Innovative Solar Still Desalination: Effects of Fans, Lenses, and Porous Materials on Thermal Performance Under Renewable Energy Integration

Abstract

Global freshwater scarcity continues to escalate due to pollution, climate change, and population growth, making innovative sustainable desalination technologies increasingly vital. Solar stills offer a simple and eco-friendly method for freshwater production by utilizing renewable energy, yet their low productivity remains a major limitation. This study experimentally evaluates and quantifies several established enhancement techniques under real climatic conditions to improve evaporation and condensation efficiency. The integration of porous materials, such as black rocks, significantly improves thermal energy storage and management by retaining absorbed heat during the daytime and releasing it gradually, resulting in an average 30% increase in daily distillate production (SD = 6 mL). Additionally, forced convection using small fans enhances humid air removal and evaporation rates, increasing the average yield by approximately 11.4% (SD = 2 mL). Optical concentration through lenses intensifies solar irradiation on the evaporation surface, achieving the highest performance with an average 50% improvement in water output (SD = 5 mL). The incorporation of Phase Change Materials (PCM) is further proposed to extend thermal stability during off-sunshine hours, with materials selected based on a melting point range of 38–45 °C. To minimize nocturnal heat loss, future designs may integrate radiative cooling materials for passive night-time condensation support, by applying a radiative cooling coating to the condenser plate to enhance passive heat rejection to the sky. Overall, the validated combined use of renewable energy-driven desalination, thermal storage media, and advanced strategies presents a practical pathway toward high-efficiency solar stills suitable for sustainable buildings and decentralized water supply systems in arid regions.

1. Introduction

Solar stills are efficient, sustainable systems for desalination and water purification, operating by harnessing solar energy to evaporate water and subsequently condense it for collection. Despite their potential, conventional solar stills frequently have low efficiency. Recent research has focused on improving solar still performance using innovative techniques, such as enhanced heat transfer, phase-change materials, nanofluids, and optimized designs, to increase freshwater yield and efficiency.

According to research, multi-basin solar stills that employ multiple stages of condensation and evaporation can significantly boost water production. The efficiency of multi-effect solar stills can be increased by up to 50% in comparison to single-basin systems [1]. Solar energy absorption and heat retention can be enhanced by modifying the geometry of solar stills, such as the evaporator’s surface area and angle of inclination. Review studies of Zayed [2] suggest that a tilt angle of 20–30 degrees maximizes solar gain. The use of selective coatings and high-absorptivity materials can enhance thermal efficiency. For example, studies of Zhao et al. [3] on the application of nanomaterial’s and carbon-based coatings have shown significant improvements in heat absorption and retention. Sharma et al. [4] have Incorporating PCMs into the design of the solar still to allow thermal energy storage and enable the solar still to operate more efficiently during non-sunny hours. Their results indicates that PCMs can maintain higher temperatures, thus enhancing evaporation rates. Hybrid systems that utilize waste heat from industrial processes or combine solar stills with reverse osmosis have been shown to increase water production by up to 60% [5]. Soltanian et al. [6] have shown in their studies that adding reflective surfaces around the solar still can concentrate sunlight onto the absorber, increasing the effective solar radiation received. Studies demonstrate that properly designed reflectors can enhance water yield significantly. Malik [7] have shown in their studies that maintaining optimal water depth in the basin is crucial for maximizing evaporation rates. Their results indicate that a water depth of around 2–3 cm is ideal for balancing evaporation and heat transfer. Suman et al. [8] have implemented smart technologies for monitoring and adjusting operational parameters to improve efficiency. They demonstrated how automated systems that adjust temperature and water levels in response to real-time data have improved performance in encouraging ways. Local climate factors, like temperature, humidity, and sun radiation, also affect efficiency of solar stills. According to Naseer et al. [9], site-specific designs that consider these environmental factors are essential for optimizing performance. Recent studies have explored innovative solar still designs, such as the use of biowaste materials for thermal enhancement and magnetic fields to improve evaporation efficiency [10,11]. Additionally, nanomaterials such as graphene and carbon nanotubes have been incorporated to enhance the thermal conductivity and absorptivity of solar still components, demonstrating significant potential to increase evaporation rates and overall performance [12]. Yadav et al.’s research [13] has shown how solar stills and photovoltaic systems can work together to increase energy efficiency. Water output has been seen to increase using hybrid systems that use excess electricity from PV panels to power additional heating components, especially during periods of low sunlight. One major focus for increasing the efficiency of solar stills has been the use of Internet of Things (IoT) technologies. Mcluret and Gnanaraj [14] have shown that automated monitoring systems that modify settings depending on real-time data (e.g., temperature, humidity) improve performance and flexibility. Mahgoub et al.’s research [15] has investigated multi-effect designs that make use of several evaporation and condensation stages. Studies have shown that these systems are far more efficient than standard single-basin designs, producing up to 70% more water. A recent study by Murali et al. [16] has demonstrated how well PCMs save thermal energy when solar radiation is minimal. PCM-integrated systems have shown longer operating durations and increased overall effectiveness. In their research, Ammar et al. [17] have underlined the importance of designing solar stills created to specific climatic conditions. The findings indicate that efficiency can be greatly increased by adjusting designs according to local patterns of solar light and temperature fluctuations.

