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

More documents

Recommendations

Info

parameters on the heat transfer and pressure drop characteristics of packed bed solar energy storage system with large sized elements as a storage material of five different shapes. It was clear from the experimental results that the spherical shape has the highest heat transfer coefficient and the second lowest friction factor coefficient amongst the other five shapes. Standish and Drinkwater [122] reported that the element shape is a significant variable in gas liquid flooding in packed column. Gauvin and Katta [123] emphasized that the major operating cost in packed bed system is directly related to the pressure drop in the bed, which can not be predicted with reliability when the elements in the bed depart from the spherical shape. Barba and Spiga [124] analyzed the behavior of encapsulated salt hydrates, used as latent energy storage in a domestic hot water tank. The discharge process of the (PCM) was analyzed for three different geometrical configurations, i.e. considering the PCM encapsulated in slab, cylindrical or spherical polyethylene containers. The influence of the geometry and the Jacob number which is defined as: Ja c T T H (2.3) s m pcm1 where T pcm1 is the wall temperature of the cooled PCM cell surface, on the ending time of solidification were investigated. Spherical capsules, operating with high Jacob numbers (not too high to compromise the efficiency of the latent heat storage system) exhibited the largest energy density and the most rapid charge and release times as shown in figure (2.3). 2.8 Desalination with humidification-dehumidification technique Basin type solar stills exhibit two main draw backs. The first is the combined loss of evaporation, convection and radiation energy received by the condenser, which accounts for most of the energy lost to the surroundings. Partial recovery of this heat for feed preheating or to be stored for extending plant the production after sun-set has been an essential approach for enhancing the performance of solar distillers. Practically full recovery is almost impossible due to thermodynamic irreversibilities. The technical and economical viability of the energy recovery system should be justified with the additional yield gain. The second drawback is the transient trend of water production and long idleness of the plant after sunset and during cloudy periods when the ambient temperature is low and the heat sink is promising (i.e. cooling seawater temperature in the condenser is low). The limited heat capacities of these distillation systems can not support the plant operation during night or cloud periods. This drawback can be 27
overcome by applying a thermal storage capacity to the system. Likely, the technical and economical viability of energy storage (e.g. sensible or latent heat storage) in comparison with only daily production has to be justified with the additional yield gain. A break through in solar desalination was introduced based on the humidificationdehumidification (HDH) principle. The humid air distillation process has already been developed and introduced first by the University of Arizona through a pilot plant work performed from 1956 to 1963, which led to construction of an experimental pilot solar energy multiple-effect humidification (MEH) plant in 1963. Multi-Effect Humidification (MEH) process was further developed over the years and a few units were constructed and tested in different countries [2]. This technique leads to a major improvement in the efficiency and compactness of solar desalination units. The efficiency of energy utilization is increased in HDH technology due to the better control over the evaporation and condensation operations. There are several advantages of HDH process as a small-scale desalination system. It is simple, modular and flexible in scale-up, works under atmospheric pressure with below-boiling temperatures, ideal for utilization of lowgrade energy and waste heat resources, can follow a highly varying input thermal power supply without upset, inexpensive construction materials can be used, and requires moderate investment and operation costs as well as low technical skills for operation and maintenance. Khedr [130] performed a techno-economical investigation on HDH desalination process and compared it with other conventional processes. The results showed that the economics of the process is quite satisfactory for capacities of more than 10 m 3 /day, which is suitable for remote arid areas. Garg et al [131] reported that the HDH technique is suitable up to 90-m 3 /day capacity, and can be developed in coastal arid zones of the country where industrial development is impeded by inadequate or unreliable water supplies. An extensive review of the literature on HDH systems was reported by Al-Hallaj and Selman [2] Bourouni et al [129], and Narayan et al [149]. It is meaningless to repeat the previous review, thus the present study uses the previously mentioned literature surveys as its point of departure and consequently will focus on abstracting the important results for the present work and to discuss the design limitations and problems experienced with the HDH system components and potential improvements. 2.8.1 Classification of HDH systems The solar humidification-dehumidification (HDH) processes, also called the multipleeffect humidification-dehumidification processes (MEH) fall under three main 28
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 44 and 45: However, the stored heat is less va
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
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 142 and 143:
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 194 and 195:
When T m becomes sufficiently highe
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
show all

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

Create successful ePaper yourself

Delete template?

Save as template?