Design, Build and Test of Solar Thermal Air Heating …...Design, Build and Test of Solar Thermal Air Heating System with Application ii | P a g e Acknowledgement First and foremost,

School of Engineering and Information Technology

Engineering Honour Thesis

Design, Build and Test of

Solar Thermal Air Heating

System with Application By

Sultan Yahya Juma Al Habsi

Instrumentation and Control Engineering and

Renewable Energy Engineering

2nd of July, 2018

A thesis submitted to Murdoch University to fulfil the requirements for the degree of Honours

Bachelor of Engineering

Academic Supervisor

Dr David Parlevliet

Unit Coordinator

Professor Parisa Bahri

Design, Build and Test of Solar Thermal Air Heating System with Application

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Declaration

I declare that this thesis project is my work including the research and design and has not

submitted to any previous education institution. The results illustrated is based on the project

mathematical modelling as well as an experimental result performed on the solar air heating

system. The methods and results described in this thesis reflect the work performed and are

supported with records in the lab books.

Sultan Yahya Juma Al Habsi

July 2018


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Acknowledgement

First and foremost, the author, is so thankful and greatest to Almighty God for, without

His blessing, this Engineering Honour Thesis would not come to reality.

A massive appreciation and respect to those who had been contributing a tremendous

value on this thesis in one way or another by their extensive in-depth knowledge and skills.

Without their knowledge, help and kind support, the thesis project would not see the light.

The author would sincerely thank:

I am gratefully indebted to honour Thesis Supervisor, Dr David Parlevliet, Senior Lecturer,

for his in-depth knowledge, guides and encouragement all over the project period. Dave, I

believe that your philosophy and patience brought out the best in me and made a deep and

lasting impression on my life. Many thanks for your continuous support and encouragement

to carry over the entire work.

Technical Support, Mr Mark Burt, Technical Officer, for his excellent technical

engineering skills, help and support in implementing and building the project. Thanks, Burt

for teaching me to overcome difficulties and focus on the target, in order to be successful.

Similarly, profound gratitude goes to Engineering Chair, Professor Parisa A. Bahri,

Professor of Engineering, School of Engineering and Information Technology, for her helpful

guides and support.

Thesis students, Abdullah Abdullah, Salman Rauf, Tawfeeq Zakaria, Heung Chan, David

Wise, Engineering Student, for their endless help and support.


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I am grateful to my friends and family for supporting me throughout these years. I look

forward to having more time to spend with you all finally. Mainly thanks to my mother for

always having the time to listen, regardless of the years of emigration and for sharing her

wisdom and for helping me always to find the open door. Thanks to my father for the years

of sacrifice and who have offered me endless encouragement and have been an enthusiastic

sounding board regardless of the topic.

Thanks to my sisters, Nazik, Thuraya and Buthaina and my brothers Zakariya, Al Mutasim,

Al Muntasir and Naser Allah for their support and advice. Thanks for the laughs you shared

on Justalk and the good times. There are more times to come!

Thanks to my incredibly patient wife Amani, for all her efforts throughout my long

working sessions over the last years, running the house and taking care of our two precious

kids; Abdul Rahman and Abdul Samad, during which, she was doing a full-time job. That was

incredible work and unbelievable support Amani. A special thanks to dear Abdul Rahman and

Abdul Samad, for always making me smile, and for entertaining and distracting me. Thank

you, naughty boys, for motivating me to keep reaching for excellence. Thank you for

everything that you are, and everything you will become.

Study Sponsor, Petroleum Development Oman (PDO), oil and gas company, for the

support and trust during the study period.


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Abstract

The adverse effects on the environment that are associated with the use of fossil fuels

has promoted the research and adoption of alternative fuels such as wind and solar. The high

costs associated with the purchase of fossil fuels have been another factor that has led to

increased use of solar energy in the past decade. This project aims to design and build solar

thermal air heating system for education purpose to teach engineering students as well as for

researchers at Murdoch University. This will help to create a greater understanding of the

different aspects of the solar air-heating system operation.

A mathematical model was created in excel spreadsheets so that the solar air-heating

system could be tested so as get an idea of what the actual system would produce regarding

maximum temperature and energy. From the results that were generated, it was observed

that the maximum temperature varied depending on the period of the year. It was also

observed that changing the configurations of the system resulted in different outlet

temperatures. For example inclusion of the recycle effect in collector led to a significant

increase in the outlet temperature of the collector; the temperature increased from 290C to

370C. Incorporation of the recycle effect in the oven resulted in even higher temperature

gains, with the temperature rising to 51%. The percentage of the recycle effect was also

observed to affect the outlet temperature, increasing the recycle percentage from 30% to

50% led to higher temperatures in all the tested cases.


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Table of Contents

Declaration ............................................................................................................................................... i

Acknowledgement .................................................................................................................................. ii

Abstract .................................................................................................................................................. iv

LIST OF TABLES ........................................................................................................................................ x

1 Chapter 1 Introduction ................................................................................................................... 1

1.1 Introduction ............................................................................................................................ 1

1.2 Aim and Objectives ................................................................................................................. 2

1.3 Significance of the Project ...................................................................................................... 3

2 Chapter 2: Literature Review .......................................................................................................... 4

2.1 Solar energy ............................................................................................................................ 4

2.1.1 Direct Methods ............................................................................................................... 5

2.1.2 Indirect Methods ............................................................................................................. 6

2.2 Solar Thermal .......................................................................................................................... 8

2.2.1 Concentrating collectors ................................................................................................. 8

2.2.2 Flat Plate Collector .......................................................................................................... 9

2.3 Solar thermal air heating ...................................................................................................... 12

2.3.1 Components of SAH ...................................................................................................... 12

2.3.2 Classification of Solar Air Heaters ................................................................................. 14

2.4 Solar thermal air heating systems......................................................................................... 16

2.4.1 Design of the solar air heating system .......................................................................... 17

2.4.2 Material selection and Fabrication ............................................................................... 18

2.4.3 Methods of testing the system ..................................................................................... 21

2.4.4 Solar incident radiation intensity enhancement on the collector ................................ 26

2.4.5 Reduction of thermal losses.......................................................................................... 27

2.5 The Recycle Method ............................................................................................................. 34

3 Chapter 3: Design of the Solar Thermal Heater ............................................................................ 41

3.1 Design of the system ............................................................................................................. 41

3.2 Solar Thermal Air Heating System Mathematical Modelling ................................................ 44

3.2.1 Stage 1: Collector balance: ............................................................................................ 45

3.2.2 Stage 2: Collector balance with recycle: ....................................................................... 46

3.2.3 Stage 3: Split point balance collector recycle ............................................................... 46


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3.2.4 Stage 4: Oven balance: .................................................................................................. 47

3.2.5 Stage 5: Oven balance with recycle: ............................................................................. 47

3.2.6 Stage 6: Split point balance for oven recycle: ............................................................... 48

3.2.7 Stage 7: Collector with oven outlet balance with recycle:............................................ 48

3.3 Components: ......................................................................................................................... 49

3.3.1 Glass selection ............................................................................................................... 49

3.3.2 Valves ............................................................................................................................ 49

3.3.3 Brush type Motor with fan blade .................................................................................. 50

3.3.4 Instruments/temperature sensor (TC, Maxim SPI interface) ....................................... 51

3.4 Arduino ................................................................................................................................. 52

3.5 LabVIEW: ............................................................................................................................... 52

