Recent progress in renewable energy based-desalination in the Middle East and North Africa MENA region

Graphical abstract

Highlights

• •Availability of RES in the MENA region was summarized.
• •Suitable renewable energy storage systems in MENA was introduced.
• •Desalination plants powered by RES in MENA were investigated.
• •Al-khafji solar seawater RO plant in KSA was discussed.

Abstract

Background

The Middle East and North Africa (MENA) countries are rapidly growing in population with very limited access to freshwater resources. To overcome this challenge, seawater desalination is proposed as an effective solution, as most MENA countries have easy access to saline water. However, desalination processes have massive demand for energy, which is mostly met by fossil fuel-driven power plants. The rapid technological advancements in renewable energy technologies, along with their gradually decreasing cost place renewable energy-driven power plants and processes as a promising alternative to conventional fuel-powered plants.

Aim of Review

In the current work, renewable energy-powered desalination in the MENA region is investigated. Various desalination technologies and renewable energy resources, particularly those available in MENA are discussed. A detailed discussion of suitable energy storage technologies for incorporation into renewable energy desalination systems is also included.

Key Scientific Concepts of Review

The progress made in implementing renewable energy into power desalination plants in MENA countries is summarized and analyzed by describing the overall trend and giving recommendations for the potential amalgamation of available renewable energies (REs) and available desalination technologies. Finally, a case study in the MENA region, the Al Khafji solar seawater reverse osmosis (SWRO) desalination plant in the Kingdom of Saudi Arabia KSA, is used to demonstrate the implementation of REs to drive desalination processes.

1. Introduction

The scarcity of freshwater resources is the most critical issue of the 21^st century. By 2030, it is expected that water scarcity will affect 40 % of the world population [1]. Unfortunately, most of the water available on Earth is saline water from oceans and seas, with less than 3 % of the global water resources being freshwater, i.e. accpeted salinity level [2], [3]. Also, less than 1 % of available freshwater can be directly used for: drinking, agriculture, and industrial applications, as the rest is locked as ice [4]. Due to population growth, urbanization, industrialization, and improved life quality in the MENA region, the demand for freshwater is both undeniable and increasingly unsustainable. The freshwater availability in MENA is very limited due to the low precipitation, hence the low availability of surface fresh water and groundwater resources; and most MENA countries historically rely on groundwater as their primary source of freshwater [5]. Groundwater is readily available across the entire region, however, due to the dry climate and high evaporation rates, along with low recharge rates, groundwater is not considered a sustainable water source [6]. This, along with the increased demand for freshwater, significantly stresses the need for a sustainable source of freshwater.

To achieve water sustainability, requirements include having a sustainable water supply that is powered by a sustainable energy source [4], [7]. In addition, it is also important that the water supplied is efficiently used in domestic, industrial, and agricultural sectors. One of the possible solutions for water sustainability is water desalination. Seawater desalination is a process where seawater undergoes either a: thermal, mechanical, electrical, or chemical treatment to extract freshwater [8], [9], [10]. International Desalination Association (IDA) has reported that Kingdom of Saudi Arabia KSA, the United Arab Emirates UAE, and the United States of America USA are the largest desalinated water producers [11]. The current annual global installed desalination capacity amounts to 97.2 million m³ from about 16,876 plants, out of the annual global cumulative desalination capacity of 114.9 million m³ from 20,971 projects [12]. In total, the MENA region embraces around 47.5 % of the world’s desalination capacity, out of which 62.3 % is for municipal applications, followed by 35 % for industrial applications.

Desalination is to a large extent an energy-extensive process, with the majority of desalination plants relying on fossil fuels as their primary energy resource [13], [14]. This is no longer a sustainable option for several reasons. One of the most important reasons is the severe environmental impacts and harmful emissions that result from fossil fuels [15], [16]. The quantity and effects of these emissions are no longer negligible, and must seriously be taken into consideration [17]. Table 1 summarizes the energy requirements and associated emissions for different desalination processes. Thermal desalination processes such as multistage flash distilation (MSF) and multieffect distilation (MED) with their high electrical and thermal energy requirements have severe environmental impacts in terms of emissions. The table shows as well that the utilization of waste heat to supply thermal energy demand results in a significant reduction of associated emissions. Another significant reason is the gradually increasing cost and consistently depleting quantities of fossil fuels, which makes them also a less economically viable option [18], [19]. Due to these undeniable disadvantages, renewable energies (REs) present themselves as a promising alternative to conventional energy sources [20].

Table 1.

Specific energy requirements of different desalination technologies and related GHGs (based on 1 m³ of desalinated water) [15], [21].

Technology	Energy, kWh	CO2, kg	NOx, g	SOx, g	PM10, g
SWRO	3.7–4.5	3.01	7.20	6.83	0.20
MSF	4kWh (+333 MJ)	23.41	28.29	27.92	2.04
MSF-WHa		1.98	4.27	14.79
MED	2kWh (+263 MJ)	18.05	21.41	26.49	1.02
MED-WHa		1.11	2.42	16.12

^aWH = Waste heat utilized for the supply of thermal energy.

The theoretical energy requirements for desalination and the work or heat needed for separation are controlled by the thermodynamics of the system. The least exergy of separation is equal to the change of Gibbs free energy of the system, which is solely a function of the concentration and composition of feed and product streams and recovery of the system [22], [23], [24]. For the actual system, this theoretical energy of separation is hardly achieved, and the actual amount of energy is much higher due to system irreversibility and efficiency, with current processes running at about 35 % of the thermodynamic limit [25]. Mistry and Lienhard have proposed a generalized equation for the least energy of separation in desalination processes which is derived from the second law of thermodynamics [26]. The generalized equation handles both heat and work as inputs to the desalination process, which was later verified by the higher thermodynamic efficiencies of work-driven processes such as reverse osmosis (RO) over thermal-driven processes such as MSF and MED, which has been realized for decades at actual plant scale. The case of having two possible energy inputs, i.e., heat and work, raises the need to define a primary energy unit in order to uniform the scale of energy consumption among different desalination processes. Shahzad et al. proposed redefining the performance ratio of energy efficiency of desalination processes to account for the different quality of energy input, i.e. higher quality work vs lower quality heat [25]. The work incorporated conversion factor for each of thermal, electrical, and renewable energies to obtain a universal performance ratio. The work was further expanded to develop a primary standard energy approach to be used to compare the energy efficiency of different desalination processes [27].

Desalination as the most extensive-energy water treatment process consumes about 75 TWh annually, which is almost 0.4 % of the global energy electricity and produces about 76 Mt-CO₂ annually and is expected to reach 218 Mt-CO₂ annually in 2040 due to increased water desalination installed capacities [25]. This high interaction has resulted in what is commonly known as the water-energy-environment nexus given the high dependence of these three pillars on one another [28], [29]. The analysis of the water-energy-environment nexus has revealed as well the impact of unlinking the nexus and the domino effect of working on reducing the energy consumption on other pillars [30]. In this regard, the utilization of REs for desalination is considered an effective tool to reduce the fossil energy burden of desalination hence reducing the interaction and minimizing the environmental impacts of desalination processes with respect to energy consumption. Although REs are believed to be more environmentally friendly, in reality, it has some associated environmental impacts as any other product, but still much less as compared to fossil energy [31], [32].

REs have been extensively studied to drive different desalination processes, with many combinations given the varieties of each, i.e., REs and desalination processes [33], [34]. The literature is rich with many works exploring different combinations of REs and desalination processes aiming at analyzing its energy efficiency, estimated fossil energy saving, water productivity, etc. [35], [36]. The combination has been further extended to include energy storage systems as a means to smooth water productivity and overcome the intermittent nature of REs. Many literature reviews have been made to deeply discuss and analyze the progress and advances in this specific area. Eltawil et al. reviewed earlier in 2009 the integration of REs into desalination processes, indicating that the penetration of the integrated system is still very low although the maturity of the two technologies [37]. This was partially associated with the high cost of REs as compared to fossil fuels and the technical experience and skills required to install and operate REs projects by that time. Table 2 below summarizes the most recent review works (over the last five years) in the literature related to RE-driven desalination processes.

Table 2.

Summary of literature work on renewable energy-driven desalination.

Remarks	Ref.
The review has explored renewable energy desalination in terms of viability with respect to productivity and cost. Small capacity plants were found to be of high cost making them uncompetitive with conventional plants. However, it can be only viable for remote locations hence eliminating water transportation and grid connectivity costs. The typical cost for different rE-driven desalination was provided. •Solar thermal RE: MED $2.5–3/m3, HDH $2.8–7/m3, SS $1.4–12/m3. •Solar PV: ED $11.2–12.6/m3, RO $12.5–16.8/m3. •Wind: MVC $5.6–8.4/m3, RO $2.1–5.6/m3 (for > 1000 m3/d), RO $7–9.8/m3(for > 50 m3/d).	2017, [38]
Exploring PV-RO, Wind-RO, and hybrid wind-PV-RO systems: Overall desalination cost is the main barrier to the wide implementation of rE-driven desalination at a large scale as compared to the current benchmark seawater desalination cost of $0.5/m3. •PV-RO: $0.825/m3 to $34.21/m3 for capacities of 0.8 m3/d to 60,000 m3/d •Wind-RO: $0.66/m3 to $15.75/m3 for capacities of 1 m3/d to 250,000 m3/d •Wind-PV-RO: $1.4/m3 to $6.12/m3 for capacities of 3 m3/d to 83,000 m3/d Hybrid systems, such as Wind-PV-RO systems seem to be more economic and present a high potential for fast implementation at a large scale. A flow chart was proposed for designing a hybrid rE-desalination system considering capacity required and location among other factors for technoeconomic assessment.	2018, [4]
The work has explored the specific application of solar energy to drive desalination processes, which was found more suitable for RO and EDR desalination. The desalination cost for seawater and brackish water feed was found as follows: •Seawater: Conventional energy $0.38–2.97/m3, PV-solar $3.45–9.9/m3, wind energy $1.1–5.5/m3 •Brackish water: Conventional energy $0.23–1.17/m3, PV-solar $4.95–11.35/m3, geothermal energy $2.2/m3	2018, [39]
The work explored the integration of membrane desalination processes to REs specifically, assessing their potential for coproduction of water and energy. The work concluded that only rE-MD process can realize coproduction of water and energy, which is enhanced by further integration of pressure retarded osmosis PRO and reversal electrodialysis RED.	2018, [40]
The work has discussed the recent progress on rE-driven desalination processes indicating PV-RO to be of the highest potential in terms of performance and efficiency. The efficiency of solar thermal desalination can be further improved by using nanofluids as operating fluids. Tidal-RO has a 31–41 % reduced cost as compared to conventional RO at 40 % recovery and 56 bars.	2018, [41]
The work explored the reliability of PV-RO systems for brackish water desalination at a small scale, indicating high reliability for up to 20 years at 1.5–3 kWh/m3, hence suggested working at a lower recovery of 30 % for lower energy consumption, and to mitigate membrane fouling as well, hence longer membrane life. Although of the lower recovery, the system will maintain technoeconomic feasibility due to a longer lifetime and less operating cost for the replacement of different parts.	2019, [42]
The work summarizes the recent developments and status of CSP-driven desalination with more focus on thermal processes as more compatible with CSP. In thermal-only integration, solar collector temperature has to match the operating temperature of the desalination process for improved performance and efficiency.	2019, [43]
The integration of the HDH desalination process with low-grade waste heat from REs, namely geothermal, solar PV, and solar thermal was explored. •Solar air-heated HDH has low productivity of 0.87 kg/m2.h, high cost of 18 $/m3, and relatively high gained output ratio (GOR) of ∼3.1. •Solar water-heated HDH has higher productivity of ∼8 kg/m2.h, higher cost of $25/m3, and lower GOR of ∼2.7. •Dual air/water-heated HDH has intermediate productivity of ∼3 kg/m2.h, higher cost of 40 $/m3, and acceptable GOR of ∼2. •Geothermal-driven HDH has the highest productivity of 200 kg/h at the lowest cost of $2.1/m3. •Refrigeration/heat pump waste heat HDH systems have good productivity of 30.4 kg/h, a high cost of $19.2/m3, and a high GOR of ∼3.8. •Photovoltaic thermal PV/T HDH system has low productivity of 0.57 kg/m2.h and a high cost of $11.5/m3 mainly due to its intermittent nature. •Power plant-driven HDH has the best performance with productivity of ∼414 kg/h, the lowest cost of $0.74/m3, and a good GOR of ∼2.	2020, [7]
The study reviewed the work related to the desalination of geothermal water using REs, including geothermal energy, to drive the desalination processes of RO and CDI. The review has indicated a wide range of reported specific energy consumption and hence cost, depending on the combination studied.	2021, [44]
The review provides a comprehensive discussion on the integrated REs-desalination systems. The works show that effective expansion of REs-driven desalination systems is mainly hindered by technoeconomic factors with recommendations to make favorable regulations for such systems. The work has compiled as well database for rE-driven desalination units, mainly at pilot and small scale. The database has been associated with information on capacity, specific energy consumption, and desalination cost.	2021, [45]
The work has reviewed the optimization of different REs-Ro systems, with RO as the most studied and lowest energy requirement. The work recommends combining the size, operability, and thermodynamics of the system, and to incorporate economic and reliability factors into the objective function. Additionally, the incorporation of demand response has been recommended as well for demand side management, which can help minimize cost while maximizing productivity.	2021, [46]
The review has focused on the applicability of combined solar PV-T to drive different desalination processes providing both electricity and heat. Solar PV/T-driven desalination was found comparatively better than individual solar PV and thermal-driven desalination. PV/T-HDH system was found to have higher generated electricity and less energy requirements. PV/T-RO system was found to have less area requirement and energy consumption of 43.33 % and 23.61 % than the PV-RO, respectively.	2021, [47]
The work has focused on summarizing and comparing the performance of wind-RO, wind-PV-RO, and wind-PV-grid-RO systems. The system performance was found to largely depend on the configuration implemented.	2022, [48]
This review has discussed the application of artificial intelligence AI for REs-driven desalination processes, more specifically for site selection, energy prediction, desalination process selection, and parameter optimization. The application of AI was found to realize cost minimization and efficiency maximization by almost 10 %, along with the release of human resources. Artificial neural networks and genetic algorithms were found to be the most utilized algorithm for such multi-objective non-linear problems.	2022, [49]

The information presented in Table 2 can be summarized as follow:

• •The topic of RE-driven desalination has drawn much attention for research and development given its importance in securing water supply in an environmentally friendly manner.
• •Most of the reported studies are either performed at modeling and simulation levels or a lab scale, with very less studies performed at the pilot scale.
• •There is a wide range of reported values for the overall desalination cost of RE-driven desalination processes, which are still much higher than those realized for conventional desalination processes.
• •The wide implementation of RE-driven desalination has been historically hindered by cost and other technological challenges. However, with the recent developments in RE technologies, hence reducing its energy cost, the implementation is closer than ever.
• •Given the complexity of RE-Desalination interaction in terms of RE and desalination plant sizing and technology selection, along with site or geographical location characteristics, there has been a need for applying different multi-criteria decision-making approaches and even artificial intelligence algorithms.

