Introduction

Energy-related issues have grown to be a serious problem on a global scale, emphasizing the urgency for sustainable and efficient energy solutions. Solar energy offers a promising solution to this challenge. However, despite the Earth receiving abundant solar radiation daily, a significant portion of this energy remains underutilized (Abdullahi et al., 2017; Adediji et al., 2021). If all the solar power that falls upon the Earth for just one hour were harnessed, it would be sufficient to sustain the energy demands of our planet for an entire year. There is a growing interest in solar energy due to its numerous applications, including cooking (Saxena et al., 2011), air heating (Saxena et al., 2015), power generation (Saxena et al., 2021), distillation (Cuce et al., 2020), and others, which can be carried out efficiently with the help of solar thermal systems (Adediji, 2022). Additionally, the utilization of solar energy can help conserve finite fossil fuels to a significant extent (Adediji et al., 2023).

Numerous technologies that can capture and store solar radiation have been developed because of the possibility of using solar energy to meet the bulk of human energy needs (Adediji et al., 2023; Adeyinka et al., 2023; Oladimeji, et al., 2020). Solar ponds have received attention as a viable means of storing heat (Saleh, 2022). A solar pond is a non-conventional energy device that serves as a heat reservoir and integrates solar collection and storage in the same configuration to absorb and store solar radiation (Poyyamozhi & Karthikeyan, 2022a). However, a significant challenge with solar ponds is their low conversion efficiency. This inefficiency primarily stems from the natural convection process within the pond. As the water at the bottom heats up, it rises to the surface, releasing its stored heat into the environment. Consequently, much of the captured solar energy is dissipated before it can be effectively stored or harnessed, limiting the potential of solar ponds as reliable energy sources (Alcaraz et al., 2016; Sathish & Jegadheeswaran, 2021b; Wu et al., 2019). Hence, industrial-scale applications would require further research and development to increase solar ponds’ performance (Kasaeian et al., 2018).

The solar pond is divided into three zones: upper convective zone (UCZ), non-convective zone (NCZ), and lower convective zone (LCZ) (Saxena & Goel, 2013), as shown in Fig. 1.

Fig. 1
figure 1

Schematic diagram of salinity and temperature profiles of the solar pond (Khalilian, 2017)

Full size image

The UCZ is a narrow layer of low salinity water (between 0.1 and 0.40 m), where vertical convection current is caused by wind and evaporation (Gupta, 1987; Sayer et al., 2017). While some of the radiation that hits the UCZ is rapidly absorbed, a proportion of it is dissipated to the atmosphere through convection and radiation heat transfer. The remaining radiation is subsequently absorbed in the NCZ before it is transmitted into the LCZ, where it is stored (Abbassi Monjezi & Campbell, 2016; Singh et al., 2011). The LCZ is notable for its higher salt concentration, which enables thermal energy storage. The temperature of the LCZ can reach 90 °C in some climatic conditions. This heat can be used for heating and electricity generation (Shahid et al., 2023).

Conversely, heat transfer in the NCZ occurs primarily through conduction (Muñoz & Almanza, 1992). The salt concentration gradient of NCZ increases downwards, and its thickness is more than that of other zones. The NCZ acts as an insulating layer to prevent heat loss in the upward direction, and its thickness depends on the desired temperature, solar transmission properties, and thermal conductance of water (Wu et al., 2019). The bottom layer of the LCZ is where the absorbed radiation is transformed and stored in the form of heat. The bottom of the LCZ is usually coated with a black film of polyethylene to prevent leakage (AL-Musawi et al., 2020). The temperature of this layer significantly increases since there are no heat losses through convection from the LCZ.

The salinity gradient indicates the stability of the solar pond. A salt gradient is desirable as it prevents thermal convection across layers and allows the pond to act as a trap for solar radiation (Sarabia et al., 2018; Saxena et al., 2022). Hence, the pond’s thermal stability is increased, leading to improvement in the overall performance and efficiency of the solar pond. The common salts used in solar ponds are sodium chloride, magnesium chloride, or sodium nitrate, and their concentration in water ranges from 20 to 30 percent at the LCZ to virtually zero at the UCZ (Das & Ganguly, 2022). The stability of the pond is increased as the temperature rises from the UCZ to the LCZ. The stability of a solar pond is important for the performance of the solar pond (Kaushika, 1984).

Types of solar ponds

There are four types of solar ponds: salt-gradient solar ponds, shallow solar ponds, gel solar ponds, and equilibrium solar ponds.

Salinity gradient solar pond

Salinity gradient solar ponds (SGSP) are relatively simple and inexpensive to construct. In a Salinity gradient solar pond, the water at the bottom of the pond is much saltier than the water at the top, creating a density gradient. Due to this gradient, the warm water at the bottom, heated by absorbed solar radiation, does not rise to the top as it typically would in other environments because it is denser than the less salty and cooler water above it. This mechanism effectively traps the heat in the lower layers of the pond, allowing the temperature at the bottom to increase significantly as shown in Fig. 2. The stored thermal energy can be harvested and utilized for various applications (Sathish & Jegadheeswaran, 2021b). This creates a natural circulation of water in the pond, with the warm water at the surface and the cool water at the bottom.

Fig. 2
figure 2

Schematic diagram of a typical Salinity-gradient solar pond. Adapted from (Lindblom, 2003)

Full size image

SGSPs have several advantages over traditional solar ponds. They are more efficient at capturing and storing solar energy and can be used in different climates and locations (Leblanc et al., 2011). However, SGSPs also have some drawbacks. They require a large amount of space and can be visually unappealing, and they can be prone to algae growth and other forms of pollution if not properly maintained.

Shallow solar pond

A well-researched example of a convecting pond is the shallow solar pond. Among the various configurations of solar ponds, shallow solar ponds present a distinctive approach due to their structural simplicity and reduced depth, spanning only a few centimeters to about a meter (Garg, 1987). This reduced depth ensures that sunlight can penetrate the entire water body, thereby heating the whole volume of water as shown in Fig. 3. The ponds’ beds are lined with dark materials to augment the absorption of solar radiation and covered by a plastic film at the top so that the film is in contact with the top surface of the water, thus preventing the cooling effect due to evaporation.