4 Chapter 4: Testing of the Solar Thermal Air Heating System ....................................................... 53

5 Chapter 5: Result and Discussion .................................................................................................. 55

6 Chapter 6: Conclusion ................................................................................................................. 101

7 Recommendation ........................................................................................................................ 102

8 References .................................................................................................................................. 104

Appendices: ......................................................................................................................................... 109

List of Figures:

Figure 1 Solar Energy Use [2] .................................................................................................................. 4

Figure 2 Illustration of OTEC and transmission of power to the land [6] ............................................... 7

Figure 3 Operation of OTEC [5] ............................................................................................................... 7

Figure 4 Concentrating collector Operation [8] ...................................................................................... 9

Figure 5 Flat Plate solar collector components [10] ............................................................................. 10

Figure 6 Illustration of Flow over the absorber [13] ............................................................................. 12

Figure 7 Non-porous SAH Design I [14] ................................................................................................. 14

Figure 8 Non-porous SAH Design II [14] ................................................................................................ 14

Figure 9 Non-porous SAH Design III [14] .............................................................................................. 15

Figure 10 Honeycomb porous matrix configuration of the SAH [14] ................................................... 16

Figure 11 Overlapped porous matrix configuration of the SAH [14] .................................................... 16

Figure 12 Variation in the efficiency of the collector with ca hange in ∆T⁄I [19] ................................. 23

Figure 13 Efficiency curve for an FPC with double glazing and selective absorber [19] ....................... 24

Figure 14 Geometry applied in the CFD study [26] ............................................................................... 28

Figure 15 Temperature and Velocity distribution contours respectively [26] ...................................... 28

Figure 16 A comparison of the smooth and Roughened Duct of the SAH [27] .................................... 30

file:///C:/Users/oman/Downloads/Sultan%20Yahya%20Juma%20Al%20Habsi%20thesis%20Design,%20Build%20and%20Test%20of%20Solar%20Thermal%20Air%20Heating%20System%20with%20Application.docx%23_Toc521504127file:///C:/Users/oman/Downloads/Sultan%20Yahya%20Juma%20Al%20Habsi%20thesis%20Design,%20Build%20and%20Test%20of%20Solar%20Thermal%20Air%20Heating%20System%20with%20Application.docx%23_Toc521504133file:///C:/Users/oman/Downloads/Sultan%20Yahya%20Juma%20Al%20Habsi%20thesis%20Design,%20Build%20and%20Test%20of%20Solar%20Thermal%20Air%20Heating%20System%20with%20Application.docx%23_Toc521504134file:///C:/Users/oman/Downloads/Sultan%20Yahya%20Juma%20Al%20Habsi%20thesis%20Design,%20Build%20and%20Test%20of%20Solar%20Thermal%20Air%20Heating%20System%20with%20Application.docx%23_Toc521504135file:///C:/Users/oman/Downloads/Sultan%20Yahya%20Juma%20Al%20Habsi%20thesis%20Design,%20Build%20and%20Test%20of%20Solar%20Thermal%20Air%20Heating%20System%20with%20Application.docx%23_Toc521504136file:///C:/Users/oman/Downloads/Sultan%20Yahya%20Juma%20Al%20Habsi%20thesis%20Design,%20Build%20and%20Test%20of%20Solar%20Thermal%20Air%20Heating%20System%20with%20Application.docx%23_Toc521504137


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Figure 17 Structure of the absorber plate with the V-down ribs placed at a 600 angle [12] ............... 31

Figure 18 Thermal efficiency against thermal parameter (T_i-T_a)/I (Km^2 W^(-1)) [12] ................... 32

Figure 19 Dimensions and Geometry used in the CFD study [29] ........................................................ 32

Figure 20 Turbulence and Temperature Contours for the roughened V-ribs [29] ............................... 33

Figure 21 Single flow double pass SAH with recycling .......................................................................... 34

Figure 22 Design of the Upward type SAH with internal recycle [30] .................................................. 36

Figure 23 Efficiency of the collector at I_o=830w/m^2 and T_fi=288 [30] .......................................... 36

Figure 24 Efficiency of the collector at I_o=830w/m^2 and T_fi=298 [30] .......................................... 37

Figure 25 Outlet Temperature at I_o=830w/m^2 and T_fi=288 [30] ................................................... 37

Figure 26 Outlet Temperature at I_o=830w/m^2 and T_fi=298 [30] ................................................... 37

Figure 27 Set-up of the double-pass double pass solar air collector with external recycle [31] .......... 39

Figure 28 Efficiency of the collector at I_o=830w/m^2 [31] ................................................................ 39

Figure 29 Design of Solar Thermal Air Heating System with Application (Cooker/Heat Storage) ........ 41

Figure 30 Design of the Solar Thermal Collector .................................................................................. 42

Figure 31 Design of the oven (Application box) .................................................................................... 43

Figure 32 Mathematical model flow diagram of the Solar Thermal Air Heating System with

Application box ..................................................................................................................................... 44

Figure 33 Butterfly Valve with Servo positioner ................................................................................... 50

Figure 34 Design of motor and fan mount and components of the brush type motor [34] ................ 51

Figure 35 Ambient Temperature (C) and Outlet Collector Temperature To,c(C) vs Time (months)

(Monthly Average 2017 30% Recycle) .................................................................................................. 55

Figure 36 Ambient Temperature (C) and Outlet Collector Temperature To,c(C) vs Time (Maximum

Radiation 2018 30% Recycle) ................................................................................................................ 55

Figure 37 Ambient Temperature (C) and Outlet Collector Temperature To,c(C) vs Time (15 hrs)

(Maximum Radiation day 2018 with doubling the collector size) ........................................................ 56

Figure 38 Ambient Temperature (C) and Outlet Collector Temperature To,c(C) vs Time (15 hrs) (

Coldest day 2017 with 30% Recycle) .................................................................................................... 56


Coldest day 2017 with 50% Recycle) .................................................................................................... 57


Coldest day 2017 with doubling the collector size) .............................................................................. 57


Hottest day 2018 with 30% Recycle) .................................................................................................... 58


Hottest day 2018 with 50% Recycle) .................................................................................................... 58


Hottest day 2018 with doubling the collector size) .............................................................................. 59

Figure 44 Collector Energy (kWh) vs Time (Months) (Monthly Average 2017 with 30% Recycle) ....... 60


(Monthly Average 2017 30% Recycle) .................................................................................................. 61


(Monthly Average 2017 with 50% Recycle) .......................................................................................... 61

file:///C:/Users/oman/Downloads/Sultan%20Yahya%20Juma%20Al%20Habsi%20thesis%20Design,%20Build%20and%20Test%20of%20Solar%20Thermal%20Air%20Heating%20System%20with%20Application.docx%23_Toc521504159file:///C:/Users/oman/Downloads/Sultan%20Yahya%20Juma%20Al%20Habsi%20thesis%20Design,%20Build%20and%20Test%20of%20Solar%20Thermal%20Air%20Heating%20System%20with%20Application.docx%23_Toc521504160


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(Monthly Average with doubling the collector size) ............................................................................. 62

Figure 48 Collector Temperature (C) and Collector Temperature with recycle (C) vs Time (15 hrs)

(Maximum Radiation 2018 with 30% Recycle) ..................................................................................... 62


(Maximum Radiation 2018 with doubling the collector size) ............................................................... 63


(Coldest day 2017 with 30% Recycle) ................................................................................................... 63


(Coldest day 2017 with 50% Recycle) ................................................................................................... 64


(Coldest day 2017 with doubling the collector size) ............................................................................. 64


