Economic feasibility of solar-powered reverse osmosis water desalination: a comparative systemic review

Due to disparities in the allocation of rainwater and drought, extreme exploitation of groundwater reservoirs has depleted water supplies in many locations. In addition, improper disposal of domestic and industrial waste leads to poor drainage and deterioration of water quality. According to studies, desalination methods are an effective solution for treating unconventional water, i.e., sea and brackish water, and making it usable in daily life. Solar-powered desalination has recently received a great deal of attention around the world. Herein, we summarized challenges and future perspectives associated with solar-powered desalination units. Some hybrid technologies are also discussed like solar-wind desalination and RO-ED crystallizer technology in Morocco and the Middle East and North Africa (MENA) region. Previously, most experimental studies focused on the use of solar energy in traditional desalination methods such as multistage flash and multi-effect distillation. Desalination with reverse osmosis has become popular due to membrane technology improvement and benefits like high recovery ratios and low energy consumption. Furthermore, it has been seen that solar energy is less expensive than the energy obtained from traditional fuels in the MENA area. This article aims to comparatively and systematically review the economic feasibility of the use of solar photovoltaic reverse osmosis in desalination in the MENA region.


Introduction
Due to the inequality in the allotment of rainwater and the drought, existing water reserves are diminishing. On the other hand, improper disposal of household and industrial waste leads to exploitation of groundwater resources (Maftouh et al. 2023a;Randhawa 2002) As freshwater supplies are minimal to meet the main needs of the population Responsible Editor: Philippe Garrigues and saltwater is not ideal for many applications, the desalination of saltwater (seawater) is a positive approach to meet the requirements (Adams et al. 2002).
Desalination is currently separated into two categories, namely, physical procedures that comprise reverse osmosis (RO) and chemical processes, including a zero-valent iron technique (ZVI) (Antia 2016). During the physical process, salt and water are physically separated. By using the least energy, it is referred to as "minimum thermodynamic energy of separation (MTES)," and efforts to reduce energy use in MTES are only worthwhile if they are also economically beneficial (Antia 2016;Cottier et al. 2010).
One of the best and gold-standard desalination techniques is RO (Wang et al. 2018), which uses high-grade energy to purify water by removing salt and other impurities using semi-permeable membranes (Charcosset 2009). The superiority of RO results from a number of advantages. First, RO is suitable to give a wide range of product capacities, from small stage-alone installations with deliveries of lower than 1 m 3 /day to large plants with deliveries of over 500,000 m 3 / day. Second, RO handles a wide range of feedwater saltness, including brackish water and seawater. Third, RO plants can give nonstop and dependable operation without time-out for extended ages of time. Fourth, RO plants operate with low specific energy consumption (SEC) ranging from 2 to 4 kWh/m 3 (Mito et al. 2019). On the other hand, thermal desalination techniques use heat, often to evaporate and condense water to purify it (Mittelman et al. 2007). The heat required to evaporate seawater makes SEC used in thermal desalination higher than that of the RO process (Chae and Kim 2017). Furthermore, thermal desalination uses low-grade energy, which is difficult to regulate due to its rapid dissipation (Ranjan and Kaushik 2013;Warsinger et al. 2015). However, the SEC of RO desalination remains substantial, accounting for most expenses. Therefore, solarpowered RO systems have been introduced to further minimize the energy costs and environmental carbon footprints of RO desalination units.
Photovoltaic (PV) solar panels are extensively used to induce electricity these days. A cooling medium similar as air or water can be used to cool PV panels, recover heat, and ameliorate conversion effectiveness. The recovered heat can also be used to drive desalination processes, an excellent option that contributes to a sustainable medium. Where electricity and heat can be co-produced through mongrel PV/thermal units (Abdelgaied et al. 2021). Former studies have shown an active cooling system for a PVpowered RO system (PV-RO) where water was injected onto a panel mounted on the back PV face to transfer heat to an underground heat exchanger. PV effectiveness is enhanced by 7.75% when their cooling approach is applied (Shalaby et al. 2022). Similarly, Shalaby was presented with a detailed overview of PV-powered RO (Shalaby 2017). The review focuses on innovative designs that have led to increased desalination unit productivity. The results show that using PV with batteries to power RO units is not provident due to high investment and operating costs. Where brackish product costs were set up to be €7.8/m 3 using battery-less PV-RO compared to €8.3/m 3 for an analogous system that was integrated with batteries (Shalaby et al. 2022).
Most studies and reviews done on renewable energy usage in water treatment have studied or reported on particular plants and evaluated the economic aspects of particular systems and particular energy sources. There is evidence of the economic benefits of renewable energy, in particular solar energy, in desalination processes but there is a lack of comparative research to evaluate the economic practicality of desalination using renewable energy from the perspective of each country's or location's uniqueness. As every country, including Morocco and other Middle East and North Africa (MENA) countries, adopt renewable energy and desalination systems, there is no data on what systems and energy sources are more economically viable than the others in the MENA region, which systems worked better in some countries than others and why and what differentiates the use of particular technologies between MENA countries.
The concept of a water circular economy has gained much attention in recent years. Conventional water is defined as water sources of natural processes of the hydrological cycle, where water can be taken from gutters, aqueducts, lakes, downfalls, and aquifers (groundwater). In discrepancy, unconventional water comes from seawater, rainwater, agrarian drainage, thermoelectric cooling water, hydraulic fracturing water, industrially reused (waste) water, domestic wastewater, and marketable waste. Due to their potential to reuse water for human activities, non-conventional sources of water are receiving more and more attention (Chen et al. 2021). Only 0.3% of the irrigation water in all of the Arab nations comes from unconventional water sources; surface water and groundwater supply 70.1% and 29.5% of the irrigation water, respectively. About 42,800 billion m 3 /year of total yearly renewable water resources were estimated in 2017, which was greater than the total amount of water withdrawn. How to gather and use the resource properly is the main issue (Chen et al. 2021).
This manuscript aims to highlight the importance of RO technology driven by renewable energy resources, in which detailed challenges associated with solar-powered RO technique are elucidated. In addition, this paper is the first to present a comparatively systematical review of photovoltaic-reverse osmosis (PV-RO) desalination powered by solar energy sources in Morocco and the MENA region, focusing on the economic feasibility of RO systems and other desalination technologies comparing economic aspects of desalination technologies powered by solar energy sources in Morocco and MENA countries.

