Introduction

The world is experiencing catastrophic droughts as a result of global warming, which is causing increased weather disruptions and a serious shortage of potable water. As a result, it is critical to produce vast amounts of drinking water in a sustainable manner, that is, without contributing to global warming1,2,3. Desalination of salt water or sea water, which is abundant in abundance, can be used to generate drinking water. Solar desalination is the most environmentally friendly option. Solar desalination is the use of a solar distillation system in which salty water is heated by solar irradiation and evaporated through a transparent surface before condensing on the transparent surface, which has a temperature lower than water vapor4,5,6,7. Many efforts are being made by research teams to make this procedure more efficient and to reduce major hazards to the future of many residents8,9,10.

The still’s efficiency was tested with various shapes and sizes. According to Jafari, a pyramid-shaped solar still outperformed a flat-plate still in terms of both water production rate and water quality11. A higher rate of purified water production was noted by Tiwari et al12 when a multi-layered design was used to improve the still’s surface area. Several methods exist for making solar stills more efficient. An enhancement to enhance the water flow characteristics and solar irradiation of the still involves including reflectors, jute cloth, and adjusting the angle of the glass. This adjustment leads to a significant rise of 72.18 percent in water production and a notable improvement of 41.51 percent in efficiency13. To further enhance the evaporation and condensation rates, another modification is to use nanomaterials, particularly nanoparticles of cuprous oxide and aluminum oxide. This causes the distillation productivity to rise by 125.0% when employing aluminum oxide nanoparticles and a considerable 133.64% when using cuprous oxide nanoparticles14.

A solar still’s efficiency was greatly improved by narrowing the gap between the absorber tray and the glass15,16. There have been numerical and experimental studies on a solar air warmer coupled with a multi-section solar distiller with interior sections17,18,19. An innovative and sustainable adaptation has been developed to enhance the advantages of classic solar stills while maintaining their desirable features such as affordability, portability, sustainability, water purity, material availability, minimal maintenance, and space efficiency20,21. Solar distillation systems with a variety of energy storage media have been analyzed for their energy and exergy efficiency. To increase the performance of solar distillation systems, many parameters and adjustments have been investigated. When two models were used to pair thermal heat storage materials, SS water production increased. It was determined how much energy, exergy, and economic output the SS generated. Using a composite heat storage medium of paraffin wax and black gravel significantly improves the performance and efficiency of SS, as shown by the experiments. Applying CM enhanced SS productivity by 3.27 L/m2, energy efficiency by 48.22%, and exergy efficiency by 3.18%, compared to SS-PCM, which had an improvement of 37.56%, 38%, and 37%, respectively22. The installation of black-painted copper plates and phosphate pellets boosted evaporation rates and daily productivity23. Acrylic, which has a low thermal conductivity, is commonly used in solar still basin construction since it has been shown to boost system yield24. Solar distillation systems with significant embodied energy, carbon mitigation, and decreased energy payback time have shown encouraging results in energy and exergy efficiency analyses25,26. A solar still with a surface area of 1 m2 can be constructed using thermal energy storage components and a non-selective coating on the absorber sheet. Every day, with and without thermal energy storage components, the solar system is put through its paces. When compared to paraffin wax alone or without paraffin wax thermal storage, investigations show that employing nanomaterials (Al2O3) distributed in paraffin wax improves the cumulative distillate production. Solar stills using nanocomposite phase transition materials have an estimated daily efficiency of 45%; solar stills using paraffin wax alone for thermal storage have an efficiency of 40%; and solar stills without thermal storage have an efficiency of 38%27. A novel v-corrugated absorber solar still with integrated phase change material is presented (PCM). The process involves melting wax and subsequently enabling its expansion through a system of elongated tubes positioned within the container. We put the system through its paces with and without the PCM and with various kinds of water in a number of different environments. The daily productivity of the still with the Phase Change Material (PCM) is 12% higher than that of the v-corrugated still without PCM at a mass flow rate of 25 kg. Furthermore, compared to the v-corrugated still, the daily production of the PCM-using wick still is 11.7% greater28.