(Hottest day 2018 with 30% Recycle) ................................................................................................... 65


(Hottest day 2018 with 50% Recycle) ................................................................................................... 65


(Hottest day 2018 with doubling the collector size) ............................................................................. 66



Figure 58 Collector Energy (kWh) vs Time (Months) (Monthly Average 2017 with doubling the

collector size) ........................................................................................................................................ 68

Figure 59 Collector Energy (kWh) vs Time (15 hrs) (Maximum Radiation 2018 30% Recycle) ............. 68

Figure 60 Collector Energy (kWh) vs Time (15 hrs) (Maximum Radiation day 2018 with doubling the

collector size) ........................................................................................................................................ 69

Figure 61 Collector Energy (kWh) vs Time (hrs) (Coldest day 2017 with 30% Recycle)........................ 69

Figure 62 Collector Energy (kWh) vs Time (hrs) (Coldest day 2017 with 50% Recycle)........................ 70

Figure 63 Collector Energy (kWh) vs Time (hrs) (Coldest day 2017 with doubling the collector size) . 70

Figure 64 Collector Energy (kWh) vs Time (15 hrs) (Hottest day 2018 with 30% Recycle) ................... 71

Figure 65 Collector Energy (kWh) vs Time (15 hrs) (Hottest day 2018 with 50% Recycle) ................... 71

Figure 66 Collector Energy (kWh) vs Time (hrs) (Hottest day 2018 with doubling the collector size) . 72

Figure 67 Oven Temperature (C) and Outlet Oven Temperature To,o(C) vs Time (months) (Monthly

Average 2017 with 30% Recycle) .......................................................................................................... 73


Average 2017 with 50% Recycle) .......................................................................................................... 73


Average with 50% Recycle) ................................................................................................................... 74

Figure 70 Oven Temperature (C) and Outlet Oven Temperature To,o(C) vs Time (15 hrs) (Maximum

Radiation day 2018 with 30% Recycle) ................................................................................................. 74


Radiation day 2018 with doubling the collector size) ........................................................................... 75

Figure 72 Oven Temperature (C) and Outlet Oven Temperature To,o(C) vs Time (15 hrs) (Coldest day

2017 with 30% Recycle) ........................................................................................................................ 75


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2017 with 50% Recycle) ........................................................................................................................ 76


2017 with doubling the collector size) .................................................................................................. 76

Figure 75 Oven Temperature (C) and Outlet Oven Temperature To,o(C) vs Time (15 hrs) (Hottest day

2018 with 30% Recycle) ........................................................................................................................ 77


2018 with 50% Recycle) ........................................................................................................................ 77



Figure 78 Oven Temperature (C) and Oven Temperature with recycle (C) vs Time (months) (Monthly

Average 2017 30% Recycle) .................................................................................................................. 79


Average 2017 50% Recycle) .................................................................................................................. 80


Average 2017 with doubling the collector size) .................................................................................... 80


Radiation day 2018 with 30% Recycle) ................................................................................................. 81


Radiation day 2018 with doubling the collector size) ........................................................................... 81

Figure 83 Oven Temperature (C) and Outlet Oven Temperature To,o(C) vs Time (15 hrs) (Coldest

day 2017 with 30% Recycle) ................................................................................................................. 82


2017 with 30% Recycle) ........................................................................................................................ 82




2018 with 30% Recycle) ........................................................................................................................ 83


2018 with 50% Recycle) ........................................................................................................................ 84



Figure 89 Oven Energy (kWh) vs Time (Months) (Monthly Average 2017 with 30% Recycle .............. 86

Figure 90 Oven Energy (kWh) vs Time (Months) (Monthly Average 2017 with 50% Recycle .............. 86

Figure 91 Oven Energy (kWh) vs Time (Months) (Monthly Average 2017 with doubling the collector

size) ....................................................................................................................................................... 87

Figure 92 Oven Energy (kWh) vs Time (15 hrs) (Maximum Radiation day 2018 with 30% Recycle) .... 87

Figure 93 Overall system recycle temperature oven to collector vs temperature oven to a collector

after oven recycle (Maximum Radiation 50% Recycle) ........................................................................ 88

Figure 94 Oven Energy (kWh) vs Time (15 hrs) (Maximum Radiation day 2018 with doubling the

collector size) ........................................................................................................................................ 88

Figure 95 Oven Energy (kWh) vs Time (15 hrs) (Coldest day 2017 with 30% Recycle) ......................... 89

Figure 96 Oven Energy (kWh) vs Time (15 hrs) (Coldest day 2017 with 50% Recycle) ......................... 89


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Figure 97 Oven Energy (kWh) vs Time (15 hrs) (Coldest day 2017 with doubling the collector size) .. 90

Figure 98 Oven Energy (kWh) vs Time (15 hrs) (Hottest day 2018 with 30% Recycle) ......................... 90

Figure 99 Oven Energy (kWh) vs Time (15 hrs) (Hottest day 2018 with 50% Recycle) ......................... 91

Figure 100 Oven Energy (kWh) vs Time (15 hrs) (Hottest day 2018 with doubling the collector size) 91

Figure 101 Overall system recycle temperature oven to the collector and temperature oven to a

collector after oven recycle vs Time (months) (Monthly Average with 30% Recycle) ......................... 92


collector after oven recycle vs Time (months) (Monthly Average 2017 with 50% Recycle) ................ 93


collector after oven recycle vs Time (months) (Monthly Average 2017 with doubling the collector

size) ....................................................................................................................................................... 93

Figure 104 Overall system recycle temperature oven to collector and temperature oven to a

collector after oven recycle vs Time (months) (Maximum Radiation day 2018 with 30% Recycle) ..... 94


collector after oven recycle vs Time (months) (Maximum Radiation day 2018 with 50% Recycle) ..... 94


collector after oven recycle vs Time (months) (Maximum Radiation day 2018 with doubling the

collector size) ........................................................................................................................................ 95


collector after oven recycle vs Time (months) (Coldest day 2017 with 30% Recycle) ......................... 95


collector after oven recycle vs Time (months) (Coldest day 2017 with 50% Recycle) ......................... 96


collector after oven recycle vs Time (months) (Coldest day 2017 with doubling the collector size) ... 96


collector after oven recycle vs Time (months) (Hottest day 2018 with 30% Recycle) ......................... 97


collector after oven recycle vs Time (months) (Hottest day 2018 with 50% Recycle) ......................... 97


collector after oven recycle vs Time (months) (Hottest day 2018 with doubling the collector size) ... 98

Figure 113 Energy of overall system recycle oven to collector and Energy of overall system recycle

oven to a collector after oven recycle vs Time (months) (Monthly Average 2017 with 30% Recycle) 98


oven to a collector after oven recycle vs Time (months) (Monthly Average 2017 with 50% Recycle) 99


oven to a collector after oven recycle vs Time (months) (Monthly Average 2017 with doubling the

collector size) ........................................................................................................................................ 99

Figure 116 Solar Thermal Air Heating System Demo .......................................................................... 102

LIST OF TABLES

Table 1. The absorbance of different colours [17]................................................................................ 19

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Table 2. The transmittance of different materials [17] ........................................................................ 21

Table 3. Conditions under which SAH are tested [19]. ......................................................................... 25

Table 4. The thermal efficiency of the SAH at varying Reynolds number [24]. .................................... 31

Table 5 Dimensions and Geometry used in the CFD study [29] ........................................................... 33

