Perspective Chapter: Technological Advances in Harnessing Energy from Renewable Sources for Water Production

Abstract

Recently, different technologies such as desalination processes have been utilized to obtain fresh water from natural sources to develop good standards of life, flourish industrial activities, and enhance civilization. Hence, this book chapter aims to cover the fundamental aspects of harnessing energy from the sun or solar cells, covering the history of this topic as well as the new related policies. A discussion of the basics of solar cell devices, performance challenges, and long-term stability will follow. This chapter will also address state-of-the-art membrane-based desalination technologies in generating fresh water from various renewable sources such as solar, wind, wave, and geothermal.

1. Introduction

Water purification from industrial waste, potable water produced from natural sources, and filtration of contaminants (chemical or biological) from drinking water are the inevitable large-scale projects of our current decade. Given that approximately 80% of the world’s population is at high risk of having inadequate access to clean water [1]. Fresh water is projected to become a scarce resource for many communities, creating a pressing need for new technological advancements in water purification for household systems. Additionally, the abundant low-grade water is suitable for producing economical and clean energy. Therefore, different technologies have been utilized in the last two decades to obtain fresh water, and there is a necessity to have a critical overview of the related processes and their corresponding mechanism toward enhancing and achieving higher yields.

In general, desalination is either referred to as thermal desalination or membrane-based processes. Desalination can be categorized into thermal process or distillation based on evaporation and condensation. In contrast, membrane-based processes use the membrane to separate salt from seawater or brackish water to generate fresh water [2].

The thermal desalination processes involve multi-stage flash distillation (MSF), multi-effect distillation (MED), humidification dehumidification (HDH), vapor compression distillation (VCD), adsorption desalination (AD), and freezing [2, 3, 4]. Among these processes, conventional MSF plants produce around 84% of the global desalination capacity, while MED plants generate about 3.5% of the world’s desalted water [5]. This thermal process consumes more energy to remove salts from seawater than membrane-based techniques. The world’s total desalination consumption capacity was predicted by 75.2 TWh of energy per year [6].

Membrane desalination techniques include reverse osmosis (RO), nano-filtration (NF), forward osmosis (FO), membrane distillation (MD), electrodialysis (ED), electrodialysis reversal (EDR), and capacitive deionization (CDI) [2, 3, 4, 7]. Their difference mainly depends on the size of contaminants in the solution that are separated or transported through the membrane, among which RO is known as the prevalent water desalination technology owing to its widespread availability in the market, cost-effectiveness, and efficiency in refining [3, 8, 9].

Every desalination process needs a different type of natural energy to operate its system. For example, wind, hydro, tidal, and wave are electricity producers, while solar, geothermal, and biomass are thermal and electrical energy producers [10]. Conventional multi-stage flash distillation (MSF) plants produce more than 80% of the global desalination capacity, while multi-effect distillation (MED) plants generate less than 4% of the world’s desalted water [5]. This thermal process is more energy-consuming compared to membrane-based techniques. Regarding specific energy consumption, the RO process requires low energy input of only about 5 kWh/m³ compared to MED and MSF (~ 20 kWh/m³ and almost 25 kWh/m³, respectively) [2]. As RO processes are energy-intensive, the high cost of water production will be alarming for billions of residents worldwide. The world’s total desalination consumption capacity was predicted to be more than 75 TWh annually [6]. More progress over the last decade has emerged in harvesting natural energy-powered desalination systems to be cost-effective, which may appreciably reduce specific energy requirements and become beneficial, eco-friendly, and sustainable compared to the traditional desalination processes [11]. Herein, the most common natural energy, such as solar, wind, geothermal, and wave energy-combined desalination systems, shall be described in the following section.

As an infinite energy resource, solar energy has been employed for centuries and is considered a panacea for making mankind less dependent on fossil fuels. In fact, the last two decades witnessed almost a 300% increase in electricity capacity from renewable energy sources, including solar energy. The trend is expected to soar until 2030 with the advent of further wind turbines or solar panel installations by reviewing the different categories of solar cell technology made as photovoltaic devices to convert solar energy into electricity. FO process (derived by a gradient in osmotic pressure between low salinity and high salinity solution) can benefit from solar energy. Similarly, different low-cost PV systems can be integrated into normal RO processes to replace traditional fossil fuels with the natural sunlight energy source. Solar-driven MD process is another technique that can thermally separate unwanted ions from water. In this process, a porous hydrophobic membrane is utilized between hot and cold solutions to produce drinking water.

ED process can be integrated with the advances in solar cell technology to prevent further global warming and generate fresh water via solar-powered ED as a promising method that requires relatively low capital cost and specific energy consumption. However, it is limited to brackish water, and it needs further optimization of the experimental parameters (on large-scale operations) and be tested with seawater with different salinities. This book chapter also talks about the contribution of new techniques for water purification and even generation, e.g., via desalination and membrane technologies. The basics of other natural energy-driven desalination systems, such as marine wave, geothermal, and wind energies, are also discussed. This is due to the ever-increasing scarcity of reachable water and the dire need to shift energy harvesting from traditional grids to renewable energy sources. This chapter will, therefore, cover fundamental aspects of harnessing energy from the sun, wind, tides, and waves, followed by a discussion of the past and recent energy technologies and their feasibility. It also highlights the advantages and disadvantages of each harnessing system and the challenges faced earlier. Hence, this chapter will present state-of-the-art technological advances in harnessing energy from renewable sources to produce drinking water.

2. Solar cells

2.1 History and new policies

The world’s first solar collector was built by Horace de Saussure in 1767 and was practically used in solar cookers in the 1830s [12]. Robert Stirling’s system, invented in 1816, offered a solar thermal electric technology that concentrates the sun’s thermal energy to produce power [13]. Edmond Becquerel’s discovery of the photovoltaic (PV) effect in 1839 opened a new window in developing research on solar energy systems [14]. A few decades later, Augustin Mouchot registered an invention for solar-powered engines in the 1860s [15]. A decade later, Willoughby Smith discovered the photoconductivity of selenium working on underwater telegraph cables [16].

In 1883, Charles Fritts fabricated the first solar cell with sunlight-to-electricity efficiency as low as one percent by using selenium wafers coated with a thin layer of gold [17]. In 1888, Edward Weston received two granted patents (patent No. US389124 and No. US389425) focusing on transforming the sun’s radiant energy into electrical energy. Meanwhile, Aleksandr Stoletov reported that solar cells based on the photoelectric effect create more power when exposed to ultraviolet light than visible light [18]. Clarence Kemp patented the first commercial solar water heater in 1891 (patent No. US451384A). Afterward, Melvin Severy received two patents on apparatus for mounting and operating thermopiles and apparatus for generating electricity by solar heat in 1894 (patent No. US527377A and US527379A, respectively).

