o, due east is
90o, south and west is 180
oand 270
o respectively. For instance, at sunrise, the
sun is positioned in the east at an azimuth of approximately 90o, while at
midday, it is located high in the sky at nearly 0oazimult. The angle is estimated
as
30
Cos
SinHCosSin 2.5
Where;
θ = azimuth angle
δ = declination angle
β = altitude angle
H = hour angle
2.3.2 Solar Intensity
Due to the earth‟s natural tilt on its axis, the amount of direct sunlight
that geographical locations receive varies with latitude locations near the
equator and its positioned at nearly 90o angle reference to the sun. Locations at
higher latitude are positioned at much greater angles and receive far less direct
sunlight unlike location at lower latitude (Duffie and Beckman, 1974). This is
the cause of variations in the amount of solar radiation received in different
locations. To determine the total solar intensity of solar radiation falling on a
surface in a given locality, it is the sum of the beam radiation (b
I ), diffuse
radiation (d
I ) and the reflected radiation (ref
I ). This is expressed as follows
refdbtIIII 2.6
Where;
t
I = total solar intensity, W/m2
bI = beam radiation, W/m
2
dI = diffuse radiation, W/m
2
refI = reflected radiation, W/m
2
31
2.3.3 Solar Collector Orientation
The angle by which the sun‟s rays strike the earth varies by
geographical location, and time of the year. This is as a result of the earth tilt
on its axis and its revolution around the sun. This angle is also influenced by
the earth‟s daily rotation which causes the sun to travel an arching path
through the sky. The amount of incident energy per unit area and day depends
on a number of factors such as the latitude, local climate, season of the year
and the inclination of the collecting surface in the direction of the sun.
Solar collectors are equipment that transforms solar radiation to some
other useful energy forms. There are two types of collectors namely; flat plate
collector and concentrating collector. For the flat plate collectors, the area
absorbing the solar radiation is the same as the area intercepting solar
radiation unlike the concentrating collector which have concave reflector to
concentrate the radiation falling on the total area of the reflector to a point
thereby increasing the energy flux at that point. The latter posses the inherent
problem of tracking because the surfaces must be oriented so that the focus,
vertex and sun are in line and this must move about in two axis namely;
horizontal and vertical axis (Duffie and Beckman, 1974).
The orientation of the solar collectors (i.e the way the collectors face
and how they are tilted) optimizes their collection ability. The highest solar
radiation that can be collected on a given sunny day is at solar noon when
direct beam radiation is least affected by the atmosphere. This is so because
solar noon is true south in the northern hemisphere and orienting the collectors
to this true south will normally maximize the performance of the solar
collector with a variation within 20o east or west (Garg, 1987).
32
Orientation of solar collector can be of the following ways as given by
Eibling in Duffie and Beckman (1974).
1. The collector can be fixed so that it is normal to solar beam at noon
on the equinox.
2. Its rotation can be horizontal, east-west axis with a single, daily
adjustment permitted so that its surface, normal coincides with the
solar beam at noon every day of the year.
3. It can as well be rotated about a horizontal east-west axis with
continuous adjustment to obtain maximum energy incidence.
4. There is rotation about a horizontal north-south axis with
continuous adjustment to obtain maximum energy incidence.
5. The rotation can also be about an axis parallel to the earth‟s axis
with continuous adjustment to obtain maximum energy incidence.
6. The collector can be rotated also about two perpendicular axes with
continuous adjustment to allow the surface normal to coincide with
solar beam at all times.
In addition to the orientation of the solar collector, the solar collectors can
follow the diurnal movement of the sun through two orientation system
namely; manual and mechanized operation system. The manual system is the
commonly used operation system in the developing countries. It depends on
the observation of the operator and making adequate adjustment of the
collector with time. The mechanized system does not need external operation.
It is either programmed to move in a predetermined manner or have detectors
that determines the system misalignment and through control systems makes
the necessary correction to realign the collector. With these two systems,
33
concentrating solar reflector (since it reflects the sun‟s rays and concentrate it
to a focal point) can easily be manipulated to get a very desirable result.
2.3.4 Measurement of Solar Radiation
Solar radiation measurement are most often made of total (beam and
diffuse) radiation in energy per unit time per unit area on a horizontal surface.
Pyranometer or pyrhelimeter is the instrument used to measure solar radiation
received from a given area. There are other expressions that are necessary in
the measurement of solar radiation. They are described as follows
For a tilted surface, the beam radiation at normal incidence (bn
I ) is
calculated using equation 2.7.
sinB
ap
bn
AI
2.7
Where;
bnI =intensity of beam solar radiation at normal incidence, W/m
2
apA = apparent solar irradiation just outside the atmosphere
B = atmosphere extinction coefficient
= altitude angle
apA and B are constants whose values are tabulated for an average day of
every of the month in the year as shown in (Hsieh, 1986).
The intensity of beam radiation (b
I ) incidence upon a surface at sea level is
given as
CosIFIbncb
2.8
Where;
34
bI = intensity of beam radiation, W/m
2
cF = air clearness factor
= zenith angle
bnI =intensity of beam radiation at normal incidence, W/m
2
The solar intensity of diffuse radiation from a clear sky falling on a tilted
surface is given as
2
1 CosCII
bnd 2.9
Where;
dI = intensity of diffuse radiation, W/m
2
C = diffuse radiation factor
= tilted angle of the surface
The reflected radiation falling on adjusted surface ref
I is expressed as
follow
dbsrrefrefIIFRI 2.10
Where;
refI = intensity of the reflected radiation, W/m
2
refR = reflectance of reflected surface
srF = shape factor between reflected and the receiving surface
bI = intensity of beam radiation, W/m
2
dI = intensity of diffuse radiation, W/m
2
35
2.4 Applications of Solar Energy
There are numerous applications of solar energy. Some of them include;
2.4.1 Solar Water Heating
Solar energy can be easily converted to heat and this is used to provide
a significant proportion of the domestic hot water demand (Mcveigh, 1977). It
consists of a flat plate collector and a storage tank.
2.4.2 Solar Pond
Solar ponds are mainly used in areas where there is considerably small
seasonal variation in solar radiation. The technology is based in natural pond
technology but an additional technique. In a natural pond or lake, when the
sun‟s rays heat up the water in the pond, the water rises from within the pond,
reaches the top and loses heat into the atmosphere by the action of convection
current. This results in the pond water temperature remaining at atmospheric
temperature but solar pond inhibits this natural phenomenon by using
dissolved salt. The concentration of dissolved salt in the solar pond increases
with depth, causing the density of water to increase toward the base of the
pond which is often painted black to absorb solar energy. The dissolved salt
makes the water too heavy to rise above the surface even when the water is
hot. The solar pond comprises of three zones namely; top zone which is at
atmospheric temperature and has little salt content, mid-zone which is hotter
than top zone and the bottom zone which is very hot and salty. The water at
the bottom zone can be collected and stored as heat.
2.4.3 Solar Distillation
This is another popular application of solar energy which is used to
purify water. It consists of shallow tray filled with salt or brackish water
covered by a sloping glass cover plate. As solar radiation heats the water in
the tray, the fresh water evapourates and when the vapour comes in contact
36
with the colder surface of the glass, it condenses and runs down the inner
surface in form of droplets and collected in a trough at a lower edge. It is
widely used in area where there is scarcity of purified drinking water.
2.4.4 Photovoltaic
This is the direct conversion of solar energy into electricity using
photovoltaic converter (solar cells). The solar cells absorb most of the solar
spectrum and convert a fraction of the radiation to electrical energy while the
remaining energy is given off as thermal energy. This technology has been
one of the major solar energy applications.
2.4.5 Solar Cookers
Solar cooker utilizes solar energy to generate heat for cooking. It is one
of the solar energy applications that are relevant to ordinary man. The solar
energy can either be collected in an insulated box (solar box cooker), focused
to concentrate solar radiation (concentrating cookers), or indirectly collected
and transferred through a medium (indirect solar cookers). Which ever form
used in collecting solar energy, solar cookers concentrates the heat to cook
food. The use of solar cookers has not been so versatile when compared to
other solar energy applications because of lack of awareness. It is mainly used
in the developed countries than in the developing countries.
There are other applications of solar energy such as water pumping,
solar refrigerators, solar driers, solar poultry brooders, solar incubators etc. but
this work did not cover those areas.
2.5 Solar Energy Resources in Nigeria
Nigeria is located in the tropical region of the equator ranging between 4
and 11 degrees north latitude (Narayanaswamy, 2001), and with its location in
the equator, it receives greater solar energy because in the tropics, solar
37
radiation is more direct and also passes through relatively less atmosphere
(Duffie and Beckman, 1974). It has been said that Nigeria is blessed with
abundance of solar energy to help alleviate its numerous energy insufficient
problems when harnessed and utilized effectively. According to Bald et.al
(2000), Nigeria is endowed with an annual average daily sunshine of
5.2KW/m2/day for 6.25 hours ranging between 3.5KW/m
2/day for 3.5 hours at
the coastal areas to 7.0KW/m2/day for 9 hours at the far northern areas.
Ikuponisi (2004) stated that this amount of energy from the sun in Nigeria is
equivalent to 1.082 million tones of its oil production per day, four thousand
times Nigeria‟s current daily crude oil production, thirteen thousand times that
of natural gas daily production and one hundred and seventeen thousand times
the amount of electric power generated in Nigeria in 1998. He analyzed his
statistics and found out that the annual solar energy insolation value is about
twenty seven times the Nigeria‟s total convectional energy resources. This
therefore shows that Nigeria with a total land mass of 9.24×103Km
2 and
average of 1.804×1015
KWh of incidence solar energy annually requires only
3.7 % of her national land area to be utilized effectively in order to collect
amount of sun energy equivalent to the nation‟s conventional energy reserve.
Only recently with the invasion of the world energy crisis in 1970s which
has brought about global realization of the need for diversity of energy from
exhaustible fossil fuel resources to other energy sources has Nigeria started
showing interest in other energy resources (Dohn, 2000). This resulted to
Federal Government of Nigeria in 1980 to established four energy research
centers with the mandate to source for other rich available energy resources in
various areas of renewable energy (Adetola, 2006). With so many researches
on renewable energy, it was found that fossil fuels are not inexhaustible and
they have environmental consequences due to their reckless consumption.
Hyro- power, coal, wind and solar energy were found to be the most available
renewable energy resources which can be adopted in Nigeria to alternate for
38
the use of fossil fuels. Out of these renewable energy sources, the most
abundant and available energy resource in Nigeria is solar energy (Bald et.al,
2000).