The studies collectively explored various innovations in solar desalination systems with a focus on energy storage and efficiency improvements. Dhivagar et al.29, crushed granite stone was used as an energy storage medium, significantly enhancing the thermal storage capacity and overall efficiency of solar desalination when integrated with solar district heating. The findings emphasized the potential of granite to reduce energy losses and improve water output due to its high thermal conductivity. Similarly, Dhivagar et al.30 investigated a solar still using a porous biomaterial for energy storage, resulting in improved water yield and reduced environmental impact. The porous surface allowed for better thermal retention, leading to increased evaporation and efficiency. The use of gravel coarse aggregate in a single-slope solar still was analyzed using the 4E framework (energy, exergy, economic, and environmental), which demonstrated a marked improvement in the system’s overall performance by storing sensible heat31. Additional enhancements to solar stills were examined in32, where graphite plate fins and magnets were integrated, significantly boosting efficiency. The combination of these materials increased water productivity by optimizing heat retention and reducing energy waste. The application of magnetic powder as an energy storage medium was assessed33, further improving the efficiency of solar desalination systems through better exergy performance and a smaller environmental footprint. Nature-inspired innovations were explored34, where snail shell biomaterials were used in solar stills to increase water production in a sustainable manner. These biomaterials not only provided energy storage benefits but also contributed to environmental sustainability. Furthermore, Suraparaju et al.35 examined the feasibility of using waste-derived materials like discarded engine oil and paraffin wax for thermal storage, showing promise for sustainable solar desalination. Composite materials such as paraffin wax and used cooking oil were also studied in36 as effective thermal energy storage solutions in solar distillation, leading to more efficient potable water generation. Another study combined discarded transmission oil with paraffin wax as a phase change material for energy storage, reinforcing the role of waste materials in enhancing solar desalination’s environmental sustainability37. Jatropha biodiesel and paraffin wax blends were explored as thermal energy storage materials to augment freshwater generation, showing notable improvements in system efficiency38. The enhancement of water yield through composite heat storage materials was also confirmed39, where experimental and thermo-economic assessments highlighted the benefits of these materials in solar stills. Finally40 and41 explored paraffin wax and repurposed materials, respectively, as phase change materials for energy storage, both yielding improvements in productivity and efficiency in solar desalination systems. The overarching conclusions from these studies indicate that using innovative, often waste-derived, energy storage materials can significantly improve the performance of solar desalination technologies while contributing to environmental sustainability.

An extensive analysis of the efficiency of a solar still using a passive evacuated tube collector is presented in this work. Compared to previous technologies, this one has a shorter time to recover its production energy and a higher efficiency in converting energy over its lifetime, according to the data. The examination of energy matrices, with consideration for economic and environmental factors, yields these conclusions42. The solar still achieves a maximum energy efficiency of 45%, 36%, and 30% for water volumes of 2, 4, and 6 L, respectively. The highest exergy efficiency is 4.7%, 3.8%, and 2.9% for water volumes of 2.4, 4, and 6 L. These findings are based on an assessment of energy, exergy, and entropy conducted by Chávez et al.43. Tiwari et al.44 found that the exergy efficiency of a single-slope solar still was lower in the winter than the summer. A symmetric dual-slope solar still is analyzed for energy and exergy while integrated with ETC in forced operation and facing east west at 28°35′ N45. Researchers discovered that as water temperature rises, the evaporative portion exergy also increases, and that both energy and exergetic efficiency are positively affected by flow rate. Thermoeconomic analysis and solar distillation with a stepped-corrugated absorber plate have both been investigated46. The average energy efficiency, exergy efficiency, and productivity of a stepped-corrugated solar still are 259.61 percent, 418.61 percent, and 147.93 percent higher than those of a conventional solar still. Kabeel et al.47 did an experiment in which they injected wild flax into two hemispherical distillers. One of the distillers had unadulterated yellow flax, while the other included flax that had been artificially colored black. The outcomes of these two distillers were contrasted with those of a traditional hemispherical distiller. The initial distiller demonstrated a 29.7% enhancement, but the subsequent distiller exhibited a 39.6% improvement. The effect of weather, design, and operation on solar still efficiency was studied by Laxmikant et al.48. They found that factors such as the amount of solar irradiation reaching the surface, the kind of solar still employed, and the temperature of the surrounding environment all contribute to the daily output of distilled water. By increasing funding for solar distillation research and development, these constraints could be eliminated, hence broadening the possible uses of the technology49. A comparison was made between the energy and economic performance of a single-slope solar still using ball marbles (BMSS) and a conventional solar still (CSS) at Karaikal, India (10.92° N, 79.83° E), in October 2020. Trials were conducted to evaluate the efficiency of solar stills under several weather conditions, including both bright and dismal days. BMSS exhibits greater evaporation and productivity compared to CSS as a result of the increased amount of sensible heat energy retained by the ball marbles in the absorber basin. On clear days, BMSS can produce 21.23% more potable water than on overcast days, and that number jumps to 22.86%. On sunny days, the BMSS can produce a maximum cumulative output of 2950 mL/m2, but on cloudy days, it drops to 1,150,000 mL/m2. According to the economic research, the payback period (PBP) for the BMSS is 5.7 months, whereas for the CSS it is 6.5 months. The price per liter of BMSS potable water is 8% less than that of CSS50. Parametrically sensitive research was carried out to assess the efficiency of a conical solar still-operated photovoltaic thermal compound parabolic concentrator with respect to environmental and design parameters51. Kabeel et al.52 examine the development and building of innovative distillers, which have a transparent conical cap positioned at various angles relative to the horizon. The objective is to attain the highest possible total production of Conical Solar Stills (CoSS). The study examines the performance of three conical solar stills with varying inclination angles (30, 45, and 60° relative to the horizon) to identify the optimal angle that produces the maximum performance for CoSS. It has been compared theoretical models with experimental results to determine how efficient a new conical solar still design was. Featuring a constant volume flowrate and a conical glass lid, this design is sure to turn heads. Comparing its efficiency to that of a conventional solar still was our primary objective. Volume flow rates of 80, 60, and 40 mL/s were all part of the investigation. By reducing the rate of salinity in the water flow, the testing findings showed that the conical solar still could attain its maximum efficiency and output53. The study focuses on enhancing the efficiency of conical solar distillers by using design modifications, including high thermal-conductivity cylindrical fins and phase change materials (PCM), to maximize water production both during the day and night. The results indicate that distillers equipped with hollow copper tubes filled with PCM achieved the highest freshwater output, improving by 54.64% compared to conventional designs, with notable improvements of 550% in night-time production54. The study examines the use of charcoal balls in hemispherical solar stills to enhance heat transfer and solar absorption, resulting in a 29.16% improvement in water productivity compared to standard designs. Both studies demonstrate the effectiveness of innovative material integration in boosting solar distillation efficiency55.

