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Amid the escalating severity of water scarcity, seawater desalination has emerged as a viable solution. Traditional seawater desalination technologies are fraught with numerous drawbacks, including high energy consumption, exorbitant costs, intricate structures, and adverse environmental impacts. In contrast, solar-powered seawater desalination technology has garnered significant attention due to its economic viability and environmental sustainability. This article undertakes a comprehensive study of the solar seawater desalination system, encompassing both direct and indirect methodologies, and delves into the strengths and weaknesses of diverse technologies, such as solar distillation, humidification-dehumidification, solar chimney, reverse osmosis, multi-effect distillation, and multi-stage flash evaporation. The direct approach harnesses solar energy to directly desalinate seawater, whereas the indirect method transforms solar energy into other energy forms for the purpose of seawater desalination. Simultaneously, an in-depth analysis of the benefits of solar-powered seawater desalination technology is conducted. Its economic merits include minimal infrastructure requirements and ease of local design, installation, and maintenance, albeit with a limited water production capacity per unit area. The costs associated with different technologies exhibit substantial variations and are subject to the influence of multiple factors. Future development trajectories are centered around enhancing efficiency, reducing costs, and achieving large-scale applications, exemplified by the exploration of hybrid rare-earth-driven desalination, forward osmosis, and dew point evaporation technologies, as well as the integration of advanced materials, intelligent control systems, and energy storage technologies.
School of Merchant Marine, Shanghai Maritime University, Shanghai, China
Hao Luo
School of Merchant Marine, Shanghai Maritime University, Shanghai, China
*Address all correspondence to: hjxu@shmtu.edu.cn
1. Introduction
Water, as the source of life, is crucial for all life forms on Earth. On the vast Earth, approximately 97.5% of water is in a salty state, meaning that only 2.5% of fresh water can meet human usage needs. For modern society, it plays a vital role in food security, health, housing, energy production, industry, and economic growth. However, with the growth of the global population and economic development, the shortage of water resources has become a major challenge facing the world. Problems such as industrial wastewater discharge, over-exploitation, and environmental pollution have led to an increasingly acute contradiction between supply and demand of freshwater resources. According to the United Nations report [1], the United Nations World Water Development Report points out that global freshwater usage is growing at a rate of just under 1% per year, driven by economic and social development and changes in consumption patterns, including diet. Although agricultural water uses accounts for about 70% of freshwater consumption, industrial (about 20%) and domestic water (about 10%) are the main areas where freshwater demand is increasing [2]. A recent study estimates that nearly 5 billion people live in areas where water security may be threatened. To meet the needs of the global population, there is an urgent need to seek alternative sources of freshwater beyond natural resources.
To address this situation, distillation processes have been used for decades. More specifically, desalination processes are being employed, where saline water is treated to obtain freshwater. Seawater desalination is essentially a process that separates freshwater from brackish or saline water. Some energy is required to run this process. Thermal energy is widely used to operate seawater desalination plants, which is derived from fossil fuels. It is apparent that these processes require a significant amount of energy, and due to high costs, complex structures, a large CO2 footprint, and global warming effects, they are not economically and ecologically viable. Moreover, it is not a permanent energy source. While many seawater desalination plants operate on fossil fuels and produce millions of liters of fresh water from saline or brackish water at the cost of climate change, leading to global warming, energy sustainability is one of today’s most pressing socio-environmental issues. Up to 80% of the world’s energy comes from fossil fuels, which has devastating effects on the environment. In most cases, the ever-growing global energy demand is met by increasing fossil fuel consumption, resulting in acidification, global warming, air pollution, and land degradation. The capacity and efficiency of energy infrastructure, along with the broad availability of affordable energy (especially electricity), are crucial for socio-economic equality [3].
The energy sector is undergoing a crucial transformation from traditional fossil fuels to renewable energy. Renewable energy, with its remarkable advantages such as sustainability and low environmental impact, is gradually becoming the core force in responding to the challenges of the energy crisis and climate change, bringing brand-new development opportunities and directions of transformation for numerous industries. The seawater desalination process, as a key technological field concerning the security and sustainable utilization of global water resources, is also in a transitional stage to meet the growing water demand of human beings. In recent years, various methods for seawater desalination have been developed and tested all over the world. Most of the development projects are concentrated in the Middle Eastern Arab countries, African countries, China, Australia, India, and other regions where freshwater resources are not abundant [4]. Among numerous seawater desalination technologies, solar energy, as one of the most abundant and widely distributed forms of renewable energy, has demonstrated huge application potential and has given birth to the research direction of solar-powered seawater desalination technology, which is a highly promising field. The solar-powered seawater desalination technology aims to utilize solar energy, a clean energy source, and convert seawater into directly usable fresh water through innovative technical means, thus providing a green, efficient, and economically viable solution to the water shortage problems in coastal areas and regions with scarce freshwater resources. Among them, the solar still, as an important part of the solar-powered seawater desalination technology system, relies on its characteristics such as relatively simple structure, convenient operation, and high reliability. In Figure 1, the compound parabolic concentrator tubular solar still. Through the concentrating effect of the compound parabolic concentrator (CPC), more solar energy can be gathered into seawater, increasing the temperature of the seawater and accelerating the evaporation process. As a result, the conversion efficiency from solar energy to thermal energy is improved and solar energy resources are utilized more effectively. Its structural design enables the processes of seawater evaporation and water vapor condensation to be carried out in a relatively stable environment. The design of the concentric tubes is conducive to the transmission and condensation of water vapor, which can continuously and stably produce fresh water. In the future, more solar desalination technologies are expected to be more widely applied.