William Coblentz received a patent in 1913 (patent No. 1077219A) for preparing a thermal generator that used light rays to generate an electric current. One year later, the existence of a barrier layer in photovoltaic devices was noted. Various research was in hand until semiconductor technology was born in Bell Laboratories in the 1950s (patent No. US2780765A), when scientists realized that silicon is more efficient for photovoltaic applications than selenium so that the produced solar cell showed an efficiency of around 6% [19]. The efficiency exceeded 10% in just 18 months [20]. The proficiency of their device was demonstrated using a solar-powered radio transmitter. This development was considered a key measure for filling the technology gap in solar energy harvesting, so The New York Times reported that the silicon solar cell “may mark the beginning of a new era, leading eventually to the realization of one of mankind’s most cherished dreams–the harnessing of the almost limitless energy of the sun for the uses of civilization.” In the same year, Texas Instruments Inc. filed a patent application on silicon p-n junctions (patent no. US2949498A), and Western Electric Co. began to sell commercial licenses for silicon photovoltaic technologies. The first commercial office building (i.e., the Bridgers-Paxton Building) using solar water heating was built in 1955. One year later, U.S. Signal Corps Laboratories focused on preparing solar panels for proposed orbiting Earth satellites, where they later developed n-on-p silicon photovoltaic cells. With this in mind, Vanguard I, Explorer III, Vanguard II, and Sputnik-3 have been launched with PV-powered systems on board as the accepted energy source for space applications and have remained so till today. In 1963, Sharp Corporation succeeded in producing practical silicon photovoltaic modules. Up to 1977, total PV manufacturing power was approximately 500 kilowatts, and ARCO Solar became the first company to produce more than 1 megawatt of photovoltaic modules in 1980 [21].

The National Renewable Energy Laboratory (NREL) was launched by the U.S. Department of Energy (DOE) in 1977 as a solar energy research institute dedicated to transforming energy through research, development, commercialization, and deployment of renewable energy and energy efficiency technologies. NREL is one of the best research-cell efficiency charts published since 1976 for a range of photovoltaic technologies on a standardized basis, confirmed by various independent test labs (e.g., NREL, AIST, JRC-ESTI, and Fraunhofer-ISE) [22, 23]. The chart includes solar cells within five families of semiconductors: multi-junction cells, single-junction gallium arsenide (GaAs) cells, crystalline silicon (Si) cells, thin-film technologies, and emerging photovoltaics, which are comprised of 28 subcategories, as summarized in Table 1. Thus far, multi-junction, single-junction gallium arsenide, and crystalline silicon cells have recorded the highest cell efficiencies, all of which take dominant positions in the market. Still, generally, these technologies are very costly today. Therefore, many researchers worldwide strive to develop thin-film technologies (e.g., CIGSs) and emerging technologies (e.g., perovskite structures), thanks mainly to their lower costs and facile fabrication processes.

No.	Photovoltaic Technologies	Subcategories	Highest Confirmed Efficiencies (%)
1	Multi-junction Cells (2-terminal, monolithic)	Four-junction or more (concentrator)	47.1
		Three-junction (concentrator)	44.4
		Three-junction (non-concentrator)	39.5
		Four-junction or more (non-concentrator)	39.2
		Two-junction (concentrator)	35.5
		Two-junction (non-concentrator)	32.9
2	Single-Junction GaAs	Concentrator	30.8
		Thin-film crystal	29.1
		Single crystal	27.8
3	Crystalline Si Cells	Single crystal (concentrator)	27.6
		Silicon heterostructures (HIT)	26.7
		Single crystal (non-concentrator)	26.1
		Multicrystalline	23.3
		Thin-film crystal	21.2
4	Thin-Film Technologies	CIGS	23.4
		CIGS (concentrator)	23.3
		CdTe	22.1
		Amorphous Si: H (stabilized)	14.0
5	Emerging PV	Perovskite/Si tandem (monolithic)	31.3
		Perovskite cells	25.7
		Perovskite/CIGS tandem (monolithic)	24.2
		Organic cells	18.2
		Quantum dot cells (various types)	18.1
		Organic tandem cells	14.2
		Dye-sensitized cells	13.0
		Inorganic cells (CZTSSe)	13.0

Table 1.

NREL’s best research-cell efficiency chart was updated on June 30, 2022. Adapted with permission from [22].

During the past decades, the public and private sectors have made numerous energy policies to influence the global solar electricity market. Many local, national, and international projects have been carried out in this regard. Flat-Plate Solar Array (FSA) was a ten-year project run in 1975 to support developing PV modules with 10% efficiency, a 20-year lifetime, and a selling price of $0.50 per watt [24]. In 1978, the US Congress enacted the Public Utility Regulatory Policies Act (PURPA) in response to the energy crisis, with oil expected to rise to over $100 per barrel to stimulate alternative energy sources. PURPA obligated electric utilities to purchase renewable energy and is still driving renewable energy development so that it accounted for over 40% of the solar energy projects built in the US as of 2017.

The International Energy Agency Photovoltaic Power Systems Program (IEA PVPS), as one of the Technology Collaboration Programs (TCP) established in 1993, has recently reported that approximately 946 GW of PV power plants were producing electricity worldwide at the end of 2021, of which around 70% have been installed during the last five years [25]. Around 174 GW of new solar capacity was established in 2021, and that figure might rise to 260 GW in 2022. It is worth mentioning that 42 countries reached at least 1 GW of solar-based electricity in 2021. Figure 1 illustrates the worldwide evolution of cumulative PV installations between 2011 and 2021. China continues to drive the global PV market, but the EU, USA, India, and Japan also play a vital role. EU ranked second in terms of newly installed solar capacity with 29 GW, after China with 55 GW.

Figure 1.

Comparing the worldwide cumulative PV installations of IEA PVPS countries (Australia, Austria, Belgium, Canada, Chile, China, Denmark, Finland, France, Germany, Italy, Japan, Malaysia, Mexico, Morocco, the Netherlands, Norway, Portugal, South Africa, Spain, Sweden, Switzerland, Thailand, Turkey, and the United States of America) with other countries between 2011 and 2021. Adapted with permission from [25].

China’s new nationally determined contribution (NDC) pledges aim to hit the renewables target of 1.2 TW wind and solar power capacity by 2030. The Solar Energy Industries Association (SEIA) has recently issued a roadmap chart to achieve the US solar manufacturing capacity of 10 GW in two years, 15 GW in three years, and 25 GW in five years on its path to 50 GW of annual production by 2030. Similarly, the European Commission increased its solar power capacity target to nearly 600 GW by 2030, with an interim target of 320 GW by 2025 [26]. Furthermore, a new solar target represents a 41% increase in ambitions compared to 420 GW under the EU’s “Fit for 55” climate plan. Solar electricity generation capacity is expected to double every two years in the decade between 2020 and 2030 [25].

