Solar-Powered Desalination as a Sustainable Long-Term Solution for the Water Scarcity Problem: Case Studies in Portugal

: The challenge of global water scarcity, exacerbated by population growth, pollution, and uneven resource distribution, demands innovative solutions. Seawater desalination, particularly Reverse Osmosis (RO) desalination technology, offers a promising remedy due to its efficiency, economic attractiveness, and enduring durability. This study explores the potential of solar-powered desalination to replace grid-imported electricity as a cost-effective solution to water scarcity, emphasizing economic and environmental aspects. We delve into the economic viability of desalination by developing a model that considers desalination capacity, input electricity prices, and specific energy consumption. Applying this model to case studies in Portugal (Porto Santo Island in the Madeira Archipelago and Algarve in the southern mainland) demonstrates that integrating photovoltaic (PV) solar energy systems to supply the electricity required in the desalination process can reduce the unit production costs of desalinated water by about 33%. The obtained unit production cost of desalinated water using solar PV input is lower than current water tariffs, underscoring the economic feasibility of this approach. The proposed solution is in line with the United Nations Sustainable Development Goals (SDGs), contributing to Goal 6 (Clean Water and Sanitation), Goal 7 (Affordable and Clean Energy), and Goal 8 (Decent Work and Economic Growth).


Introduction
In the 2030 Agenda for Sustainable Development, published in 2015, the United Nations defined Sustainable Development Goal 6 (SDG 6) as "Ensure availability and sustainable management of water and sanitation for all", as a reinforcement of their commitment to a world where the human right to safe drinking water and sanitation is ensured, and where there is improved hygiene [1].The targets associated with Goal 6 recognize that access to water worldwide is unequal, that water quality is threatened by man-made pollution, that inefficient water usage aggravates an existing water scarcity problem, and that international cooperation is required to protect this resource globally and to end inequalities in its access and security.
Water scarcity is easily recognized but hard to define.When addressing large-scale geographical units, like countries, two indicators are commonly used for this matter [2], the first being water availability per capita, a simple ratio between the existing freshwater resources (surface water and groundwater) and the population of a given area [3], and the second, the water stress ratio that measures the percentage of renewable freshwater resources withdrawn from the total available [4].The water scarcity threshold is defined according to these indicators as 1000 m 3 /cap/year, and 40%, respectively, the figure for water stress being 1700 m 3 /cap/year [3,4].According to current population growth rates and the resulting decrease in available freshwater resources per capita, the global water scarcity threshold will be reached by 2070 [5,6], based on the water availability per capita ratio.While it is more relevant to understand which geographic areas will experience severe shortages and how many people will be affected, the scenario is undeniably overwhelming.
The primary aim of this work is to ascertain the viability of solar-powered desalination as a sustainable, long-term solution to the pressing water scarcity issue, by providing an alternative for freshwater production.
From an economic perspective, we examine whether investment in solar photovoltaic (PV) technology as a renewable energy source (RES) can yield cost-effective unit product costs for desalinated water.For this purpose, a model for the unit production cost of desalinated water as a function of plant capacity, input electricity prices, and specific energy consumption is developed, allowing us to explicitly determine the economic impact of alternative power sources.
With the estimated costs of desalinated water using PV as a power source, it is possible to compare them both with the cost of desalinated water powered by grid electricity and with water distribution tariffs in practice, the latter to find out how the economics of the project aligns with current water tariffs.Hence, this model answers a gap in water management, allowing for the estimation of desalination costs during the planning and design of new desalination facilities.Considering the given variables of the model, it is possible to assess the impact of different plant capacities, electricity prices, and specific energy consumption, which are related to the chosen desalination technology but also constrained by the environmental conditions, and finally the chosen power source energy costs.These three aspects are the main drivers of the cost structure of the desalination plant operation.
The methodology is applied to two case studies in Portugal, one to the island of Porto Santo, Madeira Archipelago, and the other to the Algarve region, in southern continental Portugal.In Porto Santo, a desalination facility has been in operation for more than 40 years, and in the Algarve region, the construction of a publicly owned desalination plant is currently under assessment.Water scarcity is defined quantitatively according to the existing indicators, and installed capacity targets are set to match geographic water needs.Most economic models developed for desalination use data from desalination units in the MENA (Middle East and North Africa) region.In this work, the models are adjusted and adapted to the Atlantic region.A quantitative comparison is undertaken between the unit production cost of fresh water produced through desalination and actual water tariffs in practice.
The objectives of the work are as follows: • Investigate if solar-powered desalination can be a sustainable and long-term solution for water scarcity; • Determine whether investing in solar PV technology is cost-effective for producing desalinated water; • Create a model to calculate the unit production cost of desalinated water based on plant capacity, electricity prices, and specific energy consumption; • Provide a tool to estimate desalination costs for planning and designing new desalination facilities.
This work is groundbreaking as it analyzes the economic viability of replacing the power input of the existing Porto Santo installation with a solar PV supply.Additionally, it provides insights into the cost-effectiveness of using PV to supply the necessary electricity for the proposed desalination facility currently under assessment in the Algarve.
Portugal is currently transitioning to a renewable power system, with the goal of achieving carbon neutrality by 2045.The water scarcity problem is particularly severe in the southern regions of Portugal, such as Alentejo and Algarve, which are notably arid.As PV systems are poised to become the main source of electricity production across Portugal, especially in Alentejo and the Algarve, solar-powered desalination is a logical solution.Given that desalination consumes only a small fraction of the overall electricity produced in these areas, the integration of solar power for desalination purposes is both feasible and sensible.
The structure of the paper is as follows.Via a literature review, in Section 2, the most adequate desalination technologies are established.In Section 3, a model for the unit product cost of desalinated water is adapted and refined in order to determine the economic impact of alternative power sources.Moreover, a methodology for dimensioning PV systems is adapted for application to desalination facilities with a given capacity and specific energetic requirements.In Section 4, the model is tested with two case studies in Portugal.Finally, in Section 5, the conclusions and future work are presented.

