Experimental and theoretical analysis of solar still with solar pond for enhancing the performance of sea water desalination

All living things require water as a basic necessity. Due to increased demand for water, there will be more wars fought in the future over this scarce resource. Human and animal life is not feasible without the availability of potable water, scarcity leads to crisis (Alwan et al. 2020). The lack of water can be dangerous to one's health, and it may even result in death. The oceans cover 71% of the planet's land area. Oceans and seas contain over 97% of the world's water resources, and much of it is saltwater (Hammadi 2020). Fewer than 3% of all water supplies around the globe are considered freshwater. The great bulk of the world's clean water contains ice in glaciers and icy surfaces. Fresh water in rivers and lakes makes up 0.3% of the total water supply all around the world (Suraparaju et al. 2021). Less than 1% of the clean surface and underground water on the earth is used by humans. The quantity of purified water needed to fulfil the expanding needs of business, agriculture, and the general public is decreasing despite a growing worldwide population (Baskaran & Saravanane 2021). Economic growth and quality of life are negatively affected when there is a deficiency of safe drinking water. Since brackish water cannot be utilized to produce drinks or other commodities due to the presence of dangerous bacteria and/or dissolved salts, many developing nations are in urgent need of clean, safe drinking water (Tlili & Alkanhal 2019). Despite the fact that seawater is readily available, potable water cannot be found along various coastlines. Even life-supporting systems need potable water (Mu et al. 2019).

A lack of access to healthy water sources is a major factor in the development of severe illness. Most of the two million individuals who die each year as a result of waterborne diseases do so as children, primarily in underdeveloped nations where cholera, diarrhoea, typhoid, and malaria are the most common. For water purification, distillation is one of the methods available (Nayi & Modi 2021). There must be a source of energy, such as heat, but solar radiation can also play a role. Pure water is formed when water vapour is isolated from dissolved materials and condensed as a result of evaporation. Renewable energy may be the ideal alternative for small communities that do not have access to cheap fossil fuels to power the desalination process (Shalaby et al. 2016). Devices like solar stills can be used to distil seawater and brackish waters into clean and reusable liquids. Solar stills (SS) are commonly used in rural and isolated areas to provide modest amounts of potable water (Appadurai & Velmurugan 2017). Water scarcity is an issue all throughout the world, but it is most acute during the summer seasons (Gao et al. 2017). Summer is also a time when people drink a lot of water, and the amount of sunlight absorbed by the earth's surface is at its highest point. So, a variety of water desalination technologies are being developed, but they all demand a lot of electricity (Hidouri & Gabsi 2016).

In contrast to other saltwater desalination methods, solar desalination is well renowned for its cost-free energy, low operational expenses, and simple construction. In addition to treating brackish and contaminated water, solar desalination technology can also be applied invariably. Numerous solar still designs were explored and tested to boost daily production (Sharon et al. 2021). A new, experimental corrugated sunlight still has been created, which increases the condensation rate (Shahin et al. 2016; Abdelgaied et al. 2022). The investigations on various solar still cover angles were carried out by Cardoso et al. (2022). The maximum efficiency obtained was 27.7%. Arjunan et al. (2017) employs both internal and external reflectors in his single basin solar still (SBSS). The flat plate collection was found to be 51% more productive than a multiple slope SS (Coelho 2005). Run Kumar et al. (2021) built a solar still that was 26% more efficient than standard solar stills by using an electric blower with several wicks. The pyramid-shaped solar still developed by El-Bahi & Inan (1999) is thus 45% more efficient than the SBSS. Riahi et al. (2016) evaluated the tube-SS with a tepid water flow over the distillate cover to achieve a high productivity of 16 kg·m⁻². A single basin was used to evaluate the outcome of water cooling the glass cover; also, it was found that still efficiency may be boosted by up to 20% by appropriately configuring the film-cooling settings. A cooling water velocity of 1.5 m·s⁻¹ was measured around the glass covering (Xu et al. 2020). The essential elements affecting the efficiency of solar evaporation were examined (Rajanarthini et al. 2013). Stepped solar productivity was still boosted by 125% by using inside and outside reflectors (Dehghan et al. 2015). Katekar & Deshmukh (2020) established one-dimensional thermal modelling of a glazing solar collector flat panel model utilizing a fin-theory method. When designing their solar still, Xue & Li (2015) focused on reducing the cost per unit of usable heat flow by adjusting the fin's width and thickness. For example, Toyama et al. (1987) employed solar stills with fins constructed of old cotton rags that have been darkened to produce 7.5 kg·m⁻².

