1. Introduction
Freshwater is a scarce natural resource essential for human life. The demand for freshwater has increased sixfold over the last 100 years, steadily increasing by about 1 percent per year. This situation is even more alarming in areas where natural water resources are scarce or nonexistent. Currently, about 3.6 billion people live in areas at risk of water stress for at least one month a year, and the population could reach between 4.8 and 5.7 billion by 2050 [
1]. By 2030, a global water deficit of 40% is expected, which will be aggravated by climate change, changes in consumption patterns caused by economic development, and population growth. This means that conventional water sources such as lakes, rivers, or aquifers are no longer able to meet the water demand in many regions of the world. For this reason, obtaining freshwater is considered one of the main challenges at the global level, included in the Sustainable Development Goal (SDG) 6, one of the 17 goals established in 2015 by the United Nations General Assembly, aimed at ensuring access to clean water for current and future generations [
1,
2] Because 97.5% of surface water is saline, seawater desalination processes have positioned themselves as a high-value alternative for freshwater production, especially in areas where natural water resources are not capable of meeting water demand [
3]. Currently, there are nearly 16,000 operational desalination plants, with a daily production of approximately 95.37 million m
3/day. Seawater is most frequently used in desalination processes, accounting for 61% of the water produced, followed by brackish water and river water with 21% and 8% of the water produced, respectively, [
4]. Thermal processes such as multieffect distillation (MED) or multistage flash distillation (MSF) are responsible for the production of 18% and 7% of desalinated water worldwide [
5]. However, these technologies are considered as intensive in energy consumption, using both thermal energy as the primary energy to produce the phase change and electrical energy for operating conditions.
Table 1 summarises the energy consumption of the main thermal-based and membrane-based desalination technologies.
Exergy analysis is a tool recognised worldwide for its use in assessing the thermodynamic performance (development, evaluation, and improvement) of an energy-conversion system by identifying the location, cause, and estimation of thermodynamic inefficiencies. Its implementation in the industry field is still very low, due in part to a lack of standardisation of definitions, leading to mixed results [
8]. Different approaches for formulating the exergetic efficiency can be found in the literature [
9,
10,
11,
12,
13,
14,
15], which can be classified into two main categories: (a) total exergy efficiency or input–output and (b) consumed–produced exergy efficiency. Brodyansky et al. [
11] offered a deep analysis of the terminology issue and how these terms can lead to different results and formulations for exergy efficiency by using different examples. Sciubba et al. [
16] categorised exergy efficiencies into three general definitions. Lior and Zhang [
17] classified and made distinctions between different definitions for energy, exergy, and second-law efficiencies. They referred to the input–output efficiency as the total or overall efficiency and suggested its use for processes where a major part of the output is “useful”. Cornelissen [
18] made a comparison between three exergy efficiency definitions. The terms product exergy and fuel or consumption exergy, from the consumed–produced exergy efficiency category, have been extensively used in the literature by several authors, with slight differences [
9,
10,
11,
12,
19,
20].
In this work, two definitions of exergy efficiency are applied to the desalination components of the three main thermal desalination processes. Two of the processes represent the main conventional thermal desalination systems, and the last is one of the most researched thermal emerging technologies [
21]. The key point is the definition of the exergy of the product (the exergy produced by the system) and the exergy of the fuel (the resources expended to generate the product). In desalination technologies, where the separation of salt takes place, the chemical exergy of the material streams plays also an important role and must be considered in the definition of the exergetic parameters. This work proposes the definitions of the product, the fuel, and, consequently, the exergetic efficiency of the main thermal desalination processes.
4. Exergy Analysis
The exergetic balance of a system arises from the first and second thermodynamic laws as a combination of the energy and entropy balances, leading to determine both the quantity and quality of the resources [
15]:
The total exergy of a system can be divided into four main components: physical, chemical, kinetic, and potential exergy. Being an extensive property, the specific exergy can be defined as
At rest conditions of the system with respect to the surroundings, the kinetic exergy and potential exergy terms can be neglected: .
