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

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Q m H c ( T T1) c ( T 2 T ) (2.2) m s m l m where ΔH m is the latent heat of melting and c s and c l the are specific heats of the solid and liquid phase, respectively. 2.5.2 Advantages of LHES The main advantages of LHES over other storage systems may be summarized in the following: a) Higher energy density per unit mass and per unit volume, resulting in reduction of mass and volume of the storage unit. It has been reported [86] that phase change materials store 5 to 14 times more heat per unit volume than sensible heat storages such as water, masonry, or rock. b) The PCM temperature remains nearly constant during the phase change, which is useful for smoothing temperature variations, then keeping the subject at uniform temperature. In other words, heat extraction and storage take place at (nominally) constant temperature corresponding to the phase change temperature. c) Lower storage temperatures, which reduce the heat loss. Latent heat energy storage media are confined in three basic forms; solid-solid, solid-liquid, and solid-liquid-vapour phase change. 2.5.3 Solid-Solid Phase Change Storages The challenges to widespread use of PCM are their packaging or capsulation, high cost, and lake of technical knowledge among potential customers and users. For solid-solid phase transformation the need for encapsulation of phase change material is eliminated. This enables the high capacity properties of phase change to be exploited without the additional thermal resistance and the associated costs due to the encapsulation. In solid-solid transitions, heat is stored when the material is transformed from one crystalline structure to another. Generally, the latent heat of transformation for this transition is smaller than that of the solid-liquid phase change materials. However, there are some of them having high capacity, form-stable, that can be used as direct contact thermal storage systems in many applications in industry and in commercial and residential heating. Some of the most promising materials for solid-solid PCM are the commercially available vinyl silane graft copolymer, a polysiloxane [88], and organic solid solution of pentaerythritol, pentaglycerine, Li 2 SO 4 , and KHF 2 [92]. Some investigations that have dealt with the Polysiloxane, as a promising formstable candidate for thermal energy storage proved that it has high latent heat of 17
fusion and relatively low cost [88-91]. Such form-stable systems have many advantages over conventional phase change storage systems. For example, conventional phase change storage systems for steam applications would be larger, less efficient, and would require an intermediate working fluid since most encapsulation materials corrode in steam environment. Due to low thermal resistance, the direct contact form-stable system is also particularly suited for low grade waste energy recovery such as from industrial processes where the flow gases are between 100 and 200 °C [88]. Commercially available polymers such as high density polyethylene (HDPE) have to be usually cross-linked to make them form-stable with the help of a catalyst or by more sophisticated method. The cross-linked storage units can use the granular form that is usually available, or can utilize other shapes such as rods or other geometries [88]. High density polyethylene offers definite advantages as potential thermal energy storage material if it is rendered form-stable by cross-linking [101]. Kamamito et al. [6] concluded that cross-linked HDPE is an excellent thermal energy storage material and can be used in direct thermal contact with ethylene glycol and silicon oil. Sharma et al. [98] reported that their operating temperatures of 110 to 140°C are too high for some applications such as space and water heating, and according to his knowledge no company has commercially developed cross-linked polyethylene for heat storage applications at this time. 2.5.4 Solid-Liquid-Vapour Phase Change Storages The use of a complete solid-liquid-vapor phase change cycle will further increase the storage density, but large volume changes cause problems regarding the containment of such materials, and then rule out their potential application in thermal storage systems. Indeed, such systems are technically feasible, but quite more complicated than the simple (and passive) solid-liquid-solid cycle [103]. Furthermore, changes of phase from liquid to gas involve large changes in volume of the material and so are difficult to make practical in a closed compact solar seawater desalination and hot water storage systems. Indeed, no single attempt to use solid-liquid-gas phase change for PCM storage has been found in the literature. 2.5.5 Solid-Liquid Phase Change Storages Solid-liquid transformations are most commonly employed due to small changes in volume and high heats of fusion. It is also possible to choose a melting temperature in a wide range from a large amount of solid-liquid phase change materials. A wide variety of solid-liquid phase change materials is commercially available with temperature ranges from – 21 °C (sodium chloride solution) to more than + 200 °C (salts and eutectic salt mixtures). Phase change materials can therefore be used as a thermal storage medium for both heating and cooling. Phase change materials 18
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 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 44 and 45: However, the stored heat is less va
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
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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
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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
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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
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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
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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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