Fig. 3
figure 3

Schematic diagram of shallow solar pond. Adapted from (Garg et al., J1982)

Full size image

The advantages of shallow solar ponds, beyond their simplicity and lower cost, extend to environmental considerations. The absence of salt or other chemicals negates the potential for environmental contamination, a risk present in salt-gradient solar ponds. Furthermore, the maintenance demands are relatively low, mainly because there is no need to manage the delicate balance of a salt gradient crucial to the operation of SGSPs. However, their reduced thermal storage capacity and lower attainable temperatures restrict their applicability in processes requiring higher temperatures.

Solar gel pond

The gel solar pond has attracted much less interest than the SGSP over the past four decades, and there is a lack of scientific research on this subject. Heat loss by evaporation at the surface is limited in a solar gel pond as illustrated in Fig. 4. In this case, the salt concentration gradient is not maintained as a thick layer of polymer gel acts as NCZ and floats on the LCZ (Spyridonos et al., 2003). Saltwater is not required if the gel layer floats on fresh water. Due to the importance of the polymer gel in the gel pond, the gel should have the following properties (Wilkins, 1991):

  • Inert and non-toxic.

  • High viscosity.

  • Low volumetric expansion coefficient and high specific heat over the operating temperature range of the pond.

  • Inexpensive, chemically, and physically stable gel structure with respect to temperature.

Fig. 4
figure 4

Schematic diagram of solar gel pond. Adapted from (Rizvi et al., 2015)

Full size image

Gel solar ponds present several advantages, including reduced surface evaporation and heat loss (El-Housayni & Wilkins, 1987), low maintenance due to the absence of a salt gradient (Sayer et al., 2018), decreased costs and environmental concerns from minimal salt use (Ines & Jomâa, 2019), and enhanced heat storage from the gel’s insulating properties (Spyridonos et al., 2003). However, the major drawback of gel ponds is the high cost of gel production for commercial-scale ponds (Kasaeian et al., 2018).

Several models have been developed to simulate the thermal behavior of gel solar ponds. Such a model has been used to calculate the optimum gel thickness and its effect on the pond’s performance at a given temperature (Wilkins et al., 1986). Under average annual insolation of 250 W/m2 and the same temperature difference between the storage zone and the ambient (20 °C), the model proposed by Wang and Akbarzadeh (Wang & Akbarzadeh, 1983) predicts an optimum gel thickness of 0.62 m and an efficiency of ~ 32%. In the study by Sayer et al. (Sayer et al., 2018), a solar gel pond was compared to a Salinity gradient type. Their result showed that at the same flow rate and thickness as the NCZ, the gel pond has a higher efficiency than the SGSPs. Also, they compared the cost of a gel pond and reported that it is higher than that of SGSP.

Equilibrium solar pond

The main challenges facing the operation of solar ponds are creating and maintaining the concentration gradient within the insulating layer of solar ponds (Harel et al., 1993). An equilibrium solar pond was proposed to overcome these challenges as shown in Fig. 5. In the equilibrium solar pond, salts whose solubility in water increases strongly with temperature are used as a solute (Karim et al., 2010). Such salt solutions typically experience thermal diffusion from lower to higher temperature zones in the fluid due to a temperature gradient (negative Soret effect).

Fig. 5
figure 5

Schematic diagram of equilibrium solar pond. Adapted from (Harel et al., 1993)

Full size image

At saturated conditions, the temperature gradient caused by solar radiation will result in a concentration gradient if enough salt is present in the pond (Karim et al., 2010) to produce a stable solar pond. The equilibrium solar pond provides the following key advantages over conventional SGSPs:

  • The zero-salt flux throughout the pond eliminates the need for the make-up of salt after the pond is set up and the need for disposal of water from the pond (Ranjan & Kaushik, 2014).

  • The thermal efficiency of the equilibrium pond is increased due to the high concentration at the bottom region of the pond (Beiki & Soukhtanlou, 2019).

The solubility of salt used in an equilibrium solar pond increases with temperature; hence, there is no need to maintain the salt concentration regularly. Several salts, such as potassium nitrate (KNO3), Calcium chloride (CaCl2), borax (Na2B4O7), Ammonium nitrate (NH4NO3), and Magnesium Chloride (MgCl2) are common solutes used in equilibrium solar ponds.

The comparative study of different renewable energy sources reviewed in this study is illustrated in Table 1. The energy source, maturity, cost, efficiency, unique advantages and limitations of the energy sources were compared.

Table 1 Comparative analysis of solar ponds and different renewable energy sources (Allouhi et al., 2022; Ang et al., 2022; Guo & Ringwood, 2021; Karre et al., 2418; Odilova et al., 2023; Owusu & Asumadu-Sarkodie, 2016; Sui et al., 2019; Tursi, 2019; Vargas et al., 2019)
Full size table

The focus on SGSPs in this paper is primarily due to their potential as an efficient and cost-effective renewable energy source. Salinity gradient solar ponds utilize the natural phenomenon of salt-gradient stratification to capture and store solar energy, effectively integrating solar collection and storage in the same configuration. This unique mechanism allows the warm, salty water at the bottom of the pond, heated by absorbed solar radiation, to remain at the bottom due to its higher density, effectively trapping the heat. This stored thermal energy can then be harvested and utilized for various applications, making salinity gradient solar ponds a versatile tool in renewable energy. Despite certain drawbacks, such as the need for large space and the potential for algae growth, the benefits of salinity gradient solar ponds, including their simplicity of construction and adaptability to various climates and locations, make them a promising focus of study and development.

The overarching goal of this paper is to provide a comprehensive review of the technology, applications, types, and efficiency improvement methods of solar ponds, with a particular focus on the salt-gradient solar pond. The paper also delves into the different heat extraction mechanisms existing in a solar pond and discusses methods of improving efficiency. By offering a detailed analysis of the current trends and future research directions, the paper seeks to contribute to the ongoing efforts in optimizing these systems, exploring various approaches to increase their efficiency and make them more economical and environmentally sustainable.

Salinity gradient solar pond

A salinity gradient solar pond is a passive solar thermal technology that can store heat energy from the sun, effectively acting as a thermal battery. This system uses the natural properties of saltwater to create a thermal gradient, which traps solar heat. The types of salts used to create the salt solution in a solar pond can significantly impact its efficiency and overall performance.