2.2 Device basics and performance challenges

Light-matter interaction can be harvested as green energy by converting sunlight into electricity through the photovoltaic effect. A silicon-based solar cell is essentially a diode with an n-type and p-type silicon configuration, which contains a level of impurities, improving the silicon potential to harvest energy from the sun and convert it into electricity. The p-type silicon is produced by adding the atoms having one less electron in their outer energy level than silicon (e.g., boron), creating an electron vacancy, namely a hole. The n-type silicon is made by adding atoms having one more electron in their outer level than that of silicon (e.g., phosphorus), creating an excess free electron not involved in bonding. A depletion zone is made around the junction of the layers, where free electrons can transfer from the n-type layer to the p-type layer to fill the holes. Under the radiation of photons, these electrons obtain sufficient energy to escape to the n-type layer and remake the holes in the p-type layer, producing a flow of electricity.

Poly-crystalline silicon solar cells, the most commercially available solar energy devices, suffer from restricted light-to-electricity conversion power. As the most efficient material for sunlight conversion, single-crystalline solar cells are made from very pure silicon and are employed for space applications. However, silicon-based solar panels are still reasonably expensive to practically change the quality of people’s life. Rigidity and fragility are the other dominant Achilles’ heels for transporting silicon panels. Besides, solar panels are expected to work for 25 years with a minimum of 80% of their original power, which has already been an eye-catching challenge to be addressed [27].

Over the past years, emerging technologies have played a vital role in enhancing the performance of generic silicon solar cell technologies and laid the foundations for disruptive solar cell technologies based on thin films and nanostructures. Thus far, a wide variety of first-generation solar cells have been commercially introduced, but many scientists from around the world are still working to develop more and more efficient next-generation solar cells; perovskite, dye-sensitized, quantum dot, polymer, copper indium gallium selenide (CIGS), copper zinc tin sulfide (SZTS), cadmium telluride (CdTe), and gallium arsenide (GaAs) solar cells as cases in point. Since the significant advances in solar technology have, directly or indirectly, their roots in nanotechnology, the future of solar power generation is in the hands of nanotechnology. A cursory glance at various available solar cell technologies is provided in the following sub-sections.

2.2.1 Perovskite solar cells

As an emerging solar cell family, perovskite-based ones, referring to a large group of chemicals with the general formula of ABX₃, have been the fastest to reach higher efficiencies. Organo-halide compounds based on Pb (i.e., B) and monovalent ions such as fluorine, chlorine, bromine, and iodine (i.e., X) have shown firm promise for the facile and low-cost production of solar cells. However, perovskite solar cells are still extensively studied due to stability challenges in sunlight, moisture, and heat exposure. The components of a typical perovskite solar cell are as follows [28]:

1. a light-absorbing layer made of perovskite compounds (e.g., CH₃NH₃PbI₃)

2. a transparent conductive oxide (TCO) substrate, which is a glass coated with fluorine-doped tin oxide (FTO)

3. a titanium dioxide (TiO₂) or silicon dioxide (SiO₂) blocking layer, which restricts the charge recombination process between the FTO and the light-absorbing layer

4. a mesoporous layer composed of TiO₂, Al₂O₃, ZnO, and ZrO₂ nanoparticles as a scaffold for the perovskite layer

5. a hole transport material (e.g., Spiro-OMeTAD)

6. a layer of gold, silver, or carbon as a cathode.

2.2.2 Dye-sensitized solar cells (DSSCs)

A mesoporous layer of 20-nm TiO₂ particles deposited on an FTO glass lies at the heart of a dye-sensitized solar cell. The dye molecules (e.g., N-719) introduced onto the surface of the TiO₂ nanoparticles are in charge of producing electron–hole pairs. Photons pass through the FTO glass and get absorbed by the dye molecules, which oxidize the dyes, providing excited electrons. Afterward, the excited electrons are moved to the molecules’ lowest unoccupied molecular orbital (LOMO), leaving the holes behind in the highest occupied molecular orbital (HOMO) of the molecules. The excited electrons diffuse to the TiO₂/FTO interface, finally reaching the FTO through the external circuit. The oxidized dye molecules can accept electrons from the electrolyte to be reduced to the ground state using I⁻ ions of the electrolyte, leading to the formation of I³⁻ ions. The remaining I³⁻ ions diffuse toward the cathode, where they are reduced to I⁻ ions again [29].

2.2.3 Quantum dot solar cells

As a substitute for dyes in DSSCs, quantum dots (QDs) create a new class of solar devices called quantum dot-sensitized solar cells, featuring high-density electron injection, and tuning the band gap [30]. A broad spectrum of QDs has thus far been introduced for use in quantum dot solar cells, such as lead sulfide (PbS), cadmium selenide (CdSe), and cadmium telluride (CdTe), opening a new window for developing Schottky, multi-junction, bulk heterojunction, and depleted heterojunction solar panels [31].

2.2.4 Polymer-based solar cells

Polymers deposited on flexible substrates (e.g., polyethylene terephthalate foils) empower researchers to build flexible solar panels. In this regard, a layer of indium tin oxide (ITO) as the anode; a layer of PEDOT: PSS serves as an electron blocking layer that helps transport holes to the anode; a layer of P3HT: PBCM, in which P3HT acts as the donor material and PBCM as the acceptor material; and an aluminum layer deposited as the back electrode are the main components [32].

2.2.5 Copper indium gallium selenide (CIGS) solar cells

As a p-type semiconductor having a thickness of around 2 mm, copper indium gallium selenide is commonly employed as the light-absorbing layer in CIGS solar cells. Additionally, a layer of molybdenum which is deposited on a piece of glass by sputtering, an n-type buffer layer commonly made of cadmium sulfide (CdS), a layer of intrinsic zinc oxide that protects the CdS and CIGS layers from sputtering damage while depositing the back electrode, and a layer of aluminum-doped ZnO which is deposited as the back electrode are the other parts of a typical CIGS solar device [33].

2.2.6 Copper zinc tin sulfide (CZTS) solar cells

CZTS and CIGS solar cells enjoy the exact mechanism and general structures. The difference is that the light-absorbing layer made of CZTS with the chemical formula of Cu₂ZnSnS₄ shows a lower level of toxicity than that of the CIGS layer. Besides, the abundance of CZTS’s elements in nature compared to CIGS is a determining factor for drawing attention [34].