Numerous innovative approaches to improving the efficiency of solar stills have been the focus of recent experiments. To improve the thermal absorption and evaporation rates of solar stills, Sharma and Birla [18] coated the absorber surface with nanomaterials like graphene oxide and carbon nanotubes. According to experiments, evaporation rates can rise by up to 35% when compared to traditional materials, greatly increasing total efficiency. Abu-Zeid et al. [19] created a hybrid system in which a heating element powered by excess solar panel energy during periods of low sunlight. The study showed that hybridization was beneficial since cloudy days resulted in a 50% increase in water output. According to a review by Kasaeian et al. [20], numerous researchers have incorporated PCMs into the still’s design in order to store extra heat during the hours of maximum sunshine. The addition of PCMs increased the still’s overall water output by almost 40% over a 24 h period by enabling it to maintain higher temperatures during the night. In order to collect as much solar energy as possible, N. Kumar and A. Arun Kumar [21] have created movable reflectors that follow the sun’s path throughout the day and focus more sunlight onto the still. The investigation showed that, in comparison to static reflector systems, solar gain and water output increased by about 30%. To optimize operational parameters in real time, Pimienta [22] has put an IoT-based monitoring system in place. This system allows for modifications based on data on temperature, humidity, and sun radiation. This smart system increased efficiency by almost 20% through automated adjustments that optimized heating elements and water levels. In their review, Ahmed et al. [23] demonstrated a variety of geometric configurations, such as multi-layer designs and inclined surfaces, to improve heat retention and evaporation surface area. Several designs demonstrated a remarkable rise in evaporation efficiency, resulting in a 45% overall improvement in water production.

The current fragmented literature lacks a statistically validated comparative benchmark, hindering practitioners’ selection of reliable enhancement strategies for low-cost, decentralized solar still designs. Therefore, this study addresses this critical gap by rigorously quantifying the relative gain and stability of readily implementable single-enhancement techniques, evaluating three main approaches: fans for accelerated forced convection, porous bodies (black rocks) for effective thermal storage, and optical lenses for concentrated solar radiation, all designed to maximize the average daily distillate yield under the arid conditions of Riyadh, Saudi Arabia. This comprehensive series of experimental studies seeks to answer the primary question regarding the most effective and reliable strategy, providing the essential experimental validation necessary for optimizing future high-efficiency systems and contributing to the sustained research needed to solve the problem of water scarcity.

2. Materials and Methods

These studies utilized two solar stills, conventional and modified to provide a comparative analysis of distillation performance. The modified still was enhanced by coupling it with fans, a lens, and porous bodies. Experiments were conducted during the summer season of 2024 on the roof of the College of Engineering building at Imam Mohammad Ibn Saud Islamic University, Riyadh, Saudi Arabia (24.7136° N, 46.6753° E). The schematic views of the experimental setup are illustrated in Figure 1. Both stills are constructed primarily from weather-resistant cedar wood and feature a truncated pyramid structure with an effective evaporation area of 0.5 m². The overall external dimensions are approximately 0.5 m in length, 0.5 m in width, and 0.5 m in height. The still employs an 8 mm thick crystal glass cover sloped at 30° for optimal solar gain, while the opaque side walls are inclined at 60° to the horizontal, defining the internal geometry. The basin, located at the bottom, is 1.5 mm thick galvanized steel, lined with black paint to maximize heat absorption. To ensure high thermal efficiency and repeatability, the structure incorporates a robust insulation system: the outer cedar wood frame is 2 cm thick and houses an inner layer of Rock Wool insulation with a thickness of 5 cm. This primary insulation possesses a low thermal conductivity of 0.03 W/mK, which is critical for minimizing heat loss. The underside of the glass cover serves as the condensation surface, channeling the distilled water into a collection trough. These precise material and geometric specifications guarantee that the device structure is efficient and fully repeatable. To ensure statistical reliability, all experiments for each configuration (conventional and modified) were conducted in triplicate under comparable conditions. The reported results represent the average values derived from these repeated measurements.