Specific physical exergy values were determined by
The specific chemical exergy
as well as the enthalpy,
h, and entropy,
s, of freshwater, brine, and seawater were calculated using the seawater thermodynamic property functions proposed by Sharqawy et al. [
23].
For the exergy of the solar thermal system, the Petela expression was applied [
60]:
where
is the temperature of the sun,
A is the area of the solar collector,
is the solar radiation, and
is the temperature at dead state.
Exergy Efficiency
Exergy efficiency quantifies the performance of an energy-dependent system from the thermodynamic point of view and is traditionally perceived as as the percentage of exergy provided to the system that can be found in the product. It is fundamental to identify both the exergy supplied to the system (exergy of the fuel) and the exergy of the product. Research studies have proposed several definitions for both terms. Some of them consider the fuel simply as all the exergy introduced into the system and the product as the exergy that leaves the system. Others relate the fuel exergy with the exergy of resources and their decrease within the system and the product as the increase of the exergy produced. Both definitions are accepted by a large number of researchers but in practice lead to different results when applied to the same processes.
Gaggioli et al. [
61] referred to exergy efficiency as “true efficiency” and energy efficiency as “traditional efficiency” due to exergy efficiency’s ability to locate and quantify “true” process inefficiencies in the form of exergy destruction. This general definition gives rise to the presence in the literature of various definitions of exergy efficiency, which makes it difficult to unify the term and, consequently, to obtain homogeneous results [
15,
61].
Definitions can be classified into two main approaches: input–output exergy efficiency and consumed–produced exergy efficiency.
Marmolejo-Correa and Gundersen [
62] applied the definitions of input–output exergy and product–consumption exergy to a natural gas liquefaction process, obtaining significantly different results in both cases. A very high exergy efficiency value (98.3%) was achieved by the first definition, compared to 50.5% efficiency obtained by the second definition. The authors considered the product–consumption approach as more advanced for process evaluation.
- a.
Inlet–outlet efficiency.
Kotas [
63], under the name of rational efficiency, defined exergy efficiency as the ratio between the exergy transformations that constitute the output and the input of the system. Gundersen [
64] defined exergy efficiency as the ratio between the useful exergy produced by the system and the total exergy of the system. Rosen and Dinçer [
8] defined exergy efficiency as the ratio between the product exergy at the output of the system and the exergy at the input. Considering those definitions, the exergy balance can be expressed as
As before, the ingoing and outgoing exergy quantities may include work, heat, and cooling, with or without mass flows. The overall or total exergetic efficiency,
, should be, consequently, defined as
- b.
Consumed–produced efficiency.
The exergy supplied to the system is either delivered in the outputs or destroyed inside the system. This balance can also be expressed in terms of exergy of the fuel (the exergy of the resources), ; exergy of the product (the desired exergy output), ; exergy loss (with effluents), ; or exergy destroyed (within the system due to irreversibilities), .
The terms product exergy and fuel or consumption exergy have been widely used in the literature by several authors, with slight differences [
15,
61,
65,
66,
67], often indicated as the most appropriate definition for exergy efficiency. Szaargut et al. and Tsatsaronis et al. [
12,
14] proposed taking into account those exergy transfers that were used in the production of the desired exergy from the driving exergy of the system, for which a coherence between the purpose of the system and its exergy analysis is necessary, giving rise to the concepts of fuel exergy and product exergy. In addition, Lazaretto and Tsatsaronis [
68] proposed a systematic procedure for the definition of the exergy efficiency for process components.
Based on the common energy efficiency definition as the ratio between the product that is obtained (work, heat, refrigeration) and the resources that must be “paid” for, the consumed–produced exergetic efficiency could be defined as the ratio between the useful exergy outputs and the paid exergy inputs [
15].
Both the total efficiency and the consumed–produced efficiency definitions have been applied.
Table 4 presents the two main approaches for the common components of the thermal desalination processes presented in this work. The thermodynamic properties and exergy flows of the processes presented in this study were determined using the Engineering Equation Solver (EES) program.
The equations for the exergy efficiency of the components and the distillation units of the case studies are presented in
Table 5 and
Table 6.
