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

More documents

Recommendations

Info

8.3.4 Effect of hot water flow rate A crucial parameter in the performance of solar collectors is the mass flow rate of the working medium through the collector. With a higher mass flow, the absorber temperature decreases because of the better cooling effect rate. The decrease of the absorber temperature leads to a decrease in the heat losses to the ambient, which in turn results in a higher efficiency of the collector. It shows that the efficiency can be increased by increasing the mass flow through the collector. However, a higher mass flow also decreases the temperature rise that can be achieved through the collector. Thus, a compromise between the temperature rise and the efficiency has to be made since a high mass flow with insignificant temperature rise is generally not practical for real applications. Therefore, the influence of inlet air mass to hot water flow ratio is important to be investigated for the HDH system as a whole, due to its combined effects and opposing trends on the performance of the single components. Increasing the mass flow rate of feed seawater increases the energy efficiency of both the condenser and solar collector, while it decreases the evaporator efficiency due to lowering the operating inlet hot water temperature, which Figure 8.7: Effect of air to hot water mass flow rate ratio on the average-yearly daily distillate rate and GOR reduces the distillate rate. On the other hand, increasing hot water flow rate increases the wetted and effective interfacial areas in the evaporator, and hence improves the mass transfer rate at the liquid-gas interface as discussed earlier. However, it is expected from earlier results that the hot water mass flow rate is not the predominating factor in comparison with the effect of inlet hot water temperature on the evaporator effectiveness. Figure 8.7 shows the combined effects of hot feed water mass flow rate on the distillate rate and GOR. The maximum fresh water production rate peaks at an air to water mass flow ratio of 0.67, which is close to the optimum ratio found for the evaporator when it was studied alone (0.75). It is clearly observed that there is an opposite trend for GOR which increases with increasing the G/L hw ratio. The GOR reaches the maximum at G/L hw =4 and depicts a minimum value at G/L hw =0.5 which means that this operation point should be avoided under 169
the specified boundary and weather conditions, especially that the distillate rate also decreases below the optimum point at G/L hw =0.67. The optimum theoretical mass flow ratio would be selected as the intersection point at G/L hw =1.8. However, it is believed that the right decision depends on the energy source and cost, for instance if the energy is supplied through a waste heat source or has a very low cost the decision then maybe to adopt the highest distillation rate as the main criterion. It can also be observed that the impact of increasing G/L hw is more pronounced on the distillate rate than the GOR between the intersection point and the maximum distillation rate. Thus the optimum point would be selected as the mean value between G/L hw =0.67 and G/L hw =1, i.e. ≈0.8-0.9. 8.3.5 Effect of cooling water flow rate Due to existence of an additional heat exchanger at inlet of the condenser to bring the cooling water temperature down to the ambient conditions, the condenser performance is expected to be always better at higher mass flow rates of the coolant. But the mass flow rate has a strong effect on the outlet cooling water temperature, which in turn is extremely important since it determines the amount of latent heat recovery through the main heat exchanger downstream of the Figure 8.8: Effect of cooling water to air flow rate ratio on the average-yearly hourly distillate production and GOR condenser. In addition, the cooling water mass flow rate should be optimized in order to reduce the pumping power required for the coolant and the cooling seawater in the additional heat exchanger upstream of the condenser. The overall thermal performance of HDH system was found to be optimized at a cooling water flow rate double the air flow rate in the system as shown in figure 8.8. The optimum water to air flow rate ratio can also be considered as the same optimum ratio found for a stand-alone condenser. 8.3.6 Effect of storage medium type Characterization of the ideal thermal storage medium for a certain application is a complex task. Numerical simulation can be used to predict different effects of certain 170
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 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 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 ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?