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

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operating limits. In light of the literature survey in chapter 2, the varying parameters include mainly inlet hot water temperature to the evaporator, mass flow rates of hot water and air, and packing height. Moreover, the experiments are designed to examine both natural convection conditions and using Non-PCM (Empty plastic spheres having the same size as PCM spheres, and Hiflow industrial packing elements) elements for comparative tests. In the very early stage of the experiments, the effect of inlet cooling water mass flow rate to the condenser on plant productivity was examined under six types of boundary conditions using PCM packing without distillate cooler. The results showed that this influence is very small within the planned range of mass flow rates. Thus, the inlet cooling water mass flow rate and temperature were constantly maintained at a higher level of 670 l/h, and around 21°C respectively to exclude the effect of cooling water conditions on plant productivity. However, the effect of inlet cooling water mass flow rate and temperature as crucial parameters under real operation conditions of the HDH system will be studied numerically in the next chapters. Based on the previous calibration data, the water pumps and ventilator are accurately adjusted at the predefined flow rates to guarantee fixed boundary conditions for similar tests. Moreover, eight repeatability-prove tests were conducted on both PCM and Non-PCM packing types and found repeatable within less than 5%. The subsequent sections compare and discuss the results obtained by employing different packing types to show the influence of operation parameters on the plant performance. First of all, the thermal behavior of PCM evaporator and condenser will be discussed in comparison with the empty spheres packing through one sample of experimentation runs. The six cases or combinations of boundary conditions which are indicated in table (5.2) were selected from the experimental plan in table (A.2) in the Appendix (A). The selected experiments were conducted on both PCM and empty spheres at two levels of packing height for studying the effect of PCM compared to Non-PCM packing. Additionally, the same set of experiments were conducted on industrial packing elements called “Hiflow packing” at lower packing height under forced and natural convection. In the first and fourth case of boundary conditions in table (5.1), both of them have a higher inlet water temperature (83 ºC). The first case has high inlet mass flow rate of water (500 l/h) to the evaporator, and high air velocity (0.55 m/s), while the fourth case has the lower levels of both inlet water mass flow (250 l/h) and air velocity (0.23 m/s) in the system. This means that the first case has the highest heat capacity flow rates, while the fourth case has the lowest for both water and air in the system, but air to water mass flow ratio is nearly the same. 109
In the second case, water heat capacity flow is decreased due to lower inlet hot water mass flow rate, while the air flow rate is maintained high. When Air flow rate has two impacts on the system performance, where the energy transfer rate through dry batches between solid and air (i.e. MEHH) increases with increasing air flow rate, both sensible and latent heat transfer at the air-liquid interface increases until reaching an optimum value of air/water mass flow ratio. Beyond this point the high sensible heat transfer rate negatively influences (i.e. comes at the expense) of the mass transfer rate at the interface. Moreover, most of the heat and mass transfer take place at the upper region of the evaporator, which causes the solid packing in the lower part of the column to be colder than the gas phase. Table 5.1: Chosen variable parameters for experiments and range of values Parameter Units Low level High level Mass flow of hot water “m hw ” l/h 250 500 Corresponding water mass flux “L” kg /m 2 .s 0.55 1.1 Temperature of inflow hot water “T w2 ” ºC 65 83 Mass flow of cold water “m cw ” l/h 370 670 Air velocity “v a ” m/s 0.23 0.55 Corresponding air mass flux “G” kg /m 2 .s 0.25 0.59 Packing height “Z” m 0.38 0.78 Table 5.2: Basic sets of designed experiments according to boundary conditions Case No. Parameter 1 2 3 4 5 6 Air velocity “v a ” (m/s) 0,55 0,55 0,23 0,23 0,55 0,23 Mass flow of hot water “m hw ” 500 250 500 250 500 500 Temperature of inlet hot water “T w2 ” (ºC) 83 83 83 83 65 65 Mass flow of cold water “m cw ” (l/h) 670 670 670 670 670 670 When lowering the water mass flow rate the gas-solid interfacial area increases, while the wetted area (between water and solid) decreases. At constant inlet hot water temperature and constant air flow rate, this has two opposing effects; as it leads to higher rate of energy retrieval from solid to gas, but decreasing the water heat capacity flow rate decreases all sensible and latent heat transfer components in the system as well as thermal stratification along the packing height. In the third case of boundary conditions, water heat capacity flow rate is maintained at the highest level, while air velocity is low which lowers all sensible and latent heat components between the gas phase and both solid and liquid phases. 110
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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
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
Page 124 and 125: 5.2 Measuring Techniques K-type the
Page 126 and 127: The product condensed water was mea
Page 130 and 131: In the last two cases, the inlet ho
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
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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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