-
PhD Thesis
OPTIMAL PERFORMANCE ANALYSIS OF A SOLAR THERMAL ENERGY STORAGE
PLANT BASED ON
LIQUID AMMONIA
Submitted by
Engr. Sadaf Siddiq (08F-UET/PhD-ME-47)
Supervised by
Prof. Dr. Shahab Khushnood
Department of Mechanical Engineering Faculty of Mechanical and
Aeronautical Engineering
University of Engineering and Technology Taxila, Pakistan
July 2013
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ii
OPTIMAL PERFORMANCE ANALYSIS OF A SOLAR THERMAL ENERGY STORAGE
PLANT BASED ON
LIQUID AMMONIA
by
Engr. Sadaf Siddiq (08F-UET/PhD-ME-47)
A proposal submitted for research leading to the degree of
Doctor of Philosophy in
MECHANICAL ENGINEERING
Approved by
External Examiners
________________________________
(Engr. Dr. M. Javed Hyder) Dean of Engineering,
Pakistan Institute of Engineering & Applied Sciences Nilore,
Islamabad.
________________________________
(Engr. Dr. Ejaz M. Shahid) Associate Professor,
Department of Mechanical Engineering, University of Engineering
& Technology, Lahore.
Internal Examiner (Research Supervisor)
________________________________
(Engr. Dr. Shahab Khushnood) Professor,
Department of Mechanical Engineering, University of Engineering
& Technology, Taxila.
Department of Mechanical Engineering Faculty of Mechanical and
Aeronautical Engineering
University of Engineering & Technology Taxila, Pakistan.
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iii
DECLARATION
I declare that all material in this thesis is my own work and
that which is not, has been identified and appropriately
referenced. No material in this work has been submitted or approved
for the award of a degree by this or any other university.
Signature: _____________________________
Authors Name: ________________________
It is certified that the work in this thesis is carried out and
completed under my supervision.
Supervisor: Prof. Dr. Shahab Khushnood Department of Mechanical
Engineering Faculty of Mechanical and Aeronautical Engineering
University of Engineering and Technology Taxila, Pakistan.
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ABSTRACT
This work focuses on extending the use of a solar thermal energy
plant from an intermittent
energy source to a base load power plant by incorporating an
efficient thermal storage feature.
A reference 10 MWe solar thermal plant design is considered with
liquid ammonia as a
working fluid for energy production, in a Rankine Cycle, as well
as a thermal storage
medium. During periods of no solar insolence, the recovery
system, based on an industrial
ammonia synthesis system, is used to drive the power conversion
unit and enable continuous
operation.
A thermofluid model, based on the continuity, momentum and
energy conservation equations,
is used to carry out a numerical simulation of the plant, to
determine the process variables
and subsequently carry out an integrated plant energy recovery
analysis. The objective of this
work is to maximize the efficiency of the plant by a detailed
consideration of the most critical
process in the plant: the energy recovery unit. This is carried
out by (i) estimating the
sensitivity of non-uniform catalyst concentration in a synthesis
reactor, and (ii) obtaining an
optimal configuration from a variational Lagrangian cost
functional and applying
Pontryagins Maximum Principle. The optimal configuration is used
to recommend a re-
design of the synthesis reactor and to quantify the energy
recovery benefits emanating from
such a recommendation. Industrial optimal configurations are
achieved by carrying out the
analysis with the simulation code, Aspen Plus, to design a heat
removal system
surrounding the catalyst beds, and incorporating the effect of
standard industrial processes
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v
such as purge gas removal, quench gas recycling, and recycle
ratio to achieve the optimal
temperature profile obtained for the synthesis reactor
considered in this work.
This work quantifies the maximum energy recovery in a base-load
solar thermal plant
utilizing the existing environment of chemical process industry.
It is concluded that a one-
dimensional model, with mass and energy conservation equations
using the Temkin-Pyzhev
activity and pressure-based kinetics rate expressions, predicted
an optimal ammonia
conversion of 0.2137 with a thermal energy availability of 20
MWth. A comprehensive
process simulation using Aspen Plus predicts an optimal ammonia
conversion of 0.2762
mole fraction at exit, with two inter-bed heat exchangers having
optimal temperature drops of
205K and 95K respectively, and yielding a thermal availability
of 45.6 MWth. The thermal
energy availability of a base-load solar thermal plant can be
increased by 15% in the
ammonia conversion and over 25% in thermal energy availability
for energy recovery.
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vi
To my family . . .
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ACKNOWLEDGEMENTS
During the development of my PhD studies at University of
Engineering & Technology
Taxila, several persons and institutions collaborated directly
and indirectly with my research.
Without their support it would be impossible for me to finish my
work. That is why I wish to
dedicate this section to recognize their support.
I want to start expressing a sincere acknowledgement to my
advisor, Prof. Dr. Shahab
Khushnood because he gave me the opportunity to research under
his kind guidance and
supervision. I received motivation; encouragement and support
from him during all my
studies. I owe Special thanks to Dr. Zafar Ullah Koreshi for the
his support, guidance, and
transmitted knowledge for the completion of my work. With him, I
have learned writing
papers for conferences and journals and sharing my ideas with
the scientific community. I
also want to thank the example, motivation, inspiration and
support I received from Dr.
Tasneem M. Shah, Dr. Arshad H. Qureshi and Dr. M. Bilal
Khan.
The Grant from University of Engineering & Technology Taxila
provided the funding and
resources for the development of this research and validation of
my work. At last, but the
most important I would like to thank my family, for their
unconditional support, inspiration,
love and prayers.
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NOMENCLATURE
A Cross-sectional area (m2) ANU Austrailian National
University
pC Specific heat at constant pressure (kJ kmol-1 K-1)
rC Compression Ratio
CSP Concentrating Solar Power
E Activation energy (kJ kmol-1)
sfF Force (external, fluid to solid) 0
NF Initial nitrogen molar flow rate (kmol h-1) Gt Giga-ton (109
ton)
Hamiltonian
Enthalpy per unit mass J
Functional *
, ii JJ Molar Fluxes
K Kinetic Energy
aK Equilibrium constant
KBR Kellogg Brown and Root
L Length of synthesis reactor (m) MTD Metric tonnes per day Mtoe
Million ton of oil equivalent
MWe Megawatt electric
MWth Megawatt thermal
OEM One Equation Model
P Pressure (MPa) Linear Momentum PMP Pontryagins Maximum
Principle
PV PhotoVoltaic
Q Heat
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ix
R Universal gas constant 8.3144 kJ kmol-1 K-1
AR Reaction rate (kmol NH3 h-1 m-3 catalyst) RK-4 4th order
Runge-Kutta
S Surface Area (m2) T Temperature (K) TEM Two Equation Model TSP
Thermal Storage Plant TWh Terawatt-hours (1012 Watt-hrs) U Internal
Energy (kJ) U
Internal Energy per unit mass
W Watts
ia Activity for specie i
c Total Molar Concentration
ic Molar Concentration of Specie i
pd Particle Diameter
g Gravitational acceleration (9.81 ms-2) *
, ii jj Mass Fluxes kWe kilowatt Electric
kWchem. kilowatt chemical
kWth kilowatt thermal
m Mass (kg) 0in Initial mole flow rate of specie i (kmol
h-1)
ppm Parts per million r
Molar Production
t Time
u Control variable
v Velocity (m s-1) w
Work
x Distance along catalyst bed (m)
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x
x Normalized Distance along catalyst bed (m)
iy Mole fraction for specie i
o
iy Initial Mole fraction for specie i
Nzz, Fractional conversion of Nitrogen
Greek
rH Heat of reaction (kJ kmol-1 NH3) Extent of reaction
Potential Energy
i Fugacity coefficient for specie i
Mass Flow Rate (kghr-1)
Shear Stress
Catalyst effectiveness factor
The void space of the bed Lagrange multiplier
)(x Catalyst spatial factor )(x Optimal Temperature
Density (kg m-3) r
Vector containing state variables
Subscripts
eqm Equilibrium
i Species in a multi component system, Ni ,....4,3,2,1=
opt Optimal
s Isentropic
tot Total amount of entity in a macroscopic system
0 Evaluated at a surface
2,1 Evaluated at cross sections 1 and 2
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xi
Table of Contents ABSTRACT
.......................................................................................................................................................
IV
ACKNOWLEDGEMENTS
............................................................................................................................
VII
NOMENCLATURE
......................................................................................................................................
VIII
TABLE OF CONTENTS
..................................................................................................................................
XI
TABLE LIST
...................................................................................................................................................
XIII
FIGURE LIST
................................................................................................................................................
XIV
1 INTRODUCTION
.....................................................................................................................................
1
1.1 SOLAR ENERGY: POTENTIAL AS A RENEWABLE ENERGY SOURCE
............................................................ 2 1.2
SOLAR POWER PLANTS IN OPERATION
......................................................................................................
5
1.2.1 PV Plants
........................................................................................................................................
5 1.2.2 Solar Thermal Plants
......................................................................................................................
7
1.3 THERMAL ENERGY STORAGE REQUIREMENT
............................................................................................
9 1.4 THERMAL STORAGE MATERIALS
..............................................................................................................
9 1.5 USE OF LIQUID AMMONIA AS STORAGE MATERIAL
................................................................................
