Study of a water desalination station using the SMCEC technique: dynamic modelling and simulation

The dynamic modelling and simulation of the three sections of a water desalination facility using the Solar Multiple Condensation Evaporation Cycle (SMCEC) technique are presented. The models are obtained using thermal energy and mass balances of the different sections of the unit. The resulting distributed parametric systems of equations are transformed into a system of ordinary differential equations using the orthogonal collocation method. The parametric study in the dynamic mode and the numerical simulation allow to predict the behaviour of output of each of the three unit sections following variations in the internal signals and external perturbations.

As the operation of this type of desalination installation is very sensitive not only to variations of its input signals but also to meteorological condition perturbations, the mathematical modeling of the solar collector, the evaporation and condensation towers in the dynamic mode of operation obtained the best knowledge of the real process and the spatial distribution as well as the time evolution of the unit parameters. The parametric study carried out using the dynamic simulation of the solar collector was interesting. First, the fluid temperature at the collector exit increased with the solar flux and the fluid temperature at the entrance and second, the fluid temperature at the collector exit decreased as the fluid debit increased. Therefore, it would be recommended to work with a low Table 3 Condensation tower simulation results Case 2 Constant Constant Constant Constant Variable Case 3 Constant Variable Constant Constant Constant Case 4 Variable Constant Constant Constant Constant Operating Tc1=25°C, Dc=0.2 kg s–1 conditions G = 0.3 kg s–1 t≤20 mn, T G2 = 32°C XG2= 0.030645 t > 20 mn, TG2 = 45°C XG2 = 0.06433 G =0.3 kg s–1, T c1=25°C TG2 = 3 2 ° C XG2= 0.030645 t ≤ 20 mn, D c= 0.2 kg s–1 t>20 mn, Dc=0.4 kg s–1 Dc = 0.2 kg s–1 G = 0.3 kg s–1 TG2 = 3 2 ° C XG2 = 0.030645 t≤20 mn, T c1= 25°C t >20 mn, Tc1 = 15°C Dc = 0.2 kg s–1 Tc 1 = 2 5 ° C TG2 = 3 2 ° C XG2 = 0.030645 t≤20 mn, G = 0.3 kg s–1 t>20 mn, G = 0.15 kg s–1 Tc2 (t) Wc (t) Notes: No change No change Decreases Dc Decreases Dc 40 % reduction in cooling water temperature improved Wc by 69.87%; Use low cooling temperature for best production Increases with G Increases with G 50% variation in G produced 21.75% variation in Wc; G has more impact on Wc than Tg2 G(t) Dc(t) Tg2(t) Xg2(t) Tc1(t) Case 1 Constant Constant Variable Constant Constant Increases with Tg2 Increases with Tg2 Wc is sensitive to Tg2, A 40% rise in Tgc increased Wc by 264.5% (3.75 kg/h) water debit so as to have the highest possible temperature at the collector outlet. The optimal operation of the evaporation tower requires also a significant water debit, a high water temperature, and finally a minimum air debit at the entrance level. Therefore, the following compromises should be made: • the use of storage tank permitting the fluid circulation in a closed circuit between the solar collectors and the tank. This solution has the advantage of increasing water temperature and enabling the operation with a relatively high water debit. However, it creates a discontinuity in the process since during the operation in a closed circuit, water does not enter the evaporation tower; • the use of multiple solar collectors connected in series and parallel combinations to insure a very high water temperature due to the series set-up and a high water debit due to the parallel set-up; • the use of recycling technique. This would reclaim the amount of hot water that did not evaporate from the bottom of the evaporation tower and mix it with the hot water coming from the solar collector before injection into the evaporation tower. This technique would increase the water debit but with a limited temperature improvement. • Operation with a low air debit in the tower guarantees a high temperature at the tower exit with a significant water content. Following this study, it can be concluded that the optimal operation of the condensation tower requires at the entrance level a high humid air