Theoretical review based on the technologies developed in solar desalination.

Increasing global demand for water and energy has given rise to renewable energies that offer an environmentally friendly alternative. Solar desalination systems have become a very attractive topic, due to the fact that areas lacking fresh water have a wealth of solar energy, allowing to present a convenient, promising and viable solution, also obtaining drinking water for consumption in remote areas from water with high salinity, where it is considered to be a process free of CO2 emissions when powered by solar radiation, being sustainable and environmentally friendly. The main objective of this study is to provide an extensive review of the different solar desalination systems, evaluating the indicators, factors and technologies involved in the process, through scientific sources such as: articles, academic publications, international congresses, indexed journals, and others.


Introduction
Today, the shortage of drinking water and the increasing demand for energy is the greatest challenge facing [1], [2], due to the continuous increase in population, industrial activities and the fact that a quarter of mankind suffers from inadequate freshwater use [3], [4].The water emergency situation is very alarming, mainly in arid and remote areas, despite the fact that the planet is covered by 97% of water, however, fresh water comprises only 2.5% of the total amount, the rest being seas, glaciers, ice caps and groundwater [5] Brackish water has salinity in the range of 10,000 ppm, where the possible limit for drinking water is (500 to 1,000 ppm), so, to obtain these acceptable ranges is achieved through water desalination [6], [7], However, desalination is an energy-intensive process that requires 10,000 tons of fossil fuels per year to produce 1,000 cubic meters of water per day [8]- [10].
Eltawil et al. [11], refers that desalination is a process based on the separation of dissolved salts from seawater to convert it into fresh water suitable for consumption, industrial use or irrigation.[12].At present, there are several desalination technologies or systems that, although they have different characteristics, such as type of energy, design and productivity, all meet the same objective, which is to desalinate water [13]- [16].On the other hand, natural and human contamination of water does not imply that the total availability of water is of good quality and fit for consumption.That is why water quality is different for each region and depending on the type of contaminants subsoil [17].
Therefore, the integration of renewable energies with desalination systems is the most promising solution to overcome this water deficiency and, at the same time, to acquire the unprecedented commitment on a global scale [18], [19], to innovate water technologies and management systems, while preserving the quality of current resources, so as to reduce water demand by increasing efficiency and generally increasing the availability of freshwater quantity [20]- [24].This work explores the global potential of desalination technologies integrated with renewable energy sources, mainly those related to solar radiation, by means of a narrative review method, identifying, compiling and selecting the systems with the greatest current impact, taking into account technological advances, increased productivity, efficiency and cost reduction.

Countries with desalination plants
The countries with the largest number of desalination plants are: United States with 2174 plants covering 34% worldwide, where 72% are reverse osmosis (RO), Saudi Arabia with 32% and 2086 plants of which 65% belong to multi-stage flash distillation (MSF), Japan covers 22% with 1457 plants and 90% of these belong to RO and lastly Spain with 12% and 760 plants of which 90% correspond to RO [25], [26].(See Figure 1).

Literature review
In order to initiate an efficient bibliographic search strategy, it was necessary to use a free and controlled language, so databases, mainly SCOPUS, were consulted, using the following search equation: "solar* AND desalination*" AND "technologies*" AND "review*".Searches are also carried out with the combination of the main keywords.The literature collected was extracted in .CSV format to perform the bibliographic analysis in the VOSviewer software.The figure 2 shows the co-occurrence of keywords in order to identify the research trend in the topic.