2.2.7 Cadmium telluride (CdTe) solar cells

A cadmium telluride layer as a p-type semiconductor is used as the light-adsorbing layer. A piece of ITO-coated glass as the front electrode, a layer of polycrystalline cadmium sulfide (n-type semiconductor) serves as a window layer, and an Al or Au layer deposited as the back electrode is the other vital layer making this family of solar cells [35].

2.2.8 Gallium arsenide (GaAs) solar cells

As mentioned in the previous section, GaAs solar panels are mainly used in the aerospace industry due to their high production cost. Monocrystalline layers of n-type and p-type GaAs are the main components of this family of panels, for which various thin film structures have so far been introduced. GaAs are commonly employed in single-junction solar cells (e.g., GaAs, InP, and InGaP), double-junctions (e.g., InGaP/GaAs), triple-junctions (e.g., InGaP/GaAs/Ge, InGaP/GaAs/InGaAs, and InGaP/GaAs/InGaAsNSb), and four-junctions [36].

2.2.9 Amorphous silicon (a-Si) solar cells

Compared to crystalline silicon solar panels, the principal privilege of a-Si cells is their potential for producing highly low-weight structures using such practical approaches as chemical vapor deposition. This family of cells comprises various p-i-n junctions using n-type and p-type silicon layers. A layer of intrinsic silicon, a layer of ITO, a layer of aluminum-doped ZnO, and a layer of silver as the back electrode are the principal layers of a typical a-Si solar cell [37].

3. Solar cell-driven desalination

Solar energy is the most widely abundant natural energy in the world and can be classified into solar thermal and photovoltaic [7]. The desalination system can supply one or both sources to run its process. The direct solar collecting approaches involved solar stills and humidification-dehumidification (HD) desalination in obtaining distilled water. Solar energy conversion into electricity or thermal energy is generally conducted through indirect methods like photophilic cells to supply MSF, MED, and RO desalination systems [38]. Figure 2 presents various natural energy sources that can be used to power different desalination technologies. There has been a significant growth in research that explored the feasibility of solar energy-integrated desalination processes. An example of the solar thermal process is a solar collector. It captures the solar radiation and converts it to heat transferred into a liquid in the absorber.

Figure 2.

A diagram showing the suggested integration of natural energy sources with different desalination processes. Reproduced from Sayed et al. [39].

The heat created from the collector can be transformed into both electric output and mechanical energy, which can be used directly to power the thermal desalination process or indirectly to run the membrane-based desalination system. Some crucial limitations are insufficient condensation and the recovery of latent heat from condensation, high cost, short lifetime, and energy storage problems, which hindered the commercialization and large-scale operation [6, 7, 9, 11, 38, 40].

3.1 Solar-driven RO process

The first solar PV-powered RO prototype was installed in the north of Mexico in 1979. This system exhibited a water production capacity of about 1.5 m³/d and specific energy consumption of around 4 kwh/m³ [41]. Another solar-powered RO desalination system was installed in Oman to desalinate brackish groundwater. The plant generates 5 m³/day of freshwater during 5 h (of each day), and sometimes the maximum output becomes above 7.5 m³/day. The average freshwater production cost was predicted at US$ 6.52/m³ over the 20-year lifetime of the equipment. It is also recommended that solar tracking, adjusting the tilt angle, and continuous cleaning of the PV arrays may improve the efficiency of the solar-powered desalination system. A theoretical study was conducted to determine the increase in the clean water flow rate for the yearly tilt, monthly tilt, and single- and double-axis tracking PV panels compared to the classical flat panel [42]. It was recorded that the annual permeate gain with the yearly optimal tilt was 10% and the monthly optimal tilt of the PV panel was 19%, respectively, concerning classical flat panel installation. When adjusting the PV orientation by adding a single- and double-axis tracker to the PV panel, the yearly permeate gain rose to 43% and 62%, respectively. However, the production of the RO process can be lowered due to the reduction in the radiation intensity during the low solar energy period or at night. Some issues are the cost of the PV cell, short experimental time, low lifetime, and reduced efficiency due to high average temperature, especially in GCC countries [2]. Thus, it is necessary to use a battery storage and cooling system [43].

A pilot plant PV-BWRO combined with an additional battery storage plant was constructed in Hartha Village, Jordan [44]. This system consisted of eight PV modules with 433 Wp, two batteries (230 Ah, 12 V), a softener as a pretreatment method, and an RO filtration system. It was designed to deliver 9.3–53 L/h from brackish water at 1700 mg/L over 24 hours, while the specific energy consumption reached 1.9 kWh/m³ without using the softener. However, it was increased to 13.82 kWh/m³ when coupling the softener to the desalination unit. The PV operated the high-pressure feed pump, data acquisition system, sensors, and solar regulator. The efficiency of the PV modules was good at about 12%. The system worked continuously during the year and generated enough fresh water for the citizens in the town. Depending on the percentage of water recovery and the system operating pressure, the range of freshwater permeate was between 9.3 L/h and 53 L/h [45]. Since the battery required frequent replacement, which increased the cost of water production, it is recommended to use a fuel cell storage device (FCs). When the FCs were coupled to the PV-RO unit, the water production approached 150 m³/day [39]. It was proven that this hybrid system achieved an acceptable cost of electricity and was economically feasible. Compared to the battery, the FCs showed a longer lifetime, higher efficiency, and less ecological consequence as the carbon emission could be minimized to 71 t CO₂/year.

Rahimi et al. [46] considered different scenarios depending on the other solar panel installations into the grid for small-scale RO systems. The renewable RO unit was implemented in Bandar Abbas with high solar radiation and low cost of electricity. They assessed the impact of energy storage systems, ERDs, and membrane characteristics on the performance of the renewable RO system. It was concluded that the specific total capital cost was 1270 USD/(m³/day) compared to that for the local market (1200 USD/(m³/day)). When using the PV to deliver electricity to the power grid, the RO system achieved a unit product cost of about 1.11 US$/m³ and a payback period of around 6 years. The workable membrane was SW30HRLE-440i, with a minimum water flux of 0.9 LMH/bar, and the lowest active area of 37.2 m². It could raise the energy consumption of the RO unit, which boosted the capacity of the PV panels and the RO unit productivity. Moreover, if the PV is used only to operate the RO unit, the RO system achieves the lowest net present value. Consequently, PV is a viable natural energy source for the RO process, which was proved to be economically viable and competitive with a traditional fuel source.