The summary presented in Table 2 shows the need to have a recent overview exploring the recent advances in this specific area, considering different REs. This review considers to fulfill the following gaps in the literature related to RE-driven processes:

• •Present, discuss, and analyze the recent advances in the area of RE-driven desalination.
• •Explore and compare the combination of different REs with different desalination processes.
• •The work specifically considers the MENA region given its unique characteristics of high water desalination capacity with its huge energy and cost burden, with almost half of the world's desalination capacity.
• •The work discusses the RE potentials of the study area, the MENA region, and its alignment with different RE-driven desalination processes.
• •The work compiles, reviews, and discusses the work performed mostly at the lab and pilot scale for RE-driven desalination in the MENA area specific to each country.
• •Discuss the first-ever large-scale photovoltaic PV-RO desalination plant, Al Khafji desalination plant, starting its operation recently.

This work aims to discuss the application of different REs to efficiently drive desalination processes. The discussion starts with a description of the major desalination processes in section 2, with a focus on well-established technologies such as reverse osmosis as the benchmark membrane desalination process. Multistage flash distillation MSF and multi-effect distillation MED have been considered the benchmark thermal desalination processes. The discussion continues in section 3 to describe the different REs, namely solar (both photovoltaic and thermal), wind, hydro, bioenergy, and geothermal as the major REs. The work then continues to discuss the different energy storage systems (ESSs) in section 4 as a necessity for smooth power generation from REs. The thermal and electrical EESs are the core of the discussion given their wide applicability and suitability for RE-desalination systems. Next, in section 5 the various sources of REs in the MENA region are critically analyzed and discussed at the country level, and countries with the highest potential for each RE are highlighted. A comprehensive summary of the work on implementing different REs for desalination is then described and analyzed in section 6, along with a discussion of the obtained results. Finally, the largest RE-powered desalination plant, Al Khafji solar-powered desalination plant is discussed in section 7 as a typical case study. The work then present a concluding remarks and recommendations on the utilization of REs to drive desalination processes to maximize economic and environmental benfits.

2. Desalination processes

A wide range of desalination processes has been developed, which vary with respect to the driving force, and working principles [50]. Accordingly, many classifications can be adopted to classify such processes. Desalination processes can be classified into a thermal, membrane, and other processes with different working principles [9], [51], [52]. Thermal processes rely on heat addition or abstraction to drive the separation of freshwater in association with phase change. Thermal processes in their very simple principles mimic the natural water cycle, in which solar thermal energy drives the water evaporation from large water bodies such as oceans and seas, followed by condensation and precipitation. Examples of thermally-driven desalination processes include multistage flash distillation (MSF), multi-effect distillation (MED), thermal vapor compression (TVC), membrane distillation (MD), adsorption desalination (AD), humidification/dehumidification (HDH), and freeze desalination (FD) [53], [54], [55], [56]. Mechanically-driven desalination processes involve the application of pressure to separate water molecules through a semi-permeable tight membrane. This includes reverse osmosis (RO), nanofiltration (NF), and pressure-assisted osmosis (PAO). Alternatively, the pressure can be lowered to enhance the water vaporization as in the case of mechanical vapor compressions (MVC).

The previous processes are mainly working on separating and extracting water from saline water, which requires high energy. On the other hand, electrically-driven desalination processes work on the principle of separating ions present in water from the bulk of the solution, i.e. saline water. These processes include capacitive deionization (CDI), electrodialysis ED, and electrodialysis reversal EDR [57], [58]. Finally, some processes are not placed under any of the previous classes. This includes forward osmosis (FO), which is a natural osmotically-driven process, that mimics the natural osmosis process by which plants extract water [59], [60]. Generally, RO, NF, PAO, and FO are classified as membrane processes [61], [62]. It is worth mentioning that ED and EDR processes involve a special type of membrane which is an ion-exchange membrane that allows either cations or anions to pass, similarly MD involve hydrophobic membrane for enhanced vapor/liquid seprartion.

The three most widely used desalination technologies in MENA and worldwide are RO, MSF, and MED. However, there are emerging desalination technologies, that have not yet been fully developed for large-scale applications. These technologies include ED/EDR, FO, CDI, and FD. The RO dominates the desalination market in terms of desalination capacity and the number of plants with 69 % and 84.5 %, respectively [12], [51]. MSF and MED as the main thermal desalination processes come second with about 25 and 7.7 % for desalination capacity and the number of plants, respectively. This is mainly because most thermal desalination plants are of large capacity to enhance energy efficiency, while RO plants are very flexible and modular in size.

2.1. Reverse osmosis (RO)

The most dominant desalination process used worldwide is RO, in which saline water is pressurized and forced to pass through a tight semi-permeable membrane, which restricts the passage of solute to large extent, and permeates water. In RO desalination, the saline water is pressurized to a value higher than the osmotic pressure, to extract product water through the RO membrane, leaving out a higher concentration stream usually called brine, reject, or concentrate, which is charged back to the sea [11], [63]. The operating pressure for seawater reverse osmosis (SWRO) is typically between 50 and 70 bars, which has to be higher than the respective osmotic pressure of the brine stream. Currently, the most used membrane types are cellulose acetate/triacetate (CA/CTA), or thin-film composite membrane (TFC) mainly made of polyamide active layer over polysulfone support layer [64]. The common membrane configurations are spiral-wound (SW) and hollow-fiber (HF) membranes [65]. A typical SWRO plant, as illustrated in Fig. 1 consists of intake, pretreatment, high-pressure pumps, RO modules, energy recovery device (ERD), and product post-treatment to make it suitable for specific domestic or industrial applications. According to the IDA, RO is the fastest-growing technology and recently is being employed in the MENA rather than traditionally employed thermal desalination processes. This is mainly due to several factors, including lower energy requirements, and enhancements in membrane technologies making it suitable for high-temperature and high-salinity feedwaters [66], [67]. The Specific Energy Consumption (SEC) of SWRO, has declined from 20 kWh/m³ in the 1980 s, to less than 3–4 kWh/m³ recently making it more attractive [14], [68], [69], [70], [71].

Fig. 1.

Schematic for typical Seawater reverse osmosis process (LLP: Low-pressure pump, HPP: High-pressure pump, ERD: Energy recovery device, RO: Reverse osmosis).

2.2. Multistage flash (MSF)

MSF process was initially established in the 1950s, making it one of the earliest developed desalination processes [72], [73]. MSF, as illustrated in Fig. 2, is comprised of multistage chambers, with each chamber maintained at a lower pressure than its predecessor using a steam ejector. The process starts with increasing the saline water’s temperature. It is then introduced to the first chamber or stage, where the hot feedwater partially evaporates, due to the lowered pressure. The vapor generated is condensed, while preheating the feedwater over tubes located at the top of each stage. The process is repeated in multiple stages to maximize water productivity and energy efficiency. Most current commercial MSF plants are equipped with 10 to 30 stages, with a temperature drop of 2 °C per stage [74]. Typical MSF plants have high production capacities, ranging from 4,000 to 30,000 m³/d, and operate at high temperatures (90–120 °C) [75]. Two major issues related to MSF (and other seawater distilleries) are corrosion and scaling due to working with saline water at high operating temperatures, i.e. boiling temperature [76]. To prevent scaling, the feedwater is chemically treated to reduce and prevent the presence of calcium and magnesium salts, which commonly precipitate at high temperatures as calcium and magnesium carbonates [77]. Corrosion, however, is a common issue that requires continuous maintenance and use of high-quality materials in heat exchangers and stages, along with corrosion inhibitors to be minimized. MSF is considered the most energy-intensive desalination process, with an SEC of 7.5–12 and 2.5–4 kWh/m³ of thermal and electrical energy, respectively [14], [50], [71], [78].

Fig. 2.

Multistage flash desalination process [79].

2.3. Multi-effect distillation (MED)

MED is the earliest desalination method that was first developed in the late 1800s and dominated the desalination market until MSF emerged in the 1950s. The principle of MED is similar to that of MSF, as they both rely on evaporating feedwater. In MED, as illustrated in Fig. 3, the inlet seawater is sprayed over evaporating tubes, which are heated using steam. The evaporating tubes and seawater sprayers are positioned in vacuum vessels called “Effects” [80]. After the seawater is sprayed on the evaporating tubes, it evaporates and leaves the first effect. The evaporated water, i.e., steam, from the first effect is then used to heat the second effect, where the steam is then cooled and condensed, and later collected as pure freshwater product [81]. The process is repeated in successive effects at lowered pressure and temperatures to maximize energy efficiency and water productivity. The MED process has many advantages, including lower operating temperatures (<70 °C), and high thermal efficiency compared to that of MSF. A typical MED can have an SEC of about 4–7 and 1.5–2 kWh/m³ of thermal and electrical energy, respectively [71], [82], [83]. The main advantage of MED over MSF is the better thermal performance, however; there are several important concerns regarding MED such as the high costs and scaling tendency [84]. Nowadays, MED is gaining more attention due to advances in technology, and this attention has led to the incorporation of stages that address these concerns, such as the addition of an adsorption stage and the possibilities of several brine recovery techniques.

Fig. 3.

Schematic diagram of a MED process [79].

2.4. Other desalination processes

Thermal desalination using MSF and MED processes has dominated the desalination market from the 1950s to the 1980s, then RO membrane desalination started to take off dominating the market with a share going up to 70 % recently [12]. The main drivers for developing new desalination processes have been always to minimize cost, mainly through lowering energy consumption, to suit a wide range of feedwater qualities, brackish water, seawater, brine…etc [68]. Given the high environmental and cost burdens of fossil fuels, there has been always an effort to improve the performance of existing desalination processes, i.e. RO, MSF, and MED, along with the utilization of REs. In parallel, there has been an extraordinary effort to develop new desalination processes with promising better performance in terms of energy consumption and lower cost as well as lower environmental impacts [21], [85]. However, most of these processes are still at lab or pilot scales due to some technical and technological challenges hindering their implementation at a large scale [45], [86].

2.4.1. Forward osmosis FO

FO is a natural membrane separation process that does not require external energy input, as it utilizes the natural tendency of water to permeate through the semi-permeable membrane from the lower salinity feed to a higher salinity draw solution [59]. FO is a spontaneous process with no need for external energy, such as hydraulic pressure for other membrane processes; hence, lower energy requirements, thus lower fouling propensity, operating, and capital costs. FO energy consumption can be as low as 0.25 kWh/m³ for fluid pumping [87]. However, the main challenge hampering the development of FO is water recovery as a product from the draw solution, hence it is regenerated and recycled back [88], [89], [90]. The FO membrane still has to possess the optimized membrane characteristics of high water permeability, high selectivity, ion rejection, and small structural parameter to reduce mass transfer resistance and concentration polarization [91], [92].

2.4.2. Membrane distillation MD

MD is a combination of the two well-established desalination processes of thermal and membrane desalination, however, it is generally considered a thermally-driven process [87]. In MD feedwater is heated below its boiling point to give off vapor, which then permeates through a hydrophobic membrane, that permeates only vapor but not liquid, which is condensed as a product [54]. MD process has many configurations, namely direct contact DCMD, air gap AGMD, vacuum VMD, and sweeping gas SGMD [93], [94]. The advantages of MD can be summarized as follows [93]:

• •Complete rejection of non-volatile solutes, i.e., salts.
• •Process efficiency and energy requirements are independent or slightly dependent on feed salinity, hence suitable for high salinity feedwater and brine.
• •High concentration factor, i.e., higher brine salinity to near-saturation level.
• •No applied pressure, hence less fouling, and less membrane deterioration.
• •High feasibility to utilize various waste heat sources, as it works at temperature reasonably below the boiling point of feed (i.e., 100 °C).
• •Possibility of sensible and latent heat recovery during vapor condensation.

2.4.3. Adsorption desalination AD

AD is similar to the MD process, with the use of adsorbent material rather than a hydrophobic membrane. In AD, water vapor from the heated feedwater, below boiling point, is adsorbed onto the adsorbent during the adsorption cycle and stripped or desorbed from the adsorbent during the striping cycle [95], [96]. AD has advantages similar to those of MD in terms of performance and energy consumption [97].

2.4.4. Freeze desalination FD

FD has been proposed to utilize cold energy, in which saline feedwater is cooled to freeze water as ice that is then separated leaving brine solution, but due to the higher energy requirements for cooling it was not fully developed [98]. Attention has been given recently to FD with the increased size of the liquefied natural gas LNG market, as it is shipped in liquefied form, requiring gasification at receiving terminal, making it possible to utilize such cold energy of gasification associated with LNG to drive FD [56], [99], [100]. FD has comparable advantages to that of MD and AD [101].

