COMPUTATIONAL AND EXPERIMENTAL STUDIES ON A SOLAR CHIMNEY POWER PLANT

by

Sandeep Kumar Patel

A thesis submitted in fulfillment of the

requirements for the degree of

Master of Science in Engineering

Copyright © 2013 by Sandeep Kumar Patel

School of Engineering and Physics

Faculty of Science, Technology, and Environment

The University of the South Pacific

June 2013

i

Declaration of Originality Statement by Author

I, Sandeep Kumar Patel, hereby declare that the write up of the research project is

purely my own work without the inclusion of any other research materials that has

already been published or written. Any individuals’ work or idea that has been

included within the report has been clearly referenced and credit given to the person.

_________________

Sandeep Kumar Patel

S11031673

28/06/2013

Statement by Supervisor

I hereby confirm that the work contained in this supervised research project is the

work of Sandeep Kumar Patel unless otherwise stated.

____________________

Dr. M. Rafiuddin Ahmed

Principal Supervisor

28/06/2013

ii

Acknowledgements

First of all, I would like to thank the almighty God for giving me the

knowledge and patience to successfully finish this research. I would like to thank my

parents, Jayanti Lal Patel and Gitaben Patel, my sister, Nikita Ben Patel and my

wife, Varsha Mala for their continuous encouragement throughout the project. I

would also like to thank my uncle and aunty and my cousins for their continuous

support through the project. I sincerely thank my supervisor, Dr. M. Rafiuddin

Ahmed, for his guidance, assistance and support in my experiments, publications,

and compilation of the thesis.

I am very grateful to the University of the South Pacific, Faculty of Science

and Technology Research Committee for funding this research project. I also owe

gratitude to all the academic and technical staff members of the School of

Engineering and Physics; special thanks to Mr. Sanjay Singh and Mr. Shiu Dayal for

their guidance in technical issues and helping me with the fabrication.

I would also like to record my thanks to my colleagues Mr. Krishnil Ram,

Mr. Shivneel Prasad, Mr. Vineet Chandra, Mr. Deepak Prasad, Mr. Sandeep Reddy,

Mr. Kaushik Sharma, Mr. Epeli Naboloniwaqa, Mr. Jai Goundar, Mr. Mohammed

Faizal, Mr. Mohammed Tazil, Mr. Ronit Singh, Mr. Imran Jannif, and Mr. Shahil

Ram for helping me with the experiments and giving moral support.

I would further like to thank all those who have helped me in anyway to

accomplish my Masters Degree, a big milestone in my life.

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Publications

1. Patel, S. P., Prasad, D. and Ahmed, M. R., “Computational Studies on the Effect

of Geometric Parameters on the Performance of a Solar Chimney Power Plant”,

Energy Conversion and Management, Elsevier, (Under review).

2. Patel, S. P. and Ahmed, M. R., “Computational and Experimental Studies on a

Solar Chimney Power Plant for Power Generation in Pacific Island Countries”,

manuscript under preparation.

iv

Abstract

The solar chimney power plant (SCPP) is a renewable-energy power plant that

transforms solar energy into electricity. The SCPP consists of three essential

elements – solar air collector, chimney tower, and wind turbine(s). The present work

is aimed at optimizing the geometry of the major components of the SCPP using a

computational fluid dynamics (CFD) software ANSYS-CFX to study and improve

the flow characteristics inside the SCPP. The overall chimney height and the

collector diameter of the SCPP were kept constant at 10 m and 8 m respectively. The

collector inlet opening was varied from 0.05 m to 0.2 m. The collector outlet height

was also varied from 0.5 m to 1 m. The collector outlet diameter was also varied

from 0.6 m to 1 m. These modified collectors were tested with chimneys of different

divergence angles (0 – 3 ) and also different chimney inlet openings of 0.6 m to 1 m.

The diameter of the chimney was also varied from 0.25 m to 0.3 m. Based on the

CFD results, the best configuration was achieved using the chimney with a

divergence angle of 2 and chimney diameter of 0.25 m together with the collector

opening of 0.05 m and collector outlet diameter of 1 m. Based on the best

configuration obtained from the 10 m SCPP, a scaled down model of 1:2.5 was

modelled and simulated. The 4 m SCPP had a fixed chimney height of 4 m and a

collector diameter of 3.2 m. The collector outlet height was also kept constant at 0.2

m. the collector outlet diameter was varied from 0.24 m to 0.4 m and the chimney

throat diameter was varied from 0.10 m to 0.12 m. The collector opening was also

varied from 0.02 m to 0.08 m. This configuration was then fabricated and tested. PT

– 100 temperature sensors were used to measure temperature across the collector and

along the chimney. A pitot static tube was used to measure the dynamic pressure at

the throat. The dynamic pressure was converted into velocity. The experimental

results were then compared to the 4 m CFD results. The results were very similar. A

100 m SCPP was later modelled and simulated to predict the power available for

bigger size towers. The 100 m tower produced a maximum available power of 35.8

kW and maximum air velocity of 22.72 m/s. Such a plant will be suitable to meet the

power requirements of small islands in Pacific Island Countries where the

requirements are of the order of tens of kilowatts.

v

Table of Contents

Declaration of Originality i

Acknowledgements ......................................................................................................... ii

Publications ................................................................................................................... iii

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

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

1.1 Thesis Objectives ...................................................................................................... …2

1.2 Thesis Outline.............................................................................................................. 2

2. Literature Review ...................................................................................................... 4

2.1 History......................................................................................................................... 4

2.2 Solar Chimney Power Plant ......................................................................................... 5

2.3 Components ................................................................................................................. 6

2.3.1 Collector .............................................................................................................. 6

2.3.2 Chimney .............................................................................................................. 8

2.3.3 Turbines ............................................................................................................... 9

2.4 Thermodynamics Cycle ............................................................................................. 10

2.4.1 The Solar Chimney Power Plant as a Gas Turbine .............................................. 11

2.4.2 Air Standard Analysis of Solar Gas Turbine Cycle ............................................. 12

2.4.3 Air Standard Analysis of a Solar Chimney Power Plant Cycle ............................ 14

2.5 Solar Chimney Power Plant Theoretical and Experimental Models ............................. 18

3. Methodology ........................................................................................................... 26

3.1 Numerical Work ........................................................................................................ 26

3.1.1 CFD Code .......................................................................................................... 26

3.1.2 Full Buoyancy Model (Density Difference) ........................................................ 28

3.1.3 Boussinesq Model .............................................................................................. 29

3.1.4 Numerical Setup................................................................................................. 29

3.1.5 Geometry Generation or Modelling .................................................................... 30

3.1.6 Mesh or Grid Generation .................................................................................... 31

3.1.7 Physics Pre – Processor ...................................................................................... 33

3.1.8 Solver ................................................................................................................ 34

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3.1.9 Post – processor ................................................................................................. 35

3.2 Experimental Method................................................................................................. 35

3.2.1 Chimney Bellmouth ........................................................................................... 36

3.2.2 Solar Air Collector ............................................................................................. 40

3.2.3 Solar Chimney ................................................................................................... 43

3.2.4 Foundation ......................................................................................................... 43

3.2.5 The 4m Tall Experimental SCPP ........................................................................ 46

3.2.6 PT – 100 Temperature Sensor ............................................................................ 47

3.2.7 DaqPRO Datalogger........................................................................................... 48

3.2.8 Pitot – Static Tube .............................................................................................. 51

3.2.9 Furness Controls Digital Micromanometer FCO510 ........................................... 51

4. Results and Discussions .......................................................................................... 54

4.1 Numerical Results for the 10m SCPP ......................................................................... 54

4.2 Numerical and Experimental Results for the 4 m Tall SCPP ...................................... 69

4.3 The 100m SCPP Numerical Results ........................................................................... 79

5. Conclusions ............................................................................................................ 83

6. References .............................................................................................................. 84

vii

List of Figures

Figure 1.1: Rate of the world energy use growth [2] ............................................................ 1

Figure 2.1: The SCPP in Manzanares, Spain [11] ................................................................ 4

Figure 2.2: The SCPP collector [19] .................................................................................... 7

Figure 2.3: Schematic of an SCPP collector [20] ................................................................. 8

Figure 2.4: The chimney of the SCPP [24]........................................................................... 9

Figure 2.5: The axial flow type turbine for SCPP [26] ....................................................... 10

Figure 2.6: The T-s diagram for a solar gas turbine cycle ................................................... 12

Figure 2.7: Schematic of an SCPP ..................................................................................... 14

Figure 2.8: The T-s diagram for an SCPP cycle ................................................................. 15

Figure 3.1: Structure of ANSYS CFX................................................................................ 30

Figure 3.2: The SCPP model created in Autodesk Inventor ................................................ 31

Figure 3.3: The solar chimney mesh .................................................................................. 32

Figure 3.4: The solar air collector mesh ............................................................................. 32

Figure 3.5: Various boundaries of the SCPP ...................................................................... 34

Figure 3.6: Schematic of the 4m tall experimental SCPP ................................................... 36

Figure 3.7: The 3D view and the schematic of the chimney bellmouth ............................... 37

Figure 3.8: The flanges for the chimney bellmouth ............................................................ 37

Figure 3.9: Flat bars being bent and welded to the flange ................................................... 38

Figure 3.10: Flat bars and triangular pieces of sheet metal being welded together .............. 38

Figure 3.11: The chimney bellmouth being machined in the lathe to level the inside surface

......................................................................................................................................... 39

Figure 3.12: The chimney bellmouth fully welded together ............................................... 39

Figure 3.13: The 3D view and the schematic of the frames of the solar air collector ........... 40

Figure 3.14: Frames for the solar air collector .................................................................... 41

Figure 3.15: Perspex for the solar air collector ................................................................... 42

Figure 3.16: The solar air collector pre-allignment............................................................. 42

Figure 3.17: A section of a solar chimney .......................................................................... 43

Figure 3.18: Schematic of the SCPP foundation and footing details ................................... 44

Figure 3.19: The solar chimney foundation ........................................................................ 44

viii

Figure 3.20: Cement being poured onto the SCPP foundation ............................................ 45

Figure 3.21: Wire mesh 665 on top of the ground surface before cement is poured ............ 45

Figure 3.22: The solar air collector sitting on the black painted cement absorber................ 46

Figure 3.23: The 4m tall experimental SCPP ..................................................................... 47

Figure 3.24: PT – 100 Temperature sensor [66] ................................................................. 48

Figure 3.25: The DaqPro datalogger [67] ........................................................................... 49

Figure 3.26: A Pitot – Static tube ....................................................................................... 51

Figure 3.27: Furness Controls Digital Micromanometer..................................................... 52

Figure 4.1: Schematic diagram of the SCPP with the various parameters that were studied 54

Figure 4.2: Power available for case 5 for various collector inlet openings and various

chimney divergence angles. ............................................................................................... 56

Figure 4.3: Temperature contours on the collector for collector inlet opening of 0.05 m ..... 57

Figure 4.4: Temperature contours on the collector for a collector inlet opening of 0.2 m .... 57

Figure 4.5: Power available for cases 1, 5 and 9 for the collector inlet openings of 0.1 m and

different chimney divergence angles.................................................................................. 59

Figure 4.6: Power available for cases 3, 7 and 11 for the collector inlet openings of 0.1 m

and different chimney divergence angles ........................................................................... 60

Figure 4.7: Power available for cases 1, 3, 5, 7, 9 and 11 for the collector inlet opening of 0.1

m and different chimney divergence angles ....................................................................... 60

