Efficient passive solar desalination using cooling tower integration and thermal insulation

Abstract

Freshwater scarcity remains a critical global issue, particularly in arid regions with inadequate infrastructure. In this study, a passive, solar-powered desalination system was designed and evaluated for continuous freshwater production without reliance on fossil fuels or external electricity sources. The system integrates a solar water heater, a thermally insulated evaporation chamber, and a spiral condenser coil linked to a water-based cooling tower, enhancing daytime and nighttime water recovery. This innovation achieves higher efficiency with minimal operational cost, demonstrating significant potential for deployment in water-stressed regions. Field tests conducted in Tehran, Iran, during August and September 2023 showed a substantial increase in daily water output following the integration of the cooling tower—from 3018 to 6978 mL in August and from 2409 to 7016 mL in September—representing gains of 286.82 and 231.21%, respectively (p < 0.0001). The device also sustained overnight water production due to effective thermal insulation, yielding 1936 ± 51.55 mL of distilled water after sunset. Water quality analysis confirmed the removal of toxic elements while retaining essential minerals within safe limits. The system’s embodied carbon footprint was estimated at 0.139 kg CO₂ per liter of water produced, with zero operational emissions. Economic analysis indicated a cost of 0.526 USD/L in the first year, decreasing to 0.105 USD/L over five years, with an internal rate of return (IRR) of 26.50%. The proposed integration of a cooling tower and thermal insulation significantly enhances water yield and operational efficiency, outperforming conventional passive desalination systems in both distillate output and energy conservation. This positions the system as a superior, scalable, and sustainable solution for decentralized freshwater generation in off-grid and water-stressed regions.

Introduction

Water scarcity is a critical global challenge, increasingly exacerbated in the 21st century by climate change, population growth, and water pollution. By 2030, it is estimated that approximately 40% of the global population will experience severe water shortages^1–3. Freshwater constitutes only about 3% of the Earth’s total water resources, with the remaining 97% found in oceans and seas as saline water^3,4. Arid regions, characterized by significantly below-average annual rainfall, face acute water scarcity due to the uneven spatial and temporal distribution of precipitation⁵. This imbalance intensifies the risk of famine and negatively impacts agriculture, domestic water supply, and industrial activity⁶. In such environments, desalination represents a practical and sustainable approach to mitigating water scarcity⁵, highlighting the urgent need for efficient and scalable freshwater production technologies.

Seawater, as an abundant and renewable resource, offers a sustainable solution for freshwater generation, particularly in coastal regions⁷. Desalination technologies are employed to remove salts and other impurities from seawater, brine, or contaminated water sources to produce potable water⁸. Historically, desalination began with thermal methods involving evaporation and condensation. In these processes, thermal energy is applied to heat seawater, generating vapor that condenses into freshwater. When desalination was first introduced in the late 1850s, the primary objective was to supply water for ship boilers and drinking needs, with minimal concern for economic viability⁹.

In the 1960s, thermal desalination technologies such as multi-stage flash (MSF) and multi-effect distillation (MED) became the dominant large-scale methods through the early 2000s¹⁰. Reverse osmosis (RO), a membrane-based desalination technique, was introduced in the 1970s and was initially used for brackish water treatment. Subsequent advancements in membrane performance and durability during the 1980s enabled wider adoption of RO for seawater desalination. Since the late 1990s, membrane-based methods—particularly RO and nanofiltration (NF)—have gained dominance due to enhanced energy efficiency, scalability, and cost-effectiveness¹¹.

Today, thermal and membrane-based systems remain the two principal constitute the two primary classes of desalination technologies^12–17. The selection of an appropriate method depends on a variety of factors, including the intended application of the freshwater, the quality and salinity of the feedwater, capital and operational costs, energy consumption, opportunities for energy recovery, and the scale of the plant¹⁸.

Common thermal desalination techniques include MSF, MED, and thermal vapor compression (TVC), which operate by heating saline water to induce evaporation—often under vacuum—followed by condensation of the vapor to produce freshwater. Large-scale thermal distillation plants, particularly in the Middle East, have been operational since the 1930s¹⁹.

Despite their widespread use, conventional thermal desalination methods are highly energy-intensive and primarily dependent on fossil fuel-derived heat, contributing significantly to greenhouse gas emissions, including CO₂, CH₄, N₂O, and water vapor^7,13,20. Among these, CO₂ is the most influential driver of global temperature rise, reinforcing the need for low-emission desalination alternatives²¹. Additionally, the high energy demand of fossil fuel-powered systems elevates operating costs, limiting their applicability in low-resource settings²².

To address these challenges, solar-powered desalination technologies are under increasing investigation. Among the most studied are solar still distillation (SSD), solar chimneys (SC), and humidification–dehumidification (HDH) systems. SSD systems, typically consisting of a blackened saline water basin sealed with an inclined transparent cover, use solar radiation to induce evaporation and condensation on the inner surface of the glass. The resulting freshwater is collected as condensate^23,24. While SSD systems produce high-quality water and are fully solar-powered, daily yields are limited to approximately 2–3 L per square meter, making them suitable only for small-scale applications²⁵.

