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

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performance and its threshold value shouldn’t exceed unity. Realistic units would include an energy recovery system for lower energy consumption, as was described in the previous chapters, and will be investigated numerically in chapter 8 when considering the complete solar driven HDH plant. 5.5.1 Behavior of the PCM-supported HDH cycle Figure (5.6) shows the evolution of inlet water temperature, the outlet water and gas temperatures as well as PCM temperatures at the top, middle, and bottom layers for the evaporator and condenser as one example of the PCM system thermal behavior. As can be seen from the figure, the outlet gas, water, and PCM temperatures are increasing with time until they all approach each other at the point of steady state or thermal equilibrium after nearly two hours. Along its pass through the evaporator, hot water transfers part of its energy with the solid phase and other part with the gas mixture. Energy transport to the gas mixture includes sensible and latent heat (evaporative) components which is strongly dependent on the liquid-gas temperature difference. Energy transport to the solid phase includes two sensible heat (convection) components at the liquid-solid and gas-solid interfaces, which depend on the binary temperature differences as described in chapter 4. At the beginning of plant operation, the temperature of PCM beads is the same as ambient temperature and the differences between water temperature and both solid and gas temperatures are at its maximum values. In this early stage, one would expect a heat transfer in the direction from gas to solid phase, since the gas temperature increases very quickly compared to the solid phase due to the latent heat of evaporation. Marching in time at every specific location along the packed height, the temperature of PCM increases while the liquid-solid and gas-solid temperature differences decrease until they reach a certain limit at which the energy flows into and out of the solid phase at liquid and gas interfaces respectively are balanced. Having reached thermal balance at one point, it will move forward to the next point along the packing height from top to the bottom until full steady state conditions are reached. The same scenario takes place in the condenser with the gas phase as the heat source. It can be clearly seen that the outlet gas temperature from the evaporator is very close to the PCM top layer temperature. The outlet gas temperature from the condenser (and inlet to the evaporator), outlet water temperature from the evaporator, bottom PCM layer temperature in the condenser and bottom PCM layer temperature in the evaporator are very close to each other. Thermal stratification in the evaporator is higher than that of the condenser because of the lower heat source (gas) temperature in the condenser in comparison with the heat source (hot water) temperature in the evaporator and due to the concurrent flow in the condenser. In the concurrent flow in the condenser, it is anticipated that most 113
of the condensation takes place in the first top layers of the bed where the cooling water temperature is at its lowest level while the gas temperature is at its maximum level. Hence, the water temperature increases downwards due to condensation and cooling of humid air. This causes the temperature of the middle PCM layer to be higher than that at the top layer, as depicted in figure (5.6b), due to absorption of a large portion of the latent heat of condensation by the middle PCM layer. The middle PCM layer reaches steady state conditions earlier than PCM layers at top and bottom of the condenser, since the middle layer acts as a thermal buffer before the bottom layer. While the inlet and outlet gas temperatures approaching steady state conditions, the inlet cooling water temperature was gradually increasing over time during this experiment, which causes the retardation of the top PCM layer to reach steady state. However, it can be seen in figure (5.6) that the intermediate PCM layer temperature lies between both inlet and outlet gas temperatures in both components under the specified boundary conditions. Figure (5.7) shows the evolution of water and gas temperatures at top and bottom of the empty spheres packing unit. It can be seen that the Non-PCM system achieves steady state sooner than the PCM based one. This is self-evident due to the fact that the hollow plastic spheres have a negligible thermal storage capacity compared to PCM spheres. The plots on figure (5.8) and table (5.3) illustrate that the differences between inlet and outlet gas and inlet and outlet liquid temperatures in the evaporator and condenser of the PCM packing are higher than those of the Empty spheres packing. Table 5.3: Average values for inlet and outlet liquid and gas temperatures and productivities during steady state operation conditions (refer to figure 4.1) Productivity T w2 T w1 T w3 T w4 T a1 T a2 ΔT hw ΔT a ΔT cw (l/h) PCM 83.2 54.5 26.6 51.9 50.1 66.7 28.7 16.6 25.3 23.5 ES 82.9 57.8 21.1 47 49.8 65.3 25 15 26 20.5 Although the boundary conditions were typically the same on the evaporator side, the outlet water temperature for the PCM packing was about 3 ºC lower than that of the empty spheres. On the condenser side the inlet cooling water temperature of the PCM system was higher than that of the empty spheres, nevertheless the PCM productivity was 14.6% higher than that of the empty spheres. The plots on figure (5.9) illustrate the improvement of productivity and GOR for PCM system in this experiment under steady state conditions due to increased temperature difference between top and bottom gas states. Moreover, the performance analysis indicates that the effectiveness of PCM evaporator and condenser has increased by 3 and 6% respectively compared with the empty spheres packing. 114
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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
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 130 and 131: In the last two cases, the inlet ho
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
Page 180 and 181: 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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