STUDY OF HUMIDITY IN BOX TYPE SOLAR COOKER

WONG SIONG JI UNG

This project is submitted in partial fulfillment of the requirements for the degree of Bachelor of Engineering with Honours

(Mechanical & Manufacturing System Engineering)

Faculty of Engineering UNIVERSITI MALAYSIA SARAWAK

2005

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DEDICATION

To My Beloved Family...........

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ACKNOWLEDGEMENT

First of all, I would like to express my sincere and greatest thanks to my

supervisor, Dr. Mohd. Omar Abdullah, who encourages, guides and advices me in

carry out my final year project.

Special thanks to Mechanical Lab and Workshop technicians especially En.

Masri Zaini, En. Ryier Juen, and En. Zaidi Suhai for their advices, opinion, help and

support in carry out the experiment for the solar cooker.

I would also want to express my gratitude to my course mates, especially Mr.

Chua Keng Yew, Mr. Ong Tien Wee, Mr. Lee Chee Hooi, and Mr. Tang Chung

Hieng for their helps, advices and encouragements.

Last but not least, I wish to thanks my beloved family especially my parents

who always give encouragements and provide financial support required for this

project.

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ABSTRACT

Alternative energy is now becoming concerned globally. The energy

shortage and problem due to deforest had made people try to exploit renewable

energy in order to tackle the problem. Although solar cooker is still not common but

it had been proved that it can be a good tool if it is well designed and constructed.

Solar box cooker (SBC) that was designed and constructed by previous student, Mr.

Then Kuck Shee, still had a lot of potential for improvement. In his thesis, he did not

study the humidity in the SBC's chamber. In fact, humidity had effect on the

efficiency and performance of the SBC. Therefore, fundamental of SBC,

characteristic of humidity, and other factor that affect the performance of SBC was

studied in the present work. Then, based on the experiment result, it is found that air

in the SBC's chamber is finally saturated and this stimulate more heat lost to the

environment as the air temperature drop. Hence, the temperature of the cooker will

drop greater than the case where the air does not saturated. In order to reduce the

effect of the humidity to SBC, it is recommended that humid absorbent may be able

to be put inside the cooker chamber when cooking but it need to be made sure that

the absorbent would not affect the safety of the food. Also, ceramic can be put inside

the SBC's chamber to increase the heat storage and efficiency.

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ABSTRAK

Kini, tenaga alternatif kian mendapat tumpuan dariapada dunia sejagat.

Berikutan dengan kekurangan sumber tenaga serta kesan daripada penghutanan,

tenaga yang boleh dikitar semula telah cuba diperluaskan penggunaannya demi

menyelesaikan masalah tersebut. Walaupun "solar cooker" masih tidak umum, akan

tetapi telah dibuktikan bahawa ia boleh berfungsi dengan baik dan merupakan satu

alat yang amat berguna sekiranya direka dan dibuat dengan baik. "Solar box cooker"

(SBC) hasil rekaan dan buatan penuntut sebelum ini, iaitu En. Then Kuck Shee,

masih mempunyai banyak potensi untuk perubahsuaikan fungsinya. Selain itu,

beliau tidak membuat pengajian ke atas kelembapan yang wujud di dalam SBC.

Akan tetapi, kelembapan pada hakikatnya mepunyai pengaruh ke atas SBC. Oleh itu,

teori asas SBC, sifat-sifat kelembapan, dan factor lain yang mempengaruhi

kefungsian SBC telah dikaji dalam kertas kerja ini. Berdasarkan eksperimen yang

telah buat, udara di dalam ruangan SBC pada akhirnya akan tepu dan ini

meningkatkan kehilangan tenaga dari SBC ke sekeliling. Dengan itu, suhu SBC akan

jatuh lebih banyak berbanding dengan kes di mana udara dalam SBC yang tidak

tepu. Demi menyelesaikan masalah ini, penyerap kelembapan boleh diletakkan di

dalam ruangan SBC semasa memasak. Akan tetapi, ia perlu dipastikan bahawa tidak

akan menjejaskan keselamatan penggunaan makanan yang dimasak. Tambahan pula,

seramik atau batu-bata boleh juga diletakkan di dalam ruangan SBC untuk

meningkatan keupayaan SBC dalam merangkap tenaga untuk tempoh yang lebih

panjang.

