A review of hybrid solar desalination systems: structure and performance

As stated in Abdelkareem et al. (2018), the global desalination capacity is 86.5 million m³/day. About 60% of its capacity comes from seawater. The most common desalination technologies are RO (66%) and MSF (21%) (Henthorne 2011). The Middle East possesses the greatest amount of installed desalination capacity (Greenlee et al. 2009), with countries such as Saudi Arabia contributing 21% of the global total (about 5 million m³/day) (Radhwan & Fath 2005). However, nearly 90% of the desalination systems in this region utilize multi-stage flash (MSF) technology due to their heavy dependence on fossil fuels (Greenlee et al. 2009). Several desalination methods have achieved significant progress in recent years and are expected to continue progressing in the future, taking into account their energy, efficiency, and economic aspects. The thermal desalination methods that incorporate a phase-change mechanism encompass MSF, VC, multi-effect evaporation, and humidification–dehumidification (HDH) desalination techniques (Li et al. 2013). The energy performance of desalination plants is assessed by metrics such as specific energy or heat consumption, gained output ratio (GOR), second law efficiency, and exergy efficiency (Altmann et al. 2019). The future of desalination is experiencing a rising trajectory, primarily propelled by factors like continuously expanding industrialization, population growth, and the deterioration of current water resources (Henthorne 2011). Nevertheless, these systems require a substantial quantity of energy, ranging from 2.5 to 12 kWh/m³ (Byrne et al. 2015), and the majority of them rely on fossil fuels. In 2012, fossil fuels accounted for 81.7% of the worldwide main energy source and 67.9% of electricity production, resulting in approximately 99.5% of the overall emissions of greenhouse gases (Dincer & Acar 2015). Nevertheless, these energy sources are depleting. Hence, it is essential to employ renewable energy sources to produce environmentally benign (non-polluting) electricity, which is vital for achieving stability. The process is widely referred to as greenization, with the aim of improving efficiency, decreasing gas pollution, recovering losses, and generating numerous outputs. Therefore, the shift toward renewable energies is inevitable in order to improve human well-being and promote economic advancement (Environment America 2009). As stated in Lindeman (2015), over 1.3 billion individuals are now deprived of access to electricity, with 95.7% of this population residing in developing nations in Asia and sub-Saharan Africa. The processes of industrialization and population growth emphasize the significance of addressing the rising energy demands of society (Li & Lin 2015). The connection between energy and water is mutually dependent, as energy is essential for the production of water through any desalination process, while power-generating units are reliant on water (IEA International Energy Agency 2016). Owing to the dwindling accessibility of fossil fuels, green energies have garnered substantial popularity and affordability, courtesy of recent technical advancements. Different types of renewable energy, including solar, geothermal, etc., have been used for desalination. Solar energy is a highly abundant and dependable source compared to other forms of renewable energy (Al Suleimani & Nair 2000; Gorjian & Ghobadian 2015). On a bright sunny day, the solar radiation that reaches the Earth's surface is about 1 kW/m². This is only a tiny portion roughly 1/1,000th of the total energy the sun has released (Lovegrove & Luzzi 2003). Therefore, it can effectively tackle issues related to energy security and environmental concerns. Furthermore, numerous dry regions experiencing water scarcity and requiring sustainable food production are situated in areas with significant solar radiation levels, often ranging from 4 to 6 kWh/m² (Chaibi 2013). Therefore, solar-powered desalination is a highly appealing alternative and accounts for nearly 57% of the desalination market (Eltawil et al. 2009). The article offers a thorough examination of current and developing methods designed to decrease energy usage in greenhouses. The review encompasses several subjects, such as experimental and analytical models pertaining to heating, the cooling process, and tactics for climatic control. Furthermore, it investigates the most efficient form and alignment of greenhouses. Prior research has investigated the utilization of solar energy in greenhouses for many functions, including heating, cooling, lighting, and irrigation. However, in terms of irrigation, the research has primarily concentrated on solar-powered irrigation pumps. Alternatively, utilizing solar-powered desalination systems for greenhouses is a viable method to tackle the interconnected issues of water, energy, and food. These methods have been implemented in many nations globally and analyzed in multiple papers. These technologies can enable the operation of greenhouses in rural locations that lack connection to the energy grid, commonly known as off-grid greenhouses. In 2020, Coppitters et al. conducted an assessment of the thermal and economic efficiency of a hybrid solar and microturbine system that incorporates a desalination unit. According to their research findings, the production cost of fresh water in this power plant ranges from 1.78 to 1.92 dollars per production cubic meter, which is lower than the cost of solar water desalination (Coppitters et al. 2020). The purpose of this study is to investigate the architecture and functioning of hybrid solar desalination systems. These systems utilize solar energy to perform water purification through evaporation and condensation mechanisms. The study aims to explore the advantages of solar-powered desalination systems as a preferable alternative to traditional methods, highlighting their use of clean energy and absence of noise or sound pollution. Additionally, the cost-effectiveness of solar desalination systems for generating drinking water, particularly in desert regions and inaccessible areas, is examined. The study also emphasizes the significance of utilizing waste heat energy to lower expenses and improve the effectiveness of water desalination processes. The findings underscore the importance and rationale for implementing hybrid solar desalination systems, which rely on solar energy to efficiently handle water and energy resources.

