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).

References

[1]Waugaman DG, Kini A, Kettleborough CF. A review of desiccant cooling systems. Journal of Energy Resources Technology 1993;115:1–8.

[2]Ruthven DM. Principles of adsorption and adsorption processes. New York: Wiley; 1994. p. 5–7.

[3]Shen CM, Worek WM. Cosorption characteristics of solid absorbent. International Journal of Heat and Mass Transfer 1994;37(14):2123–9.

[4]Saito Y. Regeneration characteristics of adsorbent in the integrated desiccant/collector. Journal of Solar Energy Engineering 1993;115: 169–75.

[5]Matsuki K, Saito Y. Desiccant cooling R&D in Japan. ASHRAE Transaction: Symposia 1988;DA-88-1-2:537–51.

[6]Dupont M, Celestine B, Nguyen PH, Merigoux J, Brandon B. Desiccant solar air conditioning in tropical climates. II: field testing in Guade Lope. Solar Energy 1994;52(6):519–24.

[7]Techajunta S, Chirarattananon S, Exell RHB. Experimental in a solar simulation on solid desiccant regeneration and air dehumidification for air conditioning in a tropical humid climate. Renewable Energy 1999;17:549–68.

[8]Yoshie, R., Momoi, Y., Satake, A., Yoshino, H. and Mitamura, T., Numerical Analysis of Heat and Moisture Transfer in Desiccant Wheel for Dehumidification, Proceedings of Clima 2007 WellBeing Indoors Abstract Book, p.531, 2007.

[9]Momoi, Y., Yoshie, R., Satake, A., Yoshino, H. and Mitamura, T., Performance Prediction of Rotary Desiccant Wheel, Proceedings of the 29th AIVC Conference, Volume3, pp.297-302, 2008.

[10]Matsumoto, M., Simultaneous Heat and Moisture Transfer in Porous Wall and Analysis of Internal Condensation, Energy Conservation in Heating, Cooling, and Ventilating Buildings, Proceedings of 1977 International Seminar of Heat and Mass Transfer, Dubrounik, Vol.1, pp.45-58, 1978.

[11]Ochiai N, Sagara K, Yamanaka T, Kotani H, and Momoi Y, Numerical Simulation of Natural Ventilation Performance of Solar Chimney with Built-in Phase Change Material, Proceedings of Advanced building ventilation and environmental technology for addressing climate change issues, Volume1, pp.207-212.

[12]AIJES-H001-2006. Academic standards for measurement of moisture properties.

Nomenclature