Methodology
This review included studies focusing on the economic feasibility of reverse osmosis desalination systems powered by solar energy and other desalination technologies comparing economic aspects of solar energy use to power desalination technologies in Morocco and other Middle-East and North Africa (MENA) countries.

Inclusion criteria
All research that contained or met the goals and objectives were included. We included studies: (i) investigating the economic aspects of desalination technologies, (ii) publishing as articles in English, (iii) whose full text is available, (iv) original research studies, and (v) review and meta-analysis articles. We excluded books, theses, comments, and conference presentations were excluded from the study.

Study selection
A total of 1251 articles were considered pre-eligible. Titles, keywords, abstracts, and full text were used to shortlist them according to inclusion criteria, and duplicates were removed.

Data extraction and quality assessment
The extracted data included the study's title, year of publication, journal, and authors. Data were organized in a Microsoft excel sheet and we did further processing and fixed errors.

Results
Of 1123 articles identified from the ScienceDirect, PubMed, EBSCOhost, Mendeley, and Scopus databases, 128 articles were from other websites, organizations, and citation searching. After selection, 51 articles were selected from the databases and 3 articles from other sources, making 54 articles (Fig. 1).

Article characteristics
This review collected 11 articles reporting about Morocco, 13 articles reporting about the Middle East and North Africa Of the articles reviewed, 35 explored the use of solar energy in desalination and 22 explored renewable energy as a whole, including solar energy. Most, 38 articles reported more or less about the economic aspects of desalination, and 14 of them specifically performed economic analysis, cost analysis, or cost feasibility analysis of renewable energy-powered desalination systems. The majority, of 34 reviewed articles, studied reverse osmosis technology used in desalination with or without including other technologies. Table S1 shows the details of the selected articles provided in supplementary materials.

