advance research project (giovanni adlim 4209575)

8/11/2019 Advance Research Project (Giovanni Adlim 4209575)

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Swinburne University of Technology (Sarawak Campus)

Swinburne University of Technology

Sarawak Campus

Faculty of Engineering and Sciences

Effects of heat storage systems on the performance of

solar kiln

Bachelor of Engineering

(Mechanical)

Giovanni Adlim Mideh Jr (4209575)

May/2013


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DECLARATION

We hereby declare that this report entitled Effects on the heat storage system on the

performance of solar kilnsis the result of our own project work except for quotations and

citations which have been duly acknowledged. We also declare that is it has not been

previously or concurrently submitted for any other degree at Swinburne University of

Technology (Sarawak Campus).

Name: Giovanni Adlim Mideh Jr

ID: 4209575

Date: 20thMay 2013


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Abstract

This report consists of recommended designs and materials to be used for building a

natural indirect solar kiln. Literature review of past works suggests that, indirect

solar kiln requires heat storage to collect and store sufficient heat, and the heat

collected is channelled into a different compartment to be used for heating drying

products. Experiment was conducted to find important components for solar kiln;

such as suitable heat storage and heat collector design to efficiently store and transfer

heat to be used for drying purposes. The effectiveness of the proposed designs and

materials was tested by drying banana slices in the solar kiln, and was compared to

banana dried using conventional method. Result shows that; although outdoor drying

is faster, the quality of solar kiln drying is better. The proposed material and designwas able to store heat, but not enough to sustain heating after midnight. Several

recommendations were proposed; including adjusting the drying chambers

dimension and improve on insulation properties of the system.


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Contents

Contents .................................................................................................................................... i

I.0 Introduction........................................................................................................................ 1

1.1 Introduction to solar kilns.............................................................................................. 2

Figure 1: Classification of solar kilns and drying modes (Fudholi et al. 2010)....................... 2

Figure 2: Model design of Hybrid Solar-Biomass system (Gunasekaran et al. 2012)............. 2

1.1.1 Passive dryers.......................................................................................................... 3

Figure 3: Left: Direct heating chamber by Mursalim et al. (2003); and right Indirect natural

convection solar dryer by Bolaji (2005).................................................................................. 3

Figure 4: Natural convection solar dryer: a. mixed-mode, b. indirect mode by Simate (2003)4

1.1.2 Active dryers........................................................................................................... 5

1.2 Objectives...................................................................................................................... 6

1.3 Aims............................................................................................................................... 6

2.0 Literature review................................................................................................................ 7

2.1 Design............................................................................................................................ 7

2.1.1 Review of solar food dryer designs......................................................................... 7

Figure 6: Classification of solar dryers (Weiss &Buchingern/a)............................................. 7

2.1.2 Reviewing on types of solar air heaters development in India............................... 8

Figure 7: Solar heater types and performance parameters (Bansai, N 1999) ........................... 8

Figure 8: Natural ventilation through a roof integrated solar air heater (Mathur 1994).......... 8

2.1.3 Reviewing on different types of solar air heater..................................................... 8

Figure 9: Counter flow solar air heater with porous matrix (Mohammad, A 1996)................ 9

2.1.4 Review of optimum angle prediction for flat plate solar collector....................... 10

2.2 Heat storage................................................................................................................. 10

2.2.1 Experiment reviews on incorporating phase change materials into heat storage

material.......................................................................................................................... 10

Figure 12: Energy storage capacities of some building materials, with and without PCM(Kelly, R n/a)......................................................................................................................... 11

2.2.2 Experimental review on selecting thermal storage for testing sensible materials. 11

Figure 13: Thermal storage materials behaviour under recorded time period (Hanifa et al.

2011)...................................................................................................................................... 12

2.2.3 Experimental review of the development of thermal energy storage concrete..... 12

Figure 14: DSC curves of different materials (Dong Zhang et al. 2003)............................... 12

2.2.4 Experimental review of convective heat transfer of sand for thermal energy

storage............................................................................................................................ 13

Table 2: Sand types and heat transfer coefficient value (Golob, M 2011)............................. 13


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2.3 Insulation material....................................................................................................... 13

2.3.1 Review of the development and performance evaluation of natural thermal

insulation materials composed of renewable resources................................................. 13

Table 3: Physical and mechanical properties of investigated samples (Korjenic et al. 2011)14

2.3.2Review of rice husk characteristics as insulator.................................................... 14

Table 4: R-Value of rice husks tested with different thickness............................................. 14

2.4 Product quality............................................................................................................. 14

2.4.1 Study of the effects of different humidity level against the drying rate................ 14

2.4.2 Microorganism growth......................................................................................... 15

2.4.3 Physical appearance............................................................................................. 16

3.0 Important parameters....................................................................................................... 17

3.1 Average air flow rate and ambient temperature........................................................... 17

Figure 17: Statistics based on observations from 11/2010 to 4/2013 (Windfinder 2013)..... 17

3.3 Temperature tolerance of different drying crops......................................................... 18

3.4 Relative humidity......................................................................................................... 18

Table 6: Relative humidity for Kuching (MOSTI 2013)....................................................... 18

3.5 Solar irradiance............................................................................................................ 19

Table 7: Tabulation of geographical characteristics of selected testing location (Khatib et al.

2011)...................................................................................................................................... 19

Figure 19: Kuchings data for solar energy recorded monthly (Khatib et al. 2011).............. 19

3.6 Ventilation.................................................................................................................... 20

Figure 20: Opening heights affects passive ventilation (Autodesk Education Community

2011)...................................................................................................................................... 20

4.0 Methodology.................................................................................................................... 21

4.1 Conceptual design........................................................................................................ 23

4.1.1 SolidWork modelling............................................................................................ 23

Figure 21: Cross section view of solar kiln............................................................................ 23

Figure 22: Exploded view of solar kiln components............................................................. 23

4.1.2 SolidWork air flow simulation.............................................................................. 24

Figure 23: Simulation of air flow inside the drying chamber during the day........................ 24

4.2Heat Collector Design................................................................................................... 25

4.2.1 Experimentation on the heat storage performance of the heat collector............... 25

Figure 24: Sand and aggregates (left), and pebbles (right).................................................... 25

Figure 25: (Left) Black aluminium sheet covering the heat storage and (right);................... 25

Figure 27: Concrete aggregates surrounded with rice husks................................................. 27


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Experiment A: Drying of heat collector with exposed acrylic sheet as cover................. 27

Figure 28: Acrylic sheet used to cover the box...................................................................... 27

Figure 29: Method of measuring internal and ambient temperature...................................... 28

Experiment B: Drying of heat collector with exposed Acrylic sheet and MDF as cover28

Figure 30: Acrylic sheet with MDF (with holes)................................................................... 28

Experiment C: Testing for effectiveness of MDF board as removable lid...................... 29

4.3Solar Kiln Design.......................................................................................................... 29

4.3.1 Experimentation on the drying performance of the drying chamber.................... 29

Figure 31: Combination of drying chamber and heat collector............................................. 29

Figure 32: Method of recording temperature......................................................................... 29

Figure 33: Walls without insulator (left), and walls with insulator (right)............................ 30

