Solar-Powered Freeze-Melting Desalination Model for Water and Simultaneous Cooling Applications

Abstract

Freeze-melting (F/M) desalination presents a sustainable and energy-efficient alternative to conventional desalination methods. In this study, we evaluated two solar-powered refrigeration systems, using BaCl₂–NH₃ and NH₃–LiNO₃ sorbent–refrigerant pairs, for seawater desalination and cooling applications. The NH₃–LiNO₃ system demonstrated a superior performance, achieving evaporator temperatures below −3 °C and producing up to 8 kg/day of ice. The system operated with a significantly lower energy consumption than the 3–6 kWh/m³ required by reverse osmosis (RO). Practical tests confirmed the dual functionality of the system, providing cooling for food preservation (maintaining 4 °C for 5 h) and climate control while producing desalinated water with total dissolved solids (TDS) levels of 3650 k/m³. Although the TDS remained above the WHO potable water standard, the output is suitable for irrigation and livestock watering. These results highlight the F/M desalination system’s potential to address water scarcity and cooling needs in resource-limited, off-grid regions, contributing to sustainable desalination technologies powered by renewable energy.

1. Introduction

The increasing demand for freshwater, driven by population growth, industrialization, and climate change, has made water scarcity a critical global challenge. Over 300 million people rely on desalination technologies to meet their water needs, producing an estimated 95 million cubic meters of freshwater daily [1,2]. However, conventional desalination methods face significant challenges, including high energy consumption, environmental impacts, and operational costs [3]. For instance, the energy consumed globally by desalination plants annually is estimated to be equivalent to the total energy consumption of small- to medium-sized countries [2].

RO, which dominates the global desalination capacity (60–70%), typically consumes between 3 and 6 kWh/m³ for seawater, depending on the salinity and operational efficiency, while brackish water desalination requires significantly less energy, ranging from 1.5 to 2.5 kWh/m³ due to its lower salinity levels [4]. This high energy demand arises from the need for high-pressure operations. Additionally, RO generates large volumes of brine waste and relies on chemical pretreatment, leading to marine pollution and membrane disposal challenges [5,6].

Thermal methods, such as multi-stage flash (MSF) and multi-effect distillation (MED), consume even more energy, requiring 6–12 kWh/m³, and face issues such as scaling, corrosion, and high maintenance requirements [7,8]. Addressing these limitations necessitates the development of novel desalination technologies with low energy input and minimal environmental impact.

Freeze-melting (F/M) desalination offers a promising alternative by utilizing the latent heat of ice formation to separate salt from seawater. Salt ions are excluded from the ice matrix while freezing, concentrating the salts in the remaining brine. According to our saline diffusion model for F/M desalination, salt migration occurs due to competition between thermal and saline diffusion gradients as freezing progresses. As ice forms, salt ions are excluded and move toward the remaining liquid brine, where they become increasingly concentrated. The process depends on factors such as the initial salt concentration, freezing rates, and geometry, creating conditions where salt ions can effectively diffuse into the brine and enabling the formation of lower-salinity ice [9,10].

This process operates at near-freezing temperatures, avoiding pressures required by RO and the high temperatures of thermal methods [11,12].

Key benefits of F/M desalination include the following:

• Low energy consumption: F/M systems consume 1.5–3 kWh/m³, significantly lower than the requirements of conventional methods [13].
• No high pressure: Unlike RO, F/M desalination operates at near-freezing temperatures, eliminating the need for high-pressure pumps [14].
• Minimal environmental impact: F/M desalination reduces brine discharge and eliminates the need for chemical pretreatment, making it a more sustainable option [15].

To better illustrate the energy requirements of conventional desalination methods,

Table 1. Energy requirements and consumption of various desalination technologies.

Technology	Operation Conditions	Energy Consumption (kWh/m3)	Key Challenges
RO	High pressure	3–6	Membrane fouling, brine discharge
Multi-Effect Distillation (MED)	High temperature	6–12	Scaling, corrosion, energy-intensive
Multi-Stage Flash (MSF)	High temperature	6–10	Complex operation, maintenance
Freeze-Melting	Near-freezing temperatures	1.5–3	Ice separation, system optimization

summarizes key performance metrics, including energy consumption and operational characteristics.