11
1.5.1 Poperties of Liquid Ammonia
.......................................................................................................
12 1.5.2 Dissociation and Synthesis of Ammonia
.......................................................................................
12 1.5.3 Commercial uses of Ammonia
......................................................................................................
13 1.5.4 Industrial proprietary processes for Ammonia Production
.......................................................... 14
1.5.4.1 Haldor Topsoe Ammonia Synthesis Process
...........................................................................................
15 1.5.4.2 Kellog Brown & Roots (KBR) Advanced Ammonia Process
(KAPP).................................................... 15
1.5.4.3 Krupp Uhde GmbH Ammonia Process
...................................................................................................
16 1.5.4.4 ICI-Leading Concept Ammonia (LCA)
Process......................................................................................
17 1.5.4.5 The Linde Ammonia Concept (LAC) Ammonia (LCA) Process
............................................................ 17
1.6 THERMODYNAMIC CYCLES FOR SOLAR THERMAL POWER PLANTS
........................................................ 18 1.7
LITERATURE REVIEW
..............................................................................................................................
19 1.8 THESIS
MOTIVATION...............................................................................................................................
22 1.9 OBJECTIVES
............................................................................................................................................
23 1.10 SUMMARY OF FOLLOWING CHAPTERS
...............................................................................................
24
2 DESCRIPTION OF THE THERMAL STORAGE PLANT
..............................................................
25
2.1 PLANT FEATURES
....................................................................................................................................
25 2.1.1 Process Design
.............................................................................................................................
25 2.1.2 Opertational Parameters
..............................................................................................................
27
2.2 OVERALL PLANT LAYOUT AND DESCRIPTION
.........................................................................................
28 2.2.1 Ammonia Dissociation
..................................................................................................................
29 2.2.2 Ammonia Synthesis
.......................................................................................................................
30 2.2.3 Syn Gas and Ammonia Storage
....................................................................................................
31 2.2.4 Heat Exchangers and Transport Piping
.......................................................................................
31 2.2.5 Compressors and Pumps
..............................................................................................................
31
2.3 THERMAL STORAGE PLANT PROCESS FLOW DIAGRAM
...........................................................................
32
3 MODELLING & SIMULATION OF THERMAL STORAGE PLANT
.......................................... 34
3.1 MATHEMATICAL MODELLING
..............................................................................................................
34 3.1.1 Review of Mathematical Models of TSP
.......................................................................................
34 3.1.2 Mathematical Models for TSP
......................................................................................................
41
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3.1.2.1 TEM Model using Activity based Temkin-Pyzhev Form
(TP-A): ..........................................................
43 3.1.2.2 TEM Model using Partial Pressure based Temkin-Pyzhev
Form (TP-B): ............................................... 45
3.2 MODELING UNIT OPERATIONS
................................................................................................................
48 3.2.1 Dissociation Reactor
....................................................................................................................
48 3.2.2 Synthesis Reactor-Aspen Plus
Model............................................................................................
51 3.2.3 Synthesis Reactor- HYSYS Model
.................................................................................................
53 3.2.4 Flash Tank
....................................................................................................................................
55 3.2.5 Purge Gas & Recycle
....................................................................................................................
56 3.2.6 Heat Exchangers and Waste Heat
Recovery.................................................................................
57
3.2.6.1 Counter Flow Heat Exchanger (CF-HX):
................................................................................................
57 3.2.6.2 Thermal Heat Exchanger (SRIN-HX):
....................................................................................................
58 3.2.6.3 Thermal Heat Exchanger (SROUT-HX):
................................................................................................
58
3.3 MODELING THE INTEGRATED PLANT
......................................................................................................
59 3.4 TWO EQUATION MODEL (TEM) VALIDATION:
.......................................................................................
59
4 PLANT
OPTIMIZATION......................................................................................................................
62
4.1 REVIEW OF OPTIMIZATION TECHNIQUES
.............................................................................................
62 4.2 OPTIMAL ANALYSIS USING VARIATIONAL CALCULUS
........................................................................
72 4.3 PARAMETRIC SENSITIVITY ANALYSIS
....................................................................................................
78
4.3.1 Effect of Temperature on Dissociation
.........................................................................................
78 4.3.2 Effect of Flow Rate on Dissociation
.............................................................................................
79 4.3.3 Effect of Pressure on Synthesis
.....................................................................................................
80 4.3.4 Effect of Temperature on Synthesis
..............................................................................................
80 4.3.5 Effect of Flash Temperature on Liquid Ammonia Separation
...................................................... 82 4.3.6
Effect of Purge Fraction on Ammonia Liquification
....................................................................
83 4.3.7 Effect of Recycle Stream on Synthesis
..........................................................................................
84
5 AN OPTIMAL STORAGE PLANT
......................................................................................................
85
5.1 PROCESS MODIFICATIONS
.....................................................................................................................
85 5.1.1 Optimal Analysis Problem Formulation- Process
Modifications ................................................. 85
5.1.2 OEM using Activity based Temkin-Pehzev form (OEM-TPA)
...................................................... 86 5.1.3 OEM
using Partial Pressure based Temkin-Pehzev form (OEM-TPB)
........................................ 89 5.1.4 Process
Modifications Validation:
...............................................................................................
94
5.2 DESIGN MODIFICATIONS
.......................................................................................................................
97 5.2.1 The Proposed Design
....................................................................................................................
99 5.2.2 Design Modifications Validation:
...............................................................................................
101
6 CONCLUSIONS AND FUTURE WORK
.........................................................................................
103
REFERENCES
..................................................................................................................................................
106
APPENDIX A. AMMONIA 3D PHASE DIAGRAMS
......................................................................
121
APPENDIX B MATLAB PROGRAMS FOR AMMONIA SIMULATION
............................... 122
APPENDIX B1: MATLAB PROGRAM FOR OUTPUT OF STEADY STATE SYNTHESIS
REACTOR ....................... 123 APPENDIX B2: MATLAB PROGRAM FOR
FINDING EQUILIBRIUM CONCENTRATIONS
................................... 149 APPENDIX B3: MATLAB PROGRAM
FOR FINDING OUTPUT OF COUNTER-FLOW SYNTHESIS REACTOR ....... 153
APPENDIX B4: MATLAB PROGRAM FOR FINDING OUTPUT OF STEADY STATE
SYNTHESIS REACTOR WITH 3 CATALYST ZONES
..........................................................................................................................................
157
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Table List
Tables Page
TABLE 1.1: Worlds Largest (25MWe or above) PV Plants in
Operation .............................. 6 TABLE 1.2: Solar Themal
Plants in Operation
........................................................................
7 TABLE 1.3: Haldor Topsoe Ammonia Converter Features
................................................... 15 TABLE 2.1:
Overall Plant Design for a 10 MW(e) Baseload Plant
....................................... 26 TABLE 3.1: Equations of
change of Multi-component Mixtures in terms of the Molecular
Fluxes
..............................................................................................................................
35 TABLE 3.2: Coefficients of the correction factor polynomial in
terms of pressure .............. 38 TABLE 3.3: Input Data for
Dissociation Reactor
..................................................................
49 TABLE 3.4: Input Data for Synthesis Reactor
.......................................................................
51 TABLE 3.5: Reaction Input for Temkin-Pyzhev Power-Law Expression
in Aspen Plus .. 52 TABLE 3.6: Flash Tank Output
..............................................................................................
56 TABLE 3.7: Molar Flow Rates of Components in and out of Splitter
................................... 56 TABLE 3.8: Percentage errors
in 1-D Models compared with HYSYS and Aspen Plus61 TABLE 4.1:
Optimal solution for the exit conditions
............................................................ 67
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xiv
Figure List
Figures Page
Figure 1.1 Volume Reduction with Phase Change Materials
................................................. 10 Figure 1.2
Materials for medium and high heat storage
......................................................... 10 Figure
1.3 Energy densities for different energy carriers
....................................................... 12 Figure
2.1: Thermal Storage Plant Schematic
........................................................................
29 Figure 2.2: Array of 400 m2 Paraboloidal Solar Collectors [3]
.............................................. 30 Figure 2.3: TSP
Process Flow Diagram
..................................................................................
32 Figure 3.1 : Conversion of Nitrogen along a single-bed catalyst
........................................... 39 Figure 3.2 : Syngas
temperature in converter
.........................................................................
39 Figure 3.3 : Molar flow rate in converter
................................................................................
40 Figure 3.4 : Syngas compression requirement
........................................................................
40 Figure 3.5 : 3-Bed Homogeneous Reactor with TP-A Kinetics
............................................. 45 Figure 3.6 : 3-Bed
Homogeneous Reactor with TP-B Kinetics
.............................................. 47 Figure 3.7 : PFR
Dissociation Reactor in Aspen Plus
........................................................ 49 Figure
3.8 : Dissociation Reactor Exit Composition
.............................................................. 50
Figure 3.9 : Dissociation Reactor Temperature Profile
.......................................................... 50
Figure 3.10 : PFR Synthesis Reactor in Aspen Plus
........................................................... 51
Figure 3.11 : Synthesis Reactor Exit Composition (Aspen Plus)
....................................... 52 Figure 3.12 : Synthesis
Reactor Temperature Profile (Aspen Plus)
................................... 53 Figure 3.13 : Plug Flow
Reactor in HYSYS
.......................................................................