3.2 Solar-driven ED process

Solar-powered electrodialysis reversal is a promising alternative with lower capital cost, specific energy consumption, resistance to chlorine, and maximum recovery ratio [43, 47, 48]. The ED system involves many cell pairs full of saline water and separated by cation and anion exchange membranes [49]. Since the direct current polarity is supplied to the anode and cathode, the negative ions are transported through the anion exchange membrane. In contrast, the positive ions are transported through the cation exchange membrane. To alleviate the deposition of these ions in one of the compartments, a reversal DC polarity can be applied every 20 minutes. For instance, Al Madani et al. [50] tested a small-scale commercial-type electrodialysis stack driven by PV cells. The system’s design included 24 cell pairs arranged in four hydraulic stages and two electrical stages. The feed solution contained synthetic NaCl with different salinity ranging from 1000 to 5000 ppm and groundwater with a salinity of 3300 ppm. The experiment was run at a temperature between 10 and 40°C while the product flow rate varied from 50 to 300 gallons/day. It was found that the quality of the diluted product was enhanced at a low product flow rate while increasing the temperature further improved the quality of the diluted product. The salt rejection was the highest, about 99% for the synthetic NaCl and 95% for the groundwater at a low product flow rate of 150 gal/day.

In India, a PV-EDR pilot plant was installed to treat brackish groundwater with salinity ranging from 3600 ppm to 350 ppm [51]. Relative to solar-powered RO process, the PV-EDR system consumed lower energy input per water unit (75% less at 1000 ppm, 50% at 2000 ppm, and 30% less at 3000 ppm). It is because the ED stack consumed a direct voltage at the anode and cathode and did not require DC/AC inversion and batteries. Meanwhile, the system possessed an excellent recovery ratio of around 92%, good tolerance toward chlorine, and low feed water changes.

Ortiz et al. [52] developed a computational model for a battery-less solar PV-powered ED system. This model was used to predict the number of PV modules required, the workable configuration, and the electrical consumption for the system under optimal experimental conditions like meteorological conditions, the volume of brackish water, the concentration of brackish water, and the flow rate. The theoretical data was closer to the experimental results. For instance, when increasing the brackish water feed salinity, the desalination time, energy consumption of PV-operated ED system, and drinking water production cost were grown remarkably. At the highest feed water conductivity of 7000 μm/cm, the electrical consumption, water production cost, and desalination time approached 130 kWh/m³, 0.32 USD/m³, and 105 minutes, respectively. On the other side, the PV-operated ED system is limited to brackish water and needs further experimental investigations on the large-scale operation and economic analysis. Ultimately, the PV-powered ED system should be widely investigated on large-scale operations to optimize the experimental parameters and the design for seawater with different compositions and different locations.

3.3 Solar-driven MD process

Membrane distillation is a thermal separation process using a porous hydrophobic membrane between hot and cold solutions [53, 54]. The separation occurred due to the temperature variation between the membrane surfaces, allowing vapors to pass through the membrane and then condense on a cold compartment. As a result, pure water was produced while the salt was rejected ultimately. The MD process is classified into air gap membrane distillation, sweeping gas distillation, direct contact membrane, and vacuum membrane distillation. Extensive theoretical and experimental research was performed worldwide to assess the viability of the PV-powered MD process for generating fresh water. Guillén-Burrieza et al. [55] evaluated the performance of two prototype solar collector combined AGMD processes using 1 and 35 g/L NaCl solutions under accurate operating parameters over 2 years at Plataforma Solar de Almeria (PSA) Spain. Prototype-1 was a compact individual design, while Prototype-2 was composed of three stages connected in series. They investigated the effect of various experimental conditions on drinking water production and its quality, thermal efficiency, and recovery ratio. There was negligible impact on the water quality produced with high purity (2–5 μS/cm) if the feed flow rate, temperature, or salt concentration increased. Based on the thermodynamic analysis, both prototypes worked as excellent heat exchangers, and there was no heat loss during the experiment. Prototype 2 had higher heat recovery and water product quality when the number of modules increased. The lowest specific thermal energy consumption was 294 kWh/m³ for prototype 2 and 1805 kWh/m³ for prototype 1. The highest water permeates corresponded to 5.09 L/(h·m²), lower than the theoretical data or bench scale test. These findings indicated that the design and scaling up of the solar power MD model require further experiments to be commercialized.

Saffarini et al. [56] compared the water production cost between SP-MD systems using Direct Contact (DCMD), Air Gap (AGMD), and Vacuum (VMD) configurations to identify the correlation between many design and experimental conditions and the water production cost in the solar-driven membrane distillation process. According to a cost comparison, the water production cost varied in different MD models at a recovery ratio of 4.4%. It was 12.7 USD/m³, 18.26 USD/m³, and 16.02 USD/m³ for DCMD, AGMD, and VMD, respectively. Although the DCMD with heat recovery machine showed high conductive losses from the feed inlet to the permeate outlet, it was the best cost-effective model. Regarding the influence of the design parameters of the module, solar collector efficiency, and operation parameters, the water production cost was decreased when the feed temperature, active membrane area, and solar module efficiency were increased while decreasing air gap width and feed channel depth for the AGMD model. Increasing the feed flow rate for a laminar flow regime caused high pumping power and, therefore, high water production costs. The water cost was reduced for a turbulent flow regime when lowering the flow rate. It can be concluded that the selection of the MD model and operating parameters will impact the final water production cost.

Mohan et al. [57] developed a new design of a solar thermal poly-generation (STP) pilot plant to produce chilled water for air conditioning using an absorption chiller, clean drinking water with MD process, and domestic hot water by heat recovery, as shown in Figure 3.

Figure 3.

A schematic drawing for the solar poly-generation system. Adapted with permission from Mohan et al. [57].

Four configurations were tested using seawater with a salinity of under weather conditions of the United Arab Emirates as follows: 1- solar cooling mode, 2- cogeneration of drinking water and domestic hot water, 3- cogeneration of cooling and desalination, and 4- tri-generation. An illustration of the solar-powered MD processes is depicted in Figure 3. The experimental results showed that 25 kW of chilled energy is harnessed with a capital operation cost of around 0.6 in solar cooling mode. The solar thermal tri-generation plant used 23% more useful energy compared to the solar cooling mode. The energy recovery was estimated by 75% at 50°C for extracting domestic hot water. It was reported that the tri-generation configuration with single-stage membrane module integration generated distilled water around 4 L/h, and it was increased to 80 L/h with the dual stage (cogeneration of cooling and desalination). Furthermore, the payback period was found to be 9.08 years, and cumulative net savings were regarded as $454,000, considering an inflation rate of 10% for constructing a rooftop. According to the financial evaluation, the payback period can be reduced by 18% and cumulative net saving by 10.7% if the land cost is not included.

