Solar Energy to Water Desalination: Long-Term Experimental Studies of Solar Still in Poland
Department of Thermal and Fluid Flow Machines, Faculty of Energy and Fuels, AGH University of Krakow, al. A. Mickiewicza 30, 30-059 Krakow, Poland
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Author to whom correspondence should be addressed.
Energies 2025, 18(5), 1070; https://doi.org/10.3390/en18051070
Submission received: 6 February 2025
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Revised: 19 February 2025
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Accepted: 20 February 2025
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Published: 22 February 2025
(This article belongs to the Special Issue Solar Energy and Photovoltaic Systems: Prospect Development and Challenges)
Abstract
:Water scarcity is an escalating global issue which also affects Poland. One of the solutions to this challenge is seawater desalination, particularly using solar stills (SSs). SSs offer a sustainable, low-cost solution for desalination, but their efficiency depends mainly on local solar conditions. Therefore, this study presents long-term experimental results for a single-basin, single-slope SS’s performance in Krakow, Poland, from May to September 2022. The findings show that the SS effectively removed over 98% of total dissolved solids, with a productivity ranging from 1084 to 5014 mL/(m2·day) and water temperatures reaching up to 80.4 °C. These results highlight the feasibility of solar-powered desalination in Poland and contribute valuable data for future optimization.
1. Introduction
Water is probably one of the most crucial substances on Earth. It is used in humans’ everyday lives for domestic purposes, as well as in all kinds of industries. Globally, about 4.0·1012 m3 of freshwater is used every year [1], of which 70%, 20%, and 10% are for agriculture, industry, and domestic purposes, respectively [2]. Obviously, the total global water resources are incomparably greater than the overall water withdrawal, but almost 97% is saline water [3], which is not appropriate to be used directly neither in industry nor in the domestic or agriculture sectors. Therefore, water desalination is one of the solutions to the modern world’s water demand challenges [4,5].
Currently, there are several methods for water desalination, and the following five are the most commonly used: multi-stage flash [6], multi-effect distillation [7], reverse osmosis [8], electrodialysis [9], and nanofiltration [10]. Each of these methods uses electricity and/or thermal energy to operate. Bearing in mind that both thermal energy and electricity come from burning fossil fuels, which is related to negative effects like the emission of CO2, alternative water desalination technologies are emerging. In particular, solar-powered water desalination technologies are in high demand. Generally, solar-powered desalination can be divided into direct and indirect [11]. The idea of indirect methods is to integrate solar collectors or photovoltaics with the conventional desalination technologies that are listed at the beginning of this paragraph. In turn, in direct solar-powered desalination technologies, solar radiation is absorbed and converted into thermal energy to heat and evaporate water. There are two direct solar-powered desalination technologies, humidification–dehumidification and solar stills [11]. Humidification–dehumidification involves air flowing in a humidifier in which saline water is sprayed. Then, the humidified air flows to a dehumidifier, where the temperature of the air is lowered, causing the condensation of salt-free fresh water [12,13].
Solar stills (SSs) are the second direct solar-powered desalination technology. The working principle of SSs can be compared to the natural hydrologic cycle. Solar radiation is absorbed by the absorber, converted into heat, and transferred to saline water. Then, the salt-free water evaporates and condenses in the glass cover [14]. Water desalination in SSs possesses numerous advantages over other methods. These advantages include but are not limited to low capital and maintenance costs, simple design and implementation, and, most importantly, operation using only solar energy, without a need for electricity or heat from other sources [15]. On the other hand, SSs possess some drawbacks, of which low productivity is the most crucial. Productivity, also known as yield, refers to the volume or mass of distilled water obtained from the unit surface area of the SS per day.
An SS’s productivity depends on numerous factors, which are generally divided into meteorological, operating (e.g., water depth), and design (e.g., glass cover inclination) [16]. Among these parameters, only meteorological ones cannot be controlled, and they include solar irradiance, air temperature, and wind speed [17]. Meteorological parameters vary both daily and annually. As a result, an SS’s performance also changes across different seasons, which is often neglected in the literature.
The majority of papers In the field of SSs report results for just one or a couple of days, ignoring the rest of the year or season. There is a limited amount of research in which long-term experimental or theoretical studies were conducted. Long-term studies have only been carried out by, e.g., Muñoz et al. [18], Nian et al. [19], Ali et al. [20], Vaithilingam and Esakkimuthu [21], Nafey et al. [22], El-Sebaii and El-Naggar [23], and Sharon et al. [24].