From the previous survey there is no comparison between different storage martials used is conical solar still so, this paper aims to

  • Water productivity, system efficiency, and exergy efficiency are three metrics that may be used to compare various energy storage modules.

  • Find the optimal low-cost energy storage material.

  • Economic analysis was carried out to analyze the payback period, the selling price of distilled water, and the cost of water produced from the conical solar distiller.

This study introduces a novel approach by comparing the effectiveness of various low-cost energy storage materials, specifically glass balls, stainless steel balls, sandstones, and black gravel, in a conical solar distillation system. The research is unique in utilizing these materials of identical dimensions to enhance water evaporation and storage efficiency. Unlike previous studies that focused on single materials or design modifications, this study provides a comprehensive comparative analysis under consistent environmental conditions. The novelty of this work lies in its detailed examination of how different materials influence the energy and exergy efficiency of the system, identifying stainless steel balls as the optimal material for maximizing performance. This comparison provides a new understanding of the thermophysical behavior of different storage materials in solar distillation applications, offering a significant step toward more efficient and sustainable freshwater production technologies.

The experiments are conducted over a duration of two days. On the first day, the first distiller does not have any energy storage material, while the second distiller’s basin is filled with glass balls, and the third distiller’s basin is filled with stainless steel balls. On the second day, the first distiller is devoid of any energy storage material. The second basin of the distiller is filled with sandstones, whilst the third tank of the distiller is filled with black gravel. The ponds contain brine at a depth of 1.5 cm.

Methods and strategy for the experiment

The experiment utilizes a circular basin with an area of 0.1 m2. Additionally, a conical cap with a diameter of 40 cm and an inclination of 30 degrees is employed. The conical acrylic is positioned at the apex of the solar still to facilitate the downward movement of water droplets created on its sleek surface, which are then accumulated in the distilled water collector. Our experiment aims to enhance the productivity of a conical solar distiller by utilizing several low-cost energy storage materials, including glass balls (GB), stainless steel balls (SSB), sand stones (SS), and black gravel (BG), all of which have identical dimensions (1.5 cm).

The decision to use these specific energy storage materials in this study was based on their thermophysical properties, which directly impact the efficiency of heat transfer and water evaporation in solar distillation systems. Stainless steel was selected for its high thermal conductivity, while glass balls, sandstones, and black gravel were chosen due to their availability, low cost, and relatively moderate thermal conductivity, making them ideal for comparison. The materials were tested under identical conditions to ensure consistent results and to accurately assess their performance in terms of energy storage, water yield, and overall system efficiency. This selection provides a robust dataset for analyzing the practical applications of these materials in solar desalination.