Table 6. Data illustrating a comparison of the internal and external recycle methods [26]. ............... 38

Table 7. Comparison of the theoretical and experimental results for SAH with and without recycle

(R=0) [27]. ............................................................................................................................................. 40


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Abbreviations

SAH-Solar Air Heater

FPC-Flat Plate Collector

OTEC- Ocean Thermal Energy Conversion

Nomenclature

𝑸𝒖-Useful Energy Gain

𝑨-Area

𝝉-Transmittance of cover

𝑼𝑳-Plate to ambient loss coefficient

𝑻𝒑-average plate temperature

𝑻𝒂-Ambient temperature

𝑰-Total Irradiation

𝑻𝒇𝒊-fluid temperature at Inlet

𝑻𝒇𝒐-Fluid Temperature at outlet

𝒎-mass of collector and its contents

�̇�-mass low rate of the fluid

𝜶𝒓-Absorptance of receiver plate


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1 Chapter 1 Introduction

1.1 Introduction

The use of solar energy in thermal applications is described as one of the oldest methods

of energy transformation. The use of solar thermal energy continues to grow in popularity;

this has been prompted by the increasing attention due to climate change and BHGs. The

decline in fossil fuel resources coupled with the skyrocketing prices of these types of fuel has

led to the need to adopt alternative fuel sources. Another factor that has necessitated the

search and adoption of other sources of fuel is the desire to reduce greenhouse gas emissions

by minimising the number of fossil fuels that are utilised. In comparison to conventional

power sources such as hydropower, solar power is more expensive. However, through

continued research in the field of solar power, the overall cost of installing solar power plants

has continued to reduce. Batteries with higher efficiency in the storage of power have been

produced in recent years hence improving the overall performance of solar power plants

Energy from the sun is usually harnessed using devices referred to as solar collectors. The

different types of solar collectors that are typically used include evacuated tube collectors,

flat plate collectors and unglazed plastic collectors. A comparison of the different types of

collectors shows that the flat plate collector results in the lowest outlet temperature.

However the flat plate collector is preferred over the other collectors, this is because;

i. The design of the FPC is more straightforward and construction easy.

ii. The FPC is associated with low maintenance requirements.

iii. The materials required for the construction of the FPC are cheap.

iv. The overall cost of the flat plate collector is low due to the cheaper material, simple

design and low maintenance.


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Flat plate collectors can either be active or passive. In the active system, the fluid medium

is transported through the system using a pump while the passive system, on the other hand,

relies on gravity to move the fluid through the system

i. The efficiency at which the FPC operates depends on;

ii. The material used in the production of the absorber plate.

iii. The velocity of airflow in the collector.

iv. Thermo-hydraulic efficiency.

v. Introduction of roughness/turbulence in the collector using external bodies.

One of the main problems that are associated with the flat plate collector is the low rate

of heat transfer from the absorber to the air flowing in the collector. To overcome this

problem several design features have been incorporated into the flat plate collector, some of

these features include;

i. Use of alternate Medium or vacuum in the gas space.

ii. Enhancement of the convective heat transfer coefficient by modifying low passages.

iii. The inclusion of fins in the structure of the Solar Air Heater (SAH).

iv. Use of artificial roughness on the absorber plate.

1.2 Aim and Objectives

a) Design and build solar thermal air heating system for education purpose to teach

engineering students as well as for researchers at Murdoch University.

b) Test the mathematical modelling of the system and get an idea of what the actual

system would produce.

c) Produce seasonal profile of the system using the mathematical modelling, to get an

idea of different use of the system according to the season.


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d) Test the actual solar thermal air heating system:

i) System open loop.

ii) The effect of recycling.

iii) The effect of the roughness of an additional plate.

e) Calculate the system temperature produced by the system.

f) Calculate the energy produced by the system.

g) Calculate the efficiency.

h) Compare the results between the mathematical modelling and the actual system data.

1.3 Significance of the Project

The adverse effects on the environment that are associated with the use of fossil fuels

has promoted the research and adoption of alternative fuels such as wind and solar. The high

prices associated with the purchase of fossil fuels have been another factor that has led to

increased use of solar energy in the past decade. Some of the factors that have made solar

energy a better option as compared to other sources of alternative energy are that it is

available readily and it has no adverse effects on the environment (it is one of the cleanest

sources of energy in the world).

This project seeks to create a solar thermal air heating system for education purpose to

teach engineering students as well as for researchers at Murdoch University. Through this

system, varying factors associated with the operation of the solar air system such as the effect

of recycling will be better understood. The effect of including different design features such

as triangular or V-shaped roughness will be evaluated using this system. Thus the system will

help to create an overall better understanding of the operation of the solar thermal air system

and therefore enable improvements to be made to the current system.


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2 Chapter 2: Literature Review

2.1 Solar energy

Solar energy is described as a renewable source of energy that is readily available and

does not expose the environment to any form of degradation. Despite the vast availability of

solar energy, there are various problems associated with this source of energy. Solar energy

is described as a dilute source of energy. Even in the hottest regions on the planet, solar flux

we rarely exceed 1kW/m2 with the total radiation over a day being 7kWh/m2 [1]. Therefore

for large amounts of solar energy to be obtained a large collecting area will be required.

Installation of a large collecting area will lead to an increase in the costs involved. The other

problem is associated with the availability of solar energy. The intensity of the sun varies at

different times of the day due to the day-night cycle. In addition to this, the intensity of the

sun will vary seasonally (because of the weather) based on location. This will require the

introduction of an energy storage system which will ensure energy is available when direct

solar energy is not available. This results in a significant increase in the total cost of energy

[2].

Energy from the sun can be utilised directly or indirectly as illustrated in Figure 1.

Utilization of Solar Energy

Direct Methods Indirect Methods

Thermal Photovoltaic Power Wind Biomass Wave

energy

OTEC

Figure 1 Solar Energy Use [2]


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2.1.1 Direct Methods

Direct methods of harnessing solar power are described as those techniques where

solar power undergoes only one change for it to be usable. For example, the conversion of

solar power into electricity is a form of direct solar power [3].

2.1.1.1 Thermal

In this method, a collection device is incorporated into this system. This device is

exposed to the solar radiation directly. The collection device can either be of the absorbing

or concentrating type. The absorbing type is designed with a dark surface that is exposed to

the sun. This surface absorbs solar radiation that is transferred to the fluid (either water or

air) which is in contact with the absorber. The concentrating type is designed in such a way

that the radiation from the sun is concentrated to a focal point and the heat energy

transferred to the fluid [4].

2.1.1.2 Electrical

This is the method where devices that allow for photovoltaic conversion are used.

Solar cells are usually applied to obtain electrical power. When radiation from the sun falls on

the semiconductor device, it is converted directly into direct-current electricity [4]. The

operation of the solar cell can be described in two steps;

i. The creation of positive and negative charge pairs in the solar cell by the solar

radiation that has been absorbed by the cell.

ii. The separation of the positive and negative charges created in the solar cell by a

potential gradient.

For the creation of the charges to occur, the material used in the production of the

solar cell’s absorber must be able to absorb the energy that is associated with the photons of


6 | P a g e

light. Such materials include semiconductors such as silicon, cadmium sulphide, copper

indium sulphide, gallium arsenide among others [4]. With improving technology, the

efficiency of solar cells has improved over the years, with further advancements being

expected in the future. One of the significant advantages of solar cells is that they do not have

any moving parts which mean that little maintenance is required [4].