5. Results and Discussion
One of the most important contributions of exergy analysis is the estimation of the contributions of system components to the overall exergy destruction, because of its ability to locate the inefficiencies present in the system components.
In the MED-TVC process, the major contribution belongs to the TVC system. As the TVC system is the main component responsible for the exergy fuel in the process, this finding is not unexpected. The second major exergy destruction occurs in the seven effects due to their responsibility for the separation of salts. The implication of the pumps in exergy destruction is almost nonexistent due to the low pressure requirements of this type of process. Eshoul et al. [
57] presented a similar distribution in the contribution by component to the exergy destruction in the system, although the difference between TVC and the effects does not seem to be so pronounced. In other studies such as [
36,
69], the TVC source has consistently been identified as the main contributor for exergy destruction. However, in [
36], the effects slightly surpassed the TVC contribution for exergy destruction.
In the MSF process, the exergy destroyed in the stages represents almost the totality of the exergy destroyed. Given the small amount of chemical exergy compared to the physical, mainly thermal, exergy required to produce it, this is a logical result. The second major exergy destruction belongs to the brine heater, which also contributes mainly in the form of thermal exergy. The distribution of exergy destruction by component presented by Khoshrou et al. [
58] followed a similar pattern. However, the contribution of the stages decreased by more than 20%. Most studies present similar results, with variations mainly in the proportion of exergy destruction contributed between the stages and the brine heater [
37,
38,
39,
41]. As observed in the MED-TVC process, the pumps play a very small role in the exergetic destruction of the process due to the low pressure requirements, which was also concluded by Piacentino et al. [
26].
The highest contribution belongs to the heat exchanger with 74%, followed by the heat recovery unit with 9%. This can be observed in other studies such as Zhu et al. [
70], where the preheating system accounted for about 60% of the total exergy destroyed. As in the cases of MED-TVC and MSF, the sources of physical exergy in the form of thermal exergy, which are the main contributors to the fuel exergy in these systems, are also the main contributors to the exergy destruction in the system. As can be seen in the previous cases, the lowest contribution to the exergy destroyed corresponds to the pumps in the system since the operating pressure is close to the atmospheric pressure. Most studies are in agreement with these results [
49,
53,
54,
70,
71]. The membrane module offers small contributions of only 1% to the total destroyed exergy. These values can be justified by the assumption of the pressure and temperature losses throughout the system as negligible. Furthermore, these results are in agreement with those obtained by Mibarki et al. [
53] for a system with an AGMD configuration integrated with a solar collector, operating under similar conditions of temperature and salt concentration.
The results related to the two main approaches for the exergy efficiency of the MED-TVC, MSF, and DCMD case studies are presented in
Table 7,
Table 8 and
Table 9.
The pumps used in the MED-TVC process show similar high results according to both exergy efficiency approaches. The feed pump presents the same efficiency results for both cases, because the exergy related to the input stream is zero. In the rest of the cases, the efficiency according to the consumed–produced approach is slightly lower. In the MSF process, the same pattern can be observed regarding the pumps used in this process, with slightly lower results from the consumed–produced approach for all pumps in the system except for the feed pump, where the exergy of the input stream is zero due to dead-state conditions.
For both the MED-TVC and MSF processes, the major difference between the two approaches to exergy efficiency is observed in the distillation units. According to the input–output exergy efficiency approach, all the inlets and outlets of the units are considered.
In the MSF distillation units, the difference in the results of the two approaches is even more pronounced. The extremely small exergy associated with distillate production, compared to the physical exergy supplied to the units for its conversion, leads to a drop in the exergy efficiency of the distillation units. The disparity in the results of the two approaches in the distillation units is reflected, to a lesser extent, in the total plant efficiency result in both case studies. The exergy efficiency of the MED-TVC plant drops from 21.35% to 3.1%, while in the MSF process, it drops from 17.08% to 1.58%.