Solar stills continue to attract interest due to their simplicity, low initial cost, ease of use, and exclusive reliance on solar energy. These characteristics make them well-suited for off-grid or rural locations where water demand is modest and sunlight is abundant. However, their broader application is constrained by: (1) Low water productivity, (2) Requiring large surface areas; (3) Dependence on direct solar radiation, (4) limiting output during cloudy or low-light conditions; (5) Slow evaporation–condensation rates, reducing efficiency in high-demand settings.

Recent innovations in solar-assisted desalination have focused on hybrid systems incorporating photovoltaic (PV) modules. One such system improved daytime output but exhibited limited nighttime productivity due to the absence of thermal storage²⁶. Another study assessed economic feasibility but did not include a comprehensive life-cycle cost analysis or account for embodied carbon emissions. While meteorological variability was analysed, strategies to ensure consistent performance under fluctuating conditions were not proposed^27,28. Other works highlighted public health benefits but failed to address implementation challenges in off-grid regions²⁹, or evaluated water quality without considering essential mineral retention and the risk of over-purification³⁰.

In contrast, this study presents a fully passive solar desalination system that integrates elastomeric thermal insulation with a water-based cooling tower, enabling continuous water production during both day and night without reliance on electricity or photovoltaic input. A detailed economic evaluation is also provided, incorporating capital costs, the internal rate of return (IRR), and embodied CO₂ emissions. These contributions directly respond to the limitations identified in prior literature and offer a scalable, sustainable solution for freshwater generation in arid and semi-arid environment.

Results

The solar water desalination device was developed at the Chemistry Research Institute and assessed under the climatic conditions of Tehran. Tehran is located between 51°2′ to 51°36′ east longitude and 35°34′ to 35°50′ north latitude. The city’s elevation varies significantly, ranging from about 1050 m above sea level in the southern parts to approximately 2000 m in the north.

To assess the feasibility and performance of the solar desalination device, climatic and solar radiation data from 2023 were analyzed, collected from Mehrabad Airport. This dataset includes average daily incident shortwave solar radiation, along with average high and low daily temperatures, as presented in (Fig. 1A–C).

Fig. 1.

Environmental data from Mehrabad Airport (2023) used to evaluate solar desalination performance in August and September. (A) Daily temperature patterns throughout the year, with maximum temperatures reaching 105 °F and minimums around 70 °F in August, and ranging from 100 to 59 °F in September—conditions that significantly influence evaporation and heat loss in the system. (B) Average daily solar irradiation shows high energy availability during the testing period, with 7.4 kWh/m² in August and 7.0 kWh/m² in September. (C) Sun path diagram illustrating solar elevation angles, which help determine effective solar panel orientation and system exposure throughout the day in late summer.

Figure 1A illustrates annual temperature variation, highlighting seasonal trends and fluctuations. The desalination device was tested in August and September 2023. Peak daytime temperatures in August reached 105 °F, with nighttime lows of 70 °F, while in September, maximum temperatures averaged 100 °F and minimums around 59 °F. This temperature data is critical for evaluating the device’s thermal performance, as ambient temperature directly influences evaporation rates, thermal losses, and system efficiency. Figure 1B shows monthly solar irradiation levels, revealing a clear seasonal pattern in solar energy availability. In August, the solar irradiation level was approximately 7.4 kWh/m²/day, and in September it was 7.0 kWh/m²/day. Figure 1C depicts the sun’s position angle throughout the year, offering insights into solar elevation and the optimal orientation for solar energy capture in Tehran. Together, these datasets provide a comprehensive understanding of the solar resource potential in this region and are essential for optimizing the year-round operational strategy of the solar desalination system.

Enhanced performance of solar water desalination systems through cooling tower integration

Initially, the total volume of desalinated water produced by the device was evaluated during two periods: August 3–15 and September 3–15 (from 9:00 AM to 6:00 PM, every other day). Subsequently, a cooling tower was integrated into the system, and its performance was assessed during August 17–30 and September 17–30 under the same conditions. As shown in Fig. 2A, the total amount of water increased steadily from 9:00 AM to 6:00 PM in both configurations—with and without the cooling tower. Following the connection of the condenser to the cooling tower, the total volume of potable water increased significantly, attributed to enhanced condensation efficiency resulting from accelerated cooling of water vapor. The average daily water output without the cooling tower was 3018.14 mL/day in August and 2409.29 mL/day in September. After the integration of the cooling tower, these values increased to 6978.14 mL/day in August and 7016.86 mL/day in September. Over seven days of testing, total water production without the cooling tower was 21,127 mL in August and 16,865 mL in September. In comparison, water production with the cooling tower reached 48,847 mL in August and 49,118 mL in September. Statistical analysis using one-way ANOVA revealed a significant increase in drinking water output of 286.82% in August and 231.21% in September after connecting the system to the cooling tower (p < 0.0001), as illustrated in (Fig. 2B).

Fig. 2.