Desalination process
Desalination is a generic terminology for extracting salt from water to produce freshwater, defined as having a salt content of < 1000 mg/L (Micale et al. 2009), making unusable waters suitable for humans, livestock, agricultural, and several other uses (Beltrán and Koo-Oshima 2006). Due to increased demand for freshwater, climate change, and drought, conventional water resources are fully depleted, and desalination is becoming more popular (Hindiyeh et al. 2021). The traditional desalination method, which often involves fossil fuels, requires a large amount of energy, making the process costly (Semiat 2008). Therefore, different coupling options are emerging between desalination technologies and renewable energy sources, focusing on cost reduction and environmental protection (Fig. 2) (Curto et al. 2021).
Multi-effect distillation is a multiple-stage process. In each stage, the feed water is heated and the steam flows into tubes of the next stage (effect), with more heating and evaporation. The vapors are then condensed in desalinated water (Adams et al. 2002;El-Nashar et al. 2008).

Electrodialysis
Electrodialysis (ED) is a membrane-based salt-flushing process using an electrical field to treat brackish water or wastewater with low salinity (Wei et al. 2022).
An economic analysis of the ED process powered by fuel and PV showed that the cost of the PV-powered process Fig. 2 Coupling options between desalination technologies and renewable energy sources (Curto et al. 2021) is 30% higher than the fuel-based process (Abraham and Luthra 2011).

Desalination with Reverse Osmosis
RO is the second leading desalination technology with low SEC. This process is based on reverse osmosis, which is the movement of water from a low-concentrated area to a highly concentrated area through a semipermeable membrane. Only molecules the diameter of water molecules may flow through the tiny holes in the membrane, helping to remove all types of pollutants. Solar-driven RO plants have advantages over the traditional desalination process and other solar-driven technologies, such as high recovery ratio and low energy consumption (Ali et al. 2011). Solar photovoltaic energy that powers this process is useful for producing drinking water in remote locations where power grid links are inexistent or unstable (Alnaimat et al. 2018). As shown in Fig. 3, the RO desalination system is driven by a DC motor-powered pump that is directly connected to the PV integrated with the proposed cooling system.