As mentioned in the second paragraph, SSs suffer from a low productivity, and, thus, various approaches have been proposed to overcome this issue, such as geometry optimization [25], cover cooling [26], and using floating absorbers [27] or phase-change materials (PCMs) [28]. Researchers usually report a substantial increase in SSs’ productivity after the application of one of these methods of improvement. However, conventional SSs and enhanced SSs are often tested on different days. As raised in the previous paragraph, an SS’s productivity depends on meteorological conditions, which vary daily and seasonally. Therefore, comparing the productivity of conventional (unmodified) and modified SSs obtained on different days, and, thus, not under the same meteorological conditions, can raise some doubts. For example, Murali et al. [28] investigated a conventional SS, an SS with copper rods, an SS with copper rods filled with PCM, and an SS with copper rods filled with nano-enhanced PCM on sunny days in the January–March time frame. The authors presented the results obtained over four days claimed to have “the same solar intensities”. Nonetheless, it is worth noting that the authors only reported solar irradiance incidents on a horizontal surface. However, the solar irradiance incident on an SS’s glass cover could be different since it depends on solar declination, which, in turn, depends on the day of the year.
This paper presents the results of long-term experimental studies on a single-slope passive SS working in Krakow (Poland). The experiments were conducted in the May–September 2022 time frame. Firstly, the effect of feed water salinity on the removal efficiency and productivity was investigated. Secondly, the long-term productivity of the conventional SS and an SS enhanced with PCM was studied. The results reported in this paper can be used as a database for further studies, e.g., for developing a methodology for comparing SSs’ performances independently of meteorological parameters. In summary, the novelty of this paper lies in the fact that it examines the long-term performance of an SS, which has been overlooked in the literature. Additionally, the SS is examined in Krakow, Poland, which is rare in the literature.
2. Materials and Methods
2.1. Experimental Setup
The experimental setup, shown in Figure 1a,c, consists of an SS, a feed water tank, a distillate tank, a supporting structure, appropriate measuring devices (Table 1), and a data acquisition system. In the case of measurements with PCM, the appropriate number of pockets filled with PCM (Figure 1b) were placed on the absorber of the SS. The pockets were made of plastic, transparent on one side and black on the other. Their dimensions were as follows:
- Pocket A: 0.230 × 0.180 × 0.017 m3; PCM mass in one pocket 0.50 kg;
- Pocket B: 0.145 × 0.040 × 0.017 m3; PCM mass in one pocket 0.05 kg.
Two dimensions of the PCM pockets were used for the following reasons. The mass of PCM in the SS was 1.0 kg, 2.5 kg, and 5.0 kg. Thus, in the case of 5.0 kg of PCM, 10 “A” pockets were used. Their dimensions were 0.23 × 0.18 m2, and as the dimensions of the absorber were 1.0 × 0.5 m2, the pockets were distributed evenly over the entire surface of the absorber. The “B” pockets were smaller, as in the case of tests with 1.0 kg of PCM, the smaller sizes of the pockets allowed for the uniform distribution of PCMs on the surface of the absorber.
A detailed description of the experimental setup, including the locations of the sensors listed in Table 1, can be found in the authors’ previous papers [29,30,31,32], while the key features of the setup are provided below.
The SS is made of a stainless steel sheet. The dimensions of its absorber are 1.0 × 0.5 m2, while the height of its front wall is 0.1 m. The absorber (bottom wall) is painted black to increase its absorptance, while the rest of the walls are left in a factory state. The walls are insulated with 5 cm of styrofoam. The cover is made of glass (4 mm thickness) and inclined at 30°. The inclination angle of the glass cover should be equal to the latitude of the place where it is used to maximize annual productivity [33]. However, if a receiver of solar energy is to work mainly in the summer months, decreasing the inclination angle of the cover is advisable [34]. Therefore, an inclination angle of 30° was finally chosen. The water level in the SS is kept constant by using the feeding tank. The distilled water is collected in the distillate tank.
2.2. Experimental Procedure
All experiments were conducted at the AGH University of Krakow (latitude of 50°03′57.3″ N, longitude of 19°55′04.0″ E), located in Krakow, Poland, in the May–September 2022 time frame. Detailed descriptions of the experimental procedures are provided in Section 2.2.1 and Section 2.2.2.
2.2.1. The Effect of Water Salinity on the Removal Efficiency
The first series of experiments aimed to investigate how the salinity of feed water affected productivity and the distilled water quality. Therefore, water with three levels of salinity (0, 20, and 40 g/kg) was prepared by adding an appropriate quantity of salt (NaCl) to the tap water. In the case of 0 g/kg salinity, tap water without any further modifications was used. Then, the electrical conductivity of the water was measured and the SS was transported outside the building and placed on a dedicated platform. The SS was positioned in the south-west direction (the azimuth angle of the SS was approximately 22°). Then, 10 kg of water with a given salinity was poured into the SS, the cover was closed, and the experiment started. Each experiment began at 8 a.m. At 7 p.m., the SS was transported inside the building to protect it from unforeseen and undesirable atmospheric phenomena, such as hail, and to prevent access to the SS by unauthorized persons. Nevertheless, all measured parameters were recorded until 7 a.m. the following day. After the test was completed, the electrical conductivity of the distilled water was measured, the SS was cleaned, and the entire stand was prepared for the next experiment. The experiments with each salinity were repeated twice, which gave a total of six experiments.