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TABLE OF CONTENT

Chapter Content

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

LIST OF FIGURES

LIST OF TABLES

NOMENCLATURES

1 Introduction

1.1 Background

1.2 Objective

1.3 Methodology

2 Literature Review

2.1 Types of Solar Cooker

2.1.1 Box-style solar cooker

2.1.2 Panel solar cooker

2.1.3 Parabolic solar cooker

2.2 Physics of Solar Cooker

2.2.1 Reflection

2.2.2 Conversion

2.2.3 Retaining

2.3 Thermodynamics view of solar box cooker

2.3.1 Heat gain

2.3.2 Heat loss

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2.3.3 Heat storage

2.4 Food Safety and Solar Cooking

3 Background Study

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3.1 Solar Energy 17

3.1.1 The Sun 17

3.1.2 Solar Radiation 18

3.1.3 How much solar energy strikes the earth? 19

3.1.4 Availability of solar energy 21

3.2 Heat Transfer 25

3.2.1 Modes of heat transfer 25

3.3 Heat Flow Terms and Measurement 28

3.4 Mass Transfer 31

3.4.1 Modes of Mass Transfer 32

3.5 Moisture/ Humidity 33

3.5.1 Effect of moisture on insulation material 34

3.5.2 Effect of moisture to the performance of solar box 37

cooker

3.6 Greenhouse effect of glass trapped solar radiation 37

3.6.1 Example of Greenhouse Effect 38

3.7 Relative Humidity 39

3.7.1 The Psychrometric Chart 40

3.8 Wind Chill Effect 42

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4 Experiment or Testing Result and Discussion

4.1 Apparatus

4.2 Schedule of Experiment

4.3 Precautions

4.4 Data of Experiment

4.5 Discussion

4.5.1 Cooker temperature

4.5.2 Water temperature

4.5.3 Relative Humidity

4.6 Limitations

5 Conclusion and Recommendation

5.1 Conclusion

5.2 Recommendations

REFEENCES

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LIST OF TABLES

Table Page

1 List of used equipment and its measurement. 43 2 Experiment data 45-46

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LIST OF FIGURES

Figure Page

2.1 Basic solar box-style cooker 5 2.2 Simple and basic solar panel cooker 6 2.3 A typical parabolic solar cooker 7 2.4 Green house effect. 9 2.5 Heat radiation from warm solar box cooker 11 2.6 Escape of warm air through crack 12 2.7 Thermal mass inside of the solar box 13 2.8 Application in each temperature range and safe zone for 16

simple solar cooking 3.1 Direct, diffuse and reflected solar radiation 21 3.2 Earth rotates around the sun formed 4 seasons through 23

the year. 3.3 Sun radiation at different position of the sun 24 3.4 Annual solar radiation around the world. 24 3.5 Heat transfer from hot bodies to colder one due to 25

temperature difference 3.6 Mechanism of heat transfer at insulating material that 28

used for solar cooker wall. 3.7 The basic mechanism of mass transfer. 31 3.8 Schematic shows of re-radiation infrared wavelengths 38

that pass or impedes by car windshield. 3.9 Simplified psychrometric chart for temperature and 41

relative humidity 4.1 Comparison between ambient temperature and cooker 47

temperature 4.2 Graph showing the ambient temperature, water 48

temperature, cooker temperature, and relative humidity in cooker chamber.

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NOMENCLATURES

G- total irradiance on a surface

GB - beam (direct radiation)

Gd - diffuse (indirect radiation)

RH - Relative Humidity

SBC - Solar Box Cooker

Tr - Room Temperature

RHr - Room Relative Humidity

Ta - Ambient Temperature

RHa - Ambient Relative Humidity

Tc - Solar Cooker Temperature

RHc - Solar Cooker Relative Humidity

Tw - Water Temperature

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CHAPTER 1

INTRODUCTION

1.1 Background

People use fuel as the energy for cooking. However, many residents at the

undeveloped or rural area may face difficulty in gaining this source. They may have

to walk for a long distance to collect firewood or dung in order to cook. Thus, a lot

of time would be wasted. The worse is many of them have to use major income for

this purpose. Consequently, few decades ago, many scientists have started to

develop alternative cooking method that could help to reduce their burden.