Comparison of economic aspects of water desalination technologies in Morocco and other MENA countries
In Morocco, the Khenifra plant uses conventional and reverse osmosis (RO) systems, producing 36,290 m 3 /day of freshwater (Boulahfaa et al. 2020). The RO technique using solar energy has been reported as the best alternative, as it is low-cost and sustainable in the treatment of brackish water widely available in Morocco (El Azhar et al. 2012;Ennasri et al. 2019;Zidouri 2000). With the growing adoption of renewable energy, renewable energy desalination plants, including solar-powered plants have been installed, and Morocco has adopted PV renewable energy sources in some desalination plants (Ennasri et al. 2019). In addition, solarpowered desalination systems have been reported to reduce the economic burden usually paused by desalination energy supplies. Morocco and other countries in the Mediterranean region have abundant sunshine, but also, due to their arid climate, they have limited water sources, making the use of a solar-powered desalination system the most reliable solution (Ahdab et al. 2021;Touir et al. 2021;Walschot et al. 2020).
Morocco has adopted membrane-based desalination systems such as RO (brackish and seawater) and ED (brackish water) (Ennasri et al. 2019;Quteishat and Abu-Arabi 2006). However, RO requires less energy, lower capital cost, and greater capacity (12,000 m 3 /day) (Ennasri et al. 2019), than ED. Solar power supplies used for RO desalination cut cost in Morocco by cutting fuel imports, generating electricity for both desalination plants and other activities, and reducing carbon emissions. When comparing RO powered by wind and vapor compression (VC), two systems used for desalination in southern Morocco, Zejli et al. reported that RO used relatively less energy, reflecting lower cost (Table 1) (Zejli et al. 2004). For example, the water cost of VC was higher. However, the overall costs were lower for wind-powered VC and RO compared to grid-powered systems. They also reported that VC was used only with desalination units without the need for pretreatment.
The RO technology of the Chtouka Ait Baha plant in Morocco uses SEC of 4kWh/m 3 and the fixed water cost of about $1/m 3 on average is affordable. Furthermore, this study and other studies indicated that solar energy is costeffective when used in RO desalination plants (Kaya et al. Fig. 3 Diagrammatic representation of the PV system combined with the RO unit and cooling system (Shalaby et al. 2022) 2019, Kettani &Bandelier 2020). Compared to other MENA countries, the water cost of $1/m 3 is higher than the cost in those countries in general where desalinated water costs $0.5/m 3 with RO and except for thermal systems where the water cost is $1/m 3 for the desalination of MSF and MED in many MENA countries. This aligns with the reported thermal system being more expensive in Morocco, too (Quteishat and Abu-Arabi 2006). Regarding energy consumption, 4 kWh/m 3 is higher than 3.5 kWh/m 3 on average in Abu Dhabi and 3 kWh/m 3 in Egypt (Kaya et al. 2019;Lamei et al. 2008). In general, studies comparing solar MED and RO (PV-RO) plants found that the solar RO system is more environmentally benign and cost-effective than the solar MED system (El-Nashar et al. 2008). However, for large-scale capacities greater than 1000 m 3 /day, a solardriven MED plant was reported to be more cost-effective than a PV-RO plant (Boulahfaa et al. 2020;Ennasri et al. 2019;Ghafoor et al. 2020;Quteishat and Abu-Arabi 2006).
In the context of Morocco, solar PV energy was feasible and cheaper than concentrated solar power (CSP) due to the complexity of building the high voltage line infrastructure from the desert, which drives costs higher. It is suggested that building many CSP-driven RO plants would make building a CSP line cheaper and provide RO plants with the CSP advantage of storing heat at a lower cost. Meanwhile, studies have shown that the best alternative is to combine grid and solar energy when only solar power supply is not adequate (Filippini et al. 2019;Kettani and Bandelier 2020). On the other hand, the CSP-powered plant's economic feasibility in Jordan is challenged by subsidized water tariffs up to 80% lower cost than the actual cost, making it economically unviable to run (Al-Addous et al. 2020). A reduction of 16% in subsidies on water tariffs would make solar-powered RO economically viable, attracting more adoption and more benefits (Rachid et al. 2015;Trieb and Müller-Steinhagen 2008).
On the other hand, compared to other MENA countries, solar energy supply is favored over other renewable energy sources due to both sunlight availability and different water salinity, especially in countries bordering the Mediterranean Sea. The CSP in these countries generates both heat and electricity as in Morocco, but in countries bordering the Red and Mediterranean Seas, CSP-RO desalinated water costs $1.52-1.74/m 3 less than $1.97-2.08 from MED powered by CSP, which is a thermal process (Bank 2012). Using CSP thermal energy sources is cheaper than using thermal energy directly operating the desalination system (Darwish 2015). Similarly, Kettani & Bandelier suggested a comprehensive report with future directions on cost analysis of solar RO technology in Morocco. Their paper's primary finding is that desalinated water may be purchased for about $1/m 3 -a reasonable price. The second is that PV without storage will continue to be the least expensive choice for power production both now and in 2030. Although storage-based solutions do not appear to be as competitive right now, they may become more so if future electricity grid prices rise and flexibility needs increase. The report makes suggestions for how various technological decisions might affect the structure of the future Moroccan energy sector (Kettani and Bandelier 2020).
On the contrary, a study on a pilot plant in Ouarzazate, Morocco, with concentrated photovoltaic panels and vanadium flow batteries showed decreased plant performance and a 50% cost increase, but with an 81% reduction in carbon emissions from 7440 to 23,825 tons of CO 2 emissions eliminated compared to fossil powered plants (Biain 2021 Thi et al. 2021). Membrane-based desalination systems are favored over thermal systems because they can be adapted to water quality, produce higher capacity, and have lower capital and operational costs (Reif and Alhalabi 2015). Therefore, Morocco has been adopting more and more RO systems to solve its water supply shortage, using more versatile systems to supply all regions ( Solar-powered thermal desalination is cheaper than solar RO in countries of the Gulf Cooperation Council (GCC) where the salinity of the water is high (Bank 2012). Since RO systems use high-grade energy and are power-hungry, the measures taken, including solar energy, have yielded a significant energy cost reduction of up to 43% per m 3 . In addition to increasing fuel prices, solar energy has become . Combining different systems, either various renewable energy sources or conventional energy