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

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The performance of the condenser is mainly dependent, among other factors, on the temperature difference between the humid air mixture and the condenser surface, and the condensation surface area. The desired condenser design should enhance both above factors in addition to simplicity and economic considerations. As a direct consequence, it is important to note that effective heat recovery in such type of condensers require a larger heat transfer area for improving the overall HDH system performance. For example, Bourouni et al. [128] used 3000 m length of polypropylene tubes in the condenser. In another design Orfi et al. [156] used a seawater cooled condenser that contains two rows of long copper cylinders where longitudinal fins were soldered to their outer surfaces. The feed seawater flows inside the cylinders while the water vapor condenses on the finned surfaces. The condenser has a heat-transfer surface area of 1.5 m 2 and a total length of 28 m of the coil. The multiple paths of the cooling medium inside tubes of small diameters increase both the pressure loss and scale formation, which makes the fouling tendency inherently high. Furthermore, this configuration also has a rather restrictive coupling between the performance of the condenser and the performance of the whole system, since the mass flow rate of feed seawater (i.e. the condenser coolant) should be same as in other system components (i.e. evaporator, and solar collector). For a given solar collector area, increasing the mass flow rate of feed seawater increases the energy efficiency of both the condenser and solar collector, while decreases the evaporator effectiveness due to lowering its inlet hot water temperature. Due to existence of non-condensable gases, the condenser performs better at higher mass flow rates of cooling seawater than the optimum designed flow rate in the evaporator. The overall thermal performance of HDH system is therefore extremely restricted by such a kind of complex trade off. Direct-contact condensers have several advantages that tend to alleviate these problems. This type of condensers makes use of spray columns or packed beds in countercurrent/concurrent flow regime to bring together the cooling medium (e.g. freshwater in HDH cycle) and humid air in direct contact along the bed height. On contrary to non-contact heat exchangers, direct contact condensers exhibit higher intensity of heat transfer without heat resistance through the tube walls and provide a large specific interfacial area with a minimum pressure loss. As a result, direct contact condensation technique is widely used in industrial processes, and in chemical and nuclear applications due to the required low driving potential and high efficiency. Several research studies (136, 137, 147, 148) report that direct contact condensation of humid air is an attractive alternative for small-scale HDH desalination units. Dawoud et al. [148] argued that it is simple, inexpensive, not susceptible to fouling, and has relatively high heat transfer rate and low pressure 47
drop per unit volume. The direct contact spray condenser (DCSC) which is illustrated in figure (2.17) applies a direct heat and mass transfer between the humid air and a cooled (in an external heat exchanger) portion of condensate. Figure 2.17: A schematic direct contact spray condensers [148] Packed columns with liquid desiccant cooling is especially suitable to be driven by low temperature energy source such as solar energy and waste heat, because the energy for air dehumidification and cooling can be stored efficiently in liquid desiccants without considerable losses. Such desiccant cooling dehumidifiers require more heat exchangers and a thermal energy source for desiccant regeneration and fresh water condensate recovery. This displaces the system a way from simplicity and uneconomical for small scale category of desalination units of 1 m 3 / day distillate. In direct contact packed bed condensers, countercurrent operation provides the greatest efficiency because mass transfer driving forces are at a maximum [36]. However, for natural draft operation of HDH units, concurrent operation may be more advantageous as the pressure drop is less compared to countercurrent flow. Moreover, gas and liquid contact intensity is greatly increased at higher flow rates; thus mass transfer rates can be elevated in concurrent flow. Strigle [36] mentioned that, in those systems where there is practically no vapor pressure of a solute above the liquid phase, concurrent flow operation should be considered. 48
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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
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 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 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
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
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A HE QHE U T HE LM (4.81) Where Q
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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
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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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