Figure 4.8: Power available for cases 2, 6 and 10 for the collector inlet opening of 0.1 m and

different chimney divergence angles.................................................................................. 61

Figure 4.9: Power available for cases 1, 2, 5, 6, 9 and 10 for the collector inlet opening of 0.1

m and different chimney divergence angles ....................................................................... 62

Figure 4.10: Velocity vectors on the entire SCPP for case 3 for the collector inlet opening of

0.05 m and chimney divergence angle of 2⁰ ....................................................................... 63

Figure 4.11: Temperature contours on the entire SCPP for case 3 for the collector inlet

opening of 0.05 m and chimney divergence angle of 2⁰ ..................................................... 63

Figure 4.12: Temperature variation along the chimney height for case 3 for the collector inlet

opening of 0.05 m and chimney divergence angle of 2⁰ ..................................................... 64

Figure 4.13: Velocity variation along the chimney height for case 3 for the collector inlet

opening of 0.05 m and chimney divergence angle of 2⁰ ..................................................... 65

ix

Figure 4.14: Temperature variation along the outer radius of the collector to the center

measured at 0.025 m above ground for case 3 for the collector inlet opening of 0.05 m and

chimney divergence angle of 2⁰ ......................................................................................... 66

Figure 4.15: Temperature variation from the ground to the collector outlet at the center for

case 3 for the collector inlet opening of 0.05 m and chimney divergence angle of 2⁰ .......... 67

Figure 4.16: Velocity variation from the ground to the collector outlet at the center for case 3

for the collector inlet opening of 0.05 m and chimney divergence angle of 2⁰ .................... 67

Figure 4.17: Temperature variation from the ground to the top of the chimney for case 3 for

the collector inlet opening of 0.05 m and chimney divergence angle of 2⁰.......................... 68

Figure 4.18: Velocity variation from the ground to the top of the chimney for case 3 for the

collector inlet opening of 0.05 m and chimney divergence angle of 2⁰ ............................... 68

Figure 4.19: Temperature and pressure sensors measurement locations .............................. 70

Figure 4.20: Power available for cases A, B, C and D for the chimney divergence angle of 2

and different collector inlet openings ................................................................................. 71

Figure 4.21: Mass flow rate for cases A, B, C and D for the chimney divergence angle of 2

and different collector inlet openings ................................................................................. 72

Figure 4.22: Velocity for cases A, B, C and D for the chimney divergence angle of 2 and

different collector inlet openings ....................................................................................... 73

Figure 4.23: Velocity vectors on the entire SCPP for case D for the collector inlet opening of

0.04 m ............................................................................................................................... 74

Figure 4.24: Temperature contours on the entire SCPP for case D for the collector inlet

opening of 0.04 m ............................................................................................................. 75

Figure 4.25: Temperature variation along the outer radius of the collector to the center

measured at various locations for both experimental and CFD for case D for the collector

inlet opening of 0.04 m ..................................................................................................... 76

Figure 4.26: Temperature variation along the chimney height for both experimental and CFD

for case D for the collector inlet opening of 0.04 m ........................................................... 77

Figure 4.27: Temperature variation across the collector from 9:00 am to 8:00 pm on a typical

day .................................................................................................................................... 78

Figure 4.28: Temperature variation along the chimney from 9:00 am to 8:00 pm on a typical

day .................................................................................................................................... 79

Figure 4.29: Temperature variation from the ground to the top of the chimney for the 100 m

SCPP ................................................................................................................................ 80

Figure 4. 30: Velocity variation from the ground to the top of the chimney for the 100 m

SCPP ................................................................................................................................ 81

x

Figure 4.31: Temperature variation along the outer radius of the collector to the center

measured at 0.025 m for the 100 m SCPP .......................................................................... 82

xi

List of Tables

Table 3. 1: PT – 100 Temperature Sensor Specifications ........................................ 48

Table 3. 2: DaqPRO Datalogger Specifications ...................................................... 49

Table 3. 3: Furness Controls Digital Micromanometer Specifications .................... 52

Table 4. 1: Different Configurations of the 10 m SCPP Tested ............................... 55

Table 4. 2: Different Configurations of the 4 m SCPP Tested ................................. 69

xii

Nomenclature

Symbol Descriptions Units

A Cross-sectional area m2

Compression temperature ratio -

Specific heat capacity J / kg K

Grashoff number -

Acceleration due to gravity m / s2

Enthalpy J / kg

Mass flow rate kg / s

n Power law profile exponent -

P Power W

Prandtl number -

Pressure Pa

Rayleigh number -

Reynolds number -

Cycle pressure ratio -

S Source term -

s Entropy J / kg

T Temperature K

Time s

Vector of velocity in XYZ coordinates m / s

xiii

Velocity Magnitude m / s

v Velocity m / s

Height (or vertical distance) m

Greek Symbol Descriptions

Thermal expansion coefficient 1 / K

γ Specific heat ratio of air -

Efficiency -

ρ Density kg / m3

Stress tensor -

Thermal Conductivity W / m K

Subscripts

1 Solar gas turbine compressor inlet

1’ Solar chimney atmospheric inlet

2 Solar collector inlet

3te Turbine exit

4 Chimney exit

E Energy equations

lift Power required to lift air in the chimney

M Momentum equations

ref Reference

xiv

shaft Shaft power

tot Total

Superscripts

* Normalized quantity

‘ Solar chimney atmospheric inlet

Abbreviations

CFD Computational fluid dynamics

SCPP Solar chimney power plant

1

1. Introduction

The increase in global energy consumption and the rapid development of global

economy is known to cause serious environmental problems [1]. Compared to the

past years, the demand for electricity today is far greater than ever in both developed

and developing countries. Even today, fossil fuels are the primary fuel sources and

are still widely used for major electricity generation as shown in Figure 1.1 [2].

Many developing countries cannot afford these energy sources due to its high cost,

and nuclear power stations are an unacceptable risk in many locations around the

world. Inadequate energy supplied do not only lead to higher energy costs, but

poverty as well which commonly results in population explosions [3]. Not only are

they expensive and cause harm to the environment, fossil fuels are diminishing day

by day. With fossil energy nearing exhaustion as well as greenhouse effect and air

pollution being more severe, utilization of renewable energy technologies are

increasingly gaining great importance and is playing a major role in solving the

above problems in future [4].

Figure 1.1: Rate of the world energy use growth [2]

2

Although renewable energy related technologies are still at its primary stages, they

hold a great promise for the future. This is due to the fact that renewable energy is a

cleaner and greener source of energy and is also available in abundance. While there

are so many different sources of renewable energy, solar energy is one of the more

promising ones since the sun is the ultimate source of most renewable energy

supplies. Although solar energy has the highest available energy, only a little fraction

of the available energy is used [5]. The biggest problem with solar energy is that it is

only available in the day, but technical advancements through research has made it

possible to harness the solar energy at night by storing the solar energy available in

the day.

Although there are so many devices that have been built to harness this energy from

the sun, majority of them are very expensive to build and maintain and that is a

major issue in developing countries. The solar energy device must be simple, reliable

and cheap to build and maintain. The solar chimney power plant (SCPP) meets these

conditions very well. The solar chimney is simple and reliable since it doesn’t have

many moving parts and it is cheap since the raw materials needed to build the solar

chimney are readily available in most developing countries. The SCPP can also

produce power at night by using water bags inside the collector to store and release

heat slowly at night to provide a continuous 24 hours power supply.

1.1 Thesis Objectives

� To perform a detailed literature survey on an SCPP, their operational concepts,

the individual components, and overall performance parameters.

� To fully design an SCPP using Autodesk Inventor and test it numerically using a

commercial code ANSYS CFX.

� To fabricate and install an SCPP at the University of the South Pacific (USP).

� To experimentally determine the performance of the SCPP under various

operational conditions.

� To simulate a 100 m tall SCPP and predict the power available.

1.2 Thesis Outline

� Chapter 1 gives a general introduction on the importance of renewable energy

and how an SCPP will help dealing with the problem the world is facing.

3

� Chapter 2 gives an overview of the development and studies carried out on the

SCPP over years. Also major components of the SCPP are discussed in details.

Detailed theoretical analysis using the thermodynamic cycle is also presented in

this chapter.

� Chapter 3 provides a detailed methodology of both numerical and experimental

works of the SCPP.

� Chapter 4 presents the numerical and experimental results of the SCPP.

� Chapter 5 finally summarizes the main findings of this research.

4

2. Literature Review

2.1 History

In 1903, Isidoro Cabaynes, a Spanish artillery colonel gave one of the earliest

descriptions about the solar chimney power plant. His idea of the solar chimney

power plant was attaching some kind of wind propeller to the chimney of a house for

power generation [6].

In 1926, Prof Engineer Bernard Dubos proposed a construction of a solar chimney

power plant whose chimney will be positioned on the slope of a high mountain. In

1931, a German author, Hanns Gunther demonstrated the concept of a solar chimney

power plant technology by performing a small experiment based on plate and a spirit

lamp which acts as the heat from the Sahara desert and the small wind wheel on top

of the chimney represents the wind turbines [6, 7].

The first real concept of the solar chimney power plant was proposed by Professor

Jorg Schlaich in 1978. In 1982, a 50kW solar chimney power plant was constructed

and tested out in Manzanares, Spain [8, 9] as shown in Figure 2.1.

Figure 2.1: The SCPP in Manzanares, Spain [11]

5

The prototype had collector radius of 122m and a chimney height of 194.6m and

produced a maximum updraft air velocity of 15m/s under no load conditions [7]. The

prototype was tested for 7 years till 1989 where the tower collapsed due to rust and

storm winds affecting the guy wires which were not protected against rust [10].

2.2 Solar Chimney Power Plant

Solar thermal power plants are normally classified into two main categories: high

and low temperature power plants. This mainly depends on their temperature level.

High temperature power plants collect direct solar radiation and often use a closed

cycle thermodynamic process. These plants have very high efficiency but also high

capital costs and operational costs. Solar chimney power plants are classified as low

temperature power plant since its working fluid is kept primarily in the free

atmosphere [12]. Solar chimney power plants comprise of three major components

[13]:

� The collector (greenhouse)

� The chimney

� The power conversion unit which included turbines

The chimney is a long cylindrical structure placed in the centre of the circular

greenhouse collector [14, 15]. The turbine is normally placed at the base of the

chimney where the pressure difference is large from the outside [8]. The green house

collector is made out of transparent glass or plastic film which is supported close to

the ground and its height increases towards the centre where the chimney is placed.

The solar radiation enters the greenhouse collector and gets absorbed by the soil. The

air around the soil gets heated up and starts to rise towards the chimney where a

turbine is placed to produce power. Suction from the chimney draws more hot

buoyant air from the collector and cold air from free atmosphere replaces the hot air

through natural convection. To ensure that the solar chimney power plant works at

night, water filled tubes/bags are placed under the collector roof [16].

The solar energy is first converted into thermal energy by the soil, which is then

converted into kinetic energy by the hot air and later converted into mechanical

energy by the turbine rotors which is then eventually converted into electrical energy

[15].

6

Solar chimney power plants have notable advantages compared to other solar

thermal power plants [3]:

� The collector uses both direct and diffuse solar radiation. This is very important

for tropical countries where the sky is often overcast.

� Solar chimney power plants can operate for 24 hours with the help of water filled

tubes placed under the collector roof. During the day the soil and water filled

tubes absorb heat from solar radiation and at night the water filled tubes releases

heat slowly.

