# Reverse Electrodialysis: Potential Reduction in Energy and Emissions of Desalination
## 3 Results and Discussion
### Energy Savings to Desalination from RED  
While energy resource quality (i.e., feed concentration and temperature) and quantity (feed volume) significantly affect RED performance and thus appropriateness for implementation, it is the SWRO plant's energy demand (SEC ranging from 3.0-5.5 kWh per cubic meter of water desalted across selected emplacements) what will ultimately define the potential energy savings RED could offer to desalination. Building on RED plant's performance assessment, we estimated that the grid mix share of total desalination plants' SEC RED could relieve (i) if reached the thermodynamic limit--Gibbs free energy of mixing, Equation (A1)--and (ii) in the current state of development (Net Specific Energy). Figure 4 displays the salinity gradient energy share of total SEC of desalination plants along with the respective SEC values (given in the relevant references listed in Table 1); the Gibbs free energy per unit volume of desalted water entering the RED system--denoted in Figures 4 and 5 by the pie chart's green and yellow sectors--sets the upper energy savings to drive desalination [70]. If all salinity gradient's chemical energy was converted to useful work, almost in all scenarios, RED's energy supply could potentially save 33% to 42% to desalination plants except Fukuoka (scenario 7, Figure 4), where the exergy input hardly reaches a quarter of its specific energy demand (22%), and Glen Rocky (scenario 8, Figure 4) where, as opposed to Fukuoka, the SEC is more than halved (57%). Even so, as the overall energy balance among locations (left graph in Figure 5) infers, the net energy output of the RED plant differs from the maximum extractable work (Input exergy (In), Figure 5), given (i) RED's energy conversion process is irreversible (Pump and Internal losses, Figure 5), and (ii) a fraction of the attainable SGE remains untapped (Out, Figure 5), which altogether makes up the Gibbs free energy unused and wasted (U + W \(\Delta\)Gmix, denoted by the green sector in Figures 4 and 5). Overall, the exergy recovered for conversion (Retrieved Exergy, Figure 5)--i.e., the difference between the exergy entering and leaving the RED system--accounts for 86-90% of the input exergy, of which 26% to 31% is converted to useful work, resulting in net energy yield of the RED plant--i.e., the input exergy fraction converted to useful work--of 22-28% (Net, Figure 5); the net energy output of the RED plant thereby could meet \(\sim\)10% of SWRO plant's demand in all locations, aside from two opposed scenarios already instanced: Fukuoka (scenario 7, Figure 4), where the net energy's share is halved as it features the highest SEC along with the lowest net specific energy (Net SE, Figure 3), and Glen Rocky (scenario 8, Figure 4), where the RED plant satisfy \(\sim\)14% of the SWRO plant's needs since it is the least energy-demanding desalination plant. Interestingly, although the Al Dur facility (scenario 1) is as energy-intensive as Fukuoka, the net energy share is doubled provided the net specific energy delivered to Al Dur (0.58 kWh m\({}^{-3}\)) is twice Fukuoka's (0.28 kWh m\({}^{-3}\)). These results denote that there is room for improvements to bridge the gap between the thermodynamic limit and the actual energy the RED system produces (i.e., increase the yellow sector; Figures 4 and 5). Not only the plant layout but also the stack design of RED should be systematically optimized to make more efficient use of these waste streams, thereby increasing the RED's energy share of the SWRO plant's supply. The most suitable engineering solution for RED optimization requires minimizing the stack internal electric resistance (Figure 5), especially diluate compartments' contribution (RLC)--given its low conductivity compared to IEM (RLC and RLAEM) and concentrate compartments' (RHC)--which makes up half RED stack's internal resistance. In this regard, the use of thinner conductive spacers or profiled membranes may substantially enhance RED process efficiency but may also lead to higher hydrodynamic losses [11].