2.6 Deforestation caused by use of Biomass Fuel
Deforestation has been the most pressing environmental problem faced by
most of the Africa countries today and Nigeria has been rated as the most
affected country with a depletion rate of more than 35 ×106 m
2/year (UNDP,
2004). Nigeria has relied on firewood as the major conventional energy for
cooking. This energy source has made the country to face a huge
environmental problems due to the depletion of its forest as a result of cutting
down of trees to make up for its rising fuel for cooking. Deforestation has
negative implications to local environment due to increased soil erosion. In the
global environment, it causes fast climatic change and threatens biodiversity
since an area of rainforest when destroyed, the land surface and the overlying
air will be warmer because of the reduced evapo-transpiration and
precipitation and this results to a longer dry season which if occur over a long
period of time, changes the climate completely and makes it impossible for
forest to re-establish itself (Sudzina, 2002). The deforested area will support
dry forest and savannah as shown in Fig 2.3 and this increases the poverty
level as firewood becomes scarce and expensive and families are forced to
switch to other fuels which are expensive. Carol (2006) reported that with
current trend of deforestation in Nigeria, Nigeria will have no remaining forest
in the next twenty years and this will give rise to gross ecological problems if
not handled on time.
39
Fig 2.3: Desert encroachment caused by deforestation
Even though there is improvement in the combustion of biomass energy
source such as charcoal fired cooking stove (Ndirika, 2006), wood burning
stove (Sambo et.al, 1991), and other cooking stoves such as biogas burner,
there is still need for an alternative renewable energy such as solar cooker
which is free, abundant, less expensive, smokeless and safe in Nigeria. If
reforestation plans are combined with solar cooking program which provides
affordable renewable energy, deforestation would be reduced and the forest
reserve would be protected.
40
2.7 Types of Solar Cookers
There are three types of solar cookers namely; solar box type, panel
type cookers and parabolic or concentrating cookers.
2.7.1 Solar Box Cooker
Solar box cooker (SBC) type is the simplest of the solar cookers. It is
an insulated container with a glass cover. This kind of cooker depends on the
green house effect in which the transparent glazing unit permits passage of
shorter wavelength radiation coming from relatively low temperature heated
object (Narayanaswam, 2001). Removable lid on the cover of the box cooker
acts as the solar box door and this door is heat resistant transparent cover
which can allow solar radiation inside the box and allows the removal or
placing of the dark pot containing food inside the cooker. The commonly used
reflecting surface is mirror. Insulating material inside the box cookers is
mainly crumpled newspapers, wool, rags, dry grass, sheets of cardboard, rice
husk et.c. Inside of box cooker and the cooking pot are painted black for
maximum absorption of heat. Box cookers can attain temperature as high as
150oC over along period of time but for food containing mixture, it cannot get
hotter than 100oC (en.wikipedia.org, 2003). It can be used to warm food,
pasteurize water or milk, dry agricultural products, et.c. An example of solar
box cooker is shown in Fig 2.4.
Fig 2.4: Solar box cooker
41
2.7.2 Solar Panel Cooker
A solar panel cooker consists of a number of flat reflecting surface that
focus solar radiation to a black pot in a clear heat resistant plastic bag. One of
the reflecting surfaces serves as the floor of the cooker while the others serve
as walls. The original design was from Dr Roger Barnard of France who used
an open foil box with a dark bowl (en.wikipedia.org). Barbara Kerr, an
Arizona later developed from the basic design of Dr Barnard, a foldable model
which was suitable for back packing. This she built by replacing the bowl with
a plastic bag. Working from her model, Ber Blum, the director of solar cooker
international journal joined the volunteer engineers to develop a mass
producible solar panel cooker known as cookit (Kundapur, 2003). This type
of cooker is foldable kit made from cardboard. Solar panel cooker attains
higher temperature than the box type because of its multiple reflecting
surfaces though it requires periodical movement in other to trap sun energy. A
solar panel cooker is shown in Fig 2.5.
Fig 2.5: Solar panel cooker
42
2.7.3 Concentrating Solar Cooker
Concentrating solar cookers concentrates solar radiation to a central
point where a pot containing food to be cooked are placed. The reflecting
surface is always parabolic in shape. Geometrically, a parabola is defined as a
set of all point that is equidistance from a point known as the focus and a fixed
line called the directrix (Jennifer, 2005). With this principle, when a parabolic
cooker is aimed at the sun, all the light that falls upon its mirrored surface is
reflected to a point known as the focal point as shown in Fig 2.6.
Fig 2.6: Parabolic shape and its focus point
There are different types of designs based on this principle of parabola. The
designs are classified based on two types namely; shape of the reflector used
such as spherical reflector and cylindrical parabolic cookers. The second
classification is based on the position of the focal point on the reflector such as
shallow and deep parabolic cookers.
2.7.3.1 Spherical Parabolic Cooker
This type of parabolic solar cooker is spherically paraboloid with
highly polished, reflective surface that converge incident radiation to a focal
point. It has a support mechanism that holds the cooking vessel at the focal
point. Spherical parabolic cooker has different configurations that ranges from
43
shallow dish model to deep dish model. Some of its design has a continuous
reflecting surface while others have series of concentric rings which are
supported to focus solar radiation at a single point. Its focal point is
considerably diffuse due to optical imperfection in the reflecting surface that
resulted from imperfect fabrication (Ugwoke, 1998). Collapsible parabolic
cooker is an example of continuous reflective surface of spherical parabolic
type. The cooker opens out and closes as umbrella. Its reflector is made from
sheets of aluminized polyester. Fresnel reflector (Fig. 2.7) is a special type of
concentric ring spherical solar cooker. It is made of three to four ring of
masonite.
Fig 2.7: Fresnel spherical parabolic cooker
2.7.3.2 Cylindrical Parabolic Cooker
The shape of this type of concentrating cooker is cylindrical with cross
section of a parabola. The sun rays that strike the semi cylindrical surface are
focused at the focal point. Normally, temperature at the focal point is not as
high as that of the spherical parabolic solar cooker due to the fact that the
focused radiation is spread along the focal line (Ugwoke, 1998). Its first
design was the Prata design (Prata, 1961) and it was later developed by
Bowman (Bowman and Blatt, 1978). Their design is shown in Fig 2.8.
44
Fig 2.8: Prata and Bowman Cylindro-Parabolar
Source: Prata, 1961
2.8 History of Solar Cooking Technology
Cooking using solar energy started in the 18th and 19th centuries
(Mcveigh, 1977). This was when a successful cooker was made and was used
for cooking. The first cooker was the hot-box type which was developed by a
French-Swiss naturalist, Horance de Saussure in 1767 (Jennifer, 2005). He
performed series of experiment with an insulated, air tight wooden box
covered with two colourless window panes separated by an air tight bag with a
black base that face the sun. The outer box had lower temperature but higher
than ambient temperature while the inner most box had the highest
temperature. It was reported that the cooker was used to cook food at a
temperature of 90oF (Ugwuoke,1998).
After Horance de Saussure successful cooker, other scientists continued
in the quest to improve on the knowledge of solar cookers and in 1837, a
British astronomer Sir John Herschel was reported to develop box type solar
cooker in England to cook food (Ackermann, 1915). His box cooker was
insulated by burying it in the sand and leaving only the top exposed with two
glass covers through which the sun ray enteres the box. In 1869, a French
man, Augustin Mouchot, a professor of mathematics at the Lycee de Tour
(Adam, 1876), constructed a concentrating solar cooker that consisted of a
parabolic reflector which reflected beam radiation into the cooking pot.
45
W.adams of Bombay in India designed a cooker in 1876 (Narayanaswawy,
2001 and Adam, 1876) that consisted of octagonal, glass-enclosed oven
surrounded by glass mirror that collected the sunlight and directed it into the
enclosure. It was reported that the span of the reflector rim is 71cm and it was
able to cook the ratio of seven people in two hour in the month of January.
Other models of solar cookers were developed in the United States by C.G
Abbot (Abbot, 1939) in 1916 in which he used cylindrical parabolic reflector
to focus sunlight on black pipe filled with motor oil and the tube enclosed by a
transparent glass tube to prevent heat loss. The black pipe carries the hot oil to
a reservoir in an insulated box where the cooking pot is located. As time went
by, it was found that properly constructed solar cooker not only can cook food
thoroughly and nutritionally but also are very easy to build and use, therefore,
the United Nations and other agencies began supporting solar cooker
production and because of the new designs, programs were adopted to
introduce these designs to locations to aid those in special need. Today, solar
cookers function well than the ones designed by the earlier predecessors
because of advancement in technology and contemporary materials.
2.9 Solar Cooking In Nigeria
Survey of literature revealed that since the year 1969, various sizes of
prototype solar box cookers, ovens and concentrating solar cookers have been
developed and tested in Nigeria by researchers (Sheyin, 2005). The research
which focuses more on rural application and their performance in different
environmental condition were found to be satisfactory. The history of solar
cooking in Nigeria started when Dr Robert Metcalf, the founder of solar
cooker international toured most of African nations on solar cooking
promotion and worked with Nigerian society for the improvement of rural
people (www.wikipedia posted 2008). The organization introduced the use of
solar cookers and it was reported that around 50 families were using solar
46
cooker to pasteurize water and also cook food. Lydia Gordon Nkan of the
environmental education and her group were later reported to introduce the
fabrication and use of solar cooking technology to secondary and primary
school pupil with the ambition of involving every household on solar cooking
technology. In 2000, Josephe Odey started promoting solar cooking in Nigeria
and his activities include organization of workshops and seminars, training
programs, et.c (Odey, 2000). Over the years, there has been massive number
of cookers built to promote solar cooking technology in Nigeria.
2.9.1 Technology of Solar Cooker in Nigeria
Through research and development centers and higher institutions,
solar cooking technology has gained much acceptance in terms of designs. A
lot of work has been done on solar box cooker more than the other types of
solar cookers. An oven type solar cooker was constructed using local material
by Okeke and Ani (1989). It recorded a maximum temperature of 120oC when
the cooker was tested without load and 98oC when it was loaded with 2kg of
water. The oven has concentration ratio of 20. Another report by Ayorinde
(1989) on the analysis of a solar assisted bakery oven, showed that technically
and economically, solar energy can profitably be utilized in baking process.
Musa and Bajpai (1989) constructed a hot box solar cooker which its
performance was recorded to be very low such that a booster reflector was
later added by Ugwoke (1998) to improve its performance and on testing, the
maximum temperature of 158oC was recorded at the absorber plate. A box
type cooker was constructed by Amiyodu (1993), he introduced steam relief
line to help let off steam from cooking chambers. The experimental analysis
of the cooker recorded plate temperature of 138oC and it took 20mins to boil
eggs, 80mins to cook yam and 100mins to boil 0.22kg of rice. In Akure Ondo
state, a solar box cooker was reported to attain temperature of 50oC above
ambient on 1200ml of water for 4 hours.
47
Other researches have basically been on concentrating solar cookers.