Thus, solar radiation heats the system and promotes fresh water evaporation. Condensation occurs on the glass surface due to the lower temperature of this area, causing internal moisture to turn into liquid droplets. Ultimately, the condensate is gathered and pure water is acquired. The graphic below displays the schematic of this system. An optimal yield of superior condensate is achieved at a rate of around 2–3 L/m² on a daily basis. Consequently, this system is limited to small-scale applications (Curto et al. 2021). Solar water desalination system performance depends on system design, operational and environmental circumstances, and technical competency. Water depth, inlet water temperature, tilt angle, glass thickness, additional condensers, reflectors, phase-change materials, flat plate and evacuated tube collector (ETC) collectors, and nanofluids affect the thermal performance of the solar desalination system. Freshwater productivity depends on system design and technical expertise. Water depth, basin materials, transparent glass angle, water-glass temperature difference, and absorber area affect solar still productivity (Boukhriss et al. 2023). Desalination is the process of removing salt and minerals from saline water to produce freshwater. There are several methods of desalination, including thermal methods such as MSF distillation, MED, vapor-compression (VC), and membrane-based methods such as RO, nanofiltration (NF), MD, and forward osmosis (FO). Thermal methods such as MSF, MED, and VC involve heating saltwater until it reaches a certain temperature, followed by its introduction into a chamber with lower pressure compared to that of saturated water vapor. The sudden drop in pressure causes some of the water to evaporate, leaving behind salt and other contaminants. The steam produced is then condensed to produce freshwater. Membrane-based methods such as RO, NF, MD, and FO use semi-permeable membranes to separate salt and other contaminants from water. In RO, water molecules travel through a semi-permeable membrane in the opposite direction of natural osmosis, leading to the removal of pollutants and the creation of pure water. In NF, the membrane has smaller pores than in RO, allowing for the removal of smaller ions and molecules. In MD, water vapor particles pass through a semi-permeable membrane while the solutes in the water stay in the water flow around the unit. In FO, an osmotic pressure gradient drives the movement of water from a low-concentration solution to a high-concentration solution. Other methods of desalination include ED, which removes dissolved ions using semi-permeable membranes and an electric field, and HDH, which is employed for the purification of water on a small scale, specifically in remote and sparsely populated areas. Finally, hydration-based desalination (HY) produces gas hydrates, which are crystalline solids consisting of water and gas molecules such as nitrogen, CO₂, and methane. The cost of producing water varies depending on the desalination method used. According to a study conducted by the International Desalination Association, the cost of producing water using RO ranges from $0.50 to $2.00/m³, while the cost of producing water using thermal methods such as MSF distillation and MED ranges from $1.00 to $3.00/m³. The cost of producing water using ED is estimated to be between $0.50 and $2.00/m³. The cost of producing water using HDH is estimated to be between $1.00 and $2.00/m³. The cost of producing water using hydration-based desalination (HY) is estimated to be between $1.00 and $2.00/m³. The cost of producing water using MD is estimated to be between $0.50 and $2.00/m³. It is important to note that the cost of water production depends on various factors such as the salinity of the water, the cost of energy, the quality of the resulting freshwater, and the availability of suitable technology and energy sources (Ziolkowska 2015; Tiwari et al. 2023).