� Solar chimney power plants are very reliable and are not likely to break down

since it has very few moving parts. The structure is very robust and the only

moving part is the turbines thus require very little maintenance.

� Solar chimney power plants do not need cooling water like other conventional

power plants. This is an advantage since majority of the sunny countries have

difficulty getting fresh drinking water.

� Solar chimney power plants can even be built in less developed countries since

building materials mainly concrete and glass or plastic sheets are readily

available in sufficient quantities. Also solar chimney power plants do not need

high – tech manufacturing plants for construction and this will help less develop

countries with more job opportunities.

One of the most notable disadvantage of solar chimney power plants is poor or low

efficiency level due to solar chimney power plants converting a very little amount of

solar radiation into electricity, thus requiring a much larger collector area [3, 17].

2.3 Components

2.3.1 Collector

The collector of a solar chimney power plant is normally made up of glass or plastic

film which is stretched horizontally for several meters above ground as shown in

Figure 2.2. The height of the collector gradually increases towards the centre where

the chimney is often placed. This ensures smooth transition of hot air flowing from

the collector to the chimney, therefore reducing frictional and eddy losses. The main

purpose of the collector is to allow the transmission of short wave radiation and

blocks the long wave radiation emitted by the heated ground. This will result in the

7

heating up of air under the collector which then flows radially towards the centre

where the chimney is positioned [15, 18].

Figure 2.2: The SCPP collector [19]

The soil under the collector acts like a thermal storage for solar radiation and slowly

emits most of it during the day and some at night. The soil itself is not enough to

emit radiation throughout the night for continuous 24 hours power generation. For

continuous 24 hours operation, water filled black tubes are often placed side by side

on the soil under the collector as shown in Figure 2.3. The water filled tubes absorb

solar radiation in the day and slowly emits it at night. This is because thermal storage

of water works more efficiently than soil alone since the specific heat capacity of

water is about five times larger than that of soil and even at low water velocities –

from natural convection in tubes – the heat transfer between water tubes and water is

much more efficient than that between ground surface and soil layers underneath.

These water filled tubes are only filled once and are sealed to prevent evaporation

from taking place [16, 18, 20]. These water filled tubes are transparent on the upper

surface to allow solar radiation through and are painted black at the bottom surface

to absorb most of the solar radiation [20]. The volume of water in the tube

8

corresponds to the water layer thickness and this is selected to achieve desired power

output characteristics.

Figure 2.3: Schematic of an SCPP collector [20]

2.3.2 Chimney

The solar chimney or tower of a solar chimney power plant is a long cylindrical

structure placed at the centre of the collector and one or more turbo generators are

installed at its base for the purpose of power generation [8, 14, 15, 21, 22] as shown

in Figure 2.4. The solar chimneys normally have a specific angle of inclination, often

vertical as it is easier to build and operate [23].

The solar chimney itself is the actual thermal engine of the solar chimney power

plant. The solar chimney is like a pressure tube with low frictional losses due to its

favourable surface – volume ratio [15, 18, 20]. The updraft air velocity or the mass

flow rate of the updraft air is approximately proportional to the air temperature rise

in the collector and the solar chimney or tower height [18, 20].

9

Figure 2.4: The chimney of the SCPP [24]

2.3.3 Turbines

The turbine (or turbines) is one of the core components of solar chimney power

plants. The main purpose of the turbine is to convert the kinetic energy of heated air

into mechanical energy using the turbine rotors [20]. The conventional turbine of a

solar chimney power plant is normally placed at the base of the chimney because of

easier installation and maintenance for large scale solar chimney power plants [20].

The turbine of solar chimney power plant is usually an axial flow type turbine as

shown in Figure 2.5. The characteristics of these turbines (the number of rotor

blades, specific speeds, and turbine diameters) lie between that of a wind turbine and

a gas turbine. It has more than two to three blades of wind turbine but not as many as

gas turbines. The pitch angle of the blades can be adjusted like wind turbines but due

to the flow being enclosed in a solar chimney power plant like a gas turbine, the

turbine may have radial inflow guide vanes [20, 25]. Due to the fact that the turbines

do not work with staged velocity like free – running wind energy converters, but as

shrouded pressure – staged wind turbo generators, the specific power output is

roughly one order of magnitude higher than that of a velocity staged wind turbine.

The air velocity before and turbine is approximately the same [18]. The power output

10

achieved is proportional to the product of volume flow per unit time or the volume

flow rate and the pressure difference or pressure drop across the turbine [15, 18].

Figure 2.5: The axial flow type turbine for SCPP [26]

2.4 Thermodynamics Cycle

The thermodynamic processes in a solar chimney power plant could be presented as

a simple thermodynamic cycle obtained from the air standard cycle to give a clear

and useful understanding of the real cycles that they simulate. The working fluid in a

solar chimney power plant is air and it is assumed to behave as an ideal gas [27].

Assumptions for air standard cycle analysis

� The working fluid is dry air assumed to be behaving as an ideal gas with constant

specific heat

� The mass flow rate of the system is constant

� The compression and expansion processes in the air standard cycle are adiabatic

and reversible (isentropic)

� The change in kinetic energy of the air between inlet and exit of each component

is negligible

11

� In the combustor and passages, there are no stagnation pressure drops

� The inlet and exit atmospheric conditions are identical

� The only heat flow is the net heat flow of air in the system

2.4.1 The Solar Chimney Power Plant as a Gas Turbine

The first observation made for a solar chimney power plant cycle is that it is

ultimately a gas turbine (Joule or Brayton) cycle. Considering the gas turbine air

standard cycle, there are normally four processes involved; isentropic compression,

constant pressure heat addition, isentropic expansion and constant pressure heat

removal [27].

Although the solar chimney power plant cycle and the gas turbine cycle are basically

the same, there are some practical differences. One of the main practical differences

is that the inlet and the exit atmospheric conditions are assumed to be identical in the

gas turbine cycle, however this is not true for the solar chimney power plant cycle

since the exhaust is at the altitude of the chimney top, and the atmosphere has the

additional function of recompressing the exhaust air to the ground inlet conditions

[27]. This compression doesn’t have to be isentropic; it can be approximated by a

polytropic expression of the form:

(1)

where p is the pressure, ρ is the density and n is some exponent that is not

necessarily equal to the specific heat ratio γ.

In order to study and understand the solar chimney power plant cycle, it is very

important to simulate the solar chimney power plant cycle by a gas turbine air cycle

where the chimney is eliminated and an isentropic compressor is added. To better

simulate the environmental conditions of the solar chimney power plant, we assume

that the gas turbine plant is placed at the altitude of the chimney top. The pressure

ratio chosen for the solar chimney power plant is the ratio of the atmospheric

pressure on the ground level to the atmospheric pressure at the solar chimney top.

Now, the pressure ratio for the gas turbine and the solar chimney power plant is

equal. However, this gas turbine solar concept have other practical drawbacks such

12

as the collector pressure being higher than the local atmospheric pressure at the top

of the chimney and the large amount of power to be transmitted between the turbine

and the compressor. The low temperature solar gas turbine cycle will however serve

a useful bench mark for the solar chimney cycle.

2.4.2 Air Standard Analysis of Solar Gas Turbine Cycle The T-s diagram for a solar gas turbine cycle is shown in Figure 2.6. The T-s

diagram represents the different processes in the cycle. The analysis for the solar gas

turbine cycle is taken from Cohen et al. [28] and Archer and Saarlas [29] which were

later derived by von Backstrom and Gannon [27] to simplify the equations.

Figure 2.6: The T-s diagram for a solar gas turbine cycle

The cycle pressure ratio is defined as:

(2)

The compression temperature ratio is defined as:

(3)

13

The cycle efficiency is defined as the turbine shaft power out divided by the thermal

power or the solar energy transferred to the air moving across the solar the solar

collector:

(4)

The turbine output power or the shaft power out is defined as:

(5)

The thermal power or solar energy transferred to the air in the collector is defined as:

(6)

The cycle efficiency can be rewritten as:

(7)

The above equation clearly shows that the plant cycle efficiency is only a function of

cycle pressure ratio, r and not the cycle temperature ratio, t13 which is defined below.

The cycle temperature ratio can be defined as:

(8)

where is the compressor inlet temperature, is the temperature rise in the

combustor and is the compressor exit temperature.

The specific power normalised with T1 is defined as:

(9)

From the above equation, it can be said that the specific power depends on both

cycle pressure ratio and collector temperature rise.

14

2.4.3 Air Standard Analysis of a Solar Chimney Power Plant Cycle

The analysis of a solar chimney power plant cycle is very similar to that of a gas

turbine. One of the main differences is that the compression process does not take

place in the system but in the environment when the air starts to descend. The other

difference is that any analysis of compression inlet temperature at the top altitude

will be avoided since this is a variable temperature determined by a point that

intersects between the constant cooling pressure line from point 4 and the polytropic

(not isentropic) compression line from point 2. A much better reference temperature

is since it is the ground level temperature and can be easily measured. The

temperature will be made use of and is defined as the intersection between the

constant pressure line through point 4 and the isentropic line through point 2.

In an ideal gas turbine cycle where there are no irreversible processes, all the power

can be extracted from the flow by an ideal turbine as it expands from collector exit

pressure, to the chimney exit pressure, to obtain the shaft power .

For a solar chimney, the power required to lift the air up the chimney should also be

considered. The schematic and T-s diagram of an SCPP is shown in Figure 2.7 and

Figure 2.8 respectively.

Figure 2.7: Schematic of an SCPP

15

Figure 2.8: The T-s diagram for an SCPP cycle

The total power available is defined as:

(10)

The power required to lift the air up the chimney is defined as:

(11)

There is no heat transfer or shaft work in this section of the chimney.

The enthalpy change in the chimney is defined as:

(12)

Following from the assumption of zero friction and heat transfer, the energy

exchange is isentropic. The value of can be equated to the amount of air that has

descended again in the atmosphere after having been cooled to .

16

The amount of enthalpy gained is defined as:

(13)

The turbine output power or the shaft power out is now defined as:

(14)

The cycle efficiency is defined as:

(15)

The thermal power transferred to the air in the collector is defined as:

(16)

In the solar chimney cycle, the pressure ratio is defined in terms of the chimney exit

and the collector exit pressures:

(17)

The temperature ratio is now defined as:

(18)

The solar chimney efficiency or the cycle efficiency is now the same as for the gas

turbine and is defined as:

(19)

Equating the temperature drop with the potential energy,

(20)

The solar chimney efficiency is now defined as:

(21)

17

As seen in the above equations, the cycle efficiency of an idea solar chimney plant is

directly proportional to the chimney height, and inversely proportional to the

collector inlet temperature.

To calculate the specific power, the cycle temperature ratio is defined as:

(22)

For the solar chimney cycle, the specific power is normalized to the collector inlet

temperature :

(23)

Using equation (23) and the following expression,

(24)

The specific power can now be written as:

(25)

The equations above clearly show that the specific power is proportional to the

chimney height and the collector temperature rise, and inversely proportional to the

collector inlet temperature.

