Experimental study of a multiple-effect humidification solar desalination technique

The object of this research is to experimentally investigate the principal operating parameters of a new desalination process working with an air multiple-effect humidification–dehumidification method. A test set-up was designed and constructed to carry out and optimize this technique. The main parts of the present set-up consist of a heat equipment device (heat exchanger), a spray humidifier and a dehumidifier system. This equipment was used to simulate the seawater desalination process experimentally with an eight-stage air solar collector heating-humidifying system. The outlet temperature of the air solar collector was correlated for use in the desalination process as a solar heating device. The operating conditions studied were: ratio of water to dry air mass flow rate through the system, humidifier inlet absolute humidity, dry air mass flow rate through the system and solar irradiation or humidifier inlet air temperature. The experimental results obtained were used to put stress and correlate the influence of the different operating conditions on the behavior of the eight-stage air heating-humidifying desalination process. The ratio of water to dry air mass flow rates was optimized, precisely 45%. The value of dry air mass flow rate through the system can be also varied with solar radiation in order to have a maximum of humidity content at the end of the system and though working in an adiabatic humidification process.

The present investigation carried out on the one-stage experimental set-up has permitted optimization of the operating parameters of the solar desalination pilot plant by a multiple-effect humidification–dehumidification process under construction in Tunisia. The effect of these operating parameters on the air thermodynamic behavior flowing in the pilot plant were also studied. Recommendations for this pilot plant are: the water to air mass-flow ratio: R = 45%, the air mass flow rate: Mair = 790 kg/h, and number of heating-humidifying stages: 5. These results are specific for the Tunisian climatic conditions of 590 W/m2 average annual solar radiation. It is interesting to note that all experiments carried out on the one-stage setup of Bochum to experimentally simulate the pilot plant have used tap water. In future tests that have to be done at INRST, brackish or seawater should be used. Also, the salinity of the water has to be measured before humidification and after dehumidification in order to be certain that the water produced does not contain transported injected water. Also, the one-stage set-up used at Bochum was constructed with a spray humidifier. However, experiments carried-out at AUA (Athens) [21] proved that a pad humidifier is more appropriate for air humidification in the present project. Moreover, AUA proposed an optimized design for such a purpose [21] that will be used in the pilot plant under construction in Tunisia. Collectors were characterized and tested with normal air (water content: 10 g/kg). However, the air processed in the SOLDES pilot plant reaches an absolute humidity of about 150 g/kg. Thus, in the near future, collectors will be tested in the pilot plant with an airflow of higher vapor content in order to see problems due to water condensation in the collectors.