Figure 1.
Compound parabolic concentrator tubular solar still [5].
This book conducts research and analysis on solar desalination systems (including both indirect and direct methods) and explores novel and traditional desalination technologies. It discusses the advantages and disadvantages of various solar desalination methods based on the economic efficiency and technical data of most plants. This article is divided into six parts: Chapter One is the introduction, Chapter Two introduces the classification of solar desalination systems, Chapter Three analyzes the technical advantages of solar desalination, Chapter Four discusses the degree of commercialization of solar desalination technologies, Chapter Five provides future prospects, and Chapter Six is the summary.
The basic principle of seawater desalination is to remove salt and other impurities from seawater, converting it into fresh water suitable for drinking or industrial use. This process mainly relies on physical and chemical changes, covering a variety of technologies and methods. The most common desalination techniques include reverse osmosis, distillation, electrolysis, and membrane filtration. Reverse osmosis is one of the most widely used desalination technologies. It uses a semipermeable membrane to apply pressure to seawater, allowing water molecules to pass through while blocking larger salt ions and impurities. The efficiency of reverse osmosis depends on factors like membrane pore size, material, and applied pressure. Due to its relatively low energy consumption and high freshwater yield, reverse osmosis dominates many desalination plants. Another common technique is distillation, where seawater is heated to create steam, which is then condensed back into freshwater. Traditional distillation methods are effective but energy intensive. Modern versions, like multi-effect distillation (MED) and multi-stage flash distillation (MSF), improve efficiency by evaporating and condensing water in multiple stages, significantly increasing the output of fresh water while reducing energy consumption. Electrolysis uses electrical currents to cause a chemical reaction in seawater, separating it into hydrogen, oxygen, and leaving behind freshwater. Although it has been demonstrated in laboratory settings, its high energy demands and equipment costs prevent widespread commercial use. Membrane filtration removes most salt and impurities by physical filtration, employing techniques such as microfiltration, ultrafiltration, and nanofiltration to precisely separate components in seawater. However, membrane technology faces challenges like membrane fouling and limited lifespan. In addition to these technologies, there is growing interest in renewable energy-powered methods such as solar distillation and air evaporation. These methods harness natural solar energy or air humidity to extract fresh water through evaporation and condensation processes, offering environmentally friendly and sustainable alternatives. In summary, seawater desalination works by utilizing various physical and chemical processes to continuously remove salt and impurities, ultimately converting seawater into freshwater [6].
2.2 Solar desalination system
Precipitation is the primary source of freshwater globally, and the natural hydrological cycle that leads to precipitation can be seen as the foundational model for solar seawater desalination. Both academic research and industry practice have shown that thermal desalination processes are more suitable for large-scale applications compared to mechanical desalination processes. Solar energy is particularly well-suited for driving thermal desalination processes because it can be directly applied without any energy conversion losses. Moreover, existing thermal desalination installations can be easily retrofitted to accommodate solar energy. This makes solar energy an excellent choice for powering these processes, as shown in Figure 2 [7].
Figure 2.
Working principle of solar desalination system [7].
Solar energy can be directly converted or otherwise utilized for desalination. There are two types of collection systems: direct desalination and indirect desalination in Figure 2. Direct desalination systems use solar energy to produce distilled water right in the container of the solar collector itself. Furthermore, indirect systems combine solar collection with traditional seawater desalination methods. Solar energy generates heat for desalination, providing power to desalination plants. In resource-scarce areas, like deserts or places with brackish water, renewable energy obtained from oil and gas is used for desalinating seawater [8].