Solar desalination
In Mexico, according to Nassrullah et al. [22], there are 436 desalination plants installed in 320 locations, where only 5 use solar energy either as a power source or by using solar radiation as a heating source.Since solar desalination has become the best alternative in solving the problem of water scarcity and at the same time reducing carbon emissions while avoiding dangerous and irreversible uncontrollable effects on the world's climate and economy, sustainable energy systems, mainly solar radiation, play a very important role in the integration of renewable energy sources and efficient desalination technologies [22], there are several strategies that study this process, which are explained as follows: 2.1.Thermal System In the thermal system, heat is needed to make the change from liquid to vapor, where the level of salinity of the water does not influence, i.e., seawater is heated until it evaporates, followed by vapor condensation, thus producing fresh water and leaving residues such as water with high salinity concentration [23].The solar thermal system is classified into 2 processes, the direct process where there are the humidification-dehumidification (HDH) systems [24] and the solar stills system, on the other hand, the indirect process is composed by the multiple effect distillation (MED), multi-stage flash distillation (MSF) systems.
A. Humidification-dehumidification system (HDH) A clear example of the HDH system is the production of rain, since the sun as a heat source is used to evaporate about 16 million tons of water from the earth into the atmosphere every second [27].The vapor is carried by the wind to where it is treated through a zone and condenses at low temperature to form rain, snow, among others.The HDH method works in a similar way to the natura cycle (rain cycle), whereby the air is heated by a solar heater (SAH) in order to increase the vapor holding capacity where 1kg of air can carry 100g of water vapor at 60°C and rises to 500g at 80°C, whereby the air absorbs the salt water moisture in the humidifier and rejects it as fresh water in the dehumidifier [28], [29].This HDH system consists of two main elements, humidifier and dehumidifier, which can be driven by a variety of heat sources (see Figure 3).Source: [30] Generally, there are different types of HDH configurations in order to improve the performance of each component in the system [30]- [34].Hammadi [35], presents a theoretical study of the humidification -dehumidification (HDH) process in transient mode inside an open solar still, consisting of a longitudinal water pool, covered by a transparent glass panel with an inlet for dry air and an outlet for humid air.The results obtained show that freshwater productivity is highly affected by salt water, air currents and glass temperature.
The author Zhao et al. [36], presented a method using four-stage cross-flow dehydration and humidification (HDH) with direct contact dehumidifiers as shown in Figure 4, which results in a higher total water yield from the system as the air volume increases with air flow velocity; freshwater absorption has the highest yield of water per unit volume and the lowest cost of pure water.(See figure 4) Over the years, studies have been conducted to improve the efficiency of the various desalination systems, however, problems of scale and cost have not achieved a great result, due to the fact that traditional components are built separately, which increases their manufacturing cost [36]- [39].B. Multi-effect System (MED) MED plants consist of vertical or horizontal pipes, where the steam condenses on one side of the pipe while the other side releases the salt water caused by evaporation, this evaporated salt water is distributed over the outer pipes [40], [41].In each effect of the MED process, heat exchange tubes are wetted with seawater, so that the steam flows through the tubes and condenses, thus producing fresh water.
2.2.Concentrated solar energy system (CSP) Concentrated solar power (CSP) in the development of desalination has become the most viable and sustainable alternative as Mohammadi [42], since CSP generates heat at high temperatures, it can be used for electric power generation through power cycles or to satisfy heat needs for different applications such as desalination systems, both small and large scale for year-round fresh water production, mainly in regions of high solar radiation and saline water availability.
Research is currently focused on the search for alternative renewable energy sources.Concentrated solar power (CSP) is one of the most promising technologies for heat and power, due to its solar utilization [42], [43] 2.3.Solar collector Solar collectors are based on optical principles of reflection in order to concentrate the solar radiation in a specific area called focus, so that the energy collected from the sun's radiation can be transferred to the water due to the thermal differential, causing it to change phase to clean it [17].In these processes, the idea is to obtain better heat transfer results [44]- [47].
The solar desalination systems have been catalogued for their low efficiency due to heat loss on the collector surface, however, the parabolic trough collectors, since they concentrate the light in a single area, which makes the heat loss lower compared to ordinary collectors, have a higher efficiency [47] Authors Mehrpooya, et al. [43], developed a concentrated solar power plant with desalination processes and with absorption cooling cycle designated to supply power, fresh water and cooling, aimed at reducing carbon and dioxide emissions.This system consists of a concentrated solar thermal plant with parabolic dish collectors and steam turbine, using a multi-effect desalination process with parallel seawater feed and a single-stage water and ammonia absorption cooling system, as shown in Figure 5, resulting in the destruction of 86% of the total energy rate of the system, with an efficiency of 66.07%, while the thermal efficiency of the system was 93.91%.That is, the weakness of the energy analysis to identify power quality and irreversibility's within the system.The economic results showed that the proposed integrated structure had a payback period of 5,738 years and a net annual gain of $6.828 million per year.Source: [43] 2.4.Combined desalination and CCHP system (combined heat and cooling energy) The integration of renewable energies with desalination and combined heating, cooling and power (CCHP) systems is one of the possible alternatives, using a two-tier methodology to optimize the system.The authors Luo, et.al.[48], assume a design on a remote island in the South China Sea, which consists of the displacement method, branch and bound method which are used to solve the optimization problem by mixed integer linear programming in the design and operation stages.As a result, the capacity of the electrical storage system is lower than the thermal capacity due to the high cost/performance ratio of the electrical storage, however, it can meet the energy demands, and the authors note that the integration of a water tank improves the productivity performance.