Real Climatic Conditions of Riyadh, Saudi Arabia

Riyadh, the capital city of Saudi Arabia, experiences a desert climate characterized by high temperatures, low humidity, and minimal rainfall. Daytime temperatures regularly exceed 40 °C with record highs reaching above 50 °C. Nights can be warm, with temperatures often remaining above 30 °C. Humidity levels in Riyadh are generally low, often ranging from 10% to 30%. The humidity tends to be higher in the early morning and late evening. Winds can be strong, especially during the spring months. Riyadh receives approximately 3200 to 3500 h of sunshine per year. On average, the city enjoys about 8 to 10 h of sunshine daily, with longer durations during the summer months. The average daily solar radiation in Riyadh ranges from 4 to 8 kWh/m²/day. Riyadh typically experiences about 5.5 to 7.5 peak sun hours per day, meaning that the solar irradiance exceeds 1000 W/m² during these hours. In summer months, (June to August) higher solar radiation values often-exceeding 7 kWh/m²/day (Figure 2). In winter Months (December to February): lower values, averaging around 5 kWh/m²/day. High levels of solar radiation and extended sunshine hours make Riyadh a prime location for solar panels and other solar technologies, such as desalination.

3. Results and Discussion

To ensure the statistical reliability of the performance data, each solar still configuration (Conventional, Fans, Porous Bodies, Lenses) was tested, and the results were replicated in triplicate under similar ambient and solar conditions. This required a total of 12 full experimental days of data collection, with yields being recorded at the end of each daily cycle to calculate the validated average and standard deviation.

3.1. Effect of Using Fans to Enhance the Performance of the Solar Still

In our experiments, the fans shown in Figure 3 are powered entirely by renewable energy using direct solar panel input, ensuring a self-sustaining and environmentally friendly design. To maintain continuous operation during periods of low sunlight, rechargeable batteries are incorporated to store excess energy as part of an integrated energy storage and management system. The fans are strategically positioned to direct airflow across the water surface of the solar still, increasing evaporation by removing humid air that would otherwise accumulate above the basin [24].

Within the solar still a fan enhances air circulation, potentially moving warmer air from the evaporative surface to the condensation area. This can lead to increased evaporation rates, but not necessarily an increase in the temperature of the evaporated water itself. The temperature of the evaporated water primarily depends on the heat supplied by solar radiation and the thermal properties of the water. The fan may assist in maintaining a higher rate of evaporation, but it does not inherently raise the temperature of the water being evaporated (see Figure 4).

The daily distillate yield increased from 280 mL to approximately 312 mL, representing a performance improvement of nearly 11.4% and confirming that accelerated moisture removal enhances overall system efficiency. Building on this, future designs will integrate Phase Change Materials (PCMs), specifically those with a melting point of 38–45 °C to match basin water temperature and extend thermal stability into the evening, alongside Radiative Cooling (RC) coatings on the condenser plate to leverage passive heat rejection to the sky, thereby significantly enhancing nocturnal condensation efficiency.

3.2. Effect of Using Porous Bodies to Enhance the Performance of the Solar Still

In our experimental studies, the porous bodies employed are black natural rocks. Rocks are placed in horizontal layers on the bottom of the still. This arrangement allows for a larger surface area for evaporation and facilitates the distribution of water across the rocks, as shown in Figure 5. The integration of these porous materials significantly enhances thermal energy storage and management within the solar still. During the daytime, the rocks absorb and store solar radiation, gradually releasing the accumulated heat, which helps maintain elevated water temperatures and prolongs the evaporation process. Strategically placing these rocks inside the basin increases the water surface area exposed to air, thereby improving the evaporation rate.

The dark color of the rocks provides a high solar heat absorption capacity, allowing them to efficiently convert incoming solar energy into thermal energy. This absorbed heat raises the water temperature and sustains evaporation even during periods of low or intermittent sunlight. Additionally, the rocks act as a thermal buffer, retaining heat over extended periods and minimizing temperature fluctuations within the basin. At night or during cooler periods, this thermal inertia reduces heat loss, effectively functioning as an insulating layer and supporting continuous evaporation. The combination of heat absorption, gradual release, and insulation makes black rocks a simple yet highly effective material for improving the efficiency of solar stills and optimizing their thermal energy storage and management [25].

