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

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9 Summary and conclusions 9.1 Summary This work discusses the results of experimental and numerical studies on a humidification-dehumidification (HDH) system utilizing conductive packing media in the evaporator and condenser. The objective of using conductive packing is to achieve multiple-effects of heating/humidification (MEHH) and cooling/dehumidification (MECD) while air passing through the successive packing layers in the evaporator and condenser respectively. The major objective of this study is to examine the innovative approach for locally creating MEHH and MECD and to determine the technical and economic feasibility of applying conductive packing media in the evaporator and condenser and PCM thermal storage in HDH solar desalination plants under both steady state operation conditions. A mixed micro-macro balance transient simulation model has been established and validated against experimental measurements using COMSOL Multiphysics and MATLAB for solving fluid flow and heat and mass transfer phenomena in one spatial dimension for different components in such a loop. At the macroscale, the wetting phase is described in terms of its average properties within a small volume by using the interpenetration continua and volume averaging technique. Thus, at each point, a macroscale phase is characterized as occupying a fraction of the available volume and to have a certain interface per unit volume with other phases. Each phase in the system is described in a similar way. Precise definition of the interface shape is neither required nor possible to obtain at the macroscale. On the macro level, the continuous solid approach for modeling phase change regenerators (PCR) is applied using the apparent heat capacity formulation. All sensible and latent heat rates are defined at the common interfaces based on the local temperatures of all phases, which are coupled together and updated as a function of space and time. Thus, the MEHH and MECC can be captured at successive points along the packing height. Using the simulation tools, a detailed heat and mass transfer analysis for the dynamic performance of a PCM-supported HDH system over a wide range of operation conditions has been undertaken. Experiments, not only on laboratory scale but also as a prototype, were designed and performed to measure fundamental time dependent variables and critical parameters affecting the system performance. A system parameters analysis for the main single components as well as for the whole HDH plant with its supporting solar collector field and PCM thermal energy storage has been performed under varying weather conditions over one year for Al-Arish City on the eastern north coast of Egypt. The analysis focuses on the optimum coupling and operation conditions of the HDH system with the solar collector field and the external thermal buffer. Special attention has been paid to the effect of thermophysical properties of the packing media and heat recovery enhancement. 177
9.2 Conclusions Under most of the boundary conditions, conductive packing elements generally have positive impact on the productivity rate of the plant compared to non-conductive packing. However, this impact is more profound 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 conductive packing 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 conductive packing elements at steady state is attributed to the potential role of establishing multiple-effects of heating/humidification MEHH and cooling/condensation MECC 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, thermal conductivity and packing 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 multipleeffect mechanisms. Experimental results suggest that forced convection remarkably enhances the productivity of HDH plants compared to natural draft operation for all packing types. However, thermally 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 the double diffusion effects (density and humidity gradients), the MEHH boosts the natural air flow in the system, and hence increases its productivity. The solid phase thermal conductivity represents an influencing parameter that controls local heat and mass transfer not only at solid-liquid and solid-gas interfaces but also at liquid gas interface in the evaporator and condenser. Theoretical analysis has been carried out to clearly explore the effect and limits of solid thermal conductivity. It has been demonstrated that the productivity of the evaporator can be enhanced by the use of a conductive packing media. It was found that the proportionality of evaporation rate is neither uniform with the thermal conductivity k s nor linear with the hot water mass flow rate. The maximum evaporation rate was obtained when k s is around 1.14 W.m -1 .K -1 as for the case of Pyrex glass and fired clay brick, beyond this value the distillate rate decreases. For the condenser it was found that the packing media with higher thermal conductivities produces more condensate. Moreover, for all media, the rank of 178
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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
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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
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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 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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