2.1.2 Indirect Methods

Indirect methods are described as the techniques where the energy from the sun will

undergo more than one change so that it can be used. Indirect methods of harnessing the

solar power are possible due to the effect of the sun on the surroundings. Examples of indirect

solar power include wind energy, wave energy, OTEC among others [3].

2.1.2.1 Wind energy

Winds result from the absorption of solar energy on the surface of the earth and in

the atmosphere and effect of the earth’s rotation about its axis and revolution around the

sun. As a result of this, alternate heating and cooling cycles are created foremost to a pressure

difference that forces air to move. Wind energy is harnessed using wind turbines [4].

2.1.2.2 Wave Energy

Waves result from the interaction between the winds and the surface of the oceans.

Hence it can be said that solar energy is applied indirectly in its creation. Energy from waves

can be harnessed through the application of the oscillating water column system [4].

2.1.2.3 Ocean Thermal Energy Conversion (OTEC)

These are systems that use the difference in temperature between the water at the

surface and that which is found in the lower levels of the ocean. The difference is usually

around 200C and covers a few hundred meters. The temperature of the water at the surface


7 | P a g e

is typically reasonably constant for the first few meters due to mixing. It, however, starts

decreasing towards the lower levels. Heating of the water at the surface occurs due to solar

energy [4].

Deep cold seawater and surface water are continuously pumped to create a power

cycle that leads to the generation of electricity [5]. Figures 2 and 3 diagrammatically illustrate

the operation of the OTEC method.

Figure 2 Illustration of OTEC and transmission of power to the land [6]

Figure 3 Operation of OTEC [5]


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2.2 Solar Thermal

Thermal energy is absorbed from the sun through the use of devices referred to as

collectors. Collectors are described as particular kind of heat exchangers that enable the

collection of solar radiant energy which is then transferred to the fluid (either water or air).

The collector converts the solar radiant energy to thermal energy which is then used for

heating [1]. Solar collectors are either categorised as (i) concentrating or (ii) flat plate

collectors.

2.2.1 Concentrating collectors

This is a device that is used in the collection of solar energy with high intensity of solar

radiation on the absorbing surface. This type of collector introduces a reflecting (or refracting

surface) surface between the absorber and the radiation from the sun. This type of device is

typically applied in the generation of high temperatures that are required in industrial

applications. A concentrating collector is composed of a parabolic reflector and cylindrical

receiver although in some cases a spherical collector may be applied. A parabolic reflector

enables high temperatures to be achieved, however, for the high temperatures to be

achieved throughout the day; the use of a tracking system to steer the reflector will be

required. The tracking system ensures that the reflector always faces the sun with the receiver

being at its focus [6].

Concentration ratios of as low as 1.5 or 2 to values as high as 10000 can be used for

these types of thermal collectors, using a high ratio will result in high temperatures being

achieved. Concentrating collectors can however not be used for the diffuse component of

solar radiation; they only work for the beam component [6]. Figure.4 shows the different

components of a focusing/concentrating collector.


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Figure 4 Concentrating collector Operation [8]

2.2.2 Flat Plate Collector

The flat plate solar collector is a device that is applied in the harnessing of power from

the sun. When compared to the other solar harnessing devices such as the evacuated tube

collectors and concentrating collectors, the flat plate collector is found to have the least

efficiency. However, it is preferred over the other devices since it is associated with lower

maintenance and it also has a more straightforward design and is easy to construct. The lower

maintenance requirements and simplicity in design lead to the flat plate collector having the

lowest overall cost [7].

The flat plate collector is mainly composed of a flat surface that has a high level of

solar radiation absorptivity. The flat surface (usually referred to as the absorbing surface) is

usually made of a metal plate and is typically painted black. The energy from the absorber

plate is transferred to the working fluid which circulates in tubes across the surface of the

collector. The rear side of the flat plate collector is usually fitted with an insulator to prevent

the loss of heat. The front, on the other hand, has a transparent cover installed, to allow the

incoming solar radiations but prevent the passage of the infrared radiations from the


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absorber surface. This type of collector is fixed with no solar tracking required. It should be

ensured that the collector is oriented directly in the equator, face north in the southern

hemisphere and face south in the northern hemisphere. For use year-round, the tilt angle

should be equal to the latitude whereas in winter the angle of tilt should be more than the

latitude by approximately 100 or 150. For the summer the tilt angle should be less than the

latitude by approximately 100 or 150. With the application of the late flat collector, a

maximum of approximately 1000C can be attained above the ambient temperature [6]. Flat

plate collectors are associated with the following advantages;

i. They can absorb direct, diffuse as well as reflected components of solar radiation.

ii. Since they are fixed in tilt and oriented towards the sun, there is no need or the

application of solar tracking.

iii. They can be produced quickly, and their cost is low.

iv. They are associated with the low cost of maintenance, and their operational life is

extended.

v. Their efficiency of operation is comparatively high.

Figure 5 Flat Plate solar collector components [10]


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Based on the type of fluid that is applied, this type of collectors may be classified as

liquid heating or air heating.

2.2.2.1 Liquid heating collectors

A flat plate in which water or any other liquid is applied as the working fluid is referred

to as a liquid heating system. One of the most widely used liquid heating collector is the water

heater that is typically installed in residential houses to warm water used in bathrooms and

other applications [8].

A liquid heating system is composed of a metal box that is glass covered containing an

absorber plate on which an array of tubes is attached. The metal box has thermal insulation

introduced at its rear. Liquid is pumped from the storage tank through the tubes, picks up

heat from the absorber plate and then flows back to the storage tank. The flow used can be

thermosyphonic or forced. The collector surface area of most of the collectors that are

available commercially ranges from 1 to 2 m2. The length is typically more significant than the

width [6].

2.2.2.2 Air heating collectors

These are a type of collectors where the working fluid that is applied is air. These types

of collectors are usually applied in space heating and the drying of agricultural products such

as cereals [9].

The construction of air heating systems is quite simple and cheap; this has promoted

their popularity and application in numerous applications such as space heating and crop

drying. A conventional system used in air heating is usually composed of an absorber plate

that has another parallel plate underneath; this creates a high width to depth ratio passage

where air can flow. The radiation from the sun passes through the transparent cover and


12 | P a g e

impinges on the absorber plate that has been blackened. The absorbed heat is then

transferred to the air flowing beneath the absorber plate [6].

2.3 Solar thermal air heating

A typical solar air heater is composed of the following components;

2.3.1 Components of SAH

2.3.1.1 Absorber plate

The absorber plate is expected to have high thermal conductivity, sufficient tensile

and compressive strength and adequate resistance to corrosion. Copper is generally applied

in most applications due to its high levels of conductivity and resistance to corrosion. Other

materials that can be applied in the construction of the absorber plate include aluminium,

steel and galvanised Iron sheets and certain thermoplastics and metal ions [10].

Figure 6 Illustration of Flow over the absorber [13]

2.3.1.2 Cover Plate

The cover plate is an essential part of the SAH and plays the following roles;

i. Transmit solar energy to the absorber plate.

ii. Minimize loss of heat from the absorber plate to the surroundings.

iii. Protect the absorber plate from direct exposure to weathering.


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The materials that are applied in the construction of the cover plate are expected to

have high strength, durability, and non-degradability and be also capable of high transmission

of solar energy. The most common material used in the making of the cover plate is glass.

Tempered glass is preferred since it is more durable than the other types of glass and also has

high resistance to thermal cycling. The mechanical strength of the cover plate should also be

high so that it can be able to resist breakage from the maximum winds that will be expected

and also the snow loads. The mechanical strength of the cover plate is proportional to the

square of the thickness of the glass. The thickness of most cover plates is found to be at least

0.33cm thick [10].