Figure 34: Inclined black aluminium plane (left); and attached plastic cover (right)............ 30

4.4Assessment of dried products....................................................................................... 31

4.4.1 Drying rate............................................................................................................ 31

Figure 35: All samples are of the same thickness.................................................................. 32

Figure 36: The weight of all samples are fixed...................................................................... 32

4.4.2Case hardening....................................................................................................... 32

4.4.3 Observable microorganism growth....................................................................... 32

5.0 Results.............................................................................................................................. 33

5.1 Heat Collector Design.................................................................................................. 33

5.1.1 Experimentation on the heat storage performance of the heat collector............... 33

Experiment A: Drying of heat collector with exposed Acrylic sheet as cover................ 34

Experiment C: Testing for effectiveness of MDF board as removable lid...................... 35

Short review.......................................................................................................................... 35

5.2 Solar Kiln Design......................................................................................................... 36

5.2.1 Experimentation on the drying performance of the drying chamber.................... 36

5.3 Assessment of dried products...................................................................................... 38

5.3.1 Drying rate............................................................................................................ 38

Figure 37: Slices of bananas being measured to the same weight......................................... 38

5.3.3 Case hardening...................................................................................................... 39

Figure 39: Banana slice dried outdoors (left); and banana slice dried in solar kiln (right).... 39

5.3.4 Observable microorganism growth....................................................................... 40

Figure 40: Banana dried in solar kiln (left); and banana dried outdoors (right).................... 40

6.0 Mathematical modelling.................................................................................................. 41


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6.1 Heat transfer through the walls in heat collector......................................................... 41

6.2 Heat loss through acrylic sheet.................................................................................... 42

6.3 Heat loss in drying chamber......................................................................................... 44

70 Analysis and discussion.................................................................................................... 45

7.1 Performance analysis of concrete aggregates and sand mixtures as heat storage

materials............................................................................................................................. 45

7.2Performance analysis of heat collector and drying chamber......................................... 46

7.3 Discussion.................................................................................................................... 47

8.0 Conclusion....................................................................................................................... 48

9.0 Recommendation............................................................................................................. 48

10.0 References...................................................................................................................... 49

Appendices............................................................................................................................. 51


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Figure 1: Classification of solar kilns and drying modes (Fudholi et al. 2010)....................... 2

Figure 2: Model design of Hybrid Solar-Biomass system (Gunasekaran et al. 2012)............. 2

Figure 3: Left: Direct heating chamber by Mursalim et al. (2003); and right Indirect natural

convection solar dryer by Bolaji (2005).................................................................................. 3

Figure 4: Natural convection solar dryer: a. mixed-mode, b. indirect mode by Simate (2003)4

Figure 5: Left is indirect forced solar kiln by Al-Juamily (2007); and right shows rotary

column cylindrical dryer by Sarsilmaz et al. (2000)................................................................ 5

Figure 6: Classification of solar dryers (Weiss &Buchingern/a)............................................. 7

Figure 7: Solar heater types and performance parameters (Bansai, N 1999) ........................... 8

Figure 8: Natural ventilation through a roof integrated solar air heater (Mathur 1994).......... 8

Figure 9: Counter flow solar air heater with porous matrix (Mohammad, A 1996)................ 9

Figure 10: a. Single glazing; b. double glazing; c. counter-flow without a porous matrix; d.

counter-flow with a porous matrix (Mohammad, A 1996)...................................................... 9

Figure 11: Maximum temperature difference between the first glass cover and ambient air as

a function of air flow rate (Mohammad, A 1996).................................................................... 9

Figure 12: Energy storage capacities of some building materials, with and without PCM

(Kelly, R n/a)......................................................................................................................... 11

Figure 13: Thermal storage materials behaviour under recorded time period (Hanifa et al.

2011)...................................................................................................................................... 12

Figure 14: DSC curves of different materials (Dong Zhang et al. 2003)............................... 12

Figure 15The data for effects on green bell peppers in respond to different drying

temperature (Sigge et al. 2007).............................................................................................. 15

Figure 16: Tabulation of data for germination time for bacterium species on response on

different experimental conditions (Lattab et al. 2012)........................................................... 15

Figure 17: Statistics based on observations from 11/2010 to 4/2013 (Windfinder 2013)..... 17Figure 18: Average daily air temperature profile for month May 2013 (Windfinder 2013).. 17

Figure 19: Kuchings data for solar energy recorded monthly (Khatib et al. 2011).............. 19

Figure 20: Opening heights affects passive ventilation (Autodesk Education Community

2011)...................................................................................................................................... 20

Figure 21: Cross section view of solar kiln............................................................................ 23

Figure 22: Exploded view of solar kiln components............................................................. 23

Figure 23: Simulation of air flow inside the drying chamber during the day........................ 24

Figure 24: Sand and aggregates (left), and pebbles (right).................................................... 25

Figure 25: (Left) Black aluminium sheet covering the heat storage and (right);................... 25

Figure 26: Heat collector box................................................................................................ 26Figure 27: Concrete aggregates surrounded with rice husks................................................. 27

Figure 28: Acrylic sheet used to cover the box...................................................................... 27

Figure 29: Method of measuring internal and ambient temperature...................................... 28

Figure 30: Acrylic sheet with MDF (with holes)................................................................... 28

Figure 31: Combination of drying chamber and heat collector............................................. 29

Figure 32: Method of recording temperature......................................................................... 29

Figure 33: Walls without insulator (left), and walls with insulator (right)............................ 30

Figure 34: Inclined black aluminium plane (left); and attached plastic cover (right)............ 30

Figure 35: All samples are of the same thickness.................................................................. 32

Figure 36: The weight of all samples are fixed...................................................................... 32


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Figure 37: Slices of bananas being measured to the same weight......................................... 38

Figure 38: Weight of selected banana slice from different drying environment................... 38

Figure 39: Banana slice dried outdoors (left); and banana slice dried in solar kiln (right).... 39

Figure 40: Banana dried in solar kiln (left); and banana dried outdoors (right).................... 40

Figure 41: Conduction of heat in glazing materials(CYRO Industries 2013)........................ 43


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Acknowledgement

First and foremost, I would like to thank my project partner, Hamiruzudin Said for

all the time and efforts that we had spent together in order to complete this final year

project. I realise that without team effort, this project would end up a failure. I would

also like to express my gratitude to both of my parents because they had contributed

in terms of financial support to purchase necessary items to be used for research

needs. Next, I am grateful to Dr. Ha How Ung for the time he had spent on us while

completing this research project. Lastly, I would like to thank my friends and

colleagues for their help and support.


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I.0 Introduction

Drying is widely used as a method of preservations of agricultural products. The

concept is to remove the moisture content in foods to inhibit the growth of

microorganisms. According to Gutti et al. (2012), for effective drying of agricultural

products, temperature ranges of 45-60C is required for safe drying. These

temperatures can increase the rate of moisture removal, as well as to ensure the

quality of dried products.

In Malaysia, the most common method of drying is by open air drying; whereby

agricultural products are arranged on an open space, and exposed to sunlight. This

drying method is most preferred because of its low cost features and simplicity.

