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Heat and Moisture Behavior Simulation for Desiccant Cooling System
with Phase Change Materials and Desiccant Materials
Mayu Yamaguchi 1, Yoshihisa Momoi
1, Kazunobu Sagara
1, Toshio Yamanaka
1, and Hisashi Kotani
1
1Osaka University
Keywords: Desiccant, Dehumidification, Numerical Simulation
ABSTRACT
Recently, desiccant cooling system has attracted our attention, from the point of view of its merit of energy saving and in order
to improve indoor thermal environment. Desiccant materials produce adsorption heat by dehumidification. Therefore, the
dehumidification efficiency decreases as temperature of desiccant materials rises. The purpose of this study is to reduce the
temperature rise of the dehumidified air by using desiccant cooling system with phase change materials and desiccant materials,
and to simulate thermal and moisture behaviour of desiccant cooling system. In this paper, predictions of the dehumidification
performance for a desiccant bed packed with phase change materials and desiccant materials by using numerical analysis are
reported. First of all, this paper presents the heat and moisture transfer model. After that, influence of PCM on air temperature
and humidity through the desiccant bed and dehumidification amount is investigated. From the results it is found that for the
desiccant bed without PCM the air temperature through the desiccant bed rises immediately after dehumidification starts, and
increases up to 57 deg.C. On the other hand, for the desiccant bed with PCM it was found that the air temperature is maintained.
1. Introduction
Recently, desiccant cooling system has attracted our attention,
from the view point of its merit of energy saving and in order
to improve indoor thermal environment. Normally, air-
conditioners are widly used to provide people thermal
comfort. However, air-conditioners consume a lot of energy.
Energy saving has become a crucial issue ane various
government agencies and organizations have implemented
several campaigns and programs such as demand side
management, etc.
Actually, there are many well-known ways to reduce the
energy consumption of an air-conditioning system. One of
them is the use of a desiccant to remove part of the moisture
from either return or outdoor air in order to decrease the
latent heat load of the AC. The literature is rich, and various
reviews of this topic are available.For instance, an interesting
review on desiccant cooling systems was reported in [1] and
details about the adsorption process could be found in [2,3].
Various studies used the solarintegrated design for
regenerating the desiccant materials [4–6]. A research group
from the Asian Institute of Technology (AIT, Bangkok)
reproduced a lab-scale investigation of the integration of
solar design for daily regeneration and night absorption [7].
Desiccant dehumidifiers can remove moisture from air by
using desiccant materials (absorbents such as silica gel),
without cooling the air below its dew point. However,
desiccant materials produce adsorption heat by
dehumidification. Therefore, the dehumidification efficiency
decreases as the temperature of desiccant materials rises.
Fig.1 shows mechanism diagram of this system on
psychometric chart. The purpose of this study is to reduce the
temperature rise of the dehumidified air by using a new
desiccant cooling system with phase change materials (PCM)
and desiccant materials, and to simulate heat and moisture
behaviour of the desiccant cooling system. Fig.2 shows the
system diagram. Dehumidification unit dehumidifies outdoor
air, and we make the switch between adsorption and
desorption. We use a heat pump system for the sensible heat
load in the room, and a solar heater and waste heat of heat
pump system are used for desorbing. It can save energy. Cool
night air is used for solidifying PCM.
In this paper, predictions of the dehumidification
performance for a desiccant bed packed with PCM and
desiccant materials by using numerical analysis are reported.
First of all, this paper presents the heat and moisture transfer
model. In the previous reports [8, 9], the authors developed a
numerical model of a desiccant wheel, derived the heat and
moisture transport equations for the inside of the desiccant
element. In addition, the authors measured the equilibrium
moisture content and moisture transfer coefficient, which are
important parameters that determine the dehumidification
performance. Moreover, numerical analysis inputting these
measured values was conducted, and the validity of the
numerical analysis was discussed, by comparing the
calculation results with the experimental results of a
desiccant system. We developed a numerical model of a
desiccant bed with PCM based on the previous reports [8, 9].
Then, based on the calculation results, phenomena that occur
in the desiccant bed packed with PCM and desiccant
materials during the absorption processes is discussed, and
predict the dehumidification performance for the desiccant
bed.
