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Experimental Study on Increasing the Electrical Efficiency of Photovoltaic

Panel with Decreasing Rear Surface Temperature

Author:

Ali Hassanzadeh Naeini

Supervisor:

Associate prof. Dr. Ali Moosavi

Associate prof. Dr. Behshad Shafii

©Ali Hassanzadeh Naeini, June 2015

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Author’s declaration

I hereby declare that I am the sole author of this thesis. The work described in this thesis

has not been previously submitted for a degree in this or any other university. All and any

contributions by others are cited. This is a true copy of the thesis, including any required final

revisions, as accepted by my examiners. I understand that my thesis may be made electronically

available to the public.

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Abstract

Experimental study on increasing the electrical efficiency of photovoltaic panel with

decreasing rear surface temperature

Ali Hassanzadeh Naeini, M.Sc.

Supervisor: Ali Moosavi, Behshad Shafii

Hybrid photovoltaic thermal system (PV/T) is a pioneer mechanical and electrical system

which can produce both thermal and electrical energy for domestic or industrial purposes. As the

solar cell’s material is semiconductor, the electrical output of them reversely modifies with the

reduction of solar cell’s operating temperature. One of the most operational and cost-effective

methods that has been investigated is active cooling. The photovoltaic panel cools down with a

channel under the PV’s rear surface and a fan in the channel outlet. The producing heat can warm

up cold water of a house or a factory. In addition to active cooling, it is smart to utilize passive

cooling such as artificial roughness. Artificial roughness is one of the impressive ways to boost

the PV/T electrical efficiency and thermal performance. The unique geometry of sinus roughness

causes heat transfer enhancement and low pressure lost simultaneously.

In this study, the influence of PV tilt and channel height on both electrical efficiency and

thermal performance with active cooling (constant Reynolds number (Re=3150) in the channel

outlet), and without cooling mechanism are investigated. Therefore, the cooling mechanism

includes four different parameters which were observed in the experiment: PV tilt angle, channel

height, roughness height and active cooling. The purpose of this project is to detect the best state

in the combination of these four elements in which electrical efficiency and thermal performance

are maximum. The comparison of experimental results and numerical outputs represent good

agreement with each other. The results obtained from the tested solar panel, could only achieve

about 16.5% electrical efficiency in the basic condition, however, the best combination of

parameters causes the electrical efficiency elevates up to 22%.It means about 30watt enhancement.

Also for the thermal performance, it showed that usage of roughness increases heat transfer

dramatically with low pressure lost. The thermal performance index becomes more than 7.

Keywords: Experiment, Cooling Channel, Photovoltaic Panel

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Acknowledgements

I hereby would like to thank my brilliant supervisors, Associate professor Ali Moosavi

and Associate Professor Behshad Shafii for giving their generous guidance, intuition, inspiration

in my thesis duration. This work without kind assistance of Doctor Shafii in experimental part and

unsparing aid of Doctor Moosavi in numerical part was much harder than it used to be.

Moreover, I would like to thank Mr. Mojtaba Mirhosseini, PhD student of energy

conversion in Sharif University of technology who gave me a lot of beneficial advice for writing

my journal paper.

At last, I am eternally grateful to my beloved parents and brothers, who gave me courage

when I feared, and hope when I was disappointed.

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Dedication

I hereby dedicate my dissertation to my mother who spent all her life protecting,

encouraging and loving her children. Although I completely aware that this is so trivial in

comparison to all the eternal favor she has done for me, the intellectual property of this thesis

belongs to my beloved mother.

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Nomenclature

A Area of the PV module

Channel entrance area

Specific heat under constant pressure

net energy absorbed by the cell

electrical energy produced by photovoltaic cell

thermal energy produced by photovoltaic cell

rate of solar energy absorbed by Tedlar

G Total solar irradiation

h convection coeffcient of air

maximum current A

k Thermal conductivity

L channel length m

M mass flow rate kg/s

P wetted perimeter m

p packing factor ----

Maximum Voltage V

τg Fraction transmitted through the front glass ----

Bulk temperature in the channel K or °C

PV rear surface temperature K or °C

J/kg. K

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Subscripts and Greek letters

m mean

MP maximum power

c open circuit

PV Photovoltaic

sc short circuit

T Tedlar

0 basic condition (zero tilt without cooling)

C best cooled condition

In Channel inlet

Channel or Out Channel outlet

ϴ PV tilt angle

elec electrical

middle in the middle of the channel

max maximum

α absorb

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Table of context

Author’s declaration ....................................................................................................................... ii

Abstract .......................................................................................................................................... iii

Acknowledgements ........................................................................................................................ iv

Dedication ........................................................................................................................................ v

Nomenclature .................................................................................................................................. vi

Subscripts and Greek letters ......................................................................................................... vii

Table of context ............................................................................................................................. 13

List of tables .................................................................................................................................. xv

List of figures................................................................................................................................ xvi

Chapter 1 ......................................................................................................................................... 1

Introduction ..................................................................................................................................... 1

1.1 Energy .................................................................................................................................... 1

1.1.1 Renewable energy ........................................................................................................... 2

1.2 PV Cooling ............................................................................................................................ 9

1.2.1 Active cooling ................................................................................................................. 9

1.2.2 Passive cooling ............................................................................................................. 11

1.3 Earlier studies ...................................................................................................................... 11

1.4 Motivation and purposes ..................................................................................................... 18

Chapter 2 ....................................................................................................................................... 19

Experimental Procedures ............................................................................................................... 19

...................................................................................................................................... 19

2.1 Fabrication ........................................................................................................................... 19

2.1.1 Cutting .......................................................................................................................... 19

2.1.2 Forming ........................................................................................................................ 22

2.1.3 Assembling ................................................................................................................... 23

2.1.4 Painting ......................................................................................................................... 25

2.2 Final Set-up.................................................................................................................... 26

2.3 Equipment list: ............................................................................................................... 29

2.3.1 Photovoltaic panel ................................................................................................. 29

2.3.2 Centrifugal fan ....................................................................................................... 35

2.3.3 Thermocouple type T ............................................................................................. 36

2.3.4 Data logger ............................................................................................................ 38

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2.3.5 Dimmer .................................................................................................................. 39

2.3.6 Pyranometer ........................................................................................................... 39

2.3.7 Anemometer .......................................................................................................... 40

2.3.8 Digital multimeter .................................................................................................. 45

2.3.9 Sinus wave plates (sawtooth roughness) ........................................................ 45

2.3.10 The insulation ........................................................................................................ 47

2.4 Experimental stages ....................................................................................................... 48

Chapter 3 ....................................................................................................................................... 49

Theoretical equations ..................................................................................................................... 49

3.2 The analysis of heat transfer on Photovoltaic Panel ....................................................... 50

Energy equation for the rear channel ......................................................................................... 55

3.3 Theoretical tilt angle calculation ................................................................................... 56

Chapter 4 ....................................................................................................................................... 58

Results comparison and numerical verification ............................................................................. 58

4.2 The impact of cooling mechanisms on the electrical power .......................................... 60

4.2.1 PV tilt angle effect on electrical efficiency. .......................................................... 60

4.2.2 Channel depth effect on electrical efficiency ........................................................ 65

4.2.3 Roughness effect on electrical efficiency .............................................................. 69

4.3 The impact of cooling mechanisms on the thermal performance .................................. 73

4.3.1 PV tilt effect on thermal performance ................................................................... 73

4.3.2 Channel depth effect on thermal performance ....................................................... 76

4.3.3 Roughness effect on thermal performance ............................................................ 79

4.4 Numerical of verification of best experimental model. ................................................. 83

4.4.1 Numerical process ................................................................................................. 84

4.4.2 Plots ....................................................................................................................... 86

4.4.3 Contours ................................................................................................................. 88

4.4.4 Experimental and numerical Nusselt number comparison .................................... 90

4.5 Uncertainty .................................................................................................................... 91

Chapter 5 ....................................................................................................................................... 93

Conclusion and future research ..................................................................................................... 93

5.1 Conclusion ..................................................................................................................... 93

5.2 Future research .............................................................................................................. 94

Reference ....................................................................................................................................... 95

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List of tables

Table 2.1 Different type of thermocouple’s resolution, range and accuracy……………………....…38

Table 2.2 Lutron 4203 anemometer features………………………………………………….….......42

Table 2.3 Lutron 4203 general description………………………………………………………..….43

Table 2.4 Air velocity uncertainty number for The Lutron 4203 anemometer………………….…....43

Table 3.1 hear flex of Tedlar (PV rear surface)…………………………………………………..…..52

Table 3.2 Thermal proprieties of PV component’s material……………………………………….....53

Table 4.1 PV panel’s tilt comparison in April……………………………………………………..…63

Table 4.2 electrical power for 10 cm channel height……………………………………………..…..69

Table 4.3 P0 and Pfinal experimental data………………………………………………………….…..72

Table 4.4 Nusselt number comparison………………………………………………………….….…74

Table 4.5 the 10cm channel data with 8cm roughness is……………………………………….….…79

Table 4.6 the maximum Nusselt and basic Nusselt in zero tilt angle………………………………...81

Table 4.7 the heat flex of PV’ rear surface enter the channel……………………………………..….85

Table 4.8 PV’s rear surface temperature at 1:00 PM APRIL 19th ……………………………….…...85

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List of figures

Figure 1.1 forecast energy consumption in 2052………………………………………………………2

Figure 1.2 Comparison of oil and renewable energy in 2052……………………………………….....2

Figure 1.3 Distribution by country of research papers published using optimization algorithms studied

in this paper applied to renewable energies…………………………….……………………………...3

Figure 1.4 Global PV-based solar electrical energy production over four decades…………………...4

Figure 1.5 Schematic of various solar technologies……..…………………………………….…..…...4

Figure 1.6 Photovoltaic effect cycle……………..……………………………………………………..5

Figure 1.7 Solar cell component……..……………………………………………………………..….6

Figure 1.8 the trend of decreasing dollars per watt solar energy……………………………..…..……7

Figure 1.9 Actual image of water cooling system…..……………………….…..………………….....9

Figure 1.10schematic image of water cooling system…………………….…………………………..10

Figure 1.11 (a) Cross-sectional view of unglazed PV/thermal air (i) with Tedlar (Model I) [16], (ii)

without Tedlar (Model II). (b) Cross-sectional view of glazed PV/thermal air (i) with Tedlar (Model

