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

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In the last two cases, the inlet hot water temperature was decreased to the lower level while maintaining the hot water mass flow rate high. In the fifth case the air flow rate was also maintained high, while in the last case it was reduced to the lower level (the same as the first and third cases but under low inlet hot water temperature). The inlet hot water temperature potentially has a pronounced impact on heat and mas transfer components at all interfaces as well as thermal stratification in the bed. These potential influences of operational and design parameters have been introduced and discussed to a certain depth both on micro and macro scale levels in chapter 3, however it might be helpful for interpreting the experimental results and subsequent discussions to provide a qualitative expectation of the temperature profiles plotting them over the height of the bed. These qualitative profiles would make it possible to predict the transported mechanisms and their feedback influences on distillation rates. Given axial stratification along the packed height, the solid temperature at the top layers of the bed is higher than that of the gas phase, while the reverse may hold true at the bottom layers when the heat capacity flow of hot water is low. Thus, the expected behavior of the system in this case is that there is heat flow from gas to solid in the lower part of the column (i.e. cooling effect) and from solid to gas in the upper part. This implies that there should be a certain height of the bed at which the heat flow between gas and solid is zero (balancing point), as depicted in figure (5.4). Thus, if the system design is to be optimized so that at every specific location along the packing height there would be always heat flow from solid beads to the gas mixture in the evaporator to avoid cooling effects (and to the cooling water in the condenser), this will result in a more profound multi stage heating/humidification in the evaporator or cooling/dehumidification in the condenser. These idealized temperature profiles are illustrated qualitatively in figure (5.5) for the evaporator and condenser respectively. Under such circumstances, the conductive packing would have more profound impact on the distillate yield. 5.5 Results and discussions Since the size and thermal storage capacity of PCM packing in the evaporator and condenser are relatively small when compared to those of the used hot water storage tank, unexpected effects when comparing PCM with Non-PCM packing during steady state operation conditions led to a new focus of the study. The experimental measurements revealed a productivity enhancement of PCM packing. The interesting phenomena of locally established multiple-effects of heating/humidification and cooling/dehumidification in the evaporator and condenser respectively, as discussed in detail in chapter 3, seemed to play the main role behind these effects. Hence steady state operation became more crucial and interesting than the transient conditions, and therefore will be focused throughout the present analysis. 111
Figure 5.4: Qualitative illustration of temperature profiles in the evaporator Figure 5.5: Qualitative illustration of ideal temperature profiles, Left: evaporator, Right: condenser The main indicator to be considered throughout the evaluation and comparison between various operation scenarios and packing types is the productivity. Since the present system is a prototype unit developed to examine the comparative performance and reliability of the PCM based system, there is no energy recovery mechanism implemented in the setup. Thus the gained output ratio (GOR) in this case has no physical significance, as it is rather a measure for the plant thermal 112
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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
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1.3.2 Selection criteria of desalin
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of energy storage or hybridization
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Desalination development focuses on
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Introduction of the indirect type s
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2 Literature Survey 2.1 Introductio
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There is actually a commercial prod
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Q m H c ( T T1) c ( T 2 T ) (2.2)
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(solid-liquid) have a storage densi
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(iii) a sharp melting temperature.
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melt and freeze without segregation
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However, the stored heat is less va
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parameters on the heat transfer and
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categories. The first category is b
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to 550 kWh/m 3 of distilled water a
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The enthalpy and humidity of the sa
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Air mass flow rate [kg/h] GOR [-] 1
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On the other hand, the efficiency o
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2.8.4.2 Effect of air to water mass
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distillate rate. The system was not
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were; EnviPac Gr.1, 32 mm, and VFF
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Figure 2.15: Height versus air temp
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The performance of the condenser is
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Klausner and Mei [136] described a
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The impact of the collector cost ca
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exchanger and energy storage in one
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3 Scope of the Study This chapter p
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packing where it gets in direct con
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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
Page 124 and 125: 5.2 Measuring Techniques K-type the
Page 126 and 127: The product condensed water was mea
Page 128 and 129: operating limits. In light of the l
Page 132 and 133: 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
Page 140 and 141: The last two cases of boundary cond
Page 142 and 143: natural air flow in the system. In
Page 144 and 145: 6 Model Implementation and Validati
Page 146 and 147: x t u x u 0 0 , (6.5) and for a s
Page 148 and 149: Next time step validation were the
Page 150 and 151: 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
Page 156 and 157: temperature is at its maximum level
Page 158 and 159: Bottom Top Figure 6.7: Evolution of
Page 160 and 161: 7 Parameters Analysis of the Evapor
Page 162 and 163: Figure 7.1: Effect of packing size
Page 164 and 165: e 0.45 kPa under natural convection
Page 166 and 167: aspect ratio under constant bed vol
Page 168 and 169: Figure 7.7: Effect of inlet hot wat
Page 170 and 171: well as the time required to reach
Page 172 and 173: Figure 7.10: Effect of packing medi
Page 174 and 175: 40°C respectively. This gives flex
Page 176 and 177: the condensate rate, productivity f
Page 178 and 179: 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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