Feasibility of using desiccant cooling system in hot-humid region

Maatouk Khoukhi Civil and Architectural Engineering Department

Sultan Qaboos University Muscat, Oman

mkhoukhi@squ.edu.om

Shahad Abbas Jassim Civil and Architectural Engineering Department

Sultan Qaboos University Muscat, Oman

Abstract—This paper discuss the feasibility of using desiccant cooling cycle which combine sorptive dehumidification with heat recovery to create a cooling process in hot and humid area. The performance of the desiccant cooling system component has been determined previously. The result of the simulation shows that such system is a feasible solution for cooling buildings in hot-humid region.

Keywords-component; desiccant wheel; heat wheel; hot-humid region, cooling

I. INTRODUCTION The desiccant based air-conditioning system utilizes the

capability of desiccant materials (silica-gel coated) in removing the moisture content in the air by sorption process. All materials that attract moisture at different capacities are called desiccants [1]. The removal of moisture from the desiccant wheel (regeneration) can be done either by heating or by reducing the pressure. The main purpose of this preliminary study is to investigate the feasibility of using cycle which combines sorptive dehumidification with heat recovery to create thermal comfort. The experimental study has been conducted in laboratory in Tohoku University (Japan) to determine the performance of the desiccant system components. The simulation results were compared with the experimental ones and presented here.

II. EXPERIMENTAL FACILITY The experimental facility has a physical set-up shown in

Fig. 1. It has two controlled chambers in which chamber A is used to simulate the outdoor air condition and chamber B for the indoor air condition [2]. The first chamber has temperature range from -10°C to 40°C with accuracy of 2%. For chamber B, the operating temperature range is from 10°C to 40°C with accuracy of within 1%. For both chambers the humidity could be varied depending on the needed conditions.

The technical description of the facility is shown in Fig. 2. The source of raw air for both controlled chambers is from ambient air. It is forced to flow into the chambers by fans. If the temperature set-value is lower than that of the ambient air then the cooling process operates. Otherwise, the heating process operates. Similar process is used for the humidity control. If the ambient air humidity is high, dehumidification

process operates. Otherwise, the humidification process operates. The air is moved from the chambers to the testing one using fans.

The main components of the desiccant based system consist of desiccant dehumidifier wheel, heat recovery wheel and heater. The performance of the whole system is dependent on the performance of its components. This task has been already conducted in the laboratory in Tohoku University [2] and the main results are presented below. The parameters considered for the evaluation were the rate of volumetric flow, the regeneration temperature and the wheel rotational speed (see Table 1).

Fig.1 Experimental facilities

Table 1. Parameters used to evaluate the performance of the desiccant cooling system

Rate of volumetric flow [m3/hr]

100 200

Regeneration temperature [°C]

60 70 80

Wheel rotational speed [RPH]

5 10 20 25 30 35 40 50 60

978-1-4577-0547-2/12/$31.00 ©2012 IEEE

Fig.2 Technical description of the experimental facility.

III. EXPERIMENTAL RESULTS It has been shown that the desiccant wheel performance depends upon its rotational speed. For 100 m3/h the optimum speed is 30 RPH and for 200 m3/h the optimum speed is 50 RPH. Also the regeneration temperature is an important factor in the effectiveness of desiccant dehumidifier moisture adsorption [2]. On the other hand, the moisture removal capacity for both cooling and regeneration processes regarding the desiccant wheel is found to be maximum for high regeneration temperature around 80ºC and the desiccant wheel revolution speed between 20 to 30 RPH. The effectiveness of the heat wheel for 20 RPM wheel speed is 81% and 69% for 10 m3/h and 200 m3/h, respectively [2].

IV. MODELING AND SIMULATION The basic model of the desiccant cooling system was

implemented in TRNSYS [3]. Standard component such as desiccant wheel, the heat wheel and the heater were used to simulate the behavior of the whole system under hot-humid condition of the outside air. In TRNSYS, the model of the desiccant dehumidifier is based on analogy between the heat transfer of the sensible heat exchanger and of the desiccant heat and mass transfer.

The input data of the desiccant wheel are the two effectiveness value proposed by Banks which is discussed in TRNSYS manual.

Fig. 3 shows the schematic diagram of the model which was under the experimental investigation. This model is used for comparison with the experimental data obtained previously.

V. RESULTS AND DISCUSSION The model presented in Fig. 3 has been validated against

the experimental published data by Napoleon et al [2]. The result of the simulation is plotted in Fig. 4.

This set-up is used for comparison with the modeling done in TRNSYS. The model of the desiccant wheel in TNSYS has no provision for direct comparison with experimental results. Therefore, series of trials have been done. The final trials are presented in Table 2.

Table 2 shows the comparison between the experimental data and the simulation data. The Simulation results of the DBT AND Rh are within the accepted value compared with the experimental ones. The differences are mainly due to the initial value of some intrinsic parameters of the model such as the desiccant wheel effectiveness and the heat wheel effectiveness. Those parameters should be thoroughly investigated and adjusted in the future.