III), (ii) without Tedlar (Model IV)…………..…………………………………………….…..……..14

Figure 1.12 Chandra’s setup image ……………………………………………………………….…15

Figure 1.13 Schematics of the various PV/T models along with heat transfer coefficients…………..15

Figure 2.1 handy cutter…………….……………………………………………………….………...20

Figure 2.2 mechanical metal cutter……………….…………………………………………….…….20

Figure 2.3 incisive metal cutter……………………………………………………………………....21

Figure 2.4 Iron cutting for creating channel basis…………………………………………………....21

Figure 2.5 PV panel before cutting with Hacksaw………………………….…………………….…..21

Figure 2.6 PV panel frame after cutting…………..……………….……………………………...…..22

Figure 2.7 three meter metal former……………………………………………………………….…23

Figure 2.8 PV holder forming……………….………………………………………………………..23

Figure 2.9 Channel forming…………………………………………………………………….….…23

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Figure 2.10 PV iron basis welding……………………………………………………………….…..24

Figure 2.11 PV iron basis after welding……………….……………………………………………..24

Figure 3.12 assembling the sinus wave plate with screw…………………………………………….24

Figure 2.13 the channel and sinus wave plate after assembling...........................................………......24

Figure 2.14 Set-up Final shape………………….……………………………………………………25

Figure 2.15 locating the fan……………………………….………………………………………….25

Figure 2.16 painting the channel basis and tilt- angle shaft ………………………….………………26

Figure 2.17 Actual photograph of PV with duct and fan…………………….………….……….…...27

Figure 2.18 Dimension of channel……………….…………………..……………………………….27

Figure 2.19 Schematic figure of PV panel with channel………….………….……….……………...28

Figure 2.20 the PV panel in the box……………………..……………………………...…………….30

Figure 2.21 wavelength of polycrystalline solar cell working range………….……………...……...31

Figure 2.22 Characteristic curve of PV module. Open circuits(blue) and closed circuits(red)…..….33

Figure 2.23 mechanical characteristic of PV module……………………….……………………..…34

Figure 2.24 dimension of PV module…………………………………….……...…………………...34

Figure 2.25 the Fan figures……….………..……………………………..…………………………..35

Figure 2.26 Data logger Lutron BTM-4208SD ………………………………..…..…………………36

Figure 2.27 Thermocouple location in PV’s rear surface ……………………………….….………..37

Figure 2.28 Thermocouple location of thermocouple in the channel ………………………………...37

Figure 2.29 Dimer, made in Iran….………………….………...……………………………………..39

Figure 2.30 Pyranometer…………………………...…………………………….……………….….40

Figure 2.31 Lutron anemometer………………………….…………………………………………..41

Figure 2.32 Velocity testing standard…………..…………………………………………………….44

Figure 2.33 PV characteristic curve……………………………………………………...…………..45

Figure 2.34 Digital multimeter………………..……………………………………….……………..45

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Figure 2.35 Schematic figure of three type of roughness in the channel bed…………………………45

Figure 2.36 Sinus wave plates e=2………………………………………………………………...….46

Figure 2.37 Sinus wave plates e=4……………………………………………...………………..…..46

Figure 2.38 Sinus wave plates e=8 ………………………………………………………..…………47

Figure 2.39 the channel perimeter insulated with 5mm foam……………………..……………...….47

Figure 3.1 Schematic figure of energy equation in the channel……………………s………………..55

Figure 4.1 Irradiation and Average Panel Temperature for the whole day with cooling (19 April

2015)………………………………………………………………………………………………....59

Figure 4.2 Irradiation and Average Panel Temperature for the whole day without cooling (19 April

2015)……………………………………………………………………………………………...….59

Figure 4.3 PV tilt is zero degree…………………...……………………………………………….…60

Figure 4.4 PV tilt is 30 degree……………………………………………………..………….……...60

Figure 4.5 PV tilt is 45 degree………………………………………………………….……..…..…..61

Figure 4.6 PV tilt is 60 degree………………………………………………………………..….…....61

Figure 4.7 PV tilt comparison in April…………………………….……………………..………….62

Figure 4.8 channel with 30cm height under PV surface is under test in April

15th………………..……………………………………………………………………………....…..64

Figure 4.9 the channel with 20cm height (front view)………………………………………..….…..65

Figure 4.10 the channel with 20cm height (right-sided view)……………………………….….…....66

Figure 4.11 the channel with 10com height (front view)…………………………………….…...….66

Figure 4.12 the height determent shaft in 10cm (right-sided view)…………………………….…....66

Figure 4.13 an analogy between Electrical output and channel depth with cooling mechanism from

10AM to 6 PM……………………………………………………………………………….....…….66

Figure 4.14 an analogy between Electrical output and channel depth with cooling mechanism from

10AM to 6 PM………………………….…………………………………………………….….…...67

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Figure 4.15 proportion between Electrical output to channel depth from 10AM to 6

PM………………………………………………………………………………………...……..…...68

Figure 4.16 proportion between electrical efficiency for variable channel bed’s roughness with active

cooling………………………………………………………………………………………………..68

Figure 4.17 analogy between electrical efficiency for variable channel bed’s roughness and free

convection without channel bed’s roughness………………..…………………………………….…70

Figure 4.18 analogy between electrical efficiency for variable channel bed’s roughness with fan and

free convection without channel bed’s roughness and fan……………….……………………..……70

Figure 4.19 electrical output for variable channel bed’s roughness with convection active cooling...71

Figure 4.20 inlet temperature (Tin), outlet temperature (Tout), bulk temperature (Tm) and PV rear surface

temperature (Ts) for the 30º tilt………………..………………………………………………………74

Figure 4.21 an analogy between heat transfer coefficient and channel depth with cooling mechanism

from 10AM to 6 PM………………………………………………………………………………….75

Figure 5.22 an analogy between Nusselt Number and channel depth with cooling and without cooling

(Nufree) mechanism from 10AM to 6 PM………………………………………….………………....76

Figure 4.23 an analogy between heat transfer coefficient and channel depth with cooling mechanism

from 10AM to 6 PM.………………………………………………………………………………....77

Figure 4.24 proportion between Nusselt Number and channel depth with cooling and without

cooling(Nufree) mechanism from 10AM to 6 PM.................................................................................77

Figure 4.25 proportion of Nusselt number to channel depth from 10AM to 6 PM….………………78

Figure 4.26 heat transfer coefficient for variable channel bed’s roughness with active cooling...…..80

Figure 4.27 Nusselt number for variable channel bed’s roughness with active cooling.……….…....80

Figure 4.28 proportion be Nusselt number for variable channel bed’s roughness with fan and free

convection without channel bed’s roughness and fan ………………………………....…………….81

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Figure 4.29 PV’s rear surface under cooling (Ts-c) and without cooling (Ts-0)………………….…83

Figure 4.30 First mesh displayed in Fluent……………………………………………………….….86

Figure 4.31 channel static temperature in x direction [0<y<10, z=0.28] for 1:00 PM………...….….87

Figure 5.33 Channel Static temperature in y direction [0<z<0.57, x=0.6] for 1:00 PM………….….87

Figure 4.33 Channel Static temperature in y direction [0<z<0.57, x=0.6] for 1:00 PM…………..…88

Figure 5.34 Velocity magnitude contours in the channel inlet………………………………..………89

Figure 5.35 Temperature contours in the PV rear surface (Tedlar)……………………………..……89

Figure 5.36 Experimental and numerical Nusselt number………………………………………..….90

Figure 5.37 Uncertainty Domain for Nusselt number in channel 10cm and 8cm roughness height….92

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Chapter 1

Introduction

1.1 Energy

There is a lot of debate about what is the beating heart of the modern world. From my

perspective, the answer is energy. All the developments in industry and urban progress relate directly

or indirectly to diversity and amount of energy that a country possesses.

Throughout history, human being has always been looking for source of energy for

producing heat to cook and light to see surrounding environment in the night. As we all know, the

first resource of energy discovered by primitive when they scrubbed two flints which caused injection

and it could burn dry firewood.

In today’s world, demand for energy has been rising dramatically. Global energy demand

is set to grow by 37% by 2040 in our central scenario, but the development path for a growing world

population and economy is less energy-intensive than it used to be. [1]

Solar, wind, biomass, hydropower and tidal energy are different types of renewable energy

which are totally compatible with nature, therefore, they are NOx and CO2 free [2, 3]. NASA

researchers announced that the holes in the ozonosphere over the two poles currently occupy

approximately 28,300,000 km2, up from approximately 24,000,000 km2 in 1994.[4]

In figure 1.1, 1.2, the energy demand trend in the future up to 2052 is displayed. [5]

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1.1.1 Renewable energy

Renewable energy is generally defined as energy that comes from resources which are

naturally replenished on a human timescale such as sunlight, wind, rain, tides, waves and geothermal

heat, as sunlight, wind, rain, tides, waves, geothermal heat and biomass.[5] Renewable energy replaces

conventional fuels in four distinct areas: electricity generation, hot water/space heating, motor fuels,

and rural (off-grid) energy services.[6]

Figure 1.1 forecast energy consumption in 2052

Figure 1.2 Comparison of oil and renewable energy in 2052

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There are numerous examples around the world that countries are utilizing renewable

energy. For instance, in northwestern of Iran, Mangil city is one the best locations for producing

electricity from wind as it has high velocity wind in most of days in the year. The project for creating

wind farm has been started since 1994 by REOI (Renewable energy organization of Iran).[7][8]

Figure 4 demonstrates distribution of renewable energy by country. [9]

1.1.1.1 Solar energy

1.1.1.1.1 General description

Solar energy has various functions in modern world. It can heat houses, powerhouses and

villages, also it can provide electricity to every urban or rural houses even if they are far from el power

grid. Also consumption of electricity per decrease as by using eternal energy of sun. It also helps to

avoid the use of Freon refrigerant or other harmful gases that deplete ozone layers. Since 1970, 4%

in the total volume of ozone in Earth's stratosphere (the ozone layer) has been decreased. [10].