Categories of salt used in a solar pond

Solar pond requires a brine solution created by dissolving various salts in water. Sodium chloride is the most commonly used salt in SPs due to its increasing solubility in water with increasing temperature, thermal stability, density-increasing capabilities and high transparency to solar radiation in water solution (Sathish & Jegadheeswaran, 2021a). These properties allow sodium chloride to dissolve readily within the water present in the solar pond, creating a concentrated brine solution. This concentrated solution plays a crucial role in the density stratification process within the pond, contributing to the formation of distinct layers of varying densities. Moreover, sodium chloride aids in achieving and maintaining density stratification within the solar pond. As sunlight penetrates the pond, the bottom layers absorb heat, creating a temperature gradient. Sodium chloride enhances this stratification by increasing the density of the bottom layers, preventing convective mixing and ensuring that the thermal energy remains trapped within the lower regions of the pond.

However, due to economic and environmental concerns, researchers have utilized natural salt solutions and fertilizer salts (Sathish & Jegadheeswaran, 2021a). These alternatives can create a temperature profile comparable to that of NaCl while potentially offering enhanced cost-effectiveness (Murthy & Pandey, 2003; Pawar & Chapgaon, 1995). Jubran et al. (1999) explored the use of fertilizer salts, including urea, nitrate of potash, ammonium dihydrogen phosphate, and potassium dihydrogen phosphate, to generate convective liners on the sidewalls of solar ponds.

These compounds possess properties that align with the requirements of solar pond systems. Their high solubility ensures efficient distribution within the pond, while their ability to enhance density stratification contributes to the maintenance of thermal stability. Additionally, urea and nitrate of potash offer secondary benefits, such as serving as nutrient sources for aquatic life within the pond, thereby promoting ecosystem health and functionality. Moreover, the availability and cost-effectiveness of urea and nitrate of potash make them attractive options for incorporation into solar pond systems (Jubran et al., 1999). These compounds are widely utilized in agricultural and industrial sectors, resulting in easy accessibility and affordability for solar pond projects. Their compatibility with existing infrastructure and processes further enhances their suitability for integration into solar pond technology. Ammonium dihydrogen phosphate and potassium dihydrogen phosphate possess excellent thermal properties, including high specific heat capacity and thermal conductivity and these properties allow for efficient heat transfer and retention, enhancing the overall performance of the system. The study found that urea and nitrate of potash produced fewer convective liners on the 90° and 30° pond walls, respectively.

Similarly, ammonium dihydrogen phosphate showed the least convective liners on vertical inclinations, while potassium dihydrogen phosphate had the minimum convective layers on the 60° pond wall. Another study compared the performance of sodium chloride and magnesium chloride salts with urea (Pawar & Chapgaon, 1995). The thermal conductivity, solubility, affordability, and environmental friendliness of magnesium chloride make it an ideal candidate for use in solar ponds. Magnesium chloride also possesses hygroscopic properties, meaning it readily absorbs moisture from the atmosphere. This characteristic helps in preventing the formation of surface crusts on the solar pond, which could hinder the efficient absorption of solar radiation. The results of the comparative analysis indicated that the solar pond containing urea produced an equivalent energy and temperature output to those utilizing sodium chloride and magnesium chloride salts.

Sunny regions that are located close to natural brine sources, such as lagoons or coasts, have the potential to harness solar radiation for use in solar ponds. Hassairi et al. (2001) constructed a laboratory-scale solar pond to experimentally compare sodium chloride salt solution and natural seawater. Their result showed that the maximum temperature reached with the NaCl solution was 55 °C, whereas it was only 47 °C with natural brine in the lower convective zone (LCZ). Nie et al. (2011) conducted a study on a solar pond filled with the natural brine of the Zabuye salt lake in the Qinghai-Tibet plateau. The pond had a surface area of 2500 m2 and a depth of 3 m. The results of the study showed that the lower convective zone (LCZ) temperature of the solar pond reached 39.1 °C.

Kurt et al. (2006) investigated experimentally and theoretically the suitability of sodium carbonate (Na2CO3) salt for establishing a salinity gradient in SGSP due to its high solubility, heat capacity, affordability, environmental friendliness, and compatibility with existing infrastructure to facilitate efficient solar energy harvesting and storage. Their result confirmed that a salinity gradient can be achieved using sodium carbonate salt, which can limit convection from the bottom to the surface of the pond. Beiki et al. (2019) studied the improvement of salinity gradient solar ponds’ performance with nanoparticles inside the storage layer using three different nanoparticles—SiO2, Fe3O4, and ZnO. The maximum lower layer temperature (~ 47 °C) and thermal efficiency enhancement ratio (35.13%) were obtained for the ZnO nanofluid. This nanofluid showed the minimum light scattering, and transmittance is low, leading to the increased thermal efficiency of the pond, as compared to the SiO2 and Fe3O4 nanofluids.

Bozkurt et al. (2015) assessed the performance of a magnesium chloride-saturated solar pond. The incoming solar radiation is absorbed by water with magnesium chloride to increase the temperature of the storage zone. The high-temperature salty water at the bottom of the solar pond remains much denser than the salty water in the upper layers. Thus, the convective heat losses are prevented by gradient layers. They reported a maximum energy of 27.41% for the heat storage zone and 19.71% for the non-convective zone. Berkani et al. (2015) investigated numerically and experimentally the thermal behavior of these three ponds containing NaCl, Na2CO3, and CaCl2. The experimental results show a good agreement with those obtained by simulation with an error of less than 1.5%. Their result showed that the temperature of the SGSP containing the CaCl2 solution is higher than that of the other ponds without reaching saturation. However, the high cost of this salt (CaCl2) limits its use in SGSPs.

A summary of studies investigating alternatives to NaCl is presented in Table 2, with an overview of their findings.

Table 2 Some studies of the effect of different salt types on SGSP
Full size table

Technical considerations and requirements for implementing solar pond technology

The design of solar ponds necessitates a detailed understanding and consideration of various factors that influence their efficiency and functionality. This section provides an overview of all critical design factors, including site selection, pond structure and materials, salt type, heat extraction method, climatic and geological factors (Meneses-Brassea et al., 2022).