2.4.5. Humidification dehumidification HDH

HDH is a simple desalination process with lower energy requirements and no fouling propensity [102]. HDH is similar to MD and AD can utilize waste heat sources for its operation [103]. In HDH feedwater is preheated while condensing the product water from the humid air stream in the condenser, then feedwater is further heated in the preheater, where it gains most of its heating requirements, and finally fed to the humidifier where it is directly contacted with the coming dry fresh air to be saturated with water vapor to produce humid air stream. HDH has a gain over ratio GOR of 2.5–3.5, which is relatively lower than those for MED and MSF, which has a GOR of 10–12 and 8–10, respectively [104]. However, such lower GOR has been attained at a very low energy supply making it more economic and environmentally friendly [105].

2.4.6. Electrodialysis and electrodialysis reversal ED/EDR

ED/EDR utilize membrane separation of ions and charged molecules under electrical potential, hence it is removed from saline water [58], [106], [107]. ED/EDR has been explored for water desalination in specific applications of low salinity feed water desalination, such as brackish water [108], [109], or high salinity water, i.e., brine for minimization and increased productivity prior to final disposal [110], [111], [112]. ED/EDR poses many advantages such as the ability to utilize feedwater with higher suspended and dissolved solids, due to its high tolerance toward fouling, hence lower pretreatment requirements and high recovery [58]. However, ED/EDR has the disadvantage of relatively high direct electrical energy consumption, which is directly proportional to feed salinity and flow capacity [113]. ED/EDR is a straightforward process consisting of two electrodes, i.e., cathode and anode, with alternating chambers in between, with chambers separated by alternating order of anion ion exchange membrane AIX, which allows anions to pass through and retain cations, and cation exchange membranes CIX, which permeate only cations but not anions [114]. Anode attracts negatively charged anions, while cathode attracts positively charged cations upon application of electrical potential. In EDR, the polarity of electrodes is switched, i.e. cathode becomes the anode and vice versa, at certain time periods to lessen scaling and deposited material [58], [115].

2.4.7. Capacitive deionization CDI

CDI is similar to ED/EDR as it is driven by the electrical potential for separation of charged ions but uses electrosorption, i.e. electrostatic adsorption of ions to the electrode surface [116]. In CDI charged ions in feedwater are attracted to oppositely charged electrodes during the desalination cycle, hence producing product water, then the electrode charge is switched during the regeneration cycle to release adsorbed ions into brine [117], [118]. However, during regeneration oppositely charged ions can be adsorbed to the switched polarity electrode; thus ion exchange membrane, like those used in ED/EDR, hence called membrane capacitive deionization MCDI, has been utilized to improve regeneration efficiency [57], [116]. Most of the R&D efforts in CDI are geared to develop efficient electrodes with high electroadsorption capacity [91], [119].

2.4.8. Summary

Table 3 shows a comparison of different desalination processes. In general, the desalination methods that utilize phase change, i.e., thermal processes require high energy. On the other hand, membrane processes have less energy requirements but need adequate feedwater pretreatment. New technologies, such as FO and CDI don’t have fouling or scaling issues, but they have not been commercialized yet. The combination of two desalination technologies together, i.e., hybrid desalination, has the ultimate objective of maximizing water productivity at reduced specific energy consumption. Many configurations have been studied in the literature such as MED-TVC, MED-AD, MED-MD, FO-MD, HDH-AD, and MSF-RO [100], [120], [121], [122], [123], [124]. Desalination is an energy-extensive process that is mainly derived from conventional fossil fuels, with high energy consumption, although utilizing waste heat and energy recovery approaches [16], [125], [126]. Accordingly, desalination using fossil fuels is not considered a sustainable approach, especially with the volatility of fossil fuel prices and their decreasing availability. A key solution to this issue is to utilize renewable energies (REs) to drive desalination processes. Fig. 4 shows proposed combinations of RE with various desalination processes. According to the International Renewable Energy Agency (IRENA), renewables can meet rising energy demand, at low costs, and contribute to limiting global warming [127].

Table 3.

Comparison of various desalination approaches.

Type	Desalination Process	Advantages	Disadvantages
Thermal	SS: Solar still	•Low capital and maintenance costs. •Low external energy requirements.	­Low and intermittent productivity. ­Large land area is required.
	MSF: Multistage Flash	•High-quality desalted water. •Large production capacity. •Recovery and performance are independent of feedwater salinity.	­High energy consumption. ­Corrosion and scaling issues ­High capital cost. ­High-quality material of construction to withstand corrosion and high temperature.
	MED: Multi-Effect Distillation	•Compared to MSF •Low operating temperatures. •High thermal performance and lower energy requirements
	AD: Adsorption desalination	•Utilize waste heat. •Low energy requirements. •Less prone to scaling and fouling. •Ease integrability with MSF and MED.	­Low production rates unless coupled with MED or TVC. ­Not fully developed.
	HD: Humidification Dehumidification
	TVC: Thermal Vapor Compression	•Ease integrability with MSF and MED. •Utilize waste heat.	­High cost. ­Complex components.
	FD: Freeze desalination	•Low energy consumption. •Utilize cryogenic waste heat.	­Not fully developed. ­Has to be co-cited with LNG facilities.
	MD: Membrane Distillation	•Recovery and performance are independent of feedwater salinity. •Low operating temperature. •Utilize waste heat. •Low energy requirements.	­Membrane scaling and fouling issues. ­Temperature & concentration polarization. ­Requires heat-resistant membrane.
Mechanical	MVC: Mechanical Vapor Compression	•Lower energy cost than MSF and MED. •Suitable for high feedwater salinity. •High water recovery.	­Prone to corrosion ­Feasible only for small to medium-scale production.
	RO/NF: Reverse Osmosis/Nanofiltration	•Low energy consumption. •Low desalination costs. •No thermal energy requirements.	­Adequate pretreatment is required. ­Energy consumption is dependent on feedwater salinity. ­Membrane scaling and fouling issues. ­The material of construction should withstand high pressure.
Electrical	ED: Electrodialysis	•Low energy consumption •High water recovery •No fouling issues.	­Primarily used for low-salinity/brackish feedwater. ­Low productivity. ­Biofouling.
	CDI: Capacitive Deionization	•Low energy consumption •Low cost •Energy recoverability.	­Only feasible for low-salinity/brackish feedwater. ­Not fully developed
Others	FO: Forward Osmosis	•Low or no applied pressure. •Less membrane fouling than RO. •High water recovery.	­Requires regeneration of draw solution to extract water. ­Reverse salt flux results in loss of draw solution. ­Concentration polarization. ­Not fully developed.

Fig. 4.

Incorporation of renewable energy resources and desalination processes.

3. Renewable energies

Renewable energies (REs), or renewable energy resources (RERs), or renewables, in short, have fascinated much consideration in the last few decades as substitutes to commonly used conventional fossil energy sources. This is because they are: efficient, eco-friendly, emission-free, locally available, and became recently cost-effective as well. The share of REs in the global energy supply has been progressively increasing over the last three decades, almost doubling from 1.125 to 1.976 gigaton of oil equivalent (Gtoe) [128]. REs are sourced from renewable or continuous unlimited actions, hence can be reused continuously without harm to the environment. Among these renewable sources are the sun’s radiance, i.e., solar energy, the movement of water in rivers and oceans, i.e., hydro and tidal, and wind flow, i.e. wind, or earth internal energy, i.e., geothermal, or from continuing growth living matter, i.e., biomass, and many others [129]. The most important energy generation technologies that use renewable sources are solar photovoltaics (PV), solar thermal energy, hydroelectric energy systems, wind turbines, biomass energy systems, and geothermal energy power plants. These technologies present themselves as potential alternatives to conventional fossil fuel-driven energy generation technologies, along with being more environmentally friendly [31], [32].

Figure 5 below shows the evolution of REs electricity capacity both worldwide and in the MENA area throughout 2000–2021 showing the capacity to increase almost four times over the last two decades [130]. The increasing capacity is dominated by installations for solar and wind REs, growing from 2 % to 60 % of global RE capacity in 2000 and 2021, respectively, while the share of Hydro dropped from 94 % to 40 % over the same period. The figure shows that although of the huge REs potential in the MENA region it is still very less utilized with less than 1 % of the global REs electricity capacity dominated by hydro and solar REs. This to large extent can be explained by the region is rich with fossil energy resources, hence available at a low cost. However, many countries in the MENA area have set ambitious plans to expand REs capacity within their future development visions due to many environmental and economic drivers [131]. For instance, the KSA has inherited in its 2030 vision to change the energy mix with 50 % of the kingdom’s domestic energy demand, from the current 1 %, to be met by REs, hence displacing ∼1 million barrels of liquid fuel per day, while reducing the carbon emissions by 175 million ton per annuum (Mtpa) [132]. The UAE launched its “Energy Strategy 2050” in 2017 aiming to increase the share of REs up to 50 % of the energy mix, as compared to 7 % in 2020, while increasing energy efficiency by 40 %, hence reducing the associated carbon footprint by 70 % [133]. Similarly, Egypt has set plans for the REs to meet 20 and 42 % of electricity demand by 2022 and 2035 as compared to 10 % in 2020 [134].

Fig. 5.

Electricity capacity from renewable energies (REs) Worldwide (Kopp, #29), and the MENA region (Bottom) [130].

3.1. Solar photovoltaics (Solar PV)

Solar PV is a technology that implements P—N junctions to directly convert energy carried by the sun’s rays into electrical energy as a direct current DC. Since its invention in 1954 [135], solar PV technology has seen a huge development in terms of its efficiency and its applications [136]. There is a clear improvement in the efficiency of different PV technologies over the 1975–2020 period from the early 2–22 % reaching up to 48 % recently [137]. The DC power that is produced by the PV panels can be stored in batteries for future use, used directly to power DC loads, or used to power an AC load first by converting it to AC power, using an inverter. The PV systems’ power can drive electrically-driven desalination processes, such as RO and CDI [138], [139], [140].

3.2. Solar thermal energy (Solar-T)

Another RE technology that uses solar irradiance to produce usable power is solar thermal energy. Where the sun’s irradiance is absorbed as thermal energy, i.e., heat, in a heat transfer fluid, commonly water, which can vaporize and be used to drive a turbine producing usable work. To heat the water, various designs of solar collectors are implemented, such as: concentrating, flat plate, and evacuated collectors [43], [141], [142]. In order to use this technology to power desalination plants, two methods can be applied. The first one is using the produced work of the turbine to generate electricity, with electricity then being used to power an electrically driven desalination process, as in solar thermal- RO desalination plant. The second method is using thermal energy as a heat source to directly drive a thermal-driven desalination process such as MSF and MED [143].

3.3. Hydro energy

Hydro energy is based on extracting the kinetic and potential energy carried by a mass of water to produce usable electrical power. There are multiple types of power-generating hydroelectric systems [144]. The first is an impoundment system, in which a dam is used to collect the water of a river so that the water carries a considerable amount of potential energy. After that, the water is left to flow through a turbine that extracts the mechanical energy of water as shaft work. This energy could be transformed into electrical energy through a generator [145]. The second type is known as a run-of-river system. Here, the kinetic energy of water flowing in the rivers is directly extracted using turbines and then converted to power using generators. This electrical energy can be readily used to drive electrically-driven processes. Another type of hydroelectric energy technology is systems that utilize ocean energy. Ocean energy is defined as the kinetic, potential, and chemical energy carried by ocean water, and this energy can be extracted in different ways to produce usable electricity [146]. The four main types of ocean energies are salinity gradient energy (SGE), wave, tidal, and ocean-thermal energies.

Salinity gradient energy (SGE) is the chemical energy stored in seawater, as a result of the difference in salinity between two different bodies of water [147]. When two bodies of water with varying levels of salinity mix, a massive amount of energy is dissipated, which can be partially recovered as electrical energy using technologies such as pressure retarded osmosis (PRO) and reverse electrodialysis (RED) [148], [149]. This technology is of great potential where there is contact between high-salinity and low-salinity water bodies such as the discharge of rivers into seas and oceans, or at the discharge of treated wastewater into open seas [150].

Wave energy is defined as the kinetic energy that is carried by the moving waves of the ocean [151]. This kinetic energy can either be used directly to drive equipment that needs mechanical energy or transformed into electrical energy for use in general applications such as desalination [149]. Tidal energy is a type of ocean energy that is associated with the kinetic and potential energies of ocean water, as they rise and fall during high and low tides, respectively, with tidal generators used to extract tidal’s energy into electrical energy [152].

The ocean-thermal energy is the energy obtained from the temperature difference between the different water layers of the ocean [149]. This temperature difference can be used to run a thermodynamic cycle that produces useful work that can be used to generate electricity. There are two main types of ocean-thermal energy conversion (OTEC) systems, closed-cycle and opened-cycle [153]. In a closed-cycle system, a closed Rankine cycle with ammonia due to its low boiling point is used as the working fluid to produce useful work. In the open-cycle OTEC system, the warmer seawater of the shallow layers is directly used as the working fluid. This produced steam drives a turbine while being condensed to liquid water again [154]. What is special about an open-cycle OTEC, is that the condensed water is desalinated water, so it has a dual production of energy and freshwater [155]. This gives a further advantage of the open-cycle OTEC in comparison with other RE-driven desalination units and makes it a feasible option for countries that have access to oceans with feasible OTEC potentials, however, such energy is not a feasible option for the MENA region.

