Recent developments in thermally-driven seawater desalination: Energy efficiency improvement by hybridization of the MED and AD cycles

The energy, water and environment nexus is a crucial factor when considering the future development of desalination plants or industry in the water-stressed economies. New generation of desalination processes or plants has to meet the stringent environment discharge requirements and yet the industry remains highly energy efficient and sustainable when producing good potable water. Water sources, either brackish or seawater, have become more contaminated as feed while the demand for desalination capacities increase around the world. One immediate solution for energy efficiency improvement comes from the hybridization of the proven desalination processes to the newer processes of desalination: For example, the integration of the available thermally-driven to adsorption desalination (AD) cycles where significant thermodynamic synergy can be attained when cycles are combined. For these hybrid cycles, a quantum improvement in energy efficiency as well as in increase in water production can be expected. The advent of MED with AD cycles, or simply called the MEDAD cycles, is one such example where seawater desalination can be pursued and operated in cogeneration with the electricity production plants: The hybrid desalination cycles utilize only the low exergy bled-steam at low temperatures, complemented with waste exhaust or renewable solar thermal heat at temperatures between 60 and 80 °C. In this paper, the authors have reported their pioneered research on aspects of AD and related hybrid MEDAD cycles, both at theoretical models and experimental pilots. Using the cogeneration of electricity and desalination concept, the authors examined the cost apportionment of fuel cost by the quality or exergy of working steam for such cogeneration configurations. © 2014 Elsevier B.V. All rights reserved.

The trends in cogeneration of electricity and desalination plants have improved both the energetic and exergetic utilization of the thermally-activated processes. The authors have demonstrated the correct use of exergetic approach to fuel-cost apportionment rather than the conventional energetic analysis despite the high latent heat of condensation in the working steam. The lower exergy of bled-steam consumed by the MED desalination processes should cost significantly lesser by comparison to the high exergy steam which is needed for electricity production of turbines. The recent hybridization of the thermally-driven cycles, such as the AD to the MED cycles, have demonstrated their excellent thermodynamic synergy between the thermally-driven processes, improving the specific water production yields by more than two-fold at almost similar heat input temperatures or TBTs. The quantum improvements in water production is due mainly to the operation of the vapor suction by the AD cycles, resulting in the higher temperature differences between stages as well as the lowering of the bottom-brine temperatures to below ambient. Experiments from laboratory-scale pilots have validated the water production increase by more than 2 folds. In the authors' opinion, the hybrid desalination cycles provide the key methodology in the crucial nexus between energy, water and environment that is needed for a more efficient desalination industry.