0nyishi (1992) designed and constructed a concentrating solar cooker and the
experimental result showed a concentration ratio 37.78 and 13.69% efficiency.
Another parabolic concentrator was constructed and characterized by Eze and
Agbo (2006) using a reflector made of aluminum foil. The cooker whose
diameter measures 0.46m and base diameter 0.09m has a concentration ratio
of 11, and optimum efficiency of 0.4% on a sunny day with minimum and
maximum temperature profile of 50oC and 110
oC respectively. A parabolic
solar cooker by Musa et.al (1991) has Fresnel design with mirrors as its
reflecting surface. It was recorded that the cooker was able to boil 1kg of
water in 60mins and cook rice in 120mins at maximum temperature of 98oC.
Sulaiman et.al (2003) carried out a comparative study of various designs of
solar cookers in the north eastern part of Nigeria and found out that well
designed solar cookers have high efficiency. Nsukka has been one of the
numerous research centers for solar cookers because it is located at latitude
6o56N, and has seasonal climatic condition of dry and rainy season. The
researches on solar cookers at Nsukka which were carried out at University of
Nigeria Nsukka has shown a tremendous success since its inception. A
spherical parabolic concentrating cooker was in Nsukka designed and
constructed by Okonkwo and Mageswaran (2000). The cooker is made of
stainless steel shell having a diameter of 180cm with a focal point of 68cm
and the surface covered with aluminum foil to increase the reflectivity of the
surface. The performance evaluation of the cooker recorded maximum
temperature of 250oC on a very clear sky. Its concentration ratio is 74.
There are other projects and researches on solar cookers going on at
various institution, research centers and non governmental organizations in
order to promote the use of solar energy in Nigeria and to reduce the
dependence on the of use of biomass and liquefied natural gas as the major
energy source for cooking.
48
2.10 Benefits of Solar Cookers
Since cooking accounts for 90% of the total energy consumed in the
developing world especially in the rural domestic sector (GTZ, 2002 and
Burgos, 2008), cooking with solar energy is the most desirable option to the
developing nations such as Nigeria. The environmental benefits of solar
cooking to wood burning energy source includes that it reduces CO2 release
from the burning firewood, preserve forest reserve by reducing cutting down
of trees thereby reducing soil erosion, water pollution, loss of soil fertility and
untimely desertification. There are social benefits of using solar cookers in
areas where collecting fire wood can mean long hours of work and dangerous.
The use of solar cookers can also help improve people‟s health since it can be
used to sterilize water by heating to 65oC. This can highly be beneficial to
areas where people do not have access to safe drinking water and often suffer
sickness or death as a result of impure water consumption (Metcalf, 1999). In
addition, many people suffer respiratory and eye ailment as a result of extreme
smoky cooking condition in homes by using fuel wood. Solar cooking is
obviously smokeless and so eliminates this problem as well as reduces burns
and other fire related injuries. Another important benefit of using solar
cookers is their high temperature attainment in the case of concentrating
cookers. It has severally be reported that temperature as high as 600oC can be
attained on a very clear sky. This temperature is high enough to cook, bake
and roast any type of food stuff and this is why concentrating cookers has
more acceptability than any other type of solar cookers.
2.11 Limitations of using Concentrating Solar Cookers in Nigeria
It is a well known fact that solar cooker can not replace the other
conventional cooking systems completely because of its limitations. It can
only be used as a substitute in dry season. In Nigeria, the major barriers on the
use of concentrating solar cookers include the following;
49
1 There is low awareness on the use of solar energy for cooking.
2 There is poor technical and industrial base which does not favour
the rapid advancement of the technology of solar cookers in
Nigeria.
3 There is lack of funds on research works and project on the
development of concentrating solar cookers.
4 Unavailability of solar energy all round the day and seasonal
climatic changes restricts it usage to a given period of time.
5 Concentrating solar cookers are complicate to make and requires
more technical know how. Also, the high heat generated at the focal
point can be dangerous to eye and cause burns if they are
improperly used.
6 Concentrating solar cooker requires continuous attention in other to
keep the focal point on the cooking pot as the sun changes position
across the sky.
7 The cost of constructing a concentrating cooker is high
There have been so many research studies on concentrating solar cookers
using different materials in order to reduce cost and improve efficiency.
The materials ranged from use of mud, satellite dish or metal sheet for the
reflector shell. Use of mud has been reported to be very cheap and simple
but with low efficiency (Egger-lsura, 1979) while the use of satellite dish
or metal sheet is very costly. To overcome these limitations, there is need
to try other materials such as fiber reinforced plastic because of its known
properties and easy to mold.
50
2.12 Materials for Constructing Concentrating Solar Cookers
One of the most vital considerations in engineering design practice is
material. Materials determine the efficiency, durability and portability of any
design. In the design and construction of solar cookers, material used in
construction is of great importance. For this fact, material selection in
concentrating solar cooker is a vital issue. The reflector of the concentrating
solar cooker comprises of reflecting lining, shell, supporting and orientation
structure. Reflecting material should have maximum specular reflectance (i.e
angle of reflection equals the angle of incidence). Shell and supporting
structure has to be able to be supported and operated at different positions of
orientation and also capable of resisting structural damage in high wind and
other storm conditions in a fixed position. The most commonly a used
material in construction of concentrator shell of concentrating cookers is metal
sheet (Okonkwo and Mageswaran, 2000, Okeke and Ani, 1989) while
aluminum foil or glass mirror are used to cover the shell. One material which
has be found to be economical because of its tensile strength and light weight
in engineering design is fiber reinforced plastic (FRP) and it has not been
widely used in constructing solar cookers.
2.13 Fiber Reinforced Plastic
Fiber reinforced plastics is a fiber composite that comprises of resin and
glass fiber. The fiber serves as a reinforcement agent while resin holds the
fibers together and stabilizes the shape of the composite structure.
2.13.1 Properties of Fiber Reinforced Plastic
The strength of fiber reinforced plastic is determined by the mechanical
property, toughness property, thermal and chemical properties of the resin in
the composite. These properties in addition to the fiber contribution give the
51
fiber reinforced plastic its mechanical property. The four factors that govern
the fiber‟s contribution are;
1 Basic mechanical properties of the fiber
2 The surface interaction of fiber and resin
3 The amount of fiber in the composite.
4 The orientation of the fiber in the composite.
2.14 General Configuration of Concentrating Cookers
Solar concentrators are optical device which are very sensitive to
spatial imperfection (Garg, 1987). The optical property of the reflecting
surface is mostly affected by dirt, moisture, high temperature and other
corrosive atmospheric component. The configuration of concentrating cookers
is discussed under the concentration ratio, power and efficiency.
2.14.1 Concentration Ratio
Concentration ratio of concentrating solar cookers could be best
discussed with the definition of some terms like aperture and acceptance
angle. Aperture is defined as that plane area through which the incident solar
flux is accepted while acceptance angle is the limit to which the ray path may
deviate from a normal drawn to the aperture plane and still reach the receiver
(Duffie and Beckman, 1974). The concentration ratio is defined as the ratio of
the collecting aperture area (Aa) to the area of the receiver (Ar) which is
emitting thermal infra red radiation at the receiver temperature (Donald,
1981). Mathematically, concentration ratio can be expressed as follows;
r
a
A
AC 2.11
52
Where;
C = concentration ratio
aA = aperture area, m
2
rA = receiver area, m
2
2.14.2 Energy Efficiency of Concentrating Solar Cookers
Energy analyses of concentrating solar cookers are based on first law of
thermodynamics and are concerned only on quantity of energy used and the
efficiency of energy processes. For steady state flow process during a finite
time interval, the overall energy balance of a concentrating solar cooker can be
expressed as
Energy input = Energy output + Energy loss
2.14.2.1 Energy Input
Energy input to the cooker is the total solar energy incident upon the plane
of the solar cooker per unit time per unit area. It is important to note that for
concentrating solar cookers, solar radiation used in calculating the energy
input is beam radiation. Thus, energy input to the solar cooker can be
calculated as follows;
IAEi
2.12
where;
iE = energy input per second, W
I = total solar energy incident upon the surface of the cooker, W/m2
A = surface area of the solar cooker, m2
Garg (1982) gave further details on the energy input as follows
IArEi
2.13
Where;
53
i
E = energy input per second, W
A = net aperture area intercepting solar radiation, m2
= intercepted factor representing the fraction of specularly reflected radiation
that is intercepted by the pot
r = specular reflectance of reflector
= absorptivity of the cooking pot
I = beam radiation, W/m2
2.14.2.2 Energy Gained
The energy gained or useful energy collected by the solar cooker to the
cooking pot (cooking power) is considered as the energy output from the solar
cooker. The average heating power as given by Hafner et.al (2001) is
t
TCMQ
w
2.14
Where;
wM = mass of material, kg
C = specific heat capacity of material, J/kg.oC
t = temperature difference, oC
T = time duration, sec
2.14.2.3 Heat Loss
Heat loss in concentrating solar cookers includes heat loss by radiation,
convection and evaporation. It evaporation is considered as part of the
cooking, then the heat loss is simply given as
alTTAUQ 2.15
where;
lQ = energy loss per second, W
A = area of the cooking pot, m2
U = combined energy loss coefficient due to convection and radiation
T = cooking vessel temperature, oC
54
aT = ambient temperature,
oC
From equations 2.13 and 2.15, the rate of useful energy collection,u
E is given as
IArQa
TTAU 2.16
and the rate of useful collection per unit aperture area is given as
IArEu a
TTC
U 2.17
where;
C is concentration ratio.
From the expression of C, it can be seen that the concentration ratio increases
the useful energy collected due to reduced heat losses from the cooking vessel.
2.14.2.4 Efficiency
Efficiency of the system is the ratio of energy output to that of the energy
input. The energy efficiency of the solar cooker ( ;%) is calculated from the
following equation;
inputenergy
outputenergy
AI
tTCMw
. 2.18
Where;
= efficiency
wM = mass of material, kg
C = specific heat capacity of water, J/kg oC
t = temperature difference, oC
T = time duration, sec
I = solar beam radiation, W/m2
A = area of the collector, m2
55
CHAPTER THREE
DESCRIPTION AND DESIGN OF FIBER REINFORCED
PLASTIC CONCENTRATING SOLAR COOKER
3.1 Design Consideration
The major considerations in the design of the concentrating solar
cooker is portability and resistance to environmental condition such that the
cooker could easily be transported from one point to another and as well
withstand thermal and mechanical stress for a long time. Ultimately, the
cooker should be able to cook stable local food stuffs such as rice, eggs, and
yam. Another important consideration in the design is in the reflector
orientation since the major factor influencing the collection efficiency of
concentrating cooker is solar collector orientation. The design took into
account the axial and azimuth movement of the cooker for maximum solar
energy tracking.