Integrating renewable energy into desalination systems provides a sustainable alternative to fossil-fuel-based methods, reducing environmental impacts and operational costs. Among renewable sources, solar energy is the most widely used for desalination—solar thermal technologies such as flat-plate collectors and parabolic concentrators efficiently power thermal desalination processes such as MED and MSF. In contrast, solar photovoltaic (PV) systems supply electricity for high-pressure operations in RO systems [2,15].

Recent advancements in solar-powered desalination technologies highlight a growing interest in energy-efficient, sustainable water production systems. For instance, ref. [16] developed a solar-powered freeze desalination system capable of producing 52.8 m³/day of freshwater while recovering cold energy for air conditioning, achieving a specific energy consumption of 6.94 kWh/m³.

F/M desalination benefits significantly from solar-powered refrigeration, which can achieve the low temperatures required for ice formation using absorption and adsorption cycles. These cycles utilize sustainable refrigerants such as NH₃ and sorbents such as lithium bromide and calcium chloride, aligning with the environmentally friendly principles of the F/M process [13,16].

The dual-purpose nature of solar-powered F/M desalination—producing desalinated water and cooling energy—makes it highly suitable for diverse applications:

• Off-grid coastal communities: Provides reliable freshwater and cooling for food preservation without dependence on electricity grids.
• Agricultural regions: Supports irrigation and refrigeration of produce, reducing post-harvest losses.
• Arid, sun-rich areas: Maintains comfortable indoor conditions while providing potable water for daily use.

Recent advancements demonstrate the feasibility of such systems. For example, a solar-powered freeze desalination unit achieved a production rate of 52.8 m³/day with a specific energy consumption of 6.94 kWh/m³ while recovering cold energy for air conditioning [16]. Earlier studies, such as the Yanbu project in Saudi Arabia, successfully utilized solar energy storage with molten salts to power freeze desalination systems [13].

This study explores the dual-function potential of F/M desalination integrated with solar refrigeration, focusing on the following:

• Efficiency comparison: Evaluating two sorbent–refrigerant pairs, namely barium chloride–ammonia and ammonia–lithium nitrate (BaCl₂–NH₃ and NH₃–LiNO₃), for seawater desalination and cooling applications.
• Practical applications: Using F/M-produced ice for food preservation and climate control in off-grid, energy-constrained regions.

By addressing potable water and cooling needs, this work posits F/M desalination as a sustainable and adaptable solution for high-demand coastal and agricultural areas, advancing renewable-powered desalination technologies.

2. Materials and Methods

To evaluate the effectiveness of solar-powered refrigeration for F/M desalination, two different refrigeration systems were tested: one using a BaCl₂–NH₃ sorbent–refrigerant pair and the other using NH₃–LiNO₃, both from Sigma-Aldrich (St. Louis, MO, USA). Each system operates in an intermittent day–night cycle powered by solar energy, aiming to achieve and maintain freezing temperatures for desalination.

Table 2. Comparative performance of BaCl₂–NH₃ and NH₃–LiNO₃ solar refrigeration systems for F/M desalination.

Feature	BaCl2–NH3 System	NH3–LiNO3 System
Sorbent–Refrigerant Pair	BaCl2–NH3	LiNO3–NH3
Operating Cycle	Intermittent (day–night cycle)	Intermittent (day–night cycle)
Temperature Achieved	Above −3 °C (did not reach freezing)	Below −3 °C, sustained around −2 °C
Cooling Duration	Limited cooling at near-freezing for short durations	Up to 5 h below freezing
Ice Production Capacity	Limited capacity (not sufficient for effective desalination)	Up to 8 kg/day (sufficient for desalination)
Primary Application Suitability	Less suitable for sustained desalination or cooling	More suitable for desalination and extended cooling applications

compares each system’s key attributes and performance metrics, highlighting their cooling capacities and desalination suitability differences.

2.1. Solar Refrigeration Systems for Freeze-Melting Desalination Experiment

In this experimentation phase, two distinct solar-powered refrigeration systems were evaluated for their effectiveness in driving the F/M desalination process. Developed at the Instituto de Energías Renovables, Universidad Nacional Autónoma de México, these systems utilized NH₃ as a refrigerant in combination with either BaCl₂ or LiNO₃ as a sorbent. Both systems operated intermittently and were powered by solar energy to enable the cooling required for freezing seawater.