It has been observed that solar stills used to produce ethanol benefit from fins by a factor of 46% (Mousa & Gujarathi 2016). Production was increased by 33% when a storage container, another solar system, and two flat plate collectors were added (Appadurai & Velmurugan 2015). They demonstrated that, on an average day, a two-basin solar still generated 100% more distilled water than a single basin (Dumka & Mishra 2022). To increase production, Kumar et al. (2021) paired standard with flat plate collectors. The addition of a solar collector increased solar power's efficiency even further. It is possible that winter days can increase daily output by as much as 70–100%. It was discovered by Bie et al. (2019) that their new wick basin solar still was 90% extra prolific than previous models and 43% more productive than previous wick basin models. Cranmer (1998) utilized black rubber and also black gravel as the storage media for solar stills. An increase of 20 and 19%, respectively, in productivity was discovered in the studies with black rubber also black gravel used.

From the literature, it was found that adequate work on SBSS and desalination plants equipped with another integrated setup was not carried out. Hence, an attempt was made to develop an SBSS with three different conditions; SBSS with fin (SBSS-F), SBSS with solar pond (SBSS-FP), and integrated SBSS with fin and finned solar pond (IBSS), which is the novelty of the present study. The experimental and theoretical analyses were carried out and compared on the basis of the technique that was presented.

Experiments were conducted at Pongalur, close to Tirupur (10.9729° N, 77.3698° E), at 10° north latitude. Bay of Bengal seawater was collected for experimental purposes. The experimentation was conducted on 22 May 2023, from 9 AM to 5 PM. Four thermocouples were installed in various locations to measure basin and water temperatures, and a digital temperature indicator was connected to them. A solarimeter measures global solar radiation. The anemometer measures the productivity of distillate water. The three operating parameters of the solar still are slope angle (10°), water depth (3 cm), and ambient inlet water temperature (30 °C) were considered for this experiment (Arunkumar et al. 2021). A polyvinyl chloride (PVC) conduit and flow control valve connect the solar still to the micro solar pond to produce heated water in sections (Mahian et al. 2017). The LCZ heats a copper heat exchanger that circulates the UCZ's water. The flow control valve is opened every half-hour to receive the preheated water. To balance the water level in the UCZ, fresh water is added to the solar pond's surface. Every hour, measurements are recorded.

For the theoretical estimation of SBSS and its modification, the energy balance equation is considered. The basin energy balance equation is the sum of all the energy gained and lost through convective heat transfer, as well as side losses Q_loss that are received by the basin plate in solar still.

ε_eff = (1/ε_w + 1/ε_g)⁻¹

T_w and T_g represent water as well as glass temperature (°c), respectively.

To begin with, the temperatures of the water, basin, and glass are all considered to be ambient. Equations (6) and (7) are used to calculate temperature changes in water (d_tw), basin temperature (d_tb), and temperature changes in glass as in Equation (12). By using the MATLAB program, Equations (6) and (12) were solved. The flat plate collector's internal heat losses are equal to the solar still's energy intake.

From Figure 6(b), it was observed that the maximum freshwater productivity of SBSS-P was 0.557 L·m⁻² (Experimental) and 0.579 L·m⁻² (Theoretical) at 1 PM. The solar still's performance and efficiency can be improved by combining it with a solar pond (Mahian et al. 2017). The solar pond's heated water can be used to heat the solar still, allowing it to produce more potable water at higher temperatures. Incorporating a small solar pond into the still boosted its production, as seen in Figure 6, which is raised by an average of 27.6%.