3.2 Component Description
The sectional views of the fiber reinforced plastic concentrating solar
cooker are illustrated in Fig 3.1. It comprises of three essential parts namely
the concentrator, support and pot trestle. Each of these components has a
specific function it plays in order to ensure that the solar cooker functions
appropriately. The concentrator, otherwise known as the reflector tracks the
sun. The support holds the concentrator very firm while the pot trestle is the
portion where the pot is placed while cooking.
56
57
3.2.1 Concentrator
The main component of the concentrating solar cooker is the
concentrator which receives, reflects and concentrates solar radiation to a focal
point. The concentrator is parabolic in shape. The material used for the
construction of the concentrator is fiber reinforced plastic. This was
considered suitable because its properties and easy to mold to required size
and shape. The material is considered to be light enough to be carried from
one place to the other. FRP is used because of its surface texture which will
boost the optical efficiency of cooker. The major properties of FRP considered
were its thermal, mechanical and chemical. The cooker has a shallow
concentrator of depth 36cm, major diameter 160cm while the minor diameter
124cm as shown in Fig 3.2.
58
59
3.2.2 Support
The support of the concentrating solar cooker holds the concentrator and
also allows for manual rotation of the concentrator to follow the diurnal
movement of the sun. It has two parts namely; the base support (Fig 3.3) and
the Y shaped component (Fig 3.4). The base support is a spherical solid base
with 3m metallic hollow rod at center of it. It is inside this hollow metallic rod
that the Y component of the FRP concentrating solar cooker is fixed and rotate
freely on the base for adequate altitude tracking. The Y component of the solar
cooker carries the FRP concentrator which is screwed at two opposite sides of
the Y component to allow for azimuth tracking of the sun. It also has a locking
mechanism that holds the FRP concentrator at a desired position. It is
considered that the support has to be made of Y components in other to allow
for altitude tracking of the sun movement across the sky while the base
support carries the whole component of the solar cooker firmly.
60
61
3.2.3 Pot Trestle
The main function of the pot trestle is to carry the pot at a fixed
position where the pot receives optimum concentrated solar radiation. The
major consideration in the design of the pot trestle was the focal point, the size
of pot placement ring and horizontal adjustment of the pot trestle. Owing to
the fact that the cooker can be operated by different individuals, the trestle
consists of the rotational rod and horizontal adjustable mechanism to ensure
that focal point is maintained at any given point of usage. The diameter of the
circular part of the adjustable component was assumed 20cm considering the
average size of cooking pot for about three persons. The pot trestle is shown in
Fig 3.5.
62
3.3 Design Analysis and Calculation
3.3.1 Concentrator Area
The concentrator area is calculated using the area of the concentrator
and it is calculated using equation 3.1.
4
ba
c
DDA 3.1
Where;
cA = concentrator area, m
2
aD = major diameter, m
bD = minor diameter, m
256.1
4
124160mA
c
3.12.2 Aperture Area
The aperture area is the circular part of the pot trestle where the cooking
pot is placed to trap the concentrated solar energy. This area is calculated as
follows;
4
2
r
r
DA 3.2
Where;
Ar =Aperture area, m2
Dr = Aperture diameter, m
63
2
2
0314.04
2.0mA
r
3.12.3 Concentration Ratio
The concentration ratio of the solar cooker is estimated from the
expression given by Duffie and Beckman (1980).
r
c
A
AC 3.3
Where;
C = concentration ratio
Ac = concentrator area,m2
Ar = aperture area, m2
Substituting the values of Ac and Ar as calculated from the above equation
as 1.56m and 0.034m respectively.
034.0
56.1C
50C
The concentration ratio of the solar cooker is estimated to be 50. This
implies that the ratio of the aperture area to the area of the receiver is 50.
3.12.4 Focal Length
Focal length of a parabolic surface is calculated using equation 3.4
d
DF
16
2
3.4
64
D = concentrator diameter, m
d = depth of concentrator, m
Considering that the concentrating solar cooker elliptically shaped, the focal
length is calculated as follows;
d
DDF
ba
16 3.5
where;
F = focal length, m
aD = major diameter, m
bD = minor diameter, m
d = depth of the concentrator, m
By substituting the value ofa
D , b
D and d into equation 3.5, the focal length of
the solar cooker is calculated as;
mF 34.03616
124160
3.13 Solar Collector Orientation
It is important to note that the orientation of the concentrator and receiver
relative to the direction of propagation of solar radiation (beam radiation) is
important since the sun tracking will be required for concentrating solar
cookers. There are varieties of orienting mechanism which can be used in the
design of concentrating solar cooker so that the incident beam radiation will
be reflected to the receiver but owing to the fact that tracking of sun is through
its altitude and azimuth angle, the design of the concentrating solar cooker
was based on the selection of the altitude and azimuth angle. For cylindrical
concentrating solar cooking systems to focus beam radiation to the receiver,
the focal axis, vertex line of the reflector and the sun most lie in a plane
65
(Duffie and Beckman, 1974). The concentrator was designed in such a way to
rotate the concentrator about different axis; this axis of rotation may be north-
south, east-west or inclined and parallel to the earth‟s axis such that in
whichever case, the rate of rotation is 15o/hr.
3.4.1 Selection of Altitude and Azimuth Angles
To select the altitude and azimuth angles, the cooking time during which
the cooker is expected to be used is very important. For this design, the
estimated cooking time is between 9.00am and 3.00pm. Therefore for
allowance of one hour each, the minimum and maximum altitude angle is
taken between 8.00am and 4.00pm solar time. This is estimated using the
equation of altitude angle and azimuth angle given below.
3.4.1.1 Altitude Angle
The altitude angle is expressed by
CosHCosLCosSinLSinSin 3.6
Where;
= altitude angle
L = latitude of the locality, the latitude angle of Nsukka is 6o52.
H = hour angles
= declination angle
3.4.1.2 Declination Angle
The declination angle is calculated using the expression as
NSin 284365
36045.23 3.7
Where;
66
= declination angle
N = nth
day of the year
Using 15th
January, the declination angle is calculated as
46284365
36045.23 Sin -21.27
o
3.4.1.3 Hour Angle
Hour angle ( H ) = T1224
360 3.8
Where;
T = solar time
For 8.00am, the hour angle is calculated as
oH 60812
24
360
For 4.00pm, the hour angle is calculated as
oH 601612
24
360
The minimum altitude angle is then calculated as
oooCosCosCosSinSinSin 6027.2152627.21526
ooooCosCosCosSinSinSin 9.246027.2152627.21526
1
The maximum altitude angle is also calculated as
ooo
oo
CosCosCosSinSinSin
CosCosCosSinSinSin
8.246027.2152627.21526
6027.2152627.21526
1
67
3.4.1.4 Azimuth Angle
The calculation for the selection of minimum and maximum azimuth
angle is estimated using equation 3.9 below
Cos
SinHCosSin 3.9
Where;
= azimuth angle
H = hour angle
= altitude angle
At 8.00am, declination angle is -21.27, hour angle is 60o and the
altitude angle is 24.8o. Substituting these values into equation 3.9, the
minimum azimuth angle is calculated as
o
Cos
SinCosSin
Cos
SinCosSin
8.628.24
6027.21
8.24
6027.21
1
The maximum azimuth angle is calculated at 4.00pm. The hour angle is
-60o, declination angle is -21.27 and the altitude angle is 24.8. Therefore the
maximum angle to be set for the azimuth angle is calculated as
o
Cos
SinCosSin
Cos
SinCosSin
8.628.24
6027.21
8.24
6027.21
1
The negative sign indicated that the concentrator is oriented along the east –
west axis of the solar altitude.
68
3.5 Thermal Load and Efficiency
3.5.1 Heat Input
The amount of heat that is supplied to the cooker is calculated by using
the expression below
IAQi
3.10
where;
iQ heat input, W
I beam radiation, W/m2
A surface area of the solar cooker, m
As mentioned earlier, concentrating collector uses only beam radiation
and so to calculate the beam radiation at normal incident on a solar cooker,
ASHRAE (1981) expressed it as follows
sin
B
ap
BN
AI
3.11
where;
BNI = beam radiation at normal incident, W/m
2
apA = apparent solar irradiation just outside the atmosphere
B = atmospheric extinction coefficient
= altitude angle
The average beam radiation at 10am solar time on 15th
January for
Nsukka at latitude 6o52 is determined by estimating the hour angle,
declination angle and the altitude angle. The values of ap
A and B are obtained
from Hsieh (1986) as 1229.5W/m2 and 0.448 respectively.
69
At 10am, the hour angle ( H ) 101224
360
o30
.
For January 15th
, the nth
day (N) is 46, therefore the declination angle is
calculated from equation 3.7 as follows
27.2115284365
36045.23 Sin
The hour angle, declination angle and the latitude angle are substituted into
equation 3.6 to estimate the altitude angle.
oooCosCosCosSinSinSin 3.493027.2152627.21526
1
The beam radiation at normal incident on the concentrator (BN
I ) is estimated
as
2
3.49sin
448.0/680
5.1229mWI
BN
The beam radiation on the horizontal surface of the concentrator is estimated
from the expression
SinIIBN
3.12
Where;
I = beam radiation on the surface, W/m2
BNI = beam radiation at normal incident, W/m
2
= altitude angle
2/2.5163.499.680 mWSinI
This is the average amount of solar radiation that is estimated to fall on
the surface of the concentrator at 10am solar time 0n 15th
of January at
Nsukka. Hence, this value varies depending on the weather condition of the
day. Magneswaran (2000) suggested 12% deduction from the estimated
average beam radiation on the surface of the collector to account for the
weather variation and other possible losses due to radiation and convection.
70
Thus the estimated solar radiation ( I ) on the surface of the concentrator after
accounting for radiation and convection losses is estimated as follows
Solar radiation considering conventional 2/9.612.516
100
12mWlosses
Total beam radiation at the surface (b
I ) = 516.2-61.9 = 454.3W/m2
From this, the energy input to the solar cooker is beam radiation b
I at the
reflector multiplied by surface area of the reflector ( A ) which is
W7.70856.13.454
3.5.2 Estimated Heat Output
The concentrating solar cooker is designed for water heating and cooking.
