Experimental study on a humidification and dehumidification desalination system of solar air heater with evacuated tubes

In this paper, a small scale solar humidification–dehumidification (HDH) experimental setup based on a new kind of solar air heater with all-glass evacuated tubes is designed and tested. Firstly, a new kind of solar air heater with evacuated tubes is designed and tested for the solar HDH desalination process. The test results of solar air heater show that the cut length of efficiency and overall heat loss coefficient is 0.47 and 1.60 respectively, while air flow rate is 140 m3/h. Secondly, the humidifier and dehumidifier are designed and optimized by the mathematical design methods. Finally, a desalination pilot plant is designed and built. And then, operation characteristics are tested and analyzed. Test results show that different inlet sprayed water temperature in the pad humidifier from 9 °C to 27 °C can effectively improve relative humidity of outlet moist air from 89% to 97% and the outlet air temperature from 35 °C to 42 °C. The results are valuable in the pursuit of the optimal design for a 1000 L/day solar HDH desalination system with the new kind of solar air heater. © 2014 Elsevier B.V. All rights reserved.

Fig. 11. Temperature curves of the sprayed water temperature with 27 °C. approximately 89% in Fig. 10. At the same time, Fig. 11 shows that the outlet air temperature of the pad humidifier is increased to 42 °C compared with 35 °C in Fig. 9. That means more effective driving force in the mass and heat transfer processes because of the higher temperature and relative humidity. Through the thermodynamic behavior of moist air, the outlet air from the pad humidifier possesses higher enthalpy. Therefore, the higher thermal performance in the pad humidifier can be obtained by increasing the sprayed temperature from 9 °C to 27 °C. Here higher outlet air temperature and relative humidity can effectively increase the absolute water content of the air flow, which will improve fresh water production of the plant under the same cooling condition. Additionally, the lower relative humidity and higher temperature of outlet air from solar air heater field in Fig. 12 can improve heat and mass transfer performance between the air flow and the sprayed water in the pad humidifier. 5.4. Fresh water productivity From Figs. 11 and 12, the outlet moist air with average 42 °C and 97% relative humidity from the pad humidifier is the position with the enthalpy of 181 kJ/kg and the water content of 53.73 g/kg. Compared with Figs. 9 and 10, the outlet moist air with average 35 °C and 89% relative humidity from the pad humidifier is the position with the enthalpy of 119 kJ/kg and the water content of 32.75 g/kg. So, in the fixed air flow rate and the same cooling water, it has almost produced additionally more than 21 g fresh water per kilogram air in theoretical way. Through Fig. 12. Relative humidity curves of the sprayed water temperature with 27 °C.