One of the alternatives that have been well developed and widely distributed

or promoted for its usage is the solar cooker. Solar cooker is a simple, safe and

convenient way for cooking. The most significant advantage is its low construction

cost and the source will not instinct or renewable, and free of charge. Moreover,

most of the area that need for this alternative cooking method had great source of

solar radiation through over the year.

Solar cooker work as the cooking temperature is reached. These can be

achieved, as the solar energy gained is equal to the heat losses from the cooker.

That's mean, the cooking temperature is achieved as the solar power gained is in

balance with heat losses. Usually, heat losses from the cooker are including

reflective losses, absorption losses, transmitted loss, leakage losses, and food losses.

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In contrast, solar gain is depending on the total exposure of the cooker to the sun and

the effectiveness of the cooker in collecting and retaining the solar power.

Generally, there are three common types of solar cooker; solar box cooker,

panel cooker and parabolic cooker. Each of them has their own advantages and

disadvantages. Among them, solar box cooker is commonly used, as it is simplest,

cheapest and easiest to be used.

In designing of solar cooker, many factors need to be taken into account.

These include, applied physic, materials, shape, structure, and so on. For instance, a

solar box cooker implements greenhouse effect to gain heat. Hence, transparent

material such as glass or plastic plate is used to trap the solar radiation. In addition,

reflector is used to lead the solar radiation toward the cooker's chamber, and the

cooker is construct of insulation materials in order to reduce heat lost to surrounding.

Also, a dark pot is used for cooking because blackbody can absorb more heat than

others. Hence, it is obvious that proper design of solar cooker is essential for the

construction of an efficiency cooker. However, it should remember that low cost

with safe cooking should be emphasized in design consideration.

1.2 Objective

The main objectives of this project are as follow:

1. To study on the fundamental of solar box cooker.

2. Further study on the solar box cooker that had been designed and

constructed by a previous student namely Mr. Then Kuck Shee.

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As refer to the thesis done by Mr. Then Kuck Shee, he did not do the study

on the humidity in the solar cooker's chamber. However, in fact, there would be a lot

of moisture or water vapour inside the cooker chamber as it starts cooking. This is

because during cooking, water in the pot will evaporate into water vapour and then

released. Moreover, the temperature influences the characteristic of humidity. In

general, relative humidity reduced as temperature increased. Hence, the existing of

water vapour may affect the cooker performance and efficiency.

1.3 Methodology

In order to make this project more precise and concise, some methods were

taken and these including as listed below:

i. Study and Research

The study includes the fundamental of solar box cooker and examine on the

thesis done by Mr. Then Kuck Shee.

ii. Maintenance of previous design and constructed solar cooker

Repairing and cleaning of the solar cooker that made by Mr. Then. For

example, replacing the broken parts, and repair the broken foil.

iii. Experimental or Testing

Carry out the experiment for the solar cooker. The emphasis is on the relative

humidity data collection.

iv. Analysis

Plot the data into graphic form. Then, analyze the experiment data based on

the graphs.

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CHAPTER 2

LITERATURE REVIEW

2.1 Types of Solar Cooker

There are many types of solar cooker. Among them, the most common types

of solar cooker are box-style solar cooker, panel solar cooker and parabolic solar

cooker.

2.1.1 Box-style solar cooker

According to Tamara Dwyer (1999), box-style cookers are the most common

type made for personal use and are made in both circular and rectangular shapes.

Figure 2.1 illustrates a typical basic box cooker. He also stated that they were

consisting of an enclosed inner box covered with clear glass or plastic, a reflector,

and insulation. However, Tamara Dwyer (1999) did mention that this type of cooker

had disadvantages of low heating and slow cooking rate.