Effect of cooling tower integration on freshwater output from the solar desalination device. (A) Cumulative freshwater production between 9:00 AM and 6:00 PM during two testing periods: before (August 3–15 and September 3–15) and after (August 17–30 and September 17–30) connecting a cooling tower to the device. Water output increased steadily throughout the day in both configurations, with significantly higher values observed after cooling tower integration. (B) Average daily freshwater output before and after cooling tower integration. In August, production increased from 3018.14 to 6978.14 mL/day (286.82% increase), and in September, from 2409.29 to 7016.86 mL/day (231.21% increase). One-way ANOVA confirmed these differences were statistically significant (p < 0.0001), highlighting the cooling tower’s role in enhancing condensation and overall system efficiency.

Evaluating the effects of environment temperature on salt water temperature

To evaluate the relationship between environmental temperature and salt water temperature within the solar desalination device, a correlation analysis was conducted using data collected in August and September 2023 in Tehran. In August, a strong positive correlation was observed between the environmental temperature and the salt water temperature (r = 0.81, p < 0.0001), indicating that higher ambient temperatures significantly increased the thermal energy of the salt water (Fig. 3A). Similarly, in September, a strong positive correlation was also found (r = 0.70, p < 0.0001), although slightly lower than in August, likely due to seasonal temperature variations (Fig. 3B). Linear regression analysis for both months demonstrated that environmental heat played a critical role in influencing the internal temperature of the desalination system.

Fig. 3.

Correlation between environmental temperature and saltwater temperature inside the solar desalination device in Tehran. (A) Data from August 2023 show a strong positive correlation between ambient temperature and saltwater temperature (r = 0.81, p < 0.0001), indicating substantial heat transfer from the environment to the saltwater within the device. (B) A similarly strong positive correlation is observed in September 2023 (r = 0.70, p < 0.0001), although slightly reduced, likely due to seasonal cooling. These results suggest that environmental heat significantly contributes to the internal thermal dynamics of the desalination system during operation.

Evaluating the effects of solar radiation on water output

To evaluate the seasonal and operational performance of the solar desalination device, measurements were collected during four periods, August 3–15, August 17–30, September 3–15, and September 17–30, 2023. The main parameters assessed included solar radiation (kWh/m²), energy input (kWh), water output (L), energy used (kWh), and device efficiency (%).

August 3–15: During this period, the solar radiation ranged from 7.9 to 7.5 kWh/m². The water output varied between 3.48 L and 2.62 L, while the device efficiency ranged from 18.44 to 14.62%. A clear trend was observed, where decreasing solar radiation correlated with reduced water output and decreased efficiency. Overall efficiency levels were relatively low, suggesting suboptimal device performance during the early stages of operation.

August 17–30: In the second half of August, solar radiation remained relatively stable (7.9–7.5 kWh/m²), but the water output increased significantly, ranging from 7.39 L to 6.58 L. Efficiency during this time ranged from 39.20 to 36.76%, approximately double the efficiency recorded during August 3–15. This improvement indicates that the system became more stabilized over time, due to enhanced operational conditions because of adding cooling tower to the device.

September 3–15: In early September, solar radiation slightly decreased to a range of 7.6 to 7.0 kWh/m². Correspondingly, water output also decreased, ranging between 3.01 L and 1.785 L, and device efficiency declined from 16.58 to 10.68%. This period reflects another phase of low desalination performance, suggesting a strong dependence on solar intensity. This period reflects a phase of reduced desalination performance, highlighting the system’s sensitivity to solar intensity.

September 17–30: In the final measurement period, solar radiation again ranged from 7.6 to 7.0 kWh/m². Water output ranged from 7.44 L to 6.71 L, and efficiency remained consistently high, between 41.13% and 39.63%. Despite a slight reduction in solar radiation, the device sustained high performance, indicating that the system had become fully optimized and was able to efficiently utilize available solar energy. This period demonstrated the best performance in terms of efficiency and consistent water production.

Carbon footprint and environmental impact

While the operational carbon emissions are effectively zero due to exclusive use of solar energy, the embodied carbon footprint according to Eq. (1)—primarily from materials like aluminum, galvanized steel, solar water heater, and pumps—was estimated to be 452.5 kg CO₂.

Normalized to annual water production, the device emits:

This makes the device significantly cleaner than fossil-fuel-based desalination systems, especially over its expected 5-year lifespan.

Assessment of water quality following solar desalination: ICP-OES and ion chromatography

To evaluate the quality of the desalinated water, inductively coupled plasma optical emission spectrometry (ICP-OES) and ion chromatography analyses were performed on selected output samples at the Iran Research Institute of Chemistry and Chemical Engineering. The results (Table 1) indicated that the concentrations of most heavy metals, such as Ag, Co, Ga, Ni, and Pb, were below the detection limit (< 0.01 mg/L), suggesting effective removal of trace metals during the desalination process. The concentrations of Ba, Fe, and B were detected at 0.026, 0.097, and < 0.1 mg/L, respectively, which are within the acceptable range for drinking water standards. Essential minerals such as Mg, Ca, K, Na, and Zn were present at measurable concentrations of 0.77, 7.54, 0.38, 8.90, and 0.38 mg/L, respectively. These values indicate that, while the desalination process effectively removes contaminants, it also retains certain beneficial minerals in the output water. Overall, the desalinated water exhibited low concentrations of toxic elements, confirming its potential suitability for potable. The elemental analysis of the desalination output water demonstrates that the concentrations of toxic metals were well below the permissible limits established by the World Health Organization (WHO) and the United States environmental protection agency (EPA) drinking water guidelines.

