# Reverse Electrodialysis: Potential Reduction in Energy and Emissions of Desalination
## 3 Results and Discussion
### Net Power Density and Net Specific Energy of the RED System  
The quality (feed concentration and temperature) and availability (feed flow rate) of the local energy source (i.e., SGE) will be the drivers in RED technical performance. Using the RED system's model, we conducted simulations to estimate the optimal net power density (net power output per membrane pair area of the single-stage arrangement, Figure 2) and maximum net specific energy (net energy per cubic meter of desalted water of the single-parallel layout, Figure 2) attainable when the RED plant recovers energy from SWRO plants' concentrate effluent--with reported seasonal average concentration, temperature and volume of 0.86-1.67 M, 16-32 \({}^{\circ}\)C and 0.67-1.57 m\({}^{3}\) per m\({}^{3}\) of total permeate produced; Tables 1 and 3--and the optimal low-salinity stream that maximizes the net power output in every location (with concentration ranging from 35 to 55 mM, Table 3). The concentration and temperature of RED's inlet streams will determine the net power density of the system; the more concentrated and warmer the feeds, the larger the net power output of the RED plant (green to yellow hues in the map, Figure 3) [29; 67; 68; 69]. As such, the SWRO plants located in the Middle East (i.e., scenarios 1, 9 and 15), suppling the most concentrated and warmest brine effluent to RED, enhance the net power density (3.6-3.7 W m\({}^{-2}\) per cell pair; Figure 3) peaking to \(\sim\)4.0 W m\({}^{-2}\) per cell pair in scenario 12. Similarly, locations where the high temperature (32 \({}^{\circ}\)C, scenario 19) or the high concentration (1.67 M, scenario 11) of the concentrate feed offsets the adverse effect of its lower concentration (1.13 M) [24; 25] or temperature (20 \({}^{\circ}\)C) [24; 67; 69] bring about improved net power densities (3.5 W m\({}^{-2}\) per cell pair and 3.4 W m\({}^{-2}\) per cell pair, respectively). Conversely, the net power density almost halves (\(\sim\)2.1 W m\({}^{-2}\) per cell pair) in scenarios with opposite working conditions (e.g., scenarios 14 and 20; Figure 3), where RED draws energy from the coldest and more diluted SWRO concentrate effluents. Besides feed solutions' properties, the volume availability will be equally vital to realize substantial energy savings to desalination, as it limits the potential specific energy supply of the RED plant (which scales with bubble size in Figure 3). For instance, the scant SWRO brine volume in scenarios 11, 12 and 19 (0.67-0.75 m\({}^{3}\) m\({}^{-3}\) due to the high water recovery rate of these desalination plants of up to 60%; Table 10 hinders the energy capacity of the RED plant (\(\sim\)0.30 kWh m\({}^{-3}\), Figure 3), despite the enhanced net power density gained in these scenarios. In contrast, desalination plants in scenarios 15 and 1 supply twice the brine effluent volume (1.33-1.53 m\({}^{3}\) m\({}^{-3}\)) to that of scenarios 11, 12 and 19, which in turn doubles the net specific energy (0.46-0.58 kWh m\({}^{-3}\)). Even scenarios where RED implementation yields lower net power densities (e.g., scenarios 20, 18, 8 and 2) become appealing given its larger net specific energy (0.40-0.44 kWh m\({}^{-3}\)). Overall, scenarios 1 and 15 hold the most desirable site features for RED implementation--i.e., high concentration and temperature (high net power density)--along with large brine volume (high net specific energy), unlike scenarios 5 and 10, which combine soft temperature and/or low concentration  
Figure 3: Performance of the RED system: Net Specific Energy—i.e., Net Energy per m\({}^{3}\) of desalted water, of the series-parallel network—and Net Power Density—i.e., Net Power per membrane pair active area, of the parallel arrangement. Series arrangement: seven industrial-scale RED units in series; number of parallel RED units set according to the total flowrate of SWRO’s brine available for conversion and the flowrate of concentrate stream fed to each RED unit.