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

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However, the stored heat is less valuable, due to the low temperature level. Selecting a higher phase change temperature, the process of phase change will take place only at a higher solar gain. The number of days, with higher solar gain is less, thus the period in which latent heat storage can be used is short. On the other hand, the stored heat is much more valuable, due to the high temperature level. A double grade phase change materials combines the advantages of both versions. Another novel approach that has been suggested by Watanbe et al. [116] includes the use of three types of PCM with different melting temperatures. This allows for maximum use of the heat transfer medium energy. 2.7 Heat transfer in PCM regenerators The heat transfer process between the latent heat storage units and the heat transfer fluid during charging and discharging cycles greatly depends -among several other factors- on the type of heat exchanging surface. The crystallising (nucleating) and thickening agents, which are added to some PCM to prevent supercooling and phase segregation, lower the thermal conductivity of the PCM and increase the viscosity which inhibits convection motion in the liquid PCM [100, 106]. Therefore, an important disadvantage of the common materials used in solid-liquid phase change (paraffin, hydrated salts) is their low thermal conductivity which makes them act as self-insulator materials. That is; PCM solidifies on the interfacial heat transfer surface with the working fluid during the discharging process and acts as self-insulator. Enormous amount of research effort so far has been devoted to overcome the inherent handicaps mentioned above via some kind of heat transfer enhancement techniques: (a) Active methods such as agitators/vibrators, scrapers and slurries [102] (b) Using microencapsulated PCM. (c) Using PCM containing dispersed high conductivity particles or Lessing rings [108] (d) Using PCM graphite composite material [107] (e) Using extended surfaces such as fins and honeycombs [100, 109, 110] (f) An efficient configuration of PCM is encapsulation in hollow shells (e.g. tubes, pipes, spheres, cans with small dimensions), which are then used in a packed or fluidized bed configuration. Using such passive heat exchangers that typically consist of a large number of encapsulated elements increases the effective heat transfer area, then enhances the heat transfer rate, and a great thermal stratification in the storage could be achieved. These benefits are more obvious when the dimensions of the elements are small [85]. Phase change materials perform best in containers that –when combined PCM- total one inch (25.4 mm) in diameter or width [111], however, this statement can not 25
e generalized. Steel and polyethylene are common packaging materials, (g) Using the composite salt/ceramic thermal energy storage media concept which offers the potential of using PCM via direct contact heat exchange. A direct contact heat exchanger with an immiscible heat transfer fluid moving in the PCM has eliminated the permanent heat exchange surface and has been confirmed to prevent phase separation of the PCM [113, 114]. (h) Using a porous metal matrix such as aluminum matrix [115] as a way of improving the performance of the storage system, enhancing heat conduction without reducing significantly the stored energy The type of heat exchanger surface strongly influences the temperature gradients of the PCM in the charging and discharging of the storage. Proper designing of latent heat energy storage systems require quantitative information about the heat transfer and phase change processes inside the PCM units. Among different geometrical configurations of the PCM capsules, it is found that spherical shape has received the utmost attention of research work in literature. Some authors attributed their interest in spherical capsules to the fact that the sphere has the largest volume to surface area ratio [126], although self-insulation during solidification might hinder full exploitation of the storage, which is rather in favor of large surface area per unit volume. Hence sphere size should not be too large as mentioned earlier. Further considerations for optimum sphere size depend on handling, energy density, and pressure drop [126]. Singh et al. [121] have clarified experimentally that if packing of spherical and other shapes like cubes and tubes are compared physically, it appears that during fluid flow in the bed, fluid film may remain in contact with the maximum portion of the surface area of spherical elements as compared to other shapes. In the case of non-spherical shapes, as the working fluid strikes the surface, it may get detached from the packing surface due to presence of sharp corners and edges. Therefore lesser contact area may be available for heat transfer for nonspherical shapes. Besides, non-spherical shapes have also surface contact between material elements which reduces the area available for heat transfer. Therefore lower values of heat transfer may be expected for other non-spherical configurations as compared to the spherical material elements. Singh et al. [121] have conducted an extensive experimentation to investigate the effect of the system and operating Figure 2.3: Energy released versus time for different geometries [124] 26
Page 1: Technische Universität München In
Page 4 and 5: It is my pleasure to thank Prof. Th
Page 6 and 7: Contents List of figures ..........
