Seawater Desalination System Driven by Sustainable Energy: A Comprehensive Review

Abstract

Seawater desalination is one of the most widely used technologies for freshwater production; however, its high energy consumption remains a pressing global challenge. Both the development and utilization of sustainable energy sources are anticipated to mitigate the energy shortages associated with seawater desalination while also effectively addressing the environmental issues linked to fossil fuel usage. This study provides a comprehensive overview of the classification and evolution of traditional desalination technologies, emphasizing the advancements, progress, and challenges associated with integrating various sustainable energy sources into the desalination process. Then, the cost, efficiency, and energy consumption of desalination systems driven by sustainable energy are discussed, and it is found that even the most widely used reverse osmosis (RO) technology driven by fossil fuels has CO₂ emissions of 0.3–1.7 kgCO₂/m³ and the lowest cost of desalinated water as high as 0.01 USD/m³, suggesting the necessity and urgency of applying sustainable energy. A comparison of different seawater desalination systems driven by different sustainable energy sources is also carried out. The results reveal that although the seawater desalination system driven by sustainable energy has a lower efficiency and a higher cost than the traditional system, it has more potential from the perspective of environmental protection and sustainable development. Furthermore, the efficiency and cost of desalination technology driven by a single sustainable energy source is lower than that driven by multi-sustainable energy sources, while the efficiency of desalination systems driven by multi-sustainable energy is lower than that driven by hybrid energy, and its cost is higher than that of desalination systems driven by hybrid energy. Considering factors such as cost, efficiency, consumption, economic scale, and environmental impact, the integration of various seawater desalination technologies and various energy sources is still the most effective strategy to solve water shortage, the energy crisis, and environmental pollution at present and in the future.

1. Introduction

As fundamental requirements for human survival, energy and freshwater are crucial for social progress. With ongoing economic growth, the demand for clean water (freshwater) and energy sources continues to rise. Currently, the energy crisis [1,2] and water scarcity [3,4] represent some of the most significant challenges facing the world. Consequently, it is essential to identify efficient solutions for producing sufficient freshwater while minimizing energy consumption.

Currently, the primary sources of global freshwater include seasonal precipitation (rain, hail, and snow), deep groundwater, freshwater rivers, and lakes, as well as brackish water from the ocean. However, freshwater constitutes only 2.5% of the total water resources available worldwide, and its availability is largely dependent on regional factors. To address the scarcity of freshwater resources, there is a growing focus on the ocean, which offers abundant water supplies. Consequently, seawater desalination technology has emerged as a direct and effective solution to this issue. In terms of feed water quality, seawater is the most commonly used source, followed by brackish and river waters (see Figure 1) [5].

According to data from the International Desalination Association, by 2020 there were more than 22,757 desalination plants worldwide, of which 15,906 are in long-term operation, producing more than 95 million tons of water daily and addressing the water supply needs of approximately 300 million people. As illustrated in Figure 2, nearly half of the operational desalination plants in operation (47.5%) are located in the Middle East and North Africa, of which Saudi Arabia (15.5%), the United Arab Emirates (10.1%), and Kuwait (3.7%) account for a large proportion. East Asia and the Pacific accounted for 18.4% of the total production, primarily due to China being the largest producer in this region. North America, which operates a significant number of desalination plants, accounts for 11.9% of the world’s total desalination capacity. Based on the large production capacity of the United States, North America accounts for 11.9% of the world’s desalination plants. Western Europe accounts for 9.2% of the world’s desalination plants. In addition, the shares of Latin America and Caribbean, South Asia, Sub-Saharan Africa, and Eastern Europe are 5.7%, 3.1%, 2.4%, and 1.9%, respectively. Obviously, most desalination plants are located in economically developed areas rather than in water-scarce areas [6].

In the long run, seawater desalination provides a reliable means for humans to obtain freshwater resources and partially addresses the issue of freshwater scarcity. However, the energy source for seawater desalination has become a significant concern. Currently, fossil fuels, including coal, oil, and natural gas, dominate energy utilization [7,8,9]. With the explosive growth of the population and the rapid development of industry, the depletion of fossil energy is an increasingly pressing issue. Although energy-saving measures [10,11,12] are effective strategies to mitigate the energy crisis, they do not fundamentally resolve the underlying problem. Furthermore, the combustion of fossil fuels accounts for more than 75% of global greenhouse gas emissions and nearly 90% of all CO₂ emissions, making it a primary contributor to global warming.

Table 1. CO₂ emissions and costs for different desalination processes.

Ref.	Technology	kgCO2/m3	Cost [USD/m3]
[13]	MSF (Non-cogeneration, natural gas)	20.4–25.0	0.41–0.50
	MSF (Cogeneration, steam cycle, natural gas)	13.9–15.6	0.28–0.31
	MED (Non-cogeneration, natural gas)	11.8–17.6	0.24–0.35
	MED (Cogeneration, steam cycle, natural gas)	8.2–8.9	0.16–0.18
	RO (sea)	3.4–6.0	0.07–0.12
	RO (brackish)	0.3–1.7	0.01–0.03
Reddy et al. [14]	MSF (Cogeneration, hybrid RO, steam cycle, natural gas)	9.45	-
	MSF (Cogeneration, hybrid RO, combined cycle, natural gas)	5.56	-
	MED (Cogeneration, hybrid RO, steam cycle, natural gas)	7.33	0.15
	MED (Cogeneration, hybrid RO, combined cycle, natural gas)	4.38	-
Becker et al. [15]	SWRO	3.01	-
Raluy et al. [16]	MSF	23.41	-
	MSF (Cogeneration, combined cycle, natural gas)	9.41	0.18
	MSF (Waste heat utilized for supply of thermal energy)	1.98	0.04
	MED	18.05	-
	MED (Cogeneration, combined cycle, natural gas)	7.01	0.14
	MED (Driven waste heat)	1.19	-
	RO (4 kWh/m3)	1.78	-
	RO (Steam cycle)	2.79	-
	RO (Internal combustion engine)	2.13	0.04
	RO (Combined cycle)	1.75	-
	MED (Waste heat utilized for supply of thermal energy)	1.11	0.02

shows some studies on the CO₂ emissions and abatement costs of several mainstream seawater desalination technologies driven by fossil fuels. Obviously, the MSF technology generates a substantial amount of CO₂ during the desalination process, leading to significant abatement costs. In contrast, RO technology produces the least amount of CO₂. Therefore, it is necessary to choose a suitable seawater desalination technology from the perspective of environment and economy.

In recent decades, the concepts of environmental protection and sustainable development have gained significant popularity. Given the limitations of fossil fuels, the development and utilization of sustainable energy sources have increasingly become a focus for humanity [17,18,19]. To date, a comprehensive assessment of seawater desalination, which considers the use of sustainable energy in various freshwater production systems, has not been thoroughly explored in the existing literature. Therefore, the aim of this study is to provide an in-depth discussion of various sustainable energy sources and their associated auxiliary equipment for seawater desalination systems. In this context, the advancements in seawater desalination systems that utilize sustainable energy sources, along with their limitations, are also examined. Consequently, it is reasonable to assert that the findings of this study will guide future research and facilitate the widespread adoption of seawater desalination systems driven by sustainable energy sources across diverse settings.

2. Classification of Seawater Desalination Technology

Currently, there are over 20 types of seawater desalination technologies available globally. As illustrated in Figure 3, these technologies can be categorized into two primary groups: membrane methods and thermal methods. The membrane methods encompass reverse osmosis (RO) [20], forward osmosis (FO) [21], electrodialysis (ED) [22], membrane distillation (MD) [23], capacitive deionization (CDI) [24], etc. The thermal desalination methods encompass multi-stage flash (MSF) [25], humidification dehumidification (HDH) [26], multi-effect distillation (MED) [27], freezing desalination (FD) [28], adsorption desalination (AD) [29], etc. Additionally, MED includes methods such as mechanical vapor compression (MVC) [30], thermal vapor compression (TVC) [31], low-temperature (LT) [32], etc. Figure 4 illustrates the global desalination capacity by process, highlighting the capacity shares of the main desalination technologies [33].

2.1. Thermal Methods

2.1.1. Multi-Stage Flash (MSF)

As illustrated in Figure 5, MSF is a seawater desalination method that was developed in the 1950s to address the significant limitations of multi-effect evaporation and scaling issues. This method features equipment that is both simple and reliable, with excellent anti-scaling performance and ease of scalability. Additionally, it offers substantial operational flexibility and the capability to utilize low-grade heat energy and waste heat. The MSF system is mainly composed of a brine heater, a multi-stage flash evaporation device heat recovery device, a heat exhaust device, a seawater pre-treatment device, a non-condensable gas exhaust device, and a brine circulation pump [34]. Its working principle is as follows: The feed seawater is preheated using steam generated from the previous flash chamber before being introduced into the first flash chamber, where the pressure is lower than the saturation pressure of the hot brine. Due to this pressure differential, the preheated seawater rapidly vaporizes, producing steam that is directed to the condenser tube to be condensed into freshwater. This process is repeated across a series of flash chambers, each operating at progressively lower pressures. Ultimately, the concentrated brine is either partially discharged or mixed with the incoming seawater, forming circulating brine for the heat recovery device [35].

In recent decades, MSF technology has been gradually optimized and upgraded. Zouli et al. [36] added hematite nanoparticles to salt water to increase the evaporation and heat transfer coefficient (HTC) of MSF. Hasan et al. [37] developed a novel dynamic model for the MSF process and found that the final stage requires a longer settling time than the preceding stages. Additionally, the steam temperature has an insignificant effect on the performance ratio when compared to the inlet seawater temperature and the recycle brine flow rate. Moreover, it has been observed that the productivity of the plant can increase during the winter months in comparison to the summer months. To prevent the emission of CO₂ into the atmosphere and save fuel consumption, Mehtari et al. [38] introduced a novel design for the desired desalination process, which is based on the un-mixing of target streams.