53 Figure 3.14 : Synthesis Reactor Temperature Profile (HYSYS)
........................................ 54 Figure 3.15 : Synthesis
Reactor Exit Composition (HYSYS)
............................................ 55 Figure 3.16 : Flash
Tank in Aspen Plus
..............................................................................
55 Figure 3.17 : Splitter in Aspen Plus
....................................................................................
56 Figure 3.18 : Mixer in Aspen Plus
......................................................................................
57 Figure 3.19 : Counter Flow Heat Exchanger
(CF-HX)...........................................................
57 Figure 3.20 : Thermal Heat Exchanger (SRIN-HX)
............................................................... 58
Figure 3.21 : Thermal Heat Exchanger (SROUT-HX)
........................................................... 58
Figure 3.22 : Integrated Plant
..................................................................................................
59 Figure 3.23 : Comparison of 1-D (TP-B) model, HYSYS, and Aspen
Plus results ....... 60 Figure 4.1 : Optimization Process
...........................................................................................
63 Figure 4.2 : Mathematical Methodology to solve governing
equations ................................. 64 Figure 4.3 :
Counter-Flow Ammonia Synthesis Reactor
........................................................ 66 Figure
4.4 : Temperature & Concentration Profiles along Converter
Length ........................ 66 Figure 4.5 : GA Search Algorithm
..........................................................................................
67 Figure 4.6 : Four-Bed Synthesis Reactor
................................................................................
68 Figure 4.7 : Effect of Quench gas on conversion efficiency
................................................... 69 Figure 4.8 :
GA Algorithm for obtaining optimal temperature distribution
........................... 70
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Figure 4.9 : Optimal and Normal Ammonia Production Rates
.............................................. 71 Figure 4.10 :
Optimal and Normal Nitrogen Conversion and Reaction rates
......................... 71 Figure 4.11 : Effect of Temperature on
Dissociation
............................................................. 79
Figure 4.12 : Effect of Flow Rate on Dissociation
.................................................................
79 Figure 4.13 : Effect of Pressure on Synthesis
.........................................................................
80 Figure 4.14 : Effect of Temperature on Synthesis
..................................................................
81 Figure 4.15 :Temperature & Pressure Parametric Sensitivity
for Synthesis .......................... 81 Figure 4.16 :Effect of
Flash Temperature on Ammonia Flow Rate
....................................... 82 Figure 4.17 :Effect of
Flash Temperature on Ammonia Mole Fraction
................................. 83 Figure 4.18 :Effect of Purge
Fraction on Ammonia Liquification
......................................... 83 Figure 5.1 :
Homogeneous reactor with 1-D Model (TEM-TPA) showing gas
temperatureT ,
equilibrium temperature eqmT , and optimal temeprature optT
............................................ 87 Figure 5.2 :
Temperature in homogeneous reactor compared with one-equation
optimal
temperature optT and equilibrium temperature eqmT .
......................................................... 91 Figure
5.3 : Homogeneous reactor: (a) ammonia mole fraction, (b)
temperature profile, and (c)
hydrogen/nitrogen/ammonia mole fractions.
..................................................................
92 Figure 5.4 : Homogeneous reactor with OEM-TPA, showing gas
temperatureT , equilibrium
temperature eqmT , and optimal temperature optT .
.............................................................. 93
Figure 5.5 : The Proposed Energy Recovery Plant with Process
Modifications ................... 93 Figure 5.6 : PFR reactor beds
with cooling between beds 1 and 2
......................................... 94 Figure 5.7 : PFR
reactor beds with cooling between beds
...................................................... 95 Figure
5.8 : Effect of temperature drop in the inter-bed heat exchanger,
after the first bed, on
the ammonia mole fraction at reactor outlet.
..................................................................
95 Figure 5.9 : Effect of temperature drop in the inter-bed heat
exchangers, after the first and
second beds, on the ammonia mole fraction at reactor outlet.
........................................ 96 Figure 5.10 :
Homogeneous reactor: Nitrogen conversion in catalyst bed.
............................ 98 Figure 5.11 : Effect of varying
spatial composition in reactor beds on the mole fraction of
ammonia in the reactor compared with the reference (homogeneous)
design with spatial concentration [1.00, 1.00, 1.00]
......................................................................................
99
Figure 5.12 : Effect of varying spatial composition in reactor
beds (1.50, 1.25, 1.00); a) nitrogen conversion, b) actual, optimal
and equilibrium temperatures, c) hydrogen, nitrogen and ammonia
mole fractions.
..........................................................................
100
Figure 5.13 : Bed1: Temperature Profile with different Catalyst
Distribution ..................... 101 Figure 5.14 : Bed2:
Temperature Profile with different Catalyst Distribution
..................... 102 Figure A.1 Ammonia 3D Phase Diagram
.............................................................................
121
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Engr. Sadaf Siddiq (08F-UET/PhD-ME-47)
1
1 INTRODUCTION
In the coming centuries of the decline of the worlds fossil
energy stocks, an electricity
production mix will establish which will be inevitably dominated
increasingly by alternate &
renewable energies.
The alternate energy sources are available in form of solar
energy, wind energy,
hydroelectric, geothermal, wave and tidal power etc. The current
global energy consumption
is 15TWe per year while the solar energy potential is estimated
to be 86000 TWe per year
[46].
Solar energy can be utilized either as a direct photovoltaic
(PV) source, where the light is
converted directly into electrical energy or as concentrated
solar power where a fluid is
heated by concentrating the solar thermal energy to produce
electricity in a thermal power
plant. Solar thermal energy is concentrated using different
techniques, such as, Parabolic
Trough, dish System and power tower etc.
The success of solar thermal systems for electricity production
hinges very crucially on the
selection, mechanical design and optimal operation of an energy
storage system which can
enable the continuous operation of a power plant. The energy
storage systems being
investigated include solid graphite, encapsulated Phase Change
Materials (PCMs) in a
graphite matrix, and liquid ammonia [72].
This work focuses on extending the use of a solar thermal energy
plant from an intermittent
energy source to a base load power plant by incorporating an
efficient thermal storage feature.
A reference 10 MWe solar thermal plant design is considered with
liquid ammonia as a
working fluid for energy production in a Rankine Cycle as well
as a thermal storage medium.
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Engr. Sadaf Siddiq (08F-UET/PhD-ME-47)
2
During periods of no solar insolence, the recovery system, based
on an industrial ammonia
synthesis system, is used to drive the power conversion unit and
enable continuous operation.
The objective of this work is to increase the efficiency of
ammonia synthesis process for
maximum heat recovery and hence to improve the performance of
Solar Thermal Storage
plant.
1.1 Solar Energy: Potential as a Renewable Energy Source
Solar energy currently accounts for an installed capacity of
about 23 GWe, compared with
geothermal (installed capacity 10.7 GW), and wind (160 GW) [8].
This is insignificant in the
global scenario where in 2010, the total primary energy
consumption was 12002.4 Mtoe [8]
consisting of oil (33.8%), coal (29.6%), natural gas (23.8%),
hydroelectric (6.5%) and
nuclear (5.6%). Even though renewable sources such as solar,
geothermal and wind are not
presently significant, they offer the promise of providing clean
and sustainable energy by
mitigating the effect of the carbon release from fossil fuels,
in the form of greenhouse gases
[8], [14]. Such reductions are necessary for the environment and
are binding on states
signatory to the Kyoto Protocol [117]. Emission of greenhouse
gases (carbon dioxide,
methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and
sulphur hexafluoride) as
well as toxic and pollutant gases, also have a harmful effect on
people.
The Kyoto Protocol of 1997 [117] came into force on 16th
February 2005 and establishes
quantified limitations on greenhouse gases, to promote
sustainable development and calls for
member states to develop new forms of renewable energy and
innovative environmentally
sound technologies.
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Engr. Sadaf Siddiq (08F-UET/PhD-ME-47)
3
According to a study by the World Energy Outlook [14] a
Reference Scenario studies the
period 2006-2030 and estimates an increase in world primary
energy demand of 45% from an
annual of 11730 Mtoe to over 17010 Mtoe at 1.6% increase per
year. While oil remains the
dominant fuel, its share decreases from 34% to 30% over this
period. In the same period, gas
rises at 1.8% per year to 22% while coal, at an annual increase
of 2% rises from 26% to 29%.
Thus the fossil fuels contribute to 81% of the total primary
energy demand by 2030.
Notwithstanding the impact of a nuclear renaissance, the
contribution of nuclear power to
primary energy drops from 6% to 5%; this is an electricity
generation share from 15% to 10%
by 2030.
In this period, renewable energy sources take second place after
coal for electricity
generation. The contribution of hydropower drops from 16% to 14%
while non-hydro
renewables, growing at an average annual rate of 7.2% increase
from less than 1% to 4%.
The absolute magnitude of the non-hydro renewables increases
from 66 Mtoe in 2006 to 350
Mtoe by 2030.
The power outlook has coal contribution to electricity
generation increasing from 41% in
2006 to 44% in 2030, while the share of renewable grows from 18%
to 23% in the same
period. The worlds final electricity consumption grows from
15665 TWh to 28141 TWh at
an average annual growth of 2.5%. This corresponds, in the
Reference Scenario, to an
electricity generation of 18921 TWh in 2006 to 33265 TWh in
2030.