18

2.5 Solar Chimney Power Plant Theoretical and Experimental Models The very first theoretical model for the solar chimney power plant was developed by

Mullet [30] who presented an analysis and derived its overall efficiency. In his study

he concluded that solar chimney power plants have very low overall efficiencies and

it is vital for power generation in large scale. Based on the data and results from the

Manzanares tower prototype, Padki and Sherif [31] extrapolated solar chimney

power plants model for medium – to – large scale power generations. In their studies

they investigated the effect of various geometrical configurations on the chimney

performance and efficiency. A detailed study was undertaken by Pasumarthi and

Sherif [15] to investigate the performance characteristics of a solar chimney power

plant both theoretically and experimentally. They presented a mathematical model

which was used to study effect of various geometric parameters on the air

temperature, air velocity, and power output of the solar chimney power plant. In the

further studies conducted by Pasumarthi and Sherif [32], experimental modifications

were conducted on the collector. The modifications were extension of the collector

base and an introduction of an immediate absorber. According to them, both the

changes help in increasing air temperature and mass flow rate inside the chimney

resulting in higher power output. They also conducted a brief economic analysis on

solar chimney power plants.

The first attempt to solve a solar chimney power plant simulation by Computational

Fluid Dynamics (CFD) was made by Bernades et al. [33]. They presented numerical

analysis of natural convection in a radial solar heater operating in steady state to

predict the thermo-hydrodynamic behaviour of the device. A Finite Volume Method

in Generalized Coordinates was used to analyse the Navier-Stokes and Energy

Equations allowing a detailed visualization of the effects of geometric of optimal

geometric operational characteristics. According to results obtained by Bernades et

al. [33], curved junctions initiates well distributed-temperature fields, recirculation-

free flow as well a higher mass flows compare to straight junctions at the centre of

the base of the collector.

An analysis of the driving potential of a solar chimney power plant was presented by

Kroger and Blaine [34]. Several theoretical models were assessed and the influences

of prevailing ambient conditions were evaluated. Kroger and Blaine presented in

19

their studies that humidified air can enhance the driving potential and at certain

conditions condensation may occur.

A study including chimney friction, system, turbine and exit energy losses was

introduced by Gannon and von Backstrom [35]. For this study, a simple collector is

used to the mass flow rate and the temperature rise in the solar collector. A simple

solar chimney power plant model was fabricated and was used to compare with the

simulation. From their study, it can be concluded that the pressure drop associated

with vertical acceleration of air is about three times the pressure drop associated with

friction and also for flared chimney; the vertical pressure drop can be eliminated.

Gannon and von Backstrom [36] later proposed a turbine design based on the

requirements of a solar chimney power plant whereby the turbine was integrated

with the chimney. This was done by radially offsetting the chimney base legs so that

they can act as inlet guide vanes which will introduce pre-whirl before the rotor

reducing the exit kinetic energy. In their studies, they optimized the blades using a

surface vortex method to achieve blades of minimum chord and low drag. They

concluded from their study that their proposed turbine design can extract over 80%

of the power available in the flow. Further studies by Gannon and Backstrom [37,

38] revealed that their experimental turbine has a total-to-total efficiency of 85-90%

and total-to-static efficiency of 77-80% over the design range.

A comprehensive analytical and numerical model describing the performance of

solar chimney power plants was developed by Bernades et al. [13]. The model was

used to estimate the power output of solar chimney power plants and also to study

the effect of several ambient conditions and structural dimensions on the power

output. The results from the mathematical model were validated with the

experimental results which were then used to predict the performance characteristics

of large-scale solar chimney power plants. It can be concluded from their results that

the chimney height, the factor of pressure drop across turbine, the diameter and the

optical properties of the collector are important parameters for solar chimney power

plant design.

The effects of atmospheric winds on the performance of solar chimney power plants

were presented by Serag – Eldin [39] using a computational model. The

20

computational model comprises of governing partial differential equations

expressing conservation of mass, energy and balance of momentum and also a two

equation model of turbulence to study the flow pattern of a small-scale solar

chimney power plant in the neighbourhood. The results show that the effect of

atmospheric winds on solar chimney power plants cannot be ignored and also there

is a total degradation of the solar chimney power plant performance due to strong

winds and substantial degradation even with slow winds unless the collector inlet

height is kept significantly low.

A study based on the loss coefficient and mean exit swirl angle of the flow in the

collector-to-chimney transition region as dependent on inlet guide vanes (IGV)

stagger angle and the collector roof height was presented by Kirstein and von

Backstrom [40]. Their experiments were conducted on a small scale solar chimney

power plant and later tested with a commercial CFD code. It was found that there

was a very good agreement between the two results in terms of predicting the flow

angles, velocity components, and internal and wall static pressures. Semi-empirical

equations were also developed later to predict the loss coefficient and the turbine

mean inlet flow angles as dependent on the collector deck height and the inlet guide

vane setting angle for solar chimney power plants.

A more comprehensive model was developed by Tingzhen et al. [41] to evaluate the

performance of solar chimney power plants by investigating various parameters like

the relative static pressure, driving force, power output and efficiency. Numerical

studies were also performed to explore the geometric modifications on the system

performance based on the Manzanares prototype as an example and it shows a

reasonable agreement with the analytical model.

Pretorius and Kroger [22] evaluated the influence of a recently developed convective

heat transfer equation, a more accurate turbine inlet loss coefficient, quality collector

roof glass and various types of soil on the performance of a large-scale solar

chimney power plant. Results from his studies indicated that the new heat transfer

equation reduces the power output of the solar chimney power plant significantly.

Also, the effect of a more accurate turbine inlet loss coefficient is very minor and by

using a better quality glass can vastly improve the efficiency of the solar chimney

21

power plant. Models tested with Limestone and Sandstone soil produced virtually

comparable results to a Granite-based model. In another study performed by

Pretorius and Kroger [42], they claimed that 24hrs power production is possible and

plant power production is a function of the collector roof shape and collector inlet

height.

A study was conducted by von Backstrom and Fluri [43] to investigate analytically

the validity and applicability of the assumption that, for maximum fluid power, the

optimum ratio of the turbine pressure drop to pressure potential (available system

pressure difference) is 2/3. From their analysis and an analysis conducted by

Schlaich, both these analyses predicted that the maximum fluid power is available at

much lower flow rate and much higher turbine pressure drop than predicted by the

constant pressure potential assumption. It can be concluded from their study that the

constant pressure potential assumption may well lead to overestimating the size of

the flow passages in the plant, and designing a turbine with poor stall margin and

very high runaway speed margin.

The effect of tower area change in a solar chimney power plant was studied by

Koonsrisuk and Chitsomboon [44] using CFD technology. The results from their

study showed that the tower area change affects the efficiency and the mass flow rate

through the plant. It was found out from their study that although velocity increases

at the top of a convergent tower, the mass flow rate remains similar as that of a

constant area tower. For a divergent tower design, velocity increases near the base of

the chimney and the maximum kinetic energy also occurs at the base of the chimney.

Ninic [12] determined the dependence of the work potential on the hot air flowing

inside the collector, the air humidity and the atmospheric pressure as a function of

elevation. In his study, several collector types were analysed using dry and humid

air. Also, the effects of several chimney heights on the air work potential were

found.

The effects of solar radiation on the flow inside the solar chimney plant was

analysed by Huang et al. [45]. In their study, the Boussinesq model and the Discrete

Ordinate Model (DO) were employed and simulations were carried out. It can be

concluded from their study that the pressure throughout the system is negative, the

22

temperature difference between the collector inlet and collector outlet and the

differential pressure in the collector-chimney transition section increases with

increasing solar radiation intensity.

The use of dimensionless variables was proposed by Koonsrisuk and Chitsomboon

[46] to study the flow in a small-scale solar chimney power plant. Water and air were

chosen as working fluids for the modelling study and Computational Fluid Dynamics

(CFD) was used to obtained results. From the CFD results, air proved to be a better

working fluid for a small-scale solar chimney power plant. Also from CFD results, it

was shown that the models were dynamically similar to the prototype.

A study of a new mathematical model was developed by Wei et al. [47] and it was

based on the concept of relative static pressure. According to the authors, optimizing

local geometric dimensions between the collector outlet and the chimney inlet can

lead to an increase in local velocity, a more uniform temperature profile and a drop

in relative static pressure; thus, improving the energy conversion and reducing the

energy losses.

A sensitivity analysis on the influence of the quality, thickness, reflectance,

emissivity, shape, and insulation of the collector roof glass, the cross section of the

collector roof supports, various ground types, ground surface roughness, absorptivity

and emissivity, turbine inlet and bracing wheel loss coefficients, and the ambient

pressure and lapse rate on the performance of a large-scale (reference) solar chimney

power plant was presented by Pretorius and Kroger [48]. Results from computer

simulations indicated that collector roof insulation, emissivity and reflectance, the

ambient lapse rate, and ground absorptivity and emissivity all have a vital effect

on the power production of a solar chimney power plant.

Zhou et al. [7] conducted an experimental study of temperature field on a pilot size

SCPP. In their study, the temperature distribution across the collector at different

heights and the temperature along the chimney were measured. According to their

study, it was found that temperature inversion is produced when the solar radiation

increases from minimum and clears up when the absorber bed is heated to a high

temperature. Further studies were conducted by Zhou et al. [49] by comparing the

experimental results to a simulated study obtained form a developed mathematical

23

model. Very close correlation between the two results makes it possible to use a

mathematical model to predict the performance of an SCPP of a different scale.

Numerical simulations evaluating characteristics of heat transfer and air flow in the

solar chimney power plant with an energy storage layer including the solar radiation

and the heat storage on the ground was carried out by Tingzhen et al. [50]. It can be

concluded from their study that the ground heat storage depends on the solar

radiation incidence and also higher temperature gradients leads to more energy loss

from the ground [6].

Further studies were conducted by Koonsrisuk and Chitsomboon [51] based on the

use of dimensional analysis together with engineering intuition to combine eight

variables into a single dimensionless variable to establish dynamic similarity

between a prototype and scaled models of a solar chimney power plant. They tested

three plant configurations numerically for similarity; fully geometrically similar,

partially geometrically similar, and dissimilar types. From their studies, it was found

out that the value obtained from the physical plant through testing was almost the

same as that of numerical simulations and this provides the validity of the

proposition. Also from their studies, it was found that for a fixed solar heat flux,

different-sized models that are partially or fully geometrically alike share an equal

excess temperature across the outlet of the collector roof.

A study based on improving the flow of air in the solar chimney power plant was

undertaken by Klarin et al. [52]. They carried out basic geometry changes in a solar

chimney power plant by analysing it using CFD in a three dimensional domain. They

design various shapes for the inside and also outside of the solar chimney power

plant and tested them.

Ming et al. [53] performed further studies on the thermal performance of a solar

chimney power plant. They established a simple analysis of the air flowing through

the solar chimney power plant and also a thermodynamic cycle of the solar chimney

power plant including the environment. They also produced mathematical model of

ideal and actual cycle efficiencies for medium-sized and later for large-sized solar

chimney power plant. The results from their work posed as a theoretical guideline for

designing and building a commercial-size solar chimney power plant in China.

24

Hamdan [14, 54] performed an analytical model and a thermodynamic study of

steady airflow inside a solar chimney. He used a simplified Bernoullis equation

combined with fluid dynamics and ideal gas equation using EES solver to predict the

performance of the solar chimney power plant. The analytical model was validated

against an experimental and numerical data available and was also used to evaluate

the effect of geometric parameters on the solar chimney power plant. From his

analysis, it could be said that the height and diameter of the solar chimney are the

most important variables for solar chimney power plant design and also the collector

area has small effect on second-law efficiency but strong effect on harvested energy.

Further studies were conducted by Hamdan [55] to evaluate the use of constant

density assumption and compare it with the more realistic chimney mathematical

model. From the results obtained, it can be concluded that the constant density

assumption simplifies the analytical model but it over predicts the power output. It

can also be concluded that maximum power output depends on the turbine head.