Figure 1.3 Distribution by country used renewable energies.2013

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Photovoltaic (PV) is one of the most efficient ways of using solar energy by directly

converting it into electricity. Solar cells are devices, which are used to transmit sunlight to electricity

by the use of the photoelectric effect. A photovoltaic system consists of solar cells and ancillary

components. Researchers at the Bell Telephone Laboratories produced the first PV system by use of a

p–n junction type solar cell with 6% efficiency in 1954[11].The Common PV panel efficiencies are

between 15 and 20% [12]

Figure 1.4 Global PV-based solar electrical energy production over four decades

Figure 1.5 Schematic of various solar technologies.

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1.1.1.1.2 Photoelectric Phenomenon

The process in which two different materials produce an electrical current when any light or

sun irradiation is absorbed by the cells. Crystals such as germanium, in which electrons are usually

not free to move from atom to atom within the crystal, provides the energy needed to free some

electrons from their bound condition. Free electrons cross the junction between two dissimilar crystals

more easily in one direction than in the other, giving one side of the junction a negative charge and,

therefore, a negative voltage with respect to the other side, just as one electrode of a battery has a

negative voltage with respect to the other. The photovoltaic effect can continue to provide voltage and

current as long as light continues to fall on the two materials. This current can be used to measure the

brightness of the incident light or as a source of power in an electrical circuit, as in a solar power

system [13].

Figure 1.6 Photovoltaic effect cycle

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1.1.1.1.3 Solar Cell (photovoltaic cell)

Solar cell is a conversion system that converts sun energy into electrical energy through

the photovoltaic effect. Most of solar cells are produced from silicon which has high efficiency and

low fabrication cost in comparison to polycrystalline [13].

Solar cells have two unique features that separate them from other conversion system.

1- They do not have any reaction inside like batteries, therefore they are

compatible with nature.

2- They do not need any moving part like turbines or fan which make them

able to work silently.

Figure 1.7 Solar cell component

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Figure 1.8 demonstrates, since 2010 grace for the photovoltaic panel to produce electrical

energy has been increased dramatically since the fabricating technology has enhanced, therefore, the

initial money to produce PV panel has been decreased. Figure 1.8 demonstrates a survey which was

done in 2010 and displays predication till 2015. Now solar cell produces about 0.8 $/W.

Figure 1.8 the trend of decreasing dollars per watt solar energy

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1.1.1.1.4 Radiation (total insulation)

The electrical efficiency strongly relates to sun radiation. There are three types of radiation.

Direct radiation

Diffuse radiation

Reflected radiation

1.1.1.1.4.1 Direct radiation

It is also sometimes called "beam radiation" or "beam insulation". It is used to describe solar

radiation traveling on a straight line from the sun down to the surface of the earth. It is the most

significant radiation effect on PV’s electrical output.

1.1.1.1.4.2 Diffuse radiation

The sunlight has been scattered by molecules and particles in the atmosphere but that has

still made it down to the surface of the earth.

1.1.1.1.4.3 Reflective radiation

Sunlight that has been reflected off of non-atmospheric things such as the ground. Asphalt

reflects about 4% of the light that strikes it and a lawn about 25%. However, solar panels tend to be

tilted away from where the reflected light is going

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1.2 PV Cooling

1.2.1 Active cooling

Active cooling needs electricity to work. Among all active cooling method, two of them are

more functional and cost effective:

1- Water cooling

2- Air cooling

1.2.1.1 Water cooling

Water is the working fluid in the liquid cooling system for PV cells. Water cooling systems

contain piping to the rear surface of a PV panel. The heat from the PV cells are conducted through the

metal and absorbed by the working fluid (presuming that the working fluid is cooler than the operating

temperature of the cells). In closed-loop systems this heat is either exhausted (to cool it), or transferred

at a heat exchanger, where it flows to its application. The general cooling system is as figure 1.9.

Figure 1.9 actual image of water cooling system

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1.2.1.2 Air cooling

Another fluid that is most common in PV cooling is Air. Although, conductivity of air is

low, it is Available in every place on earth and its cooling system is less complex and expensive

compare to Water cooling.

The air cooling system is as figure 1.10.

The velocity of air plays a key role in the cooling process since as fast as velocity surpasses

the PV surface more amount of heat transfer happens per second.

Figure 1.10 schematic image of air cooling system

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1.2.2 Passive cooling

In this research, the cooling method which does not require electricity to work, called

passive cooling is studied which is more beneficial since net electrical output will increase.

1.2.2.1 Variable channel’s type and height

Channel height (depth) has a key role on the variation of Reynolds number because the flow

velocity increase which has direct effect on Nusselt number, therefore one of parameters on enhancing

heat transfer between PV rear surface and channel is the channel height.

1.2.2.2 Roughened structure in the channel

The other significant method of cooling is roughness in channel bed. It leads to fluid velocity

increase as a result, Reynolds number increase, and finally it has positive effect on Nusselt number.

1.3 Earlier studies

The pioneer research in photovoltaic technology was done by Alexander Edmond Becquerel

in 1839 who observed photovoltaic effect via an electrode in a conductive solution exposed to light.

In 1905, Albert Einstein published a paper explained the photoelectric effect on a quantum basis which

officially started the photovoltaic research. The electrical performance is primarily affected by the

material and type of PV. On April 25, 1954, Bell Labs announced the invention of the first practical

silicon solar cell. These cells had about 6% efficiency. [14][15].In 1959, Hoffman Electronics created

9% efficient solar cells and one year later they fabricated a 14% efficient solar cell. At last, in 2007,

University of Delaware announced that they built a solar cell with 42.8% efficiency, however it was

not approve by unbiased observer. [16]

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Nowadays, a common PV module’s efficiency is about 8-20% of the total solar radiation into

electricity. Efficiency fluctuation depends on the type of solar cells and climatic conditions. The rest

of the total insolation is converted into heat, which significantly increases the temperature of the PV

module and reduces the PV’s electrical efficiency. This harmful heat can be extracted by flowing

water/air beneath the PV module using thermal collector, called, photovoltaic thermal (PV/T)

collectors. Investigated on PV/T-liquid and PV/T-air collectors. Tripanagnostopoulos claimed in the

zero temperature, for his PV/T liquid collector, the efficiency of PV/T is between 38%-60% enhanced.

However, the electrical performance in the room temperature is between 6%-12% [17]. A higher

thermal yield was also found by a-Si by Ji et al. [18]. However, in other experiments a lower thermal

efficiency was found for a-Si than for c-Si. After them, Ruoss [19, 20] and Selvan [21]. Helden [22]

compared a conventional PV module, an unglazed PVT module and a glazed PVT module. The

average annual electrical efficiency was found to be 7.2%, 7.6% and 6.6%, respectively.

In water cooling systems, this heat is used or exhausted before the fluid returns to the PV cells

[23]. It is also possible to disperse nanoparticles in the liquid to create a liquid filter for PV/T

applications to utilize the efficient heat. [24] [25] [26].

In air cooling systems, Brown University started developing the PVT air collectors, however

its efficiency was not acceptable. Although this first generation technology was not a big success,

has become a motivation to the development of second generation technology. The effect of thermal

gradient on electrical efficiency of PV panel was investigated [27].

A European company [28] fabricated a PV-air cooling panel which was a new step

forward. A new type PVT-air collector was completed in European company [29, 30]. This type

of PVT air collector has thin-film. The function of this system was used for decreasing humidity

in the air.

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Tiwari evaluated the overall performance of PV-T air collector. [31], in this study, different

kind of configurations of PVT air collector (like unglazed, glazed, with and without Tedlar)

shown in Fig 1.11 were used to investigate the electrical and thermal performance. It was shown

that the glazed PVT air collector without Tedlar provides the best performance. Solar cells were

put between the glass cover and absorber plate. The air goes into the duct which is covered with

the glass cover and photovoltaic panel, then, it enters a channel created between the photovoltaic

panel and absorber. This configuration can greatly reduce the heat loss and increase its thermal

efficiency as a result the thermal efficiency of this system can reach to 60%.In this system both

electrical and thermal efficiency increases, therefore it is an excellent model that should be

investigated more.

The sun total irradiation effect on increasing temperature and enhancing electrical efficiency

and various type of sun irradiation explained by many authors in various aspect [32, 33]. It is shown

that direct irradiation has the greatest impact on electrical and thermal output. [34, 35]

There are also many different mathematical equations on calculating the sun angle to

horizontal and vertical PV panel. [36. 37.38. 39.40] each one has its priority and accuracy, however

Mittelman[36] equation is the most popular method to calculate PV panel’s tilt angle and I personally

used this equation to verifying my experimental optimum tilt angle.

14

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In the channel height issue. Farshchimonfared [41] investigated about the optimum channel

depth for different length over width and also found the mass flow rate over area for the best heat

transfer. Hegazy [42] compared four different canals for cooling the PV panel as figure 1.13. The result

is for mass flow lower than 0.02(m/s): model I has the least electrical efficiency and II, III, IV model

has the same electrical efficiency. However, for mass flow over 0.02(m/s) Model III has the best

Figure 1.11 (a) Cross-sectional view of unglazed PV/thermal air (i) with Tedlar (Model I) [16], (ii)

without Tedlar (Model II). (b) Cross-sectional view of glazed PV/thermal air (i) with Tedlar (Model III),

(ii) without Tedlar (Model IV)

15

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electrical efficiency therefore, it is the best model for utilizing. On the other hand, model III requires

a lot of electrical efficiency that has negative impact on net electrical output and its fabrication

expenditure is high. All in all, the optimum model is considered electrical efficiency, net electrical

output and fabrication expenditure is model II.

Chandra [43] for setup in figure 1.12 found the best channel length and channel duct for air

flow that will be fully developed hydrodynamic and thermal manner. The experimental experiment

showed that for flow rate 2(m/s), the optimum channel length is 3(m) and the optimum hydraulic

diameter is 0.1.