Site selection

Choosing an optimal location is influenced by the availability of sunlight, local climate conditions, and proximity to water and salt sources. Ideal sites have high solar insolation, minimal precipitation, low wind speeds, and stable geological conditions to support the structural integrity of the pond (Rghif et al., 2023). Proximity to necessary resources like water and salt can significantly reduce operational costs and logistical challenges. Moreover, thorough environmental and regulatory assessments must be conducted to ensure the site is viable in the long term. By prioritizing these key factors in site selection, the foundation for a successful solar pond installation is established, paving the way for sustainable and efficient renewable energy generation.

Pond structure and materials

The materials used in constructing the pond should be durable, non-corrosive, and capable of withstanding the specific environmental conditions of the site. The pond must include a high-quality liner to prevent leakage and contamination, and the design should facilitate easy maintenance and access (Ali et al., 2020). Additionally, the materials should provide adequate insulation to minimize thermal losses and maintain the necessary salinity gradient. Careful selection of construction materials and structural design ensures the durability and efficiency of the solar pond.

Types and availability of salts

The selection of salt type is dictated by local availability, cost, and the physical and chemical properties of the salts. These factors affect the pond's operational costs and efficiency, requiring a detailed evaluation during the design phase. Common salts like sodium chloride (NaCl) and magnesium chloride (MgCl2) are selected based on their solubility and thermal properties, which directly influence the pond's heat storage capacity and efficiency (Saxena et al., 2022). The local availability and cost of these salts are significant factors, as they influence the initial setup and ongoing maintenance costs of the pond. Furthermore, the environmental impact of salt extraction and transportation must also be considered to ensure the sustainability of the solar pond project. Efficient management of these salts is critical in maintaining the pond’s effectiveness and operational longevity.

Heat extraction method

Designing an effective thermal extraction system is crucial for harnessing the stored heat. This involves integrating heat exchangers that can efficiently transfer heat from the lower convective zone to external applications. These exchangers must be designed with materials that withstand corrosive environments and optimized for minimal disruption to the salinity gradient (Jaefarzadeh, 2006; Wu et al., 2019). Moreover, the design of these systems should accommodate the thermal load requirements of the intended applications, ensuring maximum utilization of the stored energy. By integrating robust and well-designed heat extraction systems, solar ponds can effectively contribute to sustainable energy solutions while minimizing energy losses and optimizing thermal output.

Climatic condition and geological factors

Local climate factors like solar irradiance, temperature fluctuations, and humidity impact the evaporation rates and energy efficiency of the pond. High solar irradiance and stable temperatures promote optimal thermal storage, while excessive rainfall can dilute the salinity gradient, impairing energy storage (Tawalbeh et al., 2023). Geologic factors, such as soil type, permeability, and stability, must be assessed to ensure the structural integrity of the pond. Areas prone to seismic activity or with high soil permeability may require additional engineering measures to prevent structural failures and ensure long-term viability (Ali et al., 2020).

Environmental impact analysis of solar pond

Figure 6 shows the lifecycle of solar ponds—construction, operation, and decommissioning. During construction, the extraction and processing of materials such as salt and synthetic geomembranes can disrupt local ecosystems and contribute to carbon emissions. Transporting these materials also adds to the overall carbon footprint (Hertwich, 2021). The site preparation phase may further lead to habitat loss and changes in local water drainage patterns, necessitating ecological assessments and mitigation strategies. In the operational phase, while solar ponds efficiently collect and store solar energy, they consume significant amounts of water to offset evaporation and can risk contaminating local ecosystems due to their high salinity (Tan et al., 2023). This phase also poses potential risks to biodiversity, highlighting the need for continuous environmental monitoring and adaptive management. Decommissioning involves managing large volumes of saline water and disposing or recycling non-biodegradable components like geomembranes. It is critical to treat these materials for disposal to prevent environmental pollution. Additionally, rehabilitating the site to its original state or preparing it for new uses is crucial, involving soil remediation and the restoration of native vegetation to ensure the long-term sustainability of the local environment (Rabaia et al., 2021).

Fig. 6
figure 6

Solar Pond life cycle

Full size image

Application of SGSP

Electricity generation

Salinity gradient solar ponds can provide stable and reliable energy suitable for base load power, making them advantageous over other solar technologies (Tawalbeh et al., 2023). Unlike other solar technologies that rely on direct sunlight, salinity gradient solar ponds can store solar energy for several hours or even days. The hot water at the bottom of the solar pond is pumped to a heat exchanger, which heats a working fluid that drives a turbine to generate electricity (Saxena et al., 2022). This heated working fluid then propels a turbine to create electricity as illustrated in Fig. 7. The selection of the working fluid varies based on the design specifics and type of heat engine. Water or steam is commonly employed in these engines, especially in moderate temperature applications. However, organic fluids with lower boiling points, like refrigerants, hydrocarbons, or silicone oils, might be used for lower temperature systems. SGSP can also be used with other renewable energy technologies, such as wind or solar photovoltaic systems, to provide a more stable and reliable energy supply.

Fig. 7
figure 7

Schematic diagram of electricity generation from solar pond. Adapted from (Kalogirou, 2014)

Full size image

Several research studies have shown that salinity gradient solar ponds can be efficient and cost-effective for electricity generation (Tawalbeh et al., 2023). Ding (2017) tested a salinity gradient solar pond for electricity generation using thermoelectric generators. They investigated the performance and reliability of the thermoelectric cooler available functioning as a thermoelectric generator. The research concluded that salinity gradient solar ponds can be an efficient and cost-effective technology for electricity generation. Another study by Andrews and Akbarzadeh (2005) investigated a salinity gradient solar pond using an alternative heat extraction method from the non-convecting gradient layer. Their result showed that heat extraction from the gradient layer can increase the energy efficiency of the pond for electricity generation. Hence, salinity gradient solar ponds have demonstrated great potential for electricity generation, with several advantages over other renewable energy technologies.

Seawater desalination

Salinity gradient solar ponds offer promise as a technology for seawater desalination due to their ability to generate thermal energy and maintain temperature stratification. SGSPs can efficiently desalinate seawater by utilizing hot water at the bottom of the pond pumped through a heat exchanger, which heats the seawater, causing it to evaporate (Saleh et al., 2011). The vapor is then condensed and collected as freshwater as shown in Fig. 8. The thermal energy generated is utilized to power the desalination process. However, the efficiency of the desalination process is influenced by several factors, such as the design of the pond, the salt concentration gradient, and the operating conditions.