However, a single MSF system struggles to meet actual production demands. Combining with other distillation systems can effectively enhance the efficiency and reduce the costs of seawater desalination [39]. Ali et al. [40] proposed three different modifications of the reversal MSF plant, namely, the hybrid MSF-MD system (HYB), independent cascaded blocks (ICBs), and coupled cascaded blocks (CCBs). The results indicate that HYB is the most effective structure. The FO pretreatment offers a promising and economical solution for addressing the scaling issue in MSF desalination plants, with a water production cost of only 0.48 USD/m³ [41]. In another study, El-Ashmawy et al. [42] compared five different desalination systems, including RO, MED, MSF, MED + RO, and MSF + RO. The results showed that the cost of MSF + RO is lower than that of the thermal method.

2.1.2. Multi-Effect Distillation (MED)

As shown in Figure 6, the MED system primarily consists of a condenser and multiple effects. Compared to other seawater desalination technologies, MED offers a longer operational lifespan and lower energy consumption. Its working principle is as follows: Seawater is initially pretreated to remove suspended solids and larger particulate impurities before being heated in a heat exchanger. The heated seawater is then directed to the evaporation chamber (or effect), where the pressure in each chamber gradually decreases. At these lower pressures, the seawater begins to evaporate. The steam produced in the first effect flows into the second effect. In each effect, the steam is condensed into freshwater via the cooling system, and the heat released during this process is utilized to heat the seawater in the subsequent effect. The unevaporated brine continues to flow to the next effect until it reaches the desired concentration or is completely converted into freshwater. Ultimately, the freshwater generated in the final effect is collected and stored, while the remaining concentrated liquid is either discharged or subjected to further treatment. The freshwater production rate of the MED process depends on the number of effects employed. In theory, an increased number of effects is associated with a higher freshwater production rate. However, the number of effects is limited by the minimum temperature difference between adjacent effects and the overall temperature range of the process [43].

Currently, the two most commonly used MED systems are TVC-MED and LT-MED. On the basis of the advantage that thermal vapor compressors (TVCs) can effectively utilize low-grade energy, MED-TVC has become one of the most popular technologies in the field of seawater desalination [44]. On the one hand, the TVC enhances the efficiency of the MED system; on the other hand, it leverages low-pressure exhaust to further reduce energy consumption. Additionally, the economic benefits of the MED-TVC system are also one of the factors that must be considered. Fergani et al. [45] developed a numerical model based on an exergoeconomic approach to analyze the cost of the MED-TVC system and found that the total water price is approximately 1.73 USD/m³ for a distilled water production of 55.20 kg/s. Shahouni et al. [46] proposed an electric heating model that can be combined with an optimized MED-TVC system. The results showed that the cost of the model only accounts for 40% of the cost of the parabolic trough solar collector method.

Low-temperature multi-effect distillation (LT-MED) is among the most widely employed technologies for seawater desalination. A notable advantage of this system is its capacity to generate distilled water in quantities that significantly exceed the amount of heated steam utilized in the process. Nonetheless, the performance and cost of LT-MED both remain critical factors that cannot be overlooked. To enhance the performance of the desalination system and reduce costs, Lv et al. [47] developed a superhydrophilic anodic oxide film that exhibits significantly improved corrosion resistance. Xue et al. [48] suggested preheating the feed brine by integrating LT-MED with TVC. The results indicate that the performance of the proposed system increased from 9.67 to 10.94 and the freshwater production cost decreased from 3.13 USD/m³ to 2.84 USD/m³.

2.1.3. Freezing Desalination (FD)

As shown in Figure 7, the FD system is a technology that uses the solidification and melting process of ice to obtain freshwater. It has the advantages of low energy consumption, less fouling, and less impact on the environment. Its operating principle is as follows: The brine is cooled, causing the water to begin solidifying into ice while the salt remains in the liquid solution. As the salinity of the solution increases, the freezing point decreases, leading to the continued growth of ice crystals and a further increase in salt concentration. The formed ice crystals are then separated from the high-concentration saltwater and eventually melt into freshwater. On the basic of different demands and conditions, FD technology can adopt different configurations, including direct-contact FD, suspended FD, stepwise FD, falling-film FD, block FD, etc. Each configuration has its specific operating conditions and optimized parameters to ensure ice crystal purity and desalination efficiency.

Elhefny et al. [49] examined the impact of the compressor’s isentropic efficiency and the effectiveness of the system’s heat exchangers on the work consumption of the FD system, ultimately identifying the optimal configuration for the FD system. Shishiny et al. [50] presented an innovative FD system that incorporates the effects of sweating and centrifugal brine rejection. The results indicate that the sweating period has a significant impact on both product salinity and salt rejection efficiency, achieving a reduction in salinity from 40 parts per thousand (ppt) to 0.99 ppt, along with a salt rejection rate of 95.05%. In recent years, Kalista et al. [51] reported on the process, efficiency, application, cost, and challenges of FD. Although FD technology is becoming more and more mature, its high cost is still an obstacle to its wide application.

2.1.4. Adsorption Desalination (AD)

Adsorption desalination (AD) is a technology that employs the adsorption and desorption of steam by adsorbent materials to facilitate seawater desalination. This process primarily relies on the varying adsorption capacities of these materials for steam at different temperatures, enabling desalination through cycles of adsorption and desorption. As shown in Figure 8, the main components of the AD system include evaporator, adsorption bed, condenser, and heat exchanger. The basic principle of the AD system encompasses two primary processes: the adsorption–evaporation process and the desorption–condensation process. In the adsorption–evaporation process, the adsorbent (such as silica gel or zeolite) captures the steam generated by heating seawater in the evaporator, thereby facilitating the adsorption of water from the seawater and producing desalinated water. Conversely, in the desorption–condensation process, the adsorbent containing the adsorbed steam is heated to release the steam, which is subsequently condensed into liquid freshwater in the condenser.

The advantages of AD technology include its ability to utilize low-grade heat sources, such as solar energy, geothermal energy, and industrial waste heat. The system is characterized by its simplicity, as well as low operational and maintenance costs [52]. Additionally, it is environmentally friendly without producing chemical waste. Furthermore, AD technology can be integrated with refrigeration systems to achieve the dual functions of seawater desalination and refrigeration. However, AD technology faces several challenges, including the adsorption capacity of the adsorbent, the efficiency of the adsorption–desorption cycle, and the energy consumption of the system. Researchers are continually investigating more efficient adsorption materials and optimizing system designs to enhance both the desalination efficiency and economic viability of AD systems [53,54]. In practical applications, AD systems can be optimized in various ways, such as enhancing freshwater production by increasing the number of adsorption beds or improving energy efficiency through the enhancement of heat recovery systems [55].

2.1.5. Humidification Dehumidification (HDH)

As shown in Figure 9, the main components of the HDH system include heaters, humidifiers, dehumidifiers, and cooling devices [56]. The operating principle of the HDH desalination system is founded on a straightforward heat and mass exchange process, primarily comprising two key steps: humidification and dehumidification. In the humidification process, the brine is heated and subsequently evaporated. The heated brine is pumped into the humidifier and sprayed onto the fill via sprinklers. Cold air is introduced at the bottom of the humidifier and flows upward, facilitating heat and mass exchange with the hot brine on the fill. During this process, a portion of the water evaporates from the brine and combines with the air to form hot-humid air, whereas the unevaporated brine returns to the feed tank for the next cycle. The steam in the hot-humid air is then directed into the dehumidifier. In the dehumidification process, the hot-humid air in the dehumidifier exchanges heat and mass with the cooling water. The cooling water is sprayed onto the filler of the dehumidifier through a sprinkler, coming into contact with the hot-humid air. As the air cools, the steam condenses into water droplets, which mix with the cooling water and flow into the freshwater tank, where they are collected as a freshwater product. The cooled dry air is either exhausted from the system or, in some designs, recycled [57].

Experimental studies indicate that the volume of water production and the performance of the HDH system are influenced by several operating parameters, including air circulation flow, feed liquid flow, and feed liquid temperature [58,59,60]. Optimizing these parameters can enhance both the water production efficiency and the energy utilization efficiency of the system. Additionally, the advantages of the HDH system are quite evident. It operates at low temperatures and atmospheric pressure, effectively reducing the system’s energy consumption. Furthermore, it boasts a simple structure; a high desalination rate; and stable, reliable water quality. Additionally, it can be integrated with renewable energy sources, such as solar energy [61,62,63].

2.2. Membrane Methods

2.2.1. Reverse Osmosis (RO)

RO seawater desalination is a technology that employs hydraulic pressure to draw water through a semi-permeable membrane. As shown in Figure 10, the main components of the RO system include a pre-treatment device, high-pressure pump, RO-membrane device, energy recovery device, and post-treatment device. Its working principle is as follows: Seawater undergoes pretreatment prior to entering the RO membrane to eliminate large particulate matter, suspended solids, sediment, rust, silt, algae, bacteria, and other contaminants. The pretreated seawater is then pumped into the RO membrane module using a high-pressure pump. Under this elevated pressure, water molecules from the seawater pass through the semipermeable membrane, while salts and larger molecules are retained, resulting in the formation of concentrated brine. The water molecules that pass through the semipermeable membrane are collected as freshwater, whereas the trapped concentrated brine is either discharged or treated for potential reuse. The freshwater produced by the RO process undergoes further treatment to comply with specific water quality standards.