Fig. 1. Schemitic diagram of temperature and humidity
changes on psychrometric chart
Fig. 2. PCM desiccant system
2. Heat and moisture transfer equations in the
desiccant bed
As shown in Fig.3, cutting out an infinitesimal volume from
the desiccant bed, a numerical model was produced with
reference to the theory of the simultaneous transfer of heat
and moisture proposed by Matsumoto [10] and Hokoi et al.
In this paper, we produced the numerical model of a
desiccant bed with PCM based on that model.
(1) Heat balance equation for the air of the flow passage in
the infinitesimal volume is expressed by Eq. (1)
a a a a a a a
a
d a d p a p
C u C
t x x x
S S
[W/m3](1)
(2) Moisture balance equation for the air of the flow passage
in the infinitesimal volumes is expressed by Eq. (2)
a a a a a
a
d a d
X u X X
t x x x
S X X
[kg/sm3](2)
(3) Heat balance equation for the desiccant material in the
infinitesimal volume is expressed by Eq. (3)
d d d
d d a d d a d
CS L S X X
t
[w/m3](3)
(4) Heat balance equation for the PCM in the infinitesimal
volume is expressed by Eq. (4)
p p p
p p a p
CS
t
[W/m3](4)
In the previous paper [11], the authors developed a model of
PCM. An enthalpy method (Saito, 1992) was applied in the
numerical simulation. When PCM is in melting or freezing
process, specific heat of PCM changes to follow a sine curve
of the temperature corresponding to the latent heat in Fig.4.
Sodium sulfate decahydrate (Na2SO4 10H2O; melting point
30 deg.C., freezing point 28 deg.C, latent heat of fusion 126
kJ/kg) is used as PCM. Specific heat of PCM is expressed by
Eq. (5)
θd < 28deg.C , θd > 30deg.C
3500pC [J/kgK]
28deg.C < θd < 30deg.C
360sin 28 3.5 10
2 2p dC
[J/kgK](5)
The moisture content of the desiccant material varies
according to the moisture transfer, which is induced by the
difference in absolute humidity between the desiccant
material surface and the air of the flow passage. Accordingly,
it can be expressed by Eq. (6)
d d d a d
wS X X
t
[kg/sm3](6)
The above equation system has four equations and six
variables Xa, Xb, θa, θd, θp, and w, and so the equation system
is not closed. Then, the system is closed by assuming the
local equilibrium in which the absolute humidity and
moisture content of the desiccant material surface
immediately follow the equilibrium moisture content curve as
expressed by Eq. (7)
,d dw f X [kg/kd-dryair](7)
where it is assumed that there is no heat loss, air flow is
laminar, pressure and flow rate are constant, and temperature
and moisture content are homogeneous in the thickness
direction of the desiccant material. In this paper, the
experimental formula was used as Eq. (7). Section 3 shows
details of the experiment.
Based on the calculation results with the above mentioned
numerical calculation model for a desiccant bed, phenomena
that occur in the desiccant bed during the absorption process
is discussed. The desiccant bed was divided into 20 parts in
the flow direction for calculation. Calculation was conducted
for the adsorption process. In the adsorption process, the air
with a temperature of 25 deg.C and a relative humidity of
80 % flows from the left side.
Fig. 3. Model of PCM and desiccant bed
m The amount oftotal heat storage
(20~36deg.C)
Fig. 4. Property of specific heat of PCM
Fig. 5. Divided cells and flow direction
3. Method of experiment
An experiment was conducted in order to investigate the
change in moisture content of desiccant materials with the
relative humidity. And then, we expressed the equilibrium
moisture content as Eq.(7). The desiccant materials used
silica gel of 4 mm in diameter for the experiment.
3.1 Experimental setup
An experimental equipment for the measurement of the
moisture content was installed in an artificial climate
chamber in which the room air temperature was controlled to
be 25 deg.C, and the relative humidity was varied 40%, 50%,
60%, and 70%. Based on academic standards for
measurement of moisture properties : 2006 [12], the weight
of silica gel was measured every minute. An electronic
balance (A&D GX-8000) was used to measure the weight as
shown in Fig.6.