Figure 1.13 Schematics of the various PV/T models Figure 1.12 Chandra’s setup image

16

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In the roughness structure in the channel, some numerous works have been done. Yu and

Kandlikar [44] investigated the impact of Reynolds number, channel hydraulic diameter (Dh),

roughness pitch (λ), roughness height (H) on the heat transfer enhancement. The result is that as

Reynolds number increases, Nusselt number increases dramatically, therefore it has the most influence

on the heat transfer enhancement. Channel hydraulics diameter and roughness height has less influence

but they have. Although roughness pitch does affect the heat transfer, it has influence on friction

coefficient.

Saini [45] announced that the heat transfer of the solar cells can be increased by

expanding the wall of the channel, leading to higher thermal efficiency. However, high roughness

of wall and absorber will enhance friction factor and therefore a higher pumping.

It is demonstrated that several types of ribs in the air channel can provide better

performance in heat extraction but it is also accompanied by a significant increase in friction

losses by Han [46] and Gupta [47]. Some tools like using the porous materials and perforated plates

were suggested to enhance the heat transfer in the air channel which was not successful as it

should be. The combination of different methods done by Datta [48] to increase the heat transfer in

air channel.

Another report represents a lower value of fully developed the Nusselt number in the laminar

flow, with a decreasing trend at lower Reynolds numbers in micro channel. [49, 50, 51, 52].

In the air heater, a lot of various roughness was used to enhance heat transfer. In the table

summary of major experimental work on artificially roughened solar air heater having different

roughness geometries applied on the absorber plate. [53.54]

17

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Another extra investigation [55, 56, 57] was done which presented a study of a PVT air

hybrid system, this system comprised a plane booster and a flat plat collector mounted with

photovoltaic cells (Fig 2.10). It was concluded that the electrical efficiency of photovoltaic cell

will linearly decrease with increase of the absorber temperature. The results also indicated that the

minimum area of photovoltaic cell needed to operate a pump at a given flow rate is a function of

time. The plane boosters were utilized to reflect the extra incident rays to the photovoltaic cell in

order to increase the intensity of sunlight on the photovoltaic module.

Optimization the absorber geometry for PV thermal panel was explained by Sipple [58]. It

was reported that the optimized distance between the fins is about 5 to 10 mm. The thermal

efficiency of the collector can attain to 77% with optimized geometry. As the pressure drop

increases drastically with decreasing fin spacing, this factor should also be considered in the design.

Another significant work [59] took action in a study on the performance of the double pass

solar air heater with longitudinal fins. The study showed that the thermal efficiency increases with

increasing flow rate as the heat transfer is proportional to the mass flow rate. The number and height

of fins will also increase the heat transfer rate due to the increase of heat transfer area. Hence, the

thermal efficiency is proportional to the number and height of fins. However, the entropy

generation was found to decrease with increasing height of fins. This is because the outlet

temperature increases with increase of height of fins.

Italian colleagues [60] examined three different configurations of air ducts (simple air

channel, thin aluminum sheet and rectangular fin). They experimentally and numerically proofed

their claim. Both electrical efficiency and thermal performance are discussed in the paper.

18

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A paper was published about a steady state simulation of the single and double pass

combined photovoltaic thermal air collector by Yigit [61]. His numerical studies displayed that the

double pass photovoltaic thermal collector has superior performance during the operation. The system

could improve thermal efficiency up to 10%.

The Indians [62] expanded an experimental and numerical model to develop the

performance of single glass and double glass hybrid photovoltaic thermal air heating collector. As

double layer has extra glass it can keep irradiative rays back from absorber in the channel, thermal

efficiency is better in the double layer glass.

One of the novel pioneer creations was done by Hao [63]. A solar energy model with high

heat absorption. The system consisted of a frame, a glass sheet, a wave form heating plate, a shaft, a

support, and two ducts which could progress electrical efficiency in a large amount.

1.4 Motivation and purposes

The motivation of this research is to fabricate more efficient PV/T panel to produce both

electricity and thermal energy with a low cost, therefore, this technology will be expanded all around

the world in developed and developing countries. This technology must be able to produce the large

amount of energy that human kind requires in the 21st century and it can be able to stop the earth from

destroying.

The purpose of this study is, first, finding out the best panel tilt, sinus wave, the channel depth

for maximum electrical efficiency and thermal performance. Secondly, detecting the maximum

electrical efficiency of PV panel and the maximum thermal performance index under cooling

mechanism. At last, compare Nusselt number and heat transfer coefficient of experimental with Fluent

Numerical result to verify the experimental results.

19

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Chapter 2

Experimental Procedures

2.1 Fabrication

First we needed 5 sheets of aluminum 2m*3m with 1mm thickness to create back channel

and three sinus wave and 6m length metal to create channel basis and a joint to make PV’s variable

radius mechanism.

Then, Four Steps needed to manufacture setup.

1. Cutting

2. Forming

3. Assembling

4. Painting

2.1.1 Cutting

For cutting three different tools applied:

1. metal surface cutter

I. handy cutter( Figure 2.1)

II. mechanical metal cutter(Figure 2.2)

2. incisive metal cutter (Figure 2.3)

3. Iron cutting for creating channel basis (Figure 2.4)

20

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In the first step, it is needed to cut two parts: First, the aluminum sheet is cut to specific size

to build the channel, sinus wave plates and PV holder. Secondly, I needed to cut PV’s aluminum

periphery with metal surface cutter. (Figure 2.5, 2.6).

All these parts have been done in Sharif University of technology’s (SUT) workshop in

Tehran.

Figure 2.2 mechanical metal cutter Figure 2.1 handy cutter

21

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Figure 2.4 Iron cutting for creating channel basis

Figure 2.3 incisive metal cutter

Figure 2.5 PV panel before cutting with Hacksaw

22

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\

2.1.2 Forming

After cutting, we need tools to form the system component. The forming tool in Sharif

university workshop is a three meter length metal former which can rotate aluminum sheet 90

degree (Figure 2.7).

Then, with metal former, I was able to produce sinus wave plates, PV panel holder (Figure

2.8), and aluminum channel (Figure 2.9).

All the Forming is done with SUT workshop’s machines.

Figure 2.6 PV panel frame after cutting

23

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2.1.3 Assembling

In the third part, everything is ready to put together to organize the final system.

Figure 2.7 three meter metal former Figure 2.8 PV holder forming

Figure 2.9 Channel forming

24

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In the assembling there are two parts that should be done:

I. Drilling to make holes

II. Attaching

a. Welding( Figure 2.10, 2.11)

b. Screwing(Figure 2.12, 2.13)

All the assembling is done in SUT workshop.

Figure 2.12 assembling the sinus wave plate with screw Figure 2.13 the channel and sinus wave plate after assembling

Figure 2.10 PV iron basis welding Figure 2.11 PV iron basis after welding

25

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2.1.4 Painting

The final step is painting the set for two reasons:

1. Protecting the iron PV basis from corrosion.

2. For beauty: painting make a beautiful uniform silver setup.

All the painting was done in the SUT campus with two silver color sprays.

Figure 2.15 locating the fan Figure 2.14 Set-up Final shape

26

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2.2 Final Set-up

The experimental setup is fabricated to investigate the effect of rough plate and blower on

the enhancement of heat transfer and electrical performances of the Photovoltaic system. This

system was built on the roof of the mechanical engineering department of Sharif University of

technology in Tehran. The actual photograph of the set-up is shown in Fig 4.1. A schematic

figure of the experimental set-up is shown in Fig 3.2.

Figure 2.16 painting the channel basis and tilt- angle shaft

27

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Figure 2.17 Actual photograph of PV with duct and fan

Figure 2.18 Dimension of channel for 20cm channel height

28

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The current experiment is designed to investigate how the temperature affects the electrical

efficiency and electrical power output during the operation. The 120W solar panels were used in

the experiment to generate the electricity. A direct current blower connected to the batteries, is

used to extract surrounding air to cool the panels, during the operation. Also a dimer is used so that

the flow rate can be controlled by adjusting the knob of the controller.

Solar irradiation was measured by the irradiation probe, which was put at the same level

as the solar panels. In this experiment, the air speed was measured by the anemometer and

the temperature of air and PV module was obtained by using the T-type thermocouple directly

connected to the datalogger. The voltage and current of the solar panels were directly recorded by

the multimeter.

The experiments normally operated from 10 AM to 6:00 PM (before 10AM the sun

irradiation was imperceptible). In the experiment, PV current, PV voltage, temperature of PV’s rear

surface, temperature of inlet and outlet, air speed in channel and irradiation of sunlight were measured

during the operation of system.

Figure 2.19 Schematic figure of PV panel with channel

29

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2.3 Equipment list:

The following equipment formed the experiment.

Photovoltaic Panel

Centrifugal fan

Thermocouple & Data logger

Sun irradiation sensor

Anemometer

Multimeter

Sinus wave Plates(Sawtooth roughness)

Insulation foam

2.3.1 Photovoltaic panel

The structure of crystalline silicon solar cells is presented in Fig 3.4. EVA is a kind of

copolymer of ethylene and vinyl acetate. The polymer which is used in PV modules serves to

provide the functions like structural support, electrical insulation, physical isolation/protection and

thermal conduction for the solar cell circuit.

The rear surface of photovoltaic module normally is a kind of material, called Tedlar.

The function of Tedlar is to prevent the entree of water vapor. It is a kind of polymer material, called

polyvinyl fluoride. Tedlar will also provide the functions like UV resistance, mechanical properties,

strength and durability, resistance of weathering and electrical insulation. All of these functions

will help PV panel to sustain at least 20 years and above. Part of the rear surface is normally

made as a laminated film composite and the most common structure is the structure of

30

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Tedlar/Polyester/Tedlar, also called TPT. This kind of structure can enhance the mentioned function

above.

Fig 3.5 shows that the wavelength of polycrystalline solar cell working range is between

350 nm to 1200 nm [51]. Besides the transmission issues, the reflection of the front surface of PV

panel should be low as well. A low iron glass is most usually used in the PV industry because it is

of low cost, strong, stable, highly transparent, and impervious to water and gases and the front

contact glass also has self-cleaning properties after raining.