Desalination Technologies
The common water treatment steps in a desalination plant are described in the literature as follows [7,8]: 1.
Intake: Pumps and pipes to take the water from the source and direct it to the desalination facilities; 2.
Pre-treatment: The filtration of raw water to remove solid components and the addition of chemical substances to reduce salt precipitation, the scaling of equipment surfaces, corrosion inside the desalination unit, and also biological growth and fouling; 3.
Desalination: The retrieving of freshwater from seawater.This is the most energyconsuming step of the water treatment; 4.
Post-treatment: Correct the pH by adding selected salts to meet the requirements of the final uses.
There are two main types of desalination processes: thermal-based and membranebased [9].Thermal processes, or phase-change processes, make use of a thermal source of energy, or heat, to evaporate water, leaving behind a salty brine, and then condensate the water vapor to distill potable water.This includes processes like Multi-Stage Flask Distillation (MSF), Multi-Effect Distillation (MED), Thermal Vapor Compression (TVC), Mechanical Vapor Compression (MVC), and Solar Stills (SSDs) [7].
Membrane technologies use a pressure differential to push water through a selective membrane that separates drinkable water from the contaminated water source.Among these technologies, the most popular are Reverse Osmosis (RO) and Electrodialysis (ED) [9], where electricity is used for driving high-pressure pumps that increase the pressure of the saline solution required before facing a selective membrane, or for the ionization of salts contained in the seawater, which are removed by a suitable membrane in between poles, respectively [10].Other membrane technologies like Forward Osmosis (FO) and Nanofiltration (NF) are still under development and facing challenges [7].
In terms of the feedwater source, which is indicative of the feedwater quality in relation to the total dissolved solids (TDSs), seawater (SW) [20,000-50,000 ppm TDS] and brackish water (BW) [3000-20,000 ppm TDS] represent 61% and 21% of treated water, respectively [11].The feedwater quality, together with the desalination technology used, impacts the recovery ratio (RR) of a desalination plant, as well as the energy consumption and hence the produced freshwater price.The water RR of a desalination plant is defined as the volumetric processing efficiency of the purification process.
RO is the most adaptable solution to exploit SW and BW sources.It has a high RR, and only requires electrical energy having a low energy consumption in total, making it the easiest to couple with renewable energy, as supported by several authors [12][13][14][15].Besides requiring only electrical energy and being couplable with many RESs, Curto et al. [7] also conclude that RO presents further advantages when compared to other technologies, such as a low initial investment in equipment and a modular plant structure, meaning it is easy to scale up, phasing the investment.
RO technology is based on applying excess pressure to reverse the spontaneous process of osmosis.Figure 1 shows a schematic representation of the RO process as follows: after the pre-treatment circuit, the feed is pressurized by a high-pressure pump and made to flow across the membrane surface.Part of this feed passes through the membrane, where the majority of the dissolved solids are removed following post-treatment.The remainder, together with the remaining salts, is rejected at high pressure [10].
technologies, such as a low initial investment in equipment and a modular plant structure, meaning it is easy to scale up, phasing the investment.
RO technology is based on applying excess pressure to reverse the spontaneous process of osmosis.Figure 1 shows a schematic representation of the RO process as follows: after the pre-treatment circuit, the feed is pressurized by a high-pressure pump and made to flow across the membrane surface.Part of this feed passes through the membrane, where the majority of the dissolved solids are removed following post-treatment.The remainder, together with the remaining salts, is rejected at high pressure [10].The rejected brine flow has practically the same pressure as the feed that enters the RO module, and therefore a high energy potential.In larger plants, it is economically viable to recover the rejected brine energy with a suitable energy recovery device (ERD).Such systems are called Energy Recovery Reverse Osmosis (ER-RO) systems, and even though they represent an increase in the initial investment, they can greatly reduce the operation costs [16], given that ERDs can reduce the specific energy consumption for desalination plants by 75% [8].
Dawoud et al. [17] discuss Egypt's severe water shortage due to rapid population growth and its program to utilize non-conventional water sources, including desalination.It focuses on a single-axis tracker solar-powered, battery-free, RO small pilot plant for the desalination of brackish water.The economic and technical viability and its performance in reducing energy consumption are highlighted, suggesting a promising future for solarpowered desalination in remote areas.However, the authors find the management of the inland brine water discharge challenging.

Energy Efficiency
Nowadays, RO is recognized as the most efficient desalination technology in the market, requiring a significantly lower energy consumption when compared to thermal processes like MSF and MED [7,18].This achievement was only possible due to consecutive improvements in membrane technology.Nevertheless, the electric energy required to operate a RO desalination plant is still significant, and, considering the use of grid electricity, most countries still use fossil fuels to ensure their energy mix [19].Therefore, directly or indirectly, the use of fossil fuels to power desalination represents a large portion of operational costs and is associated with harmful greenhouse gas (GHG) emissions.These aspects have been a central concern regarding the possible widespread adoption of desalination.
Seawater Reverse Osmosis (SWRO) plants with state-of-the-art equipment (pressure exchangers, variable frequency pumps, and low-pressure membranes) and in optimal conditions (i.e., a low fouling potential, temperature >15 °C, salinity 35,000 ppm) could achieve a specific energy demand of <2.5 kWh/m 3 and total demand of <3.5 kWh/m 3 [20] if the energy spent on pre-and post-treatment is considered.The rejected brine flow has practically the same pressure as the feed that enters the RO module, and therefore a high energy potential.In larger plants, it is economically viable to recover the rejected brine energy with a suitable energy recovery device (ERD).Such systems are called Energy Recovery Reverse Osmosis (ER-RO) systems, and even though they represent an increase in the initial investment, they can greatly reduce the operation costs [16], given that ERDs can reduce the specific energy consumption for desalination plants by 75% [8].
Dawoud et al. [17] discuss Egypt's severe water shortage due to rapid population growth and its program to utilize non-conventional water sources, including desalination.It focuses on a single-axis tracker solar-powered, battery-free, RO small pilot plant for the desalination of brackish water.The economic and technical viability and its performance in reducing energy consumption are highlighted, suggesting a promising future for solarpowered desalination in remote areas.However, the authors find the management of the inland brine water discharge challenging.