with renewable energy is more economically and environmentally beneficial than purely fossilpowered desalination or water transport from other locations (Kashyout et al. 2021). PV-wind hybrid desalination system is cost-effective in remote areas because it is energy efficient and results in lower net cost in small-scale plants, common in remote areas (Dawoud 2017;Fahmy et al. 2012;Shahzad et al. 2018). More evidence on the profitability of solar-powered desalination has shown that it depends on area specificities and desalination scale studies. For example, nuclear and solar energy, when used for small-scale desalination processes, are not cost-effective, but when used for largescale desalination, they are economically feasible and cost-effective (Sadeghi et al. 2019;Tahri 2001). Higher investment costs favor large-scale facilities that require a generally low cost compared to traditional fuel-driven desalination plants. This indicates that solar-powered options will be viable in the coming years (Dawoud 2017;El-Nashar 2009;Zhang et al. 2018). More recent evidence showed that solar energy usage in desalination leads to lower water costs and that solar-powered RO desalination plants significantly cost lower when subsidies are applied or not (Esmaeilion 2020; Kharraz 2020). As Morocco has committed to significantly reducing its greenhouse gas footprints, the use of renewable energy together with conventional energy in water treatment plants accelerates this achievement. In the Arab Gulf countries, the thermal desalination process has been widely favored over solar RO because of higher salinity that damages membranes in RO systems, fouling and scaling, and lower energy costs in the region (Ali et al. 2011;El-Ghonemy 2012). On the contrary, in North Africa, RO is widely used in desalination primarily because the salinity of the water is favorable (Figs. 4 and 5) (Kharraz 2020).
Despite recent advances in RO technologies that solve most technical problems, MENA countries still face challenges in adopting solar-powered systems due to the increased technology and investment costs for solar power facilities. That overshadows the lower operating and maintenance costs of these systems (Ghermandi and Messalem 2009; Keulertz and Woertz 2015; Quteishat and Abu-Arabi 2006), as they result in higher water costs compared to conventional energy (Do Thi et al. 2021). This evidence is the same as what was reported in studies from Morocco in particular. Similarly, in Iran, when comparing the grid, diesel engines and PV solar panels, RO powered by gas engines was more economical than solar-powered RO due to high water salinity increasing the SEC. However, solar energy will be economical, as carbon emission taxes will increase, making solar energy sources economically feasible alternatives (Lamei et al. 2008;Meratizamana and Godarzib 2007). However, solar energy will be economical since the emission penalty cost of fossil fuel-powered units will increase the costs of those units and make the solar system economically feasible (Lamei et al. 2008;Meratizamana and Godarzib 2007). Another challenge is brine production, which makes management and disposal both costly and environmentally hazardous. New strategies for safe disposal will reduce costs and save the environment (Jones et al. 2019). During the last decades, solar energy cost has been decreasing. With more countries committing to adopting renewable energy and fuel prices, it is expected that the demand for solar energy will increase, and mass production will reduce the cost and make it affordable and profitable (Pugsley et al. 2016;Quteishat and Abu-Arabi 2006). In the MENA region, especially in countries like Morocco that import energy, solar energy is cheaper than conventional fuel-derived energy. When used extensively in desalination, it is profitable considering the prices of fuel on the international market (El-Katiri 2014).
As in Morocco, wind energy cost in MENA countries is lower than fuel cost, and benefits are expected to increase in the long-term with the reduction of air pollution, increased fuel prices in the future, and improved climate (El-Katiri 2014; Pinto and Marques 2017;Shukla et al. 2022). Another study revealed that solar-powered RO has a return period of under 5 years to economically be profitable (Meratizamana and Godarzib 2007;Pugsley et al. 2016), which is more than 2.7 times longer than the 1.83-year payback period for the solar-powered RO estimated in Pakistan (Ghafoor et al. 2020). The largest plants in the MENA region are located in Saudi Arabia, followed by the United Arab Emirates (UAE)   (Table 3). The largest plant (Al Shuaiba) has the lowest water cost of 0.56/m 3 , which is lower than $1 (0.98-1.14 $/ m 3 )/m 3 of the largest Moroccan plant, the Chtouka desalination plant, but similar to the cost of Magtaa plant in Algeria (Kharraz 2020). Furthermore, RO systems powered by gas engines in Iran result in a water cost range of $0.89 to 0.92/m 3 (Meratizamana and Godarzib 2007), still lower than in Morocco. This indicates that the Moroccan plant is still expensive compared to plants in the MENA region. Despite the difference in cost, only the Algeria plant is based on RO technology, while the Al Shuaiba plant is based on MSF technology. Another difference is in capacity, where the capacity for the Al Shuaiba plant is 880,000 m 3 , for the Magtaa plant is 500,000 m 3 /day and for the Chtouka plant, it is 275,000 m 3 /day. Therefore, the Magtaa plant and Al Shuaiba plant cost almost two times and three times less than the Moroccan largest plant, respectively. Desalination systems tend to be more cost-feasible in large-scale plants. The RO in small-scale desalination plants leads to up to a 50% increase in cost compared to large-scale seawater desalination plants (Ghaffour et al. 2013;Tahri 2001). Other factors contributing to the difference in cost are subsidies, currency exchange, cost calculation methods used (Ghafoor et al. 2020), and the resulting electricity prices (Kariman et al. 2020).
Despite the relatively high investment costs for the use of renewable energy in the treatment of water in MENA, the reduction of the cost of PV modules in recent decades provides an attractive option (Fig. 6) (Al-Karaghouli et al. 2010;Kharraz 2020). In many MENA countries, including Morocco, coupling solar energy with desalination or combining solar, thermal, and electrical grid energy in desalination processes improved energy efficiency and reduced cost. In Abu Dhabi, 90% of seawater desalination is by MSF and MED, which are energy inefficient and more polluting. The cost analysis of the powering of RO systems with solar energy concluded that when solar photovoltaic systems are used to power RO desalination facilities, they reduce the SEC by about 10%, resulting in a reduced total cost of water (Kaya et al. 2019). Hybrid RO and CSP are more economical than PV-RO because the thermal storage of CSP improves the performance of the RO unit with low water costs, similar to a fossil-powered RO unit (Laissaoui et al. 2018).

