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

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natural air flow in the system. In the fifth case under natural air mass flow, increasing the specific surface area as of the Hiflow packing compensates and outpaces the role of MEHH, due to low mass flow rate of hot water, resulting in a higher production of 25% more than that of the PCM packing. On comparison with empty spheres, it is evident from this case, where air velocity is very low estimated at 0.1 m/s, that MEHH are nearly absent at very low hot water mass flow rates and low specific packing area. 5.6 Conclusions The performance of air humidification-dehumidification system utilizing phase change materials (PCM) as packing media both in the evaporator and condenser under forced and free convection has been studied experimentally. Performance of the PCM based system was evaluated in comparison with Non-PCM elements including empty plastic spheres having the same size as PCM spheres, and Hiflow Rings which is a commercial industrial packing. The effect of various parameters on the plant performance and productivity has been investigated. From the previous analysis it is clear that there is a strong and complex interaction between four main parameters, air to water mass flow ratios in the evaporator and condenser, packing height, heat capacity flow of inlet hot water, and thermal conductivity and PCM communication with the surrounding fluid phases which can be characterized by their respective Biot numbers. The axial thermal stratification is established in the bed due to the interaction between these parameters, which is essential for creating the multiple-effect mechanisms. Under most of the boundary conditions, PCM elements generally have positive impact on the productivity rate of the plant compared to Non-PCM packing elements. However, this impact can be seen more clearly under higher inlet water mass flow rate and higher inlet water temperature on the evaporator, and higher mass flow rate of air in the system due to low thermal conductivity and large size of the PCM elements. Although this enhancing effect of PCM elements under such higher boundary conditions is more pronounced for higher packing height, it holds also true for the lower packing height but with lower impact as well. The comparative analysis revealed that the main reason behind enhancing the system productivity with PCM elements at steady state is attributed to the potential role of establishing multiple-effects of heating/humidification and cooling/dehumidification mechanisms in the evaporator and condenser respectively as compact heat and mass exchangers. The experimental analysis has demonstrated a strong and complex interaction between four main parameters, air to water mass flow ratios in the evaporator and condenser, packing height, heat capacity flow of inlet hot water, and thermal conductivity or PCM communication with the surrounding fluid phases which can be characterized by their respective Biot numbers. The axial thermal stratification is 123 --
established in the bed due to the interaction between these parameters, which is essential for creating the multiple-effect mechanisms. The solid phase thermal conductivity represents the key parameter that controls local heat and mass transfer not only at solid-liquid and solid-gas interfaces, but also at liquid gas interface. The multiple-effect mechanism depends on the thermal conductivity of the packing elements but has no relationship with solidliquid phase change in the PCM media. The present analysis indicated that the potential role of multiple-effect mechanisms can also be realized by employing other conductive media rather than PCM media which usually have poor conductivity. However, since only one PCM candidate was examined in the present study, theoretical analysis has been carried out in parallel to clearly explore the effect and limits of solid thermal conductivity. These results will be presented in the next chapter. Packing media which have optimum thermal conductivity, smaller size, and lower cost would be ideal candidates for the present application. Nevertheless, incorporation of PCM media in the hot water storage tank could have a potential role for realizing 24 hours operation of the HDH plants. Experimental results suggest that forced convection remarkably enhances the productivity of HDH plants compared to natural draft operation for all packing types. However, the conductive packing elements have a positive impact on the natural draft operation as well especially at higher heat capacity flow of hot water. Under natural convection where air flow is induced by double diffusion effects (density and humidity gradients), the MEHH boosts the natural air flow in the system, and hence increases its productivity. Increasing the specific surface area of the packing can compensate and outpace the role of MEHH at low mass flow rates of hot water. The role of MEHH in naturally drafted flow is nearly absent at very low hot water mass flow rates and low specific packing area. Conductive packing media may have a potential that could be utilized to improve the natural flow characteristics in other important applications such as cooling towers. 124 --
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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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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 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 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 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
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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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