Experimental and numerical investigation of humidification/ dehumidification solar water desalination systems

In this work, a humidification–dehumidification process is considered as an advanced technique to design and construct a solar water desalination system. In this system, the salt water is heated in a solar collector before flashing inside an insulated chamber to be evaporated and condensed naturally later on where the rest of saline water is re-circulated into the system. Three systems are manufactured and tested in Cairo, 308N. The salt water is circulated in a separate loop for the first and second systems. In the first system, an auxiliary heater is used to switch the temperature of the salt water where it is naturally switched in the second system depending on the weather conditions. The third system is an open system in which the salt water is used as a collector fluid. In all three systems, a thermosyphon solar water heater is used as a heat source consisting of a collector and a storage tank. A numerical simulation was developed for the considered systems to predict their annual performance and desalinated water production. An agreement between the measured and simulated data was obtained. It is found that the third system is the most efficient one and it has the lowest cost. Moreover, the first one has the greatest distilled water with higher temperatures and flow rates.

A multi-effect humidification-dehumidification solar desalination system was installed and successfully tested in Cairo 308N. A numerical simulation for the system was developed and validated with experimental measurements. Three systems were compared experimentally and numerically. It is found that the solar open system with natural circulation is more efficient regardless of practical difficulties where the closed system using an auxiliary heater has the highest distilled water production. The design of the desalination chamber especially the evaporator performance has a significant effect on the system output. Nomenclature Subscripts 1, 2, . . ., 14, e1, e2 are indicated in Fig. 1 Collector area, m2 Ac Ach Desalination chamber surface area, m2 DES4368.3d 6/24/2009 13:31:13 Aco bo c CP DC DW F’UL FR F R UL FR(ta)n FR(ta) G Gtest H hch hfg IbT Id Ig IT ke L mh mL Condenser surface area, m2 Incidence angle modifier constant from ASHRAE 93-77 test result Condenser Specific heat of working fluid, kJ/kg C Distillation chamber Distilled water Product of the collector efficiency factor, F’, and heat loss coefficient, UL,W/m2C Collector heat removal factor Slope of the collector efficiency versus (Tci – Ta)/IT curve, W/m2.C Intercept of the collector efficiency versus (Tci – Ta)/IT curve Intercept efficiency corrected for non-normal incidence Collector flow rate per unit area, kg/m2 Collector flow rate per unit area at test conditions at which FRUL and FR(ta)n were determined, kg/m2 Enthalpy, J/kg Heat transfer coefficient inside the desalination chamber, W/m2.C Enthalpy difference of the air enclosed in the desalination chamber, J/kg. Beam radiation per unit area, W/m2 Horizontal diffuse radiation per unit area, W/m2 Diffuse reflected ground radiation per unit area, W/m2 Total incident radiation per unit area, W/m2 Thermal conductivity of air inside the desalination chamber, W/m.C Height of the condenser, m. Mass flow rate of hot stream entering tank, kg/s Mass flow rate of desalination chamber, kg/s md ma Nx Qu RH T Ta Tck TH,TL Ti TL Tw Tsat UL Uch Vh Vi VL b DTH DTL m g y re rw ta tan tas tan ; tab ; tan ta ; tag n Distilled water flow rate, kg/s air flow rate inside the desalination chamber, kg/s Number of equal sized collector nodes Rate of useful energy collection, W Relative humidity Temperature, 8C Ambient temperature, 8C Temperature of kth node in collector, 8C Upper and lower temperatures, 8C Temperature of ith segment, 8C Temperature of desalination chamber stream entering tank, 8C Temperature of distilled water, 8C Saturated of distilled water, 8C Overall heat loss coefficient, W/m2 Overall heat loss coefficient of the desalination chamber, W/m2 Volume of fluid entering tank from heat source over a time interval Dt, m3 Volume of ith segment, m3 Volume of fluid entering tank from desalination chamber over a time interval Dt, m3