3.4. Wind energy

Wind energy is the fastest grown RE and represented the highest installed capacities in Europe in 2018. The main principle is to extract the kinetic energy carried by the wind, which can be converted to useful mechanical work, and then to electrical energy using wind turbines [156]. Wind energy is very site-specific as affected by geography and topography, i.e., landscape. Vertical axis wind turbines (VAWT) and horizontal axis wind turbines (HAWT) are the two main types of wind turbines [157]. In VAWT, the rotation axis of the wind turbine’s rotor is vertical, which allows the turbine to make use of wind coming from all directions. However, VAWT has the disadvantages of low efficiency, harder to support, and poor performance and reliability [158]. On the other hand, HAWT with an axis of rotation parallel to the surface of the earth, horizontally oriented, is more advanced, has higher efficiency, more accessible support, better access to higher wind speeds, and better overall performance and reliability. HAWTs can receive wind from a single direction only, hence requiring good and variable alignment to face wind direction which is realized using a yaw mechanism. The power generated from wind energy can be used to derive electrically-driven desalination processes.

3.5. Biomass energy

Biomass is defined as any organic material that undergoes photosynthesis. In this, the material uses the energy of the sun’s irradiance to convert water and carbon dioxide (CO₂) into carbohydrates. These carbohydrates store energy in the form of chemical energy, known as biomass energy [159]. There are many sources of biomass fuel, such as wood residue, agricultural residue, and combustible trash. The stored biomass energy can be extracted in various methods. The most common and simplest is combustion, in which the biomass fuel is burned to obtain a fraction of its stored chemical energy as heat [160]. Despite its simplicity, this method is the least efficient in heat generation. The second method is co-firing, in which the biomass fuel is added to coal to form a fuel mixture that is less expensive and environmentally harmful than pure coal. This method is more efficient than the direct combustion of biomass and helps to reduce CO₂ emissions from coal combustion [161]. The third method is thermal processes, which involve the conversion of biomass by pyrolysis or gasification into other highly combustible fuels such as liquid biofuel and bio-syngas [162], [163]. These various forms of biomass fuels can be used as fuel for thermal power plants, which in turn can be used to drive desalination.

3.6. Geothermal energy

Geothermal energy is simply the thermal energy or heat stored within the Earth. This energy comes from the hot core of the Earth and the decay of radioactive materials buried in the deep layers of the Earth’s rocks, with the Earth’s core at a temperature of about 6,000 °C, which is significantly hotter than its outer layers [164]. This temperature difference, compared to the Earth’s crust, causes heat to flow from the core radially outside. In some places, rainwater passes through the cracks in the Earth’s crust and reaches deeper regions with significantly higher temperatures, hence heated to high temperatures.

In geothermal power plants, this heated water is extracted by digging deep into the rocks and using pipes to carry the hot water to the surface, in what is known as a production well. On the surface, the vapor is produced from this hot mixture of liquid water and water vapor, which is to be used for driving the turbines and generating useful work and electricity. At the end, the remaining brine is returned to the Earth’s deep layers, using injection wells, in a cyclic approach [165]. This ensures the water can be reheated from the hot rocks downwards, and continue the cycle. Geothermal power plants have some unique advantages compared to other REs. These include their high stability, independent of weather conditions, and their lower operating costs [166]. For desalination applications, geothermal energy can drive different desalination processes, either directly as a heat source for thermally-driven processes, or by using the generated work and power to drive other electrically-driven processes [167].

3.7. Summary

To summarize, different types of REs seem to be technically and economically feasible to drive different desalination processes. However, an appropriate selection has to be made for the most technically and economically feasible combination of the specific RE and desalination process to maximize yield at higher energy efficiency. Table 4 shows a comparison between different REs in terms of appropriateness for desalination processes, resource availability in the MENA region, and stability and predictability of the renewable resource.

Table 4.

Comparison of renewable energy technologies.

Criterion	Solar thermal	Geothermal	Biomass	Photovoltaic	Wind	Hydro
More Appropriate for	Thermal-driven processes			Electrical-driven processes
Site constraints and availability in MENA	High availability	Limited availability, site-specific	Medium availability, site-specific	High availability	Medium availability, site-specific	Limited availability, site-specific
Continuity of power output	Intermittent power output (requires energy storage)	Continuous power output	Continuous power output	Intermittent power output (requires energy storage)		Mostly continuous, Varies with the location and season
Predictability of power output	Relatively predictable	Predictable	Predictable	Relatively predictable	less predictable	Predictable

4. Energy storage systems for renewable energy-driven desalination systems

One of the main challenges for expanding the utilization of most REs is the inherited intermittent nature associated with power output [168]. Though many REs are widely available, they cannot be relied on entirely. To increase the reliability of such systems, means of energy storage (ES) have to be employed to smooth power output. ES systems help to store surplus energy during on-peak power output time and release it during off-peak times [169]. An example of this is using batteries to store excess electrical energy that is produced by solar PV panels during the daytime to be used at nighttime. There are a vast variety of ES technologies that are commonly categorized by their mechanism of energy storage [170]. Types of ES approaches include thermal energy storage (TES), electrical energy storage (EES), mechanical energy storage (MES), and chemical energy storage (CES). In the next sections, a detailed description of the ES technologies suitable for REs available in MENA regions will be discussed, while highlighting the future research potential of energy storage technologies.

4.1. Thermal energy storage TES

TES systems, store and release the heat, either as sensible, latent heat, or heat of chemical reaction. This energy storage approach is the best fit for thermal desalination processes such as MSF and MED, hence providing a smooth and stable supply of heat to the process. TES can be categorized according to the storage form to latent heat, sensible heat, and thermochemical TES [171].

4.1.1. Sensible heat thermal energy storage

This type is one of the most commonly used TES technologies. To store sensible heat in a material, the temperature should be in a range away from any phase-change temperatures. An example of this is storing thermal energy in liquid water between temperatures of 5–95 °C. Multiple specifications are needed from the sensible heat storage material, such as high specific heat capacity, being liquid or solid i.e., incompressible material, to avoid pressure increase, and a wide operating temperature range close to that of the desired application [172]. Other important factors include chemical and mechanical stability, good thermophysical properties, efficient reversibility of the heating/cooling, i.e., energy charge/discharge cycles, eco-friendly, and low cost [173].

Among the many promising materials for sensible heat TES, liquid water is one of the most used materials due to its remarkably high specific heat capacity, C_p of 4.19 J/g, along with vast availability [174], [175]. However, one of the main limitations of using liquid water is its limited liquid state temperature range, which ranges from 5 to 95 °C. Also, it would be unfeasible to use it with desalination processes that operate above that range. Accordingly, liquid water can be used efficiently as TES for low-temperature thermal processes, such as HDH, MD, and solar stills (SSs) [176]. For high-temperature applications, using other materials with a wider operating temperature range of stability is recommended. Examples of these materials are solid materials with high melting points and molten salts, as they can efficiently store thermal energy between 200 and 500 °C [174]. In addition, liquids with high boiling temperatures such as oils can be used. To properly size the TES system of a desalination plant, multiple factors must first be considered. These include the type and availability of energy source, the type of desalination technology, the desalination productivity or output required, the thermophysical properties of the storage material, the estimated period of storage i.e., storage capacity, and estimated thermal losses from the TES system [177], [178], [179].

A good example of using sensible heat TES with RE-driven desalination is energy storage in SS desalination systems [180]. SSs use the heat absorbed from the solar radiation to evaporate water, and then recollect it in a freshwater tank via an inclined glass cover. To give the SS a TES’s proficiency, the depth of water in the tank can be increased. In this case, the saline water, with a higher density will be mostly at the bottom and will stay hot for a long time after the sunlight goes, meaning the SS will stay productive for a longer time, but at less production rate [181]. Also, a thin layer of insulation can be placed on top of the water mass to reduce the amount of evaporating water during the daytime [182]. Another method for providing energy storage in SS systems is by putting a layer of solid sensible heat TES material under the water mass to store the heat from solar radiation. In one study, sand was used as TES material laid at the bottom of a SS. It was found that the SS’s productivity had increased the productivity by about 12 % enabling continuous production for almost 4 hr after sunset [183].

Another promising method of implementing sensible heat TES with RE-driven desalination systems is using salinity gradient solar ponds [184]. Solar ponds can be considered large-scale solar collectors. They store solar energy thermally in their layers, especially the lowest layer. Solar ponds are commonly divided into three zones: upper convection zone (UCZ), gradient zone, and lower convection zone (LCZ), which store solar energy as heat. Different types of salts can be used in solar ponds, such as MgCl₂ and NaNO₃, but NaCl is the most used salt [185]. The thermal energy stored in the solar pond can be used in two ways to drive the desalination processes. The first is, directly using the heat of the LCZ to drive low-temperature thermal desalination processes. The other way is to use such heat to drive a Rankine cycle for power generation, which can be used to drive electrically- and mechanically-driven processes. The brine of these desalination systems can be returned to the pond to help with energy storage [185]. Solar ponds were applied in multiple desalination projects in the MENA region. In Morocco, a solar pond was connected to a multiple distillation and flashing system, a hybrid MED-MSF system [186], [187]. This hybrid system was also connected to a thermal plant nearby, to recover the waste heat of gas exhaust from the thermal power plant [188].

4.1.2. Latent heat thermal energy storage

Latent heat TES refers to storing thermal energy in materials undergoing phase change processes, such as melting/solidification. Materials that are used for latent heat TES are known as phase change materials (PCMs) [189]. The heat absorbed and released during phase-change is much higher than that of a sensible-temperature change, as a result, PCMs can store much thermal energy in a much smaller mass or size, compared to sensible-heat TES materials [190]. There are multiple ways to incorporate PCMs in TES systems. The first method is by encapsulating the PCMs and adding these capsules to a liquid-state sensible heat TES material [191]. In this case, both the PCM and the liquid will contribute to TES. When energy is stored, the capsules will melt the PCM inside them and store latent heat, while the liquid will have its temperature increase. During discharge, the PCM will solidify and the liquid will decrease in temperature, and so forth [192]. Another way of integration is by filling a reservoir with a PCM, where a working fluid is allowed to flow over the PCM to both give and extract the thermal energy in charging and discharging scenarios, respectively.

Some important technical factors must be taken into consideration when designing latent-heat TES devices [189]. PCM selection is the most important criterion as it should well match the intended applications. There is a wide variety of PCMs, and each of them has different specifications. Material selection for a certain application includes high enthalpy of phase change, and its phase change temperature should be close to that of the desired application [193], [194]. For example, if an MSF desalination unit powered by a concentrated solar power (CSP) plant needs to be connected to a latent-heat TES device, the phase change temperature of the PCM must be close to that of the working fluid coming from the CSP plant [195], [196]. This is so that the PCM will undergo phase change as a result of the absorbed heat. As high-temperature TES is possible using PCMs, this type of energy storage can be well implemented with high-temperature thermal processes, such as MSF and MED. Also, the PCM should be of good thermophysical properties as well as be thermally, chemically, and mechanically stable. The thermal performance of PCM should be stable as well over a long lifetime of energy charge/discharge cycles. Furthermore, the TES device should be well insulated, so that the thermal losses to the environment are minimized.

Latent-heat TES devices were implemented in RE-powered desalination systems in the MENA region. A study carried out by Tabrizi et al., compared the productivity of weir-type SSs with and without thermal storage [197]. A latent-heat TES system with paraffin wax as a PCM was incorporated into the system design. The results indicated that the system’s productivity had increased significantly on cloudy days to 3.4 kg/m²d, as compared to 2.1 kg/m²d without energy storage. Another work was carried out in KSA by El-Sebaii et al. investigated the effect of integrating latent-heat TES on single basin SS’s performance [198]. It was found that increasing the mass of the PCM resulted in decreasing daytime productivity, but resulted in a significant increase in nighttime productivity, thereby increasing total daily productivity. The productivity with PCM utilized has reached 9 kg/m²d along with efficiency improvement up to 85 % as compared to 5 kg/m² day without PCM. These results prove that latent heat TES systems are a very promising energy storage option for renewable energy-powered thermal desalination systems in MENA.

4.1.3. Thermochemical thermal energy storage

Thermochemical TES systems store thermal energy in reversible chemical reactions. In this type of reaction, a change in chemical composition is combined with heat absorption/release, depending on the reaction direction [199]. This kind of TES is similar to latent-heat TES or PCM, as the heat is absorbed/released at little temperature variation. Thermochemical materials have an extremely high enthalpy of reaction, which exceeds that of many PCMs. As a result, thermochemical TES devices are expected to be much smaller in size and with superior TES capabilities [200]. To choose the thermochemical material for ES applications, multiple factors should be addressed. These factors include the material’s cost and availability, reversibility of the chemical reaction, the safety of the chemical process, reaction temperature, and enthalpy of the reaction [201]. However, thermochemical TES devices are still in the early development stage, and a lot of work is still to be done before the commercialization of such devices can occur [202]. Thermochemical TES devices can be a potential energy storage candidate for high-temperature thermal processes, such as MED and MSF.

4.2. Electrical energy storage (EES)

Various types of RE technologies generate electrical energy as a primary output, and this output can be used to drive electrically- and mechanically-driven desalination technologies. Examples of this are systems that rely on solar PV or wind energy to derive RO or ED desalination. For these systems to stay productive in weather conditions where power production is limited, such as cloudy weather or low wind speeds, electrical energy storage (EES) systems must be included in the plant design. There are multiple EES technologies currently available, however, batteries are the most developed and commercially available technology among them. Other EES include capacitors, supercapacitors, and superconducting magnetic (SMES).