Exposure to thermal shock of the cover plate also has to be considered. Thermal shock

usually results from the heating and cooling that is brought about by variation in the solar

intensity of the collectors during the different times of the day. Cloudy conditions result in

temperature variations of up to 500C or more in a matter of minutes. More heating occurs at

the centre of the cover plate than at the edges; this creates thermal stresses which can result

in breakage of the glass cover [10].

The rigidity of the glass cover is also a significant factor of consideration. The rigidity

of the cover plate is proportional to the cube of the plate’s thickness. Other materials that

can be used as cover plates include acrylic polycarbonate plastics, Tedlar and Mylar plastic

films, Lexan and other commercial plastics. The effect of the UV light, however, reduces the

operational life of the plastic materials [10].

2.3.1.3 Insulation

Insulation is incorporated into the system to minimise the loss of heat from the

absorber plate due to conduction or convection. The absorber plate is insulated at its rear or


14 | P a g e

on its sides depending on the design of absorber plate adopted. Materials that are typically

used are rock wood or glass wool [10].

2.3.2 Classification of Solar Air Heaters

Solar air heaters can be classified into porous and non-porous collectors depending

on the absorber plate design that is utilised.

2.3.2.1 Non-Porous Air Heaters

Several designs of non-porous air heaters may be applied as discussed accordingly;

The design I; - this is the type of air heater where the stream of air flows above or

below the absorber plate as shown in figure 7 below. The problem with this type of

configuration is that since the air flows between the absorber plate and cover plate, the cover

will receive much of the heat which it loses to the surroundings [11].

Design II; - For this design, the flow of air occurs below the absorber plate. A plate that is

parallel to the absorber plate is installed between the absorber and the insulation thus

creating the air passage. Heat loss in this configuration is low [11].

Air In

Absorber

Plate Glass Cover

Air out

Insulation

Absorber

Plate

Air In

Absorber Plate Glass Cover

Air out

Insulation

Figure 7 Non-porous SAH Design I [14]

Figure 8 Non-porous SAH Design II [14]


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Design III; - This is the type of air heater that is cooled on by streams of air flowing on both

sides of the absorber plate. The transfer of heat between the stream of air and the absorber

plate is usually low. This configuration of the SAH is thus associated with very low thermal

efficiency. Unless the application of selective coatings is made, the loss of heat from the

absorber plate is typically significant [11].

Design II is commonly applied in most SAH applications since it is associated with the

least levels of heat loss to the surroundings. In this design inclusion of a plate between the

absorber and the insulation would create a passage with a high aspect ratio. Heat loss to the

surrounding would be reduced significantly in this design. The insulation material on the

lower side of the collector would minimise loss of the heat absorbed by the air from the

absorber plate as it flows below it.

2.3.2.2 Porous Air Heaters

This is the configuration where the absorber plate is porous. This type of configuration

is associated with the following advantages;

i. Radiation from the sun will penetrate to greater depths, and thus more will be

absorbed.

ii. Higher solar radiation penetration will result in lower radiation losses and higher

temperature of the air stream.

Air In

Absorber

Plate Glass Cover

Air

out

Insulation

Figure 9 Non-porous SAH Design III [14]


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iii. The pressure drop will be lower than in the non-porous case.

The type of porous matrix selection must be done correctly as failure to select the ideal

matrix will result in lower thermal efficiencies. The correct matrix porosity and thickness

should be done [12]. Some of the typical matrix configurations that are applied have been

illustrated in figures 10 and 11.

2.4 Solar thermal air heating systems

A solar thermal air heating system is used to absorb heat from the sun and transfer the

heat obtained to air which can then be applied in space heating among other applications.

The efficiency of operation of such a system is obtained by establishing a relationship

between the air at the inlet and that which exists at the outlet [7].

The operation of thermal air systems can be enhanced by incorporating different design

features into the structure of the heating system. Some of the enhancements that can be

made have been discussed accordingly.

Insulation

Air Air

Clear Glass

Cover

Matrix

Air

Air

Honey Comb

Insulation

Cover

Figure 10 Honeycomb porous matrix configuration of the SAH [14]

Figure 11 Overlapped porous matrix configuration of the SAH [14]


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2.4.1 Design of the solar air heating system

The design stage of the Solar Air Heater (SAH) is an essential phase since it is during

this phase that the different factors such as structure and dimensions of the SAH are

determined. During the design stage of the SAH the different factors of consideration can be

categorised as follows;

2.4.1.1 Thermal performance

In relation to this, the different factors of consideration are listed accordingly;

Absorber plate; during the design stage the operating temperature range, maximum stagnant

temperature and costs associated with the absorber plate need to be considered. The

absorber plate selected should have good thermal performance but at the same time should

not put a strain on the available funds [13].

Absorber Coating; the absorber coating should be able to ensure that maximum energy

absorption is achieved with minimal energy being emitted.

Glazing; the primary role of the glazing material is to ensure that heat loss from the absorber

plate through convention and radiation is as low as possible. The glazing allows both short

and long wave [14].

Insulation; the primary role of insulation is to ensure that loss of heat from the

absorber/collector plates lower surface and sides is minimised as possible. The thermal

conductivity of the material used in insulation should be low [15].

2.4.1.2 Costs

The overall cost of the SAH should be kept at a reasonable value. However, the

performance of the SAH should not be compromised on account of keeping the cost low.

Proper planning and research before the purchase of materials should, therefore, be done to


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ensure that unnecessary costs, such as buying a more expensive material when a cheaper

material with relatively the same or better performance exists, are avoided [16].

2.4.1.3 Durability, maintenance and ease of installation

The SAH that is fabricated from the different components should have a high level of

durability. Durability will be dependent on the structural integrity and design of the SAH as

well as on the materials that are used.

Another important factor of consideration is the ease of maintenance and installation.

The SAH fabricated should be easy to install and integrate into any system without requiring

high levels of modification. Maintenance should also be achieved quickly; the different

components that require regular maintenance and servicing should be easily accessible. This

in addition to keeping the installation and maintenance costs low would ensure that the life

of the SAH is extended [16].

2.4.2 Material selection and Fabrication

2.4.2.1 The flat plate absorber material

The absorber is the primary element of the flat plate collector and performs three

main functions;

i. Absorb a maximum irradiance from the sun (should have excellent thermal

conductivity).

ii. Conduct the heat gained from the working fluid (air for SAH) at a minimum difference

in temperature.

iii. Lose as little heat to the surroundings as possible (should have low emissivity).


19 | P a g e

The materials that are usually used in the making of the absorber plates are copper,

aluminium and steel. However, copper and aluminium are favoured due to their high levels

of thermal conductivity.

Absorber plate materials are usually lined with a colour coating to increase the level of

absorptivity of the material. Several materials can be applied to this coating; however, black

is used in most cases due to its high level of absorbance [17]. The different colours and their

absorbance is outlined in Table 1.

Table 1. The absorbance of different colours [17]

Material Colour Absorbance (α)

White 0.07

Fresh Snow 0.13

White enamel 0.35

Green paint 0.50

Red brick 0.55

Grey paint 0.75

Black tar 0.93

Flat black 0.98

Granite 0.55

The materials and coatings that are selected have to be able to withstand prolonged

periods of high temperatures; they should also be cheap and easy to handle.