Incorporating porous bodies into a solar still design not only enhances evaporation but also improves water distribution, reduces heat loss, and may contribute to better water purification. Figure 6a,b show that the use of porous bodies increases water temperatures from 8 PM to 5 AM, while their effect during the daytime is less pronounced. This indicates that the porous bodies act as thermal moderators, stabilizing temperature fluctuations and thereby enhancing overall water production. The daily quantity of distillate in the conventional solar still is approximately 280 mL, whereas with the inclusion of porous bodies, production rises to about 364 mL, corresponding to an increase of nearly 30%.

In our experimental studies, the porous bodies employed are black natural rocks, as shown in Figure 5. The integration of these porous materials significantly enhances thermal energy storage and management within the solar still. During the daytime, the rocks absorb and store solar radiation, gradually releasing the accumulated heat, which helps maintain elevated water temperatures and prolongs the evaporation process. Strategically placing these rocks inside the basin increases the water surface area exposed to air, thereby improving the evaporation rate. The dark color of the rocks provides a higher solar heat absorption capacity, allowing them to efficiently convert incoming solar energy into thermal energy. This absorbed heat raises the water temperature and sustains evaporation even during periods of low or intermittent sunlight. Additionally, the rocks act as a thermal buffer, retaining heat over extended periods and minimizing temperature fluctuations within the basin. At night or during cooler periods, this thermal inertia reduces heat loss, effectively acting as an insulating layer and supporting continuous evaporation. The combination of heat absorption, gradual release, and insulation makes black rocks a simple yet highly effective material for improving the efficiency of solar stills and optimizing their thermal energy storage and management.

Incorporating porous materials into a solar still design can enhance evaporation, improve water distribution, reduce heat loss, and potentially contribute to water purification.

3.3. Effect of Using Lens to Enhance the Performance of the Solar Still

The Fresnel lens used in our experimental studies is a compact lens originally developed for lighthouses. It offers excellent light-gathering capability and is relatively inexpensive to manufacture due to its unique stepped design. The transmittance of this lens is approximately 92%, allowing most of the incident solar radiation to pass through and focus on a specific point, thereby increasing the efficiency of solar energy concentration. Optical concentration using Fresnel lenses intensifies solar irradiation on the evaporation surface of the solar still, enhancing the evaporation rate [26]. In our experiments, two lenses were used, each with a diameter of 300 mm, a focal length of 400 mm, and a thickness of 2 mm, and they were positioned as shown in Figure 7.

Figure 8a,b show that the maximum water temperature in the conventional solar still reaches approximately 52 °C, whereas in the solar still equipped with a lens, it exceeds 56 °C. The concentrated solar energy from the lens creates a localized heating effect within the still, resulting in higher temperatures at the water surface and accelerating the evaporation process. Consequently, both the evaporation rate and overall water production are significantly enhanced. The experimental results indicate that the daily distillate yield of the conventional solar still is about 280 mL, while the solar still with a lens produces approximately 420 mL, corresponding to an increase of 50% compared to the conventional still. Although the use of a lens can greatly improve solar still performance, there are practical considerations.

Incorporating a lens increases the complexity of the design and construction, potentially raising both costs and maintenance requirements. Additionally, the lens may require periodic cleaning to maintain optimal solar transmission and ensure consistent performance.

The comparative analysis, summarized in

Table 1. Comparative analysis of experimental setup.

Experimental Setup	Mean Yield (mL)	Standard Deviation (SD) (mL)	% Increase over Conventional
Conventional Still	280	1	0%
Still with Fans	312	2	11.4%
Still with Porous Bodies	364	6	30%
Still with Lenses	420	5	50%

and Figure 9, establishes the quantifiable effect and high reliability of each enhancement strategy. The results confirm that all experimental runs demonstrated low variability, with the Standard Deviation ranging only from 1 mL to 6 mL. Compared to the Conventional Still (average yield 280 mL), the integration of porous bodies proved highly effective for thermal management. This strategy leveraged the consistent effect of heat storage and subsequent release to achieve a stable 30% increase, resulting in an average yield of 364 mL (SD = 6 mL). Furthermore, utilizing fans for forced convection enhanced mass transfer by breaking the saturated vapor boundary layer, leading to an 11.4% increase and a mean yield of 312 mL (SD = 2 mL). Ultimately, the Still with Lenses demonstrated the maximum performance due to intensified solar irradiation, reaching the highest output of 420 mL and confirming a substantial 50% increase in average daily distillate yield (SD = 5 mL). This data rigorously validates the stability and effectiveness of each enhancement technique.

