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

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The MATLAB model describes how the solar power components are combined with the desalination block in one structure to simulate the complete plant based on the coupling strategy shown in figure 4.1. The above description assumes no heat losses during fluid flow through the HDH unit. The solar collector efficiency is modelled through introduction of an empirical correlation in equation (4.84). As mentioned previously, this correlation summarily includes the heat losses through the connecting pipes with the HDH plant and the flow mal-distribution in the collector absorber. Hence rather conservative amounts of fresh water production can be expected. Control criteria and mechanisms were implemented in the simulation model to guarantee positive entropy production in the system. For instance, the solar collector should be bypassed during night or cloudy hours to avoid heat losses to ambient during these times, the outlet temperature from the solar collector must not exceed 95ºC to avoid boiling of seawater and scale formation, and the inlet hot water temperature to the evaporator must not fall below the inlet cooling water temperature to the condenser. These two temperatures represent the margin for the time variation of temperature fields in the system. The inlet cooling water temperature to the condenser was assumed constant at ambient. Both the evaporator and condenser packed beds have diameter 0.8m and height 1.6m. The spherical PCM packing size was 40mm for both the evaporator and condenser, and 75mm for the thermal buffer. The baseline boundary conditions are listed in table 8.1 and partly on each figure. Table A.1 in the appendix presents the thermophysical properties of different types of PCM used for the present computer simulation corresponding to the evaporator, condenser, and thermal buffer. Table 8.1: Specified baseline boundary and operating conditions Parameter Units Value Mass flow of hot water “M hw ” kg/h 1000 Solar collector area “A coll ” m 2 220 Temperature of cooling and seawater water “T cw ” ºC T amb Mass flow of cold water “M cw ” kg/h 2600 Brine concentration factor “r c ” - 2 Mass flow rate of air “M a ” kg/h 1300 Geographical location - Al-Arish Various parameters have been varied to see their impact on the system performance. System performance is evaluated in terms of the GOR (see equations 2.4 and 4.84 to 4.89) and output distillate rate (ODR) as a function of cooling water to air flow rate ratio, hot water to air flow rate ratio, size of the solar collector field, 165
size of the thermal buffer, melting temperature of PCM in the thermal buffer, and brine concentration factor. The computation time was setup for the whole year with a time step 120 seconds using interpolation techniques to smoothly develop a time evolution of the average hourly input weather conditions over the entire time steps between the successive hours. The effects of transient weather conditions and climate on the system performance have been examined comparing between the two different locations in Egypt; Al- Arish, and Al-Kharga. 8.3.1 Effect of brine concentration factor (r c ) Figure 8.3 shows the variations of distillation rate and GOR under same operation conditions with the brine concentration factor for a solar collector area of 220 m 2 . The results reveal that the GOR increases as the brine concentration factor increases due to better heat recovery while the rate of distillate decreases at the same time. In closed water loop HDH processes, mixing of feed seawater and recirculated brine causes the degradation of the solar collector’s thermal efficiency due to increasing the inlet brine temperature to the collector, and hence the inlet heat capacity flow rate to the HDH system decreases. The distillate rate peaks at a value of r c =2 while the GOR peaks at a value of r c =3 beyond which the GOR saturates while the distillate rate continues to drop. Thus operating the plant at a value of r c >3 is not recommended under these circumstances. Figure 8.3: Effect of brine concentration ratio on the average-yearly distillate rate and GOR Further, scale formation is an important limiting factor to be considered when talking about brine circulation. The tendency of scale formation increases with increasing the recirculated fraction due to increasing the brine salinity. The threshold limit for this factor decreases in inverse proportion to the top brine temperature which can be reached in the plant. For instance, the threshold value for r c has been set at 1.5 for Multi-Stage Flash desalination plants working at a top brine temperature around 110ºC to avoid serious scale formation problems. Since the top brine temperature level in HDH systems is designed at a maximum value of 85ºC, a higher threshold limit of r c up to 3 probably can be tolerated. However, since the variation of GOR is 166
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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
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1 exp NTU 1 C r, cond cond (4
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A HE QHE U T HE LM (4.81) Where Q
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c M f b (4.86) M b S S f where r
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that form the study logical foundat
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5 Experimental Analysis This chapte
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5.2 Measuring Techniques K-type the
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The product condensed water was mea
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operating limits. In light of the l
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In the last two cases, the inlet ho
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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
Page 180 and 181: Figure 7.18: Effect of PCM thermal
Page 182 and 183: 8 Optimization of HDH plant In this
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
Page 214 and 215: From the above correlation, the con
Page 216 and 217: The Ergun analogy factor J h repres
Page 218 and 219: Equation (C.7) represents an overal
Page 220 and 221: Appendix D D1. Cooling tower theory
Page 222 and 223: A m w = the plan or cross sectiona
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