The amount of heat required for heating or cooking is estimated as
T
tCMQ
w 3.13
Where;
Q = quantity of heat, W
wM = mass of material, kg
C = specific heat capacity of water, J/kg oC
t = temperature difference, oC
T = time duration, sec
It was designed to boil water from an average temperature of 20oC to 95
oC in
10 minutes. The amount of heat required to heat 1kg of water is
WQ 523600
209541861
71
3.5.3 Estimated Heat Loss in FRP Concentrating Cooker
Heat loss is estimated by the expression
QQQil
3.14
where;
lQ heat loss, W
iQ heat input, W
Q heat output, W
The estimated heat loss by the cooker is then
WQl
5237.708
W7.185
3.5.4 Efficiency
The thermal efficiency of concentrating solar cookers is given by the
expression,
AI
Q
. 3.15
Where;
= efficiency of the solar cooker
Q = heat output
I = solar radiation on the surface of the collector
A = surface area of the collector
The thermal efficiency of the concentrating cooker is then estimated from
equation 3.16 as
Efficiency 74.07.708
523
%74
72
3.6 Thermal Properties of Food Materials
Design and operation of processes that involve heat transfer requires
special attention due to the heat sensitivity of food. This heat can be
transferred in three different ways namely; conduction, convection and
radiation but the ones that are predominant in heat transfer processes is the
heat conducted through the food product and the heat transferred by forced
convection between the product and moving fluid that are in contact with the
food material. The heat demand for cooking different food material therefore
varies from one material to another because of their different thermal
characteristics. It is therefore important to specify the food materials that can
be cooked with the solar cooker considering the thermal properties of food
material. These properties include specific heat, thermal conductivity and
thermal diffusivity. The thermal properties will help to determine how much
water is used in cooking, how long the food is cooked and at what temperature
the food material is to be cooked and since solar cookers performance is
dependent on so many factors such as the time of the day, the knowledge of
the thermal properties of the food material is very essential.
3.6.1 Specific Heat (cp)
Specific heat of food material is the amount of heat required to raise
one unit of the material by one degree. Equation relating specific heat, mass of
the sample (M), the amount of heat that must be added (Q), and the initial and
final temperatures of the sample (T1 and T2) is given below.
12TTMcQ
p 3.16
3.6.2 Thermal conductivity ( )
It is a property that tells how well a material conducts heat. Heat
conduction is the transfer of energy between neighboring molecules within a
material. The following equation as predicted by Fourier‟s law of heat
conduction relates the thermal conductivity to the amount of heat that flows
through the material per unit of time (Q), the cross sectional area of the
73
material through which the heat flows (A) with thickness of the food material
(x), and the temperature difference per unit of length of the conducting
material (T1 - T2).
x
TTkAQ 12 3.17
Thermal conductivity is influenced by a number of factors such as
temperature, porosity moisture content and fiber orientation of the material
(Dutta et.al, 1988).
3.6.3 Thermal diffusivity
Thermal diffusivity defines the ability of material to conduct heat
relative to its ability to store heat. It is related to thermal conductivity and
specific heat through density as follows;
pc
3.18
Empirically, the recommended food materials that can be cooked in the solar
cooker include food materials with high moisture content, high thermal
conductivity and thermal diffusivity but low specific heat. Such food material
includes, egg, rice, et.c. It is also important to note that for food mixture could
be difficult to cook because of difficulty in turning on the solar cooker so plain
food would be ultimately recommended.
3.7 Cooking Time of the Cooker
To estimate the cooking time for the solar cooker is difficult because of
the unpredictability of solar radiation. However, on an average clear weather,
it can be estimated through the sunset hour. The sunset or sunrise hour is
calculated by solving for hour angle in equation 3.8 as given by Duffie and
Beckman (1974).
tantan LCosH 3.19
where;
74
H =hour angle
L = latitude
= declination angle
Considering the hour angle at local solar noon is zero with each 15o of
longitude equivalent to one hour, the hour angle is then expressed as follows;
tantan15
1 1LCosH 3.20
Therefore, the day length use, of the concentrating solar cooker which
is the time interval between the sunrise and sunset is expressed by
Magneswaran (2000) as
Day length tantan15
2 1LCos 3.21
= declination angle for Nsukka at latitude 6o52N on 15
th January, the
declination angle from equation 3.14 is -21.27 so the day length is estimated
as
Day length 27.21tan526tan15
2 1 oCos
hours6.11
Sunrise hours hoursnoon2
6.1112
hours2.6
This implies that sun rises by 6.12am on 15th
January at Nsukka.
Sunset hoursnoon 8.172
6.1112
This implies that the sun will set at 5.48pm on 15th
January at Nsukka.
Normally, household cooking takes place between 6am and 8pm for breakfast,
11am to 2pm for lunch and 4pm to 8pm for dinner. Therefore, the use of the
cooker for preparing dinner will not be possible because there is no solar
radiation at this time of the day. For breakfast, the solar radiation available at
7am is estimated from equation 3.11. The declination angle is estimated from
equation 3.7 as
75
27.2146284365
36045.23 Sin
The hour angle ( H ) from equation 3.8 is estimated as
oH 75712
24
360
The altitude angle for Nsukka at latitude 6o52N is estimated from equation
3.6 as
oooCosCosCosSinSinSin 45.117527.2152627.21526
01
The beam radiation at normal incident on the concentrator is then estimated
using equation 3.11. Substituting the values of A and B as obtained from
Hsieh (1986) as 1229.5W/m2 and 0.448. The beam radiation at normal
incident (BN
I )
2
3.11
448.0 /71.1285.1229 mWIo
SinBN
The average beam on the horizontal surface of concentrator at 7am on 15th
January is estimated as
2/55.25
45.1171.128
mW
SinSinIIBN
This amount of solar radiation, 25.55W/m2 is low compared to the required
solar radiation for the cooker‟s operation. Hence, the cooker may not be
efficient to cook food in early morning.
For the lunch time, using the same parameters as calculated earlier for solar
time at 10.00am, the beam radiation of 454.3W/m2 can be used to heat water
requiring 523.3W quantity of heat at efficiency of 74%.
3.9 Material Selection
The performance of solar cooker depends greatly on the choice of
materials for its construction. Materials selected for the construction of the
FRP concentrating solar cooker has to be capable of satisfying a major desired
function such as
76
To reduce the bulkiness of the solar cooker
To ensure that the cooker can be able to withstanding environmental,
thermal and chemical damages.
Capable of resisting structural damage by high wind and other storm
conditions in a fixed position.
For easy operation, maintenance and reproduction.
And also to ensure that the material can be sourced locally.
To fulfill these requirements, the material for the concentrator which is the
main component of the solar cooker has to be carefully selected. The most
desirable characteristics of the concentrator are low heat absorbility and high
resistance to corrosion. Many materials were considered such as the use of
concrete for the reflector shell but due to the heaviness of these materials and
their cost, the idea was discarded. Consequently, fiber reinforced plastic a
moldable composite material which can be manipulated at any time was
selected since it can give the desired result. It is a good insulator with thermal
conductivity of 0.05W/mK. Its conductivity is low because it uses trapped
pockets of still air within it as a physical barrier in other to reduce the flow of
heat. Fiber reinforced plastic reduces heat due to convention and has
emmisivity value of 0.7. Similarly, the reflecting lining and its characteristics
were considered in the choice of the reflecting lining material and it was
thought that it would be desirable to use a reflective material with maximum
specular reflectance over a certain period of time. This is because it has been
shown (Ghai et al, 1954) that efficiency of the reflector of a concentrating
solar cooker remains unchanged with the cooking vessel temperature but
depends on specular reflectance ( r ) and intercepting factor ( ) of the
reflecting lining. It is therefore essential to have reflecting surface of highly
specular reflectance and this depends on the nature and smoothness of the
material. Generally, keeping cost and availability into consideration,
aluminize polyester film is considered to be appropriate for the reflective
lining of the concentrating solar cooker. It has specular reflectance of 85%
77
(Harrison, 2001) and is capable to be replaced. The values of specular
reflectance of various surfaces are given in Garg (1982).
3.9 Construction of FRP Concentrating Solar Cooker
The solar cooker is a direct concentrating cooker with a dish type reflector
directing intercepted solar radiation to a point of focus as shown in Fig 3.6.
The concentrated solar energy then heats up the pot set at the focal point and
heat is transferred from the pot to its content. The construction of the
components of the solar cooker is described below.
Fig 3.6: The FRP concentrating solar cooker
3.9.1 Concentrator
The concentrator shell is the major component of the concentrator and it is
made of fiber reinforced plastic (FRP). FRP is a composite material made up
of polyester resin and fiber glass. This composite was cast out from a concrete
mold. The mold was formed on a platform above a base surface. A base
78
surface is a leveled ground surface. A circular platform of 220cm and 30 high
is formed from a semi parabolic scrapper that was carved from the values in
Table 3.1. The values were generated using the equation of a parabola in
equation 3.19
fyx 42 3.19
where;
x = diameter of the concentrator
y = depth of the concentrator
f = focal length
The values are from the designed values of the focal length that is 34cm. A
graph was plotted from the values in table 4.1 as illustrated in Fig 3.7.
Table 3.1 Values for the plotting of the parabolic graph
X(cm) Y(cm)
0 0
10 0.74
20 2.94
30 6.62
40 11.76
50 18.38
60 26.47
70 36.03
80 47.06
90 59.56
100 73.53
110 88.97
120 105.88
130 124.26
140 144.12
150 165.44
160 188.24
79
Fig 3.7: Plot of parabolic scraper
0
20
40
60
80
100
120
140
160
180
200
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Diameter of dish, m
De
pth
of
dis
h,
m
80
After plotting the above values on a board, a squared aluminum sheet was cut
accurately to the shape of the semi parabola on the board.
The mold was formed using the semi-parabolic scraper. A steel rod, 45cm
height was placed at the centre of the platform and held rigidly at the base.
The scraper was fixed at the outer end of the fixed steel rod to allow easy
rotation round the platform. Gravel and concrete were carefully placed on the
platform until it reaches the height of the scraper. The concrete ensures that
the gravels are bounded to each other very well. At the point that the mixture
are at the height of the scraper, only concrete is placed on the mold and the
scraper is rotated on the surface until there is a smooth surface which
coincides with the exact geometry
of the scraper. The finished mold forms a hard and relatively smooth
parabolic surface on which the solar concentrator is formed. Fig 3.8 is a
completed mold in form of elongated convex parabolic body that rests on the
base surface.
Fig 3.8: Parabolic mold
81
3.9.2 Shell Casting
The shell of the concentrator is made of composite materials such as
polyester resin and fiber glass. Polyester resin are viscous, pale coloured
liquid consisting of a solution of polyester in a monomer which is usually
styrene while fiber glass are chopped strand mat used in reinforcement in fiber
composite. Casting of the fibre reinforced plastic from the concrete mold
involved gel coating, lamination, curing and de-molding operations.
3.9.3 Gel Coating
Gel coating is one of the pre-molding processes that involve the mapping
out of the required casting area and brushing through the surface of the mold
with resin. To map out the casting area, the design outline was placed on the
mold and its outline was traced on the mold as shown in Fig 3.9. The gel
coating operation prepares the mold for the lamination operation and ensures a
good finish on the work by improving adhesion of subsequent layer of resin
and reinforcing materials.