2.2.1 Direct desalination system
2.2.1.1 Solar distillation
Solar distillers are eco-friendly, using solar power to desalinate brine instead of fossil fuels. However, the heat and flux from the sun can be a bit unpredictable. There are mainly two types of solar distillers: passive and active. The passive solar distiller is a cost-effective way to get drinking water or distilled water. With heat transfer, the raw water evaporates, leaving behind sediment and salt deposits in the basin; when the vapor moves to the relatively cooler glass cover, it releases latent heat and turns back into pure liquid through condensation. One downside of passive solar distillation is that it does not produce enough fresh water per square meter. An active solar distiller has a glass cover, a basin, a reflector, and a pump. The glass cover, flat reflector, and basin insulation have different thicknesses. Sunlight keeps hitting the reflector, heating up the brackish water. This preheated water then goes into the distiller. As the sun’s rays hit, the water evaporates and collects on the glass cover, as shown in Figure 3 [9]. In this system, the water to be distilled is introduced into the distillation device through a pipeline from the feed water tank. When sunlight penetrates the inclined glass cover, the energy it carries is absorbed by the water in the distillation device, causing the temperature of the water to gradually increase. Water molecules gain enough energy to overcome the intermolecular forces of the liquid state and then transform into a gaseous state to form water vapor. During the upward movement of these water vapors, they encounter the relatively low-temperature inner surface of the glass cover. Due to the temperature difference between the glass cover and the external environment, condensation will occur on its surface, and the water vapor will be transformed back into liquid water to form condensed water droplets. Under the action of gravity, these water droplets slide down along the glass cover onto the internally inclined panel and finally flow into the distilled water collection container along the panel. Thus, the separation process from raw water to distilled water is completed. Impurities in the raw water are left at the bottom of the distillation device, realizing the purification and distillation of water. Despite the numerous merits of the solar direct distillation method, it is still confronted with a series of formidable challenges that urgently demand solutions in practical applications. On the one hand, the issue regarding its efficiency is rather conspicuous. Both the energy conversion efficiency and the evaporation-condensation efficiency leave much to be desired. A substantial amount of solar radiation energy is dissipated due to factors like reflection and heat dissipation. Furthermore, there exist efficiency bottlenecks in the steam condensation process, which consequently leads to a relatively low proportion of effective energy actually harnessed for water evaporation. On the other hand, this method entails a relatively large footprint. The meager water production hinges on extensive spatial requirements, and the system layout is rather dispersed. In the current context where land resources are constrained, this significantly curtails its large-scale dissemination. Simultaneously, climatic conditions also exert a pronounced impact on it. During overcast days, at night, or under extreme weather circumstances, the instability or absence of solar radiation will trigger a sharp decline in water production and may even inflict damage on the system, thereby severely undermining the stability and continuity of its operation. In addition, the vulnerability of water quality to contamination cannot be overlooked. Whether it be the leachates from the materials of the distiller or the microorganisms proliferating within, they will all give rise to the deterioration of water quality, rendering it difficult to meet potable water standards. Nevertheless, there are viable strategies to tackle these challenges. To enhance the efficiency, we can commence with optimizing the materials and structural design. Incorporating novel light-absorbing materials such as nano-coatings and refining the steam channels, while simultaneously integrating auxiliary energy sources or energy storage devices, can ensure the continuous and efficient progress of the distillation process [10]. With respect to reducing the occupied area, we should vigorously explore and develop high-efficiency and compact multi-layer or folding distillers and embrace the design concepts of integration and modularization to augment the freshwater yield per unit area. To cope with the limitations imposed by climatic conditions, we can combine energy storage technologies with backup energy systems and fortify the system’s resilience against disasters to guarantee its stable operation under diverse weather conditions. For the matter of safeguarding water quality, it is imperative to select appropriate nonpolluting materials and reinforce the procedures of water quality monitoring and disinfection treatment, thereby ensuring the quality and safety of the produced freshwater. Through the comprehensive implementation of these countermeasures, it is anticipated that the current predicaments faced by the solar direct distillation method can be surmounted, facilitating its extensive application and long-term development in the domain of water resources.
Figure 3.
Simple solar direct distillation.
2.2.1.2 Humidification-dehumidification process
The Humidification-Dehumidification (HD) process is based on the principle that air can mix with a large amount of water vapor. In this process, flowing air passes close to saline water, where it absorbs a certain amount of vapor from the air, inducing cooling. Conversely, distilled water can be recovered by bringing humid air into contact with a cold surface, causing condensation. HD technology is more suitable for desalination applications where water demand is not concentrated. The advantages of this method include reliable capacity, ease of use, and economical operating costs [11]. The principle of the humidification and dehumidification process is well reflected in Figure 4. The system first initiates the heating process. The inlet hot water is introduced into the heater and undergoes efficient heat exchange with the heating steam. The heat of the steam is accurately transferred to the inlet hot water, causing its temperature to increase significantly. The hot water that has completed the heating task is then transformed into outlet cold water and flows out, while the thermal energy of the heating steam is fully utilized to preheat for the subsequent seawater desalination process. Next comes the humidification stage. The preheated inlet seawater flows into the humidifier, and at the same time, the outside air is drawn in. Inside the humidifier, the special packing materials greatly increase the contact area and contact time between the seawater and the air, enabling the seawater to evaporate fully. The water vapor is incorporated into the air, resulting in a substantial increase in humidity, while the seawater is concentrated into brine and discharged. The high-humidity air rich in water vapor then enters the condenser. Through the effect of the temperature difference with the external environment, the condenser rapidly cools the air. The water vapor in it condenses into liquid condensate on the inner surface of the condenser when encountering the cold. These condensates are efficiently collected, and the dehumidified air is finally discharged from the system. Simultaneously, the intermittent and unstable nature of solar energy poses a challenge in maintaining a consistent and steady power supply for seawater desalination systems. To address this, state-of-the-art energy storage devices can be deployed, such as intelligent battery management systems and highly efficient thermal storage facilities, which are capable of storing surplus energy to ensure uninterrupted operation. The issues of scaling and corrosion on equipment significantly impact their service life and operational efficiency. Opting for novel anti-corrosive and anti-scaling materials, in conjunction with regular physical and chemical cleaning and maintenance procedures, can effectively mitigate these problems. However, the high equipment costs have limited its wide application. We often choose to optimize production technologies and develop low-cost yet high-performance components, such as solar collectors with enhanced absorption capabilities and advanced humidification and dehumidification devices [13].