From Figure 7(b), it was observed that the maximum productivity of SBSS-F was around 0.47 L·m⁻² (Experimental) and 0.50 L·m⁻² (Theoretical) at 2 PM. In comparison to basic solar still without fins, SBSS-F may greatly boost solar production. Adding fins, also known as heat-absorbing collectors, improves solar energy absorption and heat transfer to the water within the still. This results in increased freshwater output, quicker evaporation rates, and warmer water temperatures. From Figure 7(c), the maximum solar intensity with and without fins was observed around 711 and 686 W·m⁻², respectively, at 1 PM. The integration of fins into a solar system significantly affects sunlight intensity. The solar intensity is improved in a number of ways by the use of fins, eventually resulting in better solar energy absorption and utilization. The use of fins at the still's bottom has been proven to boost productivity by 46%. According to Figure 7, the theoretical performance has a maximum divergence of 9.1% when compared to the experimental results.

Figure 8(b) clearly shows that the maximum freshwater output was observed around 0.404 L·m⁻² (Experimental) and 0.415 L·m⁻² (Theoretical) at 1 PM for finned pond. When a solar still and a finned solar pond are used together, the efficacy of water filtration can be significantly boosted. The finned solar pond ensures continuous evaporation by supplying hot water to the solar still (Baskaran & Saravanane 2021). The solar still condenses and evaporates hot water from the solar pond's bottom layer. Since the solar still uses indirect sunlight in addition to direct sunlight to heat the water, this integration enables a more reliable and effective water purification process. Instead, even on cloudy days or during low solar radiation, it gains from the consistent supply of heated water from the finned pond (Sharon et al. 2020).

Table 1 illustrates the production growth rate for each of the systems studied. A solar still and solar pond with fins may increase a solar thermal energy system's output and performance. Installing ISBSS enhances the capacity for receiving energy and storing it, leading to good heat retention and fewer losses (Abdelgaied et al. 2022). Providing the solar still with warm water constantly improves its ability to distil and produce water. The integrated system is more dependable since it maintains constant energy production even when there is little sunlight or at night. Compared to other systems, ISBSS gives the maximum productivity (Hammadi 2020).

The design limitations and some experimental variables are related to the error. Instrument selection, calibration, condition, observation, environment, reading, and test design are all factors that can affect how accurate and precise it is. An uncertainty analysis is necessary to ensure the tests are conducted properly. Table 2 provides the instruments used in this work along with their model, making the accuracy level and range of operation.

Table 2

Instruments model no. and spec and make

Sl. No .	Instrument .	Model .	Make .	Accuracy .	Range .
1	Anemometer	AVM 03	Metrix +
2	Thermocouple	K-type	Radix
3	Measuring beaker	1L	RS Pro
4	Kipp–Zonen solarimeter	SP Lite2	Kipp-Zonen

Sl. No .	Instrument .	Model .	Make .	Accuracy .	Range .
1	Anemometer	AVM 03	Metrix +
2	Thermocouple	K-type	Radix
3	Measuring beaker	1L	RS Pro
4	Kipp–Zonen solarimeter	SP Lite2	Kipp-Zonen

View Large

The different changes were tested through outdoor experiments. The investigation analysed the SBSS, SBSS-F, SBSS-P, SBSS-FP, and ISBSS through theoretical and experimental methods. All modifications experienced increased productivity during the peak hours. The solar radiation led to a productivity increase. The large surface area exposed to solar radiation in the still and mini solar pond made the ISBSS more productive. In addition, convection increased the basin-to-water heat transfer rate. This increased the rate of thermal transfer from the basin to the water. Compared to conventional stills, using fins in the still and a mini solar pond can increase daily output by 52%. SBSS-FP and SBSS-F increased productivity by 48 and 46%, respectively.

All authors contributed equally to this work.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.