The energy cost of desalination
Energy is the main driver of the cost of the desalination process (Kariman et al. 2020). SEC is based on desalination technology and plant design parameters. It is also affected by ambient conditions such as temperature and feed water composition. As we have found out in our review, energy has been the main driver of costs in both Morocco and MENA countries, and costs were different depending on many factors specific to each country (Ghermandi and Messalem 2009;Napoli and Rioux 2016;Quteishat and Abu-Arabi 2006). Countries have explored inexpensive energy resources to meet their desalination process needs. One example is building desalination plants close to thermal power plants since the salt water must be evaporated with  (Kharraz 2020) heat. Consequently, the waste heat generated by the desalination facility as a by-product during electricity production can be recovered and used in thermal power plants. In addition, the cost of desalinated water depends on location, ambient conditions, operational and maintenance costs, and environmental costs associated with CO 2 emissions (Maftouh et al. 2023b) and subsidies (Al-Addous et al. 2020;Ghaffour et al. 2013). As we have seen, the cost of water was influenced by subsidies in Jordan, making the cost of desalination water using renewable energy uncompetitive compared to the water of other sources, making the method economically uncompetitive not because it is expensive but because subsidies provide a cheaper alternative. Some articles revealed that renewable energy-powered desalination systems are economical in large-scale desalination plants. The solarpowered large-scale RO systems can lower the cost by $0.37 to $0.74/m 3 (Sadeghi et al. 2019). On the other hand, largescale desalination is expensive, and water costs about twice as much as it does in a typical river or lake water purification operation (2011). Morocco and other MENA countries have adopted hybridization for cost reduction such as Solar-Wind hybrid desalination used in Morocco. This hybrid was reported to be more cost-effective by other studies outside Morocco (Bourouni et al. 2011;Gallardo-Vázquez et al. 2019). In 2003, a photovoltaic and wind-coupled RO plant was established in Greece for seawater desalination, and its energy requirement was reported to be 3.96 kWp of PV panels and 900 Wp of a wind turbine. When a 44.4 kWh battery was linked to PV and wind turbine systems, the energy consumption became 16.5 kWh/m 3 with a capacity of 3.12 m 3 /day. The estimated cost range of water was 23-27$/m 3 (Fahmy et al. 2012;Lachhab et al. 2021). Nayyar et al. proposed RO-ED-crystallizer (REC) for zero brine discharge seawater desalination. The cost of optimized REC water was $3/m 3 , and expenditures can be further offset by income from salt extraction. The efficiency of the Second Law for the improved REC system was 18% (Nayar et al. 2019). In addition to combining different energy sources, hybrids of different systems can also reduce specific energy consumption (SEC). Studies have been conducted with the mixture of RO and other membranes and showed various advantages of hybrid systems, such as decreased individual energy usage, improved RO membrane lifespan, improved performance, reliability, and general reduction of costs (Obotey Ezugbe and Rathilal 2020).

