Literature review

Sustainable development objectives SDGs (English acronym) that were presented at the Assembly of the United Nations Organization (^{ONU, 2015}), with emphasis on the objectives directly related to water (6. Clean water and sanitation) and energy (7. Afordable and clean energy) and the urgency of attending the poorest sectors, constitute the framework in which this work was elaborated.

A proposal was raised to feed a desalination plant with a double barrier of pumping wells that, located through the Seawat model applied by ^{Canales, Islas and Velázquez (2016)}, and based on other research, had the dual purpose of supplying drinking water to Guaymas, Empalme and Sonora populations, and to protect the aquifer of Boca Abierta Valley, Sonora, against the saline intrusion advance. This also proposes energy savings by using brackish water from wells with lower saline concentration instead of sea salt water. This description has not yet taken into account the use of solar energy that the hybrid system of high efficiency and high thermal photovoltaic concentration already offers at laboratory scale.

Regarding the application of solar energy, through photovoltaic cells for the desalination process (Figure 2) there are several international references that demonstrate the feasibility of its use. On the one hand, ^{Fthenakis, Atia, Morin, Bkayrat and Sinha (2015)}, present experiences in Saudi Arabia and on the other, ^{Zimmermann et al. (2015)} describe a hybrid system of high efficiency and thermal photovoltaic concentration (HCPVT).

Figure 2 Solar powered desalination plant in Kenya provides fresh water for 25,000 people daily. PSC Solar UK (2019). 

At present, IBM with the high thermal photovoltaic concentration (HCPVT) system (Figure 3), can achieve 90^oC to operate the membrane distillation process with which it has the capacity to obtain 30-40 liters per day per m² of reception area.

Figure 3 IBM research. High thermal photovoltaic concentration system (HCPVT). ^{Williams (2013)}). 

For ^{Sommariva (2017)}, the main problem encountered in the application of the progress in this process, presented by ^{Williams (2013)}, is related to closing the gap between research and development, as well as the commercialization of these concepts. ^{Sommariva (2017)}, synthesizes and projects desalination volumes from 2000 to present and makes a prediction until 2020, where it also mentions some costs. Water scarcity is not just a phenomenon limited to the Middle East; there are several large-scale desalination projects in other parts of the world, such as in North Africa, South Asia, as is the case of Saudi-Arabia, the Middle East, Israel and Southern California, North America.

As shown in Table 1, desalination volumes have doubled from 2000 to 2008 and are expected to triple by 2020, reflecting a compound annual growth rate of 8%. As water stress increases and the use of desalination expands outside of the areas that have recently adopted it, as the Middle East, it is predicted that volumes of desalinated water will reach 54 x10⁹ m³/ year in 2020.

Desalination is now used in more than 120 countries around the world. Over the past 30 years, several large-scale projects have demonstrated that it is technically feasible to generate large volumes of water of acceptable purity through desalination of seawater, brackish water and water reuse ^{Sommariva (2017)}.

The same author mentions that in the past the cost of seawater desalination was below USD 0.50 / m³ in many projects; however due to the increase in material costs, the total cost of desalination has subsequently increased to 1 -1.5 USD / m³.

^{Shahzad, Burhan, Ang and Ng (2017)} present advances and the state of the art in water-energy-environment links, which represents the sustainability of future desalination foundation. The desalination treatment process is the largest energy consumer, about 0.4% of total electricity (75.2 x 10⁹ kWh / year in 2016) and is the largest source of fossil fuels CO₂ emissions. Improving the efficiency of the two major sources, water and energy is important to control such emission and protect the environment.

Solar concentration power plants (CSP, English acronym) represent a promising solution for many countries in the Middle East and North Africa region (MENA) (^{Atanosovska 2016}). In 2014, there were around 2,800 desalination plants in MENA that supplied 27 x 10⁶ m³/ day, this represents an average of 10,000 m³ / day in each plant. In that year, Algeria had one of the largest plants in the region supplying 500,000 m³/ day.

According to the same author and the Association of the Solar Industry for the Middle East, the CSP projects that were in progress in 2014, reached a record of 294 MW capacity. Morocco had a massive solar power plant in Ouarzazate. In Tunisia for 2012, a solar plant was built in TuNur; while in Qatar the project of a low energy consumption desalination plant was launched.

At present, worldwide, in terms of the use of energy and environmental impact related to desalination, reverse osmosis (RO) processes are dominating the market for brackish water and, in the period 2005 to 2008, they showed a growth trend in supply going from 2.0 to 3.5 million m³ / day.

Processes to desalinate water contributed in 2016, 76 x 10⁶ tons of CO₂ (0.2% of the total emission of 36 100 x 10⁶ tons / year) globally, estimating by 2040, emissions of 218 x 10⁶ tons of CO₂. To have an idea of what this means, the author mentions that 2/3 of the required reduction has been used to keep the global temperature increase below 2 °C, the COP 21 objective, and the remaining third would be exhausted by 2050. ^{Sachs (2014)} comments that, the above-mentioned CO₂ emissions will lead to an annual increase of 2 ppm in the atmospheric concentration of this gas. In the last 150 years there was an increase of 280 to 400 ppm, so if in 25 years it rises to 450 ppm, there will be a planet with 2 °C more than before the Industrial Revolution, with the consequences of extreme weather events and the deterioration of terrestrial and marine ecosystems, already evident.

Energy requirements and technological innovations in the desalination process

For future water supplies in arid and semi-arid areas of the planet, the best choice will be the one that involves high efficiency desalination processes integrated with renewable energy sources. This is equivalent to coupling desalination technologies with renewable energy sources, with three major future benefits for the world: environmental sustainability and supply of fresh water and energy.

To achieve the goal of future sustainable water supply, high flow membranes, selective, resistant to clogging, stable and with minimal cost and manufacturing defects are required. Among the variety of efficient materials that have been proposed to improve the operation of membranes based on ceramics and polymers, of which ^{Shahzad et al. (2017)} cite several examples, the best functioning ones, the bio-inspired membranes, are detailed below.

Ceramic-based membranes and conventional polymers that are used in the RO process currently consume 