Entropy Generation Analysis of Desalination Technologies

Abstract

Increasing global demand for fresh water is driving the development and implementation of a wide variety of seawater desalination technologies. Entropy generation analysis, and specifically, Second Law efficiency, is an important tool for illustrating the influence of irreversibilities within a system on the required energy input. When defining Second Law efficiency, the useful exergy output of the system must be properly defined. For desalination systems, this is the minimum least work of separation required to extract a unit of water from a feed stream of a given salinity. In order to evaluate the Second Law efficiency, entropy generation mechanisms present in a wide range of desalination processes are analyzed. In particular, entropy generated in the run down to equilibrium of discharge streams must be considered. Physical models are applied to estimate the magnitude of entropy generation by component and individual processes. These formulations are applied to calculate the total entropy generation in several desalination systems including multiple effect distillation, multistage flash, membrane distillation, mechanical vapor compression, reverse osmosis, and humidification-dehumidification. Within each technology, the relative importance of each source of entropy generation is discussed in order to determine which should be the target of entropy generation minimization. As given here, the correct application of Second Law efficiency shows which systems operate closest to the reversible limit and helps to indicate which systems have the greatest potential for improvement.

An improved model for multiple effect distillation

“…However, the high energy consumption associated with desalination processes such as MED, especially as compared to the least work of separation [4], suggests that further research on these and other technologies is needed in order to lower the cost and increase the availability of potable water. One way to accomplish this is to combine thermal desalination systems, such as MED, with electricity production plants in a combined water-power co-generation scheme.…”

Exergy analysis of a high-temperature-steam-driven, varied-pressure, humidification–dehumidification system coupled with reverse osmosis

“…Moreover, this figure demonstrates that the dehumidifier, expander, and RO have relatively significant exergy destruction ratios, which are 0.15, 0.1, and 0.08, respectively. The fraction of the exergy lost is small at around 0.05, which is attributed to the small magnitude of the thermal disequilibrium for the desalination system considered [22].…”

“…The overall exergetic efficiency of the system is a function of total exergy destruction, exergy loss, and exergy input to the system. It is defined as [21,22] …”

Thermodynamic analysis of brine management methods: Zero-discharge desalination and salinity-gradient power production

“…Figure 7 shows two reversible, blackbox systems: one separating seawater into pure water and brine (denoted here asṁ p andṁ d,in , respectively), and the other one mixing the brine and pure water streams. A reversible desalination system requires the thermodynamic minimum amount of energy input, which is sometimes referred to as least work of separation [30,40]. In this paper, the least work is denoted byẆ least in .…”

“…Note that the control volume is treated as being sufficiently large that the process streams (i.e., feed, pure water, brine or salt) all enter and leave at the dead state temperature (T 0 ) and pressure (P 0 ), which requires heat transfer to the environment, (Q 0 ). (For discussion of the control system boundary, see [40], Sec. 2.1.)…”