Due to its lower energy consumption, higher recovery rate, and reduced water production costs, RO seawater desalination is currently the most widely used desalination technology [44], with numerous scholars contributing significantly to its advancement [64]. Hosseinipour et al. [65] and Naderi et al. [66] found that batch and hybrid reverse osmosis systems offer distinct advantages over semi-batch reverse osmosis, particularly in the context of high-recovery brackish water desalination. As various seawater desalination technologies mature, hybrid RO systems are increasingly being utilized, particularly in conjunction with forward osmosis (FO) technology [67]. Moon et al. [68] introduced a forward osmosis/crystallization/reverse osmosis (FO/Cry/RO) hybrid process to evaluate its economic feasibility under seawater and high-salinity conditions. The results indicated that the proposed hybrid system surpasses the traditional RO process in energy efficiency and cost-effectiveness for seawater desalination, particularly under high-temperature and high-salinity conditions. Patel et al. [69] proposed the concept of integrated batch reverse osmosis–forward osmosis (BRO–FO), which can enhance the recovery rate of the RO system and reduce specific energy consumption (SEC).

2.2.2. Electrodialysis (ED)

As shown in Figure 11, the main components of the ED desalination system include selective membranes, electrodes, and brine channel [70]. Its working principle is as follows: When seawater is introduced into the ED unit, cations present in the water (such as Na⁺, Ca²⁺, Mg²⁺, etc.) migrate toward the cathode (negative electrode), while anions (such as Cl⁻, SO₄²⁻, etc.) move toward the anode (positive electrode). Cations traverse a cation exchange membrane into an adjacent concentration chamber, whereas anions pass through an anion exchange membrane to another concentration chamber. Consequently, the ion concentration in the desalination chamber gradually decreases, leading to the production of freshwater, while the high-salinity water generated in the concentration chamber is either discharged.

Compared to other desalination technologies, ED systems consume relatively low energy and can operate across a wide temperature range [71]. The performance of ED systems is influenced by several factors, including the voltage gradient, the concentration of feed seawater, the concentration of the electrolyte, and the specific operating conditions. Recent studies have demonstrated that the desalination performance of the ED system can be significantly enhanced by increasing the voltage gradient, elevating the concentration of feed seawater and electrolyte, and reducing the system’s flow rate [72,73,74]. In addition, the combination of an ED system with other seawater desalination technologies is still a promising approach at present, and even in the future [75,76].

2.2.3. Forward Osmosis (FO)

As shown in Figure 12, the main components of the FO desalination system include draw solution, feed solution, semipermeable membrane, membrane assembly, draw solution recovery device, and concentrated feed. Its working principle is as follows: One side of the FO system contains the high osmotic pressure draw solution (DS), while the other side comprises the low osmotic pressure feed solution. The solute concentration in the draw solution is significantly higher than that in the feed solution, resulting in an osmotic pressure difference between the two. This osmotic pressure difference drives the migration of water molecules from the feed solution to the draw solution through the FO membrane. Water molecules passing through the membrane enter the draw liquid, resulting in the dilution of the draw liquid. This diluted draw liquid contains freshwater, which can be collected. To recycle the draw liquid, it is necessary to separate the water from the solute using various methods, such as reverse osmosis, nanofiltration, or membrane distillation, in order to restore its high osmotic pressure characteristics. The separated water can either be combined with the collected freshwater or used directly as a freshwater product. The recovered draw liquid is then sent back to the FO system for further recycling, while the separated solutes can be processed or discharged. The operating principle of the FO system renders it a low-energy consumption, high-efficiency desalination technology, particularly well-suited for application scenarios with stringent energy consumption requirements. Furthermore, due to the absence of external pressure in the FO process, the propensity for membrane fouling and scaling is significantly reduced, which contributes to an extended membrane service life and lower operating costs.

The design and operation of the forward osmosis (FO) system must take into account various factors, including the selection of membrane materials, the type and concentration of the draw solution, and the operating conditions, to ensure the system operates efficiently and produces a stable output of freshwater [77]. On the basis of a two-step protocol, Edokali et al. [78] developed a new laminar GO-based FO membrane and found that the water permeability of this membrane is much greater than the traditional membrane. Tang et al. [79] proposed a sodium alginate–graphene oxide (SA–GO) hydrogel with excellent performance in drawing water. Recent trends in research have focused on combining various technologies with FO [80]. Tashtoush et al. [81] developed a comprehensive system (PVT-PCM-FO-MD) and found the system was ideally suited for small-scale applications and for brackish water desalination in remote areas. Ali et al. [82] combined the advantages of FO’s low operating cost with MD’s ability to treat hypersaline water; the results showed that the thermal efficiency of the system increased by 28.7%.

2.2.4. Membrane Distillation (MD)

The MD system is a thermally driven membrane separation technology that effectively separates water vapor from liquid water using a microporous hydrophobic membrane. This technology has garnered significant attention due to its mild operating conditions, which include low temperature and atmospheric pressure, as well as its theoretical salt rejection rate of 100%. As illustrated in Figure 13, the main components of the FO desalination system include hydrophobic membrane, heating unit, cooling device, and feed solution circulation assembly. Its working principle is as follows: Seawater, serving as the feed solution, is heated to facilitate the evaporation of a portion of its water content. This heated seawater generates steam on one side of the membrane, which subsequently migrates through the micropores to the opposite side. Upon reaching the cooler side of the membrane, the steam condenses into liquid freshwater on the condensation surface.

MD technology can be categorized into several modes based on various feed solutions and condensate flow configurations. These modes include direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), permeate gap membrane distillation (PGMD), vacuum membrane distillation (VMD), and sweeping gas membrane distillation (SGMD), among others. Each mode possesses specific advantages and limitations that make them suitable for different application scenarios. Shekari et al. [83] employed PVDF powder for assembling membranes via the VIPS method for operating in AGMD; the results showed that the membrane permeability increased from 0.3 to 2.41 L/m²·h and salt rejection exceeds 99.5%. A comprehensive techno-economic evaluation of DCMD was conducted by Elboughdiri et al. [84]; they found that natural gas emerges as the most cost-effective fuel for steam production in a DCMD plant. Hafiz et al. [85] proposed a novel PGMD system with gap water circulation; the results showed that the proposed system significantly increases water productivity while reducing energy consumption compared to conventional PGMD configurations. Most desalination technologies are energy-intensive and present various limitations, including issues related to scaling and clogging. Consequently, innovations in unconventional desalination technologies have led to the development of numerous promising systems. Ahmed [86] investigated the viability and efficiency of an FO-AGMD integrated system of 5 m³/day capacity and found that the water recovery rate of the FO-AGMD system is much greater than that of the traditional AGMD system.

2.3. Emerging Methods

In recent years, traditional seawater desalination technologies have faced challenges in adapting to specific environmental conditions. Consequently, a variety of innovative technologies have emerged, among which redox flow desalination (RFD) stands out. This electrochemical desalination technology offers the advantages of low energy consumption and high desalination efficiency [87,88]. As shown in Figure 14, the RFD system is mainly composed of battery cell, electrode, ion exchange membrane, electrolyte, current collector, liquid storage tank, and pump. Its working principle is as follows: When the current flows through the system, the anions in the electrolyte at the anode undergo oxidation, while the cations in the electrolyte are reduced at the cathode. These redox reactions facilitate the migration of ions between the electrodes and the ion exchange membrane, effectively separating the ions from the feed water and ultimately achieving the goal of desalination.

The advantages of RFD technology include its low energy consumption, high efficiency, and environmental friendliness, making it well-suited to address the growing global issue of water scarcity. By optimizing parameters such as flow rate, reaction time, and electrolyte concentration, the desalination efficiency of the RFD system can be enhanced while simultaneously reducing energy consumption. Maclean et al. [89] reported the operational effect of flow rate on RFD and found that the increase of electrolyte channel flow rate can effectively improve the average desalination rate and reduce the energy consumption of the system. Xie et al. [90] proposed a novel method of RFD utilizing the highly aqueous-soluble and reversible redox-active compound; the results showed that the proposed method can effectively improve the desalination rate and reduce the energy consumption of the system. Xiao et al. [91] proposed a novel RFD using ionic liquid (IL) as the ecofriendly solvent; the results showed that the proposed system can effectively improve the desalination rate. As the place where the redox reaction occurs, the battery cell is the core component of the RFD system. A detailed review of RFD battery cells was recently reported by Khodadousti et al. [92].

3. Classification and Application of Sustainable Energy

As illustrated in Figure 15, sustainable energy includes a variety of sources, such as wind energy [93], solar energy [94], nuclear energy [95], ocean energy [96], biomass energy [97], hydroelectric energy [98], and geothermal energy [99]. Within these categories, solar energy is primarily classified into two types: solar photovoltaic (PV) and solar thermal. Likewise, ocean energy can be further subdivided into tidal energy [100], wave energy [101], and ocean thermal energy [102].

3.1. Seawater Desalination Process with Wind Energy

Wind energy seawater desalination can be categorized into two types: direct wind energy seawater desalination and indirect wind energy seawater desalination. Direct wind seawater desalination involves the direct utilization of mechanical energy converted from wind energy to operate the desalination unit for seawater desalination. In contrast, indirect wind energy seawater desalination first converts wind energy into electrical energy, which is then used to drive the desalination unit for seawater desalination.

The advantage of direct wind energy seawater desalination lies in its ability to bypass the conversion of mechanical energy into electrical energy and subsequently back into mechanical energy. This approach enhances energy utilization efficiency and simplifies the system structure. However, each device within this technical scheme operates as an independent system, making it unsuitable for large-scale seawater desalination plants. This limitation arises from the higher seawater quality requirements of desalination plants, as well as specific demands related to ocean currents and ecological considerations. Large-scale seawater desalination plants often require dozens or even hundreds of wind turbines, extending over tens of kilometers. The water intake points of each unit are spaced far apart, making it challenging to simultaneously meet the requirements of each unit regarding seawater quality, current conditions, and ecological factors. However, several issues arise when directly using wind energy for seawater desalination. For instance, fluctuations in wind can impact the flow rate of the pump or the stability of the compressor, resulting in the direct application of wind energy being relatively uncommon. In most instances, wind energy is harnessed to generate electricity, which is then utilized for seawater desalination.