This technique is used to speed up the evaporation process by increasing the surface area of water exposed to the solar irradiation. Figure 1 shows the schematic diagram of a conventional conical solar distiller with energy storage materials.

Fig. 1
Fig. 1The alternative text for this image may have been generated using AI.
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Schematic diagram and photo of a conical solar still.

A photo of different energy storage materials of same size of 1.50 cm are presented in Fig. 2a–d. Table 1 presents the properties of utilized glass balls (GB), stainless steel balls (SSB), sand stones (SS), and black gravel (BG).

Fig. 2
Fig. 2The alternative text for this image may have been generated using AI.Fig. 2The alternative text for this image may have been generated using AI.
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Photo of different energy storage materials.

Table 1 The properties of different energy storage materials56,57.
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The heat conductivity of stainless-steel balls is higher because of their metallic composition. The thermal conductivity of stainless steel can vary depending on the specific type of alloy and its composition, but generally falls in the range of 15–24 W/(m.K). This makes stainless steel balls much better conductors of heat than glass balls. Sandstones are sedimentary rocks that are composed of sand-sized grains of minerals, rock fragments, or organic material. The thermal conductivity of sandstone varies depending on the type of mineral grains and the porosity of the rock. In general, sandstone has a thermal conductivity in the range of 1–4 W/(m*K), which is similar to that of glass balls. Black gravel is typically made of crushed black basalt. Basalt has a lower thermal conductivity than sandstone or glass, with a thermal conductivity of 0.55 W/(m.K). This means that black gravel are not good conductors of heat and have a relatively low thermal conductivity. Glass balls are typically made of borosilicate glass, which has a thermal conductivity of about 6 W/(m.K). This makes glass balls a better conductor of heat than sandstone or glass balls, but still not as good as stainless steel.

In summary, the thermal conductivity of these materials varies depending on their material composition and structure. Stainless steel balls have the highest thermal conductivity, followed by glass balls, sandstone, and black gravel, which have the lowest thermal conductivity.

As different energy storage materials with low cost (glass balls (GB), stainless steel balls (SSB), sand stones (SS), and black gravel (BG) with equal sizes (1.5 cm)) is an excellent sensible heat storage. Figure 3 shows photograph of a basin solar still with different energy storage materials.

Fig. 3
Fig. 3The alternative text for this image may have been generated using AI.Fig. 3The alternative text for this image may have been generated using AI.
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Photograph of a basin solar still with different energy storage materials.

Experimental methodology

Experiment is conducted in two days (July 15 and 16; 2023). On the first day, the productivity of three distillers is compared. The first distiller is conventional (CCSD), the second distiller contains glass balls (CSD-GB) filling the base of its basin, and the third distiller contains stainless steel balls (CSD-SSB) filling the base of its basin. On the second day, the productivity of three stills is compared. The first still is conventional (CCSD), the second still contains sand stones (CSD-SS) that fill the base of its basin, and the third distiller (CSD-BG) contains black gravel that fills the base of its basin. Energy storage materials with low cost (glass balls (GB), stainless steel balls (SSB), sand stones (SS), and black gravel (BG) with same sizes (1.5 cm)). Figure 4 shows experimental photograph of a conical solar distillers with different energy storage materials.

Fig. 4
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Experimental photograph of a conical solar distillers with different energy storage materials.

The temperatures of the water, glass, and basin, among other parts of a solar still, may be precisely measured to within ± 0.1 °C using a K-type thermocouple. A solar power meter, which measures environmental factors including solar intensity, can measure values between zero and 1999 W/m2.

An accuracy of ± 10 W/m2 is provided by the solar power meter. The amount of water collected at hourly intervals is measured with an accuracy of ± 1 ml using digital weighing equipment.

Performance parameters

Energy efficiency

This approach, which can be used for any system analyzed from a thermal standpoint, entails comparing the energy consumed, namely in the form of desalinated water, to the energy input into the system, in the form of heat energy derived from solar irradiation55.

$$\eta_{sys} = \frac{{\dot{m}_{fw } \lambda_{fg} }}{{A_{sc} I}} \times 100$$
(1)

where \(I\) is solar irradiance, W/m2, \(\lambda_{fg}\) is latent heat of vaporization (kJ/kg), \(\dot{m}_{fw }\) is fresh water mass flow rate (kg/s), \(A_{c}\) is Solar collector area (m2).