Table 1.

Concentrations of heavy metals and essential minerals in desalinated water compared to WHO/EPA drinking water standards.

Element	Mg/l	Element	Mg/l	Element	Mg/l	Element	Mg/l
Ag	< 0.01	Al	< 0.01	As	< 0.01	B	< 0.1
Ba	0.026	Bi	< 0.01	Ca	7.54	Cd	< 0.01
Co	< 0.01	Cr	< 0.01	Cu	< 0.01	Fe	0.097
Ga	< 0.01	In	< 0.01	K	0.38	Li	< 0.01
Mg	0.77	Mn	< 0.01	Mo	< 0.01	Na	8.90
Ni	< 0.01	Pb	< 0.01	P	0.40	Sb	< 0.01
Si	< 0.01	Sn	< 0.01	Sr	< 0.01	Ti	< 0.01
Tl	< 0.01	V	< 0.01	Zn	0.38

Post-shutdown water production enabled by thermal insulation

One of the notable features of the solar desalination device is its ability to sustain water production after sunset, due to the implementation of high-performance thermal insulation. The insulation minimizes heat loss from the system, allowing the internal temperature to decline gradually over several hours after solar input ceases. As a result, water vapor continues to condense during the night, contributing to additional distilled water output without any active energy input. This passive production period, extending from approximately 6:00 p.m. to 8:00 a.m. the next day, consistently yields a measurable quantity of distilled water. Based on experimental observations, the device produced 1936 ± 51.55 mL of water overnight during August, and 1865 ± 67.06 mL during September. This extended productivity enhances both the efficiency and daily yield of the system, providing a strategic advantage in regions characterized by high diurnal temperature variation or intermittent sunlight (Fig. 4).

Fig. 4.

Overnight freshwater production enabled by thermal insulation in the solar desalination device. The system continued to produce distilled water during nighttime hours (6:00 p.m. to 8:00 a.m.) due to effective thermal insulation that slowed heat loss and sustained internal temperatures. This passive condensation phase resulted in 1936 ± 51.55 mL of overnight water output in August and 1865 ± 67.06 mL in September. The ability to maintain productivity after sunset significantly improves the device’s overall efficiency and daily yield, particularly in environments with fluctuating sunlight or high diurnal temperature variation.

Economic evaluation and cost per liter analysis

The estimated material cost for constructing the solar desalination device was approximately 1711 USD. This cost includes all major components, such as galvanized and aluminum sheets, a solar water heater, pumps, cooling tower elements, copper and iron piping, elastomeric insulation, a fan coil, and photovoltaic system (Table 2). Labor and installation costs were not included in this estimation. The relatively low material cost highlights the feasibility of developing an efficient, thermally-insulated solar desalination system using commercially available components.

Table 2.

Estimated material costs for constructing the solar desalination system.

Materials and equipment list	Number of units	Cost (USD)
Galvanized sheet	2 sheets	100
Circular linear pump	A device	60
Cooling tower pump	A device	46
White iron pipe score 25	4 branches	15
Galvanized and ironed fittings	……	10
Fan of Cooling tower	A device	10
Aluminum sheet	A sheet	25
Copper pipe 3/4	15 m	60
Plastic tub	A number	10
Solar water heater (15 tubes)	A device	200
Elastomeric insulation	4 m	20
Fan coil	A device	35
320 W solar module	2 devices	240
Battery (12 volts and 100 amp)	6	720
Controller charger (20 amp)	One device	40
Inverter (700 W)	One device	120
Total		1711 USD

Following the integration of cooling tower, the average daily water production increased to approximately 8.91 L, resulting in an estimated annual output of 3253.71 L. Based on total material cost of 1711 USD, the cost per liter of water produced during the first year of operation was calculated at approximately 0.526 USD (52.6 cents per liter), assuming continuous daily use without major maintenance requirements.

Average daily production = (8931 + 8897.571) / 2 = 8914.286 mL/day = 8.914 L/day.

Annual output = 8.914 L/day × 365 days = 3253.71 L/year.

First-year cost per liter = 1711 USD/3253.71 L = 0.526 USD/L.

Assuming uninterrupted operation, the cost per liter decreases significantly over time. If the system operates for three years, the cost per liter drops to approximately 0.175 USD, and after five years, to about 0.105 USD. This demonstrates the long-term economic viability of the device for sustainable freshwater production.

Financial performance: internal rate of return and net present value

The economic feasibility of the solar desalination device was evaluated using two standard financial indicators: the internal rate of return (IRR) and the net present value (NPV). These metrics provide insight into the long-term economic value and return on investment of the system, particularly in water-scarce or off-grid environments. All financial and environmental metrics—including IRR, NPV, embodied carbon emissions, and energy demand—were calculated using Python (v3.11) with the numpy, numpy_financial, matplotlib, and pandas libraries. The analysis was performed using custom scripts developed specifically for this study.