A more detailed numerical analysis of a solar chimney power plant was conducted

by Sangi et al. [56]. They created a mathematical model based on the Navier-Stokes,

continuity and energy equations to study the solar chimney power plant in detail. The

mathematical model created together with CFD software FLUENT were used to

study the temperature, velocity and pressure distributions in a solar chimney power

plant. The results produced were then validated with the experimental data from the

Manzanares solar chimney power plant.

An experimental investigation based on the effects of different climate on the

efficiency of a pilot size SCPP was conducted by Kasaeian et al. [57]. According to

their study, temperature inversion was observed at the bottom of the chimney after

sunrise on both hot and cold days. It was observed in their experimental study that

maximum velocity was obtained inside the chimney while the velocity at the

collector entrance or collector opening was zero.

A numerical analysis on the influence of ambient crosswind on the performance of

an SCPP was conducted by Ming et al. [58]. The results obtained showed that

ambient crosswind has a positive and a negative effect on the performance of an

SCPP. When the ambient crosswind is weak, the flow field is deteriorated and the

25

output power reduces. When the ambient crosswind is strong enough, the mass flow

rate increases, thus the output power also increases. This increase in mass flowrates

results from a wind suction effect on top of the chimney caused by the high velocity

wind (Bernoulli principle). Further numerical analysis were conducted by Ming et al.

[59] to overcome the negative effect of strong ambient crosswind by employing a

blockage a few meters away from the collector inlet opening. According to their

study, negative effects resulting from strong ambient crosswinds have been greatly

overcome by a large extent with the help of these blockages.

Li et al. [60] proposed a theoretical model to study the effects of collector radius and

chimney height on the power output of an SCPP with turbines. The theoretical model

was validated with the experimental data of the Manzanares SCPP. According to

their study, there is a limitation on the maximum collector radius as the power output

of the SCPP increases very slowly beyond that maximum radius. On the other hand,

the chimney height has no limitation at present due to the current construction

technology and also the highest chimney size investigated in the literature is only

1500 m.

Bernades and Zhou [61] analysed the sensible heat storage physical process in an

SCPP collector and also the use of water bags as heat storage. Thicknesses of water

bags were varied and simulated with and without insulations. From the results

obtained, it can be concluded that thicker water bags reduces the daily temperature

efficiently and the thermal stratification effect.

26

3. Methodology

3.1 Numerical Work

3.1.1 CFD Code

The rapid development in computer technology nowadays has forced the use of

numerical methods, for example, computational fluid dynamics (CFD) to be used in

research and experiments. CFD provides a cheaper and an accurate solution to

research problems. ANSYS CFX Version 14 was used for simulation purpose in this

research project. ANSYS CFX Version 14 uses unsteady Navier-Stokes equation in

their conservation form to solve set of equations. The instantaneous equation of mass

(continuity), momentum, and energy conservation are presented below [62].

The Continuity Equation

(26)

The Momentum Equations

(27)

where the stress tensor, , is related to the strain rate by

(28)

The Total Energy Equation

(29)

where is the total entahlpy, related to the static enthalpy h(T, p) by:

(30)

27

The term represents the work due to viscous stresses and is called the

viscous work term. This models the internal heating by viscosity in the fluid, and is

negligible in most flows.

The term represents the work due to external momentum sources and is

currently neglected.

In this research project, the flow of air inside the SCPP was due to natural

convection. CFX Version 14 can model natural and mixed convection flows by the

inclusion of buoyancy source terms. Natural convection flows occurs when the

convection of a fluid is driven only by local density variations while mixed

convection flows occurs when the convection of a fluid is driven by both a pressure

gradient and buoyancy forces [63].

Buoyancy is normally driven by variations in density and this can arise from a

number of sources. Some of the sources are:

� Variations in local temperature causing change in density; this is natural

convection.

� Variations in the mass fraction cause density variations because each

component usually has a different density. This occurs in multicomponent

flows.

� The difference in density between the phases in multiphase flows, including

particle transport modelling results in a buoyancy force.

� For a General Fluid, if density is variable (that is, defined by an expression),

a buoyancy force will arise.

� Local pressure variations also cause changes in density in case of ideal gases

and real fluids. These changes are often small and the buoyancy effect is

usually not important in the flow. Buoyancy does not necessarily need to be

modelled if there are no other sources of buoyancy.

Temperature variations which causes buoyancy forces in a mixed convection flow

can be estimated by using the ratio of Grashoff and Reynolds Numbers,

28

(31)

where is the thermal expansion coefficient. A value approaching or exceeding

unity indicates that buoyancy effects are significant in the flow, while small values

indicate that buoyancy effects can be ignored or are insignificant.

In purely natural convection problems, the Rayleigh Number (Ra) indicates the

relative strength of the buoyancy induced flow and is given by:

(32)

where Pr is the fluid Prandtl number. The laminar flow regime is generally

characterized by Ra < 108, while turbulent buoyant flow is characterized by Ra >

1010.

For buoyancy calculations, the gravity vector components in x, y and z must be set.

These are interpreted in the coordinate frame for the domain. Buoyancy effects can

be simulated using one of two available models in CFX:

� Full Buoyancy Model (Density Difference)

� Boussinesq Model

3.1.2 Full Buoyancy Model (Density Difference)

For single phase flows, this model is used when temperature and pressure variations

directly affects the fluid density. These include all ideal gases and real fluids and

when a multicomponent fluid is used. For Eulerian multiphase or particle tracking, it

is also set even if all phases have constant density. In most gases, temperature

variations significantly affect densities. A buoyancy reference temperature must be

specified as an approximate average value of the expected domain density. For

multiphase simulations, other factors must be considered [63].

For buoyancy calculations involving variable density, is evaluated directly.

This option is set automatically when the simulation involves multiphase flow,

multicomponent flow, or a fluid having density set as a function of temperature,

pressure, or other field variables.

29

3.1.3 Boussinesq Model

For many applications involving buoyancy, when the change in density over the

expected range of conditions is relatively small, it is assumed to have a constant fluid

density. This is often true for many liquids. The Boussinesq model is employed

when the fluid density is not a function of temperature or pressure [63].

Although, the Boussinesq model uses a constant density fluid model, a local

gravitational body force is applied throughout the fluid that is a linear function of

fluid thermal expansivity, and the local temperature difference with reference to a

datum called the buoyancy reference temperature. A reference temperature should be

specified as an approximate average value of the expected domain temperature.

In the Boussinesq model, a constant reference density is used for all terms other

than the buoyancy source term. The buoyancy source term is approximated as:

(33)

where is the thermal expansivity:

(34)

and is the buoyancy reference temperature.

3.1.4 Numerical Setup

The CFD work in this project was carried out using a commercial CFD software

known as ANSYS CFX. For the CFD simulation in ANSYS CFX, many processes

are involved and listed in Figure 3.1.

30

Figure 3.1: Structure of ANSYS CFX

The processes above will be discussed later in detail.

3.1.5 Geometry Generation or Modelling

The SCPP model was created using Autodesk Inventor software as shown in Figure

3.2. The SCPP model consisted of two major components, the solar chimney and the

solar collector. The solar chimney and the solar air collector were modelled

separately for ease of meshing. The model was created on the x-y plane and was

revolved around the z-axis to obtain the three-dimensional model. All dimensions are

in millimetres (mm) unless specified. The overall height of the SCPP was 10 m and

the solar air collector was 8m in diameter. A divergence orientated collector design

was used due its superior performance compared to a parallel orientated and a

convergence orientated collector design [64]. A straight chimney design and

divergence designed were both used for this project.

Geometry Generation Software (Autodesk Inventor)

Mesh Generation Software (ANSYS ICEM CFD)

ANSYS CFX - Pre (Physics Pre - processor)

ANSYS CFX - Solver (Solver)

ANSYS CFD - Post (Post - processor)

31

Figure 3.2: The SCPP model created in Autodesk Inventor

3.1.6 Mesh or Grid Generation

ANSYS ICEM CFD software was used for grid generation. The computational

domain was discretized using the ICEM CFD Hexa-mesher or user-defined meshing

method. The hexahedral grid used ensures that the results obtained are of the highest

quality and accuracy. Meshing for the solar chimney and solar air collector are

shown in Figure 3.3 and Figure 3.4. The total number of nodes for the model was

157432.

32

Figure 3.3: The solar chimney mesh

Figure 3.4: The solar air collector mesh

33

3.1.7 Physics Pre – Processor

The CFD work in this study was carried out using ANSYS CFX. ANSYS CFX is a

Reynolds Averaged Navier-Stokes Equation (RANSE) solver based on finite volume

technique. For the simulations, steady state analysis was chosen. The computational

domain was divided into two which consisted of the solar chimney and the solar air

collector. The working fluid used was air which was modelled as an ideal gas. The

entire model was built from the origin and extended in the positive y direction. The

buoyancy model was then activated by specifying the gravity of –g in the y direction

which represented real life flow. The reference pressure used was 1 atm. The heat

transfer model selected for the current simulation was total energy. This option was

chosen because change in kinetic energy is of significant importance in addition to

the changes in temperature. The boundary type at the inlet was opening with

boundary conditions of zero relative pressure and a static temperature of 303 K. The

boundary type at the outlet was also set as opening with a relative pressure of zero

and a static temperature of 303 K since the temperature at the height of 10 m does

not differ too much compared to the ground air temperature. The ground was

assigned a boundary type of wall with no-slip condition activated. The temperature

of the ground was set as 323 K. The remaining sides of the computational domain

were modelled as wall with no-slip condition. The no-slip condition ensures that the

fluid moving over the solid surfaces does not have a velocity relative to the surfaces

at the point of contact. Finally, appropriate interface region was created between the

chimney and the solar air collector. Automatic mesh connection method was selected

for the interface. All these boundary conditions are set for ideal conditions. The

simulation was run for 5000 iterations; for convergence, residual type of RMS and

the residual target value of 1 x 10-7 were set as the criteria. Figure 3.5 shows the

boundaries for the SCPP.

34

Figure 3.5: Various boundaries of the SCPP

3.1.8 Solver

The component that solves the CFD problem is called the Solver (ANSYS CFX –

Solver). The required results are produced in a non-interactive/batch process. In

ANSYS CFX Version 14, the CFD problem is solved as follows [65]:

1. The partial differential equations are integrated over all the control volumes

in the region of interest. This is the same as applying a basic conservation

law to each control volume.

2. These integral equations are converted to a system of algebraic equations by

generating a set of approximations for the terms in the integral equations.

3. The algebraic equations are solved iteratively.

Solving the equations iteratively is necessary because of the nonlinear nature of the

equations. As the solution approaches the exact solution, it is said to converge. For

each iteration, an error, or residual, is reported as a measure of the overall

conservation of the flow properties.

To determine how close the final solution is to the exact solution depends on a

number of factors. These factors include the size and shape of the control volumes

35

and the size of the final residuals. The solution process requires no user interaction

and is, therefore, usually carried out as a batch process.

The solver produces a results file that is then passed to the post-processor.

3.1.9 Post – processor

The post-processor (ANSYS CFD – Post) is the component used to analyze,

visualize and present the results interactively. ANSYS CFD – Post is a flexible,

state-of-the-art post – processor that is designed to allow easy visualization and

quantitative analysis of the CFD simulations results. Post-processing includes

anything from obtaining point values to complex animated sequences [65].

Examples of some important features of post-processors are:

� Visualization of the geometry and control volumes

� Vector plots showing the direction and magnitude of the flow

� Visualization of the variation of scalar variables through the domain

� Quantitative numerical calculations

� Animation

� Charts showing graphical plots of variables

� Hardcopy and online output.