The factors accelerating the share of renewables are climate
change, to attain the CO2 ppm
goal, the higher cost of oil and gas and energy security. Among
the renewables, hydropower
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Engr. Sadaf Siddiq (08F-UET/PhD-ME-47)
4
will continue to be the dominant while others will include wind,
solar, biomass and
geothermal energy.
The sun, as the primary source of energy for the solar system,
supplies over 30,000 TWyr/yr
which, compared with the global energy requirement of the order
of 20 TWyr/yr over the
next generation, may be considered to be a virtually
inexhaustible source [46]. Solar energy
is useable as thermal energy, bioenergy from photosynthesis, and
as a source for photovoltaic
conversion. Solar energy is truly renewable and sustainable as
it is non-depletable, carbon
emission free, scalable, readily accessible, robust and
flexible. The issues which will ensure
its place in the future energy scenario is its economic
competitiveness in comparison with
existing technologies. A key technological issue that lies at
the core of economic
competitiveness of solar energy -- thermal energy storage, is
the focus of this thesis.
For electricity generation, the solar energy options available
are photovoltaic (PV)
technology and concentrated solar power (CSP) technology. PV
technology is based on the
direct conversion of photon energy from the Sun to electricity.
Since the energy from the sun
is spread over a large range of wavelengths, a PV collector is
designed to utilize as much of
the available spectrum as possible. The primary limitation is
the detection window of the
sensor material forming the collector. The efficiency of a PV
collector has remained low
(about 20%) and thus its application has been generally limited
to mini-power requirements
such as off-grid homes [168][175]. However, larger PV plants
have been built and the total
PV technology had a global installed capacity of 6 GWe in 2006,
growing by 2009 to
15GWe and by the end of 2009 to 23 GWe, but had the disadvantage
of having the highest
generating cost (US$ 5500-9000 per kWh in 2007) compared to all
renewable technologies.
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The Reference Scenario estimates the cost to reduce to US$ 2600
per kWh by 2030. CSP
technology uses optics to focus sunlight to a small receiver
where the energy can be utilized
to convert water to superheated steam for electricity generation
in a turbine. The total
installed capacity of CSP was 354 MWe in 2006. This technology
is expected to be
comparable in cost (US$ 2-3 per kW: 2007) with gas-fired, but
generally more expensive
than coal-fired generation, wind and nuclear.
1.2 Solar Power Plants in Operation
1.2.1 PV Plants
Though PV technology is considered to be of use for small
off-grid locations, large plants
have been built and are currently in operation [46]. The PV
power generation technology saw
a 70% increase in 2008 alone, to 13GWe. Two notable areas of
growth witnessed in 2008
were the Building Integrated PV Plants (BIPV) in Europe, and the
utility-scaled PV plants (>
200 kWe), By the end of 2008, over 1800 such plants were in
operation worldwide. Several
of these plants can be considered to be large, with the 200 MWe
Huanghe Hydropower
Golmud Solar Park plant, completed in China in 2011, to be the
largest PV plant in the world.
Plants of this magnitude are currently under development in
Europe, China, India, Japan, the
United States of America and other countries. Table 1.1 presents
Worlds largest PV plants in
operation while 38 more plants with a cumulative nominal power
of about 13000 MWe are
planned or under construction and are expected to complete by
2019.
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TABLE 1.1: Worlds Largest (25MWe or above) PV Plants in
Operation
S.No. Name Country Nominal Power(MWe)
1 Huanghe Hydropower Golmud Solar Park China 200 2 Perovo Solar
Park Ukraine 100 3 Sarnia Photovoltaic Power Plan Canada 97 4
Montalto di Castro Photovoltaic Power
Station Italy 84.2
5 Solarpark Senftenberg Germany 82 6 Finsterwalde Solar Park
Germany 80.7 7 Okhotnykovo Solar Park Ukraine 80 8 Lopburi Solar
Farm Thailand 73 9 Lieberose Photovoltaic Park Germany 71.8
10 San Bellino Photovoltaic Power Plant Italy 70 11 Le Gabardan
Solar Park France 67.2 12 Olmedilla Photovoltaic Park Spain 60 13
Sault Ste Marie Solar Park Canada 60 14 Strasskirchen Solar Park
Germany 54 15 Tutow Solar Park Germany 52 16 Waldpolenz Solar Park
Germany 50 17 Longyuan Golmud Solar Park China 50 18 Hongsibao
Solar Park China 48 19 Serenissima Solar Park Italy 48 20 Copper
Mountain Solar Facility USA 47.6 21 Puertollano Photovoltaic Park
Spain 46 22 Moura photovoltaic power station Portugal 45 23 Kothen
Solar Park Germany 45 24 Avenal Solar Facility USA 42.7 25 Cellino
San Marco Solar Park Italy 40 26 Bitta Solar Park India 39.5 27
Frstenwalde Solar Park Germany 38.3 28 Ralsko Solar Park Ra 1 Czech
Republic 38 29 Reckahn Solar Park Germany 36.2 30 Alfonsine Solar
Park Italy 35.1 31 Vepek Solar Park Czech Republic 35 32 San Luis
Valley Solar Ranch USA 34.4 33 Sant'Alberto Solar Park Spain 34 34
Planta Solar La Magascona & La Magasquila Italy 33 35 Ernsthof
Solar Park Germany 32 36 Arnedo Solar Plant Spain 31.8 37 Parc
Solaire Curbans USA 30.2
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38 Long Island Solar Farm USA 30 39 Planta Solar Dulcinea Spain
30 40 Cottbus Drewitz Solar Park Germany 30 41 Agua Caliente Solar
Project USA 30 42 Gunthawad Solar Farm India 30 43 Cimarron Solar
Farm USA 30 44 Merida/Don Alvaro Solar Park Spain 29.9 45 Planta
Solar Ose de la Vega Spain 27.5 46 Webberville Solar Park USA 26.4
47 evtn Solar Park Czech Republic 26 48 Solarpark Heideblick
Germany 25.7 49 Solarpark Eiche Germany 25
1.2.2 Solar Thermal Plants
The CSP technology showed a small generation increase by 0.06GWe
to 0.5GWe by the end of 2008. The worlds largest solar site is in
California, owned by NextEra Energy Resources [45] The power
produced is 354 MWe, which is purchased by Southern California
Edison and provides to more than 230,000 homes at peak power during
the day. It is thus as large as a nuclear reactor such as CHASNUPP,
and would be sufficient for a city of the size of Islamabad. The
site is spread over 1500 acres, and has more than 900,000 mirrors.
Other large CSP plants in the range of 30-150 MWe are also located
in the United States and Spain [21],[47]. Several other countries
including Abu Dhabi, Algeria, Egypt, Israel, Portugal and Morocco
have projects underway [45]. One of the plants, a 20MWe CSP is
integrated with a 450MWe natural-gas combined-cycle plant in
Morocco. Table 1.2 lists solar thermal power plants in operation in
different parts of the world with total capacity amounting to
1702.65 MWe. The total capacity of under construction (to be
completed by 2014) solar thermal plants is 2106.9 MWe.
TABLE 1.2: Solar Themal Plants in Operation
Serial #
Name Country Capacity (MWe)
Technology
1 Solar Energy Generating Systems
USA 354 parabolic trough
2 Solnova Solar Power Station Spain 150 parabolic trough 3
Andasol solar power station Spain 150 parabolic trough
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4 Extresol Solar Power Station Spain 100 parabolic trough 5
Palma del Rio Solar Power
Station Spain 100 parabolic trough
6 Manchasol Power Station Spain 100 parabolic trough 7 Valle
Solar Power Station Spain 100 parabolic trough 8 Martin Next
Generation Solar
Energy Center USA 75 ISCC
9 Nevada Solar One USA 64 parabolic trough 10 Ibersol Ciudad
Real Spain 50 parabolic trough 11 Alvarado I Spain 50 parabolic
trough 12 La Florida Spain 50 parabolic trough 13 Majadas de Titar
Spain 50 parabolic trough 14 La Dehesa Spain 50 parabolic trough 15
Helioenergy 1 Spain 50 parabolic trough 16 Lebrija-1 Spain 50
parabolic trough 17 Solacor 1 Spain 50 parabolic trough 18 Puerto
Errado 1+2 Spain 31.4 fresnel reflector 19 Hassi R'mel integrated
solar
combined cycle power station Algeria 25 ISCC
20 PS20 solar power tower Spain 20 solar power tower 21 Kuraymat
Plant Egypt 20 ISCC 22 Beni Mathar Plant Morocco 20 ISCC 23 Yazd
integrated solar combined
cycle power station Iran 17 parabolic trough
24 Gemasolar Spain 17 solar power tower 25 PS10 solar power
tower Spain 11 solar power tower 26 Kimberlina Solar Thermal
Energy Plant USA 5 fresnel reflector
27 Sierra SunTower USA 5 solar power tower 28 Archimede solar
power plant Italy 5 parabolic trough 29 Thai Solar Energy (TSE) 1
Thailand 5 parabolic trough 30 Liddell Power Station Solar
Steam Generator Australia 2 fresnel reflector
31 Keahole Solar Power USA 2 parabolic trough 32 Maricopa Solar
USA 1.5 dish stirling 33 Jlich Solar Tower Germany 1.5 solar power
tower 34 Saguaro Solar Power Station USA 1 parabolic trough 35
Shiraz solar power plant Iran 0.25 parabolic trough Overall
Capacity 1702.65
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1.3 Thermal Energy Storage Requirement
The major drawback of CSPs at the moment is the lack of thermal
storage due to which
operation is only possible when daylight is available. Only two
plants have storage viz the
Andasol-1 [48] plant in Spain which has more than seven hours of
full-load thermal storage
capability, and a 280 MWe plant planned in Arizona which will
also have a six-hour storage
capacity.