However, open drying poses several disadvantages; such as tedious drying process

whereby farmers needed to be caution with bad weathers while drying their

agricultural products. Besides that, drying products are exposed to contamination

from insects, dusts and animals since the drying products are being left exposed at an

open space. Open drying method is not suitable to be used on products which have

high market values; such as cashew nuts, fearing the possibilities of product damage

due to uncontrollable heat.

Solar drying via solar kilns is nothing new, as it had been tested and used for

commercial purposes. Unfortunately in Malaysia, the concept of drying in solar kiln

is not popular, most probably due to the complexity of constructing the kiln and lack

of motivation by the government. This is quite unfortunate, considering the

promising potential of solar kiln in terms of improving the drying process.


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1.1 Introduction to solar kilns

Solar kilns are categorised into two parts; natural circulation solar kilns and forced

circulation solar kilns. The combination of both is termed hybrid solar kilns. They

had classified types of solar kilns, which are described in Figure 1 below:

Figure 1: Classification of solar kilns and drying modes (Fudholi et al. 2010)

Natural circulation solar kilns utilises natural air flow to facilitate moisture removal

in the drying product. Therefore, this type of solar dryer does not contain any

components which can induce internal air movement. Forced circulation solar kilns

utilises additional machinery; such as fans or ventilation fans to induce air movement

inside the solar kiln system. Hybrid dryers are made out of the combination of solar

dryer and another different dryer; for example biomass dryer.

Figure 2: Model design of Hybrid Solar-Biomass system (Gunasekaran et al. 2012)


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The design of natural circulation solar dryer takes account on many factors; such as

air flow profile during day and night drying and the relative humidity changes in the

ambient condition. This explains the complexity of natural circulation solar dryer

will differ from the design of forced circulation solar kilns. Certain factors which is

crucial in natural circulation solar kilns such as the night ambient air flow rate, does

not affect the drying performance in forced circulation solar kilns. Therefore, most

industries preferred forced circulation solar dryer to dry their high commercial values

crops over natural circulation solar kilns to reduce the risk of product damage and for

faster drying.

1.1.1 Passive dryers

Fudholi et al. (2010) had undergone a review regarding on effective drying methods

for agricultural and marine products in Malaysia. Their study was based on natural

convection solar kilns (passive dryers). According to their report, they had classified

several types of passive solar heating:

Figure 3: Left: Direct heating chamber by Mursalim et al. (2003); and right Indirect natural convection

solar dryer by Bolaji (2005)

1.1.1.1 Dir ect heating

Mursalim et al. (2003) stated that for direct solar heating cabinet, the direct heating

mechanism as proposed in the diagram causes overheating of crops due to direct

exposure of sunlight, large drying time and transmitivity of glass cover is reduced

due to the evaporation and condensation on the glass cover.


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1.1.1.2 I ndir ect heating

Based on the flaws posed on direct heating, they stated that Bolaji (2005) had

improvised on a new solar dryer using a box type absorber collector. The new

collector had a glass cover, and black absorber plate was inclined to about 20 degrees

to the horizontal to allow the heated air to rise up. Based on the new design, he

concluded that its maximum efficiency was about 60%. Maximum temperature

achievable inside the drying chamber is 57C.

1.1.1.3 Mix mode heating

Figure 4: Natural convection solar dryer: a. mixed-mode, b. indirect mode by Simate (2003)

Fudholi states that on 2003, Simate had designed and compared mix mode natural

convection solar kilns. Simate concluded that mix mode solar dryer gave a shorter

collector length, compared to indirect mode. In terms of cost, mix mode has low

operating cost. However, the drying product is 15% less compared to indirect

heating.


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1.1.2 Active dryers

Figure 5: Left is indirect forced solar kiln by Al-Juamily (2007); and right shows rotary column cylindrical

dryer by Sarsilmaz et al. (2000)

1.1.2.1 I ndir ect Heating

Al-Juamily et al. (2007) had constructed an indirect forced solar kiln, which is used

to dry agricultural products in Iraq. Basically, the design consists of solar collector

with V-groove absorption plates of two air passes.

The moisture content of the drying product was reported to have been reduced from

80% to 30% within a short period of time; which one and a half day.

1.1.2.2 Mix mode heating

Sarsilmaz et al. (2000) had conducted tests on drying apricots using rotary column

cylindrical dryers. The set consists of air blow, air heater and drying region. This is

in relation to increase drying rate, and at the same time to reduce the risk of product

damage prior to excessive heating.

Based on the review on types of solar dryer, it was observed that from time to time,

there are significant changes in the drying process, in terms of addition of equipment

and the complexity of the design of solar kilns. From time to time, improvements are

constantly being made by researchers to improve the drying performance using solar.


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1.2 Objectives

Finding a suitable heat storage material as a heat source for the natural

indirect solar kiln

Finding the best design to improve internal heat circulation inside the solar

kiln

Finding alternatives to reduce heat loss rate from the system into the

environment

1.3 Aims

To find the best natural indirect solar kiln design for drying foods

To find ways to efficiently harvest and store heat energy to be used for solar

kiln drying process

To introduce the benefits of indirect solar kiln drying on drying product

quality to local industry


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2.0 Literature review

2.1 Design

2.1.1 Review of solar food dryer designs

Weiss and Buchinger (n/a) through the Austrian Development Cooperation, had

reviewed the designs of several solar dryers to be used in agricultural business. They

had classified solar dryers into these categories:

Figure 6: Classification of solar dryers (Weiss &Buchingern/a)

Several passive indirect solar dryers were reviewed; including cabinet solar dryers

whereby the system consists of flat plate solar energy collector, connecting to a

drying chamber with an exit air for ventilation. Based on the design, air flows into

the heat collector will be warmed, and will pass through air ducts into the drying

chamber. Moist air will be discharged through the vent located at the drying

chamber.

It was reported that; this design will reduce the complexity of drying process, and

food to be dried will not be directly exposed to sun which will reduce the nutritional

value of drying products. However, the design will reduce the drying efficiency.


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2.1.2 Reviewing on types of solar air heaters development in India

Bansai, N (1999) conducted a study of solar air heater applications in India, and had

classified solar heaters into non-porous absorbers and porous absorbers. Based on the

report, Gupta and Garg (1967) studied various types of solar air heaters and their

performance parameters.

Figure 7: Solar heater types and performance parameters (Bansai, N 1999)

He stated that, an innovative way to utilise solar air heating is to induce ventilation.

He commented on the study by Mathur (1994), stating that it is possible to produce a

mass flow rate of 150 to 200 m3/h with only 1 m2of collector area, for an incident

solar radiation of 800 W/m2.

Figure 8: Natural ventilation through a roof integrated solar air heater (Mathur 1994)

2.1.3 Reviewing on different types of solar air heater

Mohamad, A (1996) had investigated on various types of air heater and how it affects

the performance of solar air heating. He stated that, the main disadvantage of using

an air heater in solar kilns was that, the heat transfer coefficient between the absorberplate and the airstream is low.


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Figure 9: Counter flow solar air heater with porous matrix (Mohammad, A 1996)

According to Mohamad, the design above will forced air to flow over the glass cover

which has been preheated, before passing to the absorber. Thermal efficiency was

drastically enhanced if compared to conventional air heater, for about 75%.