Figure 2.20 The PV panel in the

box

31

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PV panel description:

Cells Mono crystal: 3838mmx521mm square

Number of cells 75 (6x12) series connected

Typical application 12V DC

Size 5151(L) x311(W) x81(H) mm

Weight: 10.5 Kg

Front glass 3.2 mm

Figure 2.21 wavelength of polycrystalline solar cell working range

32

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Tolerance

Operating temperature -45 to 80 ℃

Hail diameter @ 80Km/h Up to 25 mm

Continuous wind speed Up to 24 m/s

Parameters

Note: defined as standard deviation of thousands measurements. Absolute power

values depend on the measuring system. They can differ by +/- 3% from one measuring

system to another.

Model: DSP521D

Rating power (Pm): 120W

Efficiency of cell ≥16.55%

Tolerance: +/- 3%

Rated current (Im): 6.45A

Rated voltage (Vm):18.6V

Short circuit Current (Isc) 6336A

Open circuit Voltage (Voc) 23.2V

33

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Characteristic curve

The line diagram shows the relation between PV current and voltage.

Figure 2.22 Characteristic curve of PV module. Open circuits(blue)

and closed circuits(red)

34

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Mechanical characteristic

The materials which formed the PV panel and their layer distance with each other.

Dimensions

Figure 2.23 mechanical characteristic of PV module

Figure 2.24 dimension of PV module

35

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2.3.2 Centrifugal fan

The fan used in the setup is 200 round/min and as it is measured the electrical power needed

for Reynolds number equal 3150 in the channel outlet (10cm channel) is 20W.

Fan general description and image as below [Figure 4.6]:

Fig 2.25 the Fan Figures

36

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2.3.3 Thermocouple type T

Type T (copper – constantan) thermocouples are suited for measurements in the −200 to

350 °C range. Type T thermocouples have a sensitivity of about 43 µV/°C. Note that copper has a

much higher thermal conductivity than the alloys generally used in thermocouple constructions, and

so it is necessary to exercise extra care with thermally anchoring type T thermocouples.

Figure 2.26 Data logger Lutron BTM-4208SD

37

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Thermocouple position located in two location:

A. In the PV’s rear surface

B. In the channel

Figure 2.27 Thermocouple location in PV’s rear surface

Figure 2.28 Thermocouple location of thermocouple in the channel

38

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2.3.4 Data logger

Electrical specification:

Electrical Specification (23+ 5 °C) of different thermocouple in the data loggers.

Table 2.1 Different type of thermocouple’s resolution, range and accuracy

39

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2.3.5 Dimmer

Dimmer is electronic tools which can change fan velocity with variation of electrical current.

Dimmer adjusts the speeds of a centrifugal fan accurately. We can choose full power speed or dim

the speed incrementally. Lowest speed selection is 10 percent of full power. Engineered for reliable,

long-lasting performance.

2.3.6 Pyranometer

Pyranometer is a tool that is able to measure total insolation. This instrument can measure

the radiation in the spectral range 100 to 2950 nm. The probe is made in USA.

The pyranometer contains two parts:

A probe

Figure 2.29 Dimmer, made in Iran

40

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A display

2.3.7 Anemometer

For determining velocity in the fan outlet, we need a standard to find the velocity flow profile therefore

I was able to find average velocity. Anemometers can be divided into two classes: those that measure

the wind's speed, and those that measure the wind's pressure; but as there is a close connection between

the pressure and the speed, an anemometer designed for one will give information about the other

one.

I used a Lutron 4203 anemometer is used:

Figure 2.30 Pyranometer

41

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In this case the anemometer is able to measure maximum or minimum velocity. The most

efficient part of this anemometer is measurement of the average velocity in different places which

helps to determine mean velocity in outlet with measurement standard in the next page.

Figure 2.31 Lutron anemometer

42

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2.3.7.1 General description

2.3.7.1.1 Features

Table 2.2 Lutron 4203 anemometer features

43

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2.3.7.1.2 General specification

2.3.7.1.3 Electrical specification (23 +/- 5 °C)

-

Table 2.3 Lutron 4203 general description

Table 2.4 Air velocity uncertainty number for The

Lutron 4203 anemometer

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2.3.7.2 Measurement standard

This is the standard used to measure fan velocity in the outlet. And from continuity, the air

velocity can be computed.

Mass transfer rate = constant

V channel = (V fan * A fan)/ A channel

Points Diameter

percent (%)

1 4.4

2 14.7

3 29.5

4 70.5

5 85.3

6 95.6

Figure 2.32 Velocity testing standard

45

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2.3.8 Digital multimeter

To measure PV output power, it is needed to measure current and voltage, therefore a

Multimeter is needed and it must be connected to PV wire.

The output power can be computed by polarization curve and MPPT and a load as below:

2.3.9 Sinus wave plates (sawtooth roughness)

2.3.9.1 Plates types

Three types of roughened surfaces are considered for the channel bed which all their angels

are the same, however the roughness length (e) alters.

The roughness length as below.

1. e =2cm

2. e=4cm

3. e=8cm

Figure 2.34 Digital multimeter

Figure 2.35 Schematic figure of three type of roughness in the channel bed

Figure 2.33 PV characteristic curve

46

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2.3.9.2 Plates images

2.3.9.2.1 e=2cm

The roughness plate with 2cm length. The image is as below.

\

2.3.9.2.2 e=4cm

The roughness plate with 4cm length. The image is as figure 3.20.

Figure 2.36 Sinus wave plates e=2

Figure 2.37 Sinus wave plates e=4

47

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2.3.9.2.3 e=8cm

The roughness plate with 8cm length. The image is as below.

2.3.10 The insulation

The channel insulated with 5milimeter foam which minimizes heat waste.

Figure 2.38 Sinus wave plates e=8

Figure 2.39s the channel perimeter insulated with 5mm foam

48

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2.4 Experimental stages

The performance of PVT system was monitored from March 2015 and May 2015. The

following is the entire experiment procedure:

1) Connect the 9w adaptor to Data logger and anemometer.

2) Connect the fan to urban electricity.

3) Put the pyranometer on the PV panel surface.

4) Connect thermocouple to datalogger and put datalogger in T-type mode in

order to read the temperature in the displayed location.

5) Connect multimeter wire to PV, then read Voltage and electrical current

quantity.

6) Determine fan outlet velocity with anemometer in the fan exit with the

mentioned equation in the fan section and we can find velocity in the channel.

7) Start reading Temperatures, the PV current and voltage

49

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Chapter 3

Theoretical equations

In 1981, MIT Lincoln Laboratory analyzed PV panel’s heat transfer. The cooling mechanism

is utilized to reduce the temperature of the PV modules. The front glass surfaces of the photovoltaic

modules are exposed to the surroundings and therefore radiation and convection need to be

considered in the heat transfer analysis of the module. As mentioned, There are several layers of

material in the PV panel. The Fourier conduction law can be implemented in analyzing the

conduction heat transfer in between these layers. The back of photovoltaic panels is attached to

the cooling duct. For that reason, forced convection is the main mechanism of heat transfer at the

back of modules. This is a transient simulation and the solar irradiation and ambient temperature

will be varied from time to time. The solar irradiation and ambient temperature is based on the

experimental data obtained on 19th April 2015.

Assumptions for numerical and experimental calculations:

1. Steady-state conditions in every hour. (Quasi steady)

2 Uniform heat flux.

3. Incompressible gas and negligible viscous dissipation.

4. Thermal properties are constant such as:

Thermal conductivity(k): k air=1.225

50

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Kinematic viscosity(ṿ): v air =1.78e-5

5. Adiabatic outer channel surface.

6. The data of solar irradiation and ambient temperature from the experiment were

used as the input of numerical simulation.

7. The ohmic losses in the solar cell are negligible.

8. Inter-reflections of insolation between the various surfaces are neglected.

9. Reynolds in the channel outlet is all the condition is turbulent and constant

(Re=3150)

10. All the walls except PV’s rear surface, are isolated for decreasing heat flux loss.

11. As temperature difference in the PV surface (ΔTS=4 K) and inside channel (ΔTm=1)

is so little, we consider Ts and Tm channel linearly along the channel. Therefore Ts and Tm

are equal to the average temperature of inlet, middle and outlet.

TS= 𝑠 𝑖𝑛 + 𝑠 𝑚𝑖𝑑 + 𝑠 𝑜𝑢𝑡

3 (3.1)

3.2 The analysis of heat transfer on Photovoltaic Panel

Total sun energy irritates to PV is: [64]

Ec = pαcτ g G (t) (3.2)

where G (t) is the solar irradiation incident on the glass cover, p is the cell packing factor

which defined as the ratio of area of solar cell to the area of blank absorber, αc is cell absorptivity to

sunlight, τg is the fraction transmitted through the front glass and low iron glass was used in the

experiment, τg =.0.95, αc= 0.75

51

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For the wavelength less than 1.1μm the absorption length is less than the thickness of typical

cell (i.e.: 260μm), hence the absorption process is completed before the radiation reaches the rear

surface. Absorptivity of the silicon solar cell can be computed through sun tables.

Polycrystalline silicon PV modules are used in the experiment. According to the Cox and

Raghuraman [55] report, the irradiation of wavelength above 1.1μm is transmitted through the

silicon cell without any absorption and this is absorbed by the back-sheet of PV module. The

insolation absorbed by the solar cell can be converted into electrical and thermal energy and the

equations are shown respectively below,

Ece= ηe p τ g I (t) (3.3)

Ect =

(1−ηe /α

c ) pα

cτ

g G(t) (3.4)\

Where Ece is electrical energy produced by photovoltaic cell, Ect is thermal energy released by

photovoltaic cell, ηe is the cell electrical efficiency and this parameter is functioned of the cell

temperature.

ɳe=ɳ0 [1- β * (Tc – T0)] (3.5) ɳ0 = Vmp Imp/ G A (3.6)

Where ηo is the nominal electrical efficiency under standard condition, A is the area of the PV

module, G is the irradiation and it is defined as 1000W/m2

for standard condition, Vmp is the

PV voltage at maximum power point and Imp is the PV current at maximum power point. All the

52

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relevant data can be obtained from the specification of PV module. To is the temperature of standard

condition, 25℃, Tc is the cell temperature, β is the temperature coefficient of silicon cell,

β=0.0045℃

ET = τ g (1− p) αT q″ (t) (3.7)

Therefore, we can compute ET with equation (3.7) which is the heat flux enters the channel, then

we can model the system numerically and find heat transfer coefficient (h) experimentally when p=0

ET is the rate of solar energy absorbed by Tedlar (rear surface) which the quantity for every hour in 19th April

Displayed in Table 4.1. τ g = 0.95, αT = 0.39 and p= 0.9.