1.4 Thermal Storage Materials
The thermal energy storage technologies can be classified [72]
by the mechanism of heat viz
(i) sensible, (ii) latent, (iii) sorptive, and (iv) chemical. In
the sensible heat storage systems,
there is the possibility of liquid (water tank, aquifier,
thermal oil) and solid systems (building
mass, concrete, and ground etc.) [5]. In the latent heat storage
systems, both organic
(parrafins) and inorganic (hydrate salts) compounds can be used.
In the sorptive, both
absorption and adsorption systems can be used. Finally, in the
chemical storage, energy can
be stored in chemical bonds which can be broken endothermically
and recovered in a
synthesis exothermically.
When single-phase heat transfer fluids such as thermal oil or
pressurized water are used, a
sensible heat storage system using concrete has been developed
and experimentally tested
[51] in the temperature range 300-400 oC and found to be an
attractive options for CSPs.
Storage materials and technology will also depend on the
temperatures in the plant [66]. For
domestic hot water and space heating, the temperatures will be
less than 100 oC; for process
heat, 100-250 oC; for electricity generation 250-1000 oC, while
for hydrogen production they
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will be in excess of 1000 oC. The storage capacity of some
pha
below [66]. It can be seen that the highest storage capacity is
for salts.
Figure 1.1 Volume Reduction with Phase Change Materials
Figure 1.2 Materials for medium and high heat storage
ME-47)
10
C. The storage capacity of some phase change materials is
shown
. It can be seen that the highest storage capacity is for
salts.
Volume Reduction with Phase Change Materials
Materials for medium and high heat storage
se change materials is shown
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Salts freeze 350-500 oC and boil at ~1000 oC. They have a high
volumetric heat capacity and
may be used, even in graphite blocks. Liquid fluoride salts are
also widely available, as they
are used in aluminium metal extraction Hall electrolysis process
in which aluminium oxide
is dissolved in cryolite which is a sodium-aluminium fluoride
salt. Fluoride salts are
compatible with graphite upto 1400 oC [176].
1.5 Use of Liquid Ammonia as Storage Material
Liquid ammonia is a candidate for large solar-thermal systems
due to the storage of thermal
energy in its chemical bonds during, for example, solar
insolation and recovery from
subsequent exothermic synthesis. To compare different storage
opportunities, the energy
storage density is a value which is useful to determine the
required size of storage for a
required amount of energy. With the kind of energy carrier, the
amount of stored energy
varies strongly. A comparison between different energy carriers
is presented in Figure 1.3
[79]. It is clear that thermo-chemical energy carriers offer the
suitable most energy densities
i.e. of the order of 10 MJ/kg.
Ammonia is an abundantly produced chemical, globally and in
Pakistan. It has an important
use as a fertilizer to boost agricultural production. Thus it is
used in a synthesis process of
natural gas with carbon dioxide resulting in the formation of
urea fertilizer, or carbamide
(NH2)2CO. In Pakistan, there are eight large urea fertilizer
plants based on the reforming and
synthesis of natural gas mainly from the Sui and Marri gas
fields. At an international price of
US$ 300/tonne, this represents an annual sales value of US$1,500
million. This amounts to
an average production of about 1600 tonnes per day (TPD) per
plant [25].
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Figure 1.3 Energy densities for different energy carriers
1.5.1 Poperties of Liquid Ammonia
Ammonia (NH3) stays in the liquid form at temperatures higher
than its melting point
73.77
oC and has a density of 681.9 kg/m3 at its boiling point -33.34
oC ; it must thus be
kept at very low temperature or stored at very high pressure
[165]. Liquid ammonia was first
produced on an industrial scale in Germany, during the First
World War, by the Haber -
Bosch process [110].
1.5.2 Dissociation and Synthesis of Ammonia
The dissociation of ammonia
223 32 HNNH + (H = 66.9 kJ/mol)
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is an endothermic reaction that can be carried out by
thermo-catalytic decomposition using
catalysts: ruthenium, indium, nickel, Fe-Al-L, Fe-Cr. Typical
temperatures are in the range of
850 1000 oC. Approximately 1.4 kW power per cubic meter of
hydrogen is typically used.
Conversely, the synthesis of ammonia from nitrogen and hydrogen
reactant gases
322 23 NHHN + (H = -92.22 kJ/mol)
is an exothermic reaction for which the pressure required is in
the range 130 250 bar, and
the temperature required is in the range 250 600 oC. High
temperature gives higher reaction
rate, but as reaction is exothermic, higher temperature
according to Le Chateliers principle
causes the reaction to move in the reverse direction hence a
reduction in product. Similarly,
higher Temperature reduces the equilibrium constant and hence
the amount of product
decreases; this is the Vant Hoff equation
RS
RTHK
oo +
=ln
An increase in pressure, however, causes a forward reaction and
is thus favorable. Synthesis
is achieved by using catalysts such as osmium, ruthenium, and
iron-based catalysts [110].
1.5.3 Commercial uses of Ammonia
Ammonia is one of the most widely produced chemicals, amounting
to over 15 million tones
in 2009. Its major uses are as fertilizer and for production of
nitrogen containing compounds
such as nitric acid. It is used as a refrigerant and in textile
processing. A very important
emerging use of ammonia is Hydrogen production, by its
decomposition, to be used in
Hydrogen Fuel cells.
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1.5.4 Industrial proprietary processes for Ammonia
Production
Global fertilizer industry produces about 170 million tones of
fertilizer nutrients every year
[42] for boosting agricultural output. Fertilizers are based on
nitrogen, phosphorus or
potassium. Nitrogen accounts for 78% of the earths atmosphere.
Since plants can not breathe
nitrogen, it must be converted to a suitable form such as
ammonia. The Haber-Bosch process,
first demonstrated by Fritz Haber in 1909 and scaled up to an
industrial process by Carl
Bosch in 1913. Both Haber and Bosch were awarded Nobel Prizes
for their inventions, and
ammonia was used in Germany in the First World War for the
manufacture of explosives. A
greater use of the Haber-Bosch process was in the manufacture of
fertilizers such as urea and
ammonium nitrate. About 70% of the ammonia produced is from
natural gas as feedstock
and the rest is mainly from coal. The Haber-Bosch process,
involving the steam reforming of
methane to produce hydrogen is used with nitrogen taken from the
air, to produce ammonia.
The typical size of urea plants is 1000 MeT per day with a
capital cost of US$ 150 million.
The total production of ammonia was 130 million tones in 2000,
produced in 80 countries
and 85% of which was used for nitrogen fertilizer production.
The largest chemical industry
in the world is in the U.S. [19], with ammonia being the most
important intermediate
chemical compound produced in 41 plants. The energy intensity
for ammonia manufacture in
the U.S. is 39.3 GJ/tonne (including feedstocks HHV). The
theoretical minimum for
ammonia production by steam reforming is 21.6 GJ/tonne which
represents the ideal goal.
The technology is now mature, with the market dominated by five
licensers-Haldor Topse,
M.W. Kellogg, Uhde, ICI, and Brown & Root, of which Haldor
Topse has a 50 per cent
world market share as supplier of the technology [42].
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1.5.4.1 Haldor Topsoe Ammonia Synthesis Process
The conventional sequence of process steps are optimized by the
introduction of improved
catalysts (KM high strength, versatile, stable and
poison-resistant catalyst, mainly magnetite
Fe3O4 with promoters mainly oxides of calcium, aluminum and
potassium, operating
temperatures 340-550 oC [21]), new equipment design (such as
improved synthesis
converters), and process optimization studies. The carbon
monoxide concentrations have
been minimized at the exit of the shift converters, and a
low-energy carbon dioxide removal
process (such as selexol) has been used. New syn converters
S-250 and S-300 are improved
versions of the previous single bed S-50 and two-bed S-200
radial flow converters. Topsoe
recommends S-300, developed in 1999, for all new plants [[21],
[24].
TABLE 1.3: Haldor Topsoe Ammonia Converter Features
Type Basic Design Comments S-50 One catalyst bed Simplest and
cheapest S-200 Two catalyst beds and one interbed
heat exchanger Commissioned in 1979; 130 units installed
S-250 Combination of the S-200 followed by the S-50
S-300 Three catalyst beds with two interbed heat exchangers
Higher conversion for same catalyst volume of S-250; installed
first in 1991.
1.5.4.2 Kellog Brown & Roots (KBR) Advanced Ammonia Process
(KAPP)
KAAP uses a traditional high-pressure heat exchange based steam
reforming process
integrated with a low-pressure advanced ammonia synthesis
process. The steam reforming of
hydrocarbon based on Kellogg Brown and Root Reforming Exchange
System (KRES) is
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carried out which reduces energy consumption and capital cost
besides reduced emissions
and enhanced reliability.