In terms of air heater with different amount of glazing, he concluded that the design

which can retain the most heat is the counter flow solar air heater with a porous

matrix.

Figure 10: a. Single glazing; b. double glazing; c. counter-

flow without a porous matrix; d. counter-flow with a

porous matrix (Mohammad, A 1996)

Figure 11: Maximum temperature difference between the

first glass cover and ambient air as a function of air flow

rate (Mohammad, A 1996)


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building materials, it was found out that the newly incorporated building material has

increased thermal storage capacity.

Figure 12: Energy storage capacities of some building materials, with and without PCM (Kelly, R n/a).

Based on the review of the report, it is understood that organic and inorganic

materials which has been incorporated with PCM has higher thermal heat capacity,

compared to without PCM.

2.2.2 Experimental review on selecting thermal storage for testing sensible

materialsHanifa et al. (2011) on their assessment of solar kiln design with heat storage

medium had undergone an experiment in the search of suitable material for thermal

storage. They had experimented on pebbles with various sizes, sands, gravel and

charcoal. Their method of experiment was; different sensible materials are being kept

in a tilted wooden box covered with glass. The result obtained was tabulated as

follow:


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Figure 13: Thermal storage materials behaviour under recorded time period (Hanifa et al. 2011)

Based on their result experiment, pebble was able to retain heat as high as about35C till 20:00 hours, while most sensible heat storage materials had their

temperature dropped to almost 30C.

2.2.3 Experimental review of the development of thermal energy storage concrete

Dong Zhang et al (2003) had tested procedures to produce thermal energy storage out

of concrete aggregates. Among the test was to determine the thermal energy storage

capacity of concrete aggregates with different porous structure by subjecting

specimens to differential scanning calorimetry(DSC). The result of the experimentwas as follow:

Figure 14: DSC curves of different materials (Dong Zhang et al. 2003)

The data shows that most of the concrete specimen has very low heat flow rate. This

means that, concrete aggregates have high heat absorbing capability.


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2.2.4 Experimental review of convective heat transfer of sand for thermal energy

storage

Golob, M(2011) had conducted an experiment to determine the heat transfer of sands

when flows over a flat plate electric heater. The heat transfer coefficient was

calculated using the equation . The result was as follow:

Table 2: Sand types and heat transfer coefficient value (Golob, M 2011)

The data shows that sands of all types has high heat transfer coefficient. Therefore, it

is hypothesised that sand is suitable to be used to absorb heat.

Garg et al (1985) stated that at sands can store thermal energy at high temperature in

the form of sensible heat. The heat energy stored by sand can be transferred using

steam or air packets as transfer medium.

2.3 Insulation material

2.3.1 Review of the development and performance evaluation of natural thermal

insulation materials composed of renewable resources

Korjenicet. al(2011) conducted series of experiments to measure the insulation

performances of materials made out from different ratios of natural fibres, binders

and shives. All specimens are dried at 105 C using the gravimetric method, and the

mechanical properties are also being determined. The results were tabulated as

below:


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Table 3: Physical and mechanical properties of investigated samples (Korjenic et al. 2011)

This experiment proves the potential of natural sources as a good thermal insulator

because of the high level of natural fibre content. However, it was discovered that the

thermal conductivity of these materials will deteriorate when is exposed to high level

of moisture.

2.3.2Review of rice husk characteristics as insulator

Constant, D (2009) reviews the characteristics of rice husk as potential heat insulator.

Testing for the properties of rice husks under separate experiments was conducted

based on the methods according to the American Society for Testing and Materials.

The thermal resistance of rice husks are being tested and the results were as follow:

Table 4: R-Value of rice husks tested with different thickness

Based on the data above, 1 inch thick of rice husk requires 120 hours for heat to

reach the other side of the husk. With the average R value of 2.8 per inch, the

experiment proves the effectiveness of rice husk as thermal insulator.

2.4 Product quality

2.4.1 Study of the effects of different humidity level against the drying rate

Sigge et al. (2007) had undergone a study of the drying rates and times of green bell

peppers (Capsicum Annuum L) in different humidity level. At 15% Relative

Humidity, the drying rate (kg water*kg-1 solid) was far higher compared to 40%

Relative Humidity 70C. Their study indicates that drying rate increases in low

humidity environment, while drying rate decreases with increased level of humidity.


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Figure 15The data for effects on green bell peppers in respond to different drying temperature (Sigge et al.2007)

2.4.2 M icroorganism growth

Lattab et al. (2012) had recently produced a scientific report regarding the effect of

storage conditions such as relative humidity, duration and temperature on the

germination time of Aspergilluscarbonarius and Penicilliumchrysogenum. Given the

duration of experimentation time of 2 to 28 days with relative humidity percentage

and temperature as the manipulated variable with time as responding variable, their

tabulation of data are as follows:

Figure 16: Tabulation of data for germination time for bacterium species on response on different

experimental conditions (Lattab et al. 2012)


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For Peniciliumchrysogenum, the minimum germination time for this bacteria was at

100% relative humidity and 2 days of storage, while the maximum is at 20% relative

humidity and 28 days of storage.

2.4.3 Physical appearance

Ali, A (2008) on his review of drying agricultural products for Practical Action

Group, stated that case hardening would mostly occurred to some food products

during drying. He described case hardening as the formation of a hard skin on the

surface of fruits, fish and some other foods which slows the rate of drying and may

allow mould growth. Fast drying during the initial period is the main cause of case

hardening, and can be prevented using cooler drying air at the beginning of the

drying process.


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3.0 Important parameters

3.1 Average air flow rate and ambient temperature

According to an online weather forecast Windfinder.com, it was reported that the

average daily ambient air flow rate is about 1.5ms-1, and the average daily ambienttemperature was about 30C.

Figure 17: Statistics based on observations from 11/2010 to 4/2013 (Windfinder 2013)

Figure 18: Average daily air temperature profile for month May 2013 (Windfinder 2013)

The average daily temperature for month May records the lowest temperature of

25C from 14:00. This is most probably due to heavy rains which occurred during

month April and middle May.


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3.3 Temperature tolerance of different drying crops

The tabulation of various types of drying crops and their recommended temperature

are based on various sources.

Subject Recommended temperature

Traditional Malay herbs (e.g Mas cotek,

DukungAnak and KayuManis)

Not exceeding 40C. Drying at temperature

freater than 40C will destabilise the

bioactive chemical components of the herb

(Globinmed 2005).

Fruits and vegetables About 60C. higher temperature could result

in case hardening, which is the outside of the

food dries hard before the inside moisture

can escape (Thomas & Berry 1997).

Fish No higher than 55C (Scalin, D 1997)

Table 5: List of drying products and recommended drying temperature

The subject for drying was based on the popularity in Malaysia, especially fish which

is widely dried in the East Coast at the Peninsular Malaysia.

3.4 Relative humidity

Based on the relative humidity table for Kuching, it was observed that during night

time, the relative humidity at ambient environment is about 90%RH at 20:00 hours.