Exergy efficiency

The exergy of a system is the maximum potential work that may be obtained from it when it is in a state of balance with its surroundings. Because exergy violates the principles of thermodynamics, the equation for system balance can be stated as19:

$$\sum {\dot{\text{E}}\text{x}}_{{{\text{in}}}} { } - \sum {\dot{\text{E}}\text{x}}_{{{\text{out}}}} = { }\sum {\dot{\text{E}}\text{x}}_{{{\text{dest}}}}$$
(2)
$${\dot{\text{E}}\text{x}}_{{{\text{sun}}}} - \left( {{\dot{\text{E}}\text{x}}_{{{\text{evap}}}} + {\dot{\text{E}}\text{x}}_{{{\text{work}}}} } \right) = {\dot{\text{E}}\text{x}}_{{{\text{dest}}}}$$
(3)

where;

The input and output exergy are given by58,59:

$${\dot{\text{E}}\text{x}}_{{{\text{in}}}} = {\dot{\text{E}}\text{x}}_{{{\text{sun}}}} = {\text{A}}_{{{\text{sc}}}} {\text{ I }}\left[ {1 - \frac{4}{3}{ }\left( {\frac{{{\text{T}}_{{{\text{amb}}}} + 273}}{{{\text{T}}_{{{\text{sun}}}} }}} \right) + \frac{1}{3}\left( {\frac{{{\text{T}}_{{{\text{amb}}}} + 273}}{{{\text{T}}_{{{\text{sun}}}} }}} \right)^{4} } \right]$$
(4)
$${\dot{\text{E}}\text{x}}_{{{\text{evap}}}} = {\dot{\text{m}}}_{{{\text{fw}}}} { }\lambda_{{{\text{fg}}}} \left[ {1 - \frac{{{\text{T}}_{{{\text{amb}}}} + 273}}{{{\text{T}}_{{\text{s}}} + 273}}} \right]$$
(5)

The water latent can be estimated from the equation60:

$$\lambda_{{{\text{fg}}}} = 2.4935 \times 10^{6} \left( {1 - 94779 \times 10^{ - 4} T_{avg} + 1.3132 \times 10^{ - 7} T_{avg}^{2} - 4.7947 \times 10^{ - 9} T_{avg}^{3} } \right)$$
(6)

where Tavg (K) is the average temperature between the basin water Tw (K) and the inner glass Tg (K).

Where \({\dot{\text{E}}\text{x}}_{{{\text{out}}}}\) is output exergy (W), \({\dot{\text{E}}\text{x}}_{{{\text{in}}}}\) is input exergy (W), \({\dot{\text{E}}\text{x}}_{{{\text{dest}}}}\) is destructive exergy (W), \({\dot{\text{E}}\text{x}}_{{{\text{evap}}}}\) is evaporative exergy (W), \({\text{T}}_{{{\text{amb}}}}\) is ambient temperature (°C), \({\text{T}}_{{{\text{sun}}}}\) is sun temperature (°C).

In the most unfavorable scenario, there would be no exergy present. Conversely, in the most ideal scenario, the operation would be reversible. In simpler terms, the exergy efficiency, also known as second-law efficiency, should vary between zero and one. The exergy efficiency of a system during a process can be characterized as follows54,61:

The least energy input required to provide the maximum reversible work output in energy-consuming systems.

$$\eta_{{{\text{Ex}}}} = \frac{{{\text{Exergy}}\;{\text{ recovered}}}}{{{\text{Exergy}}\;{\text{ supplied}}}} = 1 - \frac{{{\text{Exergy}}\;{\text{ destroyed}}}}{{{\text{Exergy}}\;{\text{ supplied}}}}$$
(7)
$$\eta_{{{\text{Ex}}}} = \frac{{{\dot{\text{E}}\text{x}}_{{{\text{out}}}} }}{{{\dot{\text{E}}\text{x}}_{{{\text{in}}}} }} = 1 - \frac{{{\dot{\text{E}}\text{x}}_{{{\text{dest}}}} }}{{{\dot{\text{E}}\text{x}}_{{{\text{in}}}} }}$$
(8)

Results and discussion

Daily and weather-related variables, such as solar irradiation and ambient air temperatures, have a considerable impact on the efficiency and productivity of solar stills.

The weather and solar irradiation data for July 15, 2023 are shown in Fig. 5. Solar stills are highly sensitive to the amount of light that reaches them. solar irradiation acts as a heat source that propels the evaporation process in solar stills. The rate of heat generation and water evaporation is both boosted by an increase in solar irradiation. The water in the basin may be heated or cooled depending on how much solar irradiation the still absorbs. The water in a basin might get hotter and evaporate faster when the solar irradiation is strong.

