Experimental and Numerical Analysis of a PCM-Supported ...

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Introduction of the indirect type solar desalination process based on the (HDH) principle led to a major improvement in the efficiency and reduction in the specific collector area. This is due to energy recovery mechanisms, extended evaporation surfaces, and enhanced heat and mass transfer coefficients due to the turbulent flow between air and water. Therefore, combining the principle of humidificationdehumidification with solar desalination appears to be an attractive method of water desalination with solar energy. The HDH technique is especially Figure 1.6: Schematic diagram of a HDH desalination cycle suited for seawater desalination when the demand for water is decentralized. Advantages of this technique include flexibility in capacity, moderate installation and operating costs, simplicity, and possibility of using low temperature energy (solar, geothermal, recovered energy or cogeneration) [2]. 1.5 Aim of the work The major aim of this study is to examine, characterize, and optimize the performance of an innovative “PCM-supported HDH system” utilizing spherical capsules of Phase Change Materials (PCM) as a packing media in the evaporator, and condenser. The system configuration comprises a closed air loop between the evaporator and condenser. The condenser is of direct contact type in a concurrent flow regime between cooling fresh water and humid air. Moreover, a solar thermal power supply, which consists of a solar collector and an external thermal storage is used to drive the HDH plant. The thermal storage is accomplished by storing to PCM with melting point around the required output temperature to guarantee 24 hours of operation. The PCM elements will be integrated in the hot water storage tank in order to reduce its volume, cost, and heat losses. The objective of integration of PCM packing in the HDH process is for improving the thermal performance of the evaporator and condenser and not for thermal energy storage. Under steady state conditions, the main difference between PCM packing and other commercially available bed filling materials such as Raschig Rings, Pall Rings, Berl Saddles or Intalox Saddles, is the thermal conductivity of PCM, but not its thermal storage capacity or solid-liquid phase change. Thus this study focuses on 11
using a conductive filling media to achieve multiple-effects of heating/humidification (MEHH) and cooling/dehumidification (MECD) while air passing through the successive PCM layers in the evaporator and condenser respectively. These phenomena are foreseen to enhance the heat and mass exchange processes and will be described in detail and focused throughout the present analysis. The study has been accomplished through comprehensive experimental and numerical investigations on the PCM-supported HDH system. A mixed micro-macro balance transient simulation model has been established and validated against experimental measurements using COMSOL Multiphysics and MATLAB for solving fluid flow and heat and mass transfer phenomena in one spatial dimension for different components in such a loop. The model is fairly general and simple, yet sufficiently accurate to be applied as a design and optimization tool for HDH plants integrated with different types of packing materials and external thermal storage systems. Using both experimental and simulation tools, a detailed heat and mass transfer analysis for the performance of the evaporator and condenser over a wide range of operation conditions under steady state has been performed and clearly documented using different types of packing materials. The dynamic performance of the PCM-supported HDH system over a wide range of operation conditions will be presented and analyzed on a macro-scale level under varying weather conditions over one year for Al-Arish City on the eastern north coast of Egypt. The analysis will focus on the optimum operation conditions of the system with and without external PCM thermal buffer. Special attention shall be paid to the effect of climatic conditions and comparison between PCM and water as storage media in the external thermal storage. 1.6 Dissertation Overview The dissertation contains 9 main chapters. Chapter 2 presents the state of the art of solar desalination by HDH technique, and thermal energy storage methods and media. In chapter 3 the proposed HDH system is introduced and scope of the present work is outlined. Chapter 4 discusses the theoretical background, underlying physics of the proposed system components, and mathematical modeling development for individual components and for the HDH system as a whole. Chapter 5 discusses the experimental results and analysis of the PCM-supported HDH cycle. The chapter 6 deals with the numerical simulation and validation of single components models as well as the HDH cycle. In Chapter 7 the simulation results and parametric analysis of the main single components (i.e. the evaporator and condenser) are discussed. Chapter 8 focuses on the numerical optimization of the HDHD system as a whole including the solar collector field and the external PCM thermal energy storage. Finally, chapter 9 summarizes and concludes the study activities and proposes future work needed for improving and implementation of the proposed system. 12
Page 1: Technische Universität München In
Page 4 and 5: It is my pleasure to thank Prof. Th
Page 6 and 7: Contents List of figures ..........
Page 8 and 9: 5.3 Functional description ........
Page 10 and 11: List of Figures Figure 1.1: Cost br
Page 12 and 13: 78cm; Top: Evaporator, Bottom: Cond
Page 14 and 15: xii
Page 16 and 17: m Mass flow rate; [kg.s -1 ] m eva
Page 18 and 19: ed c coll cond cw d d e or eff evap
Page 20 and 21: 1 Introduction The first part of th
Page 22 and 23: summarized in figure (1.2). Thermal
Page 24 and 25: 1.3.2 Selection criteria of desalin
Page 26 and 27: of energy storage or hybridization
Page 28 and 29: Desalination development focuses on
Page 32 and 33: 2 Literature Survey 2.1 Introductio
Page 34 and 35: There is actually a commercial prod
Page 36 and 37: Q m H c ( T T1) c ( T 2 T ) (2.2)
Page 38 and 39: (solid-liquid) have a storage densi
Page 40 and 41: (iii) a sharp melting temperature.