10:00 7.19E+02 2.67E+01

11:00 9.34E+02 3.46E+01

12:00 1.02E+03 3.77E+01

13:00 1.04E+03 3.85E+01

14:00 9.83E+02 3.64E+01

15:00 8.38E+02 3.10E+01

16:00 6.37E+02 2.36E+01

17:00 4.16E+02 1.54E+01

18:00 1.64E+02 6.09E+00

Table 3.1 hear flux of Tedlar (PV’s rear surface)

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αT is the absorptivity of the Tedlar which is mentioned in Table 3.2.

ReD-out = 𝑈𝑜𝑢𝑡 𝐷ℎ

𝑣 (4.8)

Where Uout is the fluid velocity i n t he c han ne l o u t l e t , 𝑣 is the kinematic viscosity

of fluid and Dh is the hydraulic diameter. The Reynolds number must be more than 2000 to have

turbulent flow. In this case, ReD = 3150, therefore the flow is turbulent and as the roughness exists in

the channel bed, we will have early transition and the flow will be fully turbulent. In conclusion, we

have fully turbulent flow in this case and the best method to calculate turbulent flow and backflow in

the roughness pitch is K-omega method in the Fluent-Ansys 15.

Material Thermal

Conductivity

(W/m-ºK)

Specific Heat

Capacity

(KJ/kg K)

Density

(Kg/m3)

Thermal

Diffusivity

(m2/s)

Tedlar (Polyvinyl

Fluoride)

0.14 1010 1450 9.56E-08

EVA(Ethylene-vinyl

acetate)

0.3836 2220 1080 1.6E-07

PET(Polyethylene

terephthalate)

0.24 1000 1455 1.65E-07

Silicon

(Polycrystalline)

148 712 2330 8.92E-05

PV Glass 1 858 2500 4.66E-07

Table 3.2 Thermal proprieties of PV component’s material

54

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Dh = 4∗𝐴

(4.9)

Dh is hydraulic diameter and Aent and P are the flow cross-sectional area and wetted perimeter.

The Nusselt number Nu is a dimensionless measure to determine the convective heat

transfer coefficient from the inside surface of a duct. It can be physically interpreted as the

dimensionless temperature gradient at surface.

Nu= ℎ 𝐷ℎ

𝑘 (4.10)

h is the convection heat transfer coefficient for air, k is the thermal conductivity of the air,

and Dh is the hydraulic diameter of duct.

55

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(4.12)

Energy equation for the rear channel

The heat transfer equation can be written [65]

Ein + Egen = Eout + Est (4.11)

As the fluid is steady state Est is zero.

As there is no thermal or mechanical energy or radioactive generation, Egen is zero.

Therefore,

Ein= Eout

𝑞′′ = ℎ 𝑠 − = 𝑐 𝑜𝑢𝑡 − 𝑖𝑛

And In one hour which heat flux is almost constant, the heat transfer coefficient will be found

using with the following equation.

ℎ = 𝑞′′

𝑠 −

Figure 3.1 Schematic figure of energy equation in the channel

(4.12)

(4.13)

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3.3 Theoretical tilt angle calculation

The most effective insolation is direct (beam) insolation which has the most impact on PV

cells’ electrical output, therefore, we must find the way to maximize the beam insolation that enters PV

panel. One of the most less expensive and functional ways is finding the best optimum PV tilt in every

month.

To maximize the PV internal solar irradiation, it is necessary that PV panel always absorbs

most of beam irradiation. For obtaining this purpose, we require an angle in which solar panel locates

exactly in the vertical direction of beam incident. The sun moves in elliptical cycle, therefore almost

every day the sun beam insolation angle alters a little, however as the angle is negligible in a day, we

compute the angle variation in a month. Therefore, we need to compute what is the best PV angle to

the horizontal axis for every month. In this case the best PV angle for April has been gathered and with

theoretical equation verified.

The mathematical equation written in the next page is one the best equations for finding the PV

angle in every month. Angels will be found with these three equations. [37]

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Hg = the monthly average of daily global solar on horizontal.

H0 = daily extraterrestrial radiation on a horizontal surface.

Hd = Horizontal sky diffuse irradiation.

𝛽=is the slope of the panel as to the horizontal plane

𝛾= is the azimuth

𝜔=is the solar hour angle

(4.13a)

(4.13b)

(4.13c)

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Chapter 4

Results comparison and numerical verification

. The parameters compare in two direction:

PV electrical power enhancement

Heat transfer effect in the channel

In heat transfer in the channel, parameters like temperature of Photovoltaic module at

different location, inlet and outlet temperature of air flow, bulk temperature, and solar irradiation

which causes heat flux investigated to see how heat transfer effect on PV system and how much heat and electricity we

can receive from PV panels to use in domestic or industrial purposes. , Photovoltaic voltage, Photovoltaic current,

and electrical efficiency data were gathered to quest how cooling affect the electrical efficiency and

thermal performance enhancement.

The PV back surface temperature is linearly increases to the sun total radiation. It is shown in

the figure 5.1 that under cooling the temperature at 10 AM starts from 293.2K and for every 100W/m2

of solar irradiation, the temperature of PV increases approximately 2.5 ºC. In the other hand, without

cooling mechanism, the temperature initiate from 316K (the temperature difference is 22.8) and the

temperature increases approximately 4 ºC which is shown in Figure 5.2.

All test are repeated three times in March, April and May to avoid the clouds and wind speed

effect on the experimental results.

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Figure 4.1 Irradiation and Average Panel Temperature for the whole day

with cooling (19 April 2015)

Figure 4.2 total Irradiation and Average Panel Temperature for the

whole day without cooling (19 April 2015)

305

310

315

320

325

330

335

0.00E+00

2.00E+02

4.00E+02

6.00E+02

8.00E+02

1.00E+03

1.20E+03

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

W/m^2

q″(w/m^2) Ts

280

285

290

295

300

305

310

315

0.00E+00

2.00E+02

4.00E+02

6.00E+02

8.00E+02

1.00E+03

1.20E+03

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

W/m^2

q″(w/m^2) Ts(k)

60

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4.2 The impact of cooling mechanisms on the electrical power

4.2.1 PV tilt angle effect on electrical efficiency.

4.2.1.1 Images

The setup contains four different tilts (angles are from horizontal axis)

zero degree(θ=0)

30 degree (θ=30)

Figure 4.3 PV tilt is zero degree

Figure 4.4 PV tilt is 30 degree

61

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45 degree(θ=45)

60 degree(θ=60)

Figure 4.5 PV tilt is 45 degree

Figure 4.6 PV tilt is 60 degree

62

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6.1.1.2 Diagrams and data

The best angle for maximum beam irradiation can be discovered from comparison of their

electrical power output and with a MPPT controller and 12w lamp, all the tests are given, when

NOCT is 45 W.

In the figure 4.7 and Table 4.1, we have their electrical power output comparison.

ᵒ

0

20

40

60

80

100

120

140

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

W P[0°] P[30°] P[45°] P[60°]

Figure 4.7 PV tilt comparison in April

63

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Time[hr] P[0°] P[30°] P[45°] P[60°]

10:00 73.4 104.5275 76.7529 67.7529

11:00 89.24 110.2 91.6716 91.1976

12:00 104.49 115.32 106.8608 106.861

13:00 117.986 118.1262 111.786 117.366

14:00 109.061 98.8944 111.3156 111.316

15:00 101.8336 80.7296 97.364 97.364

16:00 84.777 69.788 68.5344 66.08

17:00 59.8122 52.217 53.4743 53.4743

18:00 26.6754 26.6754 32.336 13.536

Average 85.25372 86.8309 83.34396 81.5497

Table 4.1 PV panel’s tilt electrical power output comparison in April

64

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After analyzing the information obtained from experimental data, we can organize the best

tilt as the next page.

1. PV angle is 30 degree.

2. PV angle is 0 degree

3. PV angle is 45 degree

4. PV angle is 60 degree.

Therefore, the best tilt for the panel in April is 30 degree.

In next tests the tilt used for testing will be 30 degree.

Additionally, the optimum angle can be calculated by the equation (2.1) mentioned in the

literature review whose the result is 28 degree, so it shows close similarity between theoretical and

experimental results

0

20

40

60

80

100

120

140

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 Average

W

P[0°] P[30°] P[45°] P[60°]

Figure 4.8 PV panel’s tilt electrical power output comparison in April

65

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4.2.2 Channel depth effect on electrical efficiency

The channel geometry directly affects the Reynolds number which absolutely influences

Nusselt number between fluid and PV rear surface. It is experimentally considered three channel

heights to investigate which one can observe lager amount of heat transfer from PV’s rear surface.

Of course it is obvious in each height we have different velocity, however, the Reynolds number

has to be constant 3150. As the dimmer utilized in the experiment, we were able to maintain mass

transfer in a constant number of 1.83m/s, in order to hold the Reynolds number constant too.

4.2.2.1 Experimental shapes

4.2.2.1.1 30cm from PV’s rear surface.

Figure 4.9 channel with 30cm height under PV surface and height index is under test in April 15th

66

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4.2.2.1.2 20cm from PV’s rear surface.

4.2.2.1.3 10cm from PV’s rear surface.

Figure 4.11 the channel with 20cm height (right-sided view) Figure 4.10 the channel with 20cm height (front

view)

Figure 4.12 the channel with 10com height (front view) Figure 4.13 the height determent shaft in 10cm (right-sided view)

67

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4.2.2.2 Chart line diagrams

In all the diagrams, Reynolds number and roughness length is constant. [To hold the

Reynolds number fixed, I utilized the dimer and change the wind velocity in every height]

Reynolds number=3150

Roughness length= 2cm

In the figures 6.13, 6.14, 6.15, channel depth is variable and the power for each one

shows on the next page:

P10( electrical power for the channel with 10cm height)

P20( electrical power for the channel with 20cm height)

P30( electrical power for the channel with 30cm height)

Figure 4.14 an analogy between Electrical output and channel depth with

cooling mechanism from 10AM to 6 PM

0

50

100

150

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

W

P10(watt) P20(watt) P30(watt)

68

``

0

50

100

150

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

w

P10(watt) P20(watt) P30(watt) Pfree(watt)

0.50.60.70.80.9

11.11.21.31.41.51.61.71.81.9

2

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

P10/Pfree P20/Pfree P30/Pfree

Figure 4.15 proportion between Electrical output and channel depth with cooling and

without free convection (Pfree) from 10AM to 6 PM

Figure 4.16 proportion between Electrical output with cooling and without cooling to

channel depth from 10AM to 6 PM

69

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The line diagram analysis demonstrates that the best channel depth for the maximum

electrical output is 10 centimeter channel. Table 4.2 displays the data.