3.2 Experimental Method

A 4 m tall SCPP was constructed for experimental purpose. The overall dimensions

of the SCPP are shown below in Figure 3.6. The components of the SCPP were

fabricated individually in-house.

36

Figure 3.6: Schematic of the 4m tall experimental SCPP

3.2.1 Chimney Bellmouth

The chimney bellmouth is a very important parameter in the design of an SCPP. The

bellmouth shape acts as a nozzle which allows the air to accelerate through to the

turbine section of the SCPP. The chimney bellmouth was the first component to be

fabricated. The schematic of the chimney bellmouth is shown in Figure 3.7.

37

Figure 3.7: The 3D view and the schematic of the chimney bellmouth

A large mild steel (MS) sheet metal plate 12mm in thickness was cut off in a shape

of a circle to provide the flanges for the chimney bellmouth and the foundation as

shown in Figure 3.8. The outer diameter of the flanges was 700 mm.

Figure 3.8: The flanges for the chimney bellmouth

A 10 mm MS flat bar was cut and bent to match the profile of the inner radius of the

chimney bellmouth as shown in Figure 3.9. The inner radius of the chimney

bellmouth was 200 mm.

38

Figure 3.9: Flat bars being bent and welded to the flange

Triangular pieces were also cut of from the 12mm MS sheet metal to fill in the gaps

as shown in Figure 3.10. A 3.2 mm diameter welding rod was then used to fill up all

the gaps between the flat bar and the triangular sheet metal pieces.

Figure 3.10: Flat bars and triangular pieces of sheet metal being welded together

The bellmouth was fitted into the lathe to smoothen and level the inside surface as

shown in Figure 3.11. A cardboard with the profile of the inner radius of the

39

chimney bellmouth drawn into it was used as a guide to exactly match the chimney

bellmouth in the lathe.

Figure 3.11: The chimney bellmouth being machined in the lathe to level the inside

surface

The chimney bellmouth was then welded to the MS flanges using a 3.2 mm diameter

welding rod as shown in Figure 3.12. The inner diameter of the flange was tapered to

match the profile curve of the inner radius of the chimney bellmouth.

Figure 3.12: The chimney bellmouth fully welded together

40

The inside of the chimney bellmouth was later filled with a Bondo body filler to

ensure all the gaps are filled and also to smoothen the inner surface. The cardboard

with the profile of the inner radius of the chimney bellmouth drawn into it was again

used to exactly match the chimney bellmouth.

3.2.2 Solar Air Collector

The solar air collector is a very important component of an SCPP. The solar air

collector helps trap the hot air and guides the hot air towards the turbine section with

the help of the chimney bellmouth. The Schematic and the 3D view of the frames of

the solar air collector is show in Figure 3.13.

Figure 3.13: The 3D view and the schematic of the frames of the solar air collector

41

The frames of the solar air collector were made of 40 mm x 40 mm x 1.6 mm

galvanized steel square tubing. The square tubing were cut into correct sizes and

welded together using a 2.5 mm welding rod to provide the frame for the solar air

collector as shown in Figure 3.14.

Figure 3.14: Frames for the solar air collector

The 40 mm x 40 mm x 1.6 mm square tubing was chosen so that it provides

structural stability due to the weight of the 6 mm clear Perspex and the weight of

people who will walking on top of it during construction and testing. Once the

square tubing is properly positioned, the clear Perspex sheets were then cut to correct

sizes as shown in Figure 3.15.

42

Figure 3.15: Perspex for the solar air collector

The Perspex were then placed on top of the square tubing frame and holes were

drilled on the Perspex and the frame to ensure correct alignment is achieved as

shown in Figure 3.16.

Figure 3.16: The solar air collector pre-alignment

43

3.2.3 Solar Chimney

The chimney is also a very important component of an SCPP. It is the actual thermal

engine of the SCPP. A 1.6 mm thick galvanized sheet metal was rolled into a cone

shape for the chimney as shown in Figure 3.17. Three different sections of 1200 mm

each were made separately and welded together to complete the 3600 mm tall

chimney.

Figure 3.17: A section of a solar chimney

The smaller end of the chimney was welded to a round flange which will be fitted to

the flange on the bellmouth. A rubber pad was cut into a circular shape and placed

between the two flanges to prevent leakage.

3.2.4 Foundation

The foundation and footing details of the SCPP are shown below in Figure 3.18. The

depth of the foundation was 1000 mm.

44

Figure 3.18: Schematic of the SCPP foundation and footing details

A 40 mm x 40 mm x 4mm thick gauge MS square tubing was used as the posts for

the SCPP with a round flange welded to the top as shown in Figure 3.19. The inner

diameter of this flange was also tapered to match the other tapered flange that is

fitted to the chimney bellmouth.

Figure 3.19: The solar chimney foundation

45

A 1000 mm deep hole was dug and the SCPP structure was placed inside and

blended cement of 1: 3 ratio to gravel and sand mix was poured on top of it as shown

in Figure 3.20.

Figure 3.20: Cement being poured onto the SCPP foundation

Wire mesh 665 was placed on the ground as shown in Figure 3.21 and cement of 50

mm thickness was poured on top of it to act as the absorber.

Figure 3.21: Wire mesh 665 on top of the ground surface before cement is poured

46

The absorber was then painted black to improve its absorbing capabilities as shown

in Figure 3.22. The black painted surface absorbs and emits heat better than a non

painted surface.

Figure 3.22: The solar air collector sitting on the black painted cement absorber

3.2.5 The 4m Tall Experimental SCPP

Four temperature sensors (PT – 100) were fitted across one side of the collector and

three PT – 100 sensors were fitted along the chimney height. A pitot static tube was

fitted a little above the throat of the chimney. The chimney was then fitted to the

collector and bolted. Eight M16 high tensile bolts were used to secure the

components together. Three 5 mm diameter galvanized steel guy wires were

suspended from the chimney top and were secure at angles of 120 on the ground

using augers. The completed 4m tall experimental SCPP is shown in Figure 3.23.

47

Figure 3.23: The 4m tall experimental SCPP

A DaqPRO datalogger was used to log the temperature data while a Furness Controls

digital micromanometer model FCO510 was used to measure the dynamic pressure

which was interfaced to a Windows XP laptop using RS 232 cable. The values of

dynamic pressures were saved in a Microsoft Excel file. Then the dynamic pressures

were converted into the corresponding velocities.

3.2.6 PT – 100 Temperature Sensor

A PT – 100 temperature sensor (2 wire) as shown in Figure 3.24 was used for

temperature measurements across the collector and along the chimney height. The

specifications of the sensor is given in Table 3.1 [66].

48

Figure 3.24: PT – 100 Temperature sensor [66]

Table 3.1: PT – 100 Temperature Sensor Specifications Range -200 – 400 ⁰C

Resolution 0.1 ⁰C (7 mΩ)

Accuracy -200 to -50 ± 0.5%

50 to 400 ± 0.5%

-50 to 50 ± 0.5 ⁰C

Teflon Cable Length 2.5 m

Teflon Cable Range -65 to 200 ⁰C

3.2.7 DaqPRO Datalogger

A DaqPRO datalogger as shown in Figure 3.25 was used to log and store

temperature data. The DaqPRO data logger is a portable battery operated acquisition

and logging system with 8 channel data logging capabilities. The specifications for

the DaqPRO data logger is shown in Table 3.2 [67].

49

Figure 3.25: The DaqPro datalogger [67]

Table 3.2: DaqPRO Datalogger Specifications

0 to 24 mA

Range 0 to 24 mA

Resolution 4.76 μA

Accuracy ± 0.5 %

Loop Impedance 21 Ω

0 to 50 mV

Range 0 to 50 mV

Resolution 3 μV

Accuracy ± 0.5 %

0 to 10 V

Range 0 to 10 V

Resolution 200 μV

Accuracy ± 0.5 %

50

Input Impedance 125 KΩ

Temperature PT – 100

Range -200 to 400 ⁰C

Resolution 0.1 ⁰C (7 mΩ)

Accuracy -200 to -50 ± 0.5 %

50 to 400 ± 0.5 %

-50 to 50 ± 0.5 ⁰C

Communication

USB 1.1 Compliant

Sampling

Capacity 512 KB

Analog Sampling Rate 1 sample/ hour to 4000 samples sec, 1

channel

Analog Sampling Resolution 16 – bit

Channel Separation 80 db

Main Machine Interface

Full Keyboard Operation – Enables manual programming of the logger

Graphics LCD 64 x 128 pixels

Power Supply

Internal Rechargeable 7.2 V NiMH battery

Built – in Battery Charger

External 9 to 12 V DC Input

Battery Life 25 hours between charges

Operating Temperature Range

0 to 50 ⁰C

Casing

Plastic ABS Box

Dimensions 182 x 100 x 28 mm

Weight 450 gr

51

3.2.8 Pitot – Static Tube

A standard pitot static tube shown in Figure 3.26 was used for pressure

measurements for the SCPP. The pitot – static tube was properly aligned so that that

flow is parallel to it. The pitot – static tube was connected to the FC0510 digital

micromanometer to measure the dynamic pressure readings.

Figure 3.26: A Pitot – Static tube

3.2.9 Furness Controls Digital Micromanometer FCO510

A Furness Controls digital micromanometer model FCO510 shown in Figure 3.27

was used to measure the dynamic pressure for the SCPP. The digital

micromanometer can be interfaced with a computer via RS 232 cable. The pressure

readings are then logged into the computer in a Microsoft Excel format. The

52

specifications of the Furness Controls digital micromanometer model FCO510 is

shown in Table 3. 3 [68].

Figure 3.27: Furness Controls Digital Micromanometer

Table 3. 3: Furness Controls Digital Micromanometer Specifications

Power Requirements Rechargeable batteries (8 hours run

time), or 12 V DC (minimum 350 mA),

or 90 – 240 V AC power 50 – 60 Hz

Instrument Range (dual) 0 – 0.8 “H2O and 0 – 8.0 “H2O (5

significant figures per range)

Accuracy Calibration to 0.25% of reading or 0.1%

of reading between 10% of lowest range

and full scale, +/- one digit.

Storage Temperature -10 to 50 C

Working Temperature 0 to 45 C

DC Outputs 18V DC 25mA for 4 to 20 mA sensors

Analog Outputs 0 to 5V DC, 12 bit resolution (1 in 4096)

Maximum Overload 10 times instrument differential

53

Maximum Static 10 bar applied to both + and - ports

simultaneously

Pneumatic Fittings 6mm OD by 4mm ID tube

Flow Devices 8” Standard Pitot Tube (other sizes

available upon request)

Absolute Pressure Range Preset value 0 to 11bar (external sensor

option n/a)

Relative Viscosity Range 0.1 to 3.0

Relative Density Range 0.1 to 3.0

Pitot K Factor Range 0.5 to 3.0

D.P. Units Pa, kPa, mmH2O, “H2O, ubar, mbar,

mmHg, “Hg, thou, NM-2, PSF, PSI

Display Average Time 1 to 20 seconds

Display Update Time 2 1⁄2 times per second

Bar Graph Update Time 5 times per second

Analog Output Update Time 2 1⁄2 times a sec

Data Logger Time Interval 1 to 3600 sec (1 hour)

Data Logger Buffer Size 300 data values

54

4. Results and Discussions

4.1 Numerical Results for the 10m SCPP

The numerical simulation results are presented in this section. The overall height of

the SCPP was 10 m and the solar air collector was 8 m in diameter. The collector

inlet opening and the chimney divergence angle were varied in this work, as shown

in Figure 4.1.