In 1982, the first wind-driven seawater desalination plant in Europe was inaugurated in France, utilizing a 4 kW fan to power a reverse osmosis system with a capacity of 0.5 m³/h [103]. Following this, a series of seawater desalination projects leveraging wind energy emerged [104,105]. In 2005, a wind-powered desalination system designed by the German company Enercon was installed on the Norwegian island of Utsira. Additionally, wind energy evaporation is a widely employed technology in seawater desalination. In 1991, the first wind-driven seawater desalination device was constructed on Borkum Island in Germany. This device utilizes a fan-driven vapor compression system, achieving daily water production of 48 m³, and functions effectively under variable speed conditions. Subsequently, Germany developed a larger wind-driven vapor compression desalination plant on Rügen Island, which operated entirely independently on fan power and had a maximum water production capacity of 480 m³/d [106].

In fact, seawater desalination technology is well-established and widely implemented. Conventional desalination methods typically rely on stable and reliable energy sources, primarily electricity. In contrast, wind energy is characterized by its inherent volatility and intermittency. As a result, the direct application of wind energy to seawater desalination presents several technical challenges that must be addressed. Consequently, numerous researchers have engaged in extensive efforts to explore and overcome these barriers. Since 1999, Carta et al. [104,107,108] have been investigating the direct application of wind power for seawater desalination, concluding that reverse osmosis (RO) technology is the most suitable method for this purpose. The German company Enercon has also conducted research in this area, developing a modular wind energy seawater desalination system that utilizes RO technology. Notably, the system features a specially designed energy recovery device, which requires only a low-pressure pump and a plunger energy collection device. As a result, the energy consumption for desalination is minimized to just 2 kW·h/m³ [109]. Aerodyn Company utilizes the kinetic energy generated by the fan to directly drive both the feed water pump and the high-pressure pump, which supply water and enhance the performance of the seawater desalination equipment. This approach eliminates the efficiency losses associated with secondary energy conversion, thereby improving overall energy utilization efficiency [110]. Pestana et al. [111] have confirmed the feasibility of direct wind energy reverse osmosis (RO) seawater desalination technology from an experimental perspective. The entire system operates without the need for any energy storage devices and has demonstrated stable operation for over 7000 h, achieving a water production rate of 40 to 43%.

Indirect wind energy seawater desalination can be categorized into two types based on different power supply modes: off-grid and grid-based. The single installed capacity of off-grid wind turbines typically ranges from 100 W to 10 kW, making them suitable as power sources for small-scale seawater desalination devices in island areas facing both water and power shortages [112]. In contrast to off-grid wind power, the successful integration of grid-based wind power into a regional power grid primarily hinges on the power system’s capacity to manage fluctuations in power supply. In general, the instantaneous current produced by a wind turbine when connected to the regional power grid can significantly affect the grid. This impact tends to increase with the capacity of the individual wind turbine. However, employing a soft cut-in method utilizing bidirectional thyristors allows for a smooth transition process that avoids inrush current [113].

The selection of wind energy seawater desalination technology primarily depends on the quality of feed water, the specifics of wind power technology, the construction site environment, and the characteristics of the desalination technology. The seawater desalination technologies that are compatible with wind power generation mainly include RO, MVC, and ED [114,115]. Among these, RO is the most widely utilized seawater desalination technology due to its low energy consumption and straightforward system design. Liu et al. [116] constructed and tested a prototype wind-powered RO desalination system that operates effectively under mild ambient wind conditions of 5 m/s or less. Estimates indicate that if reverse osmosis (RO) desalination is powered by wind technologies on a larger scale, the annual reduction in greenhouse gas (GHG) emissions could range from 754,000 to 3.71 million tons of CO₂-equivalent [117]. In contrast to RO, MVC is more appropriate for small to medium-sized seawater desalination, as it produces higher quality water with fewer chemicals required. Furthermore, MVC does not necessitate the replacement of membrane modules, and its manual operation requirements are minimal. These characteristics make MVC particularly suitable for remote areas or isolated islands [106,118,119]. However, the MVC approach tends to operate slowly and is susceptible to fouling in certain isolated wind power systems. Consequently, it is no longer appropriate for these applications. In contrast, the ED method effectively addresses these shortcomings [120].

3.2. Seawater Desalination Process with Solar Energy

As a clean and renewable energy source, solar energy delivers the equivalent of 5 million tons of coal-fired energy per second to the earth. The new efficient solar seawater desalination technology that combines solar energy and seawater desalination is a promising method. The utilization of solar energy for seawater desalination is mainly based on solar photovoltaic (PV) and solar thermal.

3.2.1. Solar Thermal

Among the existing desalination technologies, distillation desalination technology is the most mature, with high safety and a wide range of applications. Therefore, the utilization of solar thermal energy to replace traditional energy sources to heat seawater is a promising method. Currently, the most successful applications include solar thermal-MED and solar thermal-MFS [44,120].

In the early years, Farwati et al. [121] and García et al. [122] proposed to apply a solar thermal energy to an MSF system. After that, Shaaban [123] combined an integrated solar thermal combined cycle power plant with an MSF desalination unit and found that the proposed system can save up to 84.7% of the fuel compared to a conventional MSF desalination unit. On the basic of an MED system driven by solar thermal, Mukherjee et al. [124] proposed a carbon-neutral desalination process. Subsequently, Addous et al. [125] proposed to apply both solar PV and solar thermal to an MED system. Further, Zhao et al. [126] developed an MED seawater desalination system that can simultaneously utilize solar thermal energy and tidal energy.

In fact, the traditional solar thermal distillation method exhibits a low yield per unit area. This inefficiency primarily arises from the inadequate utilization of the latent heat generated during steam condensation, coupled with the low thermal efficiency of the distiller’s heat exchange tubes. Consequently, the effective utilization of condensation latent heat and the heat exchange efficiency of these tubes have emerged as critical factors in enhancing the efficiency of solar thermal distillation. To fully exploit the latent heat of condensation of steam, numerous scholars have developed a range of innovative solar thermal distillers, among which multi-stage stacked tray distillers are notable [127,128]. However, traditional multi-stage stacked tray distillers typically feature a single-angle (single-slot) design [129,130,131]. To enhance the condensation area of the multi-stage stacked tray distiller, Chen et al. [132] introduced a multi-angle (multi-slot) stacked tray distiller. Beyond the multi-angle stacked tray distiller, various effective methods exist to improve the utilization rate of steam latent heat. For instance, Kharabsheh et al. [133,134] incorporated a cooling water pipe around the distiller to facilitate condensation. Yuan et al. [135] proposed a novel closed solar thermal seawater desalination device capable of continuous operation both day and night.

3.2.2. Solar PV

The combination of solar energy and traditional seawater desalination technology consists of using solar PV power generation to drive the system to produce freshwater. Currently, solar PV power generation has emerged as the predominant technology in solar energy applications, with solar cells serving as the core components of this technology. Since the 1980s, the primary objective of solar PV power generation has been to enhance cell conversion efficiency while simultaneously reducing manufacturing costs [136,137,138,139]. Solar cells are typically composed of either crystalline silicon or thin film materials, with crystalline silicon remaining the predominant material for solar cell production today [140]. Crystalline silicon cells are categorized into single crystal silicon cells and polycrystalline silicon cells. The maximum efficiencies for single crystal silicon cells are 24.7% in laboratory settings and 20% in commercial applications, whereas polycrystalline silicon cells achieve maximum efficiencies of 20% and 18% in laboratory and commercial contexts, respectively [141]. In comparison to crystalline silicon cells, thin film cells are more cost-effective, making them the most widely utilized technology in seawater desalination systems driven by solar PV [142,143,144].

RO is the most widely utilized dialysis technology in the field of seawater desalination, accounting for over 44% of the world’s total desalinated water production, and more than 80% of seawater desalination devices employ RO technology [145]. Additionally, the energy recovery technologies developed in the early 21st century have also significantly reduced the production costs of RO membranes [146]. Consequently, utilizing solar PV power generation to drive RO systems is a promising technology. Najafi et al. [147] employed a novel least square Monte-Carlo real options analysis (LSMC-ROA) to assess PV-RO desalination plants. Ali et al. [148] presented a comprehensive technoeconomic analysis of stand-alone PV-RO systems in Pakistan and found that PV-RO systems, with multiple stages and energy recovery devices, greatly reduce the cost of the seawater desalination process. To improve the utilization rate of solar-PV power generation, Tourab et al. [149] proposed to combine RO technology and HDH technology.

3.2.3. The Auxiliary Equipment of Solar Energy

Solar energy is characterized by low density and an intermittent, uneven spatial distribution, which imposes significant demands on the collection and utilization of this resource. Currently, most solar panels are flat and positioned at a fixed angle. While this arrangement is straightforward, it results in suboptimal solar energy utilization [150,151,152]. Maximizing photoelectric conversion efficiency requires that the panels remain perpendicular to sunlight. Consequently, the development of technologies for automatic solar tracking and precise solar positioning has increasingly become a focal point of research [153,154].

Additionally, numerous scholars have made significant contributions to various aspects of solar collectors. Kalogirou et al. [155] presented a comprehensive overview of the different types of solar thermal collectors and their applications, evaluating their performance from optical, thermal, and thermodynamic perspectives. Zhai et al. [156] developed a solar energy system capable of heating and cooling and providing natural ventilation and hot water supply, proposing a design scheme for a U-type evacuated tubular solar collector series arrangement combined with heat pipe evacuated tubular solar collectors. Roca et al. [157] established low-complexity models for the solar field and thermal storage subsystems based on a low-temperature static solar collector field of the compound parabolic concentrator type, as well as a model for the distillate production rate dependent on process temperature. In terms of manufacturing processes, magnetron sputtering has proven to be an effective solution for enhancing the heat collection efficiency of flat plate collectors; its principle involves increasing the absorption rate of sunlight by depositing a selective absorber coating on the collector [158,159,160].

