The increasing freshwater shortage from one side and the growing concerns about the utilization of fossil fuel from another side, have led to developing renewable energy-based desalination technologies [1
]. Among various alternatives, solar still desalination systems are taken into account as one of the most propitious items [2
]. It is because solar still enjoys a number of advantages compared to the other rivals, including low operating [3
] and capital costs [4
]; space for installation [5
]; and availability of application in both small and large scales [6
], not having noise during operation [7
]. Considering such a wide range of advantages, different investigations have been done to analyze solar stills from different points of view [8
Attia et al. [9
] assessed solar still usage potential in arid areas in Algeria by taking energy and exergy indicators into account. According to the results, the range of 2.2–3.5 kg.m−2
for the system during the operation in winter and summer was seen, respectively.
Parsa et al. [10
] conducted experiments for analyzing the performance of a solar still enhanced by nanomaterials at higher altitudes of Tehran, which is the capital of Iran. According to the results of that study, since the rate of received ultra-violet is higher than the city, higher evaporation rate could be observed. The exergy efficiency of 9.27% and energy efficiency of 55.98% were reported as the highest values.
As a smart augmentation way to enhance the productivity, Attia et al. [11
] examined phosphate bags for energy storage. The measurement was done under the climatic condition of El Oued, which is an Algerian city. The ranges of 4.87 to 5.27 kg for fresh water production, and 22.5 to 28.0% for overall efficiency were observed.
In another study, Hassan et al. [12
] evaluated solar still performance under different saline water conditions based on energy, exergy, economic, and environmental criteria. Their recorded experimental results showed that the exergy and energy output levels had been increased up to two and three times more, respectively, when solar collectors from parabolic trough were used.
The research work carried out by Amiri et al. [13
] was another study in which parabolic trough collector was suggested for performance enhancement of a solar still. A simulation framework was provided for thermal modeling, which had been validated using the experimental data. The verified model was utilized for performance assessment during winter and summers, which indicated that 55% more fresh water was produced in summer in comparison to the winter.
Mohanraj et al. [14
] studied the performance of a water heating system by a heat pump, which was assisted by a solar still for energy storage. The study considered the energy, environmental, and economic sides, in which the range of 2.21–2.55 years for the coefficient of performance was found. The amount of CO2
production and payback period were also 26,800 kg and 21.46 months, respectively for 275 days of operation.
In addition to the mentioned studies, three investigations were performed recently by the research team on solar stills. Initially, in [15
], an innovative design for solar stills, which utilized side mirrors and tracking strategies, was proposed. It had 36.0% better efficiency and 43.1% higher distillate. Then, the performance of the provided design in [15
] was simulated using the artificial neural network (ANN), in which water temperature in the basin and distillate production were the outputs [16
]. The third work was [17
]. In [17
], the use of electric heaters and concentrating solar collectors was studied by experimental data. Both technical and economic issues were taken into consideration. Different conditions for water height in the basin were examined, while Kerman, as a city from the arid-hot climate of Iran, was the city of experiments.
According to the literature reviewed here, in addition to the provided information in the review studies, such as the works of Panchal et al. [18
], Kumar et al. [19
], Singh et al. [20
], and so on, the most focus in the research works in the field of solar stills have been on proposing the ways for system performance enhancement. In the conducted studies, usually, one city was selected for investigation, and even if more than one city was considered, it was not determined that:
When energy, economic, and technical indicators are considered at the same time, among a number of cities, with diverse meteorological and water tariffs, which one would be the best for installation of solar stills?
Considering the indicated gap and introduced the research question, this study is done. Iran, which is located in the Middle East region, Asia, is considered as the considered region, and from each climatic condition, a representative city is selected. Then, by considering annual fresh water production (AFWP), energy payback period (EBPT), and investment payback period (IPBP) as the technical, energy, and economic criteria, the priority to install solar stills is found. It is done using the analytical hierarchy process (AHP), as one of the most powerful tools for this purpose.
In this part, first, the results of AFWP, EPBP, and IPBP for different cities are reported and discussed in Section 3.1
, Section 3.2
and Section 3.3
, respectively. Then, the matrices of pairwise comparisons are given, and finally, the final scores are presented, by which the geographical preference for installation of solar still is determined.
3.1. Annual Fresh Water Production
The values of AFWP for seven investigated cities are reported in Figure 5
. As observed, the higher ambient temperature, wind speed, and received solar radiation a city has, the higher AFWP it enjoys. In addition, since increasing relative humidity leads to decreasing the heat transfer rate, at almost the same condition for other effective parameters, the city with lower relative humidity has a better AFWP. Nonetheless, the relative humidity impact on AFWP is not as strong as the two other indicated parameters.
Considering this point, Ahvaz, as the city with the highest received solar radiation and ambient temperature levels, has the highest AFWP, with a value of 27069.5 L. Zahedan and Tehran are in the second and third places with 2496.1 and 2115.3 L. AFWP for these two cities are 7.77 and 21.84% lower than Ahvaz, respectively.
The other four cities have relatively close AFWP values. The reason is the trade-off among the effective parameters for them. Bandar Abbas, Azadshahr, Rasht, and Tabriz are in the next ranks, which have 25.47, 26.46, 28.96, and 30.21% lower AFWP compared to Ahvaz, respectively.
3.2. Energy Payback Period
Both freshwater production (
) and the enthalpy difference (
) are dependent on the ambient characteristics. The hotter or dryer a climate is, the more water temperature in the basin, and consequently,
has a similar fashion. In addition, the impact of temperature is stronger than the relative humidity. As a result, Ahvaz, as the city with the highest ambient temperature level, enjoys the lowest EPBP among the cities, according to the presented results of Figure 6
demonstrates that EPBP for Ahvaz is only around a half of a year, i.e., 0.58 years. Zahedan is in the second rank, which has EPBP of 0.71. There is an EPBP of 0.87 for Tehran, which is the third city in sorting. Bandar Abbas, Azadshahr, and Rasht are in the fourth, fifth, and sixth ranks. For these cities, returning energy needs are 1.01, 1.16, and 1.23 years, respectively.
With 1.32 years, the longest EPBP is observed for Tabriz, which is the coldest investigated climate. Nonetheless, EPBP for all the cities are below 1.5 years, which is a very satisfactory limit. Such a satisfactory limit comes from the low required energy for the manufacturing processes and delivery of the solar still. It highlights the fact that they are completely justifiable items from EPBP perspective in all the cities.
3.3. Investment Payback Period
Based on the governing equations, in addition to the freshwater production (
), IPBP also depends on the amount of water tariff (
) in each city. For this case, the water tariff plays the most important role. For this reason, Tehran, as the capital of the country, which has a much higher water tariff compared to other cities, has the shortest IPBP. Figure 7
demonstrates that only 2.86 years is needed to return the investment on a solar still in this city.
Rasht is in the next place after Tehran, with 0.88 years longer IPBP. Tabriz and Bandar Abbas are the cities with the third and fourth shortest values of IPBP, which are equal to 3.81 and 3.88 years, respectively. For these cities, the water tariff is not as high as Tehran. Nonetheless, it is almost higher, and more than Ahvaz and Zahedan, which are in the last places from IPBP’s point of view.
Thanks to enjoying the most yield, Ahvaz has a lower IPBP compared to Zahedan. In Ahvaz, the value of 4.01 years is seen, while the value for Zahedan is 4.14 years.
Iran is a country in which subsidies are paid for water tariffs. Considering this point, the provided results for IPBP show a very acceptable range of IPBP for solar stills. Such great conditions from EPBP and IPBP aspects, in addition to a considerable AFWP for all the investigated cities, have demonstrated the significant potential of using solar still in diverse climatic conditions.
3.4. The Pairwise Comparisons
In order to conduct pairwise companions, people from academia, industry, and policy-making sectors are invited. From each side, two experts were involved, and during an online meeting, they determined the weights of pairwise comparison matrices. For each pairwise comparison, they discussed and reached a conclusion about each value. About the pairwise comparison of decision criteria, which is provided in Table 4
, they believe that:
Considering the fact that Iran has a great amount of fossil and renewable energy resources, EPPB is the least important parameter among the decision criteria.
Iran has serious water scarcity issues. Therefore, AFWP is the most important decision criterion.
IPBP has an importance in between. However, since IPBP is an essential factor for each investment plan, it should have a close importance to AFWP.
Moreover, the pairwise matrices of comparison of alternatives with respect to AFWP, EPBP, and IPBP are reported in Table 5
, Table 6
, and Table 7
3.5. Final Scores of Alternatives
Using the made pairwise comparisons and Expert Choice software, the final scores of alternatives are determined. It is graphically presented in Figure 8
demonstrates that among the cities, Ahvaz and Tehran have the highest preferences, with scores of 26.9 and 22.7 out of 100, respectively. Ahvaz has the most AFWP, while the highest water tariff belongs to Tehran. As discussed, AFWP has a direct relationship with ambient temperature and solar radiation, while it has a reverse impact with relative humidity and wind velocity. Therefore, the hot arid cities with high-received solar radiation and low wind-speed levels, or the ones that have almost high water tariffs are the best locations for installations of solar stills.
Moreover, based on Figure 8
, Zahedan is the third priority, which has a score of 21.1 out of 100. The lower priority of Zahedan compared to two other indicated cities, i.e., Ahvaz and Zahedan, comes from the low water tariff there. Consequently, offering incentives or low interest loans for purchasing the solar stills could be taken into account as good strategies for better justifiability.
While Ahvaz and Tehran are taken into account as the cities with high priority and Zahedan is the one with relatively high priority, Bandar Abbas, Rasht, and Azadshahr could be considered as the locations with below moderate priority. They have scores of 9.4, 6.8, and 6.7, respectively, while they have one point in common: being wet. The lowest priority is also for Tabriz, which has a score of only 6.4 out of 100.