3.2 Results of experiment
Fig.7 shows the variation of the moisture content computed
from the measurement value. From the results, it was seen
that the moisture content increases as the relative humidity is
larger. The adsorption rate decreased with time, and came to
equilibrium state in about 24 hours. And then, the equilibrium
moisture content curve was represented as shown in Fig.8.
The X-axis expresses the air relative humidity at equilibrium
state scale and the Y-axis expresses the moisture content scale.
Fig.8 also shows the results of the previous study [9]. The
current results are smaller than that of the previous study. In
this paper, the X-axis was converted the air relative humidity
into the air absolute humidity, and then the variation with the
air absolute humidity is expressed as a cubic approximate
equation.
Fig. 6. Experimental equipment
Fig. 7. Variation with time of moisture content
Fig. 8. Equilibrium moisture content curve
The moisture content of the desiccant element changes along
the curve of the equilibrium moisture content expressed by
Eq. (7), according to the relative humidity at the element
surface. The moisture transfer coefficient α'Sd has a close
relation to the moisture content of the desiccant materials.
From the results of the experiment, it can be concluded that
in order to improve the absorption performance, it is
important to use a material whose equilibrium moisture
content curve has a large gradient and a material whose
dehumidification speed is high, that is, moisture transfer
coefficient α'Sd is large.
In this section, we would discuss moisture transfer coefficient
α'Sd. Fig.9 shows moisture transfer coefficient α'Sd calculated
from the results. It was found that α'Sd increases as the
relative humidity rises. Also, it was seen that α'Sd decreases
with time. The reason is that as the moisture content become
higher, the internal pore resistance of porous materials
increases. However in order to express the variation of the
moisture transfer coefficient α'Sd, it is require to express the
α'Sd’ which is independent from the time, this experiment was
occurred under the natural convection. Therefore we have to
examine the issue of the air flow.
Fig.9. Discuss moisture transfer coefficient
4.Results and discussion of simulation
Fig.10 shows the desiccant bed used in this model. The
desiccant bed had a diameter of 150 mm and a length of 100
mm. PCM and desiccant materials of 4 mm particles were
filled in the desiccant bed. Void ratio was 0.32. Silica gel was
used as desiccant materials, and its density was measured
value. Sodium sulfate decahydrate (Na2SO4 10H2O;melting
point 30 deg.C, freezing point 28 deg.C, latent heat of fusion
126 kJ/kg) was used as PCM. The inlet air temperature was
25 deg.C and relative humidity was 80%. Moisture content of
desiccant materials started at 2%. α'Sd should vary with the
amount of desiccant materials. Therefore, α'Sd, the amount of
desiccant materials, and the velocity of the inlet air were
parameters in this paper. Table1 shows the properties of
parameters.
4.1 Effect of PCM on exit air temperature and
dehumidification performance
In this section, we discuss the effect of PCM on the exit air
temperature and the dehumidification performance. Fig.11
shows the exit air temperature variations with time for the
desiccant bed with PCM and without PCM. It was assumed
that the air velocity u was 0.5 m/s, φd : φp = 1:1, where the
filling ratio of desiccant materials was defined as φd , the
filling ratio of PCM as φp (φd + φp = 0.68, Void ratio was
0.32), and moisture transfer coefficient α’Sd was varied 5, 10,
20, and 30.
Starting at t=0, the exit air temperature of the case without
PCM rises rapidly until reach to about 57 deg.C. On the other
hand, the exit air temperature of the case with PCM increases
slowly until reach to the melting point 28 deg.C. Then PCM
adsorb energy to change phase from solid to liquid in a
constant temperature around from 28 deg.C to 30 deg.C, and
finally, the temperature will increases rapidly again. From the
results, it seems that most of PCM have changed to liquid in
about 20 minutes. Fig.12 shows the differences of humidity
ratio between the inlet and the outlet. The differences
decrease rapidly in about 10 minutes for without PCM, in
about 20 minutes for with PCM. These times agree with the
time when the exit air temperature reaches to the maximum,
in Fig.11.