Energy Efficiency
Nowadays, RO is recognized as the most efficient desalination technology in the market, requiring a significantly lower energy consumption when compared to thermal processes like MSF and MED [7,18].This achievement was only possible due to consecutive improvements in membrane technology.Nevertheless, the electric energy required to operate a RO desalination plant is still significant, and, considering the use of grid electricity, most countries still use fossil fuels to ensure their energy mix [19].Therefore, directly or indirectly, the use of fossil fuels to power desalination represents a large portion of operational costs and is associated with harmful greenhouse gas (GHG) emissions.These aspects have been a central concern regarding the possible widespread adoption of desalination.
Seawater Reverse Osmosis (SWRO) plants with state-of-the-art equipment (pressure exchangers, variable frequency pumps, and low-pressure membranes) and in optimal conditions (i.e., a low fouling potential, temperature >15 • C, salinity 35,000 ppm) could achieve a specific energy demand of < 2.5 kWh/m 3 and total demand of <3.5 kWh/m 3 [20] if the energy spent on pre-and post-treatment is considered.

Desalination Costs
Until relatively recently, desalination has been recognized as a costly method for producing freshwater, accessible only to oil-rich countries, or in desperate situations [9].Nevertheless, desalination has nowadays become accessible worldwide, with costs declining primarily because of the decreased prices of equipment, lower power consumption, advancements in system design, improved membrane life, and the accumulation of integrated operational experience [21,22].
To understand how different economic components affect the cost structure of desalinated water, we will divide the main desalination costs under consideration into capital expenditure (CAPEX) and annual running costs, or operations expenditure (OPEX-O&M).For any given technology, the CAPEX and OPEX are nonlinear functions of the plant capacity [21].Hence for a particular plant capacity and desalination technology selection, the unit product cost (UPC-USD/m 3 ) can be calculated [21,22]: where i = a(1+a) n (1+a) n −1 is the capital recovery factor, a is the discount rate, n is the project's lifetime, PP is the plant production (m 3 /year), and PA is the plant availability (%).The lifetime is usually taken as 20 to 30 years without salvage value, at a discount rate of 8%, to account for the increased risk of the project [22].
Some results obtained by Bhojwani et al. [21] according to the UPC formula, Equation ( 1), are reproduced in Table 1.The contribution of the different cost elements is presented in USD/m 3 and as a % of the total UPC.The results are presented for four capacity points expressed in m 3 /day, after making the conversion of 1, 5, 10, and 25 million gallons per day.Table 1 shows the unit production cost (UPC-USD/m 3 ) broken down into its components as a function of the plant capacity (m 3 /day).It can be seen that the UPC decreases as the plant capacity increases in a nonlinear way.

Environmental Impacts
Desalination is typically associated with three significant impacts: the discharge of effluents, the high energy consumption, and the production of GHG.When it comes to effluent discharge, two main aspects come into play: the management of concentrated salty brine and the use of various chemicals in desalination plants.These aspects are the primary contributors to marine pollution resulting from desalination, along with the intake of source water.In terms of potential onshore impacts, apart from energy consumption and the associated air quality concerns related to the use of fossil fuels, other factors such as noise pollution, land usage, and social implications should also be considered [23,24].
When brine management is absolutely required, the options for an outfall system are similar to intake ones comprising nearshore open sea and offshore submerged options [25].The outfall system should be meticulously designed to optimize the dilution and dispersion of discharged brine into the sea [23].
Air contamination triggered by the operation of desalination plants is mainly related to the emission of GHG, mainly C, delicate particulate matter (PM), acid rain gases (NOx, SOx), and other air pollutants released during the generation of steam or electricity from fossil fuels [26,27].According to Elsaid et al. [23], RESs are the only answer to reducing both the use of fossil fuels and the emission of GHG.
The environmental impacts of desalination are summarized in Figure 2.
Air contamination triggered by the operation of desalination plants is mainly related to the emission of GHG, mainly C, delicate particulate matter (PM), acid rain gases (NOx, SOx), and other air pollutants released during the generation of steam or electricity from fossil fuels [26,27].According to Elsaid et al. [23], RESs are the only answer to reducing both the use of fossil fuels and the emission of GHG.
The environmental impacts of desalination are summarized in Figure 2.

Emerging Technologies
Advancements in desalination technologies, including both existing and emerging methods, have a pivotal role in mitigating the environmental impacts associated with desalination processes [23].These advancements primarily aim at enhancing recovery rates to reduce energy consumption, overall costs, and environmental footprints.
Efforts to increase recovery rates and shared facility utilization offer several environmental benefits, such as minimized flows of feedwater and brine, reduced intake and outfall sizes, a decreased plant size, and diminished chemical requirements.Co-locating desalination plants with power plants allows for additional benefits through the utilization of waste heat and cooling water discharge [23].
Minimal or Zero Liquid Discharge (MLD or ZLD) technologies recover precious salts and freshwater from brine while minimizing the concentrate volume [28].Although current technologies are costly, future developments are expected to yield more effective ZLD techniques [29].Current ZLD technologies increase the cost by, at least, three times due to an increase in energy consumption from about 3 kWh/m 3 to more than 12 kWh/m 3 .
Hybrid technologies, such as Nanofiltration-Reverse Osmosis (NF-RO), optimize energy efficiency by integrating multiple filtration stages.NF-RO has shown improvements in RO performance, leading to increased permeate flow and SWRO recovery rates from an average of 40-50% to more than 70% [30][31][32].
Innovative developments harness energy from brine differentials [33], such as pressure-retarded osmosis (PRO) [34], which generates energy from the natural osmosis