The parabolic trough solar collector offers several advantages, including scalability, longevity, and cost-effectiveness. Currently, it represents the most advanced technology for solar thermal energy utilization, with an energy storage system (combustion system) capable of functioning both day and night [161]. Depending on the method of steam generation, parabolic trough solar collectors can be categorized into three types: flash evaporation, direct evaporation, and indirect evaporation [162]. Among these, flash evaporation has emerged as a favored technology for seawater desalination due to its simple structure, stable operation, and high efficiency. García et al. [122] conducted an economic comparison between a solar-assisted, multi-stage flash (MSF) distillation system utilizing a solar parabolic trough collector and a conventional energy MSF plant. Additionally, Sharaf et al. [163] determined that the solar parabolic trough concentrator (PTC) field can provide sufficient thermal power for an MSF plant. Furthermore, from a thermodynamic perspective, the MED-TVC system powered by a parabolic trough solar field is more suitable than a parabolic trough-RO combination [164].

3.3. Seawater Desalination with Nuclear Energy

To mitigate the reliance of seawater desalination systems on fossil fuels, nuclear energy has increasingly garnered attention from countries worldwide [165,166]. Nuclear energy desalination technology primarily encompasses nuclear technology, desalination technology, and their integration [167]. Although practical application experience with nuclear seawater desalination remains underdeveloped globally, it holds significant promise for the future [168]. Recent experiences in several Asian countries, including China, India, Japan, and Pakistan, illustrate the potential for the large-scale construction and operation of nuclear desalination complexes in semi-arid coastal regions worldwide [169,170]. Furthermore, there have been several research findings in this domain [171,172,173]. Kavvadias et al. [174] investigated the costs associated with nuclear desalination and proposed a methodology for examining the interactions among critical parameters. Faibish et al. [175] and He et al. [176] discussed the principles of utilization, applications, and challenges associated with solar and nuclear energy in desalination.

Indeed, the integration of nuclear energy with conventional seawater desalination systems represents the most prevalent technology. The combination of reverse osmosis (RO) with a nuclear steam supply system (NSSS) offers several economic and technical benefits [177]. Nevertheless, the hybrid reverse osmosis-multi-stage flash (RO-MSF) system presents potential advantages, including reduced power demand, enhanced water quality, and potentially lower operational costs compared to standalone RO plants [178].

Over the past decade, as the reliability of nuclear seawater desalination plants (NDPs) has continued to improve; there has been a corresponding increase in demand for economic benefits and system efficiency. In this context, Sadeghi et al. [179] developed a novel computer code to assess the economic aspects of various hybrid desalination schemes integrated with nuclear power plants (NPPs). Their findings indicate that utilizing the rejected water (hot stream) from the NPP’s condenser as the feed water for the desalination process represents the most effective option for optimizing the overall cost of nuclear desalination.

To this day, numerous micro- and small-scale reactor concepts are being developed globally, utilizing various nuclear technologies. Ghazaie et al. [180] proposed several coupling schemes for small modular reactors (SMRs) with hybrid desalination (HD) plants, demonstrating that the utilization of relatively hot water from the SMR condenser can lead to a reduction in total desalination costs by approximately 6.5 to 7.5%. Building on Ghazaie’s research, Sadeghi et al. [181] introduced three distinct scenarios for supplying the energy required by the desalination plant (DP): scenario 1 involves a solar plant (SP), scenario 2 employs an SMR, and scenario 3 integrates both the hybrid SMR and SP. Compared to small reactors, microreactors hold greater potential for future applications. A recent market assessment conducted by the Idaho National Laboratory indicates that seawater desalination represents a promising market for microreactors [182].

3.4. Seawater Desalination with Ocean Energy

The potential for ocean energy storage is substantial, with energy derived from tides, waves, and temperature differences exceeding the global energy demand by several multiples [183,184]. As freshwater resources become increasingly scarce, alongside growing concerns regarding energy shortages and environmental pollution, marine energy has garnered significant attention due to its abundant reserves and advantages of being pollution-free and renewable.

Marine energy seawater desalination involves the integration of marine energy utilization with seawater desalination, allowing for the full or partial support of the desalination process by marine energy. Compared to other renewable energy sources for seawater desalination, the primary advantage of marine energy desalination lies in its reliance on the ocean for both energy and raw materials, significantly enhancing the overall system efficiency. Currently, research in seawater desalination predominantly concentrates on tidal energy, wave energy, and ocean thermal energy.

3.4.1. Seawater Desalination with Tidal Energy

Tidal energy refers to the potential energy difference created by the periodic changes in ocean water levels, which result from the gravitational forces exerted by the Sun and the Moon, as well as the centrifugal force generated by the rotation of the Earth–Moon system. This form of energy is the earliest type of ocean energy harnessed by humans. Currently, tidal energy utilization technology is the most developed among all ocean energy technologies. Numerous countries, including Britain, France, Russia, and China, have established or are researching large tidal power stations [185]. However, the technology for seawater desalination using tidal energy remains underdeveloped, and many scholars are actively exploring this area.

Currently, there are two primary methods for tidal energy seawater desalination: distillation and reverse osmosis. Regarding reverse osmosis, Zhao et al. [186] designed a horizontal axis tidal current turbine specifically for this purpose, and experimental results indicate that the efficiency of the newly developed turbine can reach up to 47.6%. Utilizing the Taguchi-CFD (computational fluid dynamics) approach, Khanjanpour et al. [187] proposed a conceptual design for a tidal power reverse osmosis (TPRO) desalination unit aimed at optimizing horizontal axis tidal (HAT) turbines; their findings revealed that the weight of the optimized model is reduced by 17% compared to the baseline model. Ling et al. [188] introduced a reverse osmosis (RO) desalination system integrated with tidal energy, highlighting that this system benefits from the direct conversion of mechanical energy generated by the turbine into tidal energy. The results indicate that the proposed system can achieve a water cost reduction of 31.0 to 41.7% compared to traditional RO systems. Additionally, Delgado et al. [189] designed an innovative renewable energy (RE)-powered seawater reverse osmosis (SWRO) plant based on tidal range and photovoltaic (PV) systems. Their results demonstrate that off-grid SWRO desalination, powered by hybrid tidal/PV systems in favorable locations, can achieve actual water production that is half of the nominal production, contingent upon the appropriate selection of design parameters. After that, they proposed an innovative desalination technology designed for sustainable off-grid systems that leverages the complementary features of tidal range and solar photovoltaic (PV) energies [190]. The results indicate a significant temporary complementarity between tidal and solar PV resources in seawater desalination and recycling applications. Regarding distillation, Zhao et al. [126] developed a novel multi-effect solar distillation unit for seawater desalination that utilizes both solar and tidal energy. A distinctive aspect of this system is its ability to use tidal energy directly to power water supply, drainage, and vacuum extraction processes, thereby significantly reducing costs without the need to convert tidal energy into electrical energy.

3.4.2. Seawater Desalination with Wave Energy

Wave energy encompasses both the kinetic and potential energy of ocean surface waves. Although it possesses a substantial amount of energy, wave energy is considered the most unstable form of ocean energy. Nevertheless, with the rapid advancement of modern industry, wave energy has increasingly been harnessed by humans, particularly in the field of seawater desalination. Currently, the wave energy conversion devices applicable to seawater desalination primarily include oscillating buoys, ducks, oscillating water columns (OWC), water hammer effects, and wave jets [191].

The earliest research on wave energy seawater desalination focused on oscillating buoys. In 1976, the University of Delaware in the United States proposed a seawater desalination system known as DELBOUY. This system utilizes oscillating buoys to drive a piston pump located on the seabed, generating high-pressure seawater that is subsequently directed into an underwater reverse osmosis module for desalination [192]. The Salter duck seawater desalination system employs a vapor compression method, with the desalination equipment housed within the duck itself. This design allows the Salter duck to directly convert wave energy into mechanical energy, thereby powering the internal seawater desalination device. Crerar et al. [193] constructed an experimental platform for a small duck seawater desalination device, demonstrating its feasibility. The OWC-seawater desalination system was first proposed by India’s National Oceanic Development Department. This system initially converts wave energy into electrical energy, which then drives a reverse osmosis desalination device using a motor [194]. It has the capacity to produce between 14.4 and 21.6 m³ of freshwater per day [184].

The above research mainly focuses on the coupling performance of wave energy conversion devices and seawater desalination devices. In recent years, there has been a growing interest in the local components of wave energy desalination. Folley et al. [195,196] developed a numerical model for reverse osmosis (RO) plants that incorporates a pressure exchanger–intensifier, as well as a numerical model for an integrated wave-power and desalination plant. Cheddie et al. [197] introduced a transient one-dimensional model for wave-powered reverse osmosis (WPRO), which is based on finite volume discretization.

3.4.3. Seawater Desalination with Ocean Thermal Energy

Ocean thermal energy, also known as ocean thermal energy conversion (OTEC), refers to the energy harnessed from the temperature difference between the warm surface water, heated by solar energy, and the colder deep water in the ocean. The primary focus of research in ocean thermal energy is on power generation. In 1881, the French physicist Arsonval was the first to propose utilizing the temperature gradient between the warm surface layers and the cold deep layers of tropical oceans. Since then, various attempts have been made to convert this inexhaustible supply of thermal potential energy into mechanical energy and electricity. Lu et al. [198] developed a 50 kW OTEC experimental platform to address power generation needs. Additionally, Hu et al. [199] constructed an absorption–compression refrigeration device that enabled the gradient utilization of ocean thermal energy, thereby enhancing overall economic efficiency.