2.2.2. The Productivity of SS
The second series of experiments aimed to investigate the productivity of the SS under various weather conditions with and without PCM. The experimental procedure in this series was analogous to the first series, with the difference that tap water (10 kg) without further modifications was used and, in some tests, an appropriate quantity of PCM pockets was placed on the absorber of the SS. The tests were conducted without PCM and with 1 kg (20 “B” pockets), 2.5 kg (5 “A” pockets), and 5.0 kg (10 “A” pockets) of PCM. A total of 23 experiments were performed, of which 5 were without PCM, while 18 were with PCM.
Two PCMs were used in the experiments, paraffin wax LTP 56/20 and paraffin wax LTP 64/30 (manufacturer Polwax S.A., Jasło, Poland). Throughout this paper, the terms PCM 1 and PCM 2 will refer to paraffin wax 56/20 and paraffin wax 64/30, respectively. The thermal properties of these PCMs were investigated in the authors’ previous paper [30] and are listed in Table 2.
3. Results and Discussion
3.1. The Effect of Water Salinity on the Removal Efficiency
The SS’s productivity and the electrical conductivity of the distilled water in the function of feed water salinity are presented in Figure 2. The electrical conductivity of the distilled water varies in the range of 4.3–7.0 µS/cm, independently of the feed water salinity. Taking into account the electrical conductivity of the feed water in the range of 391–53,500 µS/cm (Table 3), the SS’s removal efficiency is more than 98%, which proves the effectiveness of the investigated SS. The removal efficiency ξ is calculated as follows:
where EC25 is the electrical conductivity at a 25 °C temperature, while the subscripts feed and dist stand for feed water and distilled water, respectively. Regarding the productivity of the SS, it varies from 1796 mL/m2/day to 4167 mL/m2/day, and increases with an increasing solar irradiation.
ξ = (EC25,feed – EC25,dist)/EC25,feed·100%,
3.2. The Productivity of SS
The SS’s productivity for 23 different experiments’ meteorological and operating parameters is presented in Table 4. The results are not listed chronologically, but they are arranged as follows: SS without PCM, SS with an increasing mass of PCM 1, and SS with an increasing mass of PCM 2.
The productivity of the SS with and without PCM varies in the range of 1084–4783 mL/m2/day and 1889–5014 mL/m2/day, respectively. Such results are obtained under solar irradiation on a horizontal surface of 2.6–6.7 kWh/m2. It is worth noting that the SS’s cover is inclined at an angle of 30°, and, thus, the actual solar radiation incident on the cover and then on the absorber of the SS differs from that of the horizontal surface. The solar irradiation on the inclined surface varies from 2.9 kWh/m2 to 7.6 kWh/m2, which is slightly higher compared to the solar irradiation on the horizontal surface.
As solar irradiance is the most important factor affecting productivity, and bearing in mind that it varied in a wide range during the experiments, the authors also calculate the specific productivity. The specific productivity is calculated by dividing the productivity by the solar irradiation on a horizontal surface, i.e., V/I, and dividing the productivity by the solar irradiation on the inclined surface, i.e., V/I30. The specific productivity V/I is 411–809 mL/kWh, while the specific productivity V/I30 is 368–772 mL/kWh.
Specific productivity is a parameter that allows for a better comparison between solar stills working with different meteorological parameters. When an SS works on two different days with the same values of I and I30 and if the values of specific productivity differ for those two cases, this denotes that some other parameters affect the productivity. For example, we can compare the productivity of the SS on 11 May 2022 and 6 June 2022. In the first case (11 May 2022), the SS worked without PCM, while in the second (6 June 2022), it worked with 5.0 kg of PCM 2. The solar irradiation on a horizontal surface is almost the same in those two cases and the same is true for the solar irradiation on an inclined surface. Also, the average air temperature and wind speed are comparable, but the SS’s productivity on 11 May 2022 is about 11% higher than that on 6 June 2022. Therefore, the decreased productivity in the second case can probably be attributed to the use of PCM.
Although solar irradiance is the factor affecting productivity the most, it should also be noted that air temperature and wind speed influence an SS’s productivity as well [35]. The daily average air temperature during the experiments varied from 17.4 °C to 28.6 °C, while the daily average wind speed was in the range of 0.0–0.4 m/s.
The correlation between productivity, solar irradiation, air temperature, and wind speed is illustrated in Figure 3 and Figure 4. Figure 3 shows the productivity in the function of solar irradiation on a horizontal surface, while Figure 4 presents the productivity in the function of solar irradiation on an inclined surface. Both figures include also air temperature (marked by colours) and wind speed (marked by markers’ sizes). Additionally, the experiments without PCM are marked with arrows. As one can see, the productivity of the SS increases with an increasing solar irradiation, both on horizontal and inclined surfaces.
Both the meteorological parameters, i.e., the solar irradiance and air temperature, as well as the SS’s operating parameters (the temperature of the absorber, water, inner and outer glass cover, vapor, and PCM when applicable) in the function of time, are presented in Figure 5 for all of the 23 experiments. Point 0 on the time axis denotes the start of the experiments, i.e., 8 a.m.