• System 1: BaCl₂–NH₃ Solar Refrigeration System (Figure 1). This system utilizes a solid–gas absorption cycle with BaCl₂ and NH₃, powered by solar energy. The system operates intermittently: during the day, solar collectors heat the reactor, causing NH₃ to desorb from the BaCl₂. At night, the system cools down, allowing NH₃ to be reabsorbed, which generates the cooling effect necessary for the F/M desalination process. This cycle effectively supports low-temperature operations and is suitable for sustainable cooling and ice production.

• System 2: LiNO₃–NH₃ Absorption System (Figure 2). The second system employs a LiNO₃–NH₃ mixture in a single-stage absorption cycle. Like the BaCl₂ system, solar energy powers the desorption process, with NH₃ desorbing from LiNO₃ during high solar irradiance periods. At night, the reabsorption phase provides cooling, enabling ice formation for desalination. Our system (Figure 3) offers comparable cooling efficiency and operates efficiently within moderate pressure and temperature ranges, enhancing the versatility of solar refrigeration-driven desalination.

The cold chamber of this system has 16 cylinders of approximately half a liter each.

2.2. Construction of Application Chambers

Following the ice production and desalination processes, two specialized chambers were constructed to demonstrate practical applications of the F/M desalination technology. These chambers leveraged the produced ice to provide sustainable cooling and air conditioning.

A food preservation chamber (Figure 4a) was designed to maintain low temperatures and is suitable for storing perishable foods. Utilizing the ice generated by the solar-powered F/M desalination system, the chamber provided consistent cooling, which could be particularly valuable in remote coastal areas where access to conventional refrigeration is limited.

A second chamber was constructed to serve as a climate-controlled space (Figure 4b), using the ice produced to maintain a stable temperature for air conditioning. This setup illustrates the dual functionality of the F/M desalination system, demonstrating that it could supply both potable water and cooling capabilities from the same energy input.

Figure 5 illustrates the cooling and desalinated water distribution system designed for dual applications: food preservation and space climatization.

The system initially places ice generated from the F/M desalination process in a chamber that cools the surrounding air. This cooled air is directed into the food preservation chamber, maintaining a low-temperature ideal for storing perishable items. Then, as the ice gradually melts, the resulting cold water is pumped through a pipeline into a secondary space—designated as the climate-controlled chamber. The cooled, unfrozen water helps maintain a comfortable temperature in this space by absorbing and dissipating ambient heat. After circulating through the climate control chamber, the desalinated water, now unfrozen and ready for general use, is stored in a designated reservoir. This water can be used for various applications, including drinking, irrigation, or other essential needs in resource-limited environments.

Freeze desalination separates salt from seawater using the phase change of water. This process consumes less energy than traditional thermal methods and is well-suited for off-grid applications in arid regions.

3. Results

3.1. BaCl₂–NH₃ Solar Refrigeration System

The temperatures in the evaporator reached a minimum but were only sufficient to cool the saline solution above 0 °C (Figure 6). Although the system achieved a slight temperature reduction, it did not reach the freezing point required for effective desalination.

Due to the limited cooling capacity and the short duration at near-freezing temperatures, this system could not sustain temperatures low enough to freeze the saline solution. The tested saline water had a freezing temperature of approximately −2.0 °C, corresponding to its salinity level. Enhancements in the cooling efficiency or operational duration would be required to achieve meaningful desalination.

3.2. NH₃–LiNO₃ Solar Refrigeration System

The NH₃–LiNO₃ system outperformed the BaCl₂–NH₃ system in terms of cooling capacity and consistency in maintaining freezing temperatures, as can be seen in Figure 7, which are crucial aspects for desalination via the F/M process (Figure 8). Although both systems use solar energy intermittently, making them environmentally sustainable, the NH₃–LiNO₃ system’s greater efficiency in reaching sub-zero temperatures makes it more suitable for regions requiring sustainable desalination solutions. However, for both systems, optimizing the sorbent–refrigerant combination and enhancing insulation might improve the cooling efficiency and operational duration, thus increasing the desalination efficacy.

The NH₃–LiNO₃ system maintained evaporator temperatures below −3 °C, and the saline solution reached −2 °C in five hours. As a result, partial freezing was achieved, with effective salt separation (Figure 9).

The NH₃–LiNO₃ system demonstrated a superior performance for freeze-melting desalination, maintaining stable freezing temperatures for effective salt separation. Kinetically, the rate of ice formation plays a vital role in excluding salts, while the system’s design influences the diffusion of salt ions during freezing. By maximizing heat transfer and ensuring efficient interactions between components, the NH₃–LiNO₃ system proves to be a promising solution for sustainable desalination, particularly in resource-limited regions.

