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

More documents

Recommendations

Info

melt and freeze without segregation since they freeze to an intimate mixture of crystals, leaving little opportunity for the components to separate [98]. Frame 2.1: Desirable properties of latent heat storage materials [94, 95] Thermo physical properties Melting temperature in the desired operating temperature range High latent heat of fusion per unit volume, so that less volume of the container is required for storing a given amount of energy High density, so that a smaller amount of PCM can be used. High specific heat to provide for additional significant sensible heat storage effects High thermal conductivity of both solid and liquid phases, so that the temperature gradients required for charging and discharging the storage material are small Small volume change, on phase transformation and small vapour pressure at operating temperatures to reduce the containment problems Congruent melting, the material should melt completely so that the solid and liquid phases are identical in composition. Otherwise, the difference in density between solid and liquid may cause segregation resulting in changes in the chemical composition of the material Kinetic properties High nucleation rate to avoid super cooling of the liquid phase i.e. the melt should crystallise at its thermodynamic freezing point. At times, the super cooling may be superposed by introducing nucleating agent or a "cold finger" in the storage material High rate of crystals growth, so that the system can meet demands of heat recovery from the storage system Chemical properties Chemical stability Complete reversible freezing/melting cycle No degradation after a large number of freezing/melting cycles Non-corrosiveness to construction materials Non-toxic, non-flammable and non-explosive materials for safety 2.6.4 Salt Mixtures The composite salt/ceramic thermal energy storage media concept offers the potential of using PCM via direct contact heat exchange and, therefore, the potential of significant cost improvement through elimination of heat exchange materials, and reduction of storage material and containment vessel size. This salt ceramic approach may be explained as micro-encapsulation of a PCM within the submicron pores of a ceramic matrix [101]. The liquid salt is retained within the solid ceramic 23
network by surface tension and capillary forces. Heat storage occurs as latent heat of the PCM and as sensible heat of the basic ceramic material and PCM. Therefore, the use of salt/ceramic materials represents not a pure latent heat but a latent/sensible hybrid storage concept [101]. A novel energy storage system consists of n-octadecane retained by capillary forces in a porous silica support [83]. This configuration eliminates the expensive heat exchange surface, provides higher energy density, and gives higher rates of energy storage and release [98]. 2.6.5 Multi Grade Phase Change Materials Sometimes the isothermal storage level might be disadvantageous such as the following cases [112]: 1) If the energy to be stored is available at much higher temperature level than the melting point of the heat storage material, the storage occurs at lower temperature level than that of the source. 2) A worse case may occur if the melting point of the PCM is higher than the temperature level of the energy source. Then there is no latent heat storage and a large amount of heat may be lost. These cases may occur during solar energy storage when the temperature of the heat transfer medium from the solar collector is higher than the melting point of the PCM applied (the storage is realized at not sufficiently high temperature) or the temperature of the transporting medium is lower than the melting temperature of the PCM (there is no storage). There are three possibilities to overcome these problems [112]: 1) Application of materials with high specific heat. The average specific heat of solid material is 3 (kJ.kg -1 .K -1 ), the specific heat of water is 4.18 (kJ.kg -1 . K -1 ), therefore, a large volume must be involved in the storage process 2) Application of two phase change materials; one with a lower melting point, and other with a higher melting point, then the storage volume can be reduced but two separate storage tanks are necessary 3) Application of crystal hydrates which crystallizes in two different hydrate forms and the two forms have different melting points which are called two grade phase change materials Calcium chloride may be a promising material to accomplish the third way mentioned above. Calcium chloride forms different crystal hydrates (CaCl 2 .6H 2 O, CaCl 2 .4H 2 O, CaCl 2 .2H 2 O) and they have different melting points of 29.9, 43.3, and 130 °C respectively [112]. If a PCM of low phase change temperature is selected the process of phase change will take place even at low solar gain. Thus the number of days, when the latent heat storage can be used will be high during the year. 24
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 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
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?