The integration of thermal and membrane water production techniques allows for the creation of a hybrid water desalination plant. The ratio of RO desalination plants to thermal desalination plants (MED) is only affected by things like the building's conditions, changes in how much electricity and water are used, the power generation unit's utilization factor, and the presence of a water transmission network. The evaluation and calculation of the capacity ratio should be performed individually for each building, taking into account the specific circumstances of each situation. Thermal desalination facilities in tropical and temperate climates have a challenge due to the variation in water consumption throughout the year, with a rise in summer and a decline in winter. Meanwhile, the demand for water remains relatively stable. Therefore, when the burden of the power generation unit is reduced, the production of thermal desalinated water (MED) is also dropped accordingly. These situations also arise when the power-generating unit is temporarily removed for periodic maintenance and repairs. Hence, a RO desalination facility is presently being built. Furthermore, thermal desalination (MED) has the capacity to improve the flexibility of water and energy production.

Hybrid power plants have the advantage of reducing water production costs. For water made using the hybrid RO technique, the TDS measurement indicates that the water quality level is between 300 and 500 parts per million (ppm). On the other hand, water obtained using the multi-effect distillation method has an exceptionally low total dissolved solids (TDS) level that is less than 10 parts per million (ppm). Therefore, combining water produced through the multi-effect distillation method with water generated through RO reduces the need for adding extra chemicals and leads to a drop in the costs associated with water production. If the power plant is used, it would be wise to use a single-path design in RO. This would result in enhanced water quality in the output of the RO process. The RO system gradually improves the quality of the water it produces. Hybrid power plants utilize the thermal technology known as MED to generate water of exceptional purity. As a consequence, there are longer time intervals between replacing the membrane, which ultimately leads to decreased operational expenses. Furthermore, another advantage of the hybrid method is that the product water and concentrated water (brine) generated by the thermal process usually have a relatively high temperature, requiring cooling for environmental purposes. The lower temperatures of the product and RO-concentrated water lower the temperature when combined. An important limitation of RO systems is the high concentration of concentrated water. Releasing a substantial quantity of water back into the ocean can result in ecological problems. Nevertheless, the combination of condensed water from thermal systems and the low concentration of hybrid power plants yields a satisfactory quality of output water that may be safely discharged into the sea. Table 1 presents a comprehensive summary of various research studies conducted in the field of hybrid desalination.

An important benefit of a fuel cell RO desalination plant is its low water requirement for fuel cell operation, as opposed to the significant water usage of conventional power plants (CPP) for cooling purposes. Fuel cell plants also help to alleviate or eliminate harmful environmental effects on aquatic ecosystems. In integrated membrane desalination (CPP) power plant systems, thermal energy discharge into the oceans is a major issue (Shakouri et al. 2021).

Figure 23(a) shows how the triple-pressure HRSG gets heat for the double-pressure steam turbine cycle (Yadav & Singh 2022). In addition, the heat that is recovered from the low-pressure HRSG is used as steam to improve the efficiency of the Med/TVC desalination plant. To get the required operating pressure for the SOFC system, the fuel–air combination containing methane and the water needed are individually pressurized using a compressor and a water pump, respectively (Wang et al. 2023). Before entering the chimney, each of these three currents is heated separately to the designated input temperature of the chimney (SOFC) (Azhar et al. 2023). Preheated fuel and steam are mixed in the mixing unit to make gas. At point 1, the mixture enters the SOFC. The gas mixture is then modified and transferred into the anode electrode to form hydrogen for electrochemistry. The cathode electrode receives equally distributed preheated air for an electrochemical process that produces energy and heat (Azhar et al. 2023). For SOFC cooling, the canal cathode uses most of the air. In addition, an inverter converts direct electricity into grid-quality alternating current. Fuel cell heat is used for adjustment and to raise the output current temperature (Wang et al. 2023). The high-temperature gas mixture and exhaust gases from the anode and cathode of the fuel cell are, respectively, routed to the rear, where the fuels (methane, hydrogen, and carbon) do not undergo a reaction with surplus air. The high-temperature combustion gases are harnessed to produce additional power for the compressors as they flow through the GT. The GT exhaust is channeled toward point two of three pre-heater sets to elevate the temperature of the incoming flow for the SOFC (Vojdani et al. 2021).