After reforming, carbon monoxide is removed from the shift
converter, and carbon dioxide is
removed from the process gas using hot potassium carbonate
solution, methyl diethanol
amine (MDEA) etc.
KAAP uses a high activity graphite supported ruthenium catalyst,
typically three stages, after
one stage of traditional iron catalyst. This is claimed to
increase the activity 10 to 20 times
enabling very high conversion at a lower pressure of 90 bar
[10].
KBR is a large player in the ammonia and urea industry. It has
been involved in the licensing,
design, engineering and/or construction of more than 200 ammonia
plants and 62 urea
projects in the range of 600 to 3500 MTPD worldwide,
representing approximately half of
current global ammonia production [23].
1.5.4.3 Krupp Uhde GmbH Ammonia Process
The Krupp Uhde Gmbh process uses the traditional reforming
process followed by a
medium-pressure ammonia synthesis loop[86].
The primary reforming is carried out at a pressure of 40 bar and
temperature range of 800-
850 oC. Enhanced reliability is attained by using a top-fired
steam reformer with high alloy
steel tubes. Process air is added in the secondary reformer
through nozzles installed in the
wall of vessel thus providing proper mixing of the air and
reformer gas. This also provides
high energy efficiency in high pressure steam generation and
superheating. As in other
processes, carbon monoxide is converted to carbon dioxide in HT
and LT shift converters,
and the MDEA or Benfield system is used for carbon dioxide
removal.
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The ammonia synthesis loop uses two radial flow ammonia
converters with three catalyst
beds, containing iron catalyst, and waste heat boiler located
downstream of each reactor. The
converters have small grain iron catalyst.
Since 1994, Uhde has built 15 new ammonia and 13 new urea plants
with annual production
capacities of more than 8 million tonnes of ammonia and 10
million tonnes of urea with
individual capacities ranging from 600 to 3,300 mtpd of ammonia
and from 1,050 to 3,500
mtpd of urea [36]. Uhde has also been awarded a contract to
build a 3300 MTPD Uhde
Dual-Pressure Process ammonia plant for Saudi Arabian Fertilizer
Company (SAFCO) in
Al Jubail, Saudi Arabia [78].
1.5.4.4 ICI-Leading Concept Ammonia (LCA) Process
In this process, ammonia synthesis takes place at low pressure
of below 100 kg/cm2g
(approximately 100 bar) using ICIs highly active cobalt promoted
catalyst. This process has
an energy consumption of approximately 7.2 Gcal/ MeT (30.1
GJ/MeT) ammonia for a 450
MeT per day plant [19].
1.5.4.5 The Linde Ammonia Concept (LAC) Ammonia (LCA)
Process
The LAC process consists essentially of a modern hydrogen plant
and a standard nitrogen
unit with a third-party license from Casale for a high
efficiency ammonia synthesis loop [34].
Ammonia Casale [16] is one of the oldest companies in the
business of synthetic ammonia
production, having been founded in Switzerland in 1921. To date
it has been active in the
design of over 150 ammonia synthesis reactors and in the
constructionof several new plants.
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The CO shift conversion is carried out in a single stage in the
tube cooled isothermal shift
converter and gas is sent to pressure swing absorption (PSA)
unit wherein the process gas is
purified to 99.99 mole % hydrogen . A low temperature air
separation in cold box is used to
produce pure nitrogen. BASFs MDEA process is also eliminated in
this process used for
CO2 removal.
The ammonia synthesis loop is based on Casale axial-radial
three-bed converter with internal
heat exchanger giving a high conversion. The energy consumption
for ammonia production
is about 29.3 GJ/ MeT [16].
Thus far, four plants based on the relatively new Linde Ammonia
Concept have been
constructed with capacities of between 230 to 1,350 MTPD of
ammonia.
1.6 Thermodynamic Cycles for Solar Thermal Power Plants
The two commonly used thermodynamic cycles for solar plants are
the Brayton and Rankine
Cycles depending on the temperatures of the working fluid. Power
towers employing PCM
salts are able to achieve very high temperatures, typically in
excess of 1000 oC which transfer
heat to inert gases such as helium, and at a lower temperature,
water is converted to
superheated steam. Such plants draw heavily from the experience
and resources available
with high temperature gas reactors in the nuclear industry.
While the thermodynamic
efficiency of such systems is high, special materials and high
safety features are required for
this technology [140].
Solar power plants based on the concentrating parabolic systems,
ordinarily use water as a
working fluid and are thus based on the Rankine Cycle [130].
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1.7 Literature Review
The literature survey covered a wide range of areas, this
section reviews the potential of solar
energy as a renewable source for a sustainable and clean energy
future, Solar thermal power
and its components, thermodynamic aspects of candidate solar
plants, thermal storage
materials, energy inputs and outputs from various thermal
storage materials, energy recovery
industrial process and energy efficiency analyses for plant
performance and design
parameters for realized Solar thermal power plants. This design
data from realized Solar
thermal power plants has been used as a starting point for
component and overall simulation,
as well as optimization formulations for carrying out
sensitivity analyses leading to an
optimal pant design. Modeling and Simulation techniques for
component and integrated plant
design are discussed in section 3.1.1 while review of
optimization techniques is presented in
section 4.1.
Concentrating solar power is a method of increasing solar power
density. CSP has been
theorized and contemplated by inventors for thousands of years.
The first documented use of
concentrated power comes from the great Greek scientist
Archimedes (287-212 B.C.) in 212
B.C. [175]. The modern solar concentration is believed to begin
by the experiments of
Athanasius Kircher (1601-1680) in seventeenth century [175].
Solar concentrators then
began being used as furnaces in chemical and metallurgical
experiments [161]. In eighteenth
and nineteenth centuries CSP applications were restricted to low
pressure steam generation
and solar pumps etc.
CSP systems can provide energy storage fully integrated within
the electricity-generating
plant [2][4][5]. Solar thermal radiation can be concentrated
using parabolic mirrors in the
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form of dishes, power towers, troughs and linear Fresnel etc. in
commercial CSP systems.
The efficiency of these mechanisms can be evaluated on the basis
of geometric concentration
ratio. The geometric concentration ratio for parabolic troughs
and linear Fresnel systems can
be up to 100 and in excess of 1000 for power towers and dishes.
This thermal energy can be
used to produce steam for immediate electricity generation, or
alternatively it can be stored
prior to electricity generation using sensible heat storage in
solids [27][49][66], molten salts
[88], phase change materials [9][39][145][153], or
thermochemical storage cycles [15].
Thermochemical energy storage for CSP is less mature than molten
salt and other thermal
storage methods, but it has the potential to achieve higher
storage densities and hence smaller
storage size. Reactions involving ammonia, hydroxides,
carbonates, hydrides, and sulfates
are the important candidates for thermochemical energy storage
[15][67]. At first,
thermochemical storage loops based on methane reforming received
considerable attention
[58][115][138][141]. Methane reforming is still under research
for solar enhancement of
natural gas [30] and hydrogen production [31]. A lot of research
is being conducted on solar
fuel production by making use of thermochemical processes
[17][38].
The concept of ammonia-based energy storage for concentrating
solar power systems was
first proposed by Carden in 1974 at the Australian National
University [174][177] followed
by the researchers at Colorado State University in early 1980s
[163].
Researchers at ANU [132][167] and Colorado State University
[163] have concluded after
theoretical analysis and experimental results [109] that dish
concentrators are the most
suitable solar receiver designs for ammonia dissociation because
they provide a
circumferentially homogenous solar flux profile [136] which can
facilitate thermochemical
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21
reactor design inferring that only simple control systems are
necessary, thus the mobile
receiver can be maintained at a light weight, and solar
transients are easy to handle
[132][137]. Feasilbility of parabolic trough systems have also
been investigated for use with
CSP employing ammonia based energy storage systems [97].
Prototype solar ammonia receiver/reactors, Mark I and Mark II
were tested in 1994 and 1998
respectively both employing a 200-mm long cavity type reactor
mounted on a 20-m2 faceted
paraboloidal dish. Haldor-Topse DNK-2R iron-cobalt catalyst was
used in the annular
catalyst beds [108]. These reactors were rated for 1.0
2.2-kWchem conversion. Recent work
is being conducted on paraboloibal dishes of area 400-m2, 489-m2
and newly constructed
500-m2 for a base load plant size of upto 10 MWe [11].
For solar collector/receiver design improvements, investigations
into convection losses from
cavity receivers have been undertaken [12][81] as these
improvements can amount to solar-
to-chemical efficiency gains of up to 7% absolute [106].
The kinetic mechanisms for the synthesis and decomposition of
ammonia have been
described by various authors for ironbased catalysts
[120][186][189][195] and for ruthenium-
based catalysts [111][121][126].