Table 6: Relative humidity for Kuching (MOSTI 2013)

According to Padfield, T (1998) on his thesis regarding selecting suitable materials to

control relative humidity, he concluded porous, absorbent walls has the ability to

moderate the indoor relative humidity derived from indoor activities such as cooking

and bathing, and that impermeable walls are more prone towards transient episodes

of condensations.


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3.5 Solar irradiance

Khatib et al. (2011) had produced a research paper on the modelling of daily solar

energy on a horizontal surface for 5 main sites in Malaysia; in which among them is

Kuching. They had tabulated the location characteristics for Kuching, which is

described in the table below:

Table 7: Tabulation of geographical characteristics of selected testing location (Khatib et al. 2011)

Figure 19: Kuchings data for solar energy recorded monthly(Khatib et al. 2011)

Based on the data above, it was observed that the lowest solar energy recordedmonthly was about 3.25KWh/m2.


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3.6 Ventilation

Passive ventilation affects the rate of moisture removal from the drying product.

Among the important parameters for wind ventilation includes volume, coefficient of

effectiveness, and the size of inlet and outlet of the room.This is to determine the rate

of stale air can be replaced by fresh air, and determines how much heat the space

gains or loses.

whereby Qwind is the airflow in m3/h, K is the coefficient of thickness, A is the

opening area, and V is the outdoor wind speed. Coefficient of effectiveness ranges

from 0 to 1, and is dependent on the angle of inlet.

Figure 20: Opening heights affects passive ventilation (Autodesk Education Community 2011)

By placing inlets low to high will promote air with different temperatures to

exchange. Hot air which contains low density will rise, while cold air which has

higher density will sink. The opening size also affects the rate of air exchange; to

produce higher inlet flow velocity, the smaller inlet can be paired with a larger outlet

opening.


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4.0 Methodology


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For this whole experiment, we are testing for suitable designs and materials to be

used to construct an all-natural solar kiln. Therefore, our solar kiln will be heavily

relying on the effects of natural external forces such as the air flow rate and the

average daily temperature.

Our objective was to fully utilise these natural external forces and maximise its

performance to improve the drying rate of agricultural products via the solar kiln.

The construction of our solar kiln will not include external artificial forces such as

fan.

As for the heat storage, we had decided to use sensible heat storage in our design.

Compared to phase change material, sensible heat storage is simpler in terms of

energy conversion and mathematical modelling. Besides that, sensible heat storage is

more economical and easily available compared to phase change materials.

These experiments were held at Swinburne Universitys field(coordinate 1.531638,

110.357408) around month April to May 2013. According to local weather forecasts,

the average weather during these period are mostly light rain, whereas the lowest and

highest ambient temperature was 24C and 32 C (Malaysian Meteorological

Department 2013). Our solar kiln was located somewhere at the middle of the field,

facing the south. The average wind speed for month April was 5 knots, while for

May was 4 knots (Windfinder 2013).

Throughout this experiment, the temperature was measured using a K-Type probe

which measures surrounding air temperature. For the heat storage, a contact sensor

was used for measuring. Both sensorsare translated using a digital reader (brand

UYIGAO UA-902C).


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4.1 Conceptual design

4.1.1 SolidWork modelling

Modelling of our solar kiln design was entirely made out from SolidWork software.

This software offers complete toolset to create, simulate, publish and manage data,

maximizing the innovation and productivity of engineering resources (SolidWorks

2013).

SolidWork was used to study the design of our solar kiln to minimise error and

production cost when making prototype. Through SolidWork, we are able to study

important factors such as the air flow characteristics and temperature changes of our

initial design and impose modifications where necessary.

Figure 21: Cross section view of solar kiln

Figure 22: Exploded view of solar kiln components

Adjustable vent

Air inlet


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4.1.2 SolidWork air flow simulation

Figure 23: Simulation of air flow inside the drying chamber during the day

Figure 1 shows the SolidWork air flow simulation based on these parameters;

ambient temperature at 33C, heat storage temperature at 0C and the external air

flow of 1.0 ms-1. The inlet was a rectangular hole located above the heat collector,

whereas the heat collector does not have inlet. The outlet for the whole system was a

rectangular hole with shutter, located at the wall of the drying chamber.

We had set the external air flow to 1 ms -1 to observe the air flow inside the drying

chamber. The air flow rate was selected based on the average wind flow rate in

Kuching for the month of April and May. The simulation shows that even though the

ambient air flow is at the lowest, there are slight air movement inside the drying

chamber because of the presence of the vent.

However, with the current software we are unable to determine the effects of heat

transfer from the heat storage into the drying chamber. Therefore, we had decided to

analyse the effects through prototyping.


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4.2Heat Collector Design

4.2.1 Experimentation on the heat storage performance of the heat collector

4.2.1.1 Heat storage material

For this experiment, we had tested for pebbles, and mixture of concrete aggregates

and sands. Based on the experiment done by Hanifa et. al (2010), pebbles and sands

was proven to possess the characteristics of slow heating and moderate rate of heat

release. They stated that bigger pebbles have the capability to absorb and release the

heat slowly. We had decided to mix sand with concrete aggregates because sand was

known to be able to heat and release heat slowly, whereas concrete aggregates have

high specific heat capacity. By combining both, we are trying to improve the

characteristics of both sands and concrete aggregates so that to be able to absorb

more heat and at the same time release it in a controlled manner.

Figure 24: Sand and aggregates (left), and pebbles (right)

Figure 25: (Left) Black aluminium sheet covering the heat storage and (right);

The whole system was covered with acrylic sheet


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By using the same box, we expose our test subjects under the sun covered with

acrylic sheet. The temperature of the trapped air was measured in every 2 hours,

using digital thermometer with K-Type probe sensor. The weight and the densities

for the materials are as follow:

Material Mass (kg)

Pebbles 3

Concrete aggregates + sand 3

Table 8: Mass and density of tested materials

4.2.1.2Prototyping

A series of experiments was conducted to determine the effects of different

arrangements or designs of our heat collector system to the heat retention

performance. These experiments were conducted around month Aprilto May 2013.

Kuching had experienced inconsistent rain pattern during this period, which is a great

disadvantage for us.

Figure 26: Heat collector box

A 1m*0.6m rectangular wooden case was built to test the heating storage capability

of different kinds of materials. The case was made out of plywood because it has low

thermal conductivity, about 0.11W/m K. The corners of the case were covered with

wood silicon and wood putty to reduce leakage. The wooden box has no inlet.

For the heat storage material, we used the ratio of4 buckets of sands and 3 buckets of

concrete aggregates. 1 bucket of sands weighed about 6 kg, while the aggregates

weighted about 7.2 kg. The total mixture would be 40kg.


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Figure 27: Concrete aggregates surrounded with rice husks

Experiment A: Drying of heat collector with exposed acrylic sheet as cover

For the first test, concrete aggregates which were being used as heat collector, were

covered with a black painted aluminium sheet, and was surrounded with rice husks.

Rice husks were chosen because; it has high content of silica making it a good

thermal insulator, and it is naturally produced. The aluminium sheet was painted in

black to improve the thermal absorptivity.

The box was covered with acrylic sheet and was being left to dry for the whole day

and was exposed during the night. The trapped air temperature was taken every 2

hours using a K-Type probe.