Figure 4.1: Schematic diagram of the SCPP with the various parameters that were studied

Different configurations of the SCPP were tested out and are made into cases for

ease of understanding as shown in Table 4.1. The collector base height was varied

from 0.5 m, 0.75 m and 1 m from the ground level. The collector outlet diameter was

varied from 0.6 m to 1 m and the chimney throat diameter was varied from 0.25 m to

0.3 m, as shown in Table 4.1. All of these combinations were tested for collector

inlet openings of 0.05 m, 0.10 m, 0.15 m and 0.20 m and divergence angles of 0 to

3 in increments of 1 .

55

Table 4.1: Different Configurations of the 10 m SCPP Tested Cases Collector Outlet

Height (m)

Collector Outlet

Diameter (m)

Chimney Throat

Diameter (m)

1 0.5 0.6 0.25

2 0.5 0.6 0.3

3 0.5 1 0.25

4 0.5 1 0.3

5 0.75 0.6 0.25

6 0.75 0.6 0.3

7 0.75 1 0.25

8 0.75 1 0.3

9 1 0.6 0.25

10 1 0.6 0.3

11 1 1 0.25

12 1 1 0.3

The power available for the turbine was calculated using:

(35)

The power available was calculated at the measurement location, shown in Figure

4.1, where the maximum air velocity was recorded.

Figure 4.2 shows the power available at different chimney divergence angles for all

collector inlet openings for case 5. The available power was the highest for the 0.05

m opening and lowest for the 0.2 m opening. The peak available power was observed

at divergence angle of 2 . It can also be noted that SCPP’s with a divergence

chimney configurations produced more available power compared to the SCPP with

a straight tower (0 divergence angle). This is due to the increase in mass flow rate

and velocity, hence higher kinetic energy compared to a straight tower [44]. The

high values of available power for 0.05 m opening are due to the high values of mass

flow rate and velocity compared to other collector openings. The high mass flow

rates and velocities are caused by very little interaction of the heated air in the solar

air collector with the ambient temperature and this creates a large heating area in the

56

solar air collector; this results in air getting heated up faster and rising through the

chimney; consequently more fresh air is drawn into the collector from the opening.

For the collector opening of 0.2 m, the air inside the solar air collector interacts more

with the ambient air and causing a lesser heating of air in the solar air collector.

Similar trends were observed for other cases. According to Pretorius and Kroger

[22], a lower collector inlet opening results in a smaller collector flow area, thus

having high collector air velocities. In real cases, atmospheric winds play a major

role in the performance of an SCPP. Strong atmospheric winds result in a total

degradation on the performance of an SCPP while weak atmospheric winds also

affect the performance of an SCPP unless the collector inlet opening is kept low

[39].

Figure 4.2: Power available for case 5 for various collector inlet openings and various

chimney divergence angles.

Figure 4.3 shows the temperature distribution on the collector for the 0.05 m opening

case. It can be seen that the temperature is higher over a large area near the center of

the collector. This caused the faster heating up of the air and flow through the

57

chimney, and drawing more fresh air, as described above. Compared to this, when

the opening is 0.2 m, the temperature is lower in the same area as the case for Figure

4.4. This causes the flow through the chimney to be less compared to the 0.05 m

opening case. Hence, the power available for the 0.05 m opening is much higher

compared to the 0.2 m opening, as shown in Figure 4.2. The “temperature stems”

inside the collector is due to the differential heating of the ground surface.

Figure 4.3: Temperature contours on the collector for collector inlet opening of 0.05 m

Figure 4.4: Temperature contours on the collector for a collector inlet opening of 0.2 m

58

Figure 4.5 shows the power available for a constant collector inlet opening of 0.1 m

at different chimney divergence angles for case 1, case 5 and case 9. The collector

outlet diameter is 0.6 m for all the cases. The available power was the highest for

case 5 (collector outlet height of 0.75 m) and lowest for case 1 (collector outlet

height of 0.5 m). The high values of available power for case 5 are due to the high

values of mass flow rate and velocity compared to other collector outlet heights. The

low values of available power in case 1 are due to the lower volume of air entering

the chimney. When the hot air near the collector opening interacts with the ambient

air outside the collector due to natural convection taking place, about 1/3 the radius

of the collector gets affected by this phenomenon and this cause less air to rise up in

the collector entering the chimney. This interaction of the heated air and the ambient

air is similar in all the three cases, but for case 5, due to the higher collector outlet

height, enough air enters the chimney with less collision between air particles. For

case 9, it is similar to case 5 but due to the larger collector outlet height; more

collisions of air particles take place since the chimney inlet diameter is small, thus

affecting the overall performance of the SCPP. Similar trends were observed for

other cases. According to a study conducted by Pretorius and Kroger [22], although

a lower collector outlet has a good ground energy extraction resulting in a higher

collector airflow velocities, it also has high energy losses through the collector roof

to the environment and vice – versa. According to them, an optimal collector

configuration should take into account the plant power output, the plant dimension

and the construction cost as optimization constraints.

59

Figure 4.5: Power available for cases 1, 5 and 9 for the collector inlet openings of 0.1 m and different chimney divergence angles

Figure 4.6 shows the power available for a constant collector opening of 0.1 m at

different chimney divergence angles for case 3, case 7 and case 11. These cases are

similar to cases 1, 5, and 9 but with a different collector outlet diameter of 1 m. The

available power was the highest for case 7 and lowest for case 3. The high values of

available power for case 7 are due to the higher mass flow rates and velocities

compared to other collector outlet heights. Also, compared to Fig. 4.5, Fig.4.6 has

higher available power peaks for all collector outlet heights. The larger collector

outlet diameter of 1 m increases the volume of air entering the chimney as the

resistance to the flow is less for this case, thus having higher available power.

Similar trends were observed for other cases. It can be concluded from Figure 4.5

and Figure 4.6 that the collector outlet diameter is a very important factor in the

design of an SCPP and by increasing the collector outlet diameter, the power

available can be increased significantly due to the higher mass flow rates and

velocities. Figure 4.7 shows a better representation of cases 1, 3, 5, 7, 9 and 11

combined together for a constant collector opening of 0.1 m at different divergence

angles. It is clearly seen that the available power for the collector outlet diameter of 1

60

m is higher than that of the collector outlet diameter of 0.6 m in all the respective

cases.

Figure 4.6: Power available for cases 3, 7 and 11 for the collector inlet openings of 0.1 m and different chimney divergence angles

[

Figure 4.7: Power available for cases 1, 3, 5, 7, 9 and 11 for the collector inlet opening of 0.1 m and different chimney divergence angles

61

Figure 4.8 shows the power available for a constant collector opening of 0.1 m at

different values of chimney divergence angle for case 2, case 6 and case 10. These

cases are similar to cases 1, 5, and 9 but with a chimney throat diameter of 0.3 m.

The available power is the highest for case 6 and lowest for case 2 showing a trend

similar to Figure 4.5. The high values of available power for case 6 are due to the

high values of mass flow rate and velocity compared to other collector outlet heights.

The low values of available power in case 2 are due to the lower volume of air

entering the chimney. By comparing Figure 4.5 and Figure 4.8, it can be noted that

the overall trends are similar but the magnitude of the power available is different.

The higher power available in case 2 compared to case 1 is mainly due to the higher

mass flow rate of air entering the chimney. However, for case 5 and case 6, even

though the mass flow rate was higher for case 6, the power available was still higher

for case 5 due to the higher velocity caused by the smaller chimney diameter which

acts as a nozzle increasing the velocity of air entering the chimney. Figure 4.9 shows

a better representation of cases 1, 2, 5, 6, 9 and 10 combined together for a constant

collector opening of 0.1 m at different values of chimney divergence angle.

Figure 4.8: Power available for cases 2, 6 and 10 for the collector inlet opening of 0.1 m and different chimney divergence angles

62

Figure 4.9: Power available for cases 1, 2, 5, 6, 9 and 10 for the collector inlet opening of 0.1 m and different chimney divergence angles

Figure 4.10 shows the velocity vectors for case 3 which has the highest available

power compared to all other cases. Case 3 has a collector opening of 0.05 m with a

collector height of 0.5 m. The collector outlet diameter was 1 m with a chimney

diameter of 0.25 m diverging at 2 . The maximum available power for this case is

14.504 W. The maximum velocity achieved is 7.864 m/s and the mass flow rate is

0.469 kg/s. Figure 4.11 shows the temperature contours for case 3. The temperature

is higher towards the center of the collector. Figure 4.12 shows the temperature

variation along the chimney for case 3. It can be seen that the temperature generally

decreases up to a height of 4 m and slightly increases afterwards. This slight increase

in temperature is very small (less than 1 K) and may be due to the friction at the wall

of the chimney. The decrease in air temperature is due to the pressure drop through

isentropic relation [46]. An experimental study conducted by Zhou et al. [7] showed

a general decrease in air temperature along the chimney height. The overall

temperature variation along the chimney is very similar for both experimental and

CFD results.

63

Figure 4.10: Velocity vectors on the entire SCPP for case 3 for the collector inlet opening of 0.05 m and chimney divergence angle of 2⁰

Figure 4.11: Temperature contours on the entire SCPP for case 3 for the collector inlet opening of 0.05 m and chimney divergence angle of 2⁰

64

Figure 4.12: Temperature variation along the chimney height for case 3 for the collector inlet opening of 0.05 m and chimney divergence angle of 2⁰

Figure 4.13 shows the velocity variation along the chimney for case 3. The velocity

generally increases to a height of 1 m and then decreases afterwards. The increase in

velocity is due to the reduction in area (nozzle effect) and decreases due to the

diverging duct. Similar trends were also observed by Koonsrisuk and Chitsomboon

[44], Sangi et al. [56] and Chergui et al. [69] in their study of an SCPP.

65

Figure 4.13: Velocity variation along the chimney height for case 3 for the collector inlet opening of 0.05 m and chimney divergence angle of 2⁰

Figure 4.14 shows the temperature at a height of 0.025 m along the radius of the

collector from the outer periphery to the center. The temperature inside the collector

increases from the ambient temperature of 303 K at the outer periphery to a

temperature of 321 K at about 0.8 m inside the collector. The temperature remains

essentially constant away from the collector edges. The temperature rise inside the

collector towards the chimney is due to the air accumulating thermal energy as it

travels towards the collector outlet/ chimney inlet [46]. These trends were very

similar to the experimental results obtained by Zhou et al. [7]. The temperature

difference was also very similar to that experimental study.

66

Figure 4.14: Temperature variation along the outer radius of the collector to the center measured at 0.025 m above ground for case 3 for the collector inlet opening of 0.05 m

and chimney divergence angle of 2⁰

Figure 4.15 shows the temperature variation from the base of the collector to the

collector outlet at the center of the tower. The temperature decreases as the air rises

up towards the chimney. The maximum temperature change inside the collector is 17

degrees. Figure 4.16 shows the velocity variation from the base of the collector to the

collector outlet at the center of the tower. The velocity increases towards the throat

of the chimney. Figure 4.17 shows the temperature from the base of the collector to

the top of the chimney. The temperature essentially drops although there is a small

increase towards the chimney exit. Previous works also reported a drop in

temperature towards the chimney outlet [46]. Figure 4.18 shows the velocity from

the base of the collector to the top of the chimney. The velocity increases till the

throat where the maximum velocity is recorded; after which the velocity starts to

decrease till close to the chimney outlet, after which there is a small increase in

velocity probably due to the temperature difference between the chimney air and the

ambient air.