Fig 3.9: Gel coating operation
82
3.9.4 Lamination Operation
Lamination operation involves the mixture of resin and other auxiliary
products such as catalyst (methyl ethyl ketone peroxide), accelerator and
fillers. Catalyst is added to the mixture of resin and filler. Catalyst does not
take part in the chemical reaction but activates the process and initiates
polymerization. Accelerator is added to the catalyzed resin to enable the
reaction proceed at a faster rate. After mixing the mixture for two minutes, the
accelerated polyester resin is poured on the surface of the mold and the
prepared reinforcement fiber is placed on it. Brushes were used to impregnate
the fiber in the resin. Two piles of chopped strand mat of the fiber were laid
during the lamination. More resins were poured on the reinforcing fiber until
all the fibers were soaked with the resin as shown in Fig 3.10. The laminate
was left to cure under atmospheric condition for 24 hours.
Fig 3.10: Lamination Operation
83
3.9.5 Curing Process
Due to the addition of catalyst to resin, the resin becomes more volatile
until it reaches a state when is it no longer liquid and has lost its ability to flow
known as the gel point. The resin continues to harden after it has gelled until
at a time when it has obtained its hardness properties. The whole process is
known as the curing of resin. The speed of cure is controlled by the quantity of
accelerator in the resin.
3.9.6 De-molding Operation
This operation involves the process of removing the laminate from the
concrete mold after it has cured. De-molding was done by spraying water
along the edges of the laminate and allowing it to penetrate inside the laminate
to avoid cracking and deforming. After removing the shell (laminate),
reflecting lining material (aluminized polyester film) was used to cover the
surface of the shell.
3.10 Operation of the Solar Cooker
The operation principle of the solar cooker is a simple one. It does not
require much technical knowledge though trained personnel who understand
its safety measures and operation principle would operate the cooker better.
The cooker can only be used when there is adequate sun energy. The operation
of the solar cooker includes the following;
1 Coupling the parts of the cooker
2 Adjusting the azimuth and altitude angles of the cooker to ensure
that the cooker is facing the sun
3 Placing the cooking black pot and its content on the cooker
4 Adjusting the cooker every 15 minutes in order to track the sun
84
3.11 Safety Measures and Maintenance of the Cooker
The safety measures when operating the solar cookers are as follows;
1 The operator of the cooker must protect the eyes from sun glares.
2 When operating the cooker, the operator has to stand beside the cooker
in other not to intercept the incoming sun rays.
3 The pot handle and lid should not be too large in other to minimize heat
loss.
4 Children should not be allowed near the cooker when in use.
For efficient performance of the cooker, the following maintenance culture
is necessary. They are;
1 Cleaning the surface of the concentrator before use in other to keep
the surface shinny for sun reflection.
2 As the reflecting material wears out, it should be replaced with a
new one.
3.12 Limitations of the Cooker
The concentrating solar cooker has the following limitations.
1 It requires bright, sunny weather for its operation
2 The duration of cooking is longer than that of the conventional
cooking stoves
3 It requires frequent adjustment in other to track sun energy
85
CHAPTER FOUR
PERFORMANCE EVALUATION OF THE
FRP CONCENTRATING SOLAR COOKER
4.1 Experimental Procedures
The concentrating solar cooker was used to conduct series of cooking
operations in other to evaluate its performance level. Certain quantities of
water and food items were boiled and cooked using the solar cooker. The
experiment took place at Nsukka in the month of January, 2008. There were
comparative test analyses on three other solar cookers which were subjected to
the same condition over time. The experiment was conducted using American
Society of Agricultural Engineering Standard (ASAE, 2003). The aim of the
ASAE standard was to achieve the following
Promote uniformity and consistency in the terms and units used to
describe, test, rate and evaluate the solar cooker, solar cooker
component and cooker operation.
Provide a common format for presentation and interpretation of test
results to facilitate communication
Provide unified measure of performance that consumers may use in
evaluating different designs when selecting a solar cooker.
4.2 Test Procedure
The procedures taken during the experiment are as follows;
1. The solar cookers were assembled in an open place for five consecutive
days at National Center for Energy Research and Development
(NCERD), University of Nigeria Nsukka
2. The solar cookers were set to face the east in the morning
86
3. The azimuth and altitude angle was set
4. The food samples/water and pots were weighed on a weighing balance
5. The pot trestle is set at the focal point and a black pot with its content is
placed on the pot trestle
6. Ambient temperature and pot content temperature measurement is
recorded every ten minuets. Solar radiation and wind speed were
obtained from data logger corresponding the time of the experiment.
This is because as of the time of the experiment, Eppley pyranometer
and anemometer were not available
4.3 Instrumentation and Measurement
There are variables that must be considered during the experimental
period according to the ASAE standard. These are controllable and
uncontrollable variable such as wind, ambient temperature, pot content,
temperature, solar altitude and azimuth.
4.3.1 Wind
Wind is one of the uncontrollable variables that its effect can greatly
affect the test result if not appropriately carried out. According to the stated
ASAE standard, the wind speed at the test of solar cookers should be less than
1.0m/s during experiment. This was considered during the cooking/heating
experiment only that the test was carried out in the month of January when the
weather condition in Nsukka is not steady. At this time, the weather was hazy
and windy so the recorded wind speed during the time of the experiment
ranged from 0.56m/s to 3.45m/s. The wind speed was obtained from data
logger at the National Center for Energy Research and Development,
University of Nigeria Nsukka.
87
4.3.2 Ambient Temperature
At the beginning of each day experiment, the ambient temperature was
recorded between 20oC to 35
oC. The temperature reading of the pot content
and the ambient were measured simultaneously at interval of ten minutes
using thermocouple wires.
4.3.3 Solar Radiation
The solar radiation data was obtained from data logger at the National
Center for Energy Research and Development, University of Nigeria Nsukka.
The data logger records solar radiation every one hour. Throughout the period
of the experiment, the solar radiation recorded ranged between the minimum
of 300W/m2 to maximum of 600W/m
2.
4.3.4 Pot Content Temperature
The pot temperature was recorded every ten minutes interval. The time
was recorded using stop watch. The pots used are exterior black painted pots.
As the pot absorb the heat at the focal point, the temperature of its content
increases with time. The pot content temperature was measured using
thermocouple wires.
4.3.5 Solar Altitude and Azimuth Angle
The solar altitude and azimuth angle were set at 10.00am at the
beginning of the test experiment at the angle of 25o and 63
o respectively. The
cooker is set at a height of 1.4m and the concentrator is adjusted every 15
minutes to follow the diural movement of the sun.
4.4 Comparative Test
In order to determine the performance efficiency of the concentrating
solar cooker, three other solar cookers namely; NCERD concentrating solar
cooker, Japanese type concentrating solar cooker, Box type solar cooker and
FRP concentrating solar cooker as shown in Fig 4.1, 4.2 and4.3 respectively
88
were used for the comparative analysis of the FRP concentrating solar cooker
(Fig4.4). These solar cookers were obtained from National Center for Energy
Research and Development, University of Nigeria Nsukka. The receiver area,
aperture area and concentration ratio of the solar cookers were calculated as
NCERD concentrating solar cooker (1.2m2,0.8m
2,0.67%), Japanese type
concentrating solar cooker (0.94m2,0.72m
2,77%) and Box type solar cooker
(0.32m2,0.36m
2,21.6%) respectively. Equal quantities of water and food
samples were introduced into the cookers at the same time and thus subjected
to the same weather conditions while the pot content temperature, ambient
temperature where measured over time. The concentration ration of the solar
cooker The solar radiation and wind speed during the experiment were
obtained from data logger station at National Center for Research and
Development, University of Nigeria Nsukka.
Fig 4.1: NCERD concentrating Fig 4.2: Japanese type concentrating
solar cooker solar cooker
Fig 4.3 Box type solar cooker Fig 4.4: FRP concentrating solar cooker
89
The solar cookers were loaded with 1kg of water and the test was conducted
for five consecutive days. The average ambient and water temperature of all
the pot in each of the cookers were recorded at interval of 10 minutes.
After the comparative test using 1kg of water, the solar cookers were
used to cook same quantities of egg, rice and yam. These tests were done
under the same climatic condition. The average ambient temperature, wind
speed and solar radiation were also measure during the test.
4.5 Presentation of Results
4.5.1 Water Heating Test
The water heating test was conducted for five consecutive days with the
solar cookers set in an open place ensuring that there is no obstacle
intercepting the incoming sun rays. Black pots with 1kg of water were placed
at the focal point of the concentrating solar cookers and the box cooker.
Thermocouple and a pyranometer coupled to a data logger were used to record
the data. The temperature was determined by measuring the water temperature
every ten minutes. The average solar radiation, wind speed and ambient
temperature over a day in January for Nsukka are shown in Fig 4.5.
Fig 4.5: Plot of solar radiation, wind speed and ambient
temperature over time
0
10
20
30
40
50
1 5 9 13 17 21
Time (hrs)
Win
d S
peed
(m
/s)
an
d T
em
pera
ture
(oC
)
0
200
400
600
800
So
lar R
ad
iati
on
(W/m
2)
Wind speed
Ambient temperature
Solar radiation
90
The experimental performance of the cookers for three different days for
heating 1kg of water are shown in Table 4.1, 4.2 and 4.3.The test started from
10.00hrs to 14.00 hrs and was evaluated under clear sky, windy weather and
on an average weather condition. From the results, it is revealed that the
factors that determined the performance of the cookers include, wind speed,
amount of solar radiation and the time of the day.
1. Wind speed – high wind speed generally decreases the performance of
concentrating solar cooker because it generates dust particles that covers
the surface of the solar cooker and as well offsets the reflector from the
focal point.
2. Amount of solar radiation – the greater the amount of solar radiation
received at a given period of time, the greater the performance of
concentrating solar cookers.
3. Time of the day – time of the day determines the amount of solar radiation
the reflector tracks. Solar radiation is highest when the hour angle is 0o i.e
at solar noon. At this time, the azimuth angle is directly north.
91
Table 4.1: Results on day one heating test
Time
Solar
radiation
(W/m2)
Ambient
temp.
