Investigation of a novel dew point indirect evaporative air conditioning system for buildings

This study aims to improve the performance of existing indirect evaporative coolers. A new dew point indirect evaporative cooler with counter-current heat/mass exchanger was developed in this research by optimal design, material selection, numerical simulation, experimental investigations and economic, environmental, regional acceptance analysis. A new dew point heat/mass exchanger using a counter-current flow pattern was designed by numerical simulation in terms of material, structure, geometrical sizes and operating conditions. The numerical results indicate that under a typical cooling design condition, i.e., 35oC dry-bulb/24oC wet-bulb temperatures, the heat exchanger could achieve a wet-bulb effectiveness of approximately 1.4. The results of numerical simulation are consistent with some published test data. Based on the numeric results and the material selection determined from a set of related tests, a prototype dew point heat/mass exchanger and the associated air cooler was designed and constructed in laboratory. Testing was carried out to evaluate the performance of the experiment prototype. The results indicate that the wet-bulb effectiveness of the prototype ranged from 55% to 110% for all test conditions. The power consumption of the prototype ranged from 10 to 50 W with energy efficiency (or COP) rated from 3 to 12. It is also found that the water consumption of the prototype was very small which ranged from 0.2-1.3 litre/h. A comparison between the numerical and experimental results was carried out and the reasons for the discrepancy were analysed. This research also investigates the feasibility, economic and environmental potential of using a dew point cooler in buildings in Europe and China. From the related studies in this thesis, it is concluded that the dew point cooler can achieve a higher performance (in terms of effectiveness and energy efficiency) than the typical indirect evaporative coolers without adding too much cost. It is found that the effectiveness and energy efficiency of the heat/mass exchanger in the cooler are largely dependent upon channel geometries, the intake air velocity, temperature, humidity and the working-to-intake air ratio but less on the feed water temperature. To maximise effectiveness and energy efficiency, it is suggested that 1) the channel height and the length of exchanger should be set below 6 mm and 1-1.2 m respectively; 2) the intake channel air velocity should be controlled to 0.5-1 m/s; and 3) the working-to-intake air ratio should be adjusted to 0.4-0.5. It is also concluded that the dew point system is suitable for most regions with dry, mild and hot climate. It is, however, unsuitable for humid regions where the system is used as a stand-alone unit. Compared to the conventional mechanical compression cooling system, the dew point system has a significantly higher potential in saving energy bills and reducing carbon emission. A project to construct an 8 kW commercial dew point cooler is currently under development with the assistance of a Chinese company. By the optimisation of material, structure and geometries, the cooler is expected to achieve a cooling output of 8 kW at the inlet air of 38oC dry-bulb/ 21oC wet-bulb temperatures, with a wet-bulb effectiveness of 1.02 at 1530 m3/h of supply air flow and 1200 m3/h of discharge air flow, whereas the power input of the unit is about 450 W and the energy efficiency (or COP) at 18.5.