## 3 Results and Discussion  
### Exergy Efficiency of the AGMD Modules  
Results from the permeate production rates for the two types of AGMD units are summarized in Figure 5 in terms of kilogram per square meter of membrane per second (kg/m\({}^{2}\)s). The flux decreases as the temperature difference decreases across the MD module, which is due to the decline in the rate of condensation and concomitant drop in transmembrane transport of vapor across the membrane as the temperature difference decreases. Higher specific heat demands accompany this increase in permeate flux with increasing \(\Delta\)T. The lower feed temperature condition generally shows a lower permeate flux, as observed for Xzero module's performance at 65 \({}^{\circ}\)C and 80 \({}^{\circ}\)C. When fluxes from the two MD modules are compared, the flux from Elixir500 MD unit is nearly three times higher than that from the Xzero MD unit at the same feed temperature and \(\Delta\)T (65 \({}^{\circ}\)C). The other thermodynamic performance parameters: exergy efficiency, exergy destruction, and specific heat input are summarized for both modules in Figures 6 and 7. For both modules types, exergy efficiency improves when the feed-coolant temperature difference goes up. For the Xzero unit at a feed of 80 \({}^{\circ}\)C, exergy efficiency more than doubled as \(\Delta\)T increased from 30 \({}^{\circ}\)C to 65 \({}^{\circ}\)C. This  
Figure 5: Permeate flux of Xzero and Elixir500 AGMD systems regarding permeate flux for different temperature differences at feed temperatures of 80 \({}^{\circ}\)C and 65 \({}^{\circ}\)C.  
increase is linked to the improved heat recovery at the heat sink for a higher \(\Delta\)T. This is evidently observed from the decrease in exergy destruction at a higher \(\Delta\)T compared to lower levels (Figure 7). The trend is similar for both feed temperatures of 80 \({}^{\circ}\)C and 65 \({}^{\circ}\)C. However, the lower feed temperature condition generally shows lower exergy efficiency and higher exergy destruction when compared to the corresponding performance at higher feed temperatures. For this MD unit, it can be said that a higher feed temperature and \(\Delta\)T favor higher flux and exergy efficiency and lower exergy destruction than lower \(\Delta\)T cases, of course at the expense of additional specific heat demand. Similarly, for the bench scale AGMD unit, the general trend in the performances with a change in \(\Delta\)T is the same as that of the results from the pilot-scale AGMD system. The effect of improved condensation in the Elixir500 module is reflected in the higher specific heat input owing to the higher heat transfer across the module. Hence, higher exergy efficiency and lower exergy destruction is  
Figure 6: Performance of Xzero and Elixir500 AGMD systems regarding exergy efficiency and specific heat demand at different feed–coolant temperature differences for fixed feed temperatures of 80 \({}^{\circ}\)C (for both modules) and 65 \({}^{\circ}\)C (only for Xzero). Figure 7: Exergy destructions and specific heat demand at different feed–coolant temperature differences for fixed feed temperatures of 80 \({}^{\circ}\)C (for both modules) and 65 \({}^{\circ}\)C (only for Xzero). achieved for the Elixir500 unit as compared to the Xzero module. The lower specific heat demand for the Xzero module results from the reduced heat transfer across the condensation plate in comparison to the other module, though the Xzero showed severe heat losses from the frames and cover to the surrounding atmosphere by free convection [8]. For both types of MD systems, it is evident that a lower temperature difference across the module reduces performance, including the permeate flux and exergy efficiency. As the pressure drop across the AGMD modules is low (0.01 to 0.02 MPa) and mass balance is achieved across the MD unit, exergy losses from pressure and concentration changes are not significant.