In recent years, there has been increasing attention on the integration of ocean thermal energy and seawater desalination. Rey et al. [200] proposed an innovative seawater distillation scheme that utilizes surface water and the cold reject stream from an ocean thermal energy conversion (OTEC) cycle. Building on Rey’s research, Yilmaz et al. [201] simulated and suggested the integration of an OTEC-based reverse osmosis desalination unit with a compression cycle. Subsequently, Xiao et al. [202] introduced an integrated OTEC system that combined power generation, refrigeration, and desalination. Soyturk et al. [203] proposed an OTEC-assisted multi-generation process utilizing a transcritical Rankine cycle (tRC) operating with carbon dioxide. Additionally, Ma et al. [204] developed an ocean thermal energy conversion–vacuum membrane distillation (OTEC–VMD) seawater desalination system that fully exploited tropical ocean thermal energy to address freshwater shortages on remote islands without the need for coupling with other heat sources.

In fact, ocean thermal energy represents a stable and abundant form of marine renewable energy that can be utilized for sustainable desalination on remote islands. However, its relatively low available temperature difference limits its application. To address this challenge and effectively harness ocean thermal energy despite its small thermocline temperature gradient, Hu et al. [205] proposed a dual-energy-driven distillation desalination system that utilized both solar and ocean thermal energy. Zhang et al. [206] developed, for the first time, a 1 kW-class solar–ocean thermal energy conversion integrated air-conditioning (S–OTEC/AC) experimental system that was capable of providing both electrical and cooling energy without any pollution. Furthermore, advancements in science and technology have opened up possibilities for the efficient utilization of other sustainable energy sources. Zhao et al. [207] introduced an innovative multi-energy complementary system that integrated seawater freezing desalination, refrigeration, and power-hydrogen-ammonia co-generation, powered by ocean thermal energy, wind energy, and solar energy.

4. Cost, Efficiency, and Energy Consumption of Seawater Desalination Driven by Sustainable Energy

In the last ten years, the share of fossil fuels in the global energy landscape has slowly diminished from 82% in 2013 to 80% in 2023. During this timeframe, the demand for energy has increased by 15%, with sustainable energy sources meeting 40% of this rise. In more developed economies, average energy consumption has fallen by about 0.5% annually over the last decade. Within this category, oil demand reached its peak in 2005, whereas coal has been on a steady decline since 2008. Moreover, natural gas usage has remained stagnant, displaying no overall growth. Nuclear energy has been decreasing at roughly half a percentage point per year, while renewable energy sources have experienced a rise of 3% annually since 2013. In emerging markets and developing nations, which collectively represent almost 85% of the global population, energy demand has surged at an annual rate of about 2.6% over the same period. This increase is fueled by a population growth of over 720 million individuals, a 50% expansion in economic size, and a 40% increase in industrial production. In light of this swift developmental pace, sustainable energy must strive more vigorously to replace oil, gas, and coal in emerging markets and developing nations than in more advanced economies.

The implementation of sustainable energy increases as the overall energy demand growth rate diminishes, leading to a peak in the use of all three fossil fuels prior to 2030 (Figure 16). A notable drop in coal demand enables natural gas to exceed coal’s share in the global energy mix by 2030. From 2023 to 2035, sustainable energy is anticipated to grow at a pace that outstrips the total energy demand. Propelled by a rise in solar photovoltaic and wind energy, clean energy is expected to emerge as the dominant energy source by the mid-2030s.

The global economy’s energy intensity has experienced a decrease attributed to technological progress, enhancements in efficiency, and changes in the economic structure on a global scale (Figure 17). The expansion of renewable energy sources plays a crucial role in boosting the efficiency of energy systems. Traditionally, the growth of gross domestic product (GDP) has outpaced the increase in energy demand, which demonstrates advancements in the energy intensity of GDP. These advancements in energy intensity persist and even intensify in our projections: global GDP maintains its expansion, yet it requires progressively less energy to sustain this growth.

The competitiveness of sustainable energy is rising in comparison to fossil fuels; indeed, in 2023, 81% of new renewable energy capacity is reported to produce electricity at costs lower than those of fossil fuel sources. The rapid rollout of renewable energy technologies continues to inspire innovations, thus establishing a beneficial cycle that enhances production efficiency and lowers costs. In 2023, the most notable reductions in costs were observed in solar photovoltaic (PV), wind, and hydropower sectors. The global average levelized cost of electricity (LCOE) for solar PV experienced a decrease of 12%, while costs associated with offshore wind and hydropower saw reductions of 7%, and onshore wind costs reduced by 3%. Specifically, the global average electricity cost for utility-scale solar PV fell to USD 0.044 per kilowatt-hour (kWh), whereas onshore wind reached USD 0.033 per kWh [176]. In 2023, newly commissioned utility-scale solar photovoltaic (PV), onshore wind, offshore wind, concentrated solar power (CSP), and hydropower all saw a reduction in their global weighted average electricity costs (refer to

Table 2. Total installed cost, capacity factor, and LCOE trends by technology, 2010 and 2023 [208].

	Total Installed Costs			Capacity Factor			Levelised Cost of Electricity
	(2023 USD/kW)			(%)			(2023 USD/kWh)
	2010	2023	Percent Change	2010	2023	Percent Change	2010	2023	Percent Change
Bioenergy	3010	2730	−9%	72	72	0%	0.084	0.072	−14%
Geothermal	3011	4589	52%	87	82	−6%	0.054	0.071	31%
Hydropower	1459	2806	92%	44	53	20%	0.043	0.057	33%
Solar PV	5310	758	−86%	14	16	14%	0.460	0.044	−90%
CSP	10453	6589	−37%	30	55	83%	0.393	0.117	−70%
Onshore wind	2272	1160	−49%	27	36	33%	0.111	0.033	−70%
Offshore wind	5409	2800	−48%	38	41	8%	0.203	0.075	−63%

). From 2022 to 2023, the global weighted average total installed cost for newly launched onshore wind projects dropped by 13%, decreasing from USD 1322.00 per kilowatt (kW) to USD 1154.00 per kW. Simultaneously, during this timeframe, the global weighted average levelized cost of electricity (LCOE) associated with these projects also experienced a decrease of 3%, falling from USD 0.035 per kilowatt-hour (kWh) to USD 0.033 per kWh (see Figure 18).

Both the International Energy Agency (IEA) and the International Renewable Energy Agency (IRENA) have stated that it is necessary to further expand the application of renewable energy. In the past few decades, technological advances have not only reduced the cost of sustainable energy utilization but also greatly improved equipment driven by sustainable energy, with an emphasis on seawater desalination systems. In addition, since seawater desalination is an energy-intensive technology, energy consumption is one of the important indicators for evaluating seawater desalination technology.

Typically, the gain output ratio (GOR) and recovery ratio (RR) are used as indicators to evaluate the efficiency (or performance) of seawater desalination systems. The GOR is the ratio of the latent heat of condensation of the produced freshwater to the total input energy, and it can be expressed as [209]:

$$\mathrm{GOR}={\displaystyle \frac{m_{\mathrm{FW}}\times h_{\mathrm{fg}}}{Q_{\mathrm{in}}}}$$

(1)

where m_FW, h_fg, and Q_in are the mass flow rate of the freshwater, the latent heat of condensation water, and the total input energy, respectively.

The RR is the amount of produced freshwater per unit of feed water, and it can be expressed as [210]:

$$\mathrm{RR}={\displaystyle \frac{m_{\mathrm{FW}}}{m_{\mathrm{SW}}}}$$

(2)

where m_SW is the mass flow rate of the feed water.

In terms of efficiency, many researchers have made significant contributions. Hrari et al. [211] investigated the performance of a concentrated photovoltaic/thermal (CPV/T) system coupled with direct contact membrane distillation (DCMD) for saline water desalination. The results indicated that when the system’s GOR reaches 2.6 or higher, the water outlet temperature from the CPV/T increases, along with the water permeate flux produced by the coupled system. Wu et al. [212] calculated the potential maximum gas-to-oil ratio (GOR) of a single-effect, closed-air open-water (CAOW) water-heated solar humidification–dehumidification (HDH) system. They found that the maximum GOR for a solar HDH desalination system is generally less than 12.5 (370 K), depending on various concentration ratios. Tariq et al. [213] proposed an innovative integrated Maisotsenko cycle-based air saturator designed for use as a humidifier in a humidification–dehumidification-type desalination system. The results indicated that this novel system configuration achieved a 46% higher RR and an 11% higher GOR compared to conventional direct-contact humidifier-based desalination plants.

In terms of cost,

Table 3. Typical costs of conventional seawater desalination.

Cost	MSF	MED	RO
Capital investment costs	1700–2900	1700–2700	1300–2500
Operation and maintenance costs	0.65–1.25	0.67–0.96	0.58–0.88
Total annualized cost	0.84–1.6	1.21–1.59	1.06–1.36

shows the approximate costs of the main desalination technologies. The total annualized cost analysis indicates that capital investment costs are the primary factor influencing the overall cost of the desalination system, whereas operation and maintenance costs have a comparatively minimal impact on the system’s total cost. Compared to other desalination technologies, the capital investment cost, operation and maintenance cost, and total annualized cost of RO are lower, which is also one of the reasons why RO technology is currently the most widely used. Further, Figure 19 shows the specific cost share of the three mainstream seawater desalination systems [214,215]. Obviously, the share of capital investment cost accounts for almost half of the total cost. For MSF and MED, the share of thermal energy cost and electrical energy cost accounts for almost a quarter of the total cost, respectively, while the operation and maintenance cost is almost a fixed value. For RO, the operation of its system is almost independent of thermal energy, and the waste produced during the RO process is another economic consideration, so the operation and maintenance cost accounts for a large share [216].

On the basis of the above analysis, the total cost calculation method of seawater desalination technology (C) is as follows:

For MSF and MED, the total cost can be expressed as:

$$C={\displaystyle \frac{O\&M+I+E_{\mathrm{T}}+E_{\mathrm{E}}}{Q_{\mathrm{distillate}}}}$$

(3)

where O&M, I, E_T, E_E, and Q_distillte are the cost of the operation and maintenance, capital repayment, electrical energy, thermal energy, and the annual water production, respectively. Among these, the capital repayment cost includes the purchase, installation, and transportation of equipment. The operation and maintenance cost includes the chemical cost and labor cost.