Batteries are a type of EES device that stores electrical energy internally as chemical energy [203]. Batteries rely on the mechanism of reduction/oxidation, i.e., redox reactions to convert the electrical energy into chemical energy, and vice versa [204]. Batteries are widely used in many applications starting from smartphones and laptops to large-scale ES in power stations [205]. There are two main types of batteries, i.e., primary and secondary batteries. Primary batteries are non-rechargeable batteries that are usually discarded after getting fully discharged. These include Zn/MnO₂ alkaline batteries, Li/FeS₂ batteries, and Zn/Air batteries [206]. The second type is secondary batteries, which can be recharged several times before they are discarded such as Li-ion, nickel-metal hydride NiMH, and lead-acid batteries, which are very suitable for the cyclic nature of RE generation [203], [207].

Fig. 6 shows a schematic of a solar PV-driven desalination system, with batteries as the main energy storage tool. In this system, the PV panels will generate electrical power during the daytime, which is used to drive the desalination unit. The excess energy is simultaneously stored in the batteries, which are to be used during cloudy days or at nighttime, to stabilize the water productivity. If a full-day operation of the desalination system is desired, proper sizing of the PV array and the battery system is crucial. A very similar case happens in wind-powered desalination systems. In such systems, not including an EES device will significantly affect the reliability of the desalination unit. However, connecting the system to the power grid is a good alternative to local energy storage depending on its availability.

Fig. 6.

Schematic of solar PV driving desalination system with battery energy storage system.

In KSA, a group from King Abdulaziz City for Science and Technology (KACST) built a PV-driven RO desalination plant, with batteries as an ESS [208]. They used SSs to increase the production of the desalination plant by desalinating the brine of the RO unit. Another study, in KSA, investigated the implementation of a hybrid PV-Wind energy system to drive RO desalination system with battery as an EES system [209]. In the minimum levelized cost of water (LCOW) optimized system, it was found that 6 batteries were needed for a 12-hour operation and 16 batteries were needed for a 24-hour operation.

4.3. Other energy storage technologies

Although energy storage either as TES or EES are the most mature and well-established ES for REs, other types of ESs could be implemented. The interest in other ES technologies comes from the high cost of batteries, along with other technical challenges upon scaling up, which makes building large-scale systems costly and challenging. The most interesting option is mechanical energy storage (MES) technologies, as they are well suited for large-scale applications and can be easily paired with electrical generators to produce the required electricity. These technologies include pumped hydro storage (PHS), compressed air energy storage (CAES), buoyancy work energy storage (BWES), and flywheels. PHS can be used directly to produce freshwater if integrated with an RO unit, which results in decreasing the capital cost of the RO system, and the environmental impact [210]. Also, CAES systems provide a cheap option for energy storage from large PV or wind-powered desalination plants. However, further investigation of the applicability of such devices needs to be carried out, as no actual systems have been tested for RE-driven desalination.

4.4. Summary

In general, ES plays a crucial role in enabling the utilization of REs full potential, hence smoothing the power supply to drive the desalination process to provide a continuous water supply. The ESs in this regard have to be sized in terms of storage capacity, power output, and response/discharge time at rated power, among many other factors [211], [212]. Figure 7 below shows a comparative schematic of different ES technologies relative to these factors.

Fig. 7.

Comparison of different energy storage systems (ESSs), discharge time vs power (Kopp, #29) [212], and power vs energy stored (bottom) [211].

5. Renewable energy resources RERs in the MENA region

The MENA region has a variety of RERs, such as solar, wind, hydro, and biomass, all of which present possible alternatives to conventional fossil fuels. Despite their availability, RERs are not well established as primary energy sources, and the main power supply is met by fossil fuels. In this section, the availability of various RERs in the MENA region is critically discussed, with insight into their potential power supply and implementation levels.

5.1. Solar PV

Due to their distinctive geographic location, most MENA countries have great solar irradiance resources that can be utilized as a primary energy source. This solar irradiance can be very valuable to many MENA countries, as it can be used for power generation, where most MENA countries need a sustainable energy source. The energy carried by solar irradiance can be extracted either using photovoltaics or by using solar thermal systems. For the first option, the MENA region has a very promising solar PV power potential, as shown in Fig. S1. Most MENA countries have agreeable solar power resources that can be utilized for generating power using PV technology. This is illustrated by the good values of specific yields in the range between 4.6 and 5.4 kWh/kWp daily and 1,680–1,972 kWh/kWp yearly [213]. The countries with the highest photovoltaic potential are Algeria, Egypt, Libya, KSA, and Yemen.

5.2. Solar thermal energy

The second method of utilizing solar energy is by using solar-thermal systems, which absorb the thermal energy component of solar irradiance and use it as a source of heat for thermally-driven applications, or indirectly to generate power, which can be used later in many applications. There are two types of solar-thermal systems, with the first is that works on absorbing all incident radiations, i.e., global irradiation, regardless of the irradiances' direction, i.e., horizontal or vertical irradiations, with an example of a flat plate solar collector. To evaluate the suitability of a specific location for such systems, the Global Horizontal Irradiance (GHI) of that location is first evaluated. The GHI is a great tool, used to estimate the solar potential of a given location and compare it to other competitive sites. Fig. S2 shows the GHI values over the MENA region. From the figure, it can be concluded that GHI increases for the regions closer to the equator- in other words- in the South of the MENA region [213].

The second type of solar thermal technology makes use of only one direction of solar irradiance, the irradiance coming directly from the sun to the device, known as the Direct Normal Irradiance (DNI). An example of this technology is the CSP technology, which is designed only to absorb the DNI. Fig. S3 shows the DNI values in the MENA region. Algeria, Egypt, Jordan, Libya, Morocco, KSA, Syria, and Yemen have the highest DNI values [213].

5.3. Wind energy

One of the most attractive RERs is wind energy. The main indicator for the availability of wind energy is the wind speed, as power density is proportional to the cubic power of wind speed, i.e., P/A = 0.5ρυ³, where P is power in W, A is wind turbine rotor area in m², ρ is air density kg/m³, and υ is the wind speed in m/s. Fig. S4 illustrates the average wind speed in the MENA region. Some locations in the MENA region have a relatively high average wind speed, which exceeds 8 m/s [214]. Fig. S5 shows the wind power density potential in the MENA region in W/m² of the wind turbine rotor area. Most MENA countries have mediocre wind energy resources, except for some areas mostly in Algeria, Egypt, Iran, Iraq, Libya, Oman, and Syria.

5.4. Hydroelectric energy

Hydroelectric energy, along with its subcategories including ocean thermal, wave, and tidal energies has attracted a drastic increase in attention in recent years. Despite its low energy cost and attractive advantages, hydroelectric energy is heavily reliant on geographic location, and cannot be utilized everywhere on the globe. For example, run-of-river energy can only be obtained when rivers are available in the country, with proper topography. Ocean thermal energy can only be utilized in regions near oceans and with a high ocean temperature gradient. In MENA, not all countries have access to usable hydroelectric sources. However, some countries are already utilizing hydropower successfully, such as Egypt and Iraq. Table S1 presents the total installed hydropower capacity and the total generation amounting to 21,430 MW and 33.6 TWh in 2018 in the MENA region, respectively, out of estimated annual potential of 182 TWh [215].

5.5. Biomass energy

Another promising RER is biomass-based fuels as bioenergy, which come from several sources, such as waste management plants, municipal wastewater treatment plants, agriculture waste treatment, and many other sources. These feedstocks are widely available in most countries, especially in agricultural countries, where agriculture is the steering wheel of the economy [216]. Table S2 shows potential biomass energy resources and estmated power capacity in MENA region countries of 27.4 mtoe and 107 TWh, respectively [217]. Only a few countries have implemented biomass energy as a considerable energy source, while the rest have not yet utilized it.

5.6. Geothermal energy

Geothermal energy is a promising RER that can be utilized in electricity generation, domestic heating and cooling, desalination, and many other applications [127]. In the MENA region, huge energy is used for air conditioning, especially in regions with a hot climate, such as the Arabian Gulf countries, where massive energy is used for desalination as well. In the last few years, increasing consideration has been given to utilizing geothermal energy, especially for desalination, more specifically to drive thermally-driven processes. Table S3 shows the total geothermal installed capacity, energy generation, and generation potential in the MENA region of 390 MWh, 1392 GWh, and 239 TWh, respectively. Despite the huge electricity generation potential in many MENA countries, such as KSA and Yemen, it is evident that in the MENA region, the only uses of geothermal energy are in direct use form and to less extent of the available potential [130]. In these cases, geothermal heat is used directly for heating or cooling purposes, but there are no applications for using geothermal energy to produce electricity.

5.7. Summary

In conclusion, it was found that MENA countries have a wide variety of RERs that can be used effectively to power desalination plants. Table 5 summarizes the availability of such RERs in the MENA region. Despite the availability of many RERs and the huge energy demand, many MENA countries still haven’t utilized most of their RE potential. Examples of this include the geothermal potential of Yemen and the hydropower potential in Iraq, and importantly solar energy over the whole region. This is due to the relatively cheaper prices of fossil fuels, and the high capital costs of RE power plants. The growth of the RE requires advances in RE technologies combined with increased fossil fuel prices. However, the recent advances in REs and ES have encouraged many countries to have REs at the core of their plans for the new installed electricity capacity and as a critical element of the national vision.

Table 5.

Summary of the availability of renewable energy resources in MENA.

Renewable energy	Availability
Solar PV	Abundant in all MENA countries, specifically Egypt, Jordan, Libya, Saudi Arabia, and Yemen.
Solar thermal	Abundant in all MENA countries, specifically Egypt, Jordan, Libya, Morocco, Saudi Arabia, and Yemen.
Wind	Available mainly in the North Africa region, specifically Algeria, Egypt, Iran, Libya, Morocco, and Syria.
Hydroelectric	Only available in countries with specific geographic topography, specifically Egypt, Iran, Iraq, and Morocco.
Biomass	Mostly available mostly in agricultural countries, specifically Egypt, Jordan, Iran, Iraq, Morocco, Saudi Arabia, and Syria.
Geothermal	Only available in countries with specific geographic topography, specifically Egypt, Jordan, Iran, Saudi Arabia, and Yemen.

6. Progress of implementing renewable energy-driven desalination in the MENA region

The desalination market has grown considerably in the last few years. In the MENA region, there has been a substantial shift towards renewables in the energy sector. Since desalination is a major energy demanding sector, RE-driven desalination has been studied extensively and implemented in many countries [218]. As a result, REs have become a considerable choice when installing new desalination capacities. Table 6 shows the different configurations of RE-driven desalination systems and their share of total RE-driven desalination installed capacity worldwide. The table shows that almost 1/3 of the capacity is driven by solar PV- RO, followed by wind-RO representing almost 1/5 of the desalination capacity. This can be attributed to the lower energy demand of the RO desalination process along with the generation of direct electrical energy from both solar PV and wind, which is used directly to drive the desalination process [219].

Table 6.

Worldwide used renewable energies-desalination technologies [219], [220].

Renewable energy-desalination system incorporation	Capacity share, %
Photovoltaic-Reverse osmosis (PV-RO)	32
Photovoltaic-Electrodialysis (PV-ED)	6
Solar thermal-Multi-effect distillation (Solar T-MED)	13
Solar thermal-Multi-stage flash (Solar T-MSF)	6
Wind-Vapor compression (Wind-VC)	5
Wind-Reverse osmosis (Wind-RO)	19
Others	19

6.1. Solar photovoltaic-powered desalination

RO desalination driven by solar PVs has been a topic of interest since the early 1980s when the first-ever commercial PV-RO pilot plant was commissioned in the MENA region in 1981 in Jeddah, KSA [221]. The PV-RO unit is driven by an 8 kWp PV system, with freshwater productivity of 3.2 m³/day. The integration of RO with PV is applicable for small, medium, and large-scale desalination units, with reasonable costs. Fthenakis et al. recently performed a theoretical case study on PV-RO in KSA [222]. The study compared three PV systems; a concentrated dual-axis tracking PV (CPV), CdTe one-axis tracking PVs, and CdTe a fixed-axis tilt PV at two different power outputs of 1 and 3 MW. Two capacities were considered: a small plant at 6,550 m³/day, and a large plant at 190,000 m³/day, showing the results obtained as present in Table S4. The Levelized cost of electricity (LCOE) was estimated to be around $0.21/kWh, a slightly higher number than the LCOE of diesel, which was $0.17/kWh at the time of the study. The desalination cost breakdown of the PV-RO system is present in Fig. S6, showing that almost 13 and 56 % of the total cost was for the PV capital and RO annualized capital costs, respectively.

Alsheghri et al. studied an RO plant powered by solar PV to secure 200 m³/day for Masdar Institute in Abu Dhabi, UAE [138]. Multiple solutions were investigated to secure stabilized power supply, including installing batteries as well as producing and storing surplus water during production time. Due to the batteries high cost, land limitations, and complexity of the larger capacity PV system, running the RO unit from the grid and having the PV produce equivalent energy to that consumed by the RO was found to be the optimum solution, as it ensures production continuity at reduced GHG emissions. The results proved that the plant can produce water at 56 m³/hour, for $0.49/m³, which is higher than subsidized customer prices for domestic water supply, bust still competative to bench mark desalination cost of $0.5/m³. Furthermore, the system reduced annual GHG emissions by 1,035 tCO₂. The work focused on the financial feasibility and cost analysis of the PV-RO project, and provided detailed multi-scenario site-specific cost estimates.

Abdallah et al. examined the performance of a small PV-RO desalination system in Amman, Jordan [223]. The experiments were conducted using a fixed-axis PV system, and east–west one-axis tracking PV, which showed a 15 % and 25 % increase in water and power production respectively. Although a clear description of the PV-RO system was provided, the work lacked financial analysis or a feasibility study. Another PV-BWRO plant was installed in Hartha Village, Jordan [224]. The plant consisted of eight PV modules with 433 Wp, 2 batteries (230 Ah, 12 V), a pre-treatment system, and an RO unit. The plant was operated 24 h a day, producing 9.3–53 l/h, depending on the operating pressure and water recovery, at a specific energy consumption SEC of 1.9 kWh/m³. The system operated smoothly throughout the year and produced water at an adequate rate. The PV system was found to produce surplus energy in the summer months. The discussed two studies have affirmed the feasibility of PV-RO as a sustainable freshwater supply in water and fossil energy-deficient countries such as Jordan. However, more studies, economic analyses, and a review of the country’s policy regarding renewables are required to fully assess the economic and technical feasibility of the PV-RO systems.