Comprehensive studies for solar energy heat [104],[106]]
recovery have been carried out on
an experimental 1-kWchem. synthesis reactor by Kreetz and
Lovegrove [106] in a laboratory-
scale high-pressure closed-loop system with a feed-gas mass flow
rate of 0.3 g s-1 at
pressures ranging from 9.3 to 19 MPa. With external pre-heating
of the feed gas, average
external wall temperature varying between 250-480C and peak
internal reactor temperatures
varying between 253-534C, the maximum reaction was reported by
Kreetz and Lovegrove
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22
[106] to have been achieved at approximately 450C. In their
optimal system, a net heat
recovery rate of 391 W was reported. The study by Kreetz and
Lovegrove [106] was extended
to a 10 kW system with an ammonia synthesis tubular reactor at a
pressure of 20 MPa and a
flow rate of 0.9 g s-1 [79]. The larger system, with a
controlled linear temperature profile in
the reactor wall, and the gas inlet temperature kept to 50 C
lower than the wall temperature
at the inlet, resulted in a maximum thermal output achieved at
an average wall temperature of
475C produced with an inlet temperature of 500 C and a slope of
-50 C m-1. Such studies
have attempted to achieve optimal heat recovery by varying the
inlet temperature arbitrarily
instead of attaining the optimal temperature suggested by
theoretical models, such as
variational methods.
1.8 Thesis Motivation
Thermal Storage plants using ammonia as storage medium can take
advantage of the well-
understood and extensively deployed ammonia dissociation and
synthesis technologies. Their
efficiency, however, will depend on the optimization of the
process parameters typical of the
system pressure and temperatures in the dissociation and
synthesis reactors taken together
with those at the solar receiver.
A lot of research has been carried out on solar collector design
and dissociation efficiencies
of more than 90% have been practically achieved using cavity
type dissociation reactors in
conjunction with paraboloidal dish type solar
receivers[105][106]. The motivation of plant
optimization is to maximize the efficiency of the plant by
maximizing the heat recovery from
the most critical process in the plant: the synthesis reactor.
This research is of great value to
industry as well because the same optimization techniques can be
used for improving
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ammonia production rates at pressures lower than the industry
standard pressures, hence
cutting the costs.
1.9 Objectives
The use of liquid ammonia, as a thermo-chemical energy storage
medium, for endothermic
dissociation by solar energy during insolence and subsequent
energy recovery by exothermic
synthesis is considered to be a strong candidate for the design
of a base-load solar thermal
power plant.
The technology of ammonia production is well established as is
the modeling and simulation
of ammonia synthesis. However, optimization of the process is an
on-going challenge as
technological innovations enable better designs resulting in
improved efficiency. As part of
this optimization challenge, this thesis considers the possible
improvement in the recovery of
exothermic thermal energy by optimization of the ammonia
synthesis process. While
ammonia production has remained almost the same for decades, the
energy consumption has
reduced as technology improvements have been incorporated
especially for the fertilizer
industry where over 90% of the energy utilization is for ammonia
synthesis [76].
The objective of the study will be achieved by:
Parametric Sensitivity studies leading to an optimized design of
a TSP
i. Numerical Simulation of Conservation Equations,
ii. Optimize physical dimensions of Synthesis Reactor,
iii. Optimal distribution of Catalyst (Optimal Control
analysis),
iv. Overall Thermal Energy Recovery Analysis.
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Engr. Sadaf Siddiq (08F-UET/PhD-ME-47)
24
1.10 Summary of Following Chapters
The necessary background information is given in Chapter 1.
Chapter 2 describes the thermal
storage plant features such as process design, operational
parameters and process flow
diagram etc. Chapter 3 deals with the modeling and simulation of
the components of thermal
storage plant while optimization of thermal storage plant has
been done in chapter four both
by variational calculus and process engineering codes. The fifth
chapter presents the optimal
TSP model, designed in the light of sensitivity and parmetric
analyses in chapter four.
Conclusions and future recommendations are presented in Chapter
6.
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25
2 DESCRIPTION OF THE THERMAL STORAGE
PLANT
2.1 Plant Features
A thermal storage plant may be used as a baseload plant, when it
operates on a continuous
basis just like a coal-fired, nuclear or hydroelectric power
plant, or as a traditional PV
intermittent solar plant. The baseload operation is only
possible if the plant has an integrated
thermal storage feature.
The major components of a baseload plant are the receiver
system, a storage system, an
energy recovery system, a power conversion unit, and associated
plant systems such as
compressors, pumps and heat exchangers.
The objective of the Thermal Storage Plant (TSP) considered here
is to maximize the overall
efficiency of the plant, which is essentially the optimization
of the ammonia synthesis
process.
2.1.1 Process Design
This section considers some basic aspects of the overall plant
design with the objective of
getting orders of magnitude. Table 2.1 shows such overall
conditions for a conceptual MS-
Excel calculation for a baseload plant of 10 MWe. It is assumed
that a solar insolation of 1
kW/m2 is available for 8 hours in a day. With 400 parabolic
dishes, each of area 400 m2 of
the type available to the ANU group [101][105], the thermal
power intercepted by the plant is
-
26
4.608 TJ in a day. These assumptions are optimistic even for the
high solar insolence of
about 20 MJ/m2 for Pakistan [150].
TABLE 2.1: Overall Plant Design for a 10 MW(e) Baseload
Plant
BASIC DATA
Dissociation Energy kJ/mol 66 Synthesis Energy kJ/mol 46.6 Power
Density Watts / m^2 1000 Insolation Hours per Day Hr 8 Dish Area
m^2 400 No. of Dishes 400 extent of dissociation reaction
0.9 Electrical Power Reqd (24hrs) MW(e) 10 Synthesis
Conversion
0.2 Rankine Cycle Effciency
0.4
POWER INPUT
Thermal Power Available/day kW-hr/m^2 per day 8
Thermal Power Available/day MJ/m^2 per day 28.8
ThPower/day on one dish MJ per dish per day 11520
ThPower/day on all dishes MJ per day 4608000
Flow of NH3 per dish mol per dish during insolation
193939.39
Flow of NH3 per dish kg NH3 per dish during insolation 3296.97
Flow of NH3 MTD NH3 during insolation 1318.79 Flow rate of NH3 kg/s
NH3 45.79 Flow rate of NH3 per dish kg/s NH3 0.1145
Electrical Energy Needed MJ(e) per day 288000 Thermal Energy
Needed MJ(th) per day 720000
POWER OUTPUT
Recoverd Synthesis Energy MJ per day 921600 Converted Synthesis
Energy MJ per day 368640
Overall Efficiency % 8
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27
Key design parameters are the thermal power intercepted by the
plant during insolence (4.6
TJ), the thermal energy needed (2.1 TJ) for a baseload of 10
MW(e), the recovered synthesis
energy (0.922 TJ) and the final converted energy (0.368 TJ). In
this scenario, three
efficiencies are assumed viz (i) the extent of dissociation
(0.9) [47], (ii) the synthesis
conversion (0.2), and (iii) the Rankine efficiency (0.4)
[[15],[79],[130].
The purpose of the present research is to estimate the best
possible synthesis conversion, by
optimizing the catalyst distribution, to investigate the
feasibility of such baseload operation.
2.1.2 Opertational Parameters
The operational parameters of TSP have to be chosen carefully as
the use of a reversible
reaction to store energy is governed by the dependency of the
thermodynamic equilibrium
composition on temperature and pressure. Conceptually, if a
sample of ammonia were heated
slowly (quasi-statically), it would begin to decompose at
temperatures of several hundred
degrees, around 700 K at 200 atmospheres (20MPa). Complete
dissociation would only be
approached asymptotically at very high temperatures. The amount
of energy absorbed at each
step would be proportional to the fraction of ammonia split.
Reversing the process and
withdrawing heat would see ammonia resynthesize, with heat
released progressively [196].
To implement this on an industrial scale, the limitations of
reaction kinetics must also be
taken into account. Reaction rates are zero at equilibrium by
definition; they increase by the
degree of departure from equilibrium (and in the direction
needed to return the system to
equilibrium) and also increase rapidly with temperature in
proportion to the well-known
-
28
Arrhenius factor. Thus, a real system absorbs heat at
temperatures higher than the
equilibrium curves suggest and then releases it at lower
temperatures.
The input temperature for the power cycle is an extremely
important issue for all thermal-
based energy storage systems, not just thermochemical ones.
Electric power generation via a
thermal cycle is limited by the second law of thermodynamics
lower temperature thermal
inputs reduce the efficiency of power generation. Thus, in
designing and examining thermal
energy storage systems, it is necessary to consider both thermal
efficiencies (energy
out/energy in) and second law efficiencies (potential for work
out/potential for work in).
A TSP will have operational parameters, pressures, temperatures
and flow rates, similar to
those in the ammonia units of urea fertilizer plants in the
chemical process industry. These
require pressures in the range of 130-250 bar and temperatures
in the range 250-600 oC for
flow rates typically of the order of 50 kg s-1 for a 1500 MTD
ammonia plant. Such high
pressures require compression which is expensive in terms of
equipment cost as well as
energy utilization
2.2 Overall Plant Layout and Description
The schematic diagram of TSP is shown in figure 2.1[2]. In this
closed loop system, a fixed
inventory of ammonia passes alternately between energy-storing
(solar dissociation) and
energy-releasing (synthesis) reactors, both of which contain a
catalyst bed. Coupled with a
Rankine power cycle, the energy-releasing reaction could be used
to produce baseload power
-
29
for the grid. At 20 MPa and 300 K, the enthalpy of reaction is
66.8 kJ/mol, equivalent to 1.09
kWh/kg of ammonia, or 2.43 MJ/L, with the corresponding density
of 0.6195 kg/L [165].