Figure 4.
Conventional humidification-dehumidification desalination process [12].
2.2.1.3 Solar chimneys
The materials for solar chimneys are usually readily available, and they require little maintenance, as well as no fuel or cooling water. However, solar chimneys are vulnerable to environmental issues such as dust or abrasion. In Figure 5, a key disadvantage of solar chimneys is the low efficiency of converting solar thermal energy into electricity [14]. Consequently, it is advisable to implement a high-efficiency air filtration system at the ingress of the chimney. Specifically, a combination of multi-layered electrostatic precipitator filters and coarse filters can be employed, which is capable of effectively impeding the ingress of dust particles and mitigating the abrasion and erosion on the inner surface of the chimney. Simultaneously, novel ceramic coating materials with excellent wear and corrosion resistance properties can be utilized on the inner wall. These materials not only augment the wear resistance but also confer protection against the corrosive effects of salts present in the seawater evaporation environment, thereby extending the operational lifespan of the chimney.
Countries with a per capita annual water consumption of 1000–1700 cubic meters are considered water-scarce. Excessive consumption by each citizen or individual will exacerbate the water crisis, worsening water-related problems. Reverse osmosis seawater desalination plants use seawater as their source and increase water pressure through electric-powered pumps. The water travels through pipes with semipermeable membranes inside. These membranes block the passage of salt, allowing the water to move forward. The water is split into two parts in Figure 6: one with high salt concentration and the other with low salt concentration, referred to as the permeate or product water. In reverse osmosis, the membrane is the most crucial element and must have the following characteristics: (1) It must withstand operational conditions; (2) It must reject a high percentage of salt to provide high-quality output; (3) It should have sufficient permeability to maintain a high flow of supplied water [16]. Meanwhile, we can organically combine solar energy with reverse osmosis, such as the solar photovoltaic-reverse osmosis technology [17]. It integrates the advantages of solar power generation and reverse osmosis membrane separation, demonstrating a unique working principle. Solar photovoltaic panels convert solar light energy into electrical energy through the photoelectric effect, providing power support for the high-pressure pumps, control systems and so on in the reverse osmosis system. During the reverse osmosis process, seawater undergoes pretreatment to remove impurities first and then is pressurized by the high-pressure pump to a pressure higher than its osmotic pressure, forcing water molecules to pass through the semipermeable membrane to form fresh water, while impurities such as salts are retained and discharged. However, this technology faces numerous challenges. The intermittence and instability of solar energy lead to fluctuations in power supply, which affects the continuous operation of the system. To address this, high-efficiency energy storage devices can be equipped, such as intelligent battery management systems and new heat storage equipment, to store excess energy and ensure a stable energy supply. The high cost of equipment limits its wide application. The manufacturing costs of solar photovoltaic panels and reverse osmosis equipment should be reduced through technological innovation and large-scale production. Meanwhile, the system design should be optimized to improve the cost-performance ratio. In addition, the salt scale and microorganisms in seawater are likely to cause blockage and pollution of the reverse osmosis membrane, reducing the membrane’s lifespan and desalination efficiency. Therefore, new anti-fouling and anti-scaling membrane materials can be adopted.
The multi-effect distillation seawater desalination technology is based on the principle of multiple utilization of steam, ingeniously achieving efficient desalination of seawater. In the process shown in Figure 7, seawater first enters the preheater and is preheated by utilizing the latent heat of condensation of the steam generated in each effect. Then it enters the first-effect evaporator. An external heat source (such as steam or waste heat) makes the seawater boil and evaporate in the first-effect evaporator. The secondary steam generated enters the second-effect evaporator as a heating heat source, and so on. The subsequent effect evaporators use the secondary steam generated by the previous effect to carry out evaporation operations one by one. The pressure and boiling point in each effect evaporator gradually decrease, thus realizing the multiple utilization of steam and greatly improving the energy utilization efficiency. Finally, fresh water is obtained through condensation. However, there are also some problems at the same time. The problem of equipment scaling is rather prominent. Calcium, magnesium and other ions in seawater are prone to precipitate and form scale layers during the evaporation process, which adhere to the heat exchange surfaces of the evaporators, reducing the heat transfer efficiency, increasing energy consumption and even possibly causing pipeline blockages. To overcome this problem, pretreatment techniques can be adopted, such as adding scale inhibitors and conducting ion exchange to soften the seawater. Meanwhile, regular chemical cleaning and physical flushing are carried out to ensure the cleanliness of the heat exchange surfaces. Then, the initial investment cost of the multi-effect distillation system is relatively high, including the purchase and installation costs of equipment such as evaporators, condensers, vacuum pumps, as well as the costs of factory building and so on, which restricts its promotion and application to a certain extent. By optimizing the system design and adopting modular and standardized production methods to improve the integration and manufacturing process level of the equipment, it is expected to reduce the equipment cost [19].
