Recent development in thermally activated desalination methods: achieving an energy efficiency less than 2.5 kWh(elec)/m(3)

摘要

Water-Energy-Environment nexus is a crucial consideration when designing seawater desalination processes, particularly for the water-stressed countries where the annual water availability is less than 250 m(3) per capita. Despite the thermodynamics limit for seawater desalination at normal conditions is about 0.78 to 1.09 kWh(elec)/m(3), the specific energy consumption of desalination of real plants is found to operate at several folds higher. Today's technological advancement in membranes, namely the reverse osmosis processes, has set an energy consumption of around 3.5-5 kWh(elec)/m(3), while the conventional perception of thermally activated processes such as MSF and MED tends to be higher. Although the higher energetic specific consumption of MED or MSF processes appeared to be higher at 60-100 kWh(thermal)/m(3), their true electricity equivalent has been converted, hitherto, using the energetic analyses where the work potential of working steam of the processes cannot be captured adequately. Thermally activated processes, such as MED and MSF, form the bottoming cycle of a cogeneration plant where both electricity and desalination processes operate in tandem in a cascaded manner. Only the bled-steam at lower exergy is extracted for the desalination processes. In this presentation, we demonstrate that in a cogen plant with 30% bled-steam for MED processes, the exergy destruction ratio is found to be less than 7% of the total available exergy that emanated from the boilers. By the exergetic approach, the equivalent electricity consumption of an average 75 kWh(thermal)/m(3) would result in an electrical equivalent of less than 2.5 kWh(elec)/m(3). Also in this presentation, the authors will elaborate the latest developments in the use of hybridization concept where the MED and the AD cycles are thermodynamically integrated and enhancing the overall efficiency of desalination.