Emerging Technologies
Advancements in desalination technologies, including both existing and emerging methods, have a pivotal role in mitigating the environmental impacts associated with desalination processes [23].These advancements primarily aim at enhancing recovery rates to reduce energy consumption, overall costs, and environmental footprints.
Efforts to increase recovery rates and shared facility utilization offer several environmental benefits, such as minimized flows of feedwater and brine, reduced intake and outfall sizes, a decreased plant size, and diminished chemical requirements.Co-locating desalination plants with power plants allows for additional benefits through the utilization of waste heat and cooling water discharge [23].
Minimal or Zero Liquid Discharge (MLD or ZLD) technologies recover precious salts and freshwater from brine while minimizing the concentrate volume [28].Although current technologies are costly, future developments are expected to yield more effective ZLD techniques [29].Current ZLD technologies increase the cost by, at least, three times due to an increase in energy consumption from about 3 kWh/m 3 to more than 12 kWh/m 3 .
Hybrid technologies, such as Nanofiltration-Reverse Osmosis (NF-RO), optimize energy efficiency by integrating multiple filtration stages.NF-RO has shown improvements in RO performance, leading to increased permeate flow and SWRO recovery rates from an average of 40-50% to more than 70% [30][31][32].
Innovative developments harness energy from brine differentials [33], such as pressureretarded osmosis (PRO) [34], which generates energy from the natural osmosis process.Forward Osmosis (FO), utilizing a draw solution with higher osmotic pressure than feedwater [35,36], shows promise in reducing capital and operational costs, albeit hindered by the search for the ideal draw solution [23].
Advancements in membrane technologies, incorporating transient channels inspired by biological and artificial structures, could significantly reduce the capital and energy costs associated with desalination processes [37].
Despite technological innovations, energy consumption remains a principal cost factor and air pollution threat in seawater desalination.The integration of RESs is crucial for cost reduction and minimizing environmental impacts [41][42][43].
Water 2024, 16, 2140 7 of 18 Comprehensive evaluation methodologies like Environmental Impact Assessment (EIA), Best Available Technique (BAT), and Life Cycle Analysis (LCA) must be adopted for the design, site selection, and analysis of environmental and social constraints [24].Hence, for achieving sustainable and greener desalination, technological advancement and RES utilization are critical.

Summary of Technologies
The considerations above resulted in a proposed classification scheme, illustrated in Figure 3.
Despite technological innovations, energy consumption remains a principal cost factor and air pollution threat in seawater desalination.The integration of RESs is crucial for cost reduction and minimizing environmental impacts [41][42][43].
Comprehensive evaluation methodologies like Environmental Impact Assessment (EIA), Best Available Technique (BAT), and Life Cycle Analysis (LCA) must be adopted for the design, site selection, and analysis of environmental and social constraints [24].Hence, for achieving sustainable and greener desalination, technological advancement and RES utilization are critical.

Summary of Technologies
The considerations above resulted in a proposed classification scheme, illustrated in Figure 3.

Technological Considerations
From a technical perspective, the state of the art review (Section 2) revealed that RO is deemed as the most energetically efficient technology, demanding a low electricity consumption, after decades of membrane technology enhancement and through the implementation of state-of-the-art pumps and energy recovery devices as auxiliary equipment.An RO plant is also more economical to build and operate than facilities for thermal desalination [44].These arguments align RO as the chosen desalination technology for this analysis, adding to the fact that this is also the technology implemented in the facility already in operation in Porto Santo Island.
Like RO, PV technology has seen remarkable developments in its efficiency in the last decades, and its worldwide adoption, driven by significant cost reductions, is practical proof of its benefits.PV produces electrical energy that can be used directly to power the pump systems in an RO plant, the main points of energy consumption.The literature around the use of RESs for desalination is consistent in identifying PV as the best match for RO.

Environmental and Geographical Factors
The characteristics of the Portuguese coast are considered the most favorable for SWRO, such as the average salinity in the Atlantic Ocean ranging from 35,800 ppm to 36,000 ppm and a temperature between 14 and 19 • C [45].Seawater is regarded as the preferred feedwater option, owing to its abundantly available source, which is especially advantageous for coastal countries and islands.Portuguese culture is also deeply rooted in sea and coastal economic activities, and hence it will be important to consider the socio-economic implications of the construction and operation of desalination facilities.Regarding the PV potential, Portugal is in a privileged position, presenting a specific PV power output, above 1500 kWh/kWp.This is the amount of power generated per unit of the installed PV capacity over the long term, and it is measured in kilowatt-hours (energy units) per installed kilowatt-peak of the system capacity (kWh/kWp).The higher this index, the better.For context, in central Norway, the average specific PV power output is 870 kWh/kWp; in central Germany, it is 1050 kWh/kWp; in central France, it is 1200 kWh/kWp; and in central Algeria, it is 1900 kWh/kWp [46].