The desalination technology continuously evaporates seawater in multiple flash chambers under different pressures using the thermal energy provided by solar power in Figure 8. In this process, preheated seawater rapidly evaporates (flashes) in a low-pressure environment. The resulting steam condenses into freshwater in a condenser, and the recovered heat is used for subsequent evaporation. Multi-stage flash technology works through cycles of evaporation and condensation. Solar-powered multi-stage flash plants can produce 6–60 liters of water per square meter per day. Heated seawater is introduced into a low-pressure space, where the ambient pressure is lower than the saturated vapor pressure corresponding to the temperature of the heated seawater. As a result, the seawater rapidly partially evaporates to produce steam, which is then condensed into freshwater [20]. At present, this particular technology is beset by the disadvantage of a relatively elevated energy consumption level. The issues of equipment scaling and corrosion have significantly eroded stability and curtailed the service life of the overall system. The initial investment outlay for the multi-stage flash system is exorbitantly high, encompassing a wide spectrum of expenditures from the acquisition of large-scale and intricate equipment, and the establishment of manufacturing facilities, to the painstaking installation of pipelines. In order to surmount these impediments, the adoption of a new generation of high-efficiency heat exchangers is warranted. This would enhance the heat recovery efficiency and mitigate heat dissipation. Concurrently, the pretreatment procedures for seawater ought to be intensified. By leveraging advanced methodologies such as filtration, ion exchange, and the addition of chemical reagents, the removal of deleterious impurities can be optimized to a greater extent.
2.2.2.4 Analysis of different solar desalination technologies
Approximately 61% of the world’s population has access to drinking water. However, in rural areas, drinking water is likely to be contaminated. Using large reservoirs to purify water often comes at the cost of harming nature. In terms of solar-powered methods, eco-friendly water purification is cost-effective, attractive, and essentially trouble-free. Solar desalination is a unique process that uses freely available solar energy as a heating medium. Desalination of seawater can only become a more popular freshwater supply option without significantly reducing energy and costs. Therefore, in places where water resources are scarce and water costs are high, considering the feasibility of desalination is most appropriate. Moreover, higher water temperatures and significantly lower salinity can greatly reduce the energy intensity of membrane desalination. However, desalination is not a magic solution to water supply problems; it does not address poor sanitation, weak management systems, or insufficient distribution.
Dai and Zhang [21] conducted a solar desalination experiment related to the humidification-dehumidification process at Northwestern Polytechnical University. They found that the system’s output is highly correlated with the temperature of the brine entering the humidifier, the mass flow rate of the brine, and the mass flow rate of process air. The thermal efficiency of the device exceeded 80%. As a result, waste heat can be reused to further enhance the desalination process. This system can be used for seawater desalination.
Kabeel and El-Said [22] studied a hybrid solar desalination device, which includes a humidification-dehumidification (HD) unit and a single-stage flash device. Their main goal was to investigate this system. The results showed that the device has significant operational flexibility between air HD and flash desalination. Experimental results indicated a considerable increase in the production efficiency of the HDH unit.
Joseph et al. [23] conducted a study on a single-stage solar desalination system. The goal of this study was to produce 10 liters of drinking water per day. The results showed that the water quality was acceptable and within tolerance limits. Additionally, the study indicated that the device produced approximately 8.5 liters of water per day, which is three times the output of a typical solar distiller.
In summary, desalination can be roughly divided into direct methods and indirect methods, as shown in Figure 9. Direct methods include solar distillation, humidification and dehumidification, and solar chimney technologies, which utilize solar radiation to desalinate seawater directly. Solar distillation is an ancient and effective method that involves heating seawater using solar energy, causing it to evaporate and then condense into fresh water. The humidification and dehumidification method extracts fresh water by controlling the humidity of air. The solar chimney creates an updraft to promote water evaporation and condensation into fresh water. Furthermore, indirect methods convert solar energy into other forms of energy to desalinate seawater, including reverse osmosis, multi-effect distillation, and multi-stage flash distillation technologies. Reverse osmosis employs solar energy to drive pump systems, pushing seawater through semipermeable membranes to remove salt and impurities. Multi-effect distillation and multi-stage flash distillation utilize the heat generated by solar energy to extract fresh water through multiple heating and condensation processes.
The main economic advantages of a solar-powered seawater desalination system are as follows: (1) It does not require much infrastructure. (2) It is easy to design, install, operate, and maintain locally. Furthermore, its main economic disadvantage is the limitation on unit size due to the large area required (about 250 m2 per 1 m3 of freshwater). Therefore, based on different solar desalination methods, the goal is to study the cost of producing 1 liter or 1 cubic meter of freshwater, as well as to analyze the economic benefits by calculating the proportion of each cost in the total cost. However, the feasibility and cost-effectiveness of desalination plants depend on the specific location and various other factors (Table 1) [31].