Al Suleimani and Nair studied a PV-BWRO system located at Heelat ar Rakah camp, Muscat, Oman [225]. The system consists of a 3.25 kWp PV module and a 200 Ah boost charge battery. The expected lifetime of 20 years at 5 m³/day. The system has 25 % less cost as compared to a conventional diesel-powered RO. The use of battery as ES media was essential for the continuous operation of off-grid PV-RO systems. The work showed a good economic potential for PV-RO in remote locations being more cost-effective compared to conventional systems. However, the cost of batteries and their limited lifetime remains a major concern. An alternative to ES is to integrate the PV-RO with backup diesel generators Although it might not be environmentally friendly or sustainable, their low cost and providing continuous productivity make them an economically and technically favorable option. Many other storage options for PV-RO systems are also available, such as fuel cells (FCs). A study of a PV-RO/FC system with 150 m³/day capacity, showed that the system was both economically viable, and achieved a reasonable cost of electricity (COE) [226]. FCs also have higher efficiencies, longer lifetimes, and lower environmental impacts, compared to batteries and generators. The work showed that using an FC reduced carbon emission by 71 tCO₂/year.

From the previous discussion, it is proved that PV-RO systems seem to be economically and technically feasible options for integrated RE-powered desalination systems. This is attributed to the best fit between the two processes, i.e., the high-efficiency power generation of PV, with output as electrical energy used to power the low-energy RO desalination process. Additionally, PV technology has reached a cost of electricity of PVs that is currently competitive with conventional fuel, but at lower GHG emissions, hence PVs are a very attractive and beneficial option in both the short and long run. On the other hand, PV-powered ED/EDR seems to be a promising option for low-salinity and brackish groundwaters. However, it has not been widely investigated on a large scale, with further research and experimental studies still required.

6.2. Solar thermal-powered desalination

The MENA region is known for having abundant direct solar radiation, which is ideal for CSP technology. According to Trieb and Steinhagen, the Middle East alone receives solar energy equivalent to 1.5 billion barrels of crude oil per year [227]. The authors also discussed that CSP-powered desalination is a promising alternative to conventional desalination and offers both sustainable and secure freshwater supply. Moreover, CSP desalination plants can produce both electricity, and freshwater at competitive prices [228], [229]. In the work of Shatat et al., the opportunities and challenges of solar seawater desalination worldwide were investigated [230]. It was concluded that the solar energy potential in the MENA could help solve the water scarcity dilemma in the region. Thermal desalination in general, and MED in particular, is the most compatible with solar thermal energy because MED is less prone to scaling and more flexible for operating at partial loads, in addition to the high thermal performance [231], [232].

Hassabou et al. analyzed the techno-economic performance of a 5,000 m³/d Parabolic Trough Collector (PTC) powered MSF plant in Egypt [231]. The PTC plant had a thermal storage system that utilizes PCM, and a backup heater that uses natural gas. The high-temperature fluid from the PTC was passed through the PCM storage tanks first and then used in the brine heater of the MSF. The analysis compared the freshwater cost of the PTC-MSF vs fossil fuel-driven MSF, which was found to be three times higher. Due to this, the authors suggested a hybrid natural gas-solar MSF system as an attractive alternative. Furthermore, Hamed et al. evaluated the technical and economic performance of a hybrid CSP-driven TVC-MED in Al-Jubail, KSA [233]. The CSP utilized Fresnel collectors of 55,737 m² and thermal power of 13.6 MW_th. Overall, the hybrid plant consisted of the Fresnel solar field; an auxiliary boiler that ran on fossil fuel to ensure continuous operation, and the TVC-MED system, where the electrical requirements of the TVC-MED were supplied by the grid, as shown in Fig. S7a. Their results showed that the hybrid plant has a Levelized Cost of Water (LCOW) of $2.2–6.8/m³, depending on the fuel price and the implementation of thermal storage, compared to $1.43–7.1/m³ for a fuel-driven system, with an equivalency cost point close to $92/bbl at DNI of 1,132 kWh/m², as shown in Fig. S7b, which dropped to $52/bbl at DNI of 1,938 kWh/m². It was shown as well that thermal storage increases the LCOW due to the associated capital and operating costs. This also shows the vital role of any subsidies offered to renewables and the effect of oil prices and taxes applied to fossil fuels such as carbon taxes.

Saffarini et al. evaluated three different solar-powered MD systems economically under the weather conditions of the UAE [234]. Namely, DCMD, AGMD, and VMD. The analysis performed has shown that DCMD was the most cost-effective. The work has indicated as well that almost 70 % of the system cost was due to the solar thermal subsystem, hence improvements in its efficiency will help to decrease the overall cost. Soomro and Kim investigated the performance of a PTC integrated with DCMD, in Abu Dhabi, UAE, as shown in Fig. 8a [235]. The simulated PTC had a capacity of 50 MWe, equipped with TES tanks. The PTC is used as a combined heat and power (CHP) system. Eventually, freshwater was produced at an average capacity of 14.33 m³/day, a cost of $0.64/m³, and average electricity production of 10.8 GWh, as shown in Fig. 8b and 8c. Similarly, Mohan et al., analyzed experimentally, the performance of a novel solar thermal poly-generation (STP) plant in Ras Al Khaimah, UAE [236]. The plant involved: evacuated tube solar collectors, an AGMD module, an absorption chiller, and a heat exchanger as shown in Fig. 9. Hot water produced from the collectors was stored in a TES tank and later used to drive the MD unit and the absorption chiller. The cold water produced by the chiller was used for cooling, through fan coil units. Moreover, the hot brine discharged from the MD process was used for producing domestic hot water. The STP system has a 23 % increase in energy efficiency, with technoecomnoic analysis indicating that land cost is a vital component that can lead to reducing the payback period by 18 % and increasing net cumulative savings by 10 %.

Fig. 8.

(a) PTC-MD integrated system, (b) Monthly average electricity production, and (c) Monthly average freshwater production [235].

Fig. 9.

(a) Schematic of Solar-Poly generation system, (b) Evacuated tube of the Parabolic Trough Collector (PTC), and (c) Components of the poly generation system [236].

Palenzuela et al. simulated and evaluated multiple configurations of CSP to power a desalination plant in Abu-Dhabi, UAE [237]. The CSP system involves a PTC field, a TES tank, and high-pressure (HP) & low-pressure (LP) turbines, as illustrated in Fig. S8. The steam generated is used to drive the HP turbine, and then the steam is to be reheated and fed to the LP turbine to increase the energy efficiency. Three systems are investigated, a PTC-RO, a PTC-LT-MED (low-temperature MED), and where the heat for the LT-MED is extracted from the reheated steam of the LP turbine. The third system is an LT-TVC-MED, in which the TVC is used to drive the MED. For all systems, the net power production of the CSP is 50MWe, and the daily-obtained water production is 48,498 m³/day. The results have shown that the PTC-LT-MED is thermodynamically more efficient than the CSP-RO, with a smaller solar field requirements at the same power and water output.

In Gaza strip, Hamdan et al. reported that water scarcity could be solved using a co-generation plant that produces both freshwater and energy [238]. The co-generation plant can be powered either via fossil fuels or solar energy. The authors offered a comparison, taking into consideration both economic, and environmental impacts. The proposed plant had a water capacity of 100 million m³/year, and a power production capacity of 2.5 TWh/year. Three major subsystems have been designed for the plant: a solar field, a power plant, and a desalination unit. CSP is the planned solar technology to be used, employing both PTC and linear Fresnel (LF) collectors, to minimize the cost of the plant, as LF has a lower cost compared to PTC, with a molten salt TES system. In terms of desalination, MSF was found to be the most suitable desalination technology for the proposed capacity and high feedwater salinity at a total estimated cost of about $1.1–1.3 billion for the CHP plant. Similarly, Abu-Jabal et al. tested a pilot solar multi-evaporation plant in Gaza [239]. The plant was fully powered by solar collectors and PV cells. The plant was considered a zero-emission and zero-discharge. The highest water production was obtained in July, with 204 l/d while in December it was the lowest at 85 l/d. Eventually, this type can be installed in rural areas, due to its rigidity and the fact that it does not require much maintenance.

A small solar desalination plant has been proposed to provide freshwater in remote arid regions in southern Algeria [240], [241]. The authors recommend SSs for multiple reasons, including their simplicity, low cost, and low maintenance requirements, making them convenient for remote communities. The feedwater is geothermal brackish water since it is available in southern Algeria. The author also experimented for several days using a small capillary film solar distiller under Algerian weather conditions, in which the system has shown productivity of 15 l/m²d.

Since Tunisia is located in an arid region with high demands for freshwater, the country has been a pioneer in the research of solar desalination since the early 1920s [242]. The first-ever solar evaporation plant was installed in 1929 in the city of Ben Garedna, with a capacity of 1,800 m³/d [243]. A feasibility study of a desalination plant at Sidi-Elhani, Tunisia was also carried out, in which the plant consisted of three sub-stations, utilizing 300 SSs. The average distillate production was 10 m³/d, using brackish water or collected rainwater [244]. Organizations such as the Tunisian Atomic Energy Commission (TAEC) and National Institute for Scientific Research (INRST) built several solar desalination plants in an attempt to examine their feasibility in the country. TAEC built several single-basin SS systems to desalinate brackish groundwater and produce freshwater in the 1960s. INRST built brackish water and seawater desalination system, driven by a hybrid solar-wind energy system at its site in Burj-Cedria, in the south of the Tunisian capital of Tunis City. The result concluded that RE-powered desalination can be tested experimentally for scientific research, but it is still not ready for commercialization, due to economical constraints [245].

Banat et al. studied the performance of a solar-driven MD plant located at the Marine Science Station in Aqaba, Jordan [246]. The system is comprised of two main loops: solar and desalination loops. The solar loop has a 72 m² field of flat plate collectors, and the desalination loop has 4 MD modules in parallel, along with PV panels to drive utilities. The heated water from the solar collectors is passed through the MD, while the cold feedwater is passed in the opposite direction. The freshwater production varied between 144 to 792 m³/day, with an SEC of 200–300 kWh_th/m³.

6.3. Wind energy desalination plants

Although wind energy is available in some regions in MENA, not much progress was made in implementing it with desalination systems. Mohsen et al. studied the potential of wind energy to drive RO desalination in Jordan [247]. The study included 11 potential locations in Jordan, based on the average wind speed, which was 1–7 m/s varying over the year, which result in a monthly water productivity range of 50–550 m³, with the highest freshwater potential found to be in Ras Muneef, while the lowest was in Deiralla.

A case study that explored the potential of wind-driven RO in Ténès, Algeria was completed by Dehmas et al. [248]. The wind speed data, site geography, and topography were analyzed. The average wind speed and power density were estimated at 6.54 m/s and 485 W/m². Based on these results, a 10 MW wind farm was the selected capacity to satisfy a 5,000 m³/d RO unit. The economic analysis demonstrated that the Wind-RO plant was feasible, and can produce electricity at a competitive cost of $0.07/kWh in 2011.

Loutatidou et al. studied a variable flow RO plant driven by wind energy in many sites near the Western Region of Abu Dhabi., UAE [249]. The key objective of the study was to evaluate the feasibility of such a system. The RO plant was considered a variable flow plant, meaning that the pressure and flow rate were varying within a particular range to coup up with the available wind power generated. Several capacities were studied to size the wind turbines. The wind speed at the evaluation site was 2.1–5.9 m/s over the year. The LCOW was $ 1.57–1.63, 1.83–1.96, and 2.09–2.11 per m³ for capacities of 7,000, 10,500, 14,000 m³/d, respectively. The study showed that the Wind-RO plant was mainly feasible as complementary to existing desalination plants, in which it could be used whenever the demand for freshwater rises. However, it is still not feasible to be used independently, as its water production is unpredictable, due to the relatively poor regional wind resources and its high variability.

6.4. Geothermal energy desalination plants

A significant effort was made on studying the potential of implementing geothermal energy to drive desalination in the MENA region. Bourouni et al. built a geothermal energy-driven aero-evapo-condensation desalination device in south Tunisia [250]. This device used geothermal energy to drive HDH seawater desalination at a small daily production of <10 m³/day [251]. Despite its low production, it can still be used for small arid and semi-arid villages, which only require a few cubic meters of water per day. In this work, the authors compared experimental data that was obtained from the pilot to the theoretical modeling data that was reported by Bourouni et al. [244], [250], [251]. The results show that the experimental data agreed with the model data for values of liquid film Reynolds number below a critical value, respective to the maximum amount of freshwater. The system was proved to be a possible solution to the problem of water scarcity in regions where access to clean potable water is difficult.

Loutatidou et al. also studied the potential of using low-enthalpy geothermal resources, which are widely available in the Arabian Gulf region to drive either RO or MED desalination plants [167]. They compared the two hypothetical desalination plants in terms of LCOW. RO was found to be cost-effective (LCOW of $2.06/m³), compared to using MED (LCOW of $2.71/m³) with the same low-enthalpy geothermal source, which is mainly due to the higher energy requirements, hence larger geothermal field required for MED desalination. However, the authors found that both PV-driven and grid-driven desalination plants are cheaper options than using geothermal energy as given in Table 7.

Table 7.

The Levelized cost of water (LCOW) for Reverse osmosis (RO) and multi-effect distillation (MED) desalination using different renewable energy resources (RERs) [167].