Figure 9 illustrates the salt concentration measurements in the ice and the remaining liquid after the F/M desalination process using the NH₃–LiNO₃ system.

During the experiment, the ice achieved salinity levels ranging between 3.65 and 8.2 kg/m³, while the remaining liquid exhibited much higher salinity levels, exceeding 170 kg/m³.

Although the F/M process reduced the feedwater salinity to an initial total dissolved solids (TDS) level of 3650 k/m³, this value remains above the WHO potable water standard of 1000 k/m³. However, it is suitable for less demanding applications such as crop irrigation and livestock watering, which tolerate higher salinity thresholds.

Further optimization could enhance the system’s efficiency in meeting potable water standards, such as refining freezing rates and improving operational conditions.

However, freeze desalination systems, including our configuration with energy recovery, typically achieve energy consumption values between 1.5 and 3 kWh/m³ [13,17], which are significantly lower than those required for RO systems [18]. Despite the higher residual salinity in our output (3650 k/m³), energy efficiency and low fouling risks make freeze desalination a promising alternative for applications such as irrigation and aquaculture.

To better evaluate the performance of the freeze-melting (F/M) desalination system, we compared its energy consumption, operational complexity, and environmental impact with conventional desalination technologies (

Table 3. Comparative performance metrics of desalination technologies, including energy consumption, operational complexity, and environmental impact.

Technology	Energy Consumption (kWh/m3)	Operational Complexity	Environmental Impact
Reverse Osmosis (RO)	3–6	High	Brine discharge, chemical pretreatment
Multi-Effect Distillation	6–12	High	Scaling, corrosion
Multi-Stage Flash	6–10	High	High energy, maintenance
Freeze-Melting (F/M)	1.5–3	Moderate	Minimal brine discharge, no chemicals

).

3.3. Proposed Mathematical Model

The assumption and detailed parameters for the F/M process [4,10] are based on the following heat transfer mechanisms: ice heat flow, convective heat flow, and diffusion heat flow.

$${\displaystyle \frac{T_i-T_{air}}{\frac{1}{h_{air}}+\frac{1}{\frac{k_c}{l_c}}+\frac{1}{\frac{k_{ice}}{x_0}}}}-h_b\left(T_b-T_{air}\right)=\rho L_f{\displaystyle \frac{dx}{dt}}$$

(1)

For axial coordinates, for the experimental device analyzed in this work, the cylindrical angular and radial equations are written as follows (Figure 10):

$${\displaystyle \frac{T_i-T_{air}}{\frac{1}{h_{air}}+\frac{1}{\frac{k_c}{l_c}}+\frac{1}{\frac{k_{ice}}{x_0}}}}-h_b\left(T_b-T_{air}\right)=\rho L_f\left({\displaystyle \frac{1}{r}}{\displaystyle \frac{\delta }{\delta r}}\left(r{\displaystyle \frac{\delta S}{\delta r}}\right)+{\displaystyle \frac{1}{r^2}}\left({\displaystyle \frac{{\delta }^{2}S}{\delta {\theta }^2}}\right)\right)$$

(2)

where S is the salt ion concentration, θ is the angle in the cylinder, and r is the radius of the tube with the concentrated brine.

The symmetric process may simplify Equation (2), assuming the S value is independent of θ, so the right side of the equation may be written as follows:

$${\displaystyle \frac{T_i-T_{air}}{\frac{1}{h_{air}}+\frac{1}{\frac{k_c}{l_c}}+\frac{1}{\frac{k_{ice}}{x_0}}}}-h_b\left(T_b-T_{air}\right)=\rho L_f\left({\displaystyle \frac{1}{r}}{\displaystyle \frac{\delta }{\delta r}}\left(r{\displaystyle \frac{\delta S}{\delta r}}\right)\right)$$

(3)

Using the same values from the previous paper [9], for brine salt separation, S₀ = 35 kg/m³, T_i = 12 °C, wall conductivity = 0.19 W/m K, ice conductivity = 2.22 W/m K, and D = 7.35 × 10⁻¹⁰ m²/s, the variation in S was 0.03 m, as shown in Figure 11.