Case Study Selection and Justification
Situated in a remote location, the island of Porto Santo is located 28 miles off the northeast coast of Madeira, approximately 460 miles from the southwestern shores of mainland Portugal, and around 350 miles from the African coastline.The island's climate can be described as oceanic, characterized by moisture and semi-arid conditions.As a consequence, freshwater resources are scarce and primarily limited to a few low-flow springs [47].
In 1970, recognizing the anticipated rise in tourist influx and the island's inherent constraints in terms of renewable freshwater resources, the regional government of Madeira made a strategic decision to invest in a desalination project for Porto Santo.Consequently, the construction of an SWRO desalination plant commenced, and in 1980 operation began [48].Initially, the facility was directly managed by the government, until the year 2000, when it was concessioned to a public corporation with the responsibility of overseeing freshwater production and distribution throughout the archipelago.After this transition, the company implemented capacity enhancements and introduced several efficiency improvements [47].
Presently, the facility operates with cutting-edge technology, including variablefrequency pumps and pressure exchanger ERDs, achieving a commendable total specific energy consumption of 3.3 kWh/m 3 [47] totally imported from the grid.The desalination plant boasts a capacity of 6500 m 3 /day, and it is considered to be equipped with state-ofthe-art technology.As mentioned before, the salinity of the Atlantic Ocean, where this plant is located, ranges from 35,800 ppm to 36,000 ppm, and the temperature is between 14 and 19 • C [45].
Notably, there are ongoing energy efficiency initiatives, primarily focused on replacing high-pressure pumps, intake pumps, and other equipment with more energy-efficient alternatives, with the aim of reducing the total specific energy consumption to below 3.0 kWh/m 3 [47].Additionally, there is mention of a solar PV installation project on the facility's roof and adjacent buildings, with the goal of reducing electricity consumption from the grid.However, further details regarding this initiative are yet to be disclosed.This case study presents a remarkable opportunity to assess the potential impact of PV implementation on the ongoing operation.
The Algarve region, akin to many areas south of the Tejo River basin on the Portuguese mainland, presently faces a recurring water stress level of 80% [49].This means that Water 2024, 16, 2140 9 of 18 80% of the internal renewable freshwater resources within the region's water basins are withdrawn annually.Compounding this challenge are extreme weather events, as the Algarve has experienced severe cyclic droughts since the 1940s [50].This endogenous water stress situation poses a significant threat to the water security of the Southern Portuguese population.
Acknowledging the gravity of the issue, the Portuguese government took action in 2022 by commissioning an Engineering, Project, and Construction (EPC) company to spearhead the development of the region's inaugural desalination facility [50].Despite recognizing the economic and social benefits of enhanced water security, various municipalities within the district exhibited reluctance to host the plant, driven by a 'not in my backyard' sentiment.This apprehension was linked to concerns about noise and landscape pollution, as well as the management of brine discharge [50].Nonetheless, by 2023, the project advanced to the Environmental Impact Assessment (EIA) phase, with a selected location in the Albufeira municipality.Notably, the desalinated water produced will serve the entire district's distribution network.The estimated cost of this project stands at 50 million euros.
While comprehensive public information about the project under assessment remains limited, the existing water stress predicament presents a unique opportunity for a scenario analysis of desalination strategies for the region.This includes estimating their economic feasibility and assessing the environmental impacts.

Methodology
Our analysis will focus on the estimation of the unit production cost of desalinated water, C d (EUR/m 3 ), as a function of desalination capacity, Q (m 3 /day), energy prices, p (EUR/kWh), and the specific electricity consumption f e (kWh/m 3 ).The model is derived from the equation proposed by Bhojwani et al. [21] for capital cost as a function of desalination capacity, converted for prices in euros (EUR) according to average 2023 exchange rates for USD to EUR set as 0.9242 (EXR UK), and Equation (1) for the unit production cost of desalinated water.The model for the unit production cost is given by Equation (2), a sum of two terms, where w e is the weight of the electricity in the total OPEX, and Y is the plant availability as a % of the days in a year: where CAPEX = mQ −c , and m = 11.879,c = 0.12453 are empirical constants.
The first term of the model explicitly factors in the contribution of the CAPEX.The second term retrieves the OPEX contribution to the cost of desalinated water by weighting in the energy cost in the total operation, through the weighting factor w e .This weighting factor can be calculated if comprehensive data are available for other key OPEX components, such as chemicals, labor, membranes, etc.

Levelized Cost of Energy
The levelized cost of energy (LCOE) is a projection for the electricity value obtained for a given power plant, weighting the investment cost, the operation and maintenance (O&M) costs, and the expected energy produced, all over the power plant lifetime [51].A minimum tariff for energy produced by a PV system can be estimated through the LCOE formula, presented in Equation ( 3).This formula requires that an investment cost, I 0 , for PV is given in EUR/kWp and a value for the utilization factor, h a (h), is set.The utilization factor (h), also known as the specific PV output (kWh/kWp), is geographically dependent but can be retrieved from global databases for almost any location [46].The annual operation and maintenance (O&M) costs, defined as c om , are usually given as a percentage of the initial investment.
Water 2024, 16, 2140 10 of 18 The LCOE for PV systems can be calculated independently of the installed power, also known as peak power, P p , but the PV power required by a desalination plant can be determined through the local specific PV output, h a , in kWh/kWp and annual energy requirements, E a (kWh/year), which in turn is dependent on desalination capacity, Q (m 3 /day), and specific energy consumption, f e (kWh/m 3 ), as follows: where The global installation cost for PV in 2022 was reported as 876 USD/kWp by IRENA [52], whereas a median value of 587 USD/kWp is also presented as an achievable mark by 2030 [53].These values correspond to 833.1 EUR/kWp in 2022, and 558.2 EUR/kWp in 2030, using the average exchange rate in 2022 for USD to EUR, set as 0.951 [54].These values will be considered as the PV system specific investment cost I 0 .
In the PV industry, plant lifetime is usually considered as 25 years, and a discount rate for capital amortization of 8% was considered [51].This is a relatively high rate that reflects the risk of the desalination plant investment over a long time period.The O&M costs of a PV power production plant can be estimated as 2% of the capital investment.Given this, the LCOE of PV will vary between both geographical locations of the considered cases in the study, according to the specific power output at each site.In this study, the shorter lifetime of several components, such as RO vessels, batteries, inverters, and high-pressure pumps, was disregarded and a global 25-year lifetime was considered.

