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

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Figure 2.15: Height versus air temperature for EnviPac [128] Figure 2.16: Height versus vapor production for EnviPac filling [128] From previous sections it can be realized that the factors which influence the air HDH process are: the flow rate of air, the flow rate of water, the inlet temperature of water to the humidifier, the inlet temperature of air to the humidifier, the inlet temperature of cooling water to the dehumidifier, the flow rate of cooling water and water to air heat capacity flow rate ratio in the system. In the following subsections some of these parameters will be discussed entirely through discussing different 45
processes that take place in the HDH plant in light of previous investigations available in the open literature. 2.8.4.5 Type of carrier gas A nonconventional approach for enhancing heat and mass transfer in both humidifier and condenser by using other carrier gases instead of air was reported by Abu Arabi and Reddy [133]. A lumped thermodynamic model analysis was performed on the effect of using different carrier gases besides air on the performance evaluation of HDH cycle. Hydrogen, helium, neon, nitrogen, oxygen, argon, and carbon dioxide were used. The investigations showed that hydrogen and helium give much better heat flux than air, while carbon dioxide and argon give much better mass flux. They concluded that carbon dioxide is recommended as a carrier gas in desalination units that based on HDH principle taking into account both heat and mass fluxes. Another advantage for using carbon dioxide is that it helps in reducing the calcium scale formation in these systems [133]. 2.8.5 Air dehumidification The condenser device may utilize either a direct contact heat exchanger between fresh cooling water and the humidified air, or condensation on a cooled surface, identical to a shell and tube heat exchanger, flat-plate heat exchangers and finned tube heat exchangers. Non-contact or surface type condensers have been widely used in HDH units for in-line preheating of feed seawater through recovery of latent heat of condensation. In this case, the cooling medium (i.e. feed seawater) flows counter-currently through a bank of tubes where it gets heated up by the condensed vapour upon the outer surface of the tubes. This design integrates both the condenser and pre-heater device in one heat exchanger, as the cooling medium is separated from the condensed vapour. However, due to existence of noncondensable gases, low effectiveness of this type of condensers may be cited as the major potential drawback. Literature on HDH systems show that it is the condenser rather than the evaporator, which limits the productivity of these units, so more attention is needed on the proper design and selection of condenser. It was noticed by Gahin et al. [144] that, in open air / closed water HDH cycles, the water evaporated is almost double the water condensed. This leads to the consideration of the use of more than one effect or increase the heat transfer surface of the condenser. Recirculation of the air after leaving the condenser to the humidifier is another alternative [145]. It was reported by Kheder [146] that an air recycling system has the advantage of more than 20% reduction in packed height of the humidifier and more stable operation of the plant. 46
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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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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 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
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
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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
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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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