The result shows that the desiccant cooling system combined with heat wheel and heater for the desiccant wheel regeneration could be an alternative solution for the indoor thermal comfort. More refinement of the setting condition of the whole system is required to achieve better extraction of moisture from the inlet air.

The simulation shows that the model is limited in term of input data of the DBT. Indeed, for very hot climate the TNSYS does not have the capability of converging towards the solution. Therefore, for such climate a pre-cooling system using indirect evaporative cooler IEC will solve the problem (see Fig. 5).

0

1

2

3

4

560

10203040506070

0 10 20 30 40 50 60 70 80 90

Rela

tive

Hum

idity

%

Dry Bulb Temperature °C

Fig. 4 Air process through the desiccant wheel, the heat wheel and the heater.

Fig. 3 Schematic diagram of the desiccant cooling model

Supply Air

Return Air Exit Air

Outdoor Air

DW

HW

F

F

F: Fan H: Heater DW: Desiccant Wheel HW: Heat Wheel

0 1 2

3 4 H 5 6

Table2. Simulation results compared with the experimental ones

The simulation of the desiccant cooling model under three different climates for temperate climate (little bit warm and humid), for the mediterranean (warm and humid) and, of the tropical climate (hot and humid for the all-year around) is presented in Table 3. As a result the DBT have been decreased and the amount of the Rh is considerably reduced for the three climates.

It is obvious that as mentioned previously that in case of

extreme harsh climate (i.e. DBT=44°C and Rh=80%) the IEC will be used before the air enters the DW to reduce the DBT and then the moisture in the air will be absorbed by the DW [5, 6].

Table 3. Simulation results in three different climates

Fig. 5 Schematic diagram of the desiccant cooling model with indirect evaporative cooler

VI. CONCLUSION

The basic model of the cooling system presented in this paper combining the desiccant wheel with heat wheel can be an appropriate solution to reduce the moisture content in the air and its DBT for thermal comfort in buildings.

The model using TRNSYS software has been validated against the experimental data done in Tohoku University in Japan.

The model has been also simulated in three different climates, and the result shows that a huge decrease of Rh can be achieved using the system with source of heat for the DW regeneration.

Further experimentation is needed to complete the study and more components will be added to the basic model such IEC for the reduction of the supply inlet temperature in case of very hot climate.

REFERENCES

[1] N. Enteria and K. Mizutani, “The role of the thermally activated desiccant cooling technologies in the issue of energy and environment,” Renewable and Sustainable Energy Reviews, vol. 15, pp. 2095-2122, 2011.

[2] N. Enteria, H. Yoshino, R. Takaki, A. Satake, A. Mochida, M. Khoukhi, R. Yoshie, T. Mitamura and S. Baba,” The experimental works and some parametric investigations of thermally activated desiccant cooling system,” Proceedings of Clima 2007 WellBeing Indoors.

[3] TRNSYS 16, Solar Energy Laboratory, University of Wisconsin-Madison, USA.

[4] M. Khoukhi, H. Yoshino, A. Mochida, N. Enteria, R. Takaki, A. Satake, R. Yoshi and T. Mitamura, “Study of the conventional twin rotor desiccant cooling system. Part 1: experimental set up,”Summaries of Technical Papers of Annual Institute of Japan, pp. 1109-1110, 2006.

[5] N. Enteria, H. Yoshino, A. Mochida, M. Khoukhi, R. Takaki, A. Satake, R. Yoshi and T. Mitamura, “Study of the conventional twin rotor desiccant cooling system. Part 2: Modeling and parametric investigation,”Summaries of Technical Papers of Annual Institute of Japan, pp. 1111-1112, 2006.

[6] S.P. Halliday, C.B. Beggs and P.A. Sleigh,”The use of solar desiccant cooling in UK: a feasibility study”, Applied Thermal Engineering, vol. 22 pp. 1327-1338, 2002.

0 1 2 3 4 5 6DBT [°C] Experimentation 30.8 59.2 33.5 26.1 52.6 80 51.9DBT [°C] Simulation 30 48 29 26 45.5 80 49RH [%] Experimentation 58.5 9.1 35.3 55.5 13.3 3.9 20.1RH [%] Simulation 60 12.5 36 55 18 3.5 12.5% Difference

DBT 2.5 18.9 13.4 0 13.4 0 5.5RH 2.5 27.2 2 1 26 10 37.8

Climates DBT [°C]

RH % Simulation results of supply

air (State 2) DBT [°C]

Rh[%]

Tropical 35 60 31.5 31Mediterranean 30 55 28.5 37Temperate 28 50 28.3 37.5

Supply Air

Return AirExit Air

Outdoor Air

DW

HW

F

F

F: Fan H: Heater DW: Desiccant Wheel HW: Heat Wheel IEC: Indirect Evaporative Cooler

0 1 2

3 4 H 5 6

IEC