Grid Tariffs
For the tariff that reflects the use of fossil fuels, considering the operation of a facility run by a public or private entity in Portugal, the grid electricity tariff for active energy in Medium Voltage (MV) supplied by the Last Resort Supplier (LRS) in each region was considered.This tariff represents the final price paid by an MV consumer and reflects the energy production cost (the clearing price of the electricity market) plus grid access costs, taxes, subsidies, and other costs.In Porto Santo, the average LRS tariff for the mentioned schedule is 0.1527 EUR/kWh [55].In mainland Portugal, and hence considered for the Algarve, the average LRS tariff is 0.1531 EUR/kWh [56].These tariffs will be referred to as grid tariffs Grid 2023 .

The Porto Santo Case Study
The desalination plant of Porto Santo, which has been amplified recently, has currently a capacity, Q, of 6500 m 3 /day [47].In 2022, the plant only produced 1.2 Mm 3 of desalinated water from a potential 2.37 Mm 3 , representing a plant availability of Y = 51%.
The reported specific electric consumption is f e = 3.3 kWh/m 3 , and the contribution of electricity for O&M costs is set as p f e = 0.38 EUR/m 3 in 2022 [47].With these two values, it is possible to retrieve the average electricity tariff taken in 2022 as p = 0.1152 €/kWh.This value is denoted as Grid 2022 and is lower than the LSR electricity tariff for 2023, so it is possible to anticipate that in 2023 the plant operation costs would be higher.
The PV specific power output in the island of Porto Santo is 1522.2kWh/kWp [46].According to Equation (3), this yields a value of LCOE 1 = 0.06221 EUR/kWh or LCOE 2 = 0.04169 EUR/kWh, considering PV system installation costs for 2022 or 2030, respectively.As mentioned before, the LCOE is considered as the minimum electricity tariff, if PV power is considered as an electricity source.These tariffs will be referred to as PV 2022 and PV 2030 .
Besides the energy contribution to the OPEX, the unit product cost of the facility's desalinated water is fully broken down [47], with information available on capital amorti-zation and maintenance, labor, and chemical-related costs.Setting these publicly available values for capital cost and operations costs, it is possible to calculate the unit product cost for each energy tariff without resorting to any model estimations.The results obtained for the unit production cost, C d , with the different tariffs PV 2022 , PV 2030 , and Grid 2023 , compared to the reported running cost of 2022 ( Grid 2022 ), are presented in Table 2.One note regarding the capital cost is that it is visible that the capital amortization only contributes EUR 0.06 for each m 3 of desalinated water, as reported by the plant managing entity.This is a remarkably low value, which is aligned with the fact that this operation has been running for more than 40 years.And although it has received several improvements, the main investment costs would be mostly amortized by now.This shows that desalination plants can have very long lifetimes, becoming a valuable investment for water security in the long term.In fact, in much of the literature reviewed, there are reports of desalination plants installed in the 1970s and 1980s still in operation in the late 2000s.
As seen in Table 2, between 2022 and 2023, there was an increase of 32.6% in electricity tariffs, from 0.1152 EUR/kWh to 0.1527 EUR/kWh, which translates into a 20% increase in the total unit product cost of the desalinated water, from 0.65 EUR/m 3 to 0.78 EUR/m 3 .
As expected, the use of PV solar energy yields a lower unit production cost, C d .Hence, the use of PV as a RES can allow for a decrease of 38.5% in C d from 0.78 EUR/m 3 to 0.48 EUR/m 3 , at the current installation cost, and by 2030, this decrease could be 47.4%, from 0.78 EUR/m 3 to 0.41 EUR/m 3 .This is a remarkable result, meaning that the use of PV energy could allow for a decrease in desalinated water production costs of more than one-third to almost half of the current unit product cost.
The company responsible for the management of the desalination plant and the distribution of water in Porto Santo practiced, in 2020, ranked tariffs starting from 1.46 EUR/m 3 for consumptions over 10 m 3 per month [57].This allows for a margin of 46% for C d = 0.78 EUR/m 3 and 72% for C d = 0.41 EUR/m 3 , comparing the produced water cost with the price for final customers.
To determine the peak power of the PV installation needed to supply the required electricity usage by the desalination plant, it is noted that the total energy used in a year by the desalination plant is 1.2 Mm 3 × 3.3 kWh/m 3 = 3.96 GWh per year.For a utilization factor of 1522.2 kWh/kWp, this yields a peak power of the PV installation, P p , of 2.6 MWp.
A critique can be made as to how the required PV power is calculated, given that the P p is dimensioned for the total annual energy consumption of the destination plant, or the amount of kWh needed to desalinate the annual volume of water produced, according to Equation (4).This calculation does not account for the intermittent character of PV technology.
There are two approaches to overcome this gap in energy supply.The first is connecting the PV power plant to the electricity grid supply of Porto Santo Island, and when the PV system produces more energy than the desalination plant is consuming, this energy would be used by the inhabitants of the island, contributing to increased energy security and still offsetting the greenhouse gas (GHG) emissions of energy production.The other approach is to consider energy storage systems, like battery banks.Nevertheless, the cost of accumulation systems is high, given that these technologies are still under development and enhancement [58].The use of battery banks in a desalination plant would backtrack the economic benefits of the PV system itself, and this equipment is also questionable from the environmental point of view, given that the life cycle of the materials required for battery production is very detrimental for the planet and the environment.
In Porto Santo, the PV system will produce the 3.96 GWh/year of electrical energy needed to feed the desalination plant.Considering an emission factor of 162 g CO2 /kWh, reported by the Portuguese authorities [59] as the specific CO 2 emissions of the electricity bought from the grid, PV production would avoid the emission of 640 tons of CO 2