Desalination technology	Energy source	LCOW ($/m3)
RO	Geothermal	2.06
	Photovoltaic PV	1.51
	Grid with subsidies	1.02
	Grid without subsidies	1.24
MED	Geothermal	2.71
	Photovoltaic PV	2.50
	Grid with subsidies	2.25
	Grid without subsidies	2.36

In Algeria, Mahmoudi et al. proposed a brackish water-greenhouse desalination system driven by geothermal energy [252]. The proposed unit heats the greenhouse and provides freshwater for agriculture or domestic use. Greenhouse desalination is a novel desalination technology, which firstly, converts the brackish water to humidity in the air using evaporators. It then collects back this humidity using condensers to produce freshwater [253]. Fig. S9 shows a schematic diagram of this desalination process, in which geothermal energy is used to heat the water before entering the evaporators to easily humidify the air inside the greenhouse. The design temperature of hot water that enters is 75 ∼90 °C, which can be easily obtained by heating the water using geothermal energy. This technology is a useful method to secure clean irrigation water for greenhouse plants in places where only brackish water or seawater is present. They concluded that this project has many attractive advantages, such as: developing the agricultural sector in arid regions and providing extra freshwater for other applications. Despite its low production rate, the system is useful for producing quantities of water enough for small villages in arid regions, making it a good alternative to other desalination technologies powered by other types of energy sources.

6.5. Hydroelectric energy desalination systems

Slocum et al. explored the potential for the development of Integrated Pumped Hydro Reverse Osmosis (IPHRO), using different RERs of tidal/wave, PV, and wind energies to pump water to heights, as a mean of energy storage, with such energy used to drive RO as shown in Fig. 10 [210]. The pumped hydro system is used both for the energy storage of tidal/wave, PV, and wind energy and for supplying saline water to the RO desalination plant. This helps in reducing pumping costs, as the water will move under the force of gravity through the semipermeable membranes of the RO. The authors reported that only 5 % of the pumped water is used for RO desalination. According to this study, the integrated PHES-RO system reduces the capital cost of the desalination unit, decreases the environmental impact of brine discharge, produces freshwater that can help in overcoming the increasing demand, and generates electricity that can be used to stabilize the grid during peak energy demand. The authors also explored many geographical locations where IPHRO is applicable; many sites in the MENA region were found to be excellent candidates, given their proximity to sea and mountains range, with good GHI and wind speed. For example, the regions around the northern Red Sea, such as Taba, Egypt, and Aqaba, Jordan, were evaluated as good sites for the application of this technology. Other sites, like Bandar Abbas, Iran, and Al-Fujairah and Khor Fakan in the UAE were also considered as potential locations for the application of this technology. Akash and Mohsen also explored the potential of integrating hydropower to drive membrane desalination in Aqaba, Jordan [254]. The seawater would be diverted from the Red Sea, desalted through RO, with the brine then being discharged in the Dead Sea, which is almost −420 m below sea level, hence providing a huge hydropower potential. This will help raise the falling Dead sea level, hence restoring its aquatic life, i.e., bringing it back to life, in addition to producing 533 million m³ of freshwater annually.

Fig. 10.

Schematic of Integrated pumped hydro reverse osmosis system [210].

6.6. Biomass energy desalination systems

Despite the wide availability of biomass resources in many MENA countries, more specifically those with intensive agriculture economies, biomass resources have not been yet been implemented to power desalination systems in the MENA region. Biomass has, however, already been used as the main energy source for desalination systems in other regions such as India [255], [256]. The availability of biomass and the freshwater scarcity in many MENA countries highly suggests that building biomass-powered desalination plants would significantly contribute to solving the water crisis. However, detailed technical and economic analyses of a biomass-powered desalination plant in MENA region should be carried out to verify the project feasibility.

6.7. Hybrid renewable energy desalination systems

Hybrid Renewable Energy Systems (HRESs) integrate two or more renewable energy technologies into one system. Combining the right RE technologies can help solving the problem of variability in power output, which is a major concern for most renewables [257]. Many studies investigated the potential of using HRES for electricity generation, cooling & heating, agriculture, and desalination. Atallah et al. analyzed an off-grid HRES to drive a 100 m³/d SWRO plant in Egypt [258]. The authors investigated the economics of eleven different system configurations, which used a hybrid system of PV integrated with other energy systems such as a wind turbine, a diesel generator, and a battery. After analyzing the eleven configurations, the best case was found to be a hybrid PV/diesel generator/battery system as the most economically feasible. Additionally, this system was found to significantly reduce carbon emissions by 94 %, compared to the case of using a diesel generator alone without energy storage. Mahmoudi et al. analyzed a hybrid PV/Wind system to drive a seawater greenhouse in Oman [259]. This type of system serves two purposes; the first one is to produce freshwater by HDH desalination and to provide a suitable climate for crop growth in the greenhouse. According to the authors, the HRES will enable 8 h of operation and water production of 297 l/day.

Mokheimer et al. optimized a hybrid Wind/PV-RO-Batteries system in Al Dhahran, KSA [209]. The RO load was 1 kW, and the operation time was either 12 hr or 24 hr. The results obtained showed a levelized cost of energy (LCOE) of 0.62 and 0.67$/kWh for the 12hr and 24hr loads, respectively, resulting in LCOW of $3.69–$3.81/m³. In another study, Al-Othman et al. simulated a novel MSF desalination unit driven by a HRES, that consisted of PTC and a SGSP [260]. PTC’s produced 76 % of the MSF energy requirements, while a 4 m deep, 0.53 km² SGSP covered the balance 24 %. Combined, the total power of the system was 63 MW, with 1,880 m³/d of freshwater produced.

6.8. Summary

It is clear from the previously reported studies that a wide range of RERs has been investigated to drive different desalination processes. Table 8 summarizes the different studies for RERs-driven desalination carried out in the MENA region, which shows clearly that solar energy, both thermal and PV are the most studied and utilized RER being of high availability and intensity. When it comes to solar thermal energy, PTCs are found to be the most utilized technology. This is due to their compatibility with various thermal processes such as MSF and MED, being considered cost-effective. FPCs are also utilized for thermal desalination, however, due to their limited temperature range (30–80 °C) and the losses that would be encountered when integrated with MSF and MED, they are not considered a suitable solution and are mostly used for low-temperature thermal desalination. For small-scale direct solar distillation and HDH, FPC can prove useful for MD. Solar PV’s were mainly used to drive RO desalination, which is found to be the most suitable and cost-effective since Solar PVs produce DC power that directly drives the HP pumps used in RO desalination. New PV technologies, such as CdTe and CPV, have been widely investigated. These technologies are more efficient than regular PVs, but not yet cost-effective. PVs were also used to drive electrodialysis.

Table 8.

A summary of the progress of implementing renewable energy sources with desalination systems.

Renewable energy resources and technology	Desalination process	Region, Country	System description	Performance	Year and Ref.
Solar Thermal, Fresnel Collector	MED-TVC	KSA	•Experimental study. •662.4 m2 controlled curved mirrors positioned north–south horizontal axis. •MED-TVC SW desalination plant of 1 million imperial gallons/day	•The plant was more cost-effective with thermal storage. •Desalination cost of $2.24–6.8/m3.	2016, [233]
Solar Thermal, Parabolic Trough	MSF	Egypt and MENA	•Theoretical study for SW desalination. •1,200 m2 variable-axis solar field. •9,600 m3 PCM thermal energy storage tank. •5,000 m3/d water productivity.	•Desalination cost of $7.6/m3 at 80 kWhth and 4 kWhe/m3. •CSP-RO more suitable for North Africa and the Mediterranean Sea regions. •CSP-MSF/MED is more suitable for the Arabian Gulf region.	2013, [231]
		Kuwait	•Experimental study for SW desalination. •220 m2 variable-axis solar field. •7,000 l PCM thermal energy storage tank. •12-stage MSF of 5,000 m3/d water productivity.	•Solar field efficiency range of 40–65 %. •Collector outlet temperature range of 55–78 °C throughout 8 am to 5 pm. •Thermal energy storage maintained a temperature range of 50–70 °C over 24hr.	1985, [261]
	MED	UAE	•Theoretical study for SW desalination. •50 MWe north–south positioned solar field. •48,498 m3/d water productivity	•Net output thermal capacity of 188 MWth •Levelized cost of energy €0.207/kWh •Levelized cost of water €0.703/m3	2011, [237]
	MED-TVC			•Net output thermal capacity of 231 MWth •Levelized cost of energy €0.237/kWh. •Levelized cost of water €0.704/m3.
	RO			•Net output thermal capacity of 205 MWth •Levelized cost of energy €0.218/kWh. •Levelized cost of water €0.644/m3.
	MED-RO	Egypt	•Theoretical study for SW desalination. •9,720 m2 variable-axis solar field producing 3,010 MWhe and 8,900 MWhth. •Energy backup from the grid. •20,600 m3/d water productivity. •NaNO3-KNO3 (40:60) mixture as PCM for thermal energy storage.	•10 % less carbon emissions. •Levelized cost of water €0.97/m3, drops €0.73/m3 at a capacity of 58,100 m3/d.	2014, [262]
	DCMD	UAE	•Theoretical study for SW desalination. •50 MWe north–south positioned solar field. •Cogeneration of power and water. •0.56 m2 active membrane area	•Permeate flux of 5.2–20 kg/m2.h. •Water productivity of 14.33 m3/d. •Variable power output of 7.71–13.5 GWh. •Levelized cost of energy $0.194–0.245/kWh. •Levelized cost of water $0.64/m3.	2018, [235]
	HDH	Egypt	•Experimental lab-scale study for SW desalination. •2 m2 solar collector.	•Water productivity of 2.7–9.5 l/d.	2004, [263]
		KSA	•Experimental lab-scale study for SW desalination. •31 m2 solar collector. •Solar photovoltaic was used to drive system utilities.	•Water productivity of 5–24.3 l/day. •Levelized cost of water $12.54/m3. •Specific energy consumption of 2.4–3.1 kWh/m3. •Low energy losses hence high system efficiency.	2018, [264]
Solar Thermal, Flat plate collectors
	MD	Jordan	•Experimental lab-scale study for SW desalination. •72 m2 solar collector. •3 m3 thermal energy storage tank. •12 Solar photovoltaic modules were used to drive system utilities.	•Water flux of 2–11 l/m2d. •Specific energy consumption of 200–300 kWhth/m3. •No chemical pre-treatment was required.	2007, [246]
Solar Thermal, Solar stills	HDH	KSA	•Experimental lab-scale study. •Novel hybrid Still/HD system	•Water flux of 20–30 l/m2 •Levelized cost of water $3.5/m3.	2015, [265]
Solar Thermal Heliostat	MED	Algeria	•Theoretical study for SW desalination. •Heliostat used 2 axis tracking system. •Water productivity of 22,568 m3/d. •Average DNI of 534 W/m2. •Solar to electric efficiency of 19 %.	•Solar field power of 177.4 MW •Solar field area of 474,501 m2 •Net power of 50 MWe	2015, [266]
	RO			•Solar field power of 199 MW •Solar field area of 532,505 m2 •Net power of 56 MWe
Solar Thermal Evacuated tubes	AGMD	UAE	•Experimental lab-scale study for SW desalination. •Water/heating/cooling poly-generation plant. •132 m2 Solar collector area. •Single-stage LiBr/H2O absorption chiller. •2.8 m2 membrane area.	•Overall system efficiency of 67–76 %. •Water productivity of 10–12.5 l/h.	2016, [236]
Solar Photovoltaic	RO	KSA	•The First-ever PV-RO plant. •Experimental pilot-scale study for SW desalination. •210 PV modules with a total of 8 kW. •Battery array of 40 of 194Ax6V lead-acid for energy storage.	•Desalination recovery of 22 %. •Water productivity of 6.54 m3/d. •PV module efficiency of 7.2 %.	1982, [221]
			•Theoretical study for SW desalination. •Three PV configurations are considered. •Two desalination capacities of 6,550 and 190,000 m3/d.	•Levelized cost of energy $0.09–0.16/kWh. •Levelized cost of water $0.85–1.39/m3 for the small and large capacities, respectively. •Annual fuel savings of 1,158,987 and 33,579,763 l of diesel for the small and large capacities, respectively. •Reduced annual carbon emissions of 3,115 and 90,241 tCO2 for the small and large capacities, respectively.	2015, [222]
			•Experimental pilot-scale study for BW desalination. •Two PV field of 1 and 10 kWp. •Equipped with batteries	•RO recovery of 30 %. •Water productivity of 4.3 m3/d. •Levelized cost of water $0.5/m3 at zero interest.	1998, [208]
		UAE	•Theoretical study for SW desalination. •Water productivity of 200 m3/d •RO recovery of 40 %. •5,038 m2 solar collector area for 8,000 PV unit. •Producing water at low cost, by selling electricity to the grid.	•Specific energy consumption of 7 kWh/m3. •Levelized cost of water $0.825/m3 •Reduced annual carbon emissions by 1,0.35 tCO2	2015, [138]
		Oman	•Experimental study for BW desalination. •23.2 m2 solar collector area for 3.25 kWp PV unit. •200 Ah @48 DC battery for energy storage.	•Levelized cost of water $6.25/m3. •Water productivity of 5–7.5 m3/d	2000, [225]
		Jordan	•Experimental study for BW desalination. •8 PV modules @433 Wp •2.6 m2 system •Two 230Ah@12VDC batteries for energy storage.	•Water productivity of 0.22–1.27 m3/d. •RO recovery of 22 %. •Specific energy consumption of 1.9 kWh/m3. •Pretreatment specific energy consumption of 11.9 kWh/m3.	2011, [224]
		Jordan	•Experimental lab-scale study for BW desalination •Two 35 Wp PV arrays.	•One-axis tracking PV increased power and water production by 15–25 % compared to the fixed-axis.	2005, [223]
	ED	Bahrain	•Experimental pilot-scale study for BW desalination. •Four 33 Wp PV modules. •24 cells in four-stage ED with 0.023 m2 effective ED membrane area. •Water productivity of 0.19–1.13 m3/d	•Increasing the feedwater temperature improves the ED performance. •Salt removal of 30–95 %.	2003, [267]
Hydropower Solar Pond/Salinity gradient	HDH	Iran	•Theoretical study for SW desalination. •Pond surface area of 10,000 m2 •Convective zones thicknesses of 0.15, 1.2, and 1.4 (from top to bottom). •Convective zones concentrations of 5 (upper) and 25 (lower) g/kg. •Equipped with two thermo-electric generators.	•Water productivity of 1.2–4.5 m3/h. •Power productivity 3.2–11.8 kW.	2019, [268]
Hybrid Solar photovoltaic & Wind	HDH	Oman	•Experimental pilot-scale study for SW desalination in a greenhouse.	•Water productivity of 0.297 m3/d •A stand-alone system without fossil fuel backup.	2008, [259]
Hybrid Solar Thermal/Salinity Gradient Solar Pond	MSF	UAE	•Theoretical study for SW desalination. •3,160 m2 solar collector area. •Convective zones thicknesses of 025, 1.25, and 2.5 (from top to bottom). •Convective zones concentrations of 5 (upper) and 25 (lower) g/kg. •System capacity of 1,880 m3/d.	•System overall recovery of 4.7 %. •PTC covers 76 % of MSF energy and the rest is by SGSP. •530,000 m2 solar pond area is required.	2018, [260]
Hybrid Solar Photovoltaic/Wind	RO	KSA	•Theoretical study for SW desalination •66 of 50Wp PV modules. •6 of 1 kW wind turbine. •16 of 253Ah Batteries.	•Levelized cost of water $3.7–3.8/m3. •Levelized cost of energy $0.672/kWh. •Water productivity of 5 m3/d.	2013, [209]
Hybrid Solar Thermal/Biomass	MED	Iran	•Theoretical study for SW desalination •Multi-generation system for cooling, heating, water production, and liquefaction of natural gas.	•Power productivity of 16.11 kWe. •Heating productivity of 28.94 kWth •Cooling productivity of 23.4 kWth •Water productivity of 0.18 m3/d •Liquefaction capacity of 0.48 m3/d •Levelized cost of water $7.95/m3	2018, [269]
Hybrid Solar Thermal/Geothermal	HDH	Iran	•Theoretical study for SW desalination.	•Water productivity of 0.23 m3/d •Levelized cost of water $27/m3	2019, [270]