For RO and ED, the total cost can be expressed as:

$$C={\displaystyle \frac{O\&M+I+E_{\mathrm{E}}}{Q_{\mathrm{product}}}}+\mathrm{SEC}\times E_{\mathrm{unit}}$$

(4)

where E_unit and Q_product are the unit power price and the annual water production, respectively.

At present, due to the high cost of collecting sustainable resources and the high technical and infrastructure requirements for sustainable energy-driven seawater desalination systems, in-depth development and research is a necessary process to reduce the cost of sustainable energy. It is expected that the cost of sustainable energy will become equal to the cost of traditional fossil fuels over the next 30 years [217,218].

In terms of energy consumption, the specific energy consumption (SEC) can effectively evaluate the energy consumption of the seawater desalination system and is the ratio of the work of the electrical auxiliary equipment to the produced freshwater, and it can be expressed as [210]:

$$\mathrm{SEC}={\displaystyle \frac{W_{\mathrm{elec}}}{m_{\mathrm{FW}}}}$$

(5)

where W_elec is the work of electrical equipment.

In addition, the SEC includes two parts: specific electrical energy consumption (SEEC) and specific thermal energy consumption (STEC). Among these, electric energy is the main energy supply of RO, ED, FD, and MVC technologies, while thermal energy is the main energy supply of MED, MSF, and TVC technologies [44]. Typically, RO technology consumes less energy than other seawater desalination technologies, while MSF and MED consume more energy than other seawater desalination technologies. A detailed SEC of seawater desalination is summarized in

Table 4. SEC of various seawater desalination technologies [44].

Ref.	Desalination Technology	SEEC (kWh/m3)	STEC (kWh/m3)	Total SEC (kWh/m3)
Semiat et al. [219]	MVC	7–12	-	7–12
	TVC	1.6–1.8	14.6	16.2–16.4
Abraham et al. [220]	ED	Salinity < 2500 ppm: 0.7–2.5 2500 < Salinity < 5000 ppm: 2.6–5.5	-	0.7–2.5 2.6–5.5
Shemer et al. [221]	SWRO	2–4	-	2–4
Shahzad et al. [222]	BWRO	1.5–2.5	-	1.5–2.5
Ghaffour et al. [223]	MSF	2.5–4	7.5–12	10–16
	MED	1.2–2	4–7	7.5–12
Mtombeni et al. [224] Babu et al. [225]	FD	8.2–11.9	-	8.2–11.9

.

The relationship between cost, efficiency, and energy consumption of seawater desalination systems driven by sustainable energy is inseparable and complementary. To further illustrate the technical efficiency, cost, and energy consumption of seawater desalination driven by sustainable energy, we investigated a large amount of public literature, as shown in

Table 5. Research work summary of various seawater desalination systems.

Ref.	Technology	Efficiency (%)	Plant Capacity (m3/day)	SEC (kWh/m3)	Cost (USD/m3)
Clarke et al. [226]	PV-RO	RR = 10	With Battery: 0.054 Without battery: 0.047	-	-
Kelley et al. [227]	PV-RO	RR = 9	0.3–0.45	-	-
Mokheimer et al. [228]	PV-RO	-	5	8–20	3.693–3.812
Peñate et al. [229]	PV-RO	RR = 70	49.92	2.3	-
Kumarasamy et al. [230]	PV-RO	-	0.7	-	-
Alsheghri et al. [231]	PV-RO	RR = 40	1344	6.99	0.825
Elasaad et al. [232]	PV-RO	RR = 33	1 (6 h)	-	-
Alghoul et al. [233]	PV-RO	-	5.1 (10 h)	1.1	-
Mostafaeipour et al. [234]	PV-RO	RR = 75	228	0.8	1.96
Ghafoor et al. [235]	PV-RO	-	4 (8 h)	-	4.34
Kettani et al. [236]	PV-RO	-	275,000	4	1
Rahimi et al. [237]	PV-RO	RR = 42	2000	3.4–5.5	0.76
	PV-RO	RR = 42	1000	-	4
Ajiwiguna et al. [238]	PV-RO	-	365,100 (10 h)	2.4	Constant demand: 2.11 Variable demand: 2.73
Sanna et al. [239]	PV-RO	RR = 19	125.04	1.53	-
Ghaithan et al. [240]	PV-RO	-	0.204	2–4	1.72–1.84
Nafey et al. [241]	Solar ORC-RO	-	1166	6.855	0.94
	Solar ORC-RO	-	1166	7.302	0.93
	Solar ORC-RO	-	1166	7.677	0.90
Kosmadakis et al. [242]	Solar ORC-RO	RR = 20	24	77.78	6.38
Delgado et al. [243]	Solar ORC-RO	RR = 45	1000	-	-
Peñate et al. [244]	Solar ORC-RO	RR = 40	2500	2.99	-
Salcedo et al. [245]	Solar ORC-RO	-	50,000	3.53	2.184
Ibarra et al. [246]	Solar ORC-RO	-	28.8	4	-
Xia et al. [247]	Solar ORC-RO	-	2512.12	3.2	-
Geng et al. [248]	Solar ORC-RO	RR = 30	78	-	-
Nihill et al. [249]	Solar ORC-RO	RR = 26	0.03	45.6	-
Igobo et al. [250]	Solar ORC-RO	-	13.3	0.34	-
Mansouri et al. [251]	Solar ORC-RO	RR = 40	2380.8	2.82	-
Geng et al. [252]	Solar ORC-RO	RR = 30	1000	-	-
Bouguecha et al. [253]	FPC-PV-MD	-	-	1609	-
	FPC-PV-MD	-	-	2342	-
Suárez et al. [254]	SGSP-MD	-		2029	-
Suárez et al. [255]	SGSP-MD	-	-	820–940	-
Dow et al. [256]	MD (Waste heat from the exhaust)	-	-	>1500	-
Uday et al. [257]	ETC-MD	-	-	277	80
Kabeel et al. [258]	ETC-MD	GOR = 0.49	-	680	-
Lokare et al. [259]	MD (Exhaust gas from NGCS)	GOR = 1.07 (T = 90 °C)	-	527–565	-
Drioli et al. [260]	MD (Waste heat from flue gas)	-	-	1296.38–2829.17	-
Ma et al. [261]	FPC-MD	GOR = 0.71	-	239	-
Amaya et al. [262]	MD (Ship engine)	GOR = 0.53	-	1189	-
Moore et al. [263]	FPC-PV- Battery-MD	-	-	-	85
Zarzoum et al. [264]	FPC-PV-MD	-	-	-	7.14
Miladi et al. [265]	SC-MD	GOR = 0.98–1.1	-	620–650	-
	SC-MD	-	-	450–520	-
	SC-MD	GOR = 1.3–1.5	-	620	-
	SC-MD	-	-	110	-
Shafieian et al. [266]	MD (Heat pipe ETC)	GOR = 0.77–0.87	-	377–450	18.6
Andrés et al. [267]	FPC-MD	-	-	266	-
Li et al. [268]	ETC-PV-MD	GOR = 0.82	-	279	18.34
Morciano et al. [269]	MD (Waste heat from diesel engine)	-	-	255	-
Bamasag et al. [270]	ETC-MD	GOR = 0.24	-	-	-
Deng et al. [271]	ETC-PV-MD	-	-	80	-
Abdelgaied et al. [272]	ETC-MD	GOR = 1.123–1.25 with PCM	-	705	-
Shafieian et al. [273]	ETC-MD	-	-	308	-
Marni et al. [274]	ETC-MD	GOR > 4.4	-	158.8	14.73
	FPC-MD	-	-	346.5	26.08
Miladi et al. [275]	FPC-MD	GOR = 0.93–1.01	-	671–699	-
Bamasag et al. [276]	ETC-MD	GOR = 0.44	-	-	-
Okati et al. [277]	MD (Geothermal + wastes heat)	GOR = 2.3	-	243.9–447.1	1.31
Soomro et al. [278]	SPT-MD	-	-	1430	0.392
Soomro et al. [279]	LFR-MD	-	-	1750	0.425
Krnac et al. [280]	CPV/T-MD	-	-	245	-
Elminshawy et al. [281]	CPV/T-MD	GOR = 3.33–1.01	-	103	22.48
Chen et al. [282]	CPV/T-MD	-	-	213.2	4.3
Liu et al. [283]	CPV/T-MD	GOR = 0.69	-	-	-
Arfi et al. [284]	MD (PTC + Anaerobic digestion biogas)	GOR = 1.721	-	1.55	0.5–1.45
Wu et al. [212]	HDH (Concentrated solar collector)	GOR < 12.5	-	-	-
Gabrielli et al. [285]	PV/T-HDH	-	-	-	28
Mohamed et al. [286]	HDH (Solar collector)	GOR = 0.86	-	-	12
Zubair et al. [287]	ETC-HDH	GOR = 1.6	-	-	32–38
Rafiei et al. [288]	HDH (PV/T and solar dish concentrator)	GOR = 0.904	-	-	-
Wu et al. [289]	HDH (Fresnel lens solar concentrator)	GOR = 2.1	-	-	-
Xiao et al. [290]	HDH (Fresnel lens solar concentrator)	GOR = 0.71	-	-	27
Wu et al. [291]	HDH (Electric heater)	GOR = 2.65	-	-	2.5
Tariq et al. [213]	STC-HDH	GOR = 0.8	-	-	30
Dave et al. [292]	HDH (Solar humidifier)	GOR = 1.01	-	-	7–35
Mohammadzade et al. [293]	HDH (Solar heater)	GOR = 2.16	-	-	-
Shalaby et al. [294]	HDH (FPC with reflectors)	-	-	-	112
Rahimi et al. [295]	ETC-FPC-HDH	-	-	-	2.1
Said et al. [296]	ETC-FPC-HDH	GOR = 1.54	-	-	11.2
Said et al. [297]	ETC-FPC-HDH	GOR = 1.24	-	-	13.9
Abdel et al. [298]	FPC/ETC/PTC-HDH	GOR = 4.23	-	-	-
Khalaf et al. [299]	FPC/ETC/PTC-HDH	GOR = 1.45	-	-	10.6
He et al. [300]	HDH (PV/T with CPC)	GOR = 1.56	-	-	-
Zhao et al. [301]	STC-HDH	GOR = 1.4	-	-	3.86
Kara et al. [302]	FPC-HDH	GOR = 0.674	-	-	-
Okati et al. [303]	SWC-SAC-HDH	-	-	-	27
Almahmoud et al. [304]	HDH (Ejector cooler + Solar collector)	GOR = 2.76	-	-	-
Anand et al. [305]	CPV/T-HDH (Vapor compression cycle)	-	-	-	130
Jabari et al. [306]	HDH (Stirling engine + Solar dish)	GOR = 2.35	-	-	-
Pourafshar et al. [307]	HDH (Heat pump + PV/T humidifier)	-	-	-	18
Kabeel et al. [308]	HDH (Solar still + Air-water heater)	GOR = 2.75	-	-	8.1
Abdullah et al. [309]	HDH (Wick solar stil + PV + PTC)	GOR = 5.7	-	-	-
Memon et al. [310]	DCMD-FPC-HDH	GOR = 1.3	-	-	-
[208]	Wind-RO	-	0.4	-	-
Moreno et al. [311]	PV-RO	-	0.8–3	15–16.3	-
Herold et al. [312]	Wind-RO	-	7.5	4.24	-
Bilstad et al. [313]	Wind-RO	-	8.5	3.4	0.8–3
Miranda et al. [314]	PV-RO	-	0.3	2.5–4	4.7–6.6
Bilton et al. [315]	PV-RO	-	0.75–1.02	-	-
Soric et al. [316]	Solar-RO	-	6.72	-	-
Manolakos et al. [317]	hybrid wind-PV-RO	-	3	-	-
Weiner et al. [318]	PV-RO	-	12–16.8	5.2–5.8	-
Gökçek et al. [319]	Wind-RO	-	24	4.4	2.96–6.46
Thomson et al. [320]	PV-RO	-	3	3.5	2
Kershman et al. [321]	Hybrid wind-PV-RO	-	300	2.3	5.6
Spyrou et al. [322]	Hybrid-wind-PV-RO	-	3840	2.53	3
Clarke et al. [226]	PV-RO	-	0.047–0.054	-	-
Delgado et al. [323]	ORC-RO	-	2.64	-	-
Nafey et al. [241]	ORC-RO	-	3480	-	-
Bruno et al. [324]	ORC-RO	-	15	-	4.3–9.5
Kumarasamy et al. [230]	PV-RO	-	0.7	-	-
Hossam et al. [325]	Hybrid-wind-PV-RO	-	150–300	1.25–1.6	4.6–7.3
Mokheimer et al. [228]	Hybrid-wind-PV-RO	-	5	3.6–3.8	8–20
Ibarra et al. [246]	SORC-RO	-	28.8	-	-
Kosmadakis et al. [242]	SORC-RO	-	-	-	6.4
Helal et al. [326]	PV-RO	-	20	7.3	7.34
Peñate et al. [244]	SORC-RO	-	2500	-	-
Ali et al. [327]	PV-ED	-	2.8	0.82	15.97
	PV-ED	-	1	1 kWh/kg of salt removing	-
	PV-ED	-	10	-	5.77
	PV-ED	-	200	0.6–1	-
	PV-ED	-	18	0.8	-
	PV-EDR-ED	-	1.14	-	-
	PV-ED	-	1.32	-	-
Ortiz et al. [328]	PV-ED	-	13.7	1.33–1.47	0.19–15.97
Fernandez et al. [329]	PV-ED	-	15	-	-
	PV-ED	-	2.8	-	-
Peñate et al. [330]	PV-EDR-ED	-	-	0.618	-
	PV-ED	-	4.3	-	-
	PV-ED	-	1–200	0.4–4	0.19–15.97
He et al. [331]	PV-EDR-ED	-	6	2.25	1.87–3.33
Gonzalez et al. [332]	PV-ED	-	3	2.16–2.86	-
Malek et al. [333]	Wind-ED	-	10	-	3.36
Tariq et al. [213]	Solar-HDH	-	0.013		-
Rafiei et al. [288]	Solar-HDH	-	0.456		-
Zhao et al. [301]	Solar-HDH	-	1.512		-
He et al. [334]	Geothermal-HDH	-	5.683		-
Rostamzadeh et al. [335]	Geothermal-HDH	-	8.83		-
Rostamzadeh et al. [336]	Geothermal-HDH	-	14.026		-
Karaghouli et al. [337]	Solar-MSF	-	1		1–5
	Solar-MED	-	>5000		2.3–2.8
	PV-RO	-	<100		11.7–15.6
	Wave-RO	-	1000–3000		0.7–1.2
	Wind-RO	-	50–2000		2–5.2
	Wind-MVC	-	<100		5.2–7.8
	Solar-MD	-	0.15–10		10.4–19.5