Page 42 and 43: melt and freeze without segregation
Page 44 and 45: However, the stored heat is less va
Page 46 and 47: parameters on the heat transfer and
Page 48 and 49: categories. The first category is b
Page 50 and 51: to 550 kWh/m 3 of distilled water a
Page 52 and 53: The enthalpy and humidity of the sa
Page 54 and 55: Air mass flow rate [kg/h] GOR [-] 1
Page 56 and 57: On the other hand, the efficiency o
Page 58 and 59: 2.8.4.2 Effect of air to water mass
Page 60 and 61: distillate rate. The system was not
Page 62 and 63: were; EnviPac Gr.1, 32 mm, and VFF
Page 64 and 65: Figure 2.15: Height versus air temp
Page 66 and 67: The performance of the condenser is
Page 68 and 69: Klausner and Mei [136] described a
Page 70 and 71: The impact of the collector cost ca
Page 72 and 73: exchanger and energy storage in one
Page 74 and 75: 3 Scope of the Study This chapter p
Page 76 and 77: packing where it gets in direct con
Page 78 and 79: In fact there is a tradeoff between
Page 80 and 81:
The energy flow between all and eac
Page 82 and 83:
4 Theoretical Analysis and Modeling
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The two-energy equation models whic
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solid-liquid phase change inside th
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assumed by conduction in both axial
Page 90 and 91:
the air pressure drop ∆P per unit
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evaporation rate per unit volume of
Page 94 and 95:
where β t , β c , ρ ref , c ref
Page 96 and 97:
the viscous transport and introduce
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liquid and solid phases, respective
Page 100 and 101:
4.2.3.3 Liquid and gas hold-ups Liq
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Several models and approaches for m
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where G=ρU d /ε g is the pore mas
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k k eff s 1 3 2 (4.55) This cor
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uncertainties associated with the f
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1 2 (4.62) d1 capp c2 c1 c2 1
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[58] evaluated the cooling tower ai
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1 exp NTU 1 C r, cond cond (4
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A HE QHE U T HE LM (4.81) Where Q
Page 118 and 119:
c M f b (4.86) M b S S f where r
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that form the study logical foundat
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5 Experimental Analysis This chapte
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5.2 Measuring Techniques K-type the
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The product condensed water was mea
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operating limits. In light of the l
Page 130 and 131:
In the last two cases, the inlet ho
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performance and its threshold value
Page 134 and 135:
It could be interpreted that this p
Page 136 and 137:
Evaporator Condenser Figure 5.8: Av
Page 138 and 139:
within the inaccuracy margins. The
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The last two cases of boundary cond
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natural air flow in the system. In
Page 144 and 145:
6 Model Implementation and Validati
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x t u x u 0 0 , (6.5) and for a s
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Next time step validation were the
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The evaporator and condenser both w
Page 152 and 153:
Figure 6.2: Evolution of inlet and
Page 154 and 155:
Figure 6.4: Evolution of inlet and
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temperature is at its maximum level
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Bottom Top Figure 6.7: Evolution of
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7 Parameters Analysis of the Evapor
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Figure 7.1: Effect of packing size
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e 0.45 kPa under natural convection
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aspect ratio under constant bed vol
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Figure 7.7: Effect of inlet hot wat
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well as the time required to reach
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Figure 7.10: Effect of packing medi
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40°C respectively. This gives flex
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the condensate rate, productivity f
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esult, rather than PCM media which
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Figure 7.18: Effect of PCM thermal
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8 Optimization of HDH plant In this
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The MATLAB model describes how the
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less steep than that of the distill
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8.3.4 Effect of hot water flow rate
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storage materials and thus it can h
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for latent heat storage would be ob
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When T m becomes sufficiently highe
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9 Summary and conclusions 9.1 Summa
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productivities at any cooling water
Page 200 and 201:
[13] Vafai K., Sozen M. An investig
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[48] Crone S., Bergins C., and Stra
Page 204 and 205:
[79] Tiwari G.N. and Yadav Y.P. (78
Page 206 and 207:
[109] Mehling H., Hiebler S. and Zi
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[139] Belessiotis V, Delyannis E (2
Page 210 and 211:
Appendices Appendix A A1. PCM Therm
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A3. Constants and Thermo-physical p
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From the above correlation, the con
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The Ergun analogy factor J h repres
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Equation (C.7) represents an overal
Page 220 and 221:
Appendix D D1. Cooling tower theory
Page 222 and 223:
A m w = the plan or cross sectiona
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