Fig. 8
figure 8

Schematic diagram of seawater desalination from solar pond. Adapted from (Alnaimat et al., 2018)

Full size image

Research studies have demonstrated the potential of salinity gradient solar ponds for seawater desalination. Lu et al. (2001) tested a salinity gradient solar pond coupled to a desalination plant at different operating conditions. Rizzuti et al. (2007) also produced fresh water with a capacity of 3000 m3/day, 60 m3/day, and 30 m3/day from desalination plants coupled with an SGSP near the Dead Sea in Margarita, de Savoya in Italy and at the University of Ancona in Italy respectively. These studies suggest that the SGSP could effectively desalinate seawater with high conversion and water recovery rates. Using SGSPs for seawater desalination can provide several advantages over traditional desalination methods. The technology requires no external energy input other than solar energy, making it sustainable and cost-effective. Moreover, SGSPs can operate in remote areas where electricity is unavailable, making them suitable for decentralized desalination. SGSPs can also reduce the environmental impact of desalination by minimizing greenhouse gas emissions associated with conventional desalination.

Industrial applications

The energy stored in SGSP can be utilized for various industrial processes such as heating, cooling, and drying. Theoretical calculations by Alcaraz et al. (2018) showed that a 500 m2 industrial SGSP constructed in a mineral processing plant (Solvay Minerales) in Granada (Spain) would achieve the temperature requirements of the flotation mineral purification stage and reduce the annual fuel consumption by more than 50%. Garrido et al. (2012) utilized an SGSP with an area of 1.43 km2 in an industrial process to generate an annual gross thermal supply of 626 GWh savings of up to 59,000 tons of diesel oil and 164,000 tons of CO2 per year. Additionally, they proposed an SGSP with an effective collecting area of 23, 240 m2 to deliver 12,300 MW h/year on site, reducing the annual diesel demand and CO2 emissions by 77% and 3300 tons of CO2 respectively.

These studies have shown that SGSPs provide a reliable and sustainable energy source with high efficiency and reliability. This technology can provide a reliable and sustainable energy source, saving industrial processes costs. SGSPs can also be used in remote areas, where grid electricity is unavailable, making them suitable for off-grid industrial applications. Moreover, using SGSPs for industrial applications can reduce the carbon footprint of industrial processes, contributing to a more sustainable future. Therefore, SGSP technology can play a crucial role in the transition towards a sustainable and low-carbon future for the industrial sector. Figure 9 shows some other applications of solar ponds.

Fig. 9
figure 9

Applications of Solar Pond

Full size image

Policy and regulatory landscape for solar pond deployment

Government policies and incentives have a significant effect on solar pond deployment, determining their feasibility, funding, and operational standards. Regulatory frameworks set guidelines on land use, environmental impact assessments, and water usage, which can either facilitate or hinder the feasibility and operation of solar pond projects (Dincer & Erdemir, 2024). These challenges dictate the technical and geographical parameters and impact the economic viability and public acceptance of solar ponds. Common regulatory challenges for solar pond deployment include:

  • Extensive use of water and land leads to scrutiny of solar pond projects under environmental protection laws.

  • Challenges in obtaining land use permits, especially in densely populated areas (Ven et al., 2021).

  • Strict regulation of water rights in areas facing water scarcity or with prioritized uses like agriculture (Bukhary et al., 2018).

  • Requirements to comply with local building codes and safety standards.

However, policies such as renewable energy incentives can motivate increased investment in renewable energy projects, including solar ponds. Incentives such as tax rebates, feed-in tariffs, low interest financing, grants, and subsidies, help reduce capital and operational costs, thus enhancing the attractiveness of solar ponds as a viable renewable energy source (Merrick, 2011).

It is important to note that government regulations for solar ponds differ across various geographical locations. Therefore, individuals interested in implementing solar pond technology should engage with local authorities and experts to understand the specific rules and incentives relevant to their project.

Case studies of successful implementation of solar ponds

Table 3 provides an overview of various solar ponds, highlighting their unique characteristics, applications, and challenges. Solar ponds employ different salts, including magnesium chloride, sodium chloride, and carbonate salts, and serve diverse purposes such as power generation, industrial heating, and desalination. However, they face distinct drawbacks such as high evaporation rates, maintenance requirements due to salt levels, and environmental concerns.

Table 3 Comparative analysis of real-world solar ponds
Full size table

The cost of constructing and maintaining a solar pond depends on several factors, including the size of the pond, the type of salt used, the location, and the specific application. A typical cost range for solar ponds is approximately $35 to $70 per square meter depending on the materials, labour, land, and ongoing maintenance required to operate the pond effectively (Bronicki, 2018).

Modeling of solar ponds

Several existing modeling approaches exist for salinity gradient solar ponds (SGSPs), including analytical, numerical, and experimental models. These models have been analyzed and optimized for the design of SGSPs for various applications. A mathematical model based on an empirical equation and a MATLAB script was developed by Alenezi (2012) to describe and simulate the thermal behavior of a SGSP. This study demonstrated that the pond’s efficiency depends on the salt gradient’s stability in the middle non-convecting zone. Giestas et al. (2014) presented a numerical model using Computational Fluid Dynamics (CFD) to represent the dynamics of a SGSP. This model is based on the Navier–Stokes equations for incompressible fluids and linked with two advection–diffusion equations. The study’s outcome demonstrated that the model effectively forecasted temperature in the three zones of the pond. In another CFD simulation setup by Anagnostopoulos et al. (2020), a SGSP model was developed, and the accuracy and computational resources of the model were compared with an existing one-dimensional MATLAB model, as well as two- and three-dimensional CFD models developed in the study. The results showed that the two- and three-dimensional models achieved significantly higher accuracy than the 1-D model. Also, these models accurately evaluated heat loss to the surroundings, solar radiation absorbed, and the thermal performance of the pond throughout the year.

