Incorporating the regenerative evaporative cooler and the desiccant rotor, a prototype of the desiccant cooling system was built in the configuration shown in Fig. 2(a). As the sensible heat rotor, an appropriate commercial product was selected for the effectiveness of 80% at the process air flow rate of 20 CMM. The direct evaporative cooler at the supply air outlet was omitted in the prototype for simplicity. Since the process in the direct evaporative cooler is adiabatic, the omission of it does not influence the cooling performance. The exterior dimension of the completed prototype is 700(W) x 800(D) x 1,900 mm(H).

The prototype was tested at the ARI condition (indoor: 27oC, 50%RH, outdoor: 35oC, 40%RH) for performance evaluation. The regeneration air temperature was 59.5oC. The ventilation ratio was

0. 3, that is, the supply air comprises of 30% outdoor air and 70% recirculation air. The supply air flow rate was 14.6 CMM and the regeneration air flow rate was 14.0 CMM. The rotation period of the desiccant rotor was set at 450 s.

The psychrometric diagram of the desiccant cooling process is depicted in Fig. 7 from the

0.03

0.025

0.02

0.015

0.01

0.005

measured data of the temperature and the humidity at various points in the desiccant cooling system. The circled numbers in the figure imply the numbered locations in Fig. 2(a). For comparison, the expected at the design stage from the simulation is also shown in the figure. It is seen that the actual operation of the prototype follows closely the process as was designed.

The cooling capacity and COP are evaluated with following equations:

Qcool P5V pout (i5 i1)

where Qcool is the cooling capacity, Qreg the regeneration thermal energy input, Welec the auxiliary electric input, i the enthalpy, and Vpout is the supply air flow rate.

The cooling capacity was evaluated as 4.41 kW and COP as 0.762. It should be noted that this performance was obtained with the regeneration air temperature of 59.5oC.

The electric input was assessed mostly for the fans. The pressure drop in the process air side was 430 Pa and that in the regeneration air side was 390 Pa. Based on the measured values of the flow rate and the pressure drop, the fan efficiency is estimated around 35%, which is quite low for the fan in this range of flow rate. By improving the fan efficiency, it is quite clear that the system efficiency can be improved substantially. As an ultimate case, the COP based only on the thermal energy input is found as 0.867.

It should be also noted that the cooling performance in Table 1 was obtained with the ventilation ratio of 0.3. If the cooling of the ventilation air is accounted in the evaluation of the cooling performance, that is, if i2 is used instead of i1 in Eq. (1), the cooling capacity is evaluated as 5.8 kW and COP as 1.0.

3. Conclusion

In this study, a prototype of the desiccant cooling system incorporating a regenerative evaporative cooler was designed, fabricated and tested for the performance evaluation. To this purpose, two important components, i. e., the regenerative evaporative cooler and the desiccant rotor were developed and assembled into the system.

The regenerative evaporative cooler is to cool a stream of air using the evaporative cooling effect without an increase in the humidity ratio. It is comprised basically of a pair of dry and wet channels and the evaporation water is supplied only to the wet channel. By redirecting a portion of the air flown out of the dry channel into the wet channel, the air can be cooled down to a temperature lower than its inlet wet-bulb temperature at the outlet end of the dry channels. The regenerative evaporative cooler was built by compiling the multiple pairs of dry and wet channels. The two channels were separated by a thin flat plate and metal fins were inserted into both the channels to extend the contact surfaces improving the compactness of the cooler. A desiccant rotor was fabricated using the polymeric desiccant newly developed in Korea Institute of Science and Technology (KIST). To fabricate the desiccant rotor, firstly the polymeric desiccant was prepared by ion modification of the super absorbent polymer and laminated by coating the

desiccant on a 0.1 mm thick polyethylene sheet. Then the sheet was corrugated and rolled up into a rotor.

Incorporating the regenerative evaporative cooler and the desiccant rotor, a prototype of the desiccant cooling system was built. The exterior dimension of the completed prototype is 700(W) x 800(D) x 1,900 mm(H). The prototype was tested at the ARI condition (indoor: 27oC, 50%RH, outdoor: 35oC, 40%RH) for performance evaluation. With the regeneration air temperature of 59.5oC and the ventilation ratio of 0.3, the cooling capacity was measured as 4.41 kW and COP as 0.762.

References

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