Figure 28: Acrylic sheet used to cover the box


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Figure 29: Method of measuring internal and ambient temperature

Experiment B: Drying of heat collector with exposed Acrylic sheet and MDF as cover

A different test was conducted, whereby an MDF board with the thickness of 10mm

with holes was being tested whether or not it is efficient enough to allow heat

entering the system, and at the same time reduce the rate of heat escaping the system

during night time.

MDF board was chosen in this experiment because of its low thermal conductivity,

which is 0.3 W/m.K. The hole was measured about 3mm in diameter. The drilled

MDF board was placed beneath the acrylic sheet throughout the day.

Figure 30: Acrylic sheet with MDF (with holes)


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Experiment C: Testing for effectiveness of MDF board as removable lid

As suggested by our project supervisor, the next experiment was conducted with a

removable insulator included in the heat storage system. The MDF board was placed

on top of the acrylic sheet during night time to see whether it can block heat escape

through the acrylic sheet.

4.3Solar Kiln Design

4.3.1 Experimentation on the drying performance of the drying chamber

4.3.1.1 Design

For this experiment, the drying chamber was connected to the heat collector. The

heat collected in the heat collector will be channelled to the drying chamber via heat

transfer. The design of the heat collector remains unchanged. The internal

temperature was measured using the K-Type probe connecting to a digital reader.

Figure 31: Combination of drying chamber and heat collector

Figure 32: Method of recording temperature


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Experiment D: Comparison of heat retention performance of drying chamber with and

without polystyrene

Figure 33: Walls without insulator (left), and walls with insulator (right)

Test was conducted to compare the heat retention performance with and without

polystyrene inside the drying chamber. The polystyrene used was 1 inch thick and is

easily available in bookstores. Polystyrene was selected because it has thermal

conductivity of 0.3W/m. K, which makes it a considerably good thermal insulator.

The whole system was dried under the sun, and the temperature inside the drying

chamber was taken every 2 hours.

Experiment E: Assessment of inclined flat plate versus non-inclined flat plate on the

drying chamber temperature profile

Figure 34: Inclined black aluminium plane (left); and attached plastic cover (right)

The heat collector was modified whereby the flat plate was inclined at about 30

degrees from base and walls insulated with polystyrene.

We had decided to replicate the double glazing feature, by attaching a plastic layer

on top of the black aluminium sheet. The aim for this was to create another air pocket


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inside the heat collector to minimise air flow. Air movement was among the cause of

heat loss inside the heat collector.

The data obtained was tabulated and compared to the previous data whereby the

orientation of the black aluminium sheet is 90 degrees horizontally.

4.4Assessment of dried products

We had decided to dry bananas in our solar kiln and compare it with bananas dried in an

open air. This is to test the effectiveness of our solar kiln in terms of producing better drying

quality and cleaner dried products.

Few slices of bananas are placed inside the drying chamber, and the mass changes were

being monitored. As for open air drying, few slices of bananas are placed on a tray and

conventional drying practice was applied i.e. products are being left under the hot sun was

brought indoors during night time.

The weather during the drying period consists of mostly cloudy day during the morning, and

heavy rain on the evening. There are few days where it was hot and humid.

Both drying products are compared based on the drying rate, the occurrence of case

hardening and observation on visible microorganism growth.

4.4.1 Drying rateTo measure the drying performance of our solar kiln, we used the Dry Oven Method

to test the moisture extraction rate for bananas dried inside the drying chamber and

also being dried in an open air.

Dry Oven Method was the simplest yet effective method to be used to determine the

moisture level in crops. Procedures include recording the initial weight of the test

subjects, and the weight of the subject was recorded daily until the weight of the test

subject remains constant. The percentage of moisture content was determined by this

formula:

, whereby, mass of water in sample is equals to the wet mass minus the dry mass.


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Figure 35: All samples are of the same thickness

Figure 36: The weight of all samples are fixed

Few slices of bananas were being tested with all the weights kept constant, which is

7.5g. For day 0, some banana slices was placed inside the drying chamber, and some

was being left to dry in an open air. The weight of the all the test subjects was taken

at about 17:00 hours. The time taken for the mass of each banana to remain constants

under different drying condition was taken and tabulated.

4.4.2Case hardening

According to Thomas and Berry (1997), case hardening can occur to dried products

which are dried in open air. Case hardening means that the outside of the food dries

hard before the inside moisture can escape. Therefore, using our naked eye we are

going to inspect the dried products for case hardening by dissecting the product and

observe the moisture level inside by touching.

4.4.3 Observable microorganism growth

By using our naked eye, we will inspect the drying products from both environments.

We are looking for possible mould or fungus growth on the products.


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5.0 Results

5.1 Heat Collector Design

5.1.1 Experimentation on the heat storage performance of the heat collector

5.1.1.1 Heat storage material

Based on the data above, it was shown that the heat absorption rate demonstrated by

pebbles is higher compared to the other. However, the pebbles also experiences

faster heat loss compared to mixture of concrete aggregates and sands.

It was observed that the mixture of concrete aggregates and sands has more stable

heat absorption and heat release rate. At about 10pm, the final temperature for the

mixture of concrete and sand is higher than pebbles.

In terms of availability; pebbles are not readily available as we need to purchase

these stones in participating shops. Besides that, based on the size of our proposed

drying chamber we need a huge amount of pebbles to maximise the heat storage

performance. Therefore, using pebbles as heat storage is not economically viable for

our project. Compared to pebbles, concrete aggregates and sands are readily

available in Swinburne civil laboratory.

Therefore, we had decided to go for the mixture of concrete aggregates and sands as

our heat storage.

0

10

20

30

40

50

60

70

12:00 AM 4:48 AM 9:36 AM 2:24 PM 7:12 PM 12:00 AM

Tempera

ture(C)

Time

Heat storage performance

Concrete

aggregates

and sands

Pebbles


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5.1.1.2 Design

Experiment A: Drying of heat collector with exposed Acrylic sheet as cover

t was observed that the highest trapped air temperature can reach up to 101C

indicating that this design was able to trap more heat during the day. Major heat loss

occurred during the evening, whereby the difference between the ambient and the

internal temperature was about 10-20 C.

Experiment B: Drying of heat collector with exposed Acrylic sheet and MDF as

cover

Based on the data obtained; the highest achievable air temperature trapped inside was

about 45 C. The temperature dropped gradually and at the end of the experiment, the

air temperature is almost the same as the ambient temperature. The temperature drop

was probably due to poor insulation of our box. However, by putting MDF board

beneath the acrylic sheet for drying, the rate of solar irradiance entering the box is

reduced so it takes time for the air inside the box to get heated up. This arrangement

0

50

100

150

12:00 AM4:48 AM 9:36 AM 2:24 PM 7:12 PM12:00 AM

Temperature(C)

Time

Rate of temperature changes

internal temperature (C)

ambient temperature(C)

0

10

20

30

40

50

12:00 AM 4:48 AM 9:36 AM 2:24 PM 7:12 PM 12:00 AM

Temperature(C)

Time


internal temperature (C)

ambient temperature(C)


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may be suitable on a very hot weather, but not for our case whereby the average

weather is cloudy and rain.