20 | P a g e

2.4.2.2 The glazing material

The glazing cover is usually installed on the upper part of the SAH. The primary

functions of the glazing cover are;

i. Minimise the radiant and convective heat from the absorber plate to the surroundings

ii. Transmit the incident solar radiation to the absorber plate, it transmits the shorter

wavelength radiation from the sun and blocks longer wavelength reradiating from the

absorber.

iii. Protects the absorber late from the surroundings.

The glass is usually the preferred material since it is associated with a low

transmittance of the longer wavelength while providing up to 90% transmittance to the short-

wave radiations. It does not allow the escape of the longwave radiation emitted from the

surface of the absorber plate. Transparent plastic is also used as a glazing material since it

provides a high short-wave transmittance. However, plastics are associated with relatively

low stability at higher SAH operating temperatures. Also, they are not durable and degrade

easily especially in adverse weather conditions, ultra-violet radiation accelerates their

degradation [17].

However, plastics due to their lower costs as compared to glass would provide a better

option in cases where cost is the critical factor of consideration.

Table 2. Outlines the transmittance values of different materials.


21 | P a g e

Table 2. The transmittance of different materials [17]

Material Transmittance (τ)

Crystal glass 0.91

Window glass 0.85

Acrylate, Plexiglass 0.84

Polycarbonate 0.84

Polyester 0.84

Polyamide 0.80

2.4.2.3 The heat insulation material

Insulation is necessary since it ensures that loss of heat from the back and sides of the

collector is reduced considerably. An increase in the temperature between the SAH and the

outside leads to an increase in heat loss. The insulation material that is selected should be

fireproof, weather resistant and dimensionally stable [18]. Different types of insulation have

been used over the years, some of these include;

i. Plywood

ii. Glass wool

iii. Polystyrene

The material used as an insulator is usually determined by its costs and the level of

insulation provided. For example polystyrene in addition to being cheap is readily available.

2.4.3 Methods of testing the system

Testing is usually done to evaluate the performance of the SAH. Different standards

have been developed to make it possible for results to be comparable. Tests are usually


22 | P a g e

carried out to determine the overall efficiency of the SAH and its heat loss coefficient. The

following tests methods are usually used;

2.4.3.1 The NBS method

NBS method was developed by the United States Standard Bureau. The use of this

method is based on equation (1);

𝜂 =𝑄𝑢 𝐴⁄

𝐼= (𝜏𝛼𝑟)𝑐 − 𝑈𝐿 (

𝑇𝑝 − 𝑇𝑎

𝐼) … … … (𝑒𝑞. 1)

𝑇𝑝 Is used to identify the operating temperature of the collector, in order to use the

temperature of the working fluid flowing through the collector in the equation a correction

factor denoted by 𝐹´is introduced [19].

Where;

𝐹´ =𝐴𝑐𝑡𝑢𝑎𝑙 𝑢𝑠𝑒𝑓𝑢𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑒𝑑

𝑈𝑠𝑒𝑓𝑢𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑒𝑑 𝑖𝑓 𝑡ℎ𝑒 𝑒𝑛𝑡𝑖𝑟𝑒 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑜𝑟 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑤𝑒𝑟𝑒 𝑎𝑡 𝑡ℎ𝑒 𝑒𝑣𝑒𝑟𝑎𝑔𝑒 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒

The factor 𝐹´ is also referred to as the collector effeciency factor. Equation (1)

becomes (2) when this factor is introduced.


𝐼= 𝐹´ [(𝜏𝛼𝑟)𝑐 − 𝑈𝐿 (

𝑇𝑚 − 𝑇𝑎𝐼

)] … … … (𝑒𝑞. 2)

Where;

𝑇𝑚 = 𝑇𝑓𝑖 + (𝑇𝑓𝑜 − 𝑇𝑓𝑖)/2

The quantities (𝑄𝑢 𝐴⁄ ) 𝐼⁄ and (𝑇𝑚 − 𝑇𝑎) 𝐼⁄ above are btained through experiments.

When the effeciency 𝜂 is ploted against (𝑇𝑚 − 𝑇𝑎) 𝐼⁄ base on equation 2 above, the slope of


23 | P a g e

the line will be a function of 𝑈𝐿 and the intercept (𝑇𝑚 − 𝑇𝑎) 𝐼⁄ = 0 will be some function of

(𝜏𝛼𝑟)𝑐 as illustrated in Figure 12.

Figure 12 Variation in the efficiency of the collector with ca hange in ∆T⁄I [19]

2.4.3.2 The ASHRAE Method

ASHRAE is stand for American Society of Heating, Refrigeration, and Air Conditioning

Engineering. This is a standard is testing for heat, ventilation and air conditioning. Based on

equation (1) it has been established that the performance of the flat plate collector can be

determined, stonable more detailed results/data on the performance of the flat plate

collector a parameter 𝐹𝑅 is introduced. The introduction of this factor alleviates the need to

determine the mean temperature of the collector plate [19].

Where;

𝐹𝑅 =𝐴𝑐𝑡𝑢𝑎𝑙 𝑢𝑠𝑒𝑓𝑢𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑒𝑑

𝑈𝑠𝑒𝑓𝑢𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑒𝑑 𝑖𝑓 𝑡ℎ𝑒 𝑒𝑛𝑡𝑖𝑟𝑒 𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑟 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑤𝑒𝑟𝑒 𝑎𝑡 𝑡ℎ𝑒 𝑖𝑛𝑙𝑒𝑡 𝑓𝑙𝑢𝑖𝑑 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒

When this factor also known as the heat removal factor is introduced into equation

(1) it becomes equation (3).


𝐼= 𝐹𝑅(𝜏𝛼𝑟)𝑐 −

𝑈𝐿𝐼

(𝑇𝑓𝑖 − 𝑇𝑎

𝐼) … … … (𝑒𝑞. 3)


24 | P a g e

Where;

𝑄𝑢 = 𝑚𝐶𝑝(𝑇𝑓𝑜 − 𝑇𝑓𝑖)

In equation (3) 𝑄𝑢, A,I and 𝑇𝑓𝑖 − 𝑇𝑎 are obtained from experiments. Based on this

equation ,plotting the effeceincy against (𝑇𝑓𝑖 − 𝑇𝑎) 𝐼⁄ a straight line is obtained. The slope

of the line is found to be equal to 𝐹𝑅𝑈𝐿 and he y intercept will be eequal to 𝐹𝑅(𝜏𝛼)𝑐.as shown

in Figure 13 a straight line fit may not be always sufficient for some flat plate collectors in

which case a polynomial fit is required due to change of 𝑈𝐿 with the temperature of the plate

[19].

Figure 13 Efficiency curve for an FPC with double glazing and selective absorber [19]

2.4.3.3 CEC Method

CEC method is similar to the NBS and ASHRAE methods; however, some additional

recommendations are included to take into account the weather conditions in Europe. In this

method, the temperature of the plate 𝑇𝑝 is replaced by 𝑇𝑚.