Similar results were presented using the consumed–produced approach for MSF systems by Nafey et al. [
38] of 1.87% and Mabrouk et al. [
39] of 2%. In both cases, only the resources necessary to produce the distillate product of the process were considered as product exergy. An analogous approach, in which the product exergy is defined as the minimum separation work or the minimum amount of exergy input required and the fuel exergy is defined as all the actual input exergy, resulted in slightly higher efficiencies of [
37] 4.2% and [
40] 5.8%. Al Ghamdi et al. [
41] obtained slightly lower results than those previously mentioned for this approach (3%), which may be due to the fact that the chemical exergy produced in the process was not taken into account.
Studies that employed the input–output approach, however, offered very high exergy efficiency values compared to the previous approaches and the results of the present study for an analogous method, with [
43] 61.6% and [
44] 70%. For the MED process, the results found for the input–output approach were even higher, at [
32] 62.7% and [
27] 97%.
Carballo et al. [
27] also examined a consumed–produced approach similar to that proposed in this study, where the increase in chemical exergy associated with the distillate production is considered as the only product of the process, obtaining an exergy efficiency in this case of 0.2%. An analogous outcome was presented in the study of Khalilzadeh et al. [
33], with a process exergy efficiency of 0.1%. Very similar values for the consumed–produced approach to those found in the present study were also presented by Moghimi et al. [
29] of 3.1% and García and Gómez et al. [
25] of 4.7%. The consumed–produced approach is considered to better fit the thermodynamic behaviour of the distillation units and consequently the overall process where only the distillate is considered as the desired product of the component, obtained from the conversion of physical exergy (mainly thermal) into chemical exergy, which was also observed by Piacentino et al. [
26].
In membrane distillation, an exergetic efficiency of 1.29% was achieved for the input–output approach, and it dropped to 0.37% for the fuel–product approach. The main reason is due to the reduced production of product water, which is characteristic of membrane distillation processes and similar to the exergetic efficiencies obtained in other thermal desalination systems. Furthermore, considering exergetic efficiencies only from membrane distillation processes, they are slightly higher compared to some of the studies that appear in the literature [
46,
47,
52,
71]. The DCMD module presents high exergetic efficiencies, considering both exergetic efficiency approaches. The efficiency of the DCMD module turned out to be similar to that presented by the VMD module of Miladi et al. [
49], reaching exergetic efficiencies greater than 90%, and to Zhu’s DCMD module, with an efficiency of 91% [
70].
6. Conclusions
In this work, a literature review of the application of exergy analysis to the main thermal desalination systems was carried out, with special emphasis on exergy models and exergy efficiency. This contribution also provides an analysis of the two main approaches to energy efficiency: input–output and consumed–produced. For this purpose, three case studies for the multieffect distillation, multistage flash distillation, and membrane distillation processes were studied.
Different exergy models and exergy efficiency approaches applied to the exergy analysis of thermal desalination systems were found in the literature, providing considerable deviations in results and negative exergy, which hinders the standardisation of exergetic analysis.
Equipment involving salt separation and, thus, an increase in chemical exergy plays a fundamental role in the exergy destruction of conventional processes. In the computation of the processes considered in this work, the equipment involved in heat transfer are the main components responsible for exergy destruction, due to the large thermal irreversibilities produced in them. The low pressure requirements of thermal desalination systems almost eliminate the contribution of pumps to exergy destruction.
In terms of exergy efficiency, input–output approach values for thermal desalination systems present higher results compared to the consumed–produced approach. However, the consumed–produced approach seems to present a better description of the thermodynamic behaviour of thermal desalination processes. The results obtained with the second consumed–produced approach for the MED-TVC, MSF, and DCMD case studies were 3.1%, 1.58%, and 0.37%, respectively, which is in line with the results found in the literature for similar approaches.
Based on the conclusions of this study, we recommend the following:
The use of the thermodynamic properties of seawater from Sharqawy et al. [
23] in exergy analysis, as they seem to better represent the thermodynamic behaviour of seawater.
The consideration of the consumed–produced exergy approach for analysis, which allows one to focus on the desired product of thermal desalination processes and therefore to better describe the final objective of this type of process.
The taking into account of the increase in chemical exergy in the definition of the exergy efficiency of the process components in which salt separation takes place.