The IRR represents the discount rate at which the net present value of future cash flows equals zero. For this analysis, an initial investment of 1,711 USD and estimated annual cost savings of 650.74 USD—based on an annual water production of 3,253.7 L at a market value of 0.20 USD/liter—were used. The calculated IRR according to Eq. (2), is approximately 26.50%, which is significantly higher than standard interest or inflation rates, indicating a strong return on investment.

The net present value (NPV) provides a monetary estimate of the project’s value by discounting future cash flows to their present value. Using a 5% annual discount rate over a five-year operational period, the NPV IRR according to Eq. (3), was calculated to be 987.06 USD. A positive NPV confirms that the device not only recovers its initial investment but also generates substantial economic value during its operational lifespan.

In addition, the break-even point—the time required to recover the full initial investment—was achieved during the third year of operation. Beyond this point, all further savings contribute directly to net financial gain.

These financial results highlight the dual advantages of the proposed solar desalination device: it offers a sustainable solution for freshwater generation while also being economically viable. The combination of a positive NPV, a strong IRR of 26.50%, and a short payback period demonstrates the system’s suitability for decentralized and resource-limited settings, where both affordability and sustainability are essential.

Cost comparison and economic indicators

The total capital cost of the system was estimated at 1,711 USD, which covers all construction materials. Based on an average daily water production of 8.91 L, equivalent to 3253.71 L/year, the water production cost (WPC) IRR according to Eq. (4), is calculated as:

Assuming a 5-year operational lifespan with no major repairs, the WPC declines over time:

The payback period—the time required to recover the investment through cost savings—was estimated at 3 years, assuming a local market value of 0.20 USD/L for potable water.

These values indicate that the proposed system is economically competitive, especially when compared to other passive solar desalination technologies, and becomes increasingly cost-effective with long-term use.

Discussion

This study provides passive a comprehensive evaluation of a solar water desalination device optimized for decentralized freshwater production. The system demonstrated strong performance under high solar irradiance and achieved significant improvements in both water output and thermal efficiency through the integration of a passive cooling tower.

Environmental data collected in August and September 2023 confirmed Tehran’s high solar potential—an essential factor for driving solar-based desalination. Correlation analysis showed a strong positive relationship between ambient and saltwater temperatures (r = 0.81 in August and r = 0.70 in September), highlighting the impact of environmental conditions on system heating.

A key finding was the substantial increase in water yield following the cooling tower integration. This increase reflects improvements in condensation performance driven by reduced condenser coil temperatures. Thermal insulation also played a vital role, enabling the device to operate effectively after sunset. In August and September, the system continued to produce water beyond daylight hours, with overnight yields of 1936 ± 51.55 mL and 1865 ± 67.06 mL, respectively.

Water quality analysis using ICP-OES and ion chromatography confirmed that the desalinated water meets drinking standards. These results affirm the system’s ability to eliminate harmful contaminants while preserving beneficial ions. Environmentally, the system operates with zero direct carbon emissions, relying entirely on solar energy. Its embodied carbon footprint was estimated at 452.5 kg CO₂, translating to 0.139 kg CO₂ per liter over the first year of operation—a favorable figure compared to traditional desalination technologies.

Economically, the system demonstrates strong long-term viability. With a total material cost of 1711 USD and no ongoing energy expenses, the cost per liter of water in the first year is approximately 0.526 USD. Over three and five years of continuous use, this cost drops to 0.175 USD and 0.105 USD/liter, respectively. Financial analysis yielded a high internal rate of return (IRR) of 26.50% and a net present value (NPV) of 987.06 USD over five years at a 5% discount rate. The break-even point was reached in the third year of operation.

Together, these results highlight the solar desalination system’s technical robustness, energy efficiency, environmental compatibility, and economic feasibility. Its scalability, low operational cost, and ability to function off-grid make it a compelling solution for sustainable freshwater production in arid and semi-arid regions.

Comparative evaluation with existing solar desalination technologies

In recent years, various strategies have been explored to enhance the performance of solar desalination systems, including the incorporation of nanomaterials, thermoelectric (TE) modules, thermal energy storage media, natural fibers, internal reflectors, and geometric modifications. While many of these approaches have led to notable gains in water productivity and thermal efficiency, they often entail trade-offs related to cost, system complexity, maintenance, and scalability.

For example, Rahbar and Esfahani (2012) investigated the use of nano-enhanced phase change materials (NEPCM) and nano-coated surfaces, reporting a productivity increase of up to 55.8% using CuO and Al₂O₃ nanoparticles, with a cost per liter (CPL) ranging from 0.10 USD to 0.104 USD. By comparison, the system presented in this study achieved a water output increase of 286.82% following the integration of a cooling tower, with a CPL ranging between 0.074 USD and 0.526 USD, depending on operational lifespan³¹.

Similarly, Mandev et al. (2024) demonstrated that thermoelectric (TE) modules could enhance solar still productivity by up to 35% through active condensation. However, TE modules require external electrical input and additional hardware, increasing system complexity and dependency on external energy sources. In contrast, the present system utilizes a passive, water-based cooling tower that operates without electricity, enhancing suitability for off-grid and resource-limited environments³².