Where;

𝑇𝑚 = 𝑇𝑓𝑖 +Δ𝑇

2… . 𝑎𝑛𝑑 Δ𝑇 = ( 𝑇𝑓𝑜 − 𝑇𝑓𝑖)

Therefore, the efficiency curve can be derived from equation (4)


25 | P a g e

𝜂 = 𝜂𝑜 − 𝑈𝑚(𝑇𝑚 − 𝑇𝑎)

𝐼… … … (𝑒𝑞. 4)

Where;

𝜂𝑜 = 𝐹´(𝜏𝛼)𝑐 𝑎𝑛𝑑 𝑈𝑚 = 𝐹

´𝑈𝐿

The efficiency of the FPC can be evaluated as a function of (𝑇𝑚 − 𝑇𝑎) 𝐼⁄ by measuring

the rate of flow though the collector. This is done by calculating the difference in temperature

between the inlet and outlet of the collector, the incident solar radiation and the ambient air

temperature [19]. This can be written as shown in equation (5)

𝜂 =�̇�𝐶𝑝Δ𝑇

𝐴𝐼… … … (𝑒𝑞. 5)

Where;

𝑇𝑚 = 𝑇𝑎, 𝜂𝑜 =�̇�𝐶𝑝Δ𝑇

𝐴𝐼

Equation (5) however does not take into account the thermal capacity effect noted as

𝑀𝐶𝑝 and which may have a considerable efect over the integrated performnace of the FPC.

Table 3. Conditions under which SAH are tested [19].

Parameters standards

NBS ASHRAE CEC

Ambient temp, Ta ° C < 30 < 30 As read

Insolation, I W/m2 > 630 > 315 > 600

Flow rate, m’ Kg/s.m2 0.02 0.02 0.02


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Range of fluid inlet

temprature Tfi ° C

= Ta + 10, 30, 50, 70 = 10, 30, 50, 70% of

stagnation

temprature

= Ta or less to

stagnation

temprature

Incident angle of

beam radiation,

degree

< 45 < 45 < 45

Pre-conditioning

period

A 30 min.

Pre-conditioning

period to allow

equilibrium to be

attained

30 min 30 min

2.4.4 Solar incident radiation intensity enhancement on the collector

The flat plate collector’s performance can be enhanced significantly through the

application of reflectors, this help to increase the total area that is exposed to the sun thus

providing a greater collection area. A concentration ratio up to 4 and a temperature of up to

1800C can be attained by the incorporation of reflective surfaces on the flat plate solar air

heater.

A study carried out by [20] & [21]proposes the application of concentrating and side

mirrors in flat plate solar collectors to increase the amount of radiation that falls on the

collector. The overall effect of this will be higher temperatures at the absorber plate. From

the studies carried out it was observed that the output performance of the collectors was

improved with temperatures above 1000C being achieved [11].


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2.4.5 Reduction of thermal losses

Thermal losses can be minimised by including the following design features into the SAH

2.4.5.1 Use of alternate Medium or vacuum in the gas space

Losses that are convective in nature can be reduced by optimising the gap space and

using an alternate medium between the two covers of the SAH. Work carried out by [20] has

shown that the use of heavy gases as the other medium can help minimise the losses of heat

by up to 34%. The use of a combination of moderate vacuum and a selective surface can help

increase the energy that is collected on a daily basis; it also makes it possible for the collector

to be operated at 1500C with a collection efficiency of up to 40% being achieved [11].

2.4.5.2 Enhancement of the convective heat transfer coefficient by modifying low

passages

Cases of low heat transfer rate from the absorber plate to the air flowing through the

ducts lead to a relatively higher temperature of the absorber plate leading to higher thermal

losses. To lower the level of thermal losses, the heat transfer coefficient between the

absorber plate and air should be increased. This can be achieved by creating modifications in

the low passages of the air heater [11].

2.4.5.3 The inclusion of fins in the structure of the SAH

Heat transfer from the absorber plate to the air can be enhanced by attaching fins

perpendicularly on the lower side of the absorber plate. The fins lead to the creation of

turbulence while at the same time increase the surface area for heat transfer. However, the

increase in heat transfer resulting from the use of fins leads to an increase in pressure drop.

Pressure drop is an essential factor that is considered during the selection of a solar air heater

[11].


28 | P a g e

A Computational Fluid Dynamic (CFD) study was carried out by [22] in which the effect

of applying fins in the structure of the solar air heater was evaluated. The geometry used in

the FD simulations was as shown in Figure 14. Figure 15 illustrates the results that were

obtained from the simulations. It can be observed that the highest temperature and velocity

were found in the fins. The use of the fins made the flow of the air over the absorber plate

turbulent which resulted in higher velocity in this region. Increased turbulence meant that

the flow would be more chaotic and disorderly which promoted mixing of the different layers

of air which were at different temperatures. The overall effect of all his was an improvement

in the thermal performance of the SAH.

Figure 14 Geometry applied in the CFD study [26]

Figure 15 Temperature and Velocity distribution contours respectively [26]


29 | P a g e

2.4.5.4 Use of artificial roughness on the absorber plate

The convective heat transfer coefficient can be improved by introducing turbulent

flow at the heat transfer surface. In use of roughened surfaces on the underside of the

absorber plate have been found to bring about an improvement in the convective heat

transfer coefficient. Artificial roughness has become popular due to its ability to increase the

heat transfer coefficient without leading to frictional losses [11]. Some of the artificial

roughness designs that have been used over the years include; V-Ribs, transverse ribs among

others.

Experimental work was carried out by [23] in which the effect of applying artificial

roughness in the design of the solar air heater was studied. An inclined circular rib was

introduced into the structure of the solar air heater on the surface of the absorber plate. The

circular rib was inclined at an angle of 450. The performance of the SAH was examined at

varying values of the Reynolds number.


30 | P a g e

Figure 16 A comparison of the smooth and Roughened Duct of the SAH [27]

The graph in Figure.16 was generated from the results that were obtained from the

experiments. The thermal efficiency of the smooth duct varied from 36% to 60% while the

roughened case varied from 40% to 72%. The thermal efficiency of the solar air heater with

the roughened duct is higher than the smooth duct. This occurs due to the artificially

roughened duct developing turbulent flow unlike in the smooth case where the flow is

laminar. When the sublayer is broken, and the turbulent flow develops, higher transfer of

heat will take place from the absorber plate to the air. Based on Table 4, it can be observed

that an increase in the Reynolds number results in higher values of thermal efficiency.


31 | P a g e

Table 4. The thermal efficiency of the SAH at varying Reynolds number [24].

Reynold Number (Re) Thermal Efficiency Snooth

Duct in (%)

Thermal Efficiency

Roughened Duct in (%)

2550 36.09 41.09

3214 41.50 53.20

4111 47.51 56.51

5127 55.27 69.74

6206 60.83 72.11

Thermal performance analysis was carried out by [9] in which they investigated the

effect that the use of V-Down Discrete Rib Roughness on the surface of the absorber plate.

The geometry and design of the v-down ribs on the absorber plate was shown in Figure 17.

Figure 17 Structure of the absorber plate with the V-down ribs placed at a 600 angle [12]


32 | P a g e

Figure 18 Thermal efficiency against thermal parameter (T_i-T_a)/I (Km^2 W^(-1)) [12]

From the graph in Figure 18, it was concluded that application of artificial roughening

using the V-ribs promoted more significant transfer of heat between the absorber plate and

the surrounding air. The primary function of the absorber plate is to bring about an increase

in velocity of the air which also has the effect of promoting turbulent flow. The overall effect

of this was improved thermal performance of the SAH as observed in Figure 18.

A CFD study was carried out by [25] in which the effect of using V-ribs to create

artificial roughness was evaluated. The model of the absorber that was used in the numerical

study was as shown in Figure 19.

Figure 19 Dimensions and Geometry used i

Documents

Design, Build and Test of Solar Thermal Air Heating …...Design, Build and Test of Solar Thermal Air Heating System with Application ii | P a g e Acknowledgement First and foremost,