The Algarve Case Study
The information publicly available about the desalination plant under study for the Albufeira municipality, in the Algarve, promises an annual 16 Mm 3 delivered, said to cover 20% of public supply needs [50].This represents a capacity, Q, of 43,835 m 3 /day.
A specific electricity consumption of f e = 3.5 kWh/m 3 was set, which is the literature average for state-of-the-art desalination plants at optimal conditions.This value accounts for total power consumption and not only the RO module.The location of the plant will be assumed as in Albufeira, given that this is the pre-selected location by the government and the most populated municipality in the Algarve.
For electricity tariff values, as mentioned earlier, the grid supply in Portugal's mainland, tariff Grid 2023 , is taken as p = 0.1531 EUR/kWh.As for the PV LCOE, considering that the PV specific power output in Albufeira is 1755.6 kWh/kWp [46], according to Equation (3), this yields values of LCOE 1 = p = 0.05394 EUR/kWh or LCOE 2 = p = 0.03615 EUR/kWh, considering PV system installation costs for 2022 or 2030, respectively.The tariffs will be referred to as PV 2022 and PV 2030 .
Considering the results obtained for the Porto Santo case study for the Grid 2023 case, a weight of energy costs in the total OPEX of w e = 60% is proposed.This adjustment is expected to translate the recent increase in electricity tariffs in Europe due to market uncertainty and scarcity of supply [60].The results obtained for the unit production cost, C d , with the different tariffs, Grid 2023 , PV 2022 , PV 2030 , are presented in Table 3.The use of PV solar energy as a power source yields a lower unit production cost, C d ; quantitatively, a reduction of 28.4%, from C d = 1.220EUR/m 3 to C d = 0.873 EUR/m 3 , is possible for 2022 PV installation prices, compared to the grid electricity tariff, and potentially a reduction of 33.6%, from C d = 1.220EUR/m 3 to C d = 0.810 EUR/m 3 , for 2030 prices.This represents a difference of around one-third in the unit product cost, which is transversal to many industries and sectors that have been opting for RESs, mainly PV, to power their operations.
Regarding the PV system needed to power this desalination facility Equation (4) wields P p = 31.90MW.Nevertheless, it is important to note that the limitations of the implementation of PV previously discussed in the Porto Santo Case Study also apply in this situation, regarding solar energy availability, and again the same solutions are available.
Considering the supply of desalinated water for public distribution, the Albufeira municipality has set different prices according to classes of water consumption.In 2021, for domestic users, up to a monthly consumption of 15 m 3 , the water unit cost was set above EUR 1.07, and for more than 15 m 3 /month, the price was 2.03 EUR/m 3 .For non-domestic users and for irrigation purposes, there is only one water consumption rank, which is priced at 1.07 EUR/m 3 .With these tariffs in practice, the unit product cost obtained for solar-powered desalination of C d = 0.873 EUR/m 3 allows for a margin of 18.4% compared to the water price 1.07 EUR/m 3 .In 2030, for C d = 0.810 EUR/m 3 , the margin between the produced water cost and end customer price could be 24.3%.
In the Algarve, the PV system needs to deliver 56 GWh/year to electrically supply the desalination plant.Considering the Portuguese grid emission factor of 162 g CO2 /kWh [59], this corresponds to avoiding the emission of 9000 tons of CO 2 .
The analysis of the Algarve case study was extended to assess the unit production cost of different plant capacities.The scenario that was just presented corresponds to the plans of the projected desalination facility currently being assessed by the Portuguese Government.It will be denoted as Scenario 2. Two other case studies will now be considered:  In Figure 4, it is possible not only to visualize the impact of lower electricity prices in the total unit production cost of desalinated water ( ) but also the effect of economies of scale as a decreasing cost trend is drawn for increasing capacities in the same electricity tariff.Nevertheless, it is visible that the energy cost ( ,  , and  ) can have much more impact on the final unit production cost than the scale factor of capacity  In Figure 4, it is possible not only to visualize the impact of lower electricity prices in the total unit production cost of desalinated water (C d ) but also the effect of economies of scale as a decreasing cost trend is drawn for increasing capacities in the same electricity tariff.Nevertheless, it is visible that the energy cost (Grid 2023 , PV 2022 , and PV 2030 ) can have much more impact on the final unit production cost than the scale factor of capacity (Q 1 = 10,400; Q 2 = 43,835; Q 3 = 102,850 m 3 /day).Lower PV energy costs allow for a produced water cost under 1 EUR/m 3 , and are much lower than the water costs considering the grid electricity tariff of 2023.Regarding the economy of scale effect, a reduction in C d from the capacity Q 1 = 10, 400 m 3 /day to the capacity Q 2 = 43,835 m 3 /day (scenarios 1 and 2, respectively) is clearly visible, but after that, only a small decrease is noted in a much larger variation in capacity until the next point (scenario 3).
When considering the initial investment required for the installation, our calculations are aligned with the information the Portuguese government has made available, pricing the Q 2 = 43,835 m 3 /day desalination plant at 50 MEUR.A bigger plant of Q 3 = 102,850 m 3 /day would cost 106 MEUR, which must have been taken into account when the Government plans were made available.

