Optimizing Adsorbents for Heat Storage Applications:

4,15

MB

Estimation of Thermodynamic Limits and Monte Carlo

184

stron

Simulations of Water Adsorption in Nanopores

5873

ID

Albert-Ludwigs-Universität Freiburg

2004

rok

Contents

Publications iii

1 Introduction 3

2 Thermodynamics of adsorption heat storage 9

2.1 Heat storage: State of the Art . 9

2.1.1 Hot water storage . 9

2.1.2 Phase Change Materials (PCMs) . 10

2.1.3 Adsorption 12

2.1.4 System concepts for adsorption heat storage . 13

2.2 General adsorption thermodynamics . 15

2.2.1 Classification of adsorption isotherms 18

2.2.2 Henry’s constant . 20

2.3 Adsorption heat storage cycle . 20

2.4 The “Dubinin approach” to materials optimization 23

2.5 Statistical thermodynamics approach to adsorbent optimization . 26

2.5.1 Langmuir model . 27

2.5.2 Reference conditions for cycle modeling 31

2.5.3 Parametric study . 32

2.5.4 Modeling energetic heterogeneity 34

2.5.5 Lattice gas models 37

2.5.6 Variation of temperature lift . 46

2.6 Summary . 50

3 Experimental results and comparison with model predictions 53

3.1 Thermogravimetric measurements 53

3.2 Henry’s constants of water on silica gels . 56

3.3 Comparison with Langmuir and lattice gas models . 59

3.4 Volumetric energy densities 61

3.5 Summary and Conclusions 67

4 Monte Carlo Simulations: Background and models employed 71

4.1 Monte Carlo sampling techniques 73

4.1.1 Random sampling 74

4.1.2 Importance sampling . 74

4.1.3 The Metropolis algorithm 75

4.1.4 The condition of detailed balance 76

4.1.5 Simulations in the grand canonical ensemble 78

4.1.6 Simulation of non-spherical molecules 79

4.2 Molecular modeling of water . 80

4.2.1 Partial charge models . 81

4.2.2 Ewald summation . 82

4.3 Tetrahedral square well (TSW) water model . 84

4.4 How to increase Sampling Efficiency . 86

4.4.1 Configurational bias algorithms . 86

4.4.2 Tests of particle exchange efficiency . 87

4.5 Data Analysis . 89

4.6 Force fields and zeolite modeling in Cerius2/Sorption 91

4.6.1 Generation of zeolite host grids . 92

4.6.2 Force fields 93

5 Simulation results and comparison with experiment 97

5.1 Simulations of water in zeolites with Cerius2 98

5.1.1 Water adsorption in zeolite NaX . 99

5.1.2 Water adsorption in zeolite ZSM-5 . 100

5.2 Properties of the TSW water model 102

5.2.1 Virial coefficient . 102

5.2.2 Radial distribution function . 104

5.3 From activated carbon to silica surface models 105

5.3.1 Activated carbon model used previously with TSW water 106

5.3.2 Previous results on adsorption properties of activated carbon model 107

5.3.3 Adaptation of slit pore model to silica gel surface structure . 109

5.3.4 Cristobalite model surface 110

5.3.5 Conclusions for modeling silica surfaces with respect to TSW water model 115

5.4 Slit pore model with amorphous silica surface 116

5.5 Adsorption properties of slit pore model . 120

5.5.1 Reference pore and scheme of parametric study . 120

5.5.2 Variation of silanol surface density 123

5.5.3 Finite size effects . 127

5.5.4 Variation of pore width 129

5.6 Conclusions for heat storage and suggested model refinements . 130

6 Summary and Outlook 137

6.1 Summary . 137

6.2 Outlook 142

A Adsorbents 147

A.1 Zeolites 147

A.1.1 Structure . 147

A.1.2 Water adsorption properties . 148

A.2 Silica Gels . 149

A.2.1 Pore structure . 150

A.2.2 Use of silica gels for adsorptive heat transformation . 150

A.2.3 SWS . 151

A.3 Others . 151

A.3.1 MCM-type adsorbents 151

A.3.2 Aluminophosphates . 152

A.3.3 Activated Alumina 152

A.3.4 Modified carbonaceous adsorbents 152

Nomenclature 155

Bibliography 161

Acknowledgements 179