(oC)
Temp. of FRP
concentrating
solar cooker
(oC)
Temp. of NCERD
concentrating
solar cooker
(oC)
Temp. of Japanese
type concentrating
solar cooker
(oC)
Temp. of
Box type
solar
cooker
(oC)
10.00 361.70 26.00 25.20 25.20 25.20 25.20
10.10 372.60 26.90 51.90 31.00 54.30 32.40
10.20 389.00 27.50 53.20 35.70 71.90 36.00
10.30 400.20 27.30 56.90 40.00 83.40 40.50
10.40 428.00 28.10 53.40 43.50 89.80 43.90
10.50 469.00 27.90 50.80 47.40 95.80 49.50
11.00 488.80 28.60 54.80 51.90 97.40 56.10
11.10 498.30 28.60 65.00 56.20 97.80 58.70
11.20 516.00 27.80 77.10 58.20 97.80 61.80
11.30 526.00 28.30 80.70 62.50 98.80 67.10
11.40 538.00 28.40 88.10 64.60 98.80 70.60
11.50 546.60 29.30 90.30 65.10 97.90 72.80
12.00 564.00 29.10 94.30 67.70 98.00 80.50
12.10 565.00 30.40 94.70 85.70 97.90 85.70
12.20 568.00 28.80 93.10 83.50 98.40 91.30
12.30 568.80 29.60 94.50 79.50 97.30 95.30
12.40 570.00 29.90 93.00 83.70 98.20 95.80
12.50 575.00 30.00 93.10 91.70 98.30 94.30
13.00 576.00 30.40 92.30 93.40 98.50 94.60
13.10 556.00 31.30 90.70 94.10 98.70 97.00
13.20 540.80 29.40 91.30 92.90 98.30 98.00
13.30 530.50 29.60 93.40 93.40 97.90 96.70
13.40 516.00 28.90 93.50 93.50 97.40 96.80
13.50 480.00 30.70 90.80 90.18 97.70 97.80
14.00 477.60 29.10 90.60 92.60 97.10 96.50
92
Table 4.2 Results on day two of heating test
Time
Solar
radiation
(W/m2)
Ambient
temp.
(oC)
Temp. of FRP
concentrating
solar cooker
(oC)
Temp. of NCERD
concentrating
solar cooker
(oC)
Temp. of Japanese
type concentrating
solar cooker
(oC)
Temp. of
Box type
solar
cooker
(oC)
10.00 392.50 25.20 23.70 23.70 23.70 23.70
10.10 412.30 24.70 54.00 32.70 60.60 36.20
10.20 436.70 25.00 55.00 45.30 65.30 49.90
10.30 456.90 26.20 56.60 55.00 69.10 53.70
10.40 483.80 26.70 58.20 60.20 71.30 61.10
10.50 506.70 26.50 58.40 61.10 74.80 65.20
11.00 539.20 26.50 61.90 63.70 80.30 71.80
11.10 550.00 26.70 66.30 64.60 87.60 75.10
11.20 571.30 27.10 67.20 67.70 88.40 76.70
11.30 580.50 26.80 69.80 70.80 93.20 76.60
11.40 598.30 27.30 70.10 72.00 95.70 78.10
11.50 612.70 28.20 73.50 77.30 94.30 78.20
12.00 630.40 28.60 78.10 79.40 95.80 78.30
12.10 635.50 28.50 67.00 83.70 96.00 80.10
12.20 638.20 29.50 74.70 88.20 98.30 80.30
12.30 643.40 28.80 73.50 89.40 98.30 83.50
12.40 656.60 29.50 80.90 85.70 97.70 84.70
12.50 650.80 29.10 86.60 87.30 97.90 85.30
13.00 645.60 29.70 89.70 90.40 98.80 87.70
13.10 631.90 30.20 90.80 92.30 97.90 89.30
13.20 628.60 32.30 92.30 90.20 97.50 91.30
13.30 593.40 31.50 94.80 91.70 97.40 93.70
13.40 590.40 33.70 92.70 91.20 98.10 95.30
13.50 577.30 30.10 90.20 92.10 97.20 94.90
14.00 571.20 31.60 89.80 91.80 97.40 95.20
93
Table 4:3: Results on day three heating test
Time
Solar
radiation
(W/m2)
Ambient
Temp.
(oC)
Temp. of FRP
concentrating
soar cooker
(oC)
Temp. of NCERD
concentrating
solar cooker
(oC)
Temp. of Japanese
type concentrating
solar cooker
(oC)
Temp. of
Box type
solar
cooker
(oC)
10.20 422.30 24.50 53.20 50.30 71.90 45.70
10.30 440.10 25.30 56.10 56.10 83.40 58.50
10.40 470.80 26.00 53.00 57.20 95.20 60.80
10.50 492.90 26.70 46.30 57.50 97.40 65.00
11.00 521.60 26.30 52.90 57.80 98.30 67.20
11.10 533.20 26.50 55.60 56.10 98.90 68.30
11.20 545.90 26.10 58.10 57.80 97.80 69.10
11.30 566.20 26.40 59.50 56.10 98.10 71.20
11.40 570.50 26.90 55.40 57.10 97.80 72.10
11.50 588.90 27.20 59.00 60.40 98.00 70.40
12.00 596.00 27.40 57.90 62.50 98.80 72.10
12.10 598.20 26.80 53.50 56.90 98.10 75.40
12.20 602.70 27.20 64.30 57.10 98.30 76.60
12.30 608.00 27.70 61.50 59.40 98.00 78.30
12.40 610.40 27.80 66.80 60.20 97.10 80.30
12.50 612.30 28.40 69.20 63.70 97.70 80.50
13.00 614.40 28.20 70.80 72.00 98.10 85.70
13.10 615.00 29.20 73.30 74.30 97.90 91.30
13.20 583.00 27.50 74.80 80.10 98.00 95.30
13.30 575.90 28.10 75.00 84.70 98.00 96.10
13.40 545.20 27.60 77.30 85.00 97.50 95.30
13.50 496.50 28.30 74.30 88.40 97.50 94.50
14.00 436.00 28.60 73.30 85.90 97.40 99.00
94
Fig 4.6 shows the performance of the cookers on a windy day. There is a
remarkable change in temperature attainment on this day compared to that of
on a clear day (Fig 4.7). It can be seen from the figure that the highest solar
radiation recorded on the 1st day, 16
th January 2008 as shown in Table 5.1 was
576W/m2 at 13.00 hrs of the day. The highest temperature attained on that day
by Japanese type concentrating solar cooker was 98.80oC at 11.40am, NCERD
concentrating solar cooker attained maximum temperature of 94.1oC at
1.10pm. The box type solar cooker and the FRP concentrating solar cooker
attained maximum temperature of 98oC and 97.3
oC at 1.20pm and 12.20am
respectively.
Fig 4.6: Plot of Solar radiation and cooker's temperature over time on a windy day
0.00
20.00
40.00
60.00
80.00
100.00
120.00
10
.00
10
.20
10
.40
11
.00
11
.20
11
.40
12
.00
12
.20
12
.40
13
.00
13
.20
13
.40
14
.00
Time (mins)
Tem
peratu
re (
oC
)
0.00
100.00200.00
300.00400.00
500.00600.00
700.00S
ola
r r
ad
iati
on
(W
/m2)
Ambient
temperature
FRP concentrating
solar cooker
NCERD
concentrating solar
cookerJapanese type
concentrating solar
cookerBox type solar
cooker
Solar radiation
95
Fig 4.7: Plot of solar radiation and cooker's temperature over time on a clear day
0.00
20.00
40.00
60.00
80.00
100.00
120.00
10
.00
10
.20
10
.40
11
.00
11
.20
11
.40
12
.00
12
.20
12
.40
13
.00
13
.20
13
.40
14
.00
Time (mins)
Tem
peratu
re (
oC
)
0.00100.00200.00300.00400.00500.00600.00700.00
So
lar R
ad
iatio
n (
W/m
2)
Ambient
temperature
FRP concentrating
solar cooker
NCERD
concentrating solar
cookerJapanese type
concentrating solar
cookerBox type solar
cooker
Solar radiiation
The temperatures are generally lower on windy days and it is more noticeable
in the new solar cooker and NCERD concentrating cooker. The box type did
not experience any temperature change on this day. This can be attributed to
the wind off setting the concentrators from their focal point so the
concentrating cookers were not able to track the sun energy efficiently.
4.5.2 Cooking Test
Apart from the water heating test conducted to evaluate the solar
cooker, several cooking tests were performed to ascertain the efficiency of the
cooker for varieties of food items. Some of the stable food in Nsukka such as
rice, beans, yam, egg were used to test the cookers. A sample of rice, yam and
eggs were cooked using the four solar cookers. The factors that affected
cooking time include the type of food, quantity of food and the prevailing
weather condition. The results of the cooking test are presented in Table 4.4
96
Table 4.4: Cooking test of four types of solar cookers
Quantity
(g)
New
concentrating
solar Cooker
NCERD
concentrating
solar cooker
Japanese
concentrating
solar cooker
Box type
solar
cooker
Egg 3 eggs 52min 48 min 27 min 100 min
Rice 212g 158 min 123 min 70 min 182 min
Yam 170g 160 min 120 min 90 min -
As can be seen Table 4.4, three eggs were cooked within 48mins for NCERD
concentrating cooker, 52mins for the FRP concentrating solar cooker and 1hr
40min for box type cooker while it took only 27mins for Japanese type
concentrating solar cooker. The days of the test was influenced by varying
wind condition that dominated the early hours of the morning.
A quantity of rice (212g) was cooked with 450cm3 of water at loading
time of 10.00am. The rice was cooked in 2hrs 38min for the FRP
concentrating solar cooker, 1hr 10mins for Japanese type concentrating solar
cooker and 2hr 3mins for NCERD concentrating solar cooker. The box type
solar cooker cooked the rice for 3hr 2mins. The rice cooked with the solar
cookers excepting the box type cooker was all palatable and homogeneous.
During the experiment it was found that 212g quantity of rice can be cooked
two to three times per day depending on the prevailing weather condition.
Fig 4.8 is the food samples cooked in the solar cookers. Comparing the result
with that of other conventional cooking systems, Sulaiman et al (2003) stated
that same quantity of rice and water can be cooked in just 28mins with
kerosene stove and 43mins with electric stove.
97
Fig 4.8: Cooked food samples using solar cookers
The third cooking test is the yam cooking test. A tuber of yam that
weighed 170g was chopped into cube and cooked with 250cm3 of water at a
loading time of 10.00am. It took the FRP concentrating solar cooker 2hrs
40mins to cook the yam, 2hrs for NCERD concentrating solar cooker and 1hr
30min for the Japanese type concentrating solar cooker to cook the yam. The
box type cooker was not able to cook the yam satisfactorily and the chopped
yams were unpalatable. During the cooking test, there was unstable weather
condition but all the food stuff cooked satisfactorily in the concentrating
cookers.
4.6 Thermal Performance
The thermal performance of solar cookers includes the boiling determination
of the power and the cooking efficiency.
4.6.1 Cooking Power
The boiling power is determined using 1kg of water. This is calculated
using equation 4.1.