Fig. 10. Properties of materials
Table 1. Calculation conditions in simulation
4.2 Effect of various α’Sd on exit air temperature and
dehumidification performance
From the results shown in Fig.11, it was found that the exit
air temperature rises largely as α’Sd becomes small, in the
first 15 minutes, for desiccant materials with PCM. On the
contrary, the differences of humidity ratio reduce. The reason
is that for the case of a smaller α’Sd desiccant materials
cannot adsorb enough heat at the inlet zone, and it also
adsorb heat at outlet zone, and so the temperature rises
during adsorb process, and then the high temperature air is
exhausted without heat removal. Therefore exit air
temperature rises in the early stages. The air temperature
variation of without PCM has the same tendency as with
PCM.
4.3 Effect of various filling ratio of desiccant material and
PCM
Fig.13 shows the comparisons of the exit air temperature by
the various filling ratio of desiccant materials and PCM, and
Fig.14 shows the differences of the humidity ratio between
the inlet and the outlet. As with 3.2, the filling ratio is
represented φd , φp. The results of φd : φp = 2:1 , 1:1, 1:2, 1:3,
1:5, and 1:10 are shown. It could be found from the results
of Fig.13 that the air temperature doesn’t rise so much when
the filling ratio of PCM is large, and the air is at constant
temperature for a long time. This is because of the difference
of heat capacity. The heat capacity is larger as the filling ratio
of PCM increases. It can be seen from Fig.14 that the
differences of humidity ratio is small when the filling ratio of
PCM is small. The reason is that the amount of desiccant
materials reduces as that of PCM increases. According to
Fig.14, for φd : φp = 1:5, and 1:10 the dehumidification
performance is worse than that of others in the early stages.
Fig. 11. Variation with time of exit air temperature
Fig. 12. Variation with time of exit air humidity ratio
On the other hand, 2:1, 1:1, 1:2, and 1:3 in the
dehumidification performance is almost same, and it is
enough.
We discuss the profiles along the flow direction in the
desiccant bed at different time. The moisture content
distribution, the PCM temperature distribution, and the air
temperature distribution at 10min, 20min, and 30min at φd :
φp = 1:1 are shown, in Fig.15, Fig.16, and Fig.17, respectively.
In a similar way, the moisture content distribution, the PCM
temperature distribution, and the air temperature distribution
at 10min, 20min, and 30min at φd : φp = 1:3 are shown, in
Fig.18, Fig.19 and Fig.20. The X-axis expresses a distance
from the inlet. A comparison of Fig.15 with Fig.18 shows that
the moisture content of 1:3 is more than that of 1:1. One of
the reasons is that the temperature is kept low due to PCM. It
can be seen from Fig.16 and Fig.19 that the position where
the PCM temperature reaches the maximum value moves to
the outlet zone with time. The dehumidification performance
is enough high in the early stage, however the performance
gets low with time. And then, desiccant materials cannot
adsorb enough at the inlet zone. As a result it also adsorbs at
the outlet zone, and then desiccant materials produce
adsorption heat by dehumidification. In the case of 1:1, the
PCM temperature increases beyond melting point at 3cm
from the inlet at 10 minutes. The reason is that PCM have
adsorbed thermal energy as sensible heat, and changed to
liquid. In contrast, in the case of 1:3 the PCM temperature is
maintain a constant temperature. In addition, in the case of
1:1 at 20 minutes, the PCM temperature increases from the
inlet zone, and reaches the maximum value at the centre of
the desiccant bed. It is because that the adsorption heat is not
adsorbed enough by PCM at the inlet zone. Therefore, the
excessive heat is transferred to the outlet zone. In the outlet
zone the PCM temperature decreases. The results of 1:3 in
Fig.19 have the similar tendency as that of 1:1 in Fig.16, but
Fig. 13. Variation with time of exit air temperature
Fig. 14. Variation with time of exit air humidity ratio
the maximum value is lower and the position the temperature
reaches to the maximum value is near the inlet zone. This is
because of the difference of the heat capacity of PCM. In
addition, in the case of 1:1 at 30 minutes, the PCM
temperature is kept increasing until the outlet. From the
results it can be found that most of PCM have already
changed to liquid. That indicates that the amount of PCM is
not enough. Fig.17 and Fig.20 the air temperature difference
between 1:1 and 1:3 is small as far as 2 cm from the inlet at
all time. Over 2 cm the air temperature increases with time.
