Abstract
Desalination of sea or brackish water sources to provide clean water supplies has now become a feasible option around the world. Escalating global populations have caused the surge of desalination applications. Desalination processes are energy intensive which results in a significant energy portfolio and associated environmental pollution for many communities. Both electrical and heat energy required for desalination processes have been reduced significantly over the recent years. However, the energy demands are still high and are expected to grow sharply with increasing population. Desalination technologies utilize various forms of energy to produce freshwater. While the process efficiency can be reported by the first law of thermodynamic analysis, this is not a true measure of the process performance as it does not account for all losses of energy. Accordingly, the second law of thermodynamics has been more useful to evaluate the performance of desalination systems. The second law of thermodynamics (exergy analysis) accounts for the available forms of energy in the process streams and energy sources with a reference environment and identifies the major losses of exergy destruction. This aids in developing efficient desalination processes by eliminating the hidden losses. This paper elaborates on exergy analysis of desalination processes to evaluate the thermodynamic efficiency of major components and process streams and identifies suitable operating conditions to minimize exergy destruction. Well-established MSF, MED, MED-TVC, RO, solar distillation, and membrane distillation technologies were discussed with case studies to illustrate the exergy performances.
Highlights
Freshwater is an essential commodity for sustaining human life on earth [1]
This study reported that inclusion of energy recovery systems may increase the exergy efficiency of the desalination process from 14.3% to 25.7%
A comprehensive study on a 24,000 m3/day capacity plant has shown that 40% and 35% exergy destruction have occurred in thermos-compressor and Multi-Effect Distillation (MED) evaporator effects, respectively [46]
Summary
Freshwater is an essential commodity for sustaining human life on earth [1]. World population has tripled during the past century while the water withdrawal rates increased by six times for various uses [2]. While the energy requirements for desalination processes can be met by various energy sources, many countries that lack water sources lack conventional energy sources such as fossil fuels This situation creates a need for efficient use of available energy sources for other beneficial processes as well as careful allocation of energy sources for freshwater production. An energy efficiency of 100% can be achieved between the two bodies, the resultant body temperature may not be same as the source from which the heat transfer occurred, or it may occur at an infinite time scale or it may never happen due to unavoidable ambient losses This means degradation of energy occurred in this process of heat transfer which is often expressed as generation of entropy. ItΔV It∆wV σ 1μ+σ−13μμ1 0−+TT+osμR31 0T40+TTl−nosR(34Tc4/0 c−l0TTn) o(s43c / cTT0os )
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