However, the performance for this arrangement could be enhanced by adjusting the

hole diameter at the MDF board.

Experiment C: Testing for effectiveness of MDF board as removable lid

In this experiment, the MDF board was placed on top of the acrylic sheet at about 4-

5pm. At the end of the experiment (9pm), the internal temperature was the same as

the ambient temperature. The data shows that, the MDF board is not an effective

heat reflector. In order to minimise heat loss, the MDF board should be placed on top

of the acrylic sheet while the internal temperature is still high (around 3pm,

depending on the internal temperature on the respective day).

Short review

Based on the experiments above, we observed that our heat storage would perform

better in a properly insulated environment. Even though we had sealed the gap

between the acrylic sheet and the box using acrylic tape, thermal leakage still occurs

in the system. The rate of heat loss via convection was minimised using rice husks

and most of the joints connecting the woods are being sealed using silicon.

However, it is difficult to minimise the rate of heat loss via radiation because it

requires us to modify the transparency of the acrylic sheet. Adjusting the acrylic

sheet surface such as glazing will affect the rate of irradiance level received by the

heat storage. Which is why in the end, we had decided not to modify the physical

appearance of the acrylic sheet.

0

20

40

60

80

12:00 AM 4:48 AM 9:36 AM 2:24 PM 7:12 PM 12:00 AMTempe

rature(C)

Time


Internal Temperature

Ambient temperature


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5.2 Solar Kiln Design

5.2.1 Experimentation on the drying performance of the drying chamber

Experiment D: Comparison of heat retention performance of drying chamber with and

without polystyrene

The data shows that without polystyrene as wall insulator, the heat retention of the

drying chamber is poor compared to insulated drying chamber using polystyrene.

The rate of temperature increase inside the drying chamber is slower compared toinsulated wall. When the drying chamber is insulated, there rate of temperature drop

decreases around noon. Based on these comparisons, adding polystyrene as wall

insulator will potentially reduce the rate of heat loss in the drying chamber.

At about 22:00 hours, the temperature inside the drying chamber for both cases is

almost equals to the ambient temperature. However, it was observed that the heat

storage surface temperature is about 31C at 22:00 hours. This indicates that, the air

temperature inside the heat collector is higher than the air inside the drying chamber.

0

10

20

30

40

50

12:00 AM 4:48 AM 9:36 AM 2:24 PM 7:12 PM 12:00 AM

Temperature(C)

Time

Rate of temperature changes in different

condition

Without Polystyrene

With polystyrene

Ambient

stone temperature


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Experiment E: Assessment of inclined flat plate versus non-inclined flat plate on the

drying chamber temperature profile

The temperature for both ambient and drying chamber was taken from 8am to 10pm.

The data above shows the temperature pattern recorded from both environments.

During the drying period, most of the time it rained heavily in the evening and hot

weather during the morning. Despite the bad weather, the data shows that during the

final hour, the internal temperature remains higher compared to the ambient

temperature.

The final temperature inside the drying kiln which is slightly higher than the ambient

temperature shows that the polystyrene layer attached to the wall had effectively

retain heat inside the drying chamber. Besides that, some minor adjustment to the

inclination of the black aluminium sheet improves the hot air flow from the heat

collector into the drying chamber.

Short review

By adjusting the orientation of the black aluminium sheet inside the drying chamber,

it improves the hot air flow rate into the drying chamber compared to the previous

design. By adding polystyrene, it helps to reduce the heat loss out from the drying

chamber. Although not fully weather proof, this design can be used to dry products

even in wet season.

0

10

20

30

40

12:00 AM 4:48 AM 9:36 AM 2:24 PM 7:12 PM 12:00 AM

Temperature(C)

Time

Temperature Changes

Dry

chamber

Ambient


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5.3 Assessment of dried products

5.3.1 Drying rate

For reference, the reference point of moisture content for our banana sample was

based on the final weight of the banana which is dried outdoors.

Figure 37: Slices of bananas being measured to the same weight

Samples dried in open air environment were labelled Product A, while product dried

inside solar kiln was labelled Product B. The mass of the exact same specimenwas

taken. The result was tabulated as follow:

Figure 38: Weight of selected banana slice from different drying environment

Moisture content:

;

Based on the formula, the bananas have average moisture content of 74%.

It takes about 10 days for the weight of the banana dried outdoors to remain constant,

and about 13 days for banana dried in the kiln. This is because of the weather

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Mass(g)

Days

Drying period

Outdoor

Drying chamber


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condition around May which is mostly heavy rain during the afternoon and hot sunny

day during the morning.

5.3.3 Case hardening

Figure 39: Banana slice dried outdoors (left); and banana slice dried in solar kiln (right)

Based on the picture above, there is observable moisture thickness at the middle part

of the dried banana which is dried in an open air environment. As for the banana

sample dried inside the solar kiln, the moisture thickness is smaller compared to the

other.

There are higher chances of case hardening will occur in products which are dried

outdoors, compared to products which are dried inside solar kiln. This is because of

the level of sunlight exposure which the surface of the drying product received. As

for solar kiln drying, drying process happens due to the hot air circulation inside the

contained space.


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5.3.4 Observable microorganism growth

Due to inconsistent weather pattern, the drying products are exposed to high moisture

level and also humid environment. This type of environment encourages the

development of microorganisms on some of the drying products.

At the end of the drying process, fungal growth was found on both samples from

both drying conditions. However, the growth rate of fungal is higher on samples

dried at the open air environment compared to the other.

Figure 40: Banana dried in solar kiln (left); and banana dried outdoors (right)

It was hypothesised that since the banana samples were dried in an open air

environment, it was exposed to inconsistent air condition and therefore creates asuitable environment for microorganism growths. As for products dried inside the

drying chamber, the polystyrene layer which reduces heat loss maintains the internal

temperature to be higher than the ambient temperature.


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6.0 Mathematical modelling

Thermal conductivity was calculated using Fouriers fundamental of linear heat

equation because it is the most convenient way to calculate heat transfer in sandwich

arrangements.

To study the rate of heat loss in the evening, the temperature records for 17:00 was

taken. The data was as follow: Heat collector had collected 36C of heated air, the

drying chambers air temperature is at 30C and the ambient temperature was at

28C. Assuming that the surface of the aluminium and the heat storage are of the

same temperature, which is at39C. The surface temperature of the acrylic sheet was

measured to be 33C.

6.1 Heat transfer through the walls in heat collector

Heat loss through the wall was calculated using the formula

, since the arrangement of all resistances are in series.

Assumption:

1. Area, A is kept constant for all resistances. A=1m2

2.

The convection heat transfer coefficient for outdoors is h1=10 W/m2K

3. The thermal conductivity coefficient for lightweight concrete aggregates is

ranging about 0.21 to 0.46 W/m2K.

4. The heat transfer was calculated from the midpoint of the heat storage to the

ambient surrounding.


