**2. Limitations of current evaluation methodologies**

At present, the conventional secondary or derived energy units, expressed in kWh of electricity or thermal heat source, are used inadequately for energy efficiency comparison between all types of desalination processes [13, 14]. This practice is insufficient because it has omitted a key aspect of energy quality embedded in the supply fuels. As demonstrated later, the assumption that all derived energy, no matter how dissimilar in forms, are deemed directly equivalent to each other which is thermodynamically inadequate. The units of energy measurement, namely kWh or 3.6 MJ, expresses merely its quantitative aspect but it has ignored the qualitative aspect of the energy used.

For example, same heat input, Q(1 kWh), is supplied to two processes, as depicted on a temperature versus entropy diagram of **Figure 1**. States 1–2 shows higher temperature process and states 3–4 at a lower temperature, i.e., T1 >> T3. Using the concept of an ideal Carnot cycle at the same heat input, the available work that could be extracted from the former process is higher than the latter. Being an isothermal cycle in a T-S diagram, i.e., ∮ <sup>12</sup>*dcQ* ¼ ∮ <sup>12</sup>*dcW and* ∮ <sup>12</sup>*dcU* ¼ 0, the energy input from a higher temperature source yields a larger amount of useful work due primarily to the better quality of heat input. This is reflected by the dissimilar

#### **Figure 1.**

*A graphical demonstration of the energetic quantity and quality to thermodynamic cycles. Despite having same quantity of energy input, say, Q = 1 kWh, a higher available work could be produced from the process of a higher input temperature that is, WA,12ba >> WA,34fe. Note that the available work constitutes the useful work, the internal and external dissipative losses incurred by the processes of cycle. At near to ambient temperature,To. The cycle-56Ji has zero available work, even it is supplied with the same heat quantity. Despite the same heat input quantity, the cycles have dissimilar amount of unavailable work, and it is attributed to the quality of energy (defined by temperature and pressure) being supplied over the limit of "dead state".*

unavailable work that were demarcated by the ambient temperature and the entropy change. Such unavailable work is also commonly known as the dissipation trapped by the "dead state". This aspect of diminishing available work with lower heat source temperature can be observed to reduce to zero at the limit, Tsource ! To. A second aspect of **Figure 1** is cascading of processes in which exhaust of processes at higher temperature can be used as a heat source for a second process operating at relatively lower temperature to optimize the cycle efficiency.

Over many decades, decision makers within the desalination industry have failed to notice the above-mentioned misconception. Should it remain uncorrected, sub-optimal decisions will be made and, in a world, seeking to become carbon neutral the implications are serious. The consequences will be inferior selection of desalination methods for the supply of large quantity of potable water in many water-stressed countries. Operating a non-optimal desalination plant over its lifespan not only burdens consumers economically with a higher unit water cost but the associated carbon dioxide emissions will be higher and probably there will be a higher discharge of chemically laden brine into the sea.

Those interested in pedagogy might care to ask the following rudimentary question, why despite many decades of advancement in science and engineering, how is it possible for the desalination industry to treat dissimilar energy quantities as if they were the same or, as the English would say, compare apples with oranges? We will seek to give an answer at the end of the paper.