T
ttCMQ
ww 12 4.1
Considering the weight of the cooking pots, the equation becomes
T
ttCMCMQ
ppww 12 4.2
98
where;
Q = boiling power, W
wM = mass of water, kg
pM = mass of pot, kg
wC = specific heat capacity of water, J/kg
oC
pC = specific heat capacity of pot, J/kg
oC
1t = initial water temperature,
oC
2t = final water temperature,
oC
T = measuring time, sec
The boiling power for each of the cookers is calculated as follows;
1. FRP concentrating solar cooker
Substituting the following values, kgMw
1 , kgMp
48.0 , CkgJCo
w/4186 ,
CkgJCo
p/980 , Ct
o2.22
1, Ct
o7.94
2and min170T to equation 5.2, the
boiling power (Q) is calculated as follows;
1700
2.227.9498048.041861Q
1700
337589
W6.198
2. Japanese type concentrating solar cooker
Substituting the following values, kgMw
1 , kgMp
35.0 , CkgJCo
w/4186 ,
CkgJCo
p/980 , Ct
o2.22
1, Ct
o5.98
2and sT min80 to equation 5.2,
the boiling power is calculated as follows;
800
2.225.9898035.041861Q
800
7.345562
W432
99
3. NCERD concentrating solar cooker
Substituting the following values, kgMw
1 , kgMp
24.0 , CkgJCo
w/4186 ,
CkgJCo
p/980 , Ct
o2.22
1, Ct
o7.91
2and sT min170 to equation 5.2,
the boiling power is calculated as follows;
1700
2.227.9198024.041861Q
1700
4.307273
W7.180
4. Box type Solar Cooker
Substituting the following values, kgMw
1 , kgMp
35.0 , CkgJCo
w/4186 ,
CkgJCo
p/980 , Ct
o2.22
1, Ct
o1.96
2and sec1600T to equation 5.2, the
boiling power (Q) is calculated as follows;
1600
2.221.9698024.041861Q
1600
68.326726
W204
4.6.2 Cooking Efficiency
Cooking efficiency is calculated from equation 5.3.
AI
Q 4.3
where;
= efficiency
Q cooking power
I = beam radiation
A = aperture area
100
Declination Angle
The test month is 15th
January, the declination angle is calculated from
equation 5.4.
NSin 284365
36045.23 4.4
where;
= declination angle
N = day number of the year
Day number for 15th
January is 46, therefore declination angle is calculated as
46284365
36045.23 Sin
o27.21
Hour Angle
The hour angle (H) at 10.00am is calculated using equation 4.5.
TH 1224
360 4.5
101224
360H
oH 30
Altitude Angle
The altitude angle can be calculated in equation 4.6.
CosHCosCosLSinSinLSin1 4.6
where;
= altitude angle
L = latitude angle, latitude of Nsukka is 6o52
= declination angle
H = hour angle
Altitude angle is calculated as
oooooCosCosCosSinSinSin 3027.2152627.21526
1
101
oSin 759.0
1
o3.49
Beam Radiation
The beam radiation at normal incidence on the cookers is calculated as
follows;
sinB
ap
BN
AI
4.7
where;
BNI = beam radiation at normal incident on the cooker
apA = apparent solar radiation solar irradiation just outside the atmosphere
B = atmospheric extinction coefficient
β = altitude angle
The values of ap
A and B are obtained from Hsieh (1986) as 1229.5 and 0.448
respectively.
3.49sin448.0
5.1229
BN
I
8057.1
5.1229BN
I
2/9.680 mWI
BN
Beam radiation on horizontal surface is calculated as follows
SinIIBN
5.8
3.499.680 SinI
2/2.516 mWI
The average beam radiation on horizontal surface of the cooker
considering12% losses due to convention and radiation is 454.3W/m2.
The efficiency for the designed concentrating cooker is as follows;
102
where;
efficiency
Q cooking power =198.6W
I beam radiation = 454.3W/m2
A aperture area
27.0
%27
56.13.454
6.198
AI
Q
103
Table 4.5: Water heating performance evaluation of the four solar cookers
Item Symbol Units FRP
concentrating
solar cooker
Japanese
type
concentrating
solar cooker
NCERD
concentrating
solar cooker
Box
type
solar
cooker 1 Mass of
water
M Kg 1 1 1 1
2 Average
ambient
temperature
Tamp oC 28.5 28.5 28.5 28.5
3 Initial
temperature
of water
t1 oC 22.2 22.2 22.2 22.2
4 Final
temperature
of water
t2 oC 94.7 98.5 91.7 96.1
5 Concentrator
area
A m2 1.56 0.94 1.2 0.32
6 Average
solar
radiation
I W/m2 516.2 516.2 516.2 516.2
7 Measurement
time
T Sec 1700 800 1700 1600
8 Average
wind speed
V m/s 1.35 1.35 1.35 1.35
9 Weight of
pots
Mp Kg 0.48 0.24 0.35
10 Specific heat
capacity of
pot
Cp J/kg.oC 980 980 980 980
11 Specific heat
capacity of
water
C J/kg.oC 4186 4186 4186 4186
12 Heating
power
Q W 198.6 432 180.7 204
13 Efficiency
(% )
- 27 90 49 21.6
104
4.7 Discussion of Results
The results of the experiment can be analyzed by the varying
temperature gradient in the cookers. The initial temperature rise varied slightly
between the cookers which can be seen in the differences in steepness of the
slopes of the graph (Fig 4.6, 4.7). The plot of temperature, solar radiation
against time indicates that temperature attainment of the concentrating cookers
depends largely on solar radiation and weather condition. It shows that the day
that had relatively low wind speed and average solar radiation recorded
highest temperature attainment for all the cookers.
The result from the heating water test and cooking test suggest that the
Japanese type concentrating solar cooker has the greatest performance when
compared to the rest of the cookers. The FRP concentrating solar cooker and
NCERD concentrating solar cookers were more susceptible to the change in
weather condition and solar radiation. The box type cooker is least affected by
wind blow and its content retains heat longer than others. All the
concentrating cookers performed at slower rate in varying weather condition
because of reliance exclusively on beam incident radiation. Their temperature
responded closely to rise and fall of solar radiation and wind speed. This can
be seen in Fig 4.7, a clear day result in which highest temperature was attained
by the concentrating cookers. For efficient cooking it is required that the
cooking pot is heated from the bottom that is, all reflected ray should strike
the cooking pot while the cooking pot remains horizontal. In winter season,
the altitude angle is low and some of the reflected ray strike side of the
cooking pot. Wind blows offset the concentrators from their focal point most
of the time and the cooker will not be able to cook the food because of high
forced convection looses. According to Dandakota et al (2007), attenuation of
solar radiation by the harmattan dust particles in respect to collector surface
occur in two stages namely;
1. on the incident radiation before reaching the collector surface
due to particle suspended in the lower part of the atmosphere
105
2. on the collector surface due to accumulated particles on the
aperture of the collector
This is one of the major factors known to affect tracking of the sun energy by
concentrating solar cookers even when a considerable amount of solar
radiation reaches the surface of the collector.
Poor performance of the new cooker and NCERD concentrating cooker has
little or nothing to do with the reflecting surfaces. This is because in Fig. 4.9
as given by Ghai et al (1953), it shows the general performance efficiency of a
concentrating solar cooker in relation to the cooking utensil, reflector and the
solar cooker. From this Figure, it is seen that the performance efficiency of a
concentrating solar cooker decreases with increase in its operating temperature
while the efficiency of reflector remains unchanged with temperature of
cooking vessel. In other words, the reflection of sun rays on the reflector has
no effect on the efficiency of the cooker rather instead it could be attributed to
the aluminize polyester film which has be known to reflect a larger portion of
incoming beam and diffuse radiation in all directions thus reducing efficiency
of the cooker (Burgos et al, 2008). Though, aluminize polyester film was used
because it is readily available, it is light and also very cheap.
106
The discrepancy in the theoretical and experimental performance of the
cookers suggested that the weather condition has a major impact on
concentrating cookers than box type cookers which gives greater buffer
between changes in solar radiation and inside cooker temperature.
Solar cooker
10
30
50
70
90
30 40 50 60 70 80 90
Fig. 4.9: Variation of efficiency of cooking utencil,
reflector, and solar cooker with
temperature of water
Eff
icie
ncy
,η
Temperature oC
Cooking utensil
Reflector
107
CHAPTER FIVE
CONCLUSION AND RECOMMENDATION
6.1 Conclusion
As the need for alternative energy sources for cooking increases, new
technologies continue to evolve on how to solve the problems. Generally,
solar cookers have a tremendous social, economic and environmental value to
every community. This is because using solar cookers rather than firewood for
cooking in areas where firewood is the major cooking energy source will help
reduce desert encroachment, drudgery in cooking and preserve the
environment and most of these areas where firewood is used for cooking are
sun rich areas which are around the equator. Solar cooker could be used in
such areas 200-300 days in a year and will be available 7-8 hours in a day
(Metcalf, 1996). The use of solar cooker will optimize the use of this
abundant, free energy resource rather than using firewood which is in short
supply and causes harmful effect on the environment.
In the course of this work, a concentrating solar cooker was designed
and constructed. It comprises of three parts namely; concentrator, support and
pot trestle. The major component, concentrator shell is made of fiber
reinforced plastic while the rest part of the solar cooker is made from steel.
The shell and the other parts are supported in various positions of orientation
without significant distortion that would affect the optical system. The solar
cooker is portable and can easily be assembled and disband. It can withstand
environmental factors such as rust. The cooker was tested together with three
other cookers at National Center for Energy Research and Development,
University of Nigeria Nsukka in the month of January 2008. It was used to
heat water and to cook some stable food stuff such as rice, yam and eggs. The
maximum temperature attained during the water heating test was 97.3oC for a
period of 2hrs. It took the cooker 2hrs 38mins to cook 212g of rice, 52mins to
108
cook eggs and 2hrs 40mins to cook 170g of chopped yam. All the food stuffs
cooked were palatable, nutritious and tasteful.
When compared with three solar cookers based on their performance,
the cooker‟s performance was fair though it was greatly affected by prevailing
weather condition. The cooking power of the concentrating cooker is
calculated to be 198.6W/m2 and efficiency of 27%.
6.2 Recommendation
In view of enormous benefit derived from using solar concentrating
cookers, the following recommendations are important;
1. There is need for further studies on this research work to include
comprehensive seasonal studies in other to obtain enough data for fair
performance evaluation.
2. There is need for the use of experimental design analysis for better
performance evaluation and results analysis.
3. Also, the use of the same size comparative solar cookers cooker would
be better done for more reliable comparison.
4. A detailed thermal analysis of the performance of the cooker should be
undertaken to obtain a computer based model for the cooker.
5. Automatic light sensitivity controller could be introduced to rotate the
receiver surface according to the direction of the sun.
6. An experimental analysis has to be undertaken to obtain the maximum
angle through which the focal point and aperture diameter of the
reflector can be tilted so that no reflected ray may miss the bottom of
the cooking vessel.
7. Concentrator should be made thicker by adding more fibers strands and
resins during recasting in other to avoid the cooker been affected by
high wind.
8. A shied should be introduced in the pot trestle to reduce convective
heat losses.
109
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