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

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e 0.45 kPa under natural convection and 2 kPa under forced convection as pointed out above. These values correspond to an electric power consumption of 0.15 kWh per m 3 of fresh water production under natural convection (virtual energy saving due to overcoming the pressure drop by natural convection), while it is around 0.4 kWh/m 3 real consumption for the forced convection. As a result, the influence of gas-mixture pressure drop on the energy input to the HDH system as well as its performance can be safely neglected in comparison with the thermal energy input requirements when the packing size is equal to or greater than 40mm. However, for the mass flow rates considered in this study, on comparing figures (7.2) and (7.3) and considering the cost and available sizes of PCM packing in the market, the packing diameter of 40mm can be considered as the optimum size and, therefore, will be fixed throughout the parametric analysis. 7.2.2 Effect of column aspect ratio The optimum void fraction is not only strongly dependent on the ratio between packing and column diameters (i.e. equation 4.43) but also on the ratio between column diameter (D bed ) and height (H), which is called the column aspect ratio due to the MEHH phenomenon. For a given packing diameter, the packed height and column diameter can combine differently to yield the same overall volume. The general design strategy is to minimize the fluid flow path length in contact with the porous medium to minimize the friction losses in the bed while simultaneously maximizing the distillation rate due to the MEHH and avoiding cooling effects which may result in the lower part of the bed as mentioned earlier in chapter 5. In order to study the impact of the column geometrical aspect ratio on fresh water productivity, first the packing volume was kept constant at V bed =0.3m 3 (which was close to the packing volume of the prescribed column dimensions 0.6m diameter and 1m packing height) and the packing height was kept constant. The column diameter was varied in both cases. Figures (7.4) and (7.5) show the effect of column geometrical aspect ratio on the hourly distillation under different hot water mass flow rates for both cases. At a lower column aspect ratio, the diameter of the tank gets smaller and hence the cross sectional area is reduced, which increases the superficial flow rate and as a result the heat and mass transfer coefficients increase. Thus, for both cases the effect of the aspect ratio parameter becomes more sensible under higher mass flow rates of hot water. In the first case at constant packing volume, as the aspect ratio increases the packing height decreases, which reduces the favourable thermal stratification in the bed and hence the evaporation rate decreases. For constant packing height, in the second case, the results reveal that increasing the aspect ratio improves the performance of the system, until a saturation point is reached. The general tendency is that an enlargement of the column diameter increases the packed volume and the 145
interfacial areas active in heat and mass transfer to the air which enhances the evaporation rate. Figure 7.4: Effect of column aspect ratio on the evaporation rate, V bed =const. However, when the maximum saturation potential of the outlet air is reached, additional enlargement of the column diameter has no positive effect any more. This means that there is an optimum diameter of the column for a certain water mass flow rate. Increasing the column diameter beyond this optimum value leads to a decrease in the thermal stratification which decreases the distillation rate. It is apparent from figure (7.5) that the optimum performance can be obtained at an aspect ratio between 0.8 for low mass flow rates and 1 for higher mass flow rates. Nevertheless, the effect of aspect ratio was further examined at a higher packing height of 2m as shown in figure (7.6). This plot obviously shows that the aspect ratio is not the sole determinant factor in column sizing but also the packing height by itself plays an important role in the evaporator performance due to the MEHH phenomenon. Moreover, the packing height has a significant influence on the optimum column aspect ratio as well. In this case, as the packing height is increased, the thermal stratification in the bed increases and the dependency of the optimum aspect ratio becomes more strongly tied to the hot water mass flow rate. Two distinct groups of mass flow rates can be classified as; below and above 1000 [l/h]. Below a mass flow rate of 1000 [l/h], the trend is fairly similar to effect of column 146
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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
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4.2.3.3 Liquid and gas hold-ups Liq
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Several models and approaches for m
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where G=ρU d /ε g is the pore mas
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k k eff s 1 3 2 (4.55) This cor
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uncertainties associated with the f
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1 2 (4.62) d1 capp c2 c1 c2 1
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[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 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 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
Page 200 and 201: [13] Vafai K., Sozen M. An investig
Page 202 and 203: [48] Crone S., Bergins C., and Stra
Page 204 and 205: [79] Tiwari G.N. and Yadav Y.P. (78
Page 206 and 207: [109] Mehling H., Hiebler S. and Zi
Page 208 and 209: [139] Belessiotis V, Delyannis E (2
Page 210 and 211: Appendices Appendix A A1. PCM Therm
Page 212 and 213: 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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