Type of technology
Type of water supply
Specificities
Cutting edge
Quote
1
New Heat Absorber Triangle Solar Distiller
Sea water
By using a new form of trapezoidal absorber to reduce the effect of less shadowing produced by the sloping side walls and volume of seawater
Up to half the efficiency compared to plate-constructed stills
A new renewable energy source combined with solar energy-driven reverse osmosis desalination with zero liquid discharge
Seawater/saltwater
The saline waste from the RO (Reverse Osmosis) process is remixed with water from the evaporator to reduce the salinity of the RO feed. The hydrogen produced in the electrolytic cell is burned, and its energy is stored in cement blocks. The evaporator operates using solar energy during sunny hours of the day. Then, the energy stored in the cement blocks replaces solar energy during the remaining hours of the day.
Using nanofluids for volumetric heating in overall seawater heating can generate localized high temperatures around the metal nanoparticles within the fluid, increasing the heating rate and enhancing the evaporation process
The dispersed metal or carbon nanoparticles in the liquid absorb sunlight and generate steam without needing to heat the entire fluid volume
The recycling of waste heat promotes solar desalination.
Seawater
Integration of solar-heated HDH cycle with diesel engine exhaust gases and discarded coolant flow
To optimize the freshwater production rate, recovery rate, specific entropy generation, and gained output ratio of a seawater desalination system while minimizing the waste heat discharged into the environment.
An integrated system based on solar chimneys and wind energy for hybrid desalination through reverse osmosis and multi-stage flash distillation with brine recovery.
Seawater
Using cascade desalination with thermal desalination and membrane desalination technologies, the heat stored in a solar chimney is utilized to produce fresh water. Additionally, a pressure-retarded osmosis subsystem is integrated to enhance desalination
Compared to traditional multi-stage flash distillation, it achieves more efficient energy use and higher freshwater production
Analysis chart of different solar desalination technologies.
Hamed [32] believes that the cost of seawater desalination varies depending on the desalination technology used and the water supply demand. Since most thermal desalination plants use coal and oil, the unit cost of desalinated water is estimated to be quite high. The cost of desalinating seawater ranges from $0.521 to $1.02 per cubic meter, while multi-effect distillation (MED) plants have a high capacity of over 91,000 m3/day. For MED plants with a daily output of 12,000 m3 to 50,000 m3, the water cost ranges from $0.960 to $1.5/m3, whereas for MED plants with a capacity of less than 100 m3/day, the unit cost per cubic meter of water ranges from $2 to $10/m3 (Table 2).
Desalination methods
Capital cost range
Cost composition factors
Cost range and characteristics of per liter of water.
Indirect solar still
The capital cost ranges from 2.52 to 282 dollars. There is a large difference among different types (for example, the single-basin solar still has a lower cost while the sun-tracking solar still has a higher cost).
Mainly equipment and material costs (such as glass, metal plates, etc.), and a small amount of maintenance costs (regular cleaning and water replenishment).
Passive CPL (Cost Per Liter) does not exceed 0.1667 US dollars. The lowest is 0.0074 US dollars for a single-effect tubular solar still.
Direct solar still
The capital cost is between 160 and 1174 dollars. The higher capital cost mainly stems from additional energy equipment (such as flat plate collectors, vacuum tube collectors).
Equipment and collector costs, maintenance costs (involving the maintenance of additional equipment), and energy-related costs.
Active CPL does not exceed 0.2696 US dollars. The active type has a minimum of 0.0066 US dollars for the point-focusing elliptical solar still.
Table 2.
Comparison of cost situations of different seawater desalination technologies [33].
According to the research by Karagiannis et al. [34], the Total Dissolved Solids (TDS) in brackish and freshwater rise between 2000.0 and 10,000 ppm, affecting the cost of water supply. For instance, brackish water in Jordan containing 2300.0 ppm has a cost of $0.26 per cubic meter, while water with 5000 ppm has a cost of $0.27 per cubic meter. Therefore, in two equivalent systems, the volume of water depends on the total TDS. When using sustainable sources, the cost of freshwater treatment is significantly higher. In some cases, it can exceed $10.3 per cubic meter, which is even more expensive than some of the priciest energy supply options. However, when desalinating seawater (SW) or brackish water (BW) using conventional energy sources, the costs are much lower (Table 3).
Typology
Renewable energy
Cost of production ($/m3)
Seawater
Conventional fuel Tidal Photovoltaic cell Solar collector
0.46–3.5 1.3–6.5 4.08–11.7 4.5–10.4
Saltwater
Conventional fuel Photovoltaic cell
0.27–1.38 5.8–13.42 2.61
Table 3.
Water production cost (US$/m3) for Ocean water and brackish water by utilizing a variety of sources of energy [35].