Saleh (2022) proposed a mathematical model to investigate the advantages of coupling a solar pond with an absorption chiller. The system design was optimized by considering key parameters such as ambient temperature, solar radiation, pond specifications, and cooling and refrigeration temperatures. The model results showed that a solar pond with an area of 3000 m2 could produce a heat rate at a temperature of 80 °C, which could drive a chiller with a cooling capacity of 126.3 kW, with an overall coefficient of performance (COP) of 0.183. The findings suggest that this type of system is feasible and suitable for cooling production, particularly in hot regions. Shahid et al. (2023) analyzed the thermal performance of SGSP under different climatic and soil conditions. Two-dimensional heat and mass transport equations were simulated using finite difference techniques utilizing MATLAB® scripts. The study examined salt distributions and temperature profiles for various factors that influence the thermal performance of SGSP. The findings revealed that the main variables significantly affecting SGSP’s thermal performance are soil conditions, including soil texture, types, moisture level, and water table depth. The modeled study identified the maximum energy extraction rate as 110 W/m2, with the lowest temperature observed in fine sand when dry at 62.48 ℃.

These models provide a deeper understanding from an analytical, numerical, and experimental approach of the performance of SGSPs for various applications and help optimize their design and operation for better efficiency and reliability.

Solar pond efficiency

Like any other energy system, the efficiency of a solar pond is influenced by various factors. Understanding these factors is crucial in optimizing the performance of the solar pond and maximizing its energy output.

Temperature

The temperature of a solar pond plays a critical role in its efficiency. The storage zone (LCZ) temperature, the warmest part of the solar pond, is where solar energy is stored as heat. The higher the temperature in this layer, the more energy can be stored and converted into useful work or power (Valderrama et al., 2022). The temperature gradient is another crucial aspect of a solar pond, managed through the varying salinity levels from the pond’s surface to the bottom (Shahid et al., 2023). This gradient prevents the upward convection of heat, which would otherwise cause the heat collected at the bottom layer to quickly dissipate to the cooler surface layer, reducing the overall efficiency of the solar pond (Shah et al., 2017). On the other hand, higher temperatures, especially at the top convective layer, can lead to increased heat losses due to evaporation or radiation, which can decrease the overall efficiency of the solar pond as the stored heat energy is lost to the environment (Andrews & Akbarzadeh, 2005). Therefore, while a higher temperature in the storage zone is beneficial for storing more solar energy, it is also crucial to maintain the temperature gradient and minimize heat losses to enhance the overall efficiency of a solar pond (Valderrama et al., 2022; Verma & Das, 2020; Wu et al., 2019).

External heat addition

The introduction of exogenous thermal energy into a solar pond, particularly the LCZ, can potentially augment the energy storage capacity within the system (Ganguly et al., 2017). Exergy is considered a more precise parameter for evaluating the performance of solar thermal energy storage systems, as proposed by Dincer and Rosen (2021). Karakilcik et al. (2013a) investigated the energy and exergy efficiency of an Integrated Solar Pond (ISP), which combined flat plate solar collectors with the solar pond. A serpentine cylindrical heat exchanger facilitated the thermal energy exchange between the LCZ and the solar collectors. Temperature sensors were placed at various depths and on the heat exchanger’s inner and outer walls. Solar collectors increased the temperature of the Heat Storage Zone (HSZ); however, increasing their number led to a decrease in the HSZ density (Karakilcik et al., 2013a).

Reflective covered surface

The use of reflective cover proves to be an efficient method to mitigate heat loss from the surface of a solar pond during the night. Additionally, it amplifies the capture of solar radiation during daylight hours, thereby enhancing the overall efficiency of the solar pond. Assari et al. (2015) conducted a study on the absorption in ponds with different shapes covered by glazing plastics, which helped retain heat within the pond. They investigated rectangular and circular shapes of SGSP with similar cross-sections and volumes. Their result showed that the maximum temperature was higher in the rectangular pond. Ruskowitz et al. (2014) suggested using buoyant floating discs to reduce evaporative losses. Sayer et al. (2017) compared evaporation rates in SGSP with and without a paraffin liquid cover. In addition to the efficiency improvements from using paraffin liquid, Nalan et al. (2008) sought to decrease heat loss from the top by attaching two foldable reflective covers, as illustrated in Fig. 10, on opposite sides of the solar pond in Isparta, Turkey. These covers served a dual purpose as insulators and reflectors. Although the insulating effect of the covers was minimal, their reflective properties significantly improved the pond’s performance. The LCZ temperature increased by 25% compared to an uncovered pond.

Fig. 10
figure 10

Schematic diagram of Reflectors adapted from (Nalan et al., 2008)

Full size image

Wall profile of the solar pond

The wall shading effect is another factor affecting the efficiency of salt gradients in solar ponds. Jaefarzadeh (2004) conducted a numerical study on the thermal performance of a salinity gradient solar pond with wall shading effects. Their result revealed that the wall shading effect reduces the sunny area and the temperature of the LCZ. Hence, to obtain a higher temperature for the storage zone, the thickness of the NCZ should be increased. Verma et al. (2019) conducted a numerical simulation of convective layers in solar desalination ponds, concluding that the angle at which sunlight strikes the wall significantly impacts the convective layer’s activity. Also, they investigated the influence of wall profile parameters and geometries on solar pond efficiency. They concluded that the vertical wall profile yielded the highest efficiency. Khalilian (2018) examined the potential and behavior of solar ponds, finding that wall shading effects reduce solar pond performance. His results showed that considering the shadow effect, the maximum temperatures of circular and square ponds were 66.8 °C and 65.8 °C, respectively. Hence, the right choice for the dimensions would decrease the shading area in the pond.

Turbidity

Turbidity, which causes the scattering of solar radiation, is another critical parameter that determines the efficiency of solar ponds. The turbidity level quantifies water clarity and significantly impacts the insolation received (AL-Musawi et al., 2020). Li et al. (2010) explored methods to reduce turbidity in solar ponds and examined the thermal performance of each approach. Wang and Seyed-Yagoobi (1994) established an empirical relationship between water clarity and solar radiation. Higher turbidity can hinder solar ponds from storing thermal energy. Techniques to improve water clarity and reduce turbidity have been studied. Algae growth can reduce the amount of sunlight entering the pond, as algae absorb light, causing instability in solar ponds (Malik et al., 2011). Hull (1990) recommended using aluminum sulfate to precipitate soluble phosphate, which minimizes algal growth in SGSP and reduces the amount of chlorine needed to be dissolved. Rahmani (2008) investigated electrocoagulation as a method for removing water turbidity. Hence, addressing the issue of turbidity not only improves energy capture efficiency but also enhances the stability of solar ponds, making it a critical area for continued research and technological innovation.