Time V(volt) I(ampere)P10(watt)

10:00 19.2 5.9 113.28

11:00 18.65 6.4 119.36

12:00 18.79 6.73 126.4567

13:00 19.85 7.07 140.3395

14:00 18.91 6.5 122.915

15:00 19.09 5.67 108.2403

16:00 19.2 4.3 82.56

17:00 18.58 4.3 79.894

18:00 19.98 2.7 53.946

The average electrical power from 10AM to 6PM is 105.22 W. which represents 21%

enhancement in electrical output compared to basic convection condition.

4.2.3 Roughness effect on electrical efficiency

4.2.3.1 Chart line diagrams

As it is shown in the previous part, the 10cm height gives the best electrical power, therefore

in this part the height in 10cm fixed and sinus wave plates alters to investigate its effect on electrical

power.

Table 4.2 electrical power for 10 cm channel height

70

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Reynolds number=3150

Channel height= 0.1m

In the figures 5.16, 5.17, 5.18, the channel bed’s roughness is adjustable and the power for

each is shows as:

P2( electrical power for the channel with 2cm length roughness)

P4(electrical power for the channel with 4cm length roughness)

P8(electrical power for the channel with 8cm length roughness)

`

Figure 4.18 analogy between electrical efficiency for variable channel bed’s

roughness and free convection without channel bed’s roughness

Figure 4.17 electrical efficiency for variable

channel bed’s roughness with active cooling

1

1.2

1.4

1.6

1.8

2

2.2

2.4

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

P2/Pfree P4/Pfree P8/Pfree

50

70

90

110

130

150

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

P2(watt) P4(watt) P8(watt)

71

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In the figures and tables, it is shown that in the four different tilts, three different channel height

and three different roughness which each on tested three times to obtain the best experimental results.

The best cooling mechanism is the PV panel with 10cm channel depth and 8cm roughness length in the

30º tilt from horizontal axis. (P final)

At last, It is required to compute electrical efficiency as a result of active and passive cooling

method that is utilized in this thesis.

P0= average electrical efficiency in the zero angle tilt without cooling mechanism.

Pmax = PV panel with 10cm channel depth and 8cm roughness length in the 30º tilt from

horizontal axis.(best cooling condition)

The details are shown in table 6.2.

20

40

60

80

100

120

140

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

P2(watt) P4(watt) P8(watt) Pfree(watt)

Figure 4.19 electrical output for variable channel bed’s roughness with active

cooling

72

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Time V(volt) I(ampere) P0 V(volt) I(ampere) Pfinal

10:00 20 3.67 73.4 19.5 5.95 116.025

11:00 19.4 4.6 89.24 18.65 6.7 124.955

12:00 19.28 5.42 104.4976 18.79 6.9 129.651

13:00 19.03 6.2 117.986 19.85 7.35 145.8975

14:00 19.1 5.71 109.061 18.91 6.85 129.5335

15:00 19.36 5.26 101.8336 19.09 5.9 112.631

16:00 19.67 4.31 84.7777 19.2 4.9 94.08

17:00 19.74 3.03 59.8122 18.58 4.5 83.61

18:00 19.33 1.38 26.6754 19.98 3.6 71.928

85.25372 112.0346

The electrical efficiency enhancement can be obtained by the equation (6.1)

ɳ 𝑙 = 𝑚𝑎𝑥− 0

0∗ 100 (5.1)

Therefore, the cooling mechanism could progress the electrical efficiency up to 31.41%. This

is a proof that cooling mechanisms are realistically able to improve PV panel technology.

Surprisingly the electrical efficiency enhance 31.5%, which about 30 watt more electricity

producing!!!

Table 4.3 P0 and Pfinal experimental data

73

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4.3 The impact of cooling mechanisms on the thermal performance

4.3.1 PV tilt effect on thermal performance

In the condition without cooling mechanism, the only way of heat transfer in the channel is

free convection which the heat transfer coefficient and Nusselt number can be computed as equation

5.2. [36]

(5.2)

𝑅𝑒′′ = 𝑔𝛽𝑞′′𝐻

𝛼𝜈𝑘𝐿

The data displayed in table 5.3 are computed by equation 5.2.

4.3.1.1 Nusselt number comparison in different tilt angle

For obtaining the PV thermal performance the most significant parameter is Nusselt number.

The comparison of Nusselt number in the figure 5.19 demonstrates which angle tilt is more efficient

for thermal performance in April. As it mentioned, all the data are gathered in March, April and May,

however all the diagrams based on the April data which were more accurate.

74

``

18

20

22

24

26

28

30

32

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Nu0 Nu30 Nu45 Nu60

Figure 4.20 Nusselt Number comparison in different tilt angle

Time Nu0 Nu30 Nu45 Nu60

10:00 28.72 29.69 29.055 27.284

11:00 30.44 31.2 30.67 28.918

12:00 31 31.7 31.2 29.45

13:00 31.08 31.78 31.28 29.526

14:00 30.94 31.47 31.055 29.393

15:00 29.71 30.45 29.93 28.2245

16:00 27.91 28.85 28.23 26.5145

17:00 25.11 26.5 25.655 23.8545

18:00 20.16 22.03 20.945 19.152

28.34111 29.29667 28.66889 26.92406

Table 4.4 Nusselt number comparison

75

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Therefore, in addition that 30º tilt is the best tilt for electrical efficiency, it the best for

thermal performance too.

Also, for the 30º tilt, the inlet temperature (Tin), outlet temperature (Tout), bulk temperature

(Tm) and PV rear surface temperature (Ts) as below.

0

10

20

30

40

50

60

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Ts(°c) Tin(°c) Tout(°c) Tm(°c)

Figure 4.21 inlet temperature (Tin), outlet temperature (Tout), bulk temperature (Tm) and PV

rear surface temperature (Ts) for the 30º tilt

76

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4.3.2 Channel depth effect on thermal performance

4.3.2.1 Data Diagrams

In this test, we keep roughness length (e=8) and Reynolds number (Re=3150) constant and

change the channel height to find the best situation in which thermal performance is maximum.

4.5

9.5

14.5

19.5

24.5

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

h10(((W/k*m^2) h20 (W/k*m^2) h 30(W/k*m^2)

Figure 4.22 an analogy between heat transfer coefficient and

channel depth with cooling mechanism from 10AM to 6 PM

77

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70

80

90

100

110

120

130

140

150

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Nu30 Nu20 Nu10

2.00E+01

4.00E+01

6.00E+01

8.00E+01

1.00E+02

1.20E+02

1.40E+02

1.60E+02

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Nu0 Nu30 Nu20 Nu10

Figure 4.23 an analogy between heat transfer coefficient and

channel depth with cooling mechanism from 10AM to 6 PM

Figure 4.24 proportion between Nusselt Number and channel depth with

cooling and without cooling (Nufree) mechanism from 10AM to 6 PM

78

``

The diagrams displays how is the trend of thermal performance in the channel with different height.

As it proofed with three experimental tests in May, April, and March. The channel with 10cm

height has the best performance compare to 20cm and 30 cm channel.

The 10cm channel data with 8cm roughness is in Table 5.5

2.00E+00

2.50E+00

3.00E+00

3.50E+00

4.00E+00

4.50E+00

5.00E+00

5.50E+00

6.00E+00

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Nu10/Nu0 Nu20/Nu0 Nu30/Nu0

Figure 4.25 proportion of Nusselt number to channel depth from 10AM to 6 PM

79

``

Ts(°c) Tm(°c) ET(w/m^2) h10(w.m^2.k) Kair(W/m.k)Dh(m) Nu

38.3 3.72E+01 26.7 24.3 0.027 0.17 153

38.7 3.67E+01 34.6 17.3 0.027 0.17 108.9259

39.2 3.70E+01 37.7 17.42 0.027 0.17 109.6815

37.8 3.59E+01 38.5 20.4 0.027 0.17 128.4444

39.75 3.76E+01 36.4 17.2 0.027 0.17 108.2963

40.3 3.85E+01 31 17.33 0.027 0.17 109.1148

36.3 3.49E+01 23.6 17.29 0.027 0.17 108.863

34.2 3.33E+01 15.4 17.43 0.027 0.17 109.7444

30.7 3.04E+01 6.09 20.03 0.027 0.17 126.1148

4.3.3 Roughness effect on thermal performance

Now, as 10cm channel obtained the best results, we keep it in the same Reynolds number

(Re=3150) and change the roughness length (e) as before.

1. e=2

2. e=4

3. e=8

The results are shown in the diagrams.

Table 4.5 the 10cm channel data with 8cm roughness is

80

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10111213141516171819202122232425

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

h8 (W/k*m^2) h4(W/k*m^2) h2(W/k*m^2)

70

80

90

100

110

120

130

140

150

160

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Nu8 Nu4 Nu2

Figure 4.26 heat transfer coefficient for variable channel

bed’s roughness with active cooling

Figure 4.27 Nusselt number for variable channel bed’s

roughness with active cooling

81

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In these parts showed that the best thermal performance occurs in the channel with 10cm

height which exactly the channel description that produced the most possible electrical efficiency

.The point is the best electrical and thermal efficiency happening at 10cm height which shows the

0.00E+00

1.00E+00

2.00E+00

3.00E+00

4.00E+00

5.00E+00

6.00E+00

7.00E+00

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Nu8/Nu0 Nu4/Nu0 Nu2/Nu0

Figure 4.28 proportion be Nusselt number for variable channel bed’s roughness with fan

and free convection without channel bed’s roughness and fan

h10 (W/k*m^2)Kair(W/m.k)Dh(m) Nu10 Nu0

24.3 0.027 0.17 153 28.729

17.3 0.027 0.17 108.9259 30.445

17.42 0.027 0.17 109.6815 30.999

20.4 0.027 0.17 128.4444 31.083

17.2 0.027 0.17 108.2963 30.947

17.33 0.027 0.17 109.1148 29.71

17.29 0.027 0.17 108.863 27.911

17.43 0.027 0.17 109.7444 25.113

20.03 0.027 0.17 126.1148 20.165

118.0206 28.34467

Table 4.6 the maximum Nusselt and

basic Nusselt in zero tilt angle

82

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close relation between electrical efficiency and thermal performance and an efficient way which are

able to enhance thermal and electrical efficiency.

