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

performance and its threshold value shouldn’t exceed unity. Realistic units would
include an energy recovery system for lower energy consumption, as was described
in the previous chapters, and will be investigated numerically in chapter 8 when
considering the complete solar driven HDH plant.
5.5.1 Behavior of the PCM-supported HDH cycle
Figure (5.6) shows the evolution of inlet water temperature, the outlet water and gas
temperatures as well as PCM temperatures at the top, middle, and bottom layers for
the evaporator and condenser as one example of the PCM system thermal behavior.
As can be seen from the figure, the outlet gas, water, and PCM temperatures are
increasing with time until they all approach each other at the point of steady state or
thermal equilibrium after nearly two hours. Along its pass through the evaporator, hot
water transfers part of its energy with the solid phase and other part with the gas
mixture. Energy transport to the gas mixture includes sensible and latent heat
(evaporative) components which is strongly dependent on the liquid-gas temperature
difference. Energy transport to the solid phase includes two sensible heat
(convection) components at the liquid-solid and gas-solid interfaces, which depend
on the binary temperature differences as described in chapter 4.
At the beginning of plant operation, the temperature of PCM beads is the same as
ambient temperature and the differences between water temperature and both solid
and gas temperatures are at its maximum values. In this early stage, one would
expect a heat transfer in the direction from gas to solid phase, since the gas
temperature increases very quickly compared to the solid phase due to the latent
heat of evaporation. Marching in time at every specific location along the packed
height, the temperature of PCM increases while the liquid-solid and gas-solid
temperature differences decrease until they reach a certain limit at which the energy
flows into and out of the solid phase at liquid and gas interfaces respectively are
balanced.
Having reached thermal balance at one point, it will move forward to the next point
along the packing height from top to the bottom until full steady state conditions are
reached. The same scenario takes place in the condenser with the gas phase as the
heat source. It can be clearly seen that the outlet gas temperature from the
evaporator is very close to the PCM top layer temperature. The outlet gas
temperature from the condenser (and inlet to the evaporator), outlet water
temperature from the evaporator, bottom PCM layer temperature in the condenser
and bottom PCM layer temperature in the evaporator are very close to each other.
Thermal stratification in the evaporator is higher than that of the condenser because
of the lower heat source (gas) temperature in the condenser in comparison with the
heat source (hot water) temperature in the evaporator and due to the concurrent flow
in the condenser. In the concurrent flow in the condenser, it is anticipated that most
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