The system includes a molten carbonate fuel cell (MCFC), GT, steam Rankine cycle (SRC), solar tower (ST), organic Rankine cycle (ORC), and MED/RO desalination units. In Figure 24, the solar cell tower and natural gas boiler (NGB) are heating the MCFC reactants. Methane (CH4), air, and water react. The NGB raises the solar field temperature from 570 to 935 °C. MCFC preheaters need this greater temperature. Higher temperatures are needed to elevate the MCFC intake to 550 °C. The reactants from the MCFC's power generation are sent to the AB, where the exhaust stack fuel is used to raise the GT input temperature to 909 °C. Combustion will occur. Power comes from AB exhaust gases supplied into the GT. The HRSG converts combustion byproducts from the GT into steam for the 545 °C SRC. After leaving the heat recovery steam generator, the reactants enter the organic Rankine cycle at 152 °C to heat the organic fluid and generate electricity. Finally, the RO desalination plant uses organic Rankine cycle power to produce fresh water. Figure 24 shows the SRC condenser replaced by a multi-stage distillation desalination device to produce fresh water. In order to summarize the stated content, it can be said that solar desalination systems have the potential to provide a sustainable and reliable source of freshwater in areas with limited access to freshwater resources. However, there are several challenges that need to be addressed to make solar desalination systems more efficient and cost-effective. One of the main challenges of solar desalination systems is their low efficiency compared to conventional desalination methods. Solar desalination systems require a large amount of energy to produce freshwater, which can be expensive and inefficient. Additionally, solar desalination systems are highly dependent on weather conditions, which can affect their performance and productivity. Another challenge of solar desalination systems is their high capital and maintenance costs. Solar desalination systems require a significant investment in infrastructure and equipment, which can be a barrier to their widespread adoption. Additionally, solar desalination systems require regular maintenance and cleaning to ensure their optimal performance, which can be time-consuming and expensive. Despite these challenges, solar desalination systems have several advantages and prospects for the future. Solar desalination systems can provide a sustainable and reliable source of freshwater in areas with limited access to freshwater resources. Additionally, solar desalination systems can reduce dependence on fossil fuels and lower GHG emissions. Furthermore, advances in technology and materials science are expected to improve the efficiency and cost-effectiveness of solar desalination systems in the future. In conclusion, solar desalination systems have several challenges and prospects for the future. While there are several challenges that need to be addressed to make solar desalination systems more efficient and cost-effective, advances in technology and materials science are expected to improve the efficiency and cost-effectiveness of solar desalination systems in the future.

Through a comprehensive analysis of the fundamental principles and real-world applications, this study emphasizes the importance of hybrid solar desalination systems in addressing water scarcity and ensuring efficient water and energy resource management. By combining technological innovations with renewable energy sources, these systems pave the way for a sustainable future and provide access to clean water in diverse and challenging environments. This study on hybrid solar desalination systems has shed light on their innovative structure and impressive performance in addressing the pressing challenge of water purification. By harnessing abundant solar energy, these systems efficiently convert saline water into fresh drinking water through evaporation and condensation mechanisms. The research findings demonstrate that solar-powered desalination systems offer a remarkable alternative to traditional methods, as they rely on clean energy sources, produce no noise pollution, and have shown cost-effectiveness, particularly in desert regions and inaccessible areas. Furthermore, the incorporation of waste heat energy into the desalination process has been highlighted as a significant breakthrough. The utilization of waste heat energy not only reduces expenses but also enhances the overall effectiveness of water desalination. This innovation holds great promise for improving the efficiency and sustainability of hybrid solar desalination systems. The study's analysis of the fundamental principles and real-world applications emphasizes the importance and rationale for implementing hybrid solar desalination systems. The combination of renewable energy sources, technological innovations, and efficient water management paves the way for a sustainable future. These systems not only contribute to water security but also align with global sustainability goals and efforts to mitigate climate change. In conclusion, the innovative structure and impressive performance of hybrid solar desalination systems showcased in this study offer a promising solution to the global water crisis. Continued research and investment in this field will further drive advancements, making these systems even more efficient, affordable, and widely accessible. With their potential to provide clean water in a sustainable manner, hybrid solar desalination systems represent a significant step toward a more water-secure and environmentally conscious future.

The author extends his appreciation to the deanship of scientific research at Shaqra University for supporting this research work.

The authors give their full consent for the publication of this manuscript.

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

The authors declare there is no conflict.