4. Commercialization development of solar thermal desalination
4.1 Direct solar desalination
Direct solar desalination technology has the advantages of relatively simple systems and low operation and maintenance costs and has good commercial application potential in some small and decentralized freshwater demand scenarios. For example, in some remote islands, coastal small villages, and other areas, small direct solar distillation devices can meet the basic domestic water needs of local residents. Their low costs and simple operation methods make these areas more likely to accept and adopt them. However, there are also some limitations. For instance, the energy conversion efficiency is relatively low. In large-scale desalination projects, a larger heat collection area is required to meet the water production requirements, which increases the floor space occupied by the equipment and the initial investment costs. Moreover, water production efficiency is greatly affected by weather conditions. In cases of insufficient sunlight such as cloudy and rainy days, the water production volume will drop significantly, resulting in insufficient stability of the water supply. Currently, there are already some practical application cases of direct solar desalination technology worldwide, but the overall scale is relatively small. In the coastal areas and islands of some developing countries, small solar distillation devices are providing domestic water for local residents [36].
4.2 Indirect solar desalination
Indirect solar desalination technology has been applied in some specific regions and scenarios. For example, in some countries in the Middle East, due to their abundant solar energy resources and the urgent need for fresh water, indirect solar desalination technology has been used to build medium-sized desalination plants.
Some of the technologies have achieved commercial production. For instance, membrane distillation technology, which is a common type of indirect solar desalination technology, has seen the development of some commercial modules. The plate-and-frame module constructed by Scarab AB, which consists of 10 cassette structures, has a certain commercial application value [37]. However, traditional reverse osmosis desalination technology and distillation desalination technology are more mature in large-scale commercial applications and occupy a relatively large share of the global desalination market. Indirect solar desalination technology has problems such as high equipment costs, relatively low energy conversion efficiency, and complex system maintenance, which lead to higher costs for desalinated water. As a result, it is at a disadvantage in commercial competition and restricts the speed of its large-scale commercial transformation.
4.3 Specific solar desalination technologies
The commercial applications of solar stills are mostly focused on small-scale projects. For example, in some remote islands, small communities in coastal arid areas, and special places like ships, direct solar stills are used to meet the basic freshwater needs of local residents or specific users. The daily water production varies from several hundred liters to thousands of liters [38]. For instance, some islands in Greece once adopted direct solar stills to provide domestic water for local residents. Its initial investment cost is relatively low. Especially for small and simple distillation devices, the costs of equipment procurement and installation are not high. For example, The equipment costs of some small solar tray stills may range from several hundred to several thousand dollars [39].
Solar photovoltaic-reverse osmosis, relatively low energy consumption, and high fresh water production make it a widely used desalination technology. However, the equipment cost is high. When solar energy is combined with reverse osmosis, the intermittency and instability of solar energy will affect the continuous operation of the system. Commercial feasibility can be improved by equipping high-efficiency energy storage equipment, reducing the manufacturing cost of solar photovoltaic panels and reverse osmosis equipment, optimizing system design, and adopting new anti-fouling and anti-scaling membrane materials. It is suitable for fresh water demands of different scales, especially for medium to large-scale applications.
The multi-effect distillation (MED) desalination technology has been applied to a certain extent worldwide, especially in regions with scarce water resources and abundant energy, such as the Middle East and North Africa, including countries like Saudi Arabia and the United Arab Emirates. However, in the global desalination market, the market share of MED technology is relatively limited. The high equipment investment and operating costs of MED technology have restricted its promotion and application in some areas. According to relevant data, in the global desalination market, the market share of multi-effect distillation (MED) technology is rather small, while the market share of the reverse osmosis (RO) technology exceeds 50% [40].
Multi-stage flash (MSF) technology is prevalently employed in large-scale seawater desalination endeavors and has witnessed extensive utilization in the Middle East region [41]. In locales such as Abu Dhabi, United Arab Emirates, expansive multi-stage flash seawater desalination and cogeneration systems have been erected, with the capacity of a single unit reaching up to 57,700 cubic meters per diem. Likewise, in Saudi Arabia and other nations, there exist numerous multi-stage flash seawater desalination plants capable of yielding hundreds of thousands of tons of fresh water daily [42], furnishing copious amounts of fresh water for urban water supply and industrial water demands. Nevertheless, the technology is encumbered by exorbitant construction costs. This is attributable to the requisite high-pressure apparatuses and corrosion-resistant materials, which incur substantial expenditures in equipment fabrication, procurement, and material acquisition. The operation and maintenance costs are also prohibitive, encompassing energy consumption, equipment upkeep, and chemical agent expenses. The multi-stage flash process is energy intensive and frequently necessitates integration with thermal power plants. Equipment maintenance demands the engagement of specialized personnel and significant financial outlays, thereby circumscribing the ambit of its commercial dissemination.
The application of solar energy to desalination processes shows potential for further growth in terms of improving performance, saving energy, or reducing costs. In particular, the use of hybrid renewable energy systems to promote desalination is of great interest to the research community [43].
