Three alternative approaches to desalinating seawater were evaluated with respect to their thermodynamic efficiencies and greenhouse-gas emissions. The technologies considered were multistage flash distillation (MSF), reverse osmosis (RO), and membrane distillation (MD). The analysis was based on published stream data from large-scale operational MSF and RO facilities and experimental-scale data for the MD process. RO was found to be the most exergy-efficient (30.1%) followed by MD (14.27%) and MSF (7.73%). RO and MD required less power consumption to produce water (3.29 kWh/m3 and 5.9 kWh/m3, respectively) compared to MSF which had a much higher energy demand (16.7 kWh/m3). Similar results were obtained when comparing equivalent carbon dioxide emissions from each process; MD and RO accounted for 5.22 and 2.91 kg CO2eq/m3, respectively, whereas MSF generated three to four times that amount. The results indicate that MD has potential as a commercially viable technique for seawater desalination provided a source of waste heat is available. This study provides an overview of the use of thermodynamic efficiency analysis to evaluate desalination processes and provides insight into where energy may be saved with developed desalination processes and areas of research for emerging desalination techniques.
Keywords: greenhouse-gas emissions, membrane-distillation desalination, minimum separation work