Chen et al. [58] proposed a systematic two-stage design strategy for the solar-powered MD system considering the actual yearly radiation intensity of Taiwan. A dynamic simulation model developed on Aspen Custom Modeler’s platform and a continuous operation control system were utilized to assess the performance of AGMD, DCMD, and VMD configurations. The first design worked under a fixed constant value of solar radiation intensity, and it was evaluated to determine the size of the compartments and the optimal experimental parameters. The second design was connected with the dynamic simulation model to change the flow rate automatically. It was revealed that the simulation results of the solar-powered MD process were in good correlation with the experimental literature results. When comparing all MD models, it was clear that the AGMD model needed the most extensive membrane distillation module, the DCMD model needed the most significant heat exchanger, and the VMD model required the most prominent solar collector. In terms of economic analysis, the water production cost of the optimal solar-powered MD system was $2.71, 5.38, and 10.41 per m³ of distilled water for AGMD, DCMD, and VMD models, respectively. Since the membrane was not expensive, its unit cost can be decreased from $90/m² to $36/m², resulting in a lower UPC of the optimal solar-driven AGMD system from $2.71/m³ to $2.04/m³. It should be noted that the price of membrane and solar is the main contributor to the capital cost and water production cost.

3.4 Solar-driven FO process

Forward osmosis is derived by the gradient in osmotic pressure between feed solution with low salinity and draw solution with high salinity [54]. This driving force allows the transport of the water molecules through a semi-permeable membrane to the draw solution side while blocking the transfer of salt ions. Forward osmosis was explored widely for water desalination. Still, the practical application of generating fresh water has been hindered due to some issues related to the membrane development, selection of draw solution, and low purity of water permeate. A novel process of integrating concentrated solar energy as the source of heat into the FO system was introduced by Razmjou et al. [59]. The tested draw solution was made of a water-absorptive layer to produce osmotic pressure, while the dewatering layer was at a lower critical solution temperature of 32°C to discharge the water being taken in during the experiment. The saline water was transported through the membrane and absorbed by the water-absorptive layer, which expanded and was recovered by a solar concentrator, as shown in Figure 4.

Figure 4.

A layout of solar concentrator combined FO process using bilayer thermo-responsive hydrogels as a draw solution and saline water as a feed solution for water generation. Adapted with permission from Razmjou et al. [59].

Layer concentrator increased the temperature of the dewatering layer to above its critical temperature leading to a high dewatering rate. Improving the energy input of the solar concentrator from 0.5 to 2 kW/m² yielded a rise in dewatering flux from 10 to 25 LMH. According to the thermodynamic evaluation, the minimum energy required for swollen dewatering hydrogel was 10.55 kWh/m³ at a swelling ratio of around 5, a mass of dry hydrogel of about 1 g, and 100% recovery during hydrogel collapse. This high energy demand is necessary to increase the temperature of the hydrogel draw solution to above its critical temperature. Even though using more FO modules and a high swelling ratio promoted the FO water permeate and dewatering flux, it would increase the required energy to above the energy consumption of the pressure-driven desalination process.

Amjad et al. [60] studied the potential of a new draw solution made of potassium-functionalized carbon nanofibers dissolved in triethylene glycol (TEG) solution with different concentrations. The main advantages of this material are high osmotic pressure and excellent absorption of solar energy for the recovery step. The FO process was operated using a flat sheet FO membrane, synthetic 3.0 wt% NaCl solution as the feed solution, and 0.2 wt% concentration in 20 vol% TEG as the draw solution. The draw solution’s recovery and freshwater extraction were carried out using simulated solar radiation flux. Based on the experimental results for the single FO process, the water flux was 13.3 LMH which was higher by 80% than that for 1 M NaCl as the draw solution against when DI water feed. When using the highest concentration of draw solution (0.2 wt% and 10 vol%), the reverse solute flux was as low as 0.031 g/L compared to that for 1 M NaCl as the draw solution against DI water feed. For the recovery step, the draw solution with the highest concentration exhibited improved photothermal efficiency by 105%, and the quality of the water permeate satisfied the quality standard of drinking water. Additionally, the efficiency of the draw solution was not changed after 5 cycles making it a favorable option for a prolonged solar-assisted FO-based desalination process.

Song et al. [61] investigated the possibility of extracting fresh water by combining the FO system with the photothermal evaporation method to regenerate the draw solution. A polyamide FO membrane was used for the FO process, and a photothermal polypyrrole nano-sponge (PPy/sponge) with large and interconnected pores to achieve high water evaporation was utilized for photothermal evaporation. The reduction of osmotic pressure due to the dilution of the draw solution in the single FO process was minimized by the interfacial water evaporation driven by solar energy. A good balance was found between the water permeate in the individual FO system of about 11.2 m·L/h and water evaporation in the photothermal evaporation method of approximately 10 mL/h. As a result, a continuous water flux around 11.7 L·m²/h² and a low reverse salt flux around 3 g/(m²·h¹) were obtained from the FO-EP system using NaCl as the draw solution with a salinity of 0.5 mol/L and light intensity of about 10 kW/m². This means that by controlling the concentration of the draw solution and light intensity, this system can produce continuous water permeate with low energy requirement and water production price.

A comprehensive statistical and experimental analysis was conducted to optimize the solar-powered FO process’s experimental conditions and energy demand. Khayet et al. [62] developed a Monte Carlo simulation method to identify the optimum operating conditions and energy consumption of the solar thermal and PV-powered FO pilot plant. These experimental conditions were the water permeate flux, the reverse solute permeate flux, and the FO-specific performance index that involves the water and reverses solute permeate fluxes associated with the energy consumption. It used 35 g/L NaCl as the draw solution, DI water as the feed water, and a commercial spiral wound membrane module. It was found that 0.83 L/min feed flow rate, 0.31 L/min draw solution flow rate, and 32.65^°C temperature were the optimum conditions and confirmed experimentally. The draw solution can be replenished using an optimized solar-powered reverse osmosis (RO) pilot plant with an optimum FO-specific performance index ranging from 25.79 to 0.62 L/g kWh under the same optimum conditions. This amount of energy represented only 14% of the total energy consumption for the solar-powered FO/RO hybrid process. In summary, to fully evaluate the system’s economic viability, more studies are needed to achieve the commercial implementation of the solar-driven FO process for water generation from saline water or wastewater. Other researchers considered other natural sources like ocean waves, wind, or geothermal to boost the efficiency of the membrane-based renewable desalination system and minimize the water production cost.