Solar chimneys
Solar chimneys are reliable devices with long operating life time, they are usually composed of a tower and an open collector on the edges, so, the incident radiation heats the ground under a collector, which in turn, heats the air inside the solar chimney, to generate a flow of hot air that can drive wind turbines and produce electricity, their maintenance costs are relatively low due to their robust and simple structure, the efficiency of these chimneys is very low, ranging from 0.5-10% of the solar energy delivery.However, there are not enough studies of the use of solar chimneys for desalination, there are only a few studies available, according to the authors Maia, et.al.[3], since, they focus on mathematical simulations and modeling, although the hybrid solar chimney-desalination system has great potential and can maximize performance, it is still a research phase.
Solar chimney plants have not had major demonstrations and have been produced intermittently with no improvement in performance, there are several projects internationally, for example, the 1500m high chimney system planned for South Africa, a 200 MW plant in China, three more 200 MW projects in the USA and a 200 MW solar chimney plant in China [49], these have only been planned and have not been built, although they promise to meet the necessary scales and better performance, taking into account that the stacks would have to have relatively low capital and investment delivery prices [50] 2.6.Combined system with renewable energy Renewable energies are intended to provide a solution to the problems mentioned above, by integrating desalination systems with renewable energy sources, which are divided into two types: • Desalination processes driven by heat from renewable energy systems.
• Membrane and distillation processes driven by electricity or mechanical power derived from renewable energies Therefore, desalination systems integrated or hybrid with renewable energies are the most relevant issue today, allowing to face the scarcity of fresh water [21], [51], [52].That is why, authors Kabeel & Said [17], offer a study on a modified solar-powered hybrid desalination system (SS-HDH).This consists of a solar still (SS) and the humidification-dehumidification (HDH) process, integrated with a solar water and air heater, where they studied the performance indicators and the total productivity of the system.Thus, this system has had a positive result on yield and productivity, because the maximum productivity was 18.25 l/m day with air and water mass flow rates.The humidification efficiency reached 79% and was affected by the increase in water mass flow rate rather than an increase in air mass flow rate, i.e., system performance is affected by air mass flow rate.Similarly, authors Morad et al. [53], conducted a literature search for methods or processes that improve the low performance of current desalination systems, where they found that increasing the salinity level of the water decreased productivity.Combined system with renewable energy.

Reverse osmosis (RO) desalination
The reverse osmosis (RO) process has several applications in industry, such as seawater desalination, food processing and wastewater treatment [15].This separation method is the process with the highest efficiency and is environmentally friendly, while maintaining the same time with low energy consumption [22].This RO process is based on the separation of two media with different solute concentrations through a pressure driven semi-permeable membrane, where water and some ion fractions can move through the membrane from the side with lower concentration to the side with higher concentration, while most salts are rejected.
Authors Wu,et.al. [23], developed a hybrid power system, based on reverse osmosis desalination coupled with a photovoltaic system and diesel fuel, in a power plant in order to meet load demand and increase water availability in Iran.Where they propose an optimal design of the autonomous photovoltaic hybrid system with diesel fuel, battery and reverse osmosis, using a metaheuristic technique based on tabu search, where results are obtained as a high efficiency being economical and environmentally friendly, however, being a single diesel system or a single photovoltaic system.

Conclusions
In this work, a theoretical review of solar desalination systems, their current technological status and the studies that focus on these systems was conducted; the review provides an overview of current desalination technologies and focuses on hybrid systems that take advantage of solar radiation, either for plants that produce only water or for freshwater cogeneration plants, the study presented that: The CSP system can vary in temperature and produce electricity, which is suitable for driving a variety of desalination systems classified as either thermal or membrane processes, membrane processes are based on electrically guided concentration gradients such as RO, and thermal systems require evaporation of water to separate the salt, such as MED or MSF, in these thermally driven systems to improve performance and reduce costs it is feasible to use the TVC or an absorption heat pump.
These CSP-hybrid systems are classified according to their application for either cogeneration of energy or water-only production.The review showed that there are several possible ways to hybridize a CSP, considering factors such as the appropriate selection of technologies (solar collectors), such as the study of the integrated desalination plant with a parabolic trough collector receiver tube, as well as showing high performance results in fresh water productivity, which was totally free of salts and suitable for consumption, improving its efficiency, due to its solar collector tracking system.
Thus, CSP-hybrid desalination plants are potentially cost-effective for the production of fresh water in an environmentally friendly manner, although significant research and advancement is still needed for these plants to be economically viable.
The studies presented various performance parameters for achieving high efficiency, such as overall thermal efficiency, freshwater productivity capacity, energy capacity, as well as evaluated the performance of various configurations and methods of operation to find the optimal scheme, considering the effect on performance as the location determines the level of available solar radiation and the characteristics of the brackish or seawater to be used, Therefore, these solar desalination systems (hybrid plants with solar energy), compared to traditional desalination plants powered by fossil fuels, show their great viability at present, contributing to reduce CO2 pollution, and decreasing environmental problems due to depletion of natural resources.
In most cases, the lack of drinking water occurs in places with abundant solar radiation, which allows the use of various methods with solar energy, because desalination plants consume a high value of electricity, although desalination is not a polluting process, the generation of electricity to meet the demand makes it necessary to link the production of water with renewable energy sources.
Finally, reverse osmosis has an advantage over other thermal systems in that this process makes use of both brine and seawater, presenting relatively low investment costs depending on the water to be desalinated, although for this process the use of physicochemical treatment is indispensable, it can be coupled with another type of system and with renewable energies to supply the energy demand needed to operate the high-pressure pumps.

Figure 1 .
Figure 1.Countries with the largest installed desalination plants

Figure 3 .
Figure 3. Schematic of simple HDH desalination system

Figure 4 .
Figure 4. Schematic of the four-stage HDH desalination system with direct contact dehumidifiers

Figure 5 .
Figure 5. Schematic system of the integration of solar thermal plant, absorption cooling system and MED cycle.