**1. Introduction**

The global demand for potable water to meet all activities of mankind, in the industrial, domestic and agricultural sectors has been increasing rapidly due primarily to three growth factors, namely (i) an increasing world' population in developing countries, (ii) the quest for higher economic growth in all economies and (iii) the over-abstraction of ground water and the degradation of existing natural water sources on land. Much of fresh water found on land, namely lakes, wetlands and rivers, is gradually being polluted by indiscriminate discharge of

man-made pollutants. By the year 2030, Global Water Intelligence [1] has projected an increase in annual potable-water demand from the current level of 5300–6900 billion cubic meters (bcm), equivalent to a compound annual growth rate of over 2%, as shown in **Table 1**. Yet, the existing sustainable potable water supply, mainly from natural precipitation sources, remains constant at 4200 bcm annually. Such a shortfall in the supply-demand of greater than 2700 bcm annually can only be met by reliable desalination methods [2]. Many ad-hoc measures to conserve water consumption and better manage the supply infrastructure can improve the water use inventory in water stressed countries [3]. However, ground water extraction rates are far greater than the rates at which they are replenished and there is over extraction from rivers [4]. Even with a degree of water re-use there will be a deficit between consumption and sustainable supply. Thus, the only practical means of meeting the future global potable water needs is by seawater desalination [5].

For seawater desalination at ambient temperature, the minimum work needed to separate dissolved salt ions of 3.5% by weight from the brine (within the solution) is termed as the thermodynamic limit (TL) of the normal seawater. Invoking the Gibbs equations for the separation process where the mass fractions of dissolved salts, the activity coefficients of water and solute are known, the theoretical work can be readily found to be 0.78 kWh\_primary energy(pe) per cubic meter of potable water or alternatively, the amount of potable water could be theoretically attained at TL is 1.282 m3 per kWh\_pe consumption [6]. The primary energy (PE) is the naturally available work and it is equivalent to the respective calorific value of fuel burned. It implies that the kWh\_pe/m3 of energy consumption at TL is totally devoid of dissipative losses as the processes are deemed ideal, i.e., the available work as described by classical thermodynamics. Unfortunately, such a concept has been grossly misinterpreted in the literature. The recent reports indicated that energy efficacy of exiting methods in seawater desalination have achieved merely 13% of the TL [7–11]. This shows that currently desalination processes are not consuming fossil fuel energy sources efficiently. Thus, there is a great motivation to improve the energy efficacy of desalination processes to meet the sustainable goals of future water supplies.

In this chapter, the authors attempt to address two challenges facing the desalination industry: Firstly, there is a need to have a common thermodynamic framework to define the absolute value of energy supplied to separation processes. The energy consumed by assorted desalination processes must incorporate both quantity and quality aspects at the respective input conditions of processes. Unfortunately, the quality of dissimilar energy supplied to assorted desalination methods hitherto has been inadvertently omitted. We accentuate that a meaningful efficacy comparison of dissimilar desalination methods can be achieved with a common thermodynamic platform of high to low temperature reservoirs. All derived energy consumption of desalination processes is equivalently transformed to the


#### **Table 1.**

*The projected demand and supply of potable water for the industry, agriculture and domestic sectors, as reported by global water intelligence (GWI) [1–3].*

#### *Performance Evaluation of Desalination Technologies at Common Energy Platform DOI: http://dx.doi.org/10.5772/intechopen.104867*

consumption of primary energy. Such procedures are predicated on either the same equivalent Carnot work output or input depending on the nature of desalination methods used. More importantly, the proposed methodology provides a direct apportionment, in the form conversion factors (CF), in the existing cogeneration power plants setting producing electricity and heat for desalination processes. Secondly, the authors opined that an optimally-designed desalination system can readily attain up to 35% of the TL, as reflected by the many plausible heat engines operating currently in other industries. A quantum improvement in the efficiency of separation processes of seawater desalination is most likely to realize either by (i) developing better performing work-driven systems such as thin-film composite materials or (ii) a higher thermodynamic synergy between the heat-driven processes. In the later section, the authors will highlight a hybrid heat-driven cycle, that were successfully tested at KAUST, attained the best energy efficacy for seawater desalination of 20% [12].