4. Other natural energy-driven desalination

4.1 Wave energy-powered desalination system

Another promising energy carrier is wave energy, and it is possible to integrate it with desalination technology [63]. The most workable design of this hybrid system is a wave-activated body to pressurize seawater [64]. Then, this pressurized seawater transfers through a reverse osmosis membrane to produce drinking water. This kind of energy can be harnessed on coastal areas or islands with high wind potential. Regardless, the total theoretical energy that can be extracted from ocean waves was predicted by 8 x106 TWh/year, which is 100 folds the total hydroelectric power production of the world [65]. Wave energy coupled with desalination technology can be divided into two classes: directly or indirectly. In the first class, the movement of waves can be transformed into high pressure to operate the desalination system. In contrast, in the second class, electricity is exploited from the energy of the ocean waves [64]. Electricity is the dominant wave energy conversion in most previous studies. The first developed converter in the past was Delbuoy which consisted of a wave-driven buoy, linear pump, and an anchor system with the RO seawater desalination process to generate fresh water [66]. The electricity generated from the Delbuoy machine has been derived from the pumps connected to the ocean floor to transport the seawater to the RO desalination system. The systems are modular and capable of absorbing the required amount of radiation starting from 6 m³ daily, depending on the plant’s location. Eventually, this simple design does not need regular maintenance, lubrication, or chemicals and is inexpensive, which makes it an alternative option to traditional electric-driven desalination systems in developing countries. The first commercial wave energy-powered desalination plant (CETO) was constructed in Perth, Australia [67]. The plant utilized a buoy that was placed under seawater. The plant was composed of 3 units connected together and linked to the grid. The project started in 2014, and the plant operation ran for 12 months. The total wave energy was converted to the electricity of about 12,000 kWh over the demonstration cycle, which was exported to the grid. Some electricity-powered RO desalination plants generate around 150 m³/day fresh water.

Recent work suggested using the Overtopping Breakwater for Wave Energy Conversion (OBREC) [68]. This work evaluated the performance of the OBREC-integrated RO desalination process for freshwater production on the Fenoarivo Atsinanana coast in Madagascar. In principle, this device converts the energy taken from the overtopping wave into electricity through heat turbines. This energy source contributed significantly to powering the RO desalination process. It was revealed that this renewable system could produce 964.3 m³ per meter wave front of drinking water. In particular, 60 L per capita per day was provided to populations using a 500 m-long OBREC breakwater connected to the desalination plant. It could be used due to many advantages like low installation cost, minimum environmental impact, and high freshwater capacity.

Extensive research has explored the potentiality of harvesting wave energy around Grand Canary Island [69, 70, 71]. A successful wave energy/desalination project in the north of Gran Canaria Island was investigated by Prieto et al. [69]. Not only the system design was considered, but other factors like the identification of the installation area for the harnessing of wave energy, the type of the converter, its price, its efficiency for generating power, and commercialization are essential. In this case study, the location was selected based on the availability of many desalination plants in the strip of the coast between Punta del Camello and Punta de Guanarteme. This area is suitable for extracting wave energy to be supplied to the desalination plant, and there was no potential conflict. The annual average wave energy approached 20 kW/m, and a clear seasonal pattern ranged from 27 kW/m in winter to 12 kW/m in summer.

However, the intermediate values during the transitional seasons were regarded as 22 kW/m in autumn and 17 kW/m in spring. Hence, this wave energy is still acceptable for deriving the desalination plant. Also, the environmental impact is insignificantly associated with a great chance of reducing fossil fuel consumption. Another renewable desalination plant project followed the experience in the north of Gran Canaria, Spain. Cabrera et al. [71] developed a model for designing and operating wave-combined seawater RO desalination plants. The simulation findings showed that this hybrid renewable system could produce an average of 1.51·10⁵ m³/year of fresh water and supply around 1370 citizens or the agricultural water demand of 37 ha.

On the other hand, the specific cost of this system is high because this hybrid technology was explored in a few research projects. Another aspect is the number of converter devices used in the desalination plant. In this study, only one converter device was connected to the desalination plant but installing more converters with competitive prices in the market would eventually decrease the capital cost. Also, if an energy recovery device was replaced with a conservative specific energy consumption of 4 kWh/m³, the water production cost could be minimized.

4.2 Geothermal energy-powered desalination process

In recent years, one of the most researched modes of integration between the natural energy source and desalination system is using geothermal energy to obtain fresh water [72]. Geothermal energy is the heat generated in the earth’s underground and transferred to the water in contact with it [11, 73]. The significant advantages of this heat are that it is naturally available, continuous, and stable, with no need for energy storage. The hot water can be utilized to power a turbine and generate electricity [11, 40]. This output electricity can be used to operate different desalination plants. A previous study revealed that geothermal power is a beneficial natural energy source that is accessible every day of the year during the day and night which can provide 8% of the total global electricity [73]. The electricity obtained from geothermal is enough to supply 17% of the world’s population. At the same time, citizens in Africa, Central/South America, and the Pacific can get all electricity from this clean energy. Karytsas et al. [74] stated that the geothermal energy extracted from the upper 2 Km of the hot rocks in Milos Island, Greece, can supply a 260 MWe geothermal power plant. The thermal energy harnessed from shallow with low enthalpy (<100°C) could run a water desalination unit providing 75–80 m³/h of drinking water and turn an Organic Rankine Cycle turbine (ORC) with an installed capacity of 470 kW. This renewable energy-driven seawater desalination plant comprised hot seawater (80,000 ppm) from the deep well, which was used to operate the ORC turbine to produce electricity. Also, this hot seawater was passed through the MED as a boiling process to desalinate seawater. The MED consists of several stages, and the water is heated by steam in tubes in each stage. As a result, the evaporated water can be used as drinking water while the remaining salt water is transported to the next stage and becomes hot and evaporated. The vapors were condensed to collect fresh water. The energy generated from the previous stage was reused in the following stage. Subsequently, the population can be supplied with sufficient drinking water from a cheap, clean energy source (i.e., 1.5 € per m³), ensuring water availability and food security. The poor efficiency of binary cycle systems when using the ORC and the Kalina cycle is a critical issue. In particular, the hot geothermal water with a temperature between 80 and 95°C was usually transferred to the desalination process providing low efficiency of around 5.5–8.5% [72].

An interesting example is given by Kaczmarczyk et al. [72], who proposed using low-enthalpy geothermal water to produce electricity for the desalination system and to supply the desalination process with a source of wastewater. This hot wastewater supplied to the RO desalination plant had low viscosity, allowing a large amount of fresh water. Also, it can be treated using other desalination technologies such as MD, MED, or MSF. The modeling results predicted that if the Kalina cycle is used for geothermal water (100 kg/s) at a temperature of 95°C and an ammonia-water mixture (89% ammonia and 11% water), a considerable amount of electricity can be exploited for about 6489 MWh/year. This electricity is sufficient to operate a geothermal energy-integrated brackish water desalination plant providing 3933 m³ of daily drinking water. It is important to note that this low-temperature geothermal water has been proven to be an innovative choice for the water desalination process leading to sustainable development in water resources management.