Hydropower is only economically feasible in specific locations, with most studies performed in Jordan, in which a pumped hydro RO system was studied and found to be feasible and beneficial. An example of Ocean energy is a Salinity Gradient, or a Solar Pond Salinity Gradient (SPSG), which is a pond with a high salt concentration at the bottom and a low concentration at the top. Geothermal energy resources in MENA are very limited to the North African region, thus making its usage in desalination only available in Algeria and Tunisia, where it is used for HDH. Hybrid renewable energy systems were used multiple timed in different places in MENA, and proved suitable for the application. The most used hybrid combination was solar PV/wind. Another potential combination that was not utilized much is solar PV/geothermal, which can overcome the intermittence of solar PV power, as geothermal power is always readily available and does not rely on variable weather conditions.

7. Case study of Al Khafji solar PV-SWRO desalination plant

Al Khafji Solar-SWRO desalination plant is a large-scale desalination plant located in Al Khafji city, North-Eastern coast of KSA, as shown in Fig. S10. It is the world’s first large-scale desalination plant that is powered by REs. This plant is connected to a grid-tied solar PV system, which works as its primary energy supplier. It takes its feedwater from the Arabian Gulf, which has a high-salinity level of 40 g/l. This plant was announced in January 2015 and was completed in November 2018, a year and a half after its expected completion date of early 2017. With an estimated desalination capacity of 60,000 m³/d and a project cost of $130 million, this desalination plant is considered one of the largest sustainable projects in the MENA region [271].

7.1. Solar PV system

The desalination plant is connected to a grid-tied silicon-based solar PV system, with a power output of 20 MW. These solar PV arrays were installed on an area of 900,000 m², located approximately a kilometer away from the SWRO plant [272]. The solar arrays are made of polycrystalline silicon solar cells, which were manufactured locally by the King Abdulaziz City for Science and Technology (KACST) [273]. This type of solar cell is attractive due to its affordability and acceptable conversion efficiency. Using RE as the main energy source will significantly help in reducing GHG emissions, as well as the operating costs of the plant. During daylight hours, when solar irradiance is available, the power that is being produced by the PV panels is necessary for the operation of the desalination plant, while excess power will be fed to the grid. During nighttime hours, the desalination plant will take its energy requirements directly from the grid. Connecting the desalination plant to the grid eliminates the need for an ES unit. Fig. 11 shows the operation scheme during daylight and nighttime hours.

Fig. 11.

Operation scheme during day hours and night hours.

7.2. Desalination plant

Al Khafji Solar-SWRO plant has a production capacity of 60,000 m³/d, a sufficient freshwater supply for a city of about 150,000 people [274]. The plant’s production is pumped to Al Khafji city to fulfill its need for freshwater. The SWRO plant has an area of 175,000 m² and is installed next to an existing MSF desalination plant. RO was chosen due to its lower energy requirements and higher recovery, hence more economically efficient. The main systems of Al Khafji SWRO desalination plant are shown in Fig. S11. Seawater is first taken from an intake that is located around 6 km away from the desalination plant. The intake tower features a jellyfish fight system, in addition to the standard prevention of entrainment, impingement, and entrapment of different aquatic creatures, avoiding withdrawing it with the feedwater. The intake tower is shown in Fig. S12. One of the key criteria in designing an intake of a desalination plant is the biofouling that occurs due to living organisms in the seawater. To overcome this problem, a special dosing regimen that takes into consideration the local seawater conditions, biofouling species, seasonal seawater quality variations, and the intake system design was implemented and named Eco-dosing. By implementing this advanced dosing system, the quantity of chemicals used was reduced by 15 %.

The second section in the SWRO plant is the pretreatment, which involves coagulation, flocculation, and dissolved air flotation (DAF). The feed seawater first goes through the coagulation and flocculation chambers for the removal of suspended solids and colloidal particles from the water. The DAF system is used to eliminate suspended matter in the feedwater, such as oils and solids. Fig. S13 shows the three parts of the pretreatment system. The next system is comprised of two stages of ultra-filtration (UF) units, that are used before the RO to get rid of all the suspended particles and macromolecules in the feedwater and to avoid fouling in the RO. After this, the feedwater flows to the two-stage RO unit. In this desalination plant, six RO trains are included to maximize the freshwater production of the plant and to allow more control of the freshwater production rate in low demand or low power supply cases. The brine leaves the RO unit and flows through an energy recovery device (ERD) to extract a fraction of its energy to the HP pump used for the SWRO feed.

Afterward, the desalinated product water flows to the post-treatment section, in which it is re-mineralized and neutralized, as shown in Fig. S14. Re-mineralization means adding ions mainly calcium and magnesium to the water so that it is suitable for drinking; while neutralization to get the pH of water within the acceptable range of 6.5–8.5. Re-mineralization is usually done by the addition of CO₂ and lime; while neutralization is achieved by either adding NaOH or HCl. A chlorine tank is used for water disinfection purposes. Al Khafji solar SWRO desalination plant presents a living example of the implementation of RER to power desalination plants. From the previous analysis, it was observed that solar PV power is indeed a reliable energy generation technology that can be used as a primary energy source for seawater desalination.

8. Conclusions and recommendations

This work summarized the progress that has been made on the implementation of renewable energy resources (RERs) to power desalination plants in the Middle East and North Africa (MENA) region. A brief description of the various renewable energy technologies and desalination processes was included. Different RERs in the MENA region were discussed as well and the following conclusions and recommendations can be summarized:

• •RERs are widely available in the MENA region, especially solar energy as the most abundant and dense RER, and they can be utilized effectively to power desalination plants.
• •Most of the work performed on RE-driven desalination considered solar energy (PV, and thermal), more specifically PV-RO.
• •Although of high potentials of wind and bioenergy in certain countries their integration to drive desalination has not been well explored.
• •Hybrid RE-driven desalination should be further studied and explored as it offers huge potential for improved performance in terms of cost and smooth productivity. One of the promising approaches is to explore Solar/Wind-RO/thermal hence maximizing water productivity at lower energy consumption.
• •Reported data for desalination cost is quite old and considers the high cost of RE used to be at the time of the study, hence an updated technoeconomic analysis considering the recent advances and cost drops in REs needs to be carried out.
• •Effective multicriteria decision-making has to be considered for site selection, RE selection, desalination process selection, and operation parameters given their importance for maximizing overall system performance.

Finally, a case study of Al Khafji Solar-SWRO plant in KSA, which is the largest RE-driven desalination plant was present. By discussing the power system, the desalination unit, and the geographical aspects of the site, it was observed that RERs, especially solar, could be successfully implemented with desalination plants as the primary energy source. As a result, it is recommended to build more solar-powered desalination plants in the MENA region, especially in the Arabian Gulf countries, as it holds almost half the worldwide desalination capacity, along with abundant solar energy resources. Also, this will further reduce the carbon emissions, and carbon footprint in these countries, which have some of the highest carbon emissions per capita worldwide. Increasing the share of RE-driven desalination in the desalination market will help in structuring a cleaner environment, and more sustainable energy and water supply for the upcoming decades.

Compliance with ethics requirements

This article does not contain any studies with human or animal subjects.

CRediT authorship contribution statement

Enas Taha Sayed: Investigation, Writing – original draft. A.G. Olabi: Supervision, Writing – review & editing. Khaled Elsaid: Investigation, Writing – review & editing. Muaz Al Radi: Writing – original draft. Rashid Alqadi: Writing – original draft. Mohammad Ali Abdelkareem: Investigation, Writing – original draft.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Biographies

Dr. Enas is associate professor at chemical engineering department, Minia university, Egypt. She got her PhD in bio-electrochemical systems from chemical and environmental engineering department, Gunma university, Gunma, Japan. Dr. Enas’ research focuses on the fabrication of high conductive, high surface area, and biocompatible electrodes of the MFCs for simultaneous wastewater treatment and electricity generation. Also she is working on the development and application of graphene in wastewater treatment and water desalination. Dr Enas published around 100 manuscripts in ISI indexed journals Dr Enas successfully secured two national and internal grants with more than 600,000 USD. Dr Enas has already supervised several master and PhD students some of them already graduated and others not yet. Dr Enas is Editor in Scientific reports (Nature).

Mohammad Abdelkareem got his PhD from Japan in 2008. Right now, he is professor in sustainable and renewable energy engineering department, university of Sharjah, UAE. He is working on the development of the different renewable energy resources and devices that can be used in wastewater treatment and water desalination. Moreover, He is working on the development of the electrodes for the different electrochemical energy conversion/storage devices, such as direct methanol fuel cells, direct urea fuel cells, microbial fuel cells, and supercapacitors. Professor Mohammad published more than 200 manuscripts in ISI indexed journals. He secured several research funds with more than 800,000 USD. He is editor in International journal of thermofluids and journal of carbon resources and conversion.

Dr. Khaled Elsaid has about 20 years of combined research and academic teaching experience that he gained by working for different research and academic institutions. Dr. Khaled Elsaid has a Ph.D. in Chemical Engineering from the University of Waterloo and has been working for Texas A&M University in Qatar for more than 15 years. Dr. Elsaid's research focuses on the chemical and physical processes of water treatment, with more focus on the desalination process in general, and membrane desalination in particular studying their efficiencies and environmental impacts. Due to the strong conjunct between water and energy, hence the water-energy nexus, the research interest has driven toward the utilization of renewable energies as clean energy sources to drive different desalination processes. The current work focuses on the optimum mapping of different desalination processes to different renewable energy hence maximizing efficiency and minimizing environmental impacts, including carbon footprint. Dr Khaled published more than 50 manuscripts in ISI indexed journals

Muaz Al Radi is a Graduate Research and Teaching Assistant (GRTA) at Khalifa University and a M.Sc. in Electrical and Computer Engineering student at Khalifa University. He received his B.Sc. degree in Sustainable and Renewable Energy Engineering from the University of Sharjah, Sharjah, UAE, with a rating of Highest Honors and a cumulative GPA of 3.97/4.00, being ranked as the department's No. 1. His research is focused on Renewable energy, Desalination, Artificial Intelligence (AI), and Robotics. He has several publications that are related to the renewable energy and desalination.

Rashid Alqadi holds a bachelor’s degree in Sustainable and Renewable Energy Engineering from the University of Sharjah, Sharjah, UAE, with a cumulative GPA of 3.40/4.00. His research interests include Renewable Energy, Desalination, Nanofluids, and energy efficiency. He is currently working in the field of sustainability and energy efficiency in the building sector.

Prof Olabi is the Director of Sustainable Energy and Power Systems Research Centre at the University of Sharjah “UoS”. Previously, he was Head of Department, Director of research institute, and many other academic and industrial position. He has supervised postgraduate research students (38 PhD) to successful completion. He has patented 2 innovative projects on PEM Fuel Call. He is the Subject Editor of the Elsevier Energy Journal, EiC of the Encyclopedia of Smart Materials (Elsevier), Editor of the Reference Module of Materials Science and Engineering (Elsevier), EiC of Renewable Energy section of Energies and board member of few other journals. Professor Olabi published more than 350 manuscripts in ISI indexed journals

Appendix A. Supplementary material

The following are the Supplementary data to this article: