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

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7 Parameters Analysis of the Evaporator and Condenser This chapter presents the results of the numerical study conducted by using the MATLAB models that quantifies the potential for heat and mass transfer efficiency and optimization parameters for the evaporator and condenser. The aim of this work is to characterize the optimum design of the evaporator and direct contact condenser packed with spherical phase change material (PCM) elements as a conductive media in the humidification-dehumidification (HDH) system. The external thermal buffer will be optimized as an integral part of the HDH plant under real weather conditions and will be presented separately in the next chapter. The stimulus for this analysis derives from recognition that any improvement in the performance of individual components and subsystems might lead to better ways of increasing the fresh water productivity of energy use. The investigated parameters are: Inlet hot and cooling water temperatures and flow rates, Packing thermal conductivity, Packing size and column aspect ratio, Water to air mass flow rate ratio, and Inlet air temperature to each component Here the evaporator and condenser packed beds, unless stated, have the dimensions of 0.6m diameter and 1m packing height, where they are filled with 40mm spherical PCM elements. The reference boundary conditions used for the present computer simulation are listed in table 7.1 and partly on each figure. Table A.1 in the appendix presents the thermophysical properties of PCM candidates in the evaporator and condenser. The simulation time was extended over 4 hours to guarantee the system reaches steady state. The results of the only last hour were considered for evaluation of all systems under investigation to refer to the analysis when the system works at steady state regime. 7.1 Boundary conditions The evaporator and condenser performance depends on many parameters, which have mutual influences and combined effects. In order to quantify the effects of several parameters, parametric studies have been carried out numerically with the aim of maximizing the evaporation and condensation rates. Hereby various parameters have been varied to examine their impact on the system performance, mainly the output distillate rate. 141
Table 7.1: Reference boundary conditions for the evaporator and condenser Parameter Units Value Mass flow of hot water “M hw ” kg/h 1000 Mass flow of cold water “M cw ” kg/h 1000 Velocity of air “v a ” m/s 0.55 Temperature of hot seawater water “T w2 ” ºC 83 Temperature of cooling water “T w3 ” ºC 25 Temperature of inlet air to the evaporator “T a1 ” ºC 25 Temperature of inlet air to the condenser “T a2 ” ºC 66 7.2 Evaporator Performance Fundamental variables and critical parameters mentioned above and the effect of using different types of packing media will be discussed and analysed. For each case, only the value of each targeted parameter was varied, while other parameters were kept constant at the specified value. 7.2.1 Effect of packing size It is known that there is an inverse relationship between packing diameter and specific surface area (i.e. packing area per unit volume of the bed; m 2 /m 3 ), as depicted in figure (7.1). For a specific column diameter, all heat and mass transfer coefficients depend on the specific area of the packing (a), as well as the packing diameter itself, according to equations (4.43) and (4.48) to (4.50). As the flow rate and diameter of spherical particles are increased under a given diameter of the packed column, the mean heat and mass transfer coefficients increase, while the interfacial areas of contact decrease, such that it becomes difficult to optimize the bed characteristics. In conventional packed beds, a is inversely related to both the porosity (ε) and particle size through equation (4.48), while the porosity in turn is directly related to the particle diameter as suggested by equation (4.43). Combination of all of these considerations with energy and mass balance equations (4.14) to (4.17) shows the opposing effects that the particle size and specific surface area have on the thermal performance of the system. The effect of packing diameter on the evaporation rate is demonstrated in figure (7.2), which clearly explains why heat and mass transfer rate densities do not increase substantially in consistence with figure (7.1) as the dimension of the sphere diameter (d) becomes smaller especially under lower mass flow rates of hot water. The productivity decreases on increasing the packing diameter, but this effect becomes more significant at higher mass flow rates. However, it is excepted that smaller packing size leads to a higher heat capacity flow between hot water and air, but it seems that its effect is not the predominating one. 142
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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
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 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 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
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
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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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