Fig. 5
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Measured ambient temperature and solar irradiation.

Heat capacity refers to the amount of heat required to raise the temperature of a material by a certain amount. The heat capacity of a material is typically measured in units of J/(K).

Glass balls have a relatively low heat capacity, with a typical value of around 840 J/(K). This means that glass balls require relatively little heat energy to raise their temperature. Stainless steel balls have a higher heat capacity than glass balls, with a typical value of around 500 J/(K) for common alloys. This means that stainless steel balls require more heat energy to raise their temperature than glass balls. Sandstones have a relatively high heat capacity, with a typical value of around 800–1200 J/(K) depending on the specific type of sandstone. This means that sandstones require a significant amount of heat energy to raise their temperature. Black gravel, which is typically made of crushed black basalt, has a heat capacity similar to that of stainless steel, with a typical value of around 510 J/(K). This appears clearly in the water temperatures throughout the day, as in the Fig. 6. The basin water temperature is a critical factor influencing the evaporation rate in solar stills. As shown in Fig. 6, stainless steel balls demonstrated the highest basin water temperature throughout the day. This outcome is attributable to their high thermal conductivity, which allows for efficient heat absorption and retention. The higher water temperature directly correlates with an increased evaporation rate, leading to higher water productivity. In contrast, materials like black gravel and sandstone, with lower thermal conductivities, resulted in lower basin temperatures, which in turn diminished their overall performance. This temperature variation highlights the importance of selecting materials with optimal heat conduction properties for maximizing solar still efficiency.

Fig. 6
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Basin water temperature along the day time.

Figure 7 depicts the hourly internal temperature of the glass cover, which exhibits sufficiently high values to enhance the rate of water evaporation, hence improving the efficiency of the desalination process. The glass cover temperature plays a role in the condensation process, where higher temperatures can accelerate water vapor condensation and improve overall distillation rates. The results indicate that the stainless-steel balls also contributed to maintaining a relatively high internal glass temperature, further promoting efficient condensation. The glass balls, while performing better than sandstones and black gravel, did not match the performance of stainless steel. This suggests that the high heat retention capacity of stainless steel not only elevates the basin water temperature but also creates a conducive environment for condensation, maximizing the system’s output.

Fig. 7
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Diurnal variation of internal glass temperature.

A solar still’s productivity, or the amount of potable water it produces, is highly sensitive to the water’s temperature in the basin. Rapid surface evaporation and increased water productivity can be achieved when the water temperature in the basin is higher. Condensation on the still surface can be accelerated and water productivity enhanced if the water in the basin is heated. A solar still’s energy storage material’s primary function is to maintain a constant temperature inside the still by collecting and retaining heat during the day and then releasing it during cooler hours or at night. Condensation on the still surface can be accelerated and water productivity enhanced if the water in the basin is heated. A solar still’s energy storage material’s primary function is to maintain a constant temperature inside the still by collecting and retaining heat during the day and then releasing it during cooler hours or at night. The heat capacity of a substance defines its ability to store heat energy per unit mass and per degree of temperature change. Materials having a high heat capacity have the ability to store a greater amount of heat energy for each unit of mass and for each degree of temperature change. As a result, they are more efficient at storing heat. Utilizing energy storage materials with a high heat capacity might enhance the efficiency of solar stills by ensuring a more stable internal temperature and minimizing heat dissipation during periods of lower temperatures throughout the day. This can lead to increased water production and improved efficiency of the solar still. As shown in Figs. 8 and 9, The daily yield was maximum CSD-GB, CSD-SSB, CSD-SS and CSD-BG are found as 8750, 9450, 7700 and 8250 mL/m2 respectively. whereas from the CCSD is found as 5750 mL/m2 with an improvement rate of 52.17, 64.35, 33.91 and 43.48% when using glass balls, stainless steel balls, sand stones and black gravel, respectively. It is obvious that the material (SSB) with higher heat capacity gives higher water productivity.

Fig. 8
Fig. 8The alternative text for this image may have been generated using AI.
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Hourly water productivity during day hours.

Fig. 9
Fig. 9The alternative text for this image may have been generated using AI.
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Accumulative water productivity along the day.