An economic evaluation of desalination with solar-driven reverse osmosis
Reviewed articles showed that the current technology of solar RO systems in the MENA region is not cost-effective in small-scale settings in general and cannot be applied everywhere due to different factors, including water salinity (Kashyout et al. 2021;Kharraz 2020;Rachid et al. 2015;Zejli et al. 2004). However, it remains an efficient and inexpensive approach to water management problems for small units in areas with limited electricity supplies (Rahimi et al. 2021). However, in these areas, costs depend on the conditions of the sun and the conditions of the water (Mohamed et al. 2008).

Capital expenses
One of the significant challenges MENA countries face in installing RO desalination systems powered by solar power is the high investment costs of this new technology, which costs too much to appreciate the benefits (Ghermandi and Messalem 2009;Napoli and Rioux 2016). Capital expenses for the solar PV-RO plant include hardware costs (on equipment and materials) and other capital costs. Therefore, it depends on customer choices, such as selecting types of equipment and site characteristics, including feed water, desalinated water quality, and regulations. The cost of solar PV-RO depends highly on the availability of solar energy and feed water solute required for desalination in a given region. However, batteries must be taken into account while designing a water treatment system. The photovoltaic/battery network is projected to account for 25% of the overall capital investment in the facility, while the water treatment plant (RO, cartridge filters, and UV disinfection) is anticipated to account for more than 50% (Helal et al. 2008). Many researchers have reported that using batteries to extend normal operating times is more economical than generating more water at peak hours (Sauer et al. 2012). Increased operational demands may allow for battery storage, and cloud occurrences can induce fast fluctuations in power generation, resulting in frequent pump performance variations. These drastic changes in flow and pressure may affect water quality and damage membranes (Pan et al. 2020; Reif and Alhalabi  . The initial, operational, and end-of-life cost of a solar-derived RO plant is shown in Table 4.

Economic aspects of a solar-powered PV-RO System
For a PV-RO system, the cost of water output per unit is influenced by several variables, including recovery ratio, the expense of equipment/machinery, hybridization configuration, government subsidies, interest rate, labor costs, etc. Therefore, solar RO costs can be significantly reduced by acting on those factors.
• Developing more economic PV cells Even if photovoltaic modules cost about 60% less than in the 1990s, solar cells that are both more inexpensive and more efficient can be made using nanotechnology (Ushasree et al. 2019).
• Design of a battery-less photovoltaic system Due to the increased inner deterioration at high temperatures, the battery performance, charging, and discharge phase reduces the average battery lifespan in cold and warm places. This decreases the water flow produced and subsequently increases the device's total cost. Because the battery performance is typically 75-80%, approximately 20-25% extra PV is necessary to maintain a constant supply. Therefore, battery-less photovoltaic systems are the sustainable solution.
• Employing an energy recovery device The energy recovery device (ERD) helps conserve power at a controlled pressure that can be passed immediately to the feed water. It contributes to the reduction of the implementation price of the device. It uses the extracted energy to increase the water pressure (Helal et al. 2008), conserving up to 57% of absolute power relative to non-recovery devices, with 95% effectiveness. Seawater desalination experiments showed high performance, versatility in service, and cost reduction of ERD systems (Elasaad et al. 2015;Subiela-Ortín et al. 2022).