The thermal performance index formula as below [66].

ɳth =

𝑁𝑢𝑜𝑝𝑡

𝑁𝑢0

𝑓𝑜𝑝𝑡

𝑓0 13

(5.3)

fopt= skin friction of 10cm channel with 8cm roughness length= 0.003(numerical result)

f0 = skin friction of channel with smooth bed channel= 0.015(numerical result)

Nuopt = average Nusselt number of 10cm channel with smooth bed channel= 118.026

Nu0 = average Nusselt number of PV with zero angle without fan= 28.344

The thermal of performance of panel is 7.12.This index show, we could enhance heat transfer

7.12 times bigger than our skin friction which is completely ideal with comparison by other work in

which thermal performance index is about 4[67].

Figure 5.28 displays PV rear surface with and without cooling. The decreasing temperature in

the best cooled condition and basic condition which shows how operating temperature affects electrical

efficiency and thermal performance is perceptible

83

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4.4 Numerical of verification of best experimental model.

For validating the experimental results, a numerical results need to be compared with the

experimental data and demonstrate how experimental outputs are accurate.

The numerical analysis of cooling mechanism has done in four steps:

1. It was designed in Spaceclaim software

2. It was meshed with ICME

Figure 4.29 PV’s rear surface under cooling (Ts-c) and

without cooling (Ts-0)

0

10

20

30

40

50

60

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Ts-c(°c) Ts-0(°c)

84

``

3. It was analyzed in Ansys fluent 15

At last the results were compared with experimental results in Microsoft Excel.

Numerical verification has been done for the best condition of channel (10cm) and roughness

length (e=8) in April.

At last net electrical output is equal:

P net, max = P produced, max – P fan, max (5.4)

E fan, max = 15 W.

E produced, max = 29.78

Therefore the net electrical output is 14.75 W.

4.4.1 Numerical process

The main description of numerical analysis is:

The general description: Pressure-based, absolute velocity in steady state condition.

Equations: Energy, Velocity x, y, z direction, Continuity, SST k-omega for viscosity.

After 5th time fragmentizing mesh, the result became mesh-independent.

The boundary conditions (figure 5.28) are:

PV rear surface(constant heat flux)

85

``

It is quasi steady and has constant heat flux every hour and the material is Tedlar. tg=0.95,

αt= 0.39, p =0.9

q″(w/m^2)ET(W/M^2)7.19E+02 26.7

9.34E+02 34.6

1.02E+03 37.7

1.04E+03 38.5

9.83E+02 36.4

8.38E+02 31

6.37E+02 23.6

4.16E+02 15.4

1.64E+02 6.09 .

Inlet(pressure inlet)

It is pressure inlet and the temperature is constant as below.

Time Ts(°c)

10:00 30.3

11:00 32.7

12:00 35.5

13:00 37.5

14:00 35.5

15:00 38

16:00 35.8

17:00 32.5

18:00 27.5

Table 4.8 PV’s rear surface

temperature at 1:00 PM APRIL 19th

Table 4.7 the heat flux of PV’ rear surface enter the channel

86

``

Outlet(velocity inlet)

As the channel height is 10cm for Reynolds number equal 5, velocity has to be -1.36.

All other boundaries (right wall, left wall, channel bed) are isolated and there heat flux is zero

material is Aluminum.

4.4.2 Plots

The schematic figure and plots are shown:

Figure 4.30 First mesh displayed in Fluent

87

``

Figure 4.31 channel static temperature in x direction [0<y<10, z=0.28] for 1:00 PM

Figure 4.32 PV Static temperature in z direction [y=10, x=0.6] for 1:00 PM

88

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4.4.3 Contours

The simulation displays the velocity magnitude in the channel’s inlet and PV’s rear surface

temperature under cooling mechanism in Figure 5.32 and 5.33. The figure 5.32 completely

demonstrates when speed in outlet is 1.36m/s, as a result of roughness in channel bed. Velocity in first

roughness edge goes up 4.13 which causes very fast heat transfer in the channel, therefore, the PV cools

down. In figure 5.33, PV surface temperature is displayed in order to show that the PV temperate

difference at the highest level is 5K. This temperature difference in the 1.15meter length of PV is very

little and temperature rises almost linearly in PV and of course in the channel as it is shown in figure

5.30.

Figure 4.33 Channel Static temperature in y direction [0<z<0.57, x=0.6] for

1:00 PM

89

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Figure 4.34 Velocity magnitude contours in the channel inlet

Figure 4.35 Temperature contours in the PV rear surface (Tedlar)

90

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4.4.4 Experimental and numerical Nusselt number comparison

At last, we need a proof that our experimental data is correct with small difference. The

experimental data that are gathered for the optimum condition (10 cm channel and 8cm roughness

length) compared with numerical result as below

And the Nusselt number comparison is:

100

110

120

130

140

150

160

170

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Nuexperimental Nutheoritical

Figure 4.36 Experimental and numerical Nusselt number

91

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As it is clear in the diagrams, theoretical and experimental results have admissible overlap.

Therefore experimental data are correct in the uncertainty domain. The uncertainty calculation is in the

next part.

4.5 Uncertainty

In the experiment research, we need to compute the uncertainty in the result. For analyzing the

uncertainty value in the final result, many method have been discovered. In the uncertainty computing,

it is considered that all the errors happen at the same time, however, it happens rarely.

The most accurate method for estimating the uncertainty value is Kline and McClintock method

[67].

.It is formulated as below:

2 2 2 0.5

1 2

1 2

(( ) ( ) .... ( ) )R n

n

R R RW w w w

X X X

(6.1)

Now, we compute Nusselt number with (6.1) equation above.

𝑁𝑢 =𝑞" 𝐷ℎ

𝑠 − 𝑤

92

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90

100

110

120

130

140

150

160

170

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00Nuexperimental Numin Numax

WNU =

(𝜕𝑁𝑢

𝜕𝑞”∗ 𝑞”)

+ 𝜕𝑁𝑢

𝜕𝐷ℎ∗ _𝐷ℎ +

𝜕𝑁𝑢

𝜕 𝑠∗ _ 𝑠 +

𝜕𝑁𝑢

𝜕 𝑤∗ _ 𝑤

+ 𝜕𝑁𝑢

𝜕 ∗ _ 0 5

Now, the uncertainty for heat flux is negligible for the Pyranometer, the uncertainty for PV rear

surface temperature and bulk temperature is 0.1 centigrade for T- thermocouple and the uncertainty

value for Hydraulic diameter of the channel is 1milimeter for the design ruler. Uncertainty value for

the thermal Conductivity is negligible.

The point chart for the Nusselt number uncertainty for the best channel condition (H=10cm,

e=8cm).

The Figure 6.34 demonstrates the domain of Nusselt number that another experimental

measurement must be between these ranges.

Figure 4.37 Uncertainty Domain for Nusselt number in

channel 10cm and 8cm roughness height

93

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Chapter 5

Conclusion and future research

5.1 Conclusion

At the end, the experimental results which were repeated three times in March, April and May

represented the fact that passive and active cooling is operational and cost-effective way of increasing

the hybrid PV/T system.

The following deductions are drawn based on experimental results:

1. The experimental data of best state of channel and roughness fabricated in this

experiments the Nusselt number agrees with numerical predictions in the developing

turbulent flow.

2. The electrical efficiency and heat transfer coefficient heightened substantially by the

artificial roughness height in the channel, yet, it increased pressure drop poorly. It is

proofed as artificial roughness gets higher, the Nusselt number and electrical output

increase too.

3. The channel height is found to be one the most significant factors which has great

impact on cooling. It is a cost-effective way which could improve electrical output and

Nusselt number in an impressive amount. It showed, as channel height became smaller,

both electrical and thermal performance progressed.

4. The adjustable PV tilt angle, is also so important since the panel can absorb the

maximum total insolation in every month. It is showed in April the 30 degree tilt is the

best.

94

``

5. In detail, it is expressed how significant the PV cell’s temperature and influences on

electrical efficiency. It is showed, at 1:00 PM, when cell operating temperature is

56 ºC, the electrical power is 117 watt, however, after utilizing cooling mechanism at

1:00 PM, in the electrical output is 144W in 42 ºC tempearatue.

6. Earlier transition from laminar flow to turbulent flow as existence of roughened

channel displayed in velocity magnitude contour to enhance. The roughness and

channel height both increased the flow velocity up to 4.13 m/s which is almost 3times

more than the velocity that Centrifugal fan sucks in channel outlet (1.36 m/s).

7. The net electrical output is 14.78W.

5.2 Future research

I personally recommend five different method to enhance the electrical efficiency and thermal

performance of PV panes. First, in order to avoid sharp edges, it is recommended that sinus wave

become smoother so that flow slip easily with fewer pressure lost. Second method is to stimulate earlier

transition, smooth ribbon in the channel inlet is effective to make turbulent flow earlier and of course

it increases heat transfer and electrical efficiency. Third, for the fan electricity supply, my suggestion

is using a solar thermoelectric which can produce electricity with panel’s output heat. Furthermore, in

addition to cool down the rear surface of PV, it is useful to cool down the upper surface of panel which

is in front the sun with water cooling. This method in addition to cooling the PV, water works as shield

which prevent some wavelength of sun irradiation from going out which cause thermal performance

boosting .At last decreasing the channel height in the micro- channel size will enhance electrical and

thermal output significantly.

95

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