The percentage increase ($P$) in distillate yield for each modified still was determined relative to the Conventional Still ($Y_{CS}$) using Equation (1).

$$\mathit{P}={\displaystyle \frac{{\mathit{Y}}_{\mathit{M}\mathit{S}}-{\mathit{Y}}_{\mathit{C}\mathit{S}}}{{\mathit{Y}}_{\mathit{C}\mathit{S}}}}\times \mathbf{100}$$

(1)

where $Y_{MS}$ is the mean yield of the modified design. All yields are expressed in milliliters (mL). The incorporation of black porous rocks enhanced the effective thermal mass, storing solar energy during peak irradiation and releasing it gradually after sunset. This sustained the evaporation process and improved productivity. The Still with Porous Bodies exhibited a consistent 30% increase in yield compared with the Conventional Still (SD = 6 mL). Each configuration was tested in triplicate ($n=3$) under comparable clear-sky conditions. Mean yields and standard deviations were used to assess repeatability, and error bars in Figure 8 represent SD values. Measurement uncertainties were determined from sensor specifications (K-type thermocouples for temperature and a calibrated pyranometer for irradiance). The small SD values confirm high measurement consistency and reliable experimental control.

The effectiveness and efficiency of the developed solar still techniques (

Table 2. Advantages and limitations of developed solar still techniques.

Method	Advantages	Limitations
Fans	Improved Airflow: By moving air around, fans can accelerate evaporation rates and aid in the still’s humidity removal. Cooling Effect: Fans can help improve condensation on the cover by moving air around the region of interest. Temperature Control: The temperature of the still can be regulated with the help of fans, especially in warm climates. Using fans increase the daily distillate water production by about 11%.	Energy Requirement: Fans may require electricity or another power source, which off-grid applications may not be able to provide. Maintenance: Over time, mechanical components may need to be replaced or wear out, which could lower reliability.
Porous bodies	Water Retention: Porous materials can absorb water and release it slowly, enhancing evaporation. Increased Surface Area: Efficiency may be improved by the porous structure’s wide surface area for evaporation. Thermal Properties: The ability of porous materials to retain heat raises the still’s overall temperature. Using porous bodies increase the daily distillate water production by about 30%.	Material Restrictions: Not all porous materials are appropriate since they have the potential to break down or release unwanted compounds into the distilled water. Weight and Handling: Certain porous bodies can be weighty or challenging to work with in a still.
Lenses	Solar energy concentration: Sunlight can be directed onto a still by lenses, which raises temperatures and greatly accelerates evaporation. Reduced Area Requirement: Lenses can increase the efficiency of smaller stills by concentrating sunlight. High Efficiency: In areas where sunshine is plentiful, using lenses can lead to extremely high evaporation rates. Using lenses increase the daily distillate water production by about 50%.	Complexity: Because lenses need to be installed and aligned precisely, their design and execution can be more complicated than other approaches. Cost and Accessibility: High-quality lenses may be more costly and not always easily accessible.

) can be evaluated by comparing the use of fans, porous bodies, and lenses. The advantages and limitations of each technique are summarized in the table below. The choice among fans, porous bodies, and lenses depends on local conditions, resource availability, and specific objectives. Porous bodies provide a simple, low-energy solution, making them suitable for basic designs. Lenses are preferable when maximizing evaporation is the primary goal and resources allow. Fans, although requiring more electricity and maintenance, can significantly enhance overall performance.

4. Conclusions

This study successfully resolves a critical research gap by providing the first statistically validated comparative benchmark of single-enhancement techniques for solar stills, confirming the high reliability of all methods with minimal SD values ranging from 2 to 6 mL across triplicate trials. The data established the Still with Lenses as the maximum-output strategy 50% increase, the Still with Porous Bodies as the most stable passive solution 30% increase, and the Still with Fans as a reliable 11.4% gain. This rigorous quantification offers practitioners a clear, data-driven hierarchy for implementation. Building on this benchmark, future research must now focus on Hybrid Integration (e.g., combining Lenses with 38–45 °C PCMs), comprehensive economic analyses and long-term durability studies, and optimizing condensation via radiative cooling coatings to achieve truly high-efficiency, sustainable desalination systems.