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

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When T m becomes sufficiently higher than the inlet hot water temperature to the store, i.e. at T m =80°C, the PCM storage behaves as a sensible heat storage. Of course, when this happens, the regenerator is not operating efficiently in the sense of the thermal storage concept due to the adopted once-through arrangement in the simulated system. When T m at intermediate points of 50, 60, and 70°C becomes more close to the inlet hot water temperature, phase transition of PCM will take place only in the first layers at the highest temperature of inlet hot water to the store during sunny (charging) hours. As pointed out in the previous section, this type of operation damps the thermal quality of the working medium at the storage entrance during sunny hours and hence, increases the temperature gradients inside the store which is a counterproductive effect in the once-through arrangement. However, the advantages of the once-through setup is its simplicity both in construction and operation, and lower investment and operation costs, since only one tank is needed in comparison with the cyclic generators, in which at least two tanks with a sophisticated control mechanisms are necessary. In spite of the fact that the simple system considered in the study proved to have a quite limited performance as stratification cannot be utilised effectively, simplicity and reduced costs remain major aims in realizing decentralized small-scale desalination units. One is worth noting that the once-through storage will be superheated and supercooled much more often than a cyclic regenerator, which will also result in higher stresses on the material and an overall degradation of the storage efficiency in a short life time. One possible measure to mitigate these negative consequences and improve the overall efficiency of both thermal storage and HDH system is to use a simple concentrating solar power collector (e.g. evacuated tube collector; ETC) instead of the flat plate collector FPC suggested in the study. FPC have low cost but low exit temperatures and efficiencies are low for the required temperature quality of the present application when compared with the ETC. Evacuated tube collectors are more expensive than FPC, thus the initial capital costs are high but operational costs are relatively low. Nevertheless, the improved efficiency of the ETC results in a lower collector area and hence the investment costs are partly compensated. However, it is believed that the final judgement and selection of the best overall plant setup and ideal components should be based on a comprehensive technoeconomical analysis. 175
8.4 Conclusions The results obtained from the parametric study on the HDH system indicates that the crucial parameters have been rigorously defined for practical purposes under real weather conditions. The ideal HDH performance is approached for: brine concentration factor (r c )=2, air to hot water mass flow ratio=1.5, cooling water to air mass flow ratio=2, thermal storage volume 3-5 m 3 per m 3 of daily fresh water production capacity depending on the local climatic conditions. The once-through flow arrangement in the external PCM thermal buffer with a flat plat collector has a limited efficiency when combined with the HDH plant in comparison with water storages with the same volumetric capacity. For optimal operation of the external PCM thermal buffer, Stefan number must be small enough, as this means that there is a great amount of latent heat that can be stored (relative to sensible heat). The physical properties of useful phase change compounds, and the usable temperature ranges generally limit Ste to O (10 −2 ). However, the practical average yearly limit of Ste in the present application is O (1), which means that with this larger value of Ste the amount of sensible heat is much higher than the latent heat. Utilization of PCM in the thermal storage is recommended only for high energy density and in applications where there is a narrow operational temperature change around the melting range, as the latent heat becomes much larger than the sensible heat. It is therefore of interest to look in future studies at the techno-economical aspects for different simulation scenarios and coupling setups of the external thermal buffer and concentrating solar power collectors with the HDH system. 176
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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
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It could be interpreted that this p
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Evaporator Condenser Figure 5.8: Av
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within the inaccuracy margins. The
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The last two cases of boundary cond
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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 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 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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