Prospects of the solar-powered RO desalination system
New RO technology to make "fouling-resistant" membranes and make RO capable of desalinating water with high salinity would be vital in cost reduction. Besides this, using fossil fuels and other non-green energy sources might increase the emission of greenhouse gases and lead to climate change. Fossil fuel prices are expected to continue to rise, making it economically unfeasible for countries that import energy, such as Morocco. As MENA countries become more industrialized, their energy demand and their carbon emission footprints will increase with their expenses for energy and water supply and for tackling climate change impacts. The potential of its location under the sun and its commitment to reduce carbon emissions make solar energy-powered desalination systems the only sustainable solution. Considering the arid climate in these countries with freshwater scarcity, there is an increasing need to treat wastewater, seawater, and brackish water for satisfying the increasing demand (Maftouh et al. 2022;Quteishat and Abu-Arabi 2006).
All of these point to the expected increase in the installation of solar-powered desalination plants in the MENA region and increased investments to acquire more advanced energy and cost-efficient technologies. Solar power RO desalination systems should be promoted and subsidized to become a massively adopted system, focusing on their potential benefits and removing barriers to their installation (Ali et al. 2011). Proper site analysis, including the water storage or salinity issue, available resources (electricity, gas reservoirs, solar energy), and the requirement of desalinization plants (small or large), should be performed in the area that needs fresh water. This will allow the introduction of area-specific efficient, and cost-effective desalination methods.

Conclusion
In regions where only seawater or brackish water is a source of drinking water, desalination systems can be used to provide drinking water. Several technologies have been established, and many more research and development methods for desalination are being explored. With increasing freshwater demand and climate change worldwide, solar-powered RO systems present preferred alternatives to fossil-fueled desalination systems and other water purification methods. Countries located in the sunbelt region like Morocco and other countries in the Middle East and Africa are benefited from using solar energy in desalination. This review comparatively explored the economic aspects of solar energy sources in desalination methods and other energy sources in Morocco and other MENA countries.
The higher investment costs of solar-powered desalination methods remain challenging for Morocco and other MENA countries. This review showed that desalinated water cost in Morocco is higher than the costs in other MENA countries, but different factors, including subsidies in some MENA countries, influence the water costs. RO systems are more cost-effective in MENA countries with lower water salinity, which explains why North African countries use RO systems more than GCC countries where water salinity is higher, preferring thermal systems. Renewable energy use in desalination is economically feasible according to the specific needs and water conditions of each country. Our investigation discovered that energy was the primary cost driver in both Morocco and the MENA region and that costs vary according to a variety of factors that were particular to each nation. It has been determined that desalination systems fueled by renewable energy are cost-effective in large-scale desalination plants. Large-scale RO systems powered by solar energy can reduce costs by $0.37 to $0.74/m 3 . Hybrid desalination facilities have been implemented by Morocco and other MENA nations. The anticipated cost of water for a photovoltaic and wind-coupled RO plant is 23-27$/ m 3 , whereas the cost of water for an RO-ED-crystallizer desalination plant is $3/m 3 . An analysis revealed that solar RO systems are an effective and affordable solution to water management issues for small units in places with scarce electrical supplies. In general, solar RO is cost-effective, efficient, reliable, and environmentally safe. Their benefits are expected to align with the future global direction of climate protection. Despite benefits and increasing trends toward the solar-driven RO process, its adoption in desalination plants worldwide is still limited. This indicates the need for more efforts and extensive research exploring the barriers and providing solutions to ensure that all potential benefits are unlocked for freshwater accessibility worldwide.