Environmental Aspects
Regarding environmental aspects, the literature reviewed agreed with the conclusion that desalination can be safe for the environment, if appropriate planning, monitoring, and mitigation measures are put into practice.
Despite not assessing the environmental impacts of the desalination projects to a full extent, it is known that the Porto Santo facility uses infiltration galleries for its intake system [48].Given the size of the plant and its capacity, it is appropriate for the implementation of such a system, which in the literature is remarked as one of the least disruptive for marine ecosystems [61].As for the Algarve, one of the aspects that has delayed a construction decision is precisely the environmental studies that raise awareness for the potential endangerment of natural parks, and such considerations are being studied under an Environmental Impact Assessment in order to establish the best course of action for the implementation of this project [50], showing the current Government' compromise in respecting environmental regulations.
Regarding public opinion, it is possible that there is a lack of information, considering that desalination has come a long way, and the use of state-of-the-art RO technology has very different environmental impacts when compared to the first thermal plants installed.Therefore, the social impact can be minimized through divulgation and education about the improvements desalination technologies have achieved, and the involvement of communities in the planning process, presenting the many benefits increased freshwater availability can bring to society.
It is important that awareness is raised of water scarcity and the need for appropriate water management, and that desalination can be a sustainable solution for completing the water supply after water efficiency measures have been taken.If the public is informed about the economic value of desalinated water, and the benefits that water security can bring to social and economic development, complemented by the demystification of the environmental risks of desalination, social opinion could regard this technology as a valuable public investment.Furthermore, the investment in such innovative technology and in RES systems are drivers of quality job creation.And the consequent water and energy security can nurture economic growth.

Conclusions
Addressing the challenges of freshwater production necessitates innovative solutions, with seawater desalination emerging as a logical choice due to the virtually limitless availability of seawater.Reverse Osmosis (RO) stands as the predominant method, acclaimed for its efficiency, cost-effectiveness, reduced pre-treatment requirements, and streamlined outfall management.
A model for calculating the unit production cost of desalinated water has been successfully formulated and tested.This model predominantly relies on factors such as capacity, energy prices, and specific energy consumption.It facilitates the estimation of initial investments and the analysis of capital amortization's contribution to the unit product cost.Furthermore, it highlights the critical role of energy costs and provides a framework for assessing the economic impact of alternative energy sources.Providing a tool for researchers, planners, and decision-makers to estimate water production costs during the design of new desalination facilities is an outcome of this work.
Regarding the two case studies assessed in Portugal-Porto Santo, in Madeira, and the Algarve-a significant unit production cost reduction of about 33% resulted when considering the implementation of PV-powered desalination systems to replace the grid supply and 2022 PV installation prices.This is a consequence of the pivotal role of energy consumption in the desalination process.For energy costs calculated with predicted 2030 PV prices, the average reduction in desalinated water cost rises to 40.5%.
In sum, this analysis has revealed that desalination can be an economic answer to ensure water supply, allowing for margins above 18% between the unit production cost and end consumer price for the considered case studies, and its environmental threats can be reduced to a non-harming level, complemented by the fact that the use of PV solar energy contributes to further decreasing the costs of desalinated water and the mitigation of the associated environmental impacts.These two premises allow the conclusion that solar-powered desalination can be a sustainable long-term solution to the water scarcity problem, if proper strategies are applied in the development of location-specific solutions.
To conclude on the sustainability aspects of this work, the PV-powered RO desalination system under analysis greatly contributes to increasing, not only water availability and security, but also energy.These two resources, combined, foster economic growth, qualified job creation, and human health, also creating the possibility of increasing food production and security.Translating this to the United Nations Sustainable Development Goals framework, the proposed solution contributes towards the achievement of targets in Goal 6 [Clean Water and Sanitation], Goal 7 [Affordable and Clean Energy], and Goal 8 [Decent Work and Economic Growth].Indirectly, the impact may extend to all the other SDGs.
Desalination should be integrated into comprehensive water management strategies, fostering active participation from consumers across various sectors.This collaborative approach is crucial for instilling responsible consumption habits, especially as investments in water recycling and purification are made to increase availability.Establishing and embracing such practices, while engaging all stakeholders in water consumption and management, will be imperative in ongoing efforts to combat water scarcity in the future.

Figure 1 .
Figure 1.Schematic representation of an RO desalination unit.

Figure 1 .
Figure 1.Schematic representation of an RO desalination unit.

Figure 3 .
Figure 3. Desalination technologies according to process type and energy required.Figure 3. Desalination technologies according to process type and energy required.

Figure 3 .
Figure 3. Desalination technologies according to process type and energy required.Figure 3. Desalination technologies according to process type and energy required.

• Scenario 1 -
Ensuring basic supply: The World Health Organization (WHO) sets the minimum amount of freshwater for emergency situations at 20 l/day per person.Applied to the Algarve population (467,495 people as of the 2021 Census), this yields 9349.9 m 3 /day.Considering a plant utilization factor of Y = 90%, a desalination plant to suppress these needs would require a capacity Q 1 of 10,400 m 3 /day.• Scenario 3-Reducing water stress: Water stress in the Algarve region is currently over 80%, meaning that every year the water retrieved from the internal renewable freshwater resources represents 80% of the total available.The criticality ratio that represents water stress is set at 40%; hence, half the water withdrawn in the Algarve yearly is at risk.Given the data available, this scenario will consider that the desalinated water would suppress half the municipal consumption in order to reduce the average water stress to a lower risk level of 40%.Considering a plant utilization factor of Y = 90%, a desalination plant to suppress these needs would require a capacity Q 3 of 102,850 m 3 /day.The unit production cost obtained for the three capacity scenarios (Q 1 = 10,400;Q 2 =43,835; Q 3 = 102,850 m 3 /day) in the Algarve case study, for the different electricity prices in analysis (Grid 2023 , PV 2022 , and PV 2030 ), are illustrated for comparison in Figure 4. Water 2024, 16, x FOR PEER REVIEW 14 of 18 The unit production cost obtained for the three capacity scenarios (  = 10,400;  = 43,835;  = 102,850 m 3 /day) in the Algarve case study, for the different electricity prices in analysis ( ,  , and  ), are illustrated for comparison in Figure 4.

Table 2 .
Unit production cost, C d EUR/m 3 , in Porto Santo case study, using publicly available data (*).

Table 3 .
Unit production cost, C d EUR/m 3 , in Algarve case study, using model estimation data.