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

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3 Scope of the Study This chapter presents the features of the proposed PCM-Supported HDH system under focus of the present study. The predicted phenomena of multi-effect of heating/humidification (MEHH) in the evaporator and multi-effect of cooling/dehumidification (MECD) in the condenser are illustrated. The scope, objectives of the study, and the adopted methodology will be presented. 3.1 Scope of the study As reported in the literature, thermal energy storage plays an important role in securing a high thermal performance of solar driven desalination units. A new approach in this context is the use of encapsulated phase change material (PCM) as packing media both in the evaporator and condenser while consequently applying free or forced convection. In the present study, an HDH system with fully integrated packed bed phase change regenerators in both evaporator and condenser has been proposed. Moreover, a solar thermal power supply 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 storage tank volume, cost, and heat losses. The main objective of using PCM elements in the evaporator and condenser was for heat storage as a backup during transient solar irradiation behavior for part-time night operation and cloudy hours. During analysis of steady state conditions, it was discovered that multiple-effects of heating/humidification (MEHH) and cooling/ dehumidification (MECD) while air passing through the successive PCM layers in the evaporator and condenser respectively seem to play an important role in system efficiency. The multiple-effects phenomena are attributed to existence of conductive packing media which act as heat and mass exchangers in the two columns. Thus, the focus of the study lies on the thermal conductivity rather than the thermal capacity or solid-liquid phase change processes of the packing. However, these interesting phenomena will be discussed in detail in the next sections and throughout the experimental and numerical analysis in the next chapters. 3.2 Proposed system and operation cycle The operation cycle and flow diagram of the proposed system and its processes is illustrated schematically in figure (3.1). The plant configuration comprises three closed loops; the air loop, the hot water loop, and the cooling water loop. Hot water is sprayed into the top of the evaporation tower to form a thin liquid film over the 55
spherical packing while in contact with low humidity counter current air stream. Partial evaporation and convective heat transfer cools the brine down, which leaves the evaporation unit concentrated at a lower temperature. Part of the outlet warm brine is blown down to keep the salinity concentration within a certain limit, while the other part is re-circulated back and mixed with the seawater makeup, and hence the cycle is repeated. The brine and seawater makeup is heated up through recovery of latent heat of condensation in a heat exchanger downstream of the condenser. The brine is then heated up further through a solar collector during the day time or bypassed to the external thermal buffer to avoid heat losses through the collector during the night time. The solar collector should be over designed to store the excessive energy during day time in the external thermal buffer to be utilized for night operation. Figure 3.1: Schematic layout of the PCM-Supported HDH desalination unit (AquaTube) The humidified air is guided by forced convection into the air cooler (dehumidifier) where the sub-cooled water is gently sprayed in tiny droplets at the top of the condenser and trickles downward by gravity concurrently with the hot humidified air. The liquid droplets eventually form a liquid film on the surface of the spherical 56
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Technische Universität München In
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It is my pleasure to thank Prof. Th
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Contents List of figures ..........
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5.3 Functional description ........
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List of Figures Figure 1.1: Cost br
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78cm; Top: Evaporator, Bottom: Cond
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xii
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m Mass flow rate; [kg.s -1 ] m eva
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ed c coll cond cw d d e or eff evap
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1 Introduction The first part of th
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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 30 and 31: Introduction of the indirect type s
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 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
Page 84 and 85: The two-energy equation models whic
Page 86 and 87: solid-liquid phase change inside th
Page 88 and 89: assumed by conduction in both axial
Page 90 and 91: the air pressure drop ∆P per unit
Page 92 and 93: 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
Page 98 and 99: liquid and solid phases, respective
Page 100 and 101: 4.2.3.3 Liquid and gas hold-ups Liq
Page 102 and 103: Several models and approaches for m
Page 104 and 105: where G=ρU d /ε g is the pore mas
Page 106 and 107: k k eff s 1 3 2 (4.55) This cor
Page 108 and 109: uncertainties associated with the f
Page 110 and 111: 1 2 (4.62) d1 capp c2 c1 c2 1
Page 112 and 113: [58] evaluated the cooling tower ai
Page 114 and 115: 1 exp NTU 1 C r, cond cond (4
Page 116 and 117: 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
Page 120 and 121: that form the study logical foundat
Page 122 and 123: 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
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In the last two cases, the inlet ho
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performance and its threshold value
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It could be interpreted that this p
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Evaporator Condenser Figure 5.8: Av
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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
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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
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Figure 6.2: Evolution of inlet and
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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
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[13] Vafai K., Sozen M. An investig
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[48] Crone S., Bergins C., and Stra
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[79] Tiwari G.N. and Yadav Y.P. (78
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[109] Mehling H., Hiebler S. and Zi
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[139] Belessiotis V, Delyannis E (2
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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
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Appendix D D1. Cooling tower theory
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A m w = the plan or cross sectiona
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