Figure 2.1: Thermal Storage Plant Schematic
2.2.1 Ammonia Dissociation
Having the advantage of solar concentration of 3000 suns [105],
a mirrored paraboloidal dish
focuses solar radiation onto a dissociation reactor (cavity
type) through which anhydrous
ammonia is pumped. The reactor contains an annular catalyst bed
which facilitates the
dissociation of ammonia at requisite temperature and pressure
into gaseous nitrogen and
hydrogen termed syngas. The fact that the ammonia dissociation
reaction has no possible
side reactions makes solar dissociation reactors easy to control
and implement [2][160].
Typically, 400 such reactors mounted on paraboloidal dishes, of
area 400 m2 each, are used
in an array patteren to feed the ammonia synthesis reactor.
-
30
Figure 2.2: Array of 400 m2 Paraboloidal Solar Collectors
[3]
2.2.2 Ammonia Synthesis
A reactor is used for energy recovery from the exothermic
synthesis reaction in which syngas
is synthesized to produce ammonia in the presence of an annular
catalyst bed. Since
ammonia synthesis is a developed technology for more than 100
years, synthesis reactors
used for TSP are based on standard and proprietary industrial
technologies from companies
that include Haldor-Topsoe, Kellogg Brown & Root (KBR),
AkzoNobel (formerly Imperial
Chemical Industries (ICI)), and Cassal [16],[21],[23],[24].
For the reference TSP in this work, KBR Advanced Ammonia Process
(KAAP) synthesis
convertor is chosen. In the KBR Advanced Ammonia Process
(KAAP)[23], the synthesis
converter uses a combination of catalysts to maximize the
conversion and heat recovery, such
as one stage of traditional magnetite catalyst, followed by
three stages of a proprietary
KAAP catalyst consisting of ruthenium on a stable,
high-surface-area graphite carbon base
(KBR). This KAAP catalyst has an intrinsic activity ten to
twenty times higher than
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31
conventional magnetite catalyst and is used to lower the
synthesis operating pressure to 90
bar which is one-half to two-thirds the operating pressure of a
conventional magnetite
ammonia synthesis loop and hence cutting plant costs.
2.2.3 Syn Gas and Ammonia Storage
The closed-loop TSP operates at a pressure (150 bar) above
ambient temperature saturation
pressure of ammonia and the ammonia fraction in storage is
present largely as a liquid which
causes automatic phase separation of ammonia. Thus, a common
storage tank can be used to
store both syngas and liquid ammonia.
2.2.4 Heat Exchangers and Transport Piping
The heat exchangers shown in Fig. 2.1 serve to transfer heat
from exiting reaction products to
the cold incoming reactants. In this way, the transport piping
and energy storage volume are
all operated at close to ambient temperature, reducing thermal
losses from the system, as well
as eliminating the need for costly specialized equipment.
2.2.5 Compressors and Pumps
Compressors are used for the pressure management of high
pressure storage vessel and
synthesis loop. In the dissociation part of the system, a liquid
ammonia feed pump is
incorporated with each paraboloidal dish. These pumps are used
to control the actual process
conditions within the ammonia dissociation reactor. Mass flow
control aims for 80% of the
ammonia feed being dissociated on average.
-
32
2.3 Thermal Storage Plant Process Flow Diagram
Figure 2.3 presents a simplified process flow diagram of thermal
storage plant. The input
stream (DIS-IN) to the solar driven dissociation reactor
(DISRCTR), consisting of liquid
ammonia, is pumped from the high pressure storage tank
(S-TANK).
The stream DIS-IN is pre-heated by passing it through the
counter flow heat exchanger (CF-
HX) in order to increase its temperature. The output syngas
stream (DIS-OUT), consisting of
nitrogen, hydrogen and small amounts of other gases, looses heat
in heat exchanger (CF-HX)
and is fed into storage tank (S-TANK). The feed-stream (FEED1)
from storage tank is
compressed to the pressure required for synthesis, 150 bar. Due
to the unfavourable reaction
equilibrium, only part of the Syngas is converted to ammonia on
a single pass through the
Synthesis Reactor (SYNRCTR). Since the unconverted Syngas is
valuable, the majority of it
is recycled back to the SYNRCTR. A Mixer is used to combine the
Recycle Stream (FEED2)
and fresh stream FEED1.
Figure 2.3: TSP Process Flow Diagram
-
33
This mixed stream (MIX-OUT), heated in SRIN-HX to a temperature
of 370 oC and is fed
into the catalyst-containing synthesis reactor (SYNRCTR) where
the synthesis reaction, in
the forward direction, converts nitrogen and hydrogen into
ammonia and hence producing
energy.
The effluent stream passes through the recovery heat exchanger
(SROUT-HX) into the
Knock-Out drum FLASH, where the liquid ammonia is sent back to
the storage tank through
stream PRODNH3 and stream VAPOR is carried to the purging
system. The VAPOR stream
from Flash tank (FLASHT) contain traces of undesirable gasses
such as Argon, Carbon
Monoxide and Carbon Dioxide. Argon has high partial pressure
while Carbon Monoxide and
Carbon Dioxide are poisons for the Catalyst. Some of the cycle
gas must be purged from the
Synthesis Loop. Otherwise, the argon that enters the loop in the
Syngas has no way to leave
and will build up in concentration. This will reduce the rate of
the ammonia synthesis
reaction to an unacceptable level. To prevent this from
happening, a small amount of the
cycle gas must be purged, the amount being determined by the
amount of argon in the feed
and its acceptable level in the Synthesis Converter feed
(generally about 10 mol %). A
splitter is used to divide the VAPOR stream into PURGE and
RECYCLE streams.
Another re-cycle compressor (RCOMP) is required at this stage to
restore the pressure to the
required level till the stream (FEED2) is mixed with the feed
stream (FEED1) and enters as
stream MIXER-OUT.
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34
3 MODELLING & SIMULATION OF THERMAL
STORAGE PLANT
3.1 Mathematical Modelling
3.1.1 Review of Mathematical Models of TSP
The synthesis of ammonia can be modeled using the laws of
conservation of mass,
momentum and energy for non-isothermal multi-component systems
undergoing chemical
reactions and mass transfer [85]. In the case of unsteady flow
the governing equations are:
Mass:
0222021 +=+= Svmdtd
tot
3.1
Mass of Species i:
Nirmdtd
totiiiitoti ,......3,2,1,021, =++=
3.2
Momentum:
sftottot FFgmSpv
vSpv
v
dtd
+++
+
= 022222
22
11111
21 )()(
3.3
(Total) energy:
QQwHghv
vHghv
vUKdtd
tottottot ++++
++
=++
02222
32
1111
31 )
21()
21()(
3.4
-
35
In terms of molar quantities, the continuity equation is
expressed in terms of the molar
concentration c, and the mole fractions yi as
=
=+=
+ N
iiiii NiRyRJyvt
yc
1
**,.....3,2,1)()(
3.5
TABLE 3.1: Equations of change of Multi-component Mixtures in
terms of the Molecular Fluxes
Total mass: )( vDtD
=
Species mass: (i=1,2,3,..N) ii
i rjDt
D+= )(
Momentum: g
DtDv += ][
Energy: )(])[()()()21( 2 gvvpvqvU
DtD
=+
The above have been expressed by Dashti [64] as
( )
==
=+
=
pp
NHrp
o
N
NH
dv
dv
vdxdP
RHdxdT
vC
AFR
dxdz
2
323
22 )1(75.11150
0)(/2
3
2
3
3.6
A simpler analysis ignores the pressure drop in flow reducing to
the conservation equations
for mass and energy with reaction kinetics, used by Yuguo [152]
and Dashti [64]
AFR
dxdz
o
N
NH
/23
=
3.7
0)(3
=+ NHrp RHdxdT
vC
3.8
-
36
For the reaction kinetics, the Temkin-Pyzhev [64],[152] form for
the synthesis reaction rate
as a function of the pressure, temperature, and activities is
used
= 5.1
5.12
2
3
3
2
232
H
NH
NH
HNaNHA
a
a
a
aaKkRR
3.9
where the activities are defined as Pya iii = . The individual
activities are:
[ ]{ }
+++=
+++=
+=
+
262532
262633
300/)941.5011901.0(2)98.151263.0()541.08402.3(
102761216.0101142945.0104487672.0102028538.01438996.0104775207.010270727.010295896.0102028538.093431737.0
300exp
3
2
5.0129.0
2
PXTXPXTXPXTXPXTX
eePePe
NH
N
PTTTH
3.10
The Arrhenius rate form is given as:
=
RTEkk o exp
3.11
and the specific heat capacities of hydrogen, nitrogen, methane
and argon of the syngas (T in
Kelvin, Cp in J/mol-K) are expressed as:
=
++=
+=
+=
9675.4*184.4)1063.210303.0102.175.4(184.4
)106861.01001930.01003753.0903.6(184.4)102079.01009563.01004567.0952.6(184.4
392524
392522
332222
ArCTXTXTXCHC
TXTXTXNCTXTXTXHC
P
p
p
p
3.12
For ammonia, the Shomate equations [28] are given as:
232
3 *** tE
tDtCtBANHCP ++++=
3.13
with T in the range 298-1400 K, and .1000
Tt =
-
37
18917.0;921168.1;37599.15;77119.49;99563.19 ===== EDCBA
have