Other studies have focused on improving evaporation efficiency through structural enhancements. Systems employing internal reflectors, trays, or dual-basin geometries have reported productivity gains ranging from 29 to 108%. For example, a tray-based distiller using PCM and internal reflectors achieved a thermal efficiency of 51.5%. While these designs effectively optimize solar energy capture, they typically lack configurations that support extended or nighttime operation^33,34. In contrast, the current system maintained high thermal performance during nighttime, producing up to 1936 mL of freshwater overnight— an uncommon feature among conventional solar stills.

Cost-conscious approaches using natural porous materials, such as pond fibers or palm leaf fibers, have also shown promise by improving evaporation through capillary action and increased surface area. Alshqirate et al. (2023) reported productivity improvements of up to 44.5% and CPL reductions of approximately 30%. However, these materials are susceptible to biological degradation, require frequent replacement, and may pose sanitation concerns³⁵. The present system avoids these drawbacks by employing durable, non-organic components that require minimal maintenance and provide long-term operational reliability.

Moreover, some researchers have evaluated portable or Plexiglas-based solar stills under challenging conditions such as winter or low-radiation periods. These studies often report reduced performance due to wind effects or insufficient solar input³¹. By comparison, this system was tested during August and September in Tehran, a region with high solar irradiance and ambient temperatures, and consistently achieved daily outputs exceeding 6.5 L, even when solar radiation dropped to 7.0 kWh/m², demonstrating strong operational stability.

Srivastava and Agrawal (2013) explored the benefits of double-basin solar stills, reporting a 59.9% increase in daily water yield and improved thermal efficiency. However, such geometric modifications often increase fabrication complexity and reduce scalability³⁶. In comparison, the current system achieves comparable or superior performance by strategically combining thermal insulation with passive cooling—without compromising simplicity or affordability.

Beyond performance metrics, the proposed system also offers advantages in terms of maintenance and operational reliability. The design features passive mechanisms and non-organic materials that reduce maintenance frequency to basic periodic cleaning and visual inspection of tubing and collector surfaces, typically required once every 2–4 weeks³⁷. Compared to membrane-based systems like reverse osmosis (RO), which require routine membrane flushing, chemical cleaning, and frequent replacement⁹, the maintenance complexity of current device is minimal. In terms of biofouling and scaling, the absence of membrane surfaces and the use of elevated operating temperatures naturally inhibit microbial growth⁷. However, scaling due to mineral precipitation remains a risk and can be mitigated through simple prefiltration or periodic flushing³⁸. The system includes only basic filtration requirements, such as a coarse mesh or sediment filter at the inlet, in contrast to the multi-stage pre-treatment necessary in conventional RO systems³⁹.

Regarding durability, the system is constructed from corrosion-resistant and long-lasting materials such as aluminum, galvanized steel, copper pipes, and elastomeric insulation. These components have an estimated lifespans of 5 to 10 years, depending on environmental conditions and maintenance⁴⁰. The system’s pump units may require replacement after 3–5 years⁴¹. These projected lifespans support cost-effective, long-term deployment in remote or resource-limited areas. Furthermore, the system’s modular design and reliance on commercially available components make it highly adaptable for large-scale implementation. Units can be replicated or expanded in a decentralized manner without dependence on grid electricity or complex control systems, making them suitable for rural and off-grid applications^23,42.

Limitations

While the results of this study demonstrate the promising performance and feasibility of the solar desalination device, several limitations should be acknowledged:

1. Seasonal constraints: The device was tested only during the summer and early autumn months (August and September), under high solar radiation conditions. Its performance during winter and spring—when solar intensity and ambient temperatures are significantly lower— has not been evaluated. This limits the applicability of the findings across all seasons.

2. Geographical limitation: The experimental setup was optimized for Tehran’s specific climatic conditions. Variations in altitude, humidity, and solar angle in other regions may affect performance, and further testing is nessesary to assess adaptability across different environments.

3. Limited operational duration: The evaluation was conducted over relatively short intervals (about one month per condition). Long-term performance, durability of components (e.g., insulation, condenser coil, solar collector), and potential degradation over multiple years were not assessed and could influence real-world viability.

4. Lack of automated control system: The current prototype relies on manual operation and fixed settings for pump speed, condenser temperature, and water level control. Introducing automated control systems may enhance performance and energy efficiency, but such features were not explored in this study.

5. Water quality sampling scope: Although water quality was assessed using ICP-OES and ion chromatography, the sampling was limited to selected batches. Continuous monitoring over a longer duration would provide a more comprehensive understanding of the consistency and safety of water quality.

6. Economic analysis assumptions: The financial evaluation excluded labor, maintenance, and installation costs, and assumed uninterrupted daily operation. Real-world applications may involve additional costs, seasonal downtime, or maintenance issues that could impact the overall cost-effectiveness.

7. Single cooling tower configuration: Only one configuration of the cooling system was tested. Exploring alternative condenser materials, airflow rates, or passive cooling strategies could further improve efficiency but were beyond the scope of this work.

Experimental setup and methodology

A detailed description of the present system is provided in this section. A general view of the desalination plant is presented in (Fig. 5). In this experimental research, a desalination tank was made by 2 mm galvanized sheet metal, welded together with an inverter welder. To minimize heat exchange with the environment, the tank was insulated with 6 mm thick elastomeric insulation. Additionally, a 120-liter polyethylene tub measuring 0.4 × 0.3 × 1 m was placed inside the galvanized tank and overed with 6 mm elastomeric insulation to further reduce thermal losses (Fig. 5A). This configuration effectively minimized heat exchange between the desalination system and the surrounding environment.

Fig. 5.

Photograph and labeled components of the passive solar desalination system. The system consists of several integrated units: (A) main desalination chamber, insulated with elastomeric material, containing the heat exchanger and polyethylene tub; (B) top and side views of the chamber showing freshwater outlet connections; (C) inlet and outlet connections of the heat exchanger and cooling tower; (D) vacuum tube solar water heater, which supplies thermal energy to heat the saltwater in the main chamber; (E) circulation pump that transfers heated fluid from the solar water heater to the evaporator coil; (F) secondary pump that circulates water from the cooling tower to the condenser; (H-1) cooling tower containing egg-crate packing to increase the air–water contact surface area and enhance vapor condensation efficiency; (H-2) single-phase blower fan (model C80) that supplies air to the cooling tower. Blue arrows indicate the inlet and outlet lines between the solar water heater and the plate-and-tube copper heat exchanger. Black arrows indicate connections between the heat exchanger and the polyethylene tub. Green arrows show the inlet and outlet of the cooling tower. The white arrow marks the monitoring tube for the brine water level. Red arrows indicate the outlet for desalinated water.

Then a plate-and-tube copper heat exchanger (evaporator coil) was used to heat the brine. The heat exchanger was positioned inside a polyethylene tub (Fig. 5A) and connected to a 150-liter solar water heater with 15 vacuum tubes through flexible hoses and connecting pipes (Fig. 5D). The solar water heater was conected to the evaporator coil in a closed-loop cycle. In this configuration, hot water circulated from the solar water heater through the brine coil, facilitating the heating of the brine and enabling the evaporation process. A Grundfos UPS25-50 single-phase centrifugal pump was used to circulate water from the solar water heater into the evaporator coil. Hoses and connecting pipes were insulated with 6 mm thick elastomeric insulation (Fig. 5E).

The performance of the desalination device was evaluated with and without a cooling tower connection. For the assembly of the cooling tower with the desalination device and construct the condenser, 15 m of 3/4-inch copper pipe were used. The pipe was arranged in a spiral and joined using U-shaped copper fittings. To enhance the contact area between the steam and the condenser surface, two aluminum sheets (1 mm thick, measuring 0.45 × 1.3 m) were installed above and below the copper pipe. The inlet and outlet of the copper condenser were connected to the cooling tower.

The cooling tower tank was fabricated from four 2 mm galvanized sheets (each 50 × 70 cm) and one 70 × 70 cm sheet, assembled using inverter welding. To make the cooling tower water diffuser, a 60 cm long, 1.5-inch pipe with a 1.5 mm gap at the end was used. Additionally, five 30 cm pipes, each 2.1 inches in diameter, were fitted with two 2.1-inch welding bushings. A plastic carrier nozzle was installed on each bushings to atomize the water, thereby increasing the contact surface between the water and air for enhanced cooling. A 1/2 floater was used to regulate water level within the tower. Egg-crate packing was added to the tower to increase air–water contact area and enhance cooling efficiency (Fig. 5H-1). A single-phase blower fan (model C80) was used to supply air to the cooling tower (Fig. 5H-2). The fan has dimensions of 30 × 30 cm and blowed air into the water droplets, to facilitate the cooling process. A Pentax IHP pump was used to circulate water from the cooling tower to the condenser (Fig. 5F). White 25-gauge steel pipes were used to connect the exchangers to the water heater and the cooling tower. These pipes provided favorable heat insulation properties due to their thickness and plasticity. For additional insulation, the pipes are covered with 6 mm thick elastomeric pipe insulation to minimize heat loss.

The electricity for the Pentax IHP pump, the blower fan, and the Grundfos UPS25-50 centrifugal pump was supplied by a photovoltaic system. A solar panel that generates direct current (DC) electricity from sunlight was connected to a photovoltaic charge controller. The charge controller regulated the voltage and current from the solar panel to safely charge the battery and prevent overcharging. The battery, which stores the DC electricity, was then connected to an inverter to convert the DC power into alternating current (AC) power for operating the devices.

Summary of the desalination process

The desalination system utilizes solar energy as the primary heat source. A working fluid inside the solar water heater is heated by sunlight and circulated through the copper tubes of a plate-and-tube heat exchanger, which is located within a polyethylen tub inside the desalination tank. A circulation pump ensures continuous flow of the heated fluid, transfering thermal energy to the saltwater in the plastic tub.