Heat extraction mechanisms in solar ponds

Extraction of the stored heat for uses like space heating and electricity generation is one of the main objectives of solar pond design and construction. Heat extraction in solar ponds can be divided into two: Direct heat extraction method and Indirect heat extraction method.

Direct heat extraction method

In direct heat extraction, heat is extracted from the system by pumping hot brine from the top of the LCZ via an external heat exchanger before returning the hot brine to the bottom of the LCZ at a cooled temperature, as shown in Fig. 11. The velocity of the brine being pumped must be controlled to stop the gradient layer from eroding (Leblanc et al., 2011). Using CFD, Angeli et al. (2006) designed a novel idea for removing heat from the solar pond. They positioned an external heat exchanger between the sides of a U-shaped solar pond that served as the inlet and outlet. The U-shaped solar pond had the advantages of shortening pipelines and saving energy because there was no need to utilize a pump. Yaakob et al. (2011) also suggested enhancing the solar pond’s effectiveness by removing the hot brine from NCZ using an external thermosiphon heat exchanger. They discovered that the solar pond’s efficiency can be increased by up to 30%.

Fig. 11
figure 11

Direct heat extraction method using an external heat exchanger (Leblanc et al., 2011)

Full size image

Indirect heat extraction method

The indirect heat extraction method extracts thermal energy from SGSP without direct interaction with the pond’s fluid layers, as shown in Fig. 12. In this setup, internal heat exchangers are commonly employed to transfer the heat to a secondary fluid, which can be used for various applications (Tawalbeh et al., 2023). The solar pond’s design and the energy stored in the SGSP determine how much useful energy can be extracted from them (El-Sebaii et al., 2011). Jaefarzadeh (2006) examined the thermal effectiveness of a tiny solar pond with an internal heat exchanger. The thermal efficiency of the pond is defined as the ratio of the heat extracted from the SGSP to the total solar insolation reaching the upper surface of the pond. The results indicated a steady state thermal efficiency of 10%.

Fig. 12
figure 12

Indirect heat extraction method using an internal heat exchanger (Leblanc et al., 2011)

Full size image

The effectiveness of heat extraction from a salinity gradient solar pond using two distinct approaches was assessed in three weather scenarios with various daily ambient temperature conditions. The results showed that when the heat extraction was carried out by the Lateral heat exchanger, regardless of the weather conditions, the solar pond’s instantaneous efficiency increased (Alcaraz, 2016). A transient modeling approach was developed to predict the thermal behavior of a solar pond during heat extraction operation (Aramesh et al., 2017). Their model result showed a good march with the experimental data with less than 3% uncertainty. Ould Dah et al. (2010) suggested removing the heat from NCZ. As a result, the solar pond efficiency was enhanced, but the stability of the lower interface was disturbed by the heat extraction from the NCZ. Leblanc et al. (2011) also conducted similar work on heat extraction from NCZ. Comparing the results of their practical and theoretical assessments to the traditional method of heat extraction from the LCZ, they found an increase in overall thermal efficiency of up to 55%.

Strategies to improve the efficiency of solar ponds

Use of phase change materials (PCM)

The thermal performance of a solar pond is affected by the amount of heat lost to the ground through the LCZ, where the heat is stored. Several studies have focused on minimizing heat losses, maximizing energy storage, and ensuring the system’s long-term stability. All these objectives can be achieved by incorporating a PCM into a solar pond. However, different PCMs have different thermal properties, which can affect the thermal efficiency of a solar pond. The impact of the presence and absence of phase change materials on long-term heat storage of a salinity gradient solar pond was investigated experimentally and numerically (Sogukpinar et al., 2023). Colarossi & Principi (2022) conducted an experimental study using PCMs in a tiny solar pond’s LCZ. After 6 h of heating, they observed that the LCZ attained temperatures of around 54.2 °C without PCM and 51.7 °C with PCM.

Beik et al. (2019) tested two small-scale solar ponds with PCM at the bottom of the LCZ for 75 days. They found that the temperature measurements of the Lower Convective Zone (LCZ) from the numerical and experimental analyses of the ponds displayed a strong correlation during the heat extraction phase. Poyyamozhi and Karthikeyan (2022b) experimentally tested a solar pond model with and without PCM by monitoring the temperatures at the three zones. With PCM, the average daily and nightly temperature change for a month at the solar pond was 3 ℃, while it was 7 ℃ without PCM. Based on all the findings, they concluded that the solar pond with PCM capsules can store solar energy effectively.

Paraffin Wax was used as the PCM to study the transient evolution of the heat and salinity characteristics of two pilot salt-gradient solar ponds by Assari et al. (2022). Their result showed that the solar pond with PCM had greater thermal and salinity stability, a higher average outlet temperature for the internal heat exchanger, and less temperature drop during heat extraction. To maintain the required temperature of the heat stored in the solar pond in the summer for a longer period, Bozkurt (2022) researched the use of PCM. Three different PCMs were utilized in the solar pond as insulation for this purpose, each with a different phase change temperature and set of thermophysical characteristics. The solar pond’s heat storage ratio (HSR) when paraffin C18, capric acid, and paraffin 44 were utilized as PCM was maximal in December at 32.22%, 34.85%, and 47.81%, respectively.

All these studies showed that long-term heat energy storage in solar ponds can be enhanced with PCMs. A summary of studies that investigated the efficiency of using different PCMs in a solar pond is shown in Table 4.

Table 4 Summary of recent studies where PCMs were used to optimize the efficiency of solar ponds
Full size table

Use of porous media

According to the salt diffusion experiment, adding porous media slows the salt’s upward diffusion, preserving the salt gradient. These porous media can be used in different forms. Hill and Carr (2013) discovered that porous solar ponds are generally more stable than nonporous ones in some instances. Their result showed that with 60% of the porous material in the LCZ, the maximum temperature that can be stored is optimized. Shi et al. (2011) investigated the impacts of porous media on solar ponds’ thermal and salt diffusion. They showed that porous media at the bottom of the solar pond can enhance its heat insulation capacity. Wang et al. (2015) experimentally investigated the effect of porous material on the salt diffusion under constant LCZ temperature. Salt diffusion was reduced by adding the porous medium in the experimental temperature range compared to the conditions without adding the porous medium. The use of porous media in solar ponds.