5.1.1 Reverse oasis with photovoltaic and wind energy
The advantage of a hybrid PV-wind system is that each source can compensate for the lack of availability of the other. This is relevant considering the variable nature of both and ensures more continuous energy availability. So far, there have been some studies that have optimized hybrid solar PV systems-wind power systems, taking into account variables such as water availability, load, location of solar wind farms, and their size [44]. In the only experimental study conducted in 2001 that combined photovoltaic-wind energy with desalination, Weiner et al. experimentally designed and operated a small, stand-alone brackish water reverse osmosis (BWRO) plant that combined solar photovoltaic and wind energy. Mokheimer et al. [45] simulated a small-scale hybrid wind/solar PV RO desalination system with a production capacity of 5 m3/day. They found that the cost of a hybrid powertrain could be reduced if multiple wind turbines were used. While the efficiency of the entire system depends on feed salinity, this also determines the load demand. Similarly, Alberman and ElAmin investigated solar PV/wind/diesel hybrids in remote areas of Saudi Arabia. They found that despite the progress made in reducing PV costs over the past few decades, only diesel systems are the most cost-efficient. This shows that there is still untapped potential for renewables-driven desalination.
5.2 Other desalination processes
5.2.1 Positive infiltration
Forward osmosis is a salinity gradient-driven or gravity-driven desalination process in which the differential pressure across the semipermeable membrane results in selective water transport. Due to its ability to naturally diffuse water from the feed into a higher concentration solution, it does not require hydraulics, making it a low energy alternative to RO desalination. As a result, it has the potential to significantly reduce the energy costs associated with desalination. Recently there have been several FO units, most of them in China, and one of them has a capacity of 2800 m3/day, the highest capacity of any FO unit installed to date. Khaydarov [46] introduced the concept of solar FO in 2007, using various fluids of a solar cell system, a solar heat exchanger, a pretreatment unit, and FO equipment. Razmjou et al. [47] investigated the feasibility of bilayer polymer hydrogels as extractants for FOs using solar energy concentration. The hydrogel consists of an absorbent layer that provides osmotic pressure and a dehydrated layer that absorbs water during the release of FO in Figure 10.
The future trends in solar seawater desalination include a shift toward less-explored desalination technologies. Dew point evaporation is one such technique with the potential for solar desalination. In dew point evaporation, saturated vapor is used as a carrier gas to evaporate water from the saline feed as distillate. This process offers the advantage of energy reuse, depending on the use of heat exchangers and improved process design [48].
Solar-powered desalination is a rapidly growing research area that has made significant strides in recent years. The increasing desalination capacity, combined with the competing demands for decarbonization and mitigating the adverse effects of global warming, has driven efforts to use renewable energy for desalination. Solar energy, in particular, is an attractive option because of its potential to power desalination, especially in regions where freshwater scarcity and solar radiation coexist.
The interest in direct solar desalination has surged due to advancements in novel photothermal materials. Graphene-based and metal/ceramic nanostructures have facilitated evaporation through localized heating. Current leading solar desalination technologies include solar distillation, reverse osmosis, multi-stage flash distillation, multi-effect distillation, and solar chimneys. Each of these technologies has its own advantages and disadvantages, tailored to different scales and demand scenarios.
Solar distillation is suitable for small-scale, distributed systems and is especially useful in remote, resource-scarce regions. On the other hand, reverse osmosis, with its higher energy efficiency and water production capacity, is more appropriate for medium to large-scale applications. Multi-stage flash and multi-effect distillation technologies, known for their efficient energy usage, are typically employed in large industrial facilities.
From a cost-effectiveness perspective, while the initial investment for solar desalination technologies is relatively high, ongoing technological advancements and the promotion of large-scale applications have gradually reduced the cost per unit of water produced, with operational costs becoming more manageable. In regions rich in solar resources, these technologies demonstrate strong economic feasibility. Additionally, as global attention on low-carbon economies and renewable energy grows, solar desalination technologies are expected to receive more policy support, further lowering economic barriers.
In terms of environmental and social benefits, solar desalination technologies can significantly reduce greenhouse gas emissions compared to traditional fossil fuel-powered desalination methods, thus minimizing environmental impact. Moreover, these technologies can effectively alleviate water scarcity in many regions, particularly in arid and semi-arid areas, where their social benefits are especially pronounced. By providing a stable supply of freshwater, solar desalination technologies can improve the life quality of residents and support regional economic development.
Future developments will focus on improving efficiency, reducing costs, and achieving large-scale applications. Researchers are exploring the integration of advanced materials, smart control systems, and energy storage technologies with existing solar desalination systems to enhance efficiency and system reliability. For instance, the application of nanomaterials is expected to significantly boost photothermal conversion efficiency and evaporation rates. Meanwhile, energy storage technologies can mitigate the intermittency of solar energy, ensuring continuous system operation. Furthermore, the development of multi-energy integration systems, such as combining solar energy with wind, geothermal, or other renewable sources, is a key area for future research.
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Written By
Huijin Xu and Hao Luo
Submitted: 17 September 2024Reviewed: 27 January 2025Published: 13 February 2025
Open access peer-reviewed chapter