The hourly water productivity, as shown in Fig. 8, further emphasizes the efficiency of stainless steel balls. The material’s superior thermal properties facilitated higher productivity during peak solar irradiation hours, reaching a maximum yield of 9450 mL/m2. In comparison, glass balls and black gravel showed moderate performance, while sandstones lagged behind. The results underscore the significance of using materials with high heat transfer capabilities for optimizing solar still performance. Moreover, the ability of these materials to maintain stable temperatures even during non-peak hours contributed to a more consistent distillation process. Accumulative water productivity, depicted in Fig. 9, reveals that stainless steel balls consistently outperformed the other materials over the course of the day. The accumulative yield clearly demonstrates the sustained advantage of using materials with higher thermal conductivity. The performance of glass balls, while commendable, fell short compared to stainless steel, further reinforcing the need to select materials based on their heat storage and release characteristics. The results also show that, despite the lower thermal conductivity of black gravel and sandstones, these materials still provided measurable improvements over the conventional solar still design (CCSD), indicating that any sensible heat storage material is beneficial to the overall productivity.

Raising the water temperature in the basin can improve the efficiency of a solar still, which in turn increases water productivity. Figure 10 clearly demonstrates the impact of utilizing aluminum balls as an energy storage material on the overall efficiency of the system. Since the solar irradiation remains constant over the testing days, the primary factor that affects the efficiency of the system is the water production. CCSD, CSD-SS, CSD-GB, CSD-BG and CSD-SSB give an average system efficiency of 30.37%, 39.1%, 49.5%, 42.57% and 54.06% respectively with an improvement rate about 78.01%, 63.23%, 39.8% and 28.74% for CSD-SSB, CSD-GB, CSD-BG and CSD-SS respectively. The energy efficiency of the system, as illustrated in Fig. 10, directly correlates with the water productivity findings. Stainless steel balls exhibited the highest energy efficiency, achieving 54.06%, which is substantially higher than the other materials. The enhanced energy efficiency can be attributed to the material’s ability to absorb and store more heat, allowing for better utilization of the available solar energy. Glass balls and black gravel, while effective, showed a lower energy efficiency due to their comparatively lower heat capacity and thermal conductivity. The performance of sandstones was the least favorable, highlighting the limitations of using materials with low thermal conductivity in solar distillation systems.

Fig. 10
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Hourly energy efficiency.

Exergy efficiency measures the extent to which a system can effectively convert available energy into useful work. Exergy efficiency, in the context of a solar still, refers to the system’s ability to effectively convert solar energy into usable water. Utilizing energy storage materials that possess a high capacity to store heat and a high ability to transmit heat can enhance the exergy efficiency of a solar still by minimizing heat loss and optimizing heat transfer efficiency. Hence, it can be concluded that the utilization of energy stored materials in general has enhanced the overall efficiency and exergy efficiency in comparison to the conventional design, as seen in Fig. 11. The average exergy efficiency is about 0.89%, 3.93%, 3.03%, 2.38% and 1.97% for CCSD, CSD-SSB, CSD-GB, CSD-BG and CSD-SS respectively with an improvement about 341.1%, 240.16%, 168.1% and 121.7 for CSD-SSB, CSD-GB, CSD-BG and CSD-SS % respectively compared to CCSD. Stainless steel balls once again outperformed the other materials, achieving an exergy efficiency of 3.93%. The superior exergy efficiency of stainless steel is a reflection of its ability to minimize energy loss through efficient heat transfer. Glass balls, black gravel, and sandstones, although beneficial, did not match this level of performance, indicating that stainless steel is the most effective material for optimizing exergy efficiency in solar distillation systems. The conventional solar still (CCSD), with an exergy efficiency of only 0.89%, demonstrates the clear advantage of incorporating energy storage materials into the system.

Fig. 11
Fig. 11The alternative text for this image may have been generated using AI.
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Hourly exergy efficiency.

The findings from Figs. 6, 7, 8, 9, 10, 11 provide a clear indication that selecting the appropriate energy storage material is crucial for optimizing the performance of conical solar stills. Stainless steel balls, due to their high thermal conductivity and heat capacity, significantly enhance both the energy and exergy efficiencies of the system, leading to greater water productivity. Glass balls and black gravel, while useful, do not offer the same level of performance, suggesting that future designs should focus on incorporating materials with higher heat retention capabilities. The implications of these findings extend beyond the experimental setup. In practical applications, regions facing freshwater scarcity could benefit from adopting solar stills integrated with stainless steel balls to maximize water production. Moreover, the high efficiency and productivity demonstrated by these materials suggest that they could be used in large-scale desalination systems, providing a sustainable solution to global water shortages.