Adding a layer of coal cinder at the bottom of the LCZ of the solar pond

Adding coal cinder, a byproduct of coal combustion, to the LCZ was suggested in the study done by Wang et al. (2011) to increase the temperature of the pond. The increase in temperature is due to the improved absorptivity of solar radiation in the LCZ caused by the presence of coal cinder. The coal cinder acts as a porous medium that enhances the heat storage capacity of the LCZ, reducing heat loss and improving the thermal efficiency of the solar pond. Wang et al. (2014) conducted simulations and experimental studies to explore the improvement of thermal efficiency of SGSP by adding a layer of coal cinder. Their findings demonstrate that, compared to the conventional bottom treatment, adding coal cinder to the bottom of the SGSP raises the temperature of the LCZ. Both experimental and simulated results have a high degree of similarity.

Hence, adding a layer of coal cinder at the bottom of the LCZ of a solar pond is an effective method to increase the pond’s temperature and enhance its thermal performance. However, the specific conditions of the solar pond and the type of porous medium used should be considered when implementing this method. Furthermore, the long-term effects of adding coal cinder to the LCZ, such as the accumulation of impurities or the degradation of the coal cinder, may need to be further studied.

Incorporating solar collectors

Although the overall performance is improved when flat-plate solar collectors and solar ponds are combined, the economic viability has not been thoroughly examined. Karakilcik et al. (2013) connected four solar collectors with a solar pond and found that as the number of collectors increased, so did the energy and exergy efficiency. Alcaraz et al. (2018) presented an experimental analysis of the effectiveness of an SGSP by incorporating solar collectors. Their findings showed that using solar collectors as an additional heat source for the solar pond significantly increased daily energy efficiency, especially during the cold season tests. Bozkurt and Karakilcik (2012) studied the heat storage performance of an integrated solar pond and collector system. Ali et al. (2020) performed an experimental assessment of the thermal performance of the mini solar pond integrated with thermal collectors. The results confirmed that the thermal performance was increased with the system’s maximum total efficiency of 37.67% in September. They recommended studying the effects of nanofluid on the performance of the solar pond and the effect of increasing the number of solar collectors.

Incorporating solar collectors into a Salinity gradient solar pond can enhance efficiency by increasing heat collection and enabling higher temperature gradients; however, it may also lead to increased complexity and costs and the potential for disrupted thermal stratification if not properly managed.

Use of an external magnetic field

Tian et al. (2021) studied the effect of adding a magnetic field to a solar gradient solar pond. By applying a magnetic field, the LCZ’s average temperature can rise and delay the homogenization of the salt concentration. Average LCZ temperatures and salt mass fractions were 2.93 °C and 0.9% higher, respectively than the solar pond devoid of a magnetic field. After 35 h of illumination, the magnetic-field-free solar pond had reached a thermally unstable stage; while the thickness of NCZ was reduced by 14.75%, the thickness of LCZ increased by 42.5%. Using an external magnetic field in SGSP could enhance the pond’s thermal conductivity and convection currents. However, research on its long-term effects is limited and would require careful experimentation to understand the interactions with the salt gradient and the magnetic properties of the salts.

Use of shading

Jaefarzadeh (2004) studied the thermal behavior of a small-scale salinity-gradient solar pond with wall shading effect using a finite difference model. The model for a vertical wall square pond showed details of the areas affected by wall shading and its effect on reducing the sunny area. The results are reasonable for predicting the temperature of the LCZ and the temperature profile in the depth of the pond. The sensitivity analysis reveals that the wall shading effect significantly reduces the sunny area and the temperature of the LCZ. An experimental study of energy distribution, energy efficiency, and energy efficiency ratios with shading impact on each zone of a small rectangular solar pond was investigated by Karakilcik et al. (2013b). Their results showed that the highest energy efficiencies for both shaded and unshaded cases occurred in August, with the UCZ recording 4.22% and 4.30%, NCZ recording 13.79% and 16.58%, and the lower convective zone recording 28.11% and 37.25%, respectively. These findings confirmed that eliminating the effect of shading can enhance the solar pond’s storage efficiency. Another study was carried out by Khalilian (Khalilian, 2017) investigating the energy distribution and efficiency of a square solar pond through numerical and experimental approaches. Energy efficiency analysis was conducted for the inner zones of the pond, with and without the shading effect, using appropriate equations. The results indicate that the highest energy efficiencies for the LCZ occurred in July, with 9.32% and 6.57% for the cases with and without shading area, respectively.

The shading effect in SGSP mitigates excessive evaporation and stabilizes the thermal layers by reducing surface temperature fluctuations, thus maintaining the desired salinity gradient and enhancing the efficiency of the solar pond. However, overuse of shading can lead to insufficient solar heat gain, diminishing the energy storage capacity of the pond and potentially affecting the overall thermal output and efficiency.

Conclusion and prospects

The progress in solar pond technology was reviewed with a concentration on Salinity gradient solar ponds. This study has systematically surveyed the performance improvements in SGSPs through innovative methodologies and design optimizations. The following points are the conclusive remarks of the present work:

  • SGSPs are promising technologies for collecting, storing, and distributing thermal energy. They are crucial in residential and industrial heating, desalination, and electricity production systems.

  • The use of alternative salts, such as natural salt solution and fertilizer salt, has proven to be more effective than sodium chloride (NaCl).

  • Factors such as temperature, turbidity, and wall profiles are important parameters that influence the efficiency of SGSPs.

  • SGSP shading reduces excessive evaporation and stabilizes the thermal layers by reducing surface temperature fluctuations, but overshading can result in low performance.

  • Porous media and PCMs are the most widely used strategies for improving the thermal efficiency of SGSP.

  • Incorporating solar collectors into SGSP has been tested to improve energy and exergy efficiency, albeit at the expense of additional cost to the system.

Prospects for SGSP research will be focused on incorporating emerging technologies and materials to boost the efficiency of thermal energy collection and storage, improving materials and designs to reduce heat loss, and developing more cost-effective construction and maintenance techniques. Advancements are anticipated in developing selective membranes and nanofluids to enhance thermal management. The application of artificial intelligence is set to revolutionize efficiency optimization and maintenance, enabling real-time data analysis and decision-making to adjust operational parameters to maximize performance. Furthermore, integrating SGSP with geothermal energy sources can create a more stable and continuous energy supply.