67

Figure 4.15: Temperature variation from the ground to the collector outlet at the

center for case 3 for the collector inlet opening of 0.05 m and chimney divergence angle of 2⁰

Figure 4.16: Velocity variation from the ground to the collector outlet at the center for case 3 for the collector inlet opening of 0.05 m and chimney divergence angle of 2⁰

68

Figure 4.17: Temperature variation from the ground to the top of the chimney for case 3 for the collector inlet opening of 0.05 m and chimney divergence angle of 2⁰

Figure 4.18: Velocity variation from the ground to the top of the chimney for case 3 for the collector inlet opening of 0.05 m and chimney divergence angle of 2⁰

69

4.2 Numerical and Experimental Results for the 4 m Tall SCPP

The numerical simulation and the experimental results of the 4 m SCPP are

presented in this section.

A scaled down model of 1:2.5 of the 10 m SCPP was simulated and presented in this

section. The overall height of the SCPP was 4 m and the overall collector diameter

was 3.2 m. Divergence angle of 2 was chosen since it produced best results in the

10 m tower. The collector outlet height was kept constant at 0.2 m. The collector

outlet diameter, the collector opening height and the chimney throat diameter were

varied. The collector outlet diameter was varied from 0.24 m to 0.4 m and the

collector inlet opening was varied from 0.02 m to 0.10 m at increments of 0.02 m.

The chimney throat diameter was varied from 0.10 m to 0.12 m. Different

configurations of the 4m tall SCPP were tested out and are made into cases for ease

of understanding as shown in Table 4.2. All these cases were tested with all the

collector inlet openings. The locations for temperature sensors and the pressure

sensor are shown in Figure 4.19.

Table 4.2: Different Configurations of the 4 m SCPP Tested Cases Collector Outlet

Diameter (m)

Chimney Throat

Diameter (m)

A 0.24 0.10

B 0.4 0.10

C 0.24 0.12

D 0.4 0.12

70

Figure 4.19: Temperature and pressure sensors measurement locations

Figure 4.20 shows the power available for all different configurations of the 4 m

SCPP. It can be clearly seen that highest power available of 0.57 W was achieved in

case B at collector inlet opening of 0.02 m. Case D had higher available power in

almost all collector inlet openings except for collector inlet opening of 0.02 m.

Collector inlet opening of 0.02 m had the highest available power in almost all cases

and the collector inlet opening of 0.10 m had the lowest available power in all cases

except for case D where the highest available power was achieved at collector inlet

opening of 0.04 m. The larger collector outlet diameter of 0.4 m had higher available

power compared to the smaller collector outlet diameter of 0.24 m having the same

chimney throat diameter in all respective cases. This is due to the higher volume of

air being allowed to enter the chimney. It can also be noted that chimney throat

diameter of 0.12 m had higher available power compared to chimney throat diameter

of 0.10 m in almost all collector inlet openings.

71

Figure 4.20: Power available for cases A, B, C and D for the chimney divergence angle of 2 and different collector inlet openings

Figure 4.21 shows the mass flow rate for all different configurations of the 4 m

SCPP. It can be clearly seen that highest mass flow rate of 0.063 kg/s was achieved

in case D at collector inlet opening of 0.04 m. Case D had higher mass flow rate in

all collector inlet openings. Collector inlet opening of 0.02 m had the highest mass

flow rate in almost all cases and the collector inlet opening of 0.10 m had the lowest

mass flow rate in all cases except for case D where the highest flow rate was

achieved at collector inlet opening of 0.04 m. The larger collector outlet diameter of

0.4 m had higher mass flow rate compared to the smaller collector outlet diameter of

0.24 m having the same chimney throat diameter in all respective cases. It can also

be noted that chimney throat diameter of 0.12 m had higher mass flow rate

compared to chimney throat diameter of 0.10 m in almost all collector inlet

openings.

72

Figure 4.21: Mass flow rate for cases A, B, C and D for the chimney divergence angle of 2 and different collector inlet openings

Figure 4.22 shows the air velocity for all different configurations of the 4 m SCPP. It

can be clearly seen that highest velocity of 4.673 m/s was achieved in case B at

collector inlet opening of 0.02 m. Case B had higher velocity in all collector inlet

openings. Collector inlet opening of 0.02 m had the highest velocity in almost all

cases and the collector inlet opening of 0.10 m had the lowest velocity in all cases

except for case D where the highest velocity was achieved at collector inlet opening

of 0.04 m. The larger collector outlet diameter of 0.4 m had higher velocity

compared to the smaller collector outlet diameter of 0.24 m having the same

chimney throat diameter in all respective cases. It can also be noted that chimney

throat diameter of 0.10 m had higher velocity compared to chimney throat diameter

of 0.12 m in almost all collector inlet openings.

73

Figure 4.22: Velocity for cases A, B, C and D for the chimney divergence angle of 2 and different collector inlet openings

Based on the simulation results, case D with collector inlet opening of 0.04 m was

selected for fabrication due to the high mass flow rate and high velocity, hence high

available power. Figure 4.23 shows the velocity vectors for case D. The highest

velocity achieved was 4.05 m/s just above the chimney throat diameter.

74

Figure 4.23: Velocity vectors on the entire SCPP for case D for the collector inlet opening of 0.04 m

Figure 4.24 shows the temperature contours for case D. The temperature of air inside

the collector slowly increases from outer radius to the inner periphery. Very high

temperature is achieved in the center of the collector due to hot air rising quickly and

accelerating towards the chimney bellmouth. The temperature along the chimney

decreases very slightly.

75

Figure 4.24: Temperature contours on the entire SCPP for case D for the collector inlet opening of 0.04 m

Figure 4.25 shows the temperature variations across the collector for both the CFX

results and the experiment results. The temperature was measured at 1.57 pm on the

6th of December, 2013. The average ambient temperature was 30⁰C and the average

ground temperature was 50⁰C which matched the conditions used in the simulation.

It can be seen that temperature increases towards the center of the collector for both

cases and the trends are similar. The CFX results show a larger temperature

difference compared to the experimental results. The temperature readings inside the

collector for the experimental case were generally higher than that of the CFX case.

This is due to the temperature sensor being very close to the absorber.

76

Figure 4.25: Temperature variation along the outer radius of the collector to the center

measured at various locations for both experimental and CFD for case D for the collector inlet opening of 0.04 m

Figure 4.26 shows the temperature variations along the chimney for both the CFX

results and experimental results. The temperature was measured at 1.57 pm on the 6th

of December, 2013. The average ambient temperature was 30⁰C and the average

ground temperature was 50⁰C which matched the conditions used in the simulation.

The average air velocity near the chimney throat was 3.78m/s. It can be seen that

temperature decreases along the chimney height and both the trends are similar. The

decrease in temperature along the chimney is very low (approximately 1 ) for both

the CFX results and experimental results.

77

Figure 4.26: Temperature variation along the chimney height for both experimental

and CFD for case D for the collector inlet opening of 0.04 m

Figure 4.27 shows the temperature variation across the collector at every 1 hour

interval from 9.00 am to 8.00 pm on the 6th of December, 2013. The average ambient

temperature was 30 C. There is a general increase in temperature from the outer

periphery towards the center except for the temperature variation at 3.00 pm and

4.00 pm. This may be due to temperature sensors 1 – 3 being exposed to direct

sunlight.

78

Figure 4.27: Temperature variation across the collector from 9:00 am to 8:00 pm on a typical day

Figure 4.28 shows the temperature variation along the chimney height at every 1

hour interval from 9.00 am to 8.00 pm on the 6th of December, 2013. The average

ambient temperature was 30 C. There is a general decrease in temperature from the

chimney bottom to the chimney top except for the temperature variation at 1.00 pm.

This high value of temperature at the outer periphery is caused by the PT – 100

sensor being exposed to direct sunlight. This is not the correct temperature of the air

inside the collector. The PT – 100 sensor at the chimney bottom shows the correct

temperature since it’s not being affected by direct sunlight due to the shadow

produced by the chimney.

79

Figure 4.28: Temperature variation along the chimney from 9:00 am to 8:00 pm on a typical day

4.3 The 100m SCPP Numerical Results The numerical simulation results for the 100 m SCPP are presented in this section. A

scaled up model of 10:1 of the 10 m SCPP was simulated and presented in this

section. The overall height of the SCPP was 100 m and the overall collector diameter

was 80 m. The collector inlet opening was 0.5 m and the collector outlet height was

5 m. The collector outlet diameter was 10 m and the chimney throat diameter was

2.5 m. The chimney divergence angle was 2 .

Figure 4.29 shows the temperature variation from the ground to the top of the

chimney. It can be clearly seen that the temperature decreases with height. The

decrease in temperature with height is about 3 C. Similar trends were observed with

studies conducted by Koonsrisuk and Chitsomboon [44, 46] on a similar sized tower.

80

Figure 4.29: Temperature variation from the ground to the top of the chimney for the 100 m SCPP

Figure 4. 30 shows the velocity variation from the ground to the top of the chimney.

It can be seen that the velocity intially increases from the ground to the chimney

throat and then decreases with increase in height. The increase in velocity is due to

the shape of the chimney bellmouth which acts as a nozzle. The maximum kinetic

energy is achieved just above the chimney throat. The maximum velocity achieved

was 22.72 m/s. Similar trends were observed with studies conducted by Koonsrisuk

and Chitsomboon [44] on a similar sized tower.

81

Figure 4. 30: Velocity variation from the ground to the top of the chimney for the 100 m SCPP

Figure 4.31 shows the temperature at a height of 0.025 m along the radius of the

collector from the outer periphery to the center. The temperature inside the collector

increases from the ambient temperature of 303 K at the outer periphery to a

temperature of 318 K at about halfway inside the collector. The temperature then

decreases as it move towards center of the collector due to the hot air moving up fast

through the chimney. The air in the middle of the collector rises up very quickly and

causes abrupt changes in temperature in accordance to the conservation of energy

principle [44].

82

Figure 4.31: Temperature variation along the outer radius of the collector to the center measured at 0.025 m for the 100 m SCPP

This SCPP design gives a maximum available power of 35.8 kW. The maximum

velocity achieved in the tower was 22.72 m/s and the maximum mass flow rate

achieved was 137.31 kg/s. Such a plant will be suitable to meet the power

requirements of small islands in Pacific Island Countries where the requirements are

of the order of tens of kilowatts.

83

5. Conclusions

The effect of various geometric parameters on an SCPP are presented. It can be

concluded that increasing the collector inlet opening affects the overall performance

of an SCPP. Smaller collector inlet openings performs better than larger collector

openings. The collector outlet height is also a very important parameter in the design

of an SCPP. The collector outlet should not be too high or too low. It should be at an

optimum height to provide best performance for an SCPP. The collector outlet

diameter/chimney inlet diameter is also an important parameter in the design of an

SCPP. This parameter determines the amount of air entering the chimney which has

a direct relationship to the power available. The chimney throat diameter is also an

important parameter since it determines the amount of air that will be interacting

with the turbine. A larger chimney throat diameter will allow a larger turbine to be

fitted thus, more power can be extracted. The shape of the chimney is an important

parameter in the design of an SCPP. The divergent chimney design performs better

than a straight tower or a converging tower in terms of mass flow rate and kinetic

energy. All these geometrical parameters will help improve the performance of an

SCPP. For future studies, a multiple turbine system with a divergent tower can be

designed and simulated to help extract more power.

84

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