As the saltwater heats up, it begins to evaporate, producing steam. The rising steam comes into contact with an aluminum plate situated just below a spiral-shaped copper condenser coil positioned at the top of the desalination tank. This condenser coil is connected to a cooling tower and is maintained at a temperature of approximately 17 °C. The temperature difference enables the steam to condense upon contact with the coil, converting it into liquid water. The performance of the device was evaluated both witand without the integration of a cooling tower.

The condensed freshwater then flows into a 5 cm wide collection tray inclined at 15 degrees, which directs the purified water to an outlet leading to a freshwater storage container. Additionally, a secondary freshwater outlet is installed at the bottom of the desalination tank to collect any condensed water that accumulates there. The required power for the fan and both pumps was supplied by a photovoltaic system (Fig. 6).

Fig. 6.

Schematic of a solar-powered desalination system. The system uses solar energy to heat a working fluid in a vacuum tube solar water heater. This fluid circulates through heat exchanger placed inside a polyethylene tub containing saltwater. As heat is transferred, the saltwater evaporates, generating steam. The steam rises and condenses upon contact with a cooled copper coil connected to a cooling tower, producing freshwater. A photovoltaic system powers the circulation pumps and fan. Freshwater is collected through inclined trays and bottom outlets for storage.

Sample preparation and analytical methods for water quality assessment

Water samples collected from the desalination system were analyzed to determine their elemental and ionic composition. The analytical workflow is illustrated in (Fig. 7). Samples were first filtered using a 0.45 μm membrane to remove suspended solids and then stored at 4 °C in sterile containers until analysis. Trace metals such as Pb, Cd, and Zn were quantified using inductively coupled plasma optical emission spectrometry (ICP-OES). Major anions and cations, including Na⁺, K⁺, Cl⁻, and SO₄²⁻, were measured using ion chromatography. The results were subsequently processed and interpreted to assess the safety and suitability of the desalinated water for human consumption, based on WHO and EPA drinking water guidelines.

Fig. 7.

Stepwise workflow of the water quality analysis process using ICP-OES and ion chromatography: Workflow of the analytical procedure used to evaluate the quality of desalinated water. The process includes sample collection, membrane filtration (0.45 μm), storage at 4 °C, and analysis by ICP-OES for trace metals and ion chromatography for major anions and cations. Final data were processed and interpreted to assess compliance with drinking water standards.

Embodied carbon emissions per liter

The environmental impact of the system was evaluated based on embodied CO₂ emissions per liter of water produced⁴³:

1

Where:

E_total = Total embodied CO₂ emissions (kg CO₂).

V_annual × L = Total water production over system life.

Financial performance: internal rate of return and net present value

The economic feasibility of the solar desalination device was evaluated using two standard financial indicators: the net present value (NPV) and the internal rate of return (IRR). These metrics provide insight into the system’s long-term value and return on investment, particularly for deployment in water-scarce or off-grid environments.

All financial and environmental metrics—including NPV, IRR, and water production cost (WPC)—were calculated using Python (v3.11) with the numpy, numpy_financial, matplotlib, and pandas libraries. The analysis was performed using custom scripts developed specifically for this study (see appendix A).

Internal rate of return (IRR)

The IRR is the discount rate at which the NPV equals zero⁴⁴:

2

This metric was computed using the npf.irr() function in Python. The resulting IRR reflects the effective return rate from the water savings generated over the device’s lifespan.

Net present value (NPV)

NPV quantifies the present value of expected cost savings over the system’s lifespan, adjusted for the time value of money⁴⁴:

3

S = Annual cost sa.

vings (USD/year).

r = Discount ratet = Year of operation.

L = Lifespan (years).

Water production cost (WPC)

WPC indicates the cost of producing one liter of water over the system’s operating life⁴⁵:

4

Where:

C_initial = Initial capital cost (USD).

V_annual = Annual water production (L/year).

L = Lifespan of the system (years).

Abbreviations

CO₂

Carbon dioxide

E_total

Total embodied CO₂ emissions (kg)

V_annual

Annual water production (L/year)

L

System lifespan (years) and Liter

WPC

Water production cost (USD/L)

NPV

Net present value (USD)

IRR

Internal rate of return (%)

r

Discount rate

S

Annual cost savings (USD/year)

t

Year of operation

C_initial

Initial capital cost (USD)

kWh

Kilowatt-hour

kg

Kilogram

USD

United states dollar

ICP-OES

Inductively coupled plasma optical emission spectrometry

PV

Photovoltaic

HDH

Humidification-dehumidification

SSD

Solar still desalination

TE

Thermoelectric

RO

Reverse osmosis

Appendix A – Python code for IRR calculation

import numpy_financial as npf

initial_cost = -1711

annual_savings = 650.74

cash_flows = [initial_cost] + [annual_savings] * 5

irr = npf.irr(cash_flows)

print(f"Internal Rate of Return: {irr * 100:.2f}%")

Author contributions

M.B. conducted the experiments, analyzed the data, prepared the funding, and wrote the manuscript. K.R. contributed to the study design and interpretation of the results. R.E. assisted in experimental design and data analysis. All authors reviewed and approved the final version of the manuscript.

Data availability

The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.