Future work should focus on exploring hybrid systems that combine stainless steel balls with other advanced materials, such as phase change materials (PCMs), to further enhance system performance. Additionally, long-term studies evaluating the durability of these materials under varying environmental conditions would provide valuable insights into their practical application in real-world settings.

Analysis of economic factors

The total expense of the experimental arrangement is outlined in Table 2.

Table 2 Price of manufacturing a solar still.
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The payback period (\({\varvec{n}}_{{\varvec{p}}}\)) represents an estimated timeframe for the complete recovery of the initial investment, along with a specified rate of return (i), through revenues, savings, and other monetary benefits62. In this context, “P” denotes the initial project investment, and “NCF” represents the estimated annual net cash flow.

$$NCF = cash\; inflows - cash \;outflows$$
(9)

where.

  • Cash in flows represents water price

  • Cash out flows = Operation cost (\(C_{op}\)) + maintenance cost (\(C_{min}\))

For no return, i = 0%; annual uniform NCF:

$${\varvec{n}}_{{\varvec{p}}} = \frac{{\varvec{P}}}{{{\varvec{NCF}}}}$$
(10)

Hence, the payback period of CCSD, CSD-GB, CSD-SSB, CSD-SS and CSD-BG are 26, 16, 18, 18 and 20 days.

Table 3 presents a comparison between different systems of solar stills, either with a different design and the use of factors and materials added to it to improve its performance. It is clear in some systems the best utilization of energy, which shows a clear impact on exergy efficiency, and some systems improve their performance and total production per day of water.

Table 3 A comparison between different design of solar still.
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Conclusions

This study aimed to determine the most effective energy storage materials for improving the performance of a conical solar distillation system. The following key conclusions were drawn:

  • Stainless steel balls (CSD-SSB) provided the highest water productivity at 9.45 L/m2/day, followed by glass balls (CSD-GB) at 8.75 L/m2/day, black gravel (CSD-BG) at 8.25 L/m2/day, and sandstones (CSD-SS) at 7.70 L/m2/day. The conventional system (CCSD) had the lowest yield at 5.75 L/m2/day.

  • The highest energy efficiency was achieved with stainless steel balls (54.06%), followed by glass balls (49.5%), black gravel (42.57%), and sandstones (39.1%). The conventional system’s energy efficiency was 30.37%.

  • The average exergy efficiency was highest for stainless steel balls (3.93%), followed by glass balls (3.03%), black gravel (2.38%), and sandstones (1.97%). The conventional system recorded the lowest exergy efficiency at 0.89%.

  • Compared to the conventional system, stainless steel balls improved water productivity by 64.35%, glass balls by 52.17%, sandstones by 33.91%, and black gravel by 43.48%.

  • The payback periods for the different systems were: 16 days for CSD-GB, 18 days for CSD-SSB and CSD-SS, and 20 days for CSD-BG, compared to 26 days for the conventional system.

  • Conclusively, using conical solar energy with stainless steel balls as an economical energy storage substance (\(\emptyset 1.5\;{\text{ cm}}\)) is still optimal with water productivity 9450 mL with improvement in system efficiency 78.01% and exergy efficiency 341.1%.

Practical recommendations

Based on the findings, solar distillation systems can benefit from the integration of stainless steel balls as energy storage materials to maximize water productivity and efficiency. For regions facing freshwater scarcity, especially those with high solar irradiation, incorporating stainless steel balls in conical solar stills presents a cost-effective solution. Additionally, policymakers and engineers should consider adopting this technology in large-scale desalination projects to address global water shortages. Further research should explore hybrid systems that combine these materials with other innovative technologies to further enhance system performance.

Limitations and future work

  • The study was limited to a specific set of materials. Future research should consider a broader range of advanced materials, including nanocomposites, to further enhance system performance.

  • The experiments were conducted over a short time frame. Longer-term tests are necessary to evaluate the durability and long-term effectiveness of the materials under varying environmental conditions.

  • Further investigation into hybrid systems that combine the energy storage materials with other innovative technologies could lead to further improvements in efficiency.

  • Testing in different climates and regions with varying solar irradiation levels would provide a better understanding of how these materials perform in different environmental conditions.

  • While the study addressed cost-effectiveness, a more detailed economic analysis covering the full life cycle of the system and materials is recommended to ensure scalability and practical implementation.

By addressing these limitations, future research can offer more comprehensive solutions to improve solar desalination systems for global freshwater needs.