# Reverse Electrodialysis: Potential Reduction in Energy and Emissions of Desalination
## 4 Conclusions  
This work assesses the potential energy and emissions savings SGE-RED application, an emerging renewable power technology, could offer to desalination. Using the RED system model, we estimated the technical performance of an up-scaled RED system implemented in 20 medium-to-large-sized SWRO desalination plants spread worldwide with SEC varying from 3.0-5.5 kWh per m\({}^{3}\) of desalted water. Building on RED plant's performance assessment, we estimated the grid mix share of total desalination plant energy demand the RED plant could replace in its current state of development (i.e., net specific energy) and if captured all salinity gradient's input exergy. Finally, we quantified the GHG emissions abatement range related to the energy supply of desalination plants, by contrasting the GWP\({}_{100}\) metric when (i) the local electricity mix powers desalination in full and (ii) when RED partly feeds desalination plants in its current state of development (net energy) and if reached the thermodynamic limit (retrieved exergy). Findings reveal that more saline and warmer SWRO concentrate effluents yield enhanced net power densities, yet source availability restrains specific energy supply in some locations. Desalination plants with high water recovery rate deliver a high-saline, but scarce concentrate stream that increases the net power density, though curbs the specific energy of the RED plant, thus making them less suited locations for RED implementation. The most promising scenarios are those where the RED plant attains high power densities (3.6-3.7 W m\({}^{-2}\) per cell pair) while retains high specific energy (0.46-0.58 kWh per m\({}^{3}\) of total permeate). Our estimates indicate that nearly in all locations, RED could supply just under two-fifths of total desalination plant energy demand if harnessing all input exergy--well over half in locations where the RED plant delivers high specific energy to the least energy-intensive SWRO plants. Energy conversion process irreversibility alongside untapped fraction of the input exergy, however, declines by one-fourth (\(\sim\)10% up to 14%) the RED plant's share of total desalination plant energy supply. Accounting for over half of the RED stack's internal losses, an optimized design of diluate compartments and development of IEMs with advanced properties (high conductivity and selectivity) would enhance energy conversion efficiency, thus increasing RED's share of SWRO plant energy supply. Environmentally speaking, the local grid mix's share RED saves to desalination plants outweighs RED's environmental burden contribution to the overall energy-related GHG emissions of desalination plants. On average, in its current state of development, RED could abate by roughly 10% desalination plant emissions--up to over a third if progress on RED allowed reaching the thermodynamic limit. RED integration in the most emission-intensive SWRO plants provides a sizeable emissions abatement--up to 1.95 kg CO\({}_{2}\)-eq m\({}^{-3}\) if harnessing all retrieved exergy--though a lower relative reduction in some locations. Even though the emissions emitted per m\({}^{3}\) of permeate produced were higher, RED application in large-sized desalination plants would avoid further absolute GHG emissions than in medium-sized ones. Collectively, RED could potentially abate 0.32 to 1.18 Mt CO\({}_{2}\)-eq per year, representing 0.4% to 1.6% of global operational desalination plant emissions in 2015. Overall, these findings indicate that RED energy recovery from SWRO concentrate effluents could bring desalination sector sizeable energy and emissions savings provided future advancements bring RED technology closer to its thermodynamic limit. **Author Contributions:** Conceptualization, C.T., M.F. and R.I.; methodology, C.T., M.F. and R.I.; software, C.T. ; validation, C.T. ; formal analysis, C.T. ; investigation, C.T. ; resources, M.F. and R.I.; data curation, C.T. ; writing--original draft preparation, C.T. ; writing--review and editing, M.F., R.I. and I.O. ; visualization, C.T.