# Reverse Electrodialysis: Potential Reduction in Energy and Emissions of Desalination
## 3 Results and Discussion
### GHG Emissions Savings to Desalination from RED  
To size the emissions RED could save to desalination, we compare the GHG emissions for the BaU energy supply scenario (Mix, denoted by dots in Figure 6), where the local grid mix fully powers the SWRO plant, to that of the Mix+RED scenarios, when RED replaces a fraction of the regional grid mix both in (i) the \"optimistic\" scenario (Mix+RED emissions' lower limit, Figure 6), where RED transforms all exergy retrieved for conversion into useful work (Retrieved Exergy, Equation (A1)) owing to future advancements in RED technology, and (ii) the \"current\" scenario (Mix+RED emissions' upper limit, Figure 6), reflecting the current state of RED technology (Net Energy computed with the RED system's model). We applied the 100-year Global Warming Potential metric --GWP\({}_{100}\), kg CO\({}_{2}\)-eq per m\({}^{3}\) of desalted water, Equation (A4)--to quantify the GHG emissions for each energy supply scenarios--i.e., BaU and Mix+RED--and location--i.e., scenarios 1 to 20. Figure 4: Salinity Gradient Energy share of SWRO desalination plant’s energy consumption. The actual SWRO plant’s SEC value in kWh per m\({}^{3}\) of desalted water is also displayed (relevant references in Table 1). Figure 5: Global-wide energy balance of the RED plant (**left**), and relative contribution to the internal resistive losses of the 1 st RED stack (**right**). RED's salinity gradient energy retrieval from desalination's concentrate effluent is a promising route to balance out carbon emissions from this energy-intensive sector; this is particularly true for those locations deeply reliant on fossil fuels (Table A1 and Figure A1) as some Middle East regions (scenarios 9, 15), Australia (scenarios 18 and 20), China and India (scenarios 14 and 13, respectively). Besides the local electricity portfolio, RED's ability to cut down desalination's carbon emissions hinges on (i) the technical and environmental performance of RED and (ii) the energy required to drive desalination. Concerning the environmental performance of RED, since all scenarios use the same number of series RED units per parallel branch each working with equal feed conditions--equal concentration, temperature and flow rate per branch, so the net power scales with the number of parallel branches--the differences of RED's GWP per kWh of energy produced among sites depend only on the RED plant's power output (i.e., the power density)--Equation (A6). Hence, the places where RED reaches large power densities (scenarios 12, 15 and 1; Figure 3) will earn lower GWP per kWh (Figure A2); the opposite true for scenarios 20 and 14. Even so, the grid mix share RED saves to SWRO plants greatly offsets the environmental burden allocated to RED, given its little relative contribution, below ~2%, to the overall energy-related GWP of desalination plants both in the \"optimistic\" and the \"current\" energy supply scenarios (Equation (A4)). As a result, on average, current-state RED could reduce desalination's GWP by ~10% raising to ~34% if RED technology realized future advancements. The lower RED's GWP per kWh (Figure A2), but higher grid mix's GWP per kWh (Figure A1) and lower RED's energy share (Figure 4) in scenario 7, lead RED to save a minimum of ~5% to ~20% in \"current\" and \"optimistic\" supply scenarios, respectively, as opposed to scenario 8, where desalination's emissions savings rise to ~14% (\"current\") up to ~51% (\"optimistic\") compared to the BaU scenario. Although RED provides the largest relative emissions saving in scenario 8--stem from the greatest RED energy share of total SWRO SEC; Figure 4--RED application in emission-intensive desalination plants--as those of scenarios 13, 9, 15 and 20 with high SWRO SEC and electricity mix GWP per kWh--would bring in sizable emission  
Figure 6: Electricity related GWP per m\({}^{3}\) of permeate according to locations (scenarios 1 to 20) for each energy supply scenario.