Dr. Sadaf Siddiq 08F UET PhD ME 47

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

ii

OPTIMAL PERFORMANCE ANALYSIS OF A SOLAR THERMAL ENERGY STORAGE PLANT BASED ON

LIQUID AMMONIA

by


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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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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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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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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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)

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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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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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


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.


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.


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


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.


5

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.


6

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


7

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


8

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


9

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

Engr. Sadaf Siddiq (08F-UET/PhD-ME

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


11

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].


12

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)


13

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.


14

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].


15

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


16

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.


17

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.


18

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].


19

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


20

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


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


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


23

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.


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.

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

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

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.

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

Dr. Sadaf Siddiq 08F UET PhD ME 47

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