Page 8 and 9: 5.3 Functional description ........
Page 10 and 11: List of Figures Figure 1.1: Cost br
Page 12 and 13: 78cm; Top: Evaporator, Bottom: Cond
Page 14 and 15: xii
Page 16 and 17: m Mass flow rate; [kg.s -1 ] m eva
Page 18 and 19: ed c coll cond cw d d e or eff evap
Page 20 and 21: 1 Introduction The first part of th
Page 22 and 23: summarized in figure (1.2). Thermal
Page 24 and 25: 1.3.2 Selection criteria of desalin
Page 26 and 27: of energy storage or hybridization
Page 28 and 29: Desalination development focuses on
Page 30 and 31: Introduction of the indirect type s
Page 32 and 33: 2 Literature Survey 2.1 Introductio
Page 34 and 35: There is actually a commercial prod
Page 36 and 37: Q m H c ( T T1) c ( T 2 T ) (2.2)
Page 38 and 39: (solid-liquid) have a storage densi
Page 40 and 41: (iii) a sharp melting temperature.
Page 42 and 43: melt and freeze without segregation
Page 46 and 47: parameters on the heat transfer and
Page 48 and 49: categories. The first category is b
Page 50 and 51: to 550 kWh/m 3 of distilled water a
Page 52 and 53: The enthalpy and humidity of the sa
Page 54 and 55: Air mass flow rate [kg/h] GOR [-] 1
Page 56 and 57: On the other hand, the efficiency o
Page 58 and 59: 2.8.4.2 Effect of air to water mass
Page 60 and 61: distillate rate. The system was not
Page 62 and 63: were; EnviPac Gr.1, 32 mm, and VFF
Page 64 and 65: Figure 2.15: Height versus air temp
Page 66 and 67: The performance of the condenser is
Page 68 and 69: Klausner and Mei [136] described a
Page 70 and 71: The impact of the collector cost ca
Page 72 and 73: exchanger and energy storage in one
Page 74 and 75: 3 Scope of the Study This chapter p
Page 76 and 77: packing where it gets in direct con
Page 78 and 79: In fact there is a tradeoff between
Page 80 and 81: The energy flow between all and eac
Page 82 and 83: 4 Theoretical Analysis and Modeling
Page 84 and 85: The two-energy equation models whic
Page 86 and 87: solid-liquid phase change inside th
Page 88 and 89: assumed by conduction in both axial
Page 90 and 91: the air pressure drop ∆P per unit
Page 92 and 93: evaporation rate per unit volume of
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
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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
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
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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
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operating limits. In light of the l
Page 130 and 131:
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
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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
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x t u x u 0 0 , (6.5) and for a s
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Next time step validation were the
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The evaporator and condenser both w
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Figure 6.2: Evolution of inlet and
Page 154 and 155:
Figure 6.4: Evolution of inlet and
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temperature is at its maximum level
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Bottom Top Figure 6.7: Evolution of
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7 Parameters Analysis of the Evapor
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Figure 7.1: Effect of packing size
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e 0.45 kPa under natural convection
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aspect ratio under constant bed vol
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Figure 7.7: Effect of inlet hot wat
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well as the time required to reach
Page 172 and 173:
Figure 7.10: Effect of packing medi
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40°C respectively. This gives flex
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the condensate rate, productivity f
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esult, rather than PCM media which
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Figure 7.18: Effect of PCM thermal
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8 Optimization of HDH plant In this
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The MATLAB model describes how the
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less steep than that of the distill
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8.3.4 Effect of hot water flow rate
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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
Page 200 and 201:
[13] Vafai K., Sozen M. An investig
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[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
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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
Page 216 and 217:
The Ergun analogy factor J h repres
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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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