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Figure 2.1: Basic solar box-style cooker. [Extracted from http: //solarcooking. org/]

2.1.2 Panel solar cooker

As explained by Tamara Dwyer (1999) at his website, panel cooker consist

of flat reflective panels, which focus the sunlight on a cooking vessel. This is the

easiest and least costly cooker, but they are unstable in high winds and will greatly

affected by the sun position or availability of solar radiation. A simple panel solar

cooker is shown in Figure 2.2. According to Solar Cooker International, the main

advantage of panel cookers is their ease of construction. On the other hand, the

major disadvantage is that the light actually is not projected to a single focus point

due to the arbitrary fashion of the shape of reflective panels thus the food is cooked

unevenly and very slowly.

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Figure 2.2: Simple and basic solar panel cooker. [Extracted from

http: //solarcooking. org/]

2.1.3 Parabolic solar cooker

This solar cooker consists of one parabolic reflective panel. This will cause

the light hitting any part of its surface will reflect to one focus point located at a

certain distance in front of the cooker's center. According to Tamara Dwyer (1999),

parabolic cookers reach higher temperatures and cook more quickly than solar box

cookers, but it is harder to be constructed and used. The design of this cooker

requires more precision; otherwise, it will not function. Also, when using this

cooker, the temperature needs to be controlled to avoid overheat and burning of

food. In addition, the risk of burns and eye injury is greater with homemade

parabolic designs. However, if it is used correctly with great care, it could be

excellent for cooking. Figure 2.3 shows a typical parabolic solar cooker.

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Figure 2.3: a typical parabolic solar cooker. [Extracted from http: //solarcooking. org/]

2.2 Physics of Solar Cooker

According to Natasha Lindo and Toli Lerios (2004), the fundamental physics

of the solar cooker are reflection, converting, and retaining.

2.2.1 Reflection

A good designed solar cooker need to be able to direct the greatest possible

amount of sun's light rays to the food. Moreover, it should also can accommodate for

the sun's varying position and capture enough light to cook the food. This can be

done by the implementation of reflector.

The materials used for reflection vary with the region and resources

available. Reflective materials that could be used include mirrors, aluminum foil,

aluminum siding and certain types of metallic paint. However, different type of

model of solar cooker will require different design for its effectiveness.

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2.2.2 Conversion

Besides, the solar cooker should can effectively absorb and convert the sun's

rays into heat (infrared radiation). This can be accomplished by using dark materials

as the dark surface absorbs more heat than others. For example, use of dark pots.

This principle is suitable throughout the different models of cookers because

they all use a pot to cook. In addition, some models consist black bottom to be

placed in the cooker to produce more heat energy thus increase the effectiveness of

the cooker.

2.2.3 Retaining

Retaining absorbed heat in the cooker is also essential to ensure the cooker is

efficiency for cooking. Normally, insulator that acts as a heat barrier is used. This

will increase the heat capacity and thus reduce the cooking time. Most common used

insulators in solar cookers are, glass, plastic, wood, cardboard, or still air barrier for

solar box cookers, or a plastic bag for open cookers.

However, different models of cooker may use different quantities of

insulator. For example, the parabolic and panel models use little insulation; thus,

they achieve lower temperature as compare to models with more insulation. The

solar box cooker makes good use of insulation, and thus it is the most efficient

model.

An additional method to retain heat in the cooker is placing adobe or bricks

in the cooking volume. These materials have a high heat capacity that keeps heat

captured for long amounts of time. Also, the thicker the pots used, the greater the

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amount of heat, which they absorb. Both of these methods will essentially allow for

food to cook after the sun has gone down, at the expense of longer preheating.

2.3 Thermodynamics view of solar box cooker

There are many factors that will affect the efficiency of solar cooker which

should be properly take into account during designing a solar box cooker.

Nevertheless, the important one is the thermodynamics principle of the cooker, as it

had the affect on other considerations of solar box cooker design. For this view, as

mentioned by Mark Aalfs, (1992), the considered heating principles including:

1. Heat gain

2. Heat loss

3. Heat storage

2.3.1 Heat gain

From a thermodynamically point of view the function of a solar box cooker

is posses heat gain based on the greenhouse effect as illustrated below.