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Thickness:

Plywood= 0.01m

Insulated layer = 0.1m

Aluminium = 0.0001m

Concrete = 0.3m

Thermal conductivity (W/m2K):

Plywood = 0.13

Insulated layer= 0.0359 for rice husk

Aluminium= 205

Concrete = 0.21

6.2 Heat loss through acrylic sheet

Heat loss through the acrylic sheet occurs through convection which is described in

through the equation

; whereby

R total= R air+ Racrylic sheet + Rtrapped air

Heat loss also occurs when hot object radiates heat energy to its cooler surrounding,

which can be expressed using the equation,

;whereby =

0.86 for acrylic sheet,

.

Ac

rylicsheet


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6.3 Heat loss in drying chamber

The heat loss is described though the equation;

; whereby the area of the walls and polystyrene would be

0.4225m2

Theoretically, the heat loss rate would be

= 6.3081 W.


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70 Analysis and discussion

7.1 Performance analysis of concrete aggregates and sand mixtures as

heat storage materials

The results obtained from heat storage performance experiment shows the surface

temperature for both pebbles and mixtures of concrete and sands can reach up to

60C. However, it was observed that pebbles releases heat faster compared to the

mixtures of sand and concretes.

In terms of physical appearances, pebbles have more space gaps compared to the

concrete and sand mixtures. The pebbles used for the test consists of black, round-

shaped (about 1.5 cm average radius) with smooth surface. The concrete used for the

test are mostly lightweight concretes, and the sands are coarse.

For the sand and aggregate mixtures, two different heat storage materials will absorb

heat from the sun at the same time. Compared to concrete aggregates, sands have

lower thermal diffusivity and therefore will heat up slower compared to concrete

aggregates. Based on the thermodynamic principle where the temperature gradients

goes from hot to cold, the hotter element in the mixture will heat up the colder

element until it reaches equilibrium state. This internal heat transfer process will then

continue to maintain the temperature of the mixtures.

As mentioned by Garg et al. (1985), sands can store thermal energy at high

temperature in the form of sensible heat. The heat energy stored by sand can be

transferred using steam or air packets as transfer medium. In this case, heat energy

stored by sand will be transferred to the concrete aggregates, thus maintaining the

mixtures temperature to the maximum.

However since the pebble bed consists of only singular material, the process of

thermal equilibrium takes place directly between the external air and the pebbles.

Since the pebble has higher surface temperature compared to the surrounding air, this

will increase the temperature gradient and therefore will increase the heat loss rate,

which is also described through the equation


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7.2Performance analysis of heat collector and drying chamber

Based on the trapped air temperature records that we obtained through the heat

collector experiment, we observed that the air temperature can get as high as 101C.

This proves the effectiveness of our heat collector design to harvest heat energy from

the sun. However, this would also cause huge temperature difference between the

trapped air and the ambient air, resulting in temperature drop during evening.

The data tabulated in the Heat Storage Experiment shows that the heat loss rate

increases during the evening, and by 22:00 the temperature difference between

internal and ambient is about 8C. The internal temperature was expected to drop

further more, and most probably will be equals to ambient temperature by midnight.

The data from Experiment E also indicates that at 22:00 hours, the temperature

difference between the air in drying chamber and external environment is only about

5C, and was expected to drop further after that. The temperature was measured in a

drying chamber insulated with a 1-inch thick polystyrene.

deally, the temperature difference between the drying chambers air temperature and

the external environment temperature should be large at 22:00 hours to ensure that

there is continuous heat circulating inside the solar kiln system. Besides that, there is

not enough heat in the heat collector to induce air movement, since it requires a

larger temperature gradient based on the equation

.

This proves that the insulation for the whole system was not perfect, but was able to

minimise the heat transfer rate to the external environment. The 10cm thick rice husk

surrounding the heat collector will only reduce the heat transfer rate from heat

storage to the plywood wall via conduction. Hot air from heat collector still escapes

through the acrylic sheet by the means of convection and radiation. Aside from that,

the 1-inch polystyrene attached to the walls of the drying chamber is not enough to

reduce the heat transfer rate from the drying chamber into the external environment.

Therefore, the heat transfer through the acrylic sheet should be addressed to

minimise the heat loss inside the heat collector. Several failed approaches was done

to minimise the heat loss; including using MDF board as removable insulator, and

also adding MDF board as another layer beneath the acrylic sheet.


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7.3 Discussion

This experiment proves the potential of sands and concrete aggregates as a great

thermal storage. As demonstrated through our experiment, instead of using single

type of material the combination of 2 different types of sensible heat storage can

actually prolonged the heat retention period of the heat storage.

The high air temperature records inside the heat collector suggest that our design was

able to generate hot air, but failed to efficiently store and channel the heat energy.

Insulation is the main problem which causes heat loss from all over the system. The

experimental results have shown that the rate of heat transfer reduced by the

insulators was not enough to maintain the desired heat for longer hours. Therefore,

the heat energy obtained during the day was not able to last longer than 22:00 hours.

Based on the data tabulated, the conversion of the flat aluminium plat from

horizontal to 30 and the addition of plastic sheet to replicate the double glazing

effect of the heat storage were able to facilitate the heat transfer from the heat

collector into the drying chamber. As recommended by Mohammad, A (1996), heat

collector design with more than one layer can retain heat more than a single layer

because the outer layer will be separated by a thin layer of air packet before passing

through the heated area.

Meanwhile, size could be one of the factors which cause temperature drop within the

system. The 0.65m*0.65m*0.5m drying chamber could be too large, that it requires

large amount of heated air to fill up the space in the drying chamber. This design

could not be efficient, considering the limited heat air source from the solar collector.

As mentioned by Thomas and Berry (1997), drying temperature above 60C can

cause case hardening. Since the maximum achievable drying chamber was about30C, this means that our drying chamber is a suitable environment for drying foods.

During wet season, solar kiln drying is better compared to open air drying because it

reduces the chances of microorganism growth and product damage. Factors such as

the level of humidity and moisture environment was minimised inside the drying

chamber, therefore reduce the growth rate of microorganisms and fungus.


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8.0 Conclusion

The performance of heat storage system for natural indirect solar kiln can be

upgraded by mixing more than one types of elements; preferably with different

specific heat capacity.

The performance of natural indirect solar kiln depends on the insulation design of the

whole system. Heat storage capacity alone cannot ensure there is enough heat to

maintain sufficient temperature for drying. Efficient heat circulation and tight

insulation will improve the drying performance of the solar kiln.

Solar kiln drying products differs from conventional open air drying products in

terms of the drying quality and the level of hygiene. For wet season, solar kiln drying

is better because of the low degree of exposure to external environment factors.

9.0 Recommendation

1. The insulation properties of the whole system can be improved by:

a. Using thicker polystyrene as another insulation layer for the drying

chamber walls

b. Double glaze the heat collector by adding another layer of acrylic

sheet inside the box. Ensure that the space gaps between the sheets are

tightly insulated.

2. The heat transfer efficiency of the whole system can be improved by:

a. Adjusting the location and size of the air inlet from the heat storage

into the drying chamber. Ensure the air inlet is not too big to allow

heat escape, or too small to limit air flow.

b. Reduce the size of the drying chamber. This is to ensure the heated air

will have enough kinetic energy to fill up the entire space in the

drying chamber.


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10.0 References

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Appendices

Gantt chart

Project 1

Project 2

advance research project (giovanni adlim 4209575)

Documents