.

5. Small-Scale Desalination Technologies

Seawater desalination systems can be divided into three categories based on freshwater productivity, namely super large-scale desalination (SLSD) systems, large-scale desalination (LSD) systems, medium-scale desalination (MSD) systems, and small-scale desalination (SSD) systems [338,339]. Typically, systems with a water production capacity greater than 100,000 m³/day belong to an SLSD system, systems with a water production capacity greater than 10,000 m³/day belong to an LSD system, systems with a water production capacity between 1000 and 10,000 m³/day belong to an MSD system, and systems with a water production capacity less than 1000 m³/day belong to an SSD system [340,341].

At present, most of the seawater desalination systems in the world are LSD and MSD systems, while SSD systems account for a small share [342]. In addition, the transportation and configuration costs of LSD and MSD desalination systems are far greater than those of SSD systems, and with the improvement of human living standards and the advancement of science and technology, the application range of SSD systems has gradually increased, especially in households. Due to the constraints of the overall system configuration, the common technologies of SSD systems mainly include RO, MED, HDH, MD, and ED. The advantages of an SSD system are also very obvious. First of all, its volume is small and it is easy to transport and install, and its operation is very convenient. In addition, it has a low-cost and low-energy consumption. More importantly, it can be driven by renewable energy. Therefore, more and more scholars have focused their attention on SSD systems.

Alghoul et al. [233] proposed the use of solar energy to drive RO-SSD and found that the proposed system had lower energy consumption than other seawater desalination systems. Bourouni et al. [343] employed geothermal energy to drive MED-SSD; the results showed that the freshwater production cost of the system was only 1.2 USD/m³. Nakatak et al. [344] and Suchithra et al. [345] used wind energy and tidal energy to drive RO-SSD, respectively. With the improvement of the utilization rate of various sustainable energy sources, mixing various sustainable energy sources to drive seawater desalination systems is a promising solution, which not only improves the utilization rate of energy but also reduces the cost of seawater desalination systems. Mohamed et al. [346] designed a RO-SSD system driven by PV and wind energy; the results showed that the maximum water production capacity of the system reached 9000 L/day. Setiawan et al. investigated a RO-SSD system driven by combined solar–wind–diesel energy and found that the system could produce 5000 L/day of freshwater. In terms of economy, Mousa et al. [347] evaluated the cost of an RO-SSD system driven by a solar–wind combined system; the results showed that the cost of the combined system was only 1.21 USD/m³. More detailed studies on the cost of small-scale seawater desalination systems driven by sustainable energy are shown in Figure 20 [120].

6. Conclusions

Population growth and industrial advancement are the primary drivers behind the depletion of global water resources, which in turn fosters the rapid development of seawater desalination technology. Currently, seawater desalination stands as one of the most prevalent methods for producing freshwater, leveraging abundant marine resources. In recent decades, the efficiency of freshwater production has consistently improved, while the costs associated with seawater desalination technology have continued to decline. Nevertheless, it is important to note that seawater desalination remains an energy-intensive process.

Seawater desalination technology driven by sustainable energy is a promising method to solve the shortage of water resources and energy in remote and arid areas. Contrary to the actual demand, seawater desalination plants are mainly distributed in coastal areas and economically developed areas.

Compared with traditional seawater desalination systems, the cost of seawater desalination systems driven by sustainable energy is higher and its efficiency is lower. Furthermore, the cost of seawater desalination systems driven by multiple sustainable energy sources is higher than that of systems driven by single sustainable energy sources, while the efficiency is just the opposite. The efficiency of desalination systems driven by multiple sustainable energy sources is also lower than that of systems driven by integrated energy.

Solar energy is the most widely utilized and cost-effective sustainable technology for seawater desalination. Additionally, RO is the most widely used technology at present. Without considering the initial investment and wastewater treatment, its operating cost is lower than other sea desalination technologies. In terms of energy consumption, the SEC of RO is almost the lowest among all seawater desalination technologies, while the SEC of MED and MSF is significantly higher than that of other seawater desalination technologies. The energy of TVC, MSF, and MED mainly comes from thermal energy, while the energy of MVC, ED, SWRO, BWRO, and FD mainly comes from electrical energy.

In terms of scale, both the cost and energy consumption of SSD are lower than those of LSD and MSD. Furthermore, the cost of SSD driven by sustainable energy is still higher than that of SSD driven by traditional energy.

In summary, seawater desalination technology driven by sustainable energy represents a promising solution for addressing the freshwater resource shortage and the energy crisis, particularly from the perspectives of environmental protection and sustainable development. By utilizing clean and abundant energy sources, the energy footprint of desalination processes can be significantly reduced, enhancing their environmental sustainability and economic viability.