This is because of the excessive heat from PCM.
We should make switch while the dehumidification
performance is enough high and before PCM have changed.
Therefore, such time range is defined as the effective time.
Here, the time until PCM have changed means while the exit
air temperature is lower than 31 deg.C. And also, during the
effective time, the differences of the humidity ratio are more
than 6 g/kg-dryair. This is because that the amount of the
dehumidification required for dehumidification to the air of
25 deg.C, 50% from 25 deg.C 80% is 6g/kg-dryair. Fig.21
shows the relationship between the effective time and the
amount of dehumidification. The X-axis expresses the
effective time and The Y-axis expresses the amount of the
dehumidification during the effective time. The effective time
is longer as the filling ratio of PCM increases, and also the
amount of dehumidification increases. The slope of a line
connecting points of the origin represents the amount of
dehumidification per hour. It can be seen that the more PCM
than 1:2 the smaller slope in Fig.21. From the results the
dehumidification effectiveness is getting low. Moreover, in
the case of 1:10, the effective time is the shortest of all and
the amount of dehumidification is small. The reason is that
the dehumidification performance of 1:10 is worse than
others. It is assumed that the better filling ratio exists φd : φp =
between 1:2 and 1:3 in this condition.
Fig. 15. Moisture content distribution of [1:1]
Fig. 16. PCM temperature distribution of [1:1]
Fig. 17. Air temperature distribution of [1:1]
4.4 Effect of various velocity
From the results of 3.3 the filling ratio φd : φp = 1:3 is better.
In this section, we discuss the effect of the various air
velocity through the desiccant bed on the exit air temperature
and the dehumidification performance, in the case of 1:3.
Fig.22 shows the comparisons of the exit air temperature by
the various air velocity of 0.1, 0.2, 0.3, 0.5, and 0.6 m/s and
Fig.23 shows differences of humidity ratio between the inlet
and the outlet. It can be seen that the air temperature
increases when the air velocity is large in Fig.22, on the
contrary, the amount of the dehumidification reduces.
However, the air flow and the amount of the dehumidification
vary with the air velocity. Fig.24 shows the amount of
dehumidification per hour. It can be found that the amount of
dehumidification per hour decreases as the air velocity
increases. The energy required for a fan increases with the air
velocity. Thus, the air velocity is one of the most important
factors.
5.Conclusion
A numerical model produced with reference to the theory of
the simultaneous transfer of heat and moisture is represented.
Fig. 18. Moisture content distribution of [1:3]
Fig. 19. PCM temperature distribution of [1:3]
Fig. 20. Air temperature distribution of [1:3]
By using this model, the dehumidification performance for a
desiccant bed packed with PCM and desiccant materials has
been studied numerically. Based on the results of the
calculation, the spatial distribution and time variation of the
moisture content, the PCM temperature, the air temperature,
and the relative humidity are clarified. And then it is
indicated that by analyzing the variations in the air
temperature and the relative humidity, it is possible to easily
estimate the air temperature and the relative humidity at the
outlet of the desiccant bed. From the results, it was found that
the exit air temperature is kept low due to PCM, and the
amount of dehumidification increases. Also, it was found that
the filling ratio, α’Sd, the air velocity are important factors for
the dehumidification performance and consumption of energy.
For the future, the validity of the numerical simulation
method for the desiccant bed will be examined by the
comparison between the experiment and the numerical
simulation of the desiccant dehumidifier. And then
dehumidification performance will be further generalized for
the air flow rate and the filling ratio. In addition, we would
investigate a numerical method which simulates combined
heat and moisture transfer process during moisture desorption
and PCM freezing.
Fig. 21. Dehumidification performance evaluation
Fig. 22. Variation with time of exit air temperature
Fig. 23. Variation with time of exit air humidity ratio
Fig. 24. Variation with time of dehumidification amount per
hour
Acknowledgements
This work was partly supported by TOSTEM Foundation for Construction Materials Industry Promotion (09-53, Representative Y. Momoi).
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Nomenclature