In other words, a combination of solar and geothermal energies provided an attractive option to run the desalination plant aiming to accelerate electricity, improve the efficiency of the ORC module, and lower the extraction of geothermal sources [75, 76, 77]. In Calise et al. study [75], the solar power was derived from the ORC, and the geothermal power was supplied to the MED process for drinking water production. In addition, geothermal energy was also used to power the Thermal Recovery Subsystem (TRS) connected to a cooling or heating device. Accordingly, generating electricity of about 4.60 × 10³ MWh/year was possible, and heat recovery for heating and cooling was regarded as 8.31 × 10⁴ MWh/year.

A significant breakthrough in the renewable desalination process is using a solar still as a humidifier and a ground heat exchanger as a condenser connected to an underground tube containing humid air [77]. The condensation of the humid air occurred in the condenser, and the resultant clean water was collected in the drainage pipe. A description of the system is shown in Figure 5. This freshwater could be used for irrigation or as drinking water. Since hot air with high temperature flows out continuously from the heat exchanger, it can be employed for cooling or heating purposes. Considering the theoretical analysis, it was concluded that the generation of drinking water was increased by 30.35% due to using a solar collector.

Figure 5.

A schematic diagram of (A) the geothermal energy-driven desalination system containing a solar desalinator as a humidifier and a ground heat exchanger as a condenser (B) a flowchart of the system’s compartments linked together. Adapted with permission from Okati et al. [77].

However, it was very low, around 4.45%, when using a solar air collector. Another suggestion is extracting the hot air flowing from the underground heat exchanger as its temperature remained high through the year at a depth of a meter. This can be accounted for by the fact that this hot air can provide extra energy and boost the performance of the solar photovoltaic and heating, ventilation, and air conditioning (HVAC) cycles. In this framework, the geothermal energy-powered desalination plant is economically profitable and environmentally reasonable for drinking water and irrigation water production in countries facing a freshwater crisis.

4.3 Wind energy-powered desalination system

The combination of wind energy and the desalination system is promising to produce fresh water at competitive cost and sustainability [78]. Compared with solar energy, the wind source is widely available in coastal and mountain areas and can supply power to desalination facilities without interruption [79]. One of the benefits is that the energy storage device is unessential. And its simple design, ease of installation, and limited maintenance price make it technically and economically competitive among other renewable energy sources. It could compete with other renewable systems in terms of a minimum environmental impact of about 75%, especially in locations with abundant wind power [78, 79]. The market’s most popular wind power machine is the wind turbine, which is commonly coupled to the desalination plant for drinking water production. An exciting project on the combination of wind energy and RO desalination process was implemented in Drenec Island, France, in 1990. A wind turbine powered the seawater RO desalination process; rated at 10 kW. A wind turbine with a 5-kW can deliver energy of about 1500 Ah, 24 V to many batteries, and 1.8 kW to the RO desalination process [80].

Numerous types of research have been conducted on wind energy-coupled RO desalination systems such as AERODESA, SDAWES, and AEROGEDESA [80]. Work by Serrano-Tovar et al. [81] proposed using a wind farm-powered seawater RO desalination plant installed by Soslaires Canarias S.L. in 2002. Since the desalination plant cannot get all the necessary electricity capacity from the wind farm, part of the generated electricity from the wind farm is delivered to the electricity grid of Gran Canaria Island, and some electricity powers the desalination plant from the grid. The desalination plant and water pumping are operated by an electrical grid associated with an onshore wind power farm (2.64 MW). It should be noted that this desalination plant consumed an electrical grid when the wind power deteriorated. The seawater desalination plant is based on the RO process with wind electricity-derived water capacity of around 5000 m³/day. This product water was supplied to the agricultural land for irrigating 230 ha of fresh vegetables and fruits. In the study presented here, the optimum design of renewable energy combined desalination process, the product water capacity, the final efficiency, and the capital cost remained a real challenge. Besides, when an electrical grid powers the desalination plant, the performance of the combined system can be influenced by variations in power and sometimes cutoff caused by wind energy. Consequently, the desalination plant required a storage device like a battery, diesel generator, or flywheels to prevent power interruption [82].

Accordingly, Melian-Martel et al. [83] recommended deriving the entire water cycle on the Island involving drawing out groundwater, seawater desalination, water pumping, and distribution with abundant wind energy. The desalination process requires about 18% of the total electricity of the Island while producing groundwater, distribution, and storage processes require 17%. To determine the suitable renewable configuration without an energy storage device, the author suggested two different strategies: utilizing the current water supply network derived from seawater and groundwater, which was decentralized and using the wind-powered large-scale seawater RO desalination plant associated with a centralized water storage system. The main findings were that the design based on the first strategy yielded a growth of wind integration of around 22%, but it consumed a tremendous amount of groundwater of about 45% of total water extraction for irrigation. Conversely, the design based on the second strategy produced sufficient water capacity to meet the water requirement for the Island. In this case, the annual wind integration was increased by around 84%, achieving a higher contribution from natural energy. This means that significant potential exists in the sustainable management of aquifer water when combining wind energy-driven SWRO desalination plants with a centralized storage system. Considering the capital cost, additional storage devices are expensive, raising the water production cost.

5. Conclusions

The fast climate change and the ramifications of being on the verge of 1.9°C of global warming include longer warm seasons that insinuate shorter water resources. The great worldwide demand for potable water makes desalination inevitable. Some examples of RO desalination plants have recorded almost 150 m³ of freshwater production daily. While RO is known as the most widespread membrane-based desalination process due to its superiority over thermal process (i.e., low-cost, easy-access/implement, and high efficiency), a combination of electrochemical schemes with electrodialysis reversal or capacitive deionization can even provide a higher rejection capability toward higher quality potable water. In general, the energy required for desalination processes can be supplied from natural wind, ocean waves, solar light, or geothermal, as discussed in this chapter.

Having discussed the basics of solar cell device fabrication, reviewing their state-of-the-art categories (such as multi-junction, thin-film crystals, silicon heterostructures, dye-sensitized, organic or inorganic, CIGS, perovskite/Si tandem, and quantum dot-based cells), and the horizon ahead for the future of extremely low-price roll-to-roll solar panels, one can foresee the perspective of next-generation solar-driven electrolyzers that can generate potable water while charging lithium- or even sodium-based battery of an automobile. In fact, freshwater can be a side-product of a fuel cell, while the future of large-scale high-pressure-output electrolyzer systems can be a source of energy for water desalination methods.

Conflict of interest

The authors declare no conflict of interest.