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

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The evaporator and condenser both were packed with the respective PCM spheres HS58 and HS34 as mentioned earlier. Similar tests were performed with empty spheres packing of the same size (air capsules). Table 6.2 presents the specific boundary and operating conditions for these four test cases. The measured initial temperatures, inlet cooling water, and hot air temperatures as a function of time were used as input data for the simulation model. Table 6.2: Specified boundary and operating conditions for model validation Parameter Units Value Mass flow of hot water “M hw ” l/h 500 Corresponding hot water mass flux “L hw ” kg /(m 2 .s) 1.1 Temperature of inflow hot water “T w2 ” ºC 83 Mass flow of cold water “M cw ” l/h 670 Corresponding cold water mass flux “L cw ” kg /(m 2 .s) 1.48 Air superficial velocity “v asup ” m/s 0.55 Corresponding air mass flux “G” kg /m 2 .s 0.59 Figures (6.2), (6.3) and (6.4) show the evolution of inlet and outlet temperatures of each phase in comparison with experimental measurements for PCM packing and empty spheres packing respectively at a packing height of 78cm. From figures (6.2), (6.3) and (6.4), it is observed that the results obtained from the present model are in fairly good agreement with the measured data. However, the simulation results for the PCM temperature profiles deviate relatively from the results obtained by experiments due to segregation problems occurred in the PCM packing in a number of spheres during experiments. This certainly influences the real thermophysical properties of PCM candidates while the simulation model makes use of the ideal prescribed properties set by the PCM supplier. The characteristic knee for phase change appears clearly during the phase change process in the simulation results in figure (6.3) but not at all in the experiments for both the evaporator and condenser. Consequently, the outlet gas and liquid temperatures in experiments approach the steady state more quickly than the simulation, which could be due to the considering the phase change heat capacity of PCM packing in the simulation model, while there is no phase change in the experiment due to segregation, which happened randomly in sporadic packing elements in most of layers. During transient periods, the temperature profile for the solid phase, and consequently temperature profiles for liquid and gas phase, in the simulation is different from that in experiments due to segregation, as can be clearly seen in the first hour in figure (6.3). Once the system reaches steady state, the deviation between experimental measurements and numerical predictions for the outlet liquid and gas temperatures 131
decreases. The percentage deviation for the evaporator and condenser lies within ±5% at low and high operating temperatures. However, it can be observed that all the temperature fields for fluid phases follow the PCM temperature trends, as they remain constant during the isothermal phase change processes. Up to this point, the PCM role as a temporary heat storage and heat exchanger become obvious both experimentally and theoretically regardless whether it has negative or positive effects under the present boundary and geometrical conditions and thermophysical properties of designated PCM beads. Figure (6.4) shows that the outlet liquid and gas temperatures of simulations are in better agreement with those of experiments for empty spheres than with PCM spheres. The maximum temperature deviation between simulation and experimental results for empty spheres packing is less than 3K at steady state for the outlet liquid temperature. Compared with the empty spheres packing, the system of PCM packing took longer time to reach steady sate due to the huge heat capacities of PCM. However, this huge heat capacity should have an impact only during the transient period or startup of the plant. Therefore, the deviation between measured and simulated temperatures decreases when the system approaches steady state conditions. Table 6.3: Measured and simulated total productivities [l] High packing, H=0.78m Low packing, H=0.39m PCM Empty spheres PCM Empty spheres Experiments 90.5 79.0 72.0 67.5 Simulations 90.9 79.4 73.7 67.6 The accumulated productivities over time and final total productivities for each case including the start-up period are shown in Figure (6.6) and Table (6.3), respectively. The accumulated productivities are linearly related with the operation time both for simulations and experiments, which means the fresh water production rates are almost constant for each case. The total fresh water productivities for simulations are in good agreement with experiment for all cases. From the present results, the positive impact of PCM beads on the productivity can be seen clearly especially for the higher packing height. The PCM packing increases the productivities by 6.7% and 13.3% for lower (39cm) and higher (78cm) packing heights respectively in comparison with empty spheres packing. Moreover, when the packing height is doubled under same boundary conditions for each packing type, the productivity increases by 25.7% and 17.0% for PCM packing and empty spheres packing respectively. 132
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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
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The energy flow between all and eac
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4 Theoretical Analysis and Modeling
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The two-energy equation models whic
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solid-liquid phase change inside th
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assumed by conduction in both axial
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the air pressure drop ∆P per unit
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evaporation rate per unit volume of
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where β t , β c , ρ ref , c ref
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the viscous transport and introduce
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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 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 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
Page 180 and 181: Figure 7.18: Effect of PCM thermal
Page 182 and 183: 8 Optimization of HDH plant In this
Page 184 and 185: The MATLAB model describes how the
Page 186 and 187: less steep than that of the distill
Page 188 and 189: 8.3.4 Effect of hot water flow rate
Page 190 and 191: storage materials and thus it can h
Page 192 and 193: for latent heat storage would be ob
Page 194 and 195: When T m becomes sufficiently highe
Page 196 and 197: 9 Summary and conclusions 9.1 Summa
Page 198 and 199: 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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