Figure 2.4: Green house effect. [Extracted from http: //solarcooking. org/J

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When the sun is shines through a transparent material such as glass or plastic,

visible light can easily pass through the glass, and is absorbed and reflected by

materials within the enclosed space.

The light energy that is absorbed by dark pots and the dark absorber plate

underneath the pots is converted into longer wavelength heat energy and radiates

from the interior materials. However, most of this radiant energy, which is of longer

wavelength, is unable to pass back out through the glass and therefore trapped within

the enclosed space. On the other hand, the reflected light is either absorbed by other

materials within the space or, because it does not change wavelength, passes back

out through the glass. Subsequently, the heat is conducted through those materials to

heat and cook the food.

Furthermore, according to Barbara Prosser Kerr (1991), both direct and

reflected sunlight passing through the glazing onto the dark pot produce heat on the

pot sides that flows directly into food. Sunlight also falls on the reflective oven sides

and the dark bottom of the oven. Reflective sides in an SBC throw additional light

onto the dark pots in the center as well as adding to the sunlight on the dark tray. In

addition, the hot air in the chamber transfers additional heat to the pots.

2.3.2 Heat loss

Heat within a solar box cooker is lost in three fundamental ways: conduction,

radiation, and convection.

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a) Conduction

Heat within a solar box is lost when it travels through the molecules

of tin foil, glass, cardboard, air, and insulation, to the air outside of the box.

For instance, the solar heated absorber plate conducts heat to the bottoms of

the pots and then cause heat loss via conduction through the bottom of the

cooker. In order to prevent this, the absorber plate is raised from the bottom

using small insulating spacers as shown in figure 2.5.

Figure 2.5: Heat radiation from warm solar box cooker. [Extracted from

http: //solarcooking. org/]

b) Radiation

Things that are warm or hot within a solar box cooker give off heat

waves, or radiate heat to their surroundings (refer figure 2.5). These heat

waves are radiated through air or space. Most of the radiant heat given off by

the warm pots within a solar box is reflected from the foil and glass back to

the pots and bottom tray. Glass traps radiant heat better than most plastics.

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c) Convection

Molecules of air move in and out of the box through cracks. Heated

air molecules within a solar box escape, primarily through the cracks around

the top lid, a side "oven door" opening, or construction imperfections. Cooler

air from outside the box also enters through these openings.

Figure 2.6: Escape of warm air through crack. [Extracted from

http: //solarcooking. org/]

2.3.3 Heat storage

With increased density and weight of the materials within the insulated shell

of a solar box cooker, the capacity of the box to hold heat also increased. The

interior of a box including heavy materials such as rocks, bricks, heavy pans, water,

or heavy foods will take longer to heat up because of this additional heat storage

capacity. The incoming energy is stored as heat in these heavy materials, slowing

down the heating of the air in the box. These dense materials, charged with heat, will

radiate that heat within the box, keeping it warm for a longer period at the day's end.

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Figure 2.7: Thermal mass inside of the solar box. [Extracted from

http: //solarcooking. org/]

2.4 Food Safety and Solar Cooking

According Barbara Kerr (1991) to Food safety for food cooked by any

method requires meeting specific rigid conditions. Cooked food at temperatures

between 52°C (125°F) and 10°C (50°F) can encourage grow of harmful bacteria. This

temperature range is known as the danger zone. To protect against food poisoning,

microbiologists and home economists strongly recommend that food be kept either

above or below these temperatures despite what method of cooking is used. In

cooked food held at room temperature, there is a chance of Bacillus cereus food

poisoning, a major intestinal illness. Worse, if the food is not thoroughly reheated

before consumption, there is a chance of deadly botulism poisoning or salmonella.

Even if it is reheated, when cooked food has been in the danger zone for three to

four hours, there remains a risk of food poisoning in solar cooked food as in food

cooked by any other method.

It has been prove that placing raw refrigerated or frozen food, even chicken

or other meat, in an SBC in the morning for several hours before the sun begins to

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