Figure 12 presents the distribution of salt ions as a function of radius for a pipe with a 3 cm diameter. The salt ion concentration was displaced towards the cylinder center to 1.03 × 10⁻² m, with a higher salt ion concentration value of 0.0733 g/m³. This value diminishes to 3590 g/m³ close to the pipe wall in a non-linear shape.

Figure 13 illustrates a similar trend for a process involving a pipe diameter of 4 cm. In these operating conditions, the salt ion concentration near the wall reaches 3500 g/m³. The concentration (S) increases non-linearly at a distance of 1.5 × 10⁻² m. This behavior aligns with expectations because a larger radius creates a greater volume, which lowers the salt concentration at the center of the cylinder compared to a smaller pipe.

The salinity profiles in Figure 11, Figure 12 and Figure 13 reflect localized ion concentrations at different points along the ice–brine interface during freezing. These values are lower than the bulk salinity of the liquid or frozen phases, as they represent transient diffusion patterns rather than the overall salt content of the remaining brine or desalinated ice.

Figure 14 shows precisely the same way the ionic salt concentration is a function of the cylinder radius with no linear behavior, and the S value is lower than in the previous radius evaluations because the r-axis is larger than in the other experimental sections. Therefore, the ionic salt gradient volume is lower than the smaller volumes.

3.4. Food Preservation Chamber and Climate-Controlled Space

The design of the food preservation chamber ensured low temperatures for storing perishable items. This setup included 19.42 kg of ice produced by the F/M desalination process, filling the chamber. The ice effectively maintained a temperature of 4 °C for 5 h, providing optimal food preservation conditions. Afterward, the temperature gradually increased but remained below 23 °C for 4 h before rising further.

This performance demonstrates the ability of F/M desalination-derived ice to deliver sustained cooling, making it a reliable solution for regions without conventional refrigeration. Additionally, the setup included a second space configured as a climate-controlled area, using the same ice to maintain comfortable indoor conditions. This demonstrates the dual functionality of the F/M desalination system, which produces potable water and provides cooling power for climate control. While specific temperature and duration data for this space were unavailable, the successful use of desalinated ice underscores the versatility of the F/M process for water purification and cooling applications.

3.5. Scalability

The techno-economics of solar-driven F/M desalination technology at the global scale will critically depend on several factors. First, the modular system allows small municipalities to install basic systems, with more systems added as water needs grow, so it is scalable and cost-effective. The technology can integrate with current water treatment facilities, improving supply without building new infrastructure. In sunny areas, local solar energy decreases the dependence on synthetic energy and leads to lower operating costs, with estimated solar energy consumption requirements of 6 to 10 kWh/m³ compared to the 30–90 kWh/m³ required in actual reference methods. Involving the community in training will ensure that they can operate and maintain it sustainably, while pilot projects will give them some performance data and results. Supportive policies, funding, and continued research to make production more efficient and affordable are key to scaling up production. Using these elements, F/M desalination could meet more communities’ utility and supply needs, particularly in resource-limited and arid regions, while supporting global initiatives tied to clean water access and lower carbon emissions requirements.

4. Conclusions

This study validates the Freeze-Melting (F/M) desalination method as a low-energy, sustainable alternative to conventional desalination techniques. Among the tested systems, the NH₃–LiNO₃ solar refrigeration system exhibited superior performance, maintaining sub-zero temperatures required for ice formation and achieving desalination. The energy required is significantly lower than the energy requirements of RO and thermal desalination methods, highlighting the energy efficiency of the F/M process.

The system’s dual functionality was demonstrated through practical applications, including the following: food preservation with sustained temperatures of 4 °C for up to 5 h, ensuring optimal conditions for storing perishable goods; climate control via the utilization of residual cooling energy to maintain indoor comfort; and desalination through the production of ice with salinity levels of 3650 k/m³, suitable for irrigation and livestock watering.

The broader implications of this work include addressing water scarcity and cooling needs in off-grid, resource-limited regions such as rural agricultural areas, isolated coastal communities, and arid climates with abundant solar energy. Integrating renewable solar power into the F/M process reduces reliance on fossil fuels, offering an environmentally sustainable solution for freshwater production and cooling.

Future work should focus on improving system insulation, optimizing freezing rates, and scaling up the technology for larger applications. Further integration of hybrid renewable energy sources, such as solar–wind systems, may enhance the efficiency and operational reliability, paving the way for the widespread adoption of F/M desalination technologies.