Regarding solar irradiance, the greatest and the smoothest results are reported for 10 May 2022 (Figure 5a), 18 May 2022 (Figure 5c), 19 May 2022 (Figure 5d), and 25 July 2022 (Figure 5f). A smooth line indicates a lack of clouds and a high share of direct solar radiation, which, in turn, promotes the productivity of the SS. The peak solar irradiance during the abovementioned experiments reaches about 900 W/m2, while the solar irradiation on a horizontal surface is 5.90 kWh/m2/day, 6.69 kWh/m2/day, 6.44 kWh/m2/day, and 6.13 kWh/m2/day on 10 May 2022, 18 May 2022, 19 May 2022, and 25 July 2022, respectively. The SS’s productivity on those days reaches the highest observed values, i.e., 4393 mL/m2/day, 4769 mL/m2/day, 5014 mL/m2/day, and 4783 mL/m2/day, respectively.
The air temperature is the second important meteorological parameter that affects the SS’s performance. As illustrated in Figure 5, the air temperature varies with time during the experiments and also between individual experiments. However, during all of the experiments, the air temperature increases until it reaches its peak, and then slowly decreases. There is also an apparent sudden drop in the air temperature after 11 h from the experiment’s beginning. This drop is a result of moving the SS into the building, for reasons that were discussed in Section 2.2.
In summary, solar irradiance, air temperature, and wind speed are key operational factors influencing the performance of the SS. Among these, solar irradiance is the most critical parameter, exhibiting a direct correlation with productivity—a higher irradiance results in an increased productivity. However, the specific impacts of air temperature and wind speed remain inconclusive, as no experimental conditions isolated these variables while keeping all other parameters constant.
Regarding the SS’s operating temperatures, it can be observed that the temperature changes in the absorber, water, inner and outer glass cover, vapor, and PCM are analogous to changes in solar irradiance. Additionally, the lines illustrating the absorber and water temperatures are relatively smooth, while the outer glass cover’s temperature fluctuates, which is a result of gusts of wind. Also, in some cases, e.g., Figure 5h,i, strong fluctuations in the vapor and PCM temperatures are observed. These fluctuations are caused by the solar radiation incident occurring directly on those temperature sensors.
The water temperature can reach values as high as 80.4 °C. A high water temperature promotes water evaporation, and the higher the water temperature, the higher its evaporation rate [30]. Additionally, a high water temperature inactivates various protozoa and bacteria, e.g., E. coli, rotavirus, and Salmonella typhi are inactivated at 60 °C, while Hepatitis A virus is inactivated at 65 °C [36]. Therefore, the water temperatures achieved in the SS are high enough to inactivate many biological impurities, which is beneficial for the further use of water for sanitary and living purposes.
3.3. Comparison to Literature
To place the results obtained in this study in a broader context, the authors compare the results of the study with results reported in the literature, which is presented in Table 5. The table includes only studies on single-slope and single-basin solar stills. As can be seen, the results of this study are consistent with the previous findings in the literature. It is also worth noting that comprehensive meteorological data have not been reported in many studies. As mentioned in the previous sections, meteorological parameters, especially solar irradiance, affect an SS’s productivity. Therefore, knowing these parameters is crucial when comparing the performances of different solar stills.
4. Conclusions
This paper presents the results of experimental studies on the performance of single-basin, single-slope SS in Krakow, Poland, in the May–September 2022 time frame.
The results confirm that this SS is a viable method for water desalination, achieving over a 98% removal efficiency for total dissolved solids. The distilled water maintained a low electrical conductivity (4.3–7.0 μS/cm), regardless of the feed water salinity (0–40 g/kg). This indicates a good potential for SSs in water desalination, especially in applications where an extremely high water quality is required, e.g., hydrogen production by electrolysis. Additionally, SSs are supposed to purify water from biological impurities (e.g., various bacteria) due to the high temperature (by up to 80.4 °C) reached by the water. This research also revealed that the SS’s productivity in Polish weather conditions was satisfactory and could reach a value of up to 5014 mL/m2/day.
Despite the promising results, this work has also some minor limitations. Firstly, the SS was investigated in the May–September time frame, so further research on its year-round performance is required. Secondly, this work focuses only on the performance parameters of the SS, lacking economic and life-cycle assessments, which need to be examined. There are also a lot of possibilities, different from the use of PCMs, for improving the performance of the SS, which can be investigated in the future.
This work also demonstrated that an SS’s performance depends on meteorological parameters, especially solar irradiance. These meteorological parameters fluctuate each day, which affects and makes it difficult to compare the results obtained on various days. Therefore, future work should concentrate on developing a methodology that would allow for comparing an SS’s performance independently of meteorological parameters.
Author Contributions
Conceptualization, E.R. and Ł.M.; methodology, E.R.; software, E.R.; validation, E.R. and Ł.M.; formal analysis, E.R. and Ł.M.; investigation, E.R.; resources, E.R.; data curation, E.R.; writing—original draft preparation, E.R.; writing—review and editing, Ł.M.; visualization, E.R.; supervision, Ł.M.; project administration, Ł.M.; funding acquisition, Ł.M. All authors have read and agreed to the published version of the manuscript.
Funding
This research was funded by the Ministry of Science and Higher Education, Poland, grant AGH number 16.16.210.476.
Data Availability Statement
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.
Conflicts of Interest
The authors declare no conflicts of interest.
Abbreviations
The following abbreviations are used in this manuscript:
| PCM | Phase-change material |
| SS | Solar still |
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Figure 1.
Experimental setup: (a) front view on the solar still; (b) pockets filled with PCM; and (c) scheme of cross-section: 1—feedwater tank; 2—pipe system; 3—shut-off valve; 4—three-way valve; 5—glass cover; 6—distillate trough; 7—distillate channel; 8—PCM; 9—thermal insulation; 10—distillate tank; 11—water level sensor; and T—temperature. Subscripts: air—ambient air; b—absorber; dw—distilled water; f1, f2—humid air; fw—feedwater; gi—inner glass cover; go—outer glass cover; PCM—phase-change material; and w—water.
Figure 1.
Experimental setup: (a) front view on the solar still; (b) pockets filled with PCM; and (c) scheme of cross-section: 1—feedwater tank; 2—pipe system; 3—shut-off valve; 4—three-way valve; 5—glass cover; 6—distillate trough; 7—distillate channel; 8—PCM; 9—thermal insulation; 10—distillate tank; 11—water level sensor; and T—temperature. Subscripts: air—ambient air; b—absorber; dw—distilled water; f1, f2—humid air; fw—feedwater; gi—inner glass cover; go—outer glass cover; PCM—phase-change material; and w—water.
Figure 2.
Solar irradiation, SS’s productivity, and distilled water electrical conductivity in the function of feed water salinity.
Figure 2.
Solar irradiation, SS’s productivity, and distilled water electrical conductivity in the function of feed water salinity.
Figure 3.
SS’s productivity in the function of solar irradiation on horizontal surface, air temperature, and wind velocity (experiments without PCM are marked with arrows).
Figure 3.
SS’s productivity in the function of solar irradiation on horizontal surface, air temperature, and wind velocity (experiments without PCM are marked with arrows).
Figure 4.
SS’s productivity in the function of solar irradiation on an inclined surface, air temperature, and wind velocity (experiments without PCM are marked with arrows).
Figure 4.
SS’s productivity in the function of solar irradiation on an inclined surface, air temperature, and wind velocity (experiments without PCM are marked with arrows).
Figure 5.
Weather conditions and SS’s working temperatures: (a) 10.05.2022; (b) 11.05.2022; (c) 18.05.2022; (d) 19.05.2022; (e) 5.09.2022; (f) 25.07.2022; (g) 27.07.2022; (h) 9.08.2022; (i) 10.08.2022; (j) 17.08.2022; (k) 18.08.2022; (l) 25.08.2022; (m) 30.08.2022; (n) 27.06.2022; (o) 30.06.2022; (p) 4.07.2022; (q) 1.06.2022; (r) 15.06.2022; (s) 22.06.2022; (t) 23.06.2022; (u) 31.05.2022; (v) 3.06.2022; and (w) 6.06.2022.
Figure 5.
Weather conditions and SS’s working temperatures: (a) 10.05.2022; (b) 11.05.2022; (c) 18.05.2022; (d) 19.05.2022; (e) 5.09.2022; (f) 25.07.2022; (g) 27.07.2022; (h) 9.08.2022; (i) 10.08.2022; (j) 17.08.2022; (k) 18.08.2022; (l) 25.08.2022; (m) 30.08.2022; (n) 27.06.2022; (o) 30.06.2022; (p) 4.07.2022; (q) 1.06.2022; (r) 15.06.2022; (s) 22.06.2022; (t) 23.06.2022; (u) 31.05.2022; (v) 3.06.2022; and (w) 6.06.2022.
Table 1.
Measuring devices.
| Parameter | Device | Range | Uncertainty |
|---|---|---|---|
| The temperature of the absorber and inner and outer glass cover | RTD Pt100 (TP878.3, Delta Ohm, Padua, Italy) | −30 to +200 °C | ±(0.1 °C + 0.1% |T|) °C |
| The temperature of the water, PCM, and ambient air | RTD Pt1000 (SN108, Comet System, Rožnov pod Radhoštěm, Czech Republic) | 0 to +150 °C | ±0.3 °C |
| Distilled water volume | Float liquid level sensor (EIEWIN, Gliwice, Poland) | 0 to 2365 mL | ±30 mL |
| Solar irradiance | Pyranometer (SR05-D2A2, Hukseflux, Delft, The Netherlands) | 0 to 1600 W/m2 | ±8.4% of the measured value |
| Wind speed | Wind speed sensor (WSS 100/4–20 mA, NAVIS elektronika, Kamnik, Slovenia) | 0.7 to 50.0 m/s | ±0.15 m/s or 2.5% of measured value |
| Electrical conductivity | Conductivity measuring device (Greisinger GLF 100, GHM Messtechnik GmbH, Regenstauf, Germany) | 0.0–100 mS/cm | ±(0.5% of the measured value + 0.5% of the measuring range) |
Table 2.
PCMs’ thermal properties.
| PCM | Peak Melting Temperature, °C | Peak Solidification Temperature, °C | Latent Heat of Fusion, J/g | Thermal Conductivity, W/(mK) |
|---|---|---|---|---|
| 1 | 55.8 | 53.3 | 192.7 | 0.251 |
| 2 | 59.9 | 58.6 | 181.5 | 0.267 |
Table 3.
SS’s performance in the function of feed water salinity.
| Feed Water Salinity, g/kg | Date | Electrical Conductivity, μS/cm | ξ, % | V, mL/m2/day | I, kWh/m2/day | Tair, °C | ||
|---|---|---|---|---|---|---|---|---|
| Feed Water | Distilled Water | Brine | ||||||
| 0 | 7 July 2022 | 391.0 | 4.3 | 459.0 | 98.89 | 1796 | 2.72 | 22.1 |
| 0 | 18 July 2022 | 398.3 | 6.0 | 498.0 | 98.49 | 4070 | 5.08 | 23.7 |
| 20 | 19 July 2022 | 31,600.0 | 5.3 | 42,900.0 | 99.98 | 3694 | 4.51 | 26.4 |
| 20 | 20 July 2022 | 30,633.3 | 6.7 | 45,800.0 | 99.98 | 3977 | 4.73 | 29.0 |
| 40 | 21 July 2022 | 53,500.0 | 7.0 | 69,000.0 | 99.99 | 4167 | 4.99 | 29.1 |
| 40 | 22 July 2022 | 52,800.0 | 6.3 | 55,633.3 | 99.99 | 2416 | 3.23 | 27.5 |
ξ—removal efficiency; V—productivity; I—solar irradiation on a horizontal surface; and Tair—average air temperature.
Table 4.
Long-term SS performance.
| No. | Date | PCM | mPCM, kg | I, kWh/m2 | I30, kWh/m2 | Tair, °C | vavg, m/s | Tw,max, °C | TPCM,max, °C | V, mL/m2/day | V/I, mL/kWh | V/I30, mL/kWh |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 10 May 2022 | - | 0.0 | 5.90 | 6.85 | 19.6 | 0.30 | 75.1 | - | 4393 | 745 | 641 |
| 2 | 11 May 2022 | - | 0.0 | 4.79 | 5.10 | 22.7 | 0.36 | 70.3 | - | 3072 | 641 | 602 |
| 3 | 18 May 2022 | - | 0.0 | 6.69 | 7.59 | 17.4 | 0.35 | 75.1 | - | 4769 | 713 | 628 |
| 4 | 19 May 2022 | - | 0.0 | 6.44 | 7.26 | 21.3 | 0.30 | 79.6 | - | 5014 | 779 | 691 |
| 5 | 5 September 2022 | - | 0.0 | 3.15 | 3.60 | 19.7 | 0.22 | 65.3 | - | 1889 | 600 | 525 |
| 6 | 25 July 2022 | 1 | 1.0 | 6.13 | 6.87 | 24.9 | 0.16 | 80.4 | 84.1 | 4783 | 780 | 696 |
| 7 | 27 July 2022 | 1 | 1.0 | 3.55 | 3.59 | 22.4 | 0.14 | 59.4 | 60.4 | 2459 | 693 | 685 |
| 8 | 9 August 2022 | 1 | 2.5 | 3.85 | 4.21 | 22.6 | 0.42 | 59.1 | 74.1 | 2315 | 601 | 550 |
| 9 | 10 August 2022 | 1 | 2.5 | 4.75 | 5.34 | 22.7 | 0.38 | 63.3 | 76.3 | 2742 | 577 | 513 |
| 10 | 17 August 2022 | 1 | 2.5 | 3.27 | 3.54 | 24.9 | 0.23 | 59.8 | 74.5 | 1795 | 549 | 507 |
| 11 | 18 August 2022 | 1 | 5.0 | 4.60 | 5.22 | 25.2 | 0.34 | 66.2 | 73.4 | 2601 | 565 | 498 |
| 12 | 25 August 2022 | 1 | 5.0 | 3.73 | 4.21 | 24.3 | 0.23 | 60.7 | 63.6 | 1938 | 520 | 461 |
| 13 | 30 August 2022 | 1 | 5.0 | 2.64 | 2.95 | 21.3 | 0.17 | 52.8 | 59.9 | 1084 | 411 | 368 |
| 14 | 27 June 2022 | 2 | 1.0 | 5.45 | 5.81 | 27.0 | 0.29 | 80.0 | 84.5 | 4168 | 765 | 718 |
| 15 | 30 June 2022 | 2 | 1.0 | 4.74 | 4.97 | 28.6 | 0.14 | 78.4 | 82.7 | 3836 | 809 | 772 |
| 16 | 4 July 2022 | 2 | 1.0 | 5.52 | 5.82 | 26.7 | 0.00 | 79.1 | 82.3 | 4308 | 780 | 741 |
| 17 | 1 June 2022 | 2 | 2.5 | 5.15 | 5.74 | 20.0 | 0.34 | 70.7 | 79.3 | 3164 | 614 | 552 |
| 18 | 15 June 2022 | 2 | 2.5 | 6.05 | 6.52 | 21.1 | 0.30 | 70.5 | 80.4 | 4018 | 664 | 616 |
| 19 | 22 June 2022 | 2 | 2.5 | 5.88 | 6.25 | 22.3 | 0.41 | 70.4 | 80.0 | 3784 | 644 | 606 |
| 20 | 23 June 2022 | 2 | 2.5 | 5.91 | 6.38 | 23.5 | 0.32 | 72.3 | 80.2 | 3737 | 632 | 585 |
| 21 | 31 May 2022 | 2 | 5.0 | 5.32 | 5.75 | 19.5 | 0.34 | 65.2 | 72.0 | 3162 | 594 | 550 |
| 22 | 3 June 2022 | 2 | 5.0 | 6.44 | 7.05 | 21.1 | 0.40 | 70.2 | 78.0 | 3873 | 601 | 550 |
| 23 | 6 June 2022 | 2 | 5.0 | 4.74 | 5.07 | 21.9 | 0.27 | 67.9 | 74.7 | 2739 | 578 | 540 |
mPCM—PCM mass; I—solar irradiation on a horizontal surface; I30—solar irradiation on an inclined surface; Tair—daily average air temperature; vavg—daily average wind speed; Tw,max—maximum water temperature; TPCM,max—maximum PCM temperature; and V—productivity.
Table 5.
SS’s performance literature comparison.
| Ref. | Location | Date | I, kWh/m2 | Tair, °C | V, mL/m2/day | Remarks |
|---|---|---|---|---|---|---|
| Nafey et al. [22] | Suez City, Egypt | January | 3.85 | 23.6 | 2350 | Different water depths (from 1.0 cm to 3.0 cm) |
| February | 4.56 | 23.7 | 2810 | |||
| March | 5.75 | 24.0 | 3490 | |||
| April | 6.52 | 24.4 | 4750 | |||
| May | 7.71 | 28.5 | 5830 | |||
| June | 7.63 | 31.0 | 5900 | |||
| July | 7.41 | 32.4 | 5390 | |||
| August | 6.94 | 34.0 | 5380 | |||
| September | 6.45 | 30.8 | 4960 | |||
| October | 5.59 | 28.7 | 4100 | |||
| November | 4.20 | 25.0 | 2400 | |||
| December | 4.00 | 24.0 | 2500 | |||
| Nagaraju et al. [37] | Vijayawada, Andhra Pradesh, India | n.d. | n.d. | n.d. | 907 | - |
| n.d. | n.d. | n.d. | 1043 | |||
| n.d. | n.d. | n.d. | 1050 | |||
| Kumar et al. [38] | Coimbatore, Tamilnadu, India | 19 April 2019 | 6.92 | 31.5 | 5467 | No PCM |
| 19 April 2019 | 6.92 | 31.5 | 8267 | With 5 kg of PCM | ||
| 19 April 2019 | 6.92 | 31.5 | 9133 | With 5 kg of nano-enhanced PCM | ||
| Vaithilingam and Esakkimuthu [21] | Chennai, India | 13 March 2013 | 6.255 | 32.4 | 2970 | Different water depths (from 1.0 cm to 2.5 cm) |
| 22 March 2013 | 6.261 | 34.1 | 2666 | |||
| 4 April 2013 | 6.237 | 35.0 | 2494 | |||
| 18 April 2013 | 6.249 | 35.9 | 1840 | |||
| Kumar and Prakash [39] | India | n.d. | 8.64 | 35.8 | 1444 | Water depth of 3 cm, no PCM |
| n.d. | 2163 | Water depth of 3 cm, with 17 kg of PCM | ||||
| n.d. | 7.90 | 39.2 | 1062 | Water depth of 6 cm, no PCM | ||
| n.d. | 1159 | Water depth of 6 cm, with 17 kg of PCM | ||||
| n.d. | 7.52 | 35.8 | 736 | Water depth of 9 cm, no PCM | ||
| n.d. | 949 | Water depth of 9 cm, with 17 kg of PCM | ||||
| n.d. | 7.99 | 35.7 | 728 | Water depth of 12 cm, no PCM | ||
| n.d. | 737 | Water depth of 12 cm, with 17 kg of PCM | ||||
| n.d. | 8.64 | 37.2 | 1035 | Water depth of 15 cm, no PCM | ||
| n.d. | 1039 | Water depth of 15 cm, with 17 kg of PCM | ||||
| Jahanpanah et al. [40] | Tehran, Iran | 3 October 2019 | n.d. | n.d. | 2917 | No PCM |
| 4 October 2019 | 6.7 | n.d. | 3150 | With 3 kg of PCM | ||
| 5 October 2019 | n.d. | n.d. | 3800 | With 6 kg of PCM | ||
| Hameed [41] | Najaf, Iraq | 16 October 2021 | 4.7 | 30.2 | 2218 | - |
| Ali et al. [20] | Karachi, Pakistan | January 2022 | n.d. | n.d. | 1719 | Results for representative days of the month for the year 2022; simulations previously validated by experimental studies. |
| February 2022 | n.d. | n.d. | 2021 | |||
| March 2022 | 5.8 | n.d. | 2046 | |||
| April 2022 | n.d. | n.d. | 2010 | |||
| May 2022 | n.d. | n.d. | 2104 | |||
| June 2022 | 6.3 | n.d. | 1732 | |||
| July 2022 | n.d. | n.d. | 1598 | |||
| August 2022 | n.d. | n.d. | 1587 | |||
| September 2022 | 6.3 | n.d. | 2125 | |||
| October 2022 | n.d. | n.d. | 1939 | |||
| November 2022 | n.d. | n.d. | 1667 | |||
| December 2022 | 3.6 | n.d. | 1519 | |||
| This work | Krakow, Poland | 10 May 2022 | 5.90 | 19.6 | 4393 | No PCM |
| 11 May 2022 | 4.79 | 22.7 | 3072 | |||
| 18 May 2022 | 6.69 | 17.4 | 4769 | |||
| 19 May 2022 | 6.44 | 21.3 | 5014 | |||
| 5 September 2022 | 3.15 | 19.7 | 1889 | |||
| 25 July 2022 | 6.13 | 80.4 | 4783 | with 1.0 kg of PCM 1 | ||
| 27 July 2022 | 3.55 | 59.4 | 2459 | with 1.0 kg of PCM 1 | ||
| 9 August 2022 | 3.85 | 59.1 | 2315 | with 2.5 kg of PCM 1 | ||
| 10 August 2022 | 4.75 | 63.3 | 2742 | with 2.5 kg of PCM 1 | ||
| 17 August 2022 | 3.27 | 59.8 | 1795 | with 2.5 kg of PCM 1 | ||
| 18 August 2022 | 4.60 | 66.2 | 2601 | with 5.0 kg of PCM 1 | ||
| 25 August 2022 | 3.73 | 60.7 | 1938 | with 5.0 kg of PCM 1 | ||
| 30 August 2022 | 2.64 | 52.8 | 1084 | with 5.0 kg of PCM 1 | ||
| 27 June 2022 | 5.45 | 80.0 | 4168 | with 1.0 kg of PCM 2 | ||
| 30 June 2022 | 4.74 | 78.4 | 3836 | with 1.0 kg of PCM 2 | ||
| 4 July 2022 | 5.52 | 79.1 | 4308 | with 1.0 kg of PCM 2 | ||
| 1 June 2022 | 5.15 | 70.7 | 3164 | with 2.5 kg of PCM 2 | ||
| 15 June 2022 | 6.05 | 70.5 | 4018 | with 2.5 kg of PCM 2 | ||
| 22 June 2022 | 5.88 | 70.4 | 3784 | with 2.5 kg of PCM 2 | ||
| 23 June 2022 | 5.91 | 72.3 | 3737 | with 2.5 kg of PCM 2 | ||
| 31 May 2022 | 5.32 | 65.2 | 3162 | with 5.0 kg of PCM 2 | ||
| 3 June 2022 | 6.44 | 70.2 | 3873 | with 5.0 kg of PCM 2 | ||
| 6 June 2022 | 4.74 | 67.9 | 2739 | with 5.0 kg of PCM 2 |
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Mika, Ł.; Radomska, E. Solar Energy to Water Desalination: Long-Term Experimental Studies of Solar Still in Poland. Energies 2025, 18, 1070. https://doi.org/10.3390/en18051070
AMA Style
Mika Ł, Radomska E. Solar Energy to Water Desalination: Long-Term Experimental Studies of Solar Still in Poland. Energies. 2025; 18(5):1070. https://doi.org/10.3390/en18051070
Chicago/Turabian StyleMika, Łukasz, and Ewelina Radomska. 2025. "Solar Energy to Water Desalination: Long-Term Experimental Studies of Solar Still in Poland" Energies 18, no. 5: 1070. https://doi.org/10.3390/en18051070
APA StyleMika, Ł., & Radomska, E. (2025). Solar Energy to Water Desalination: Long-Term Experimental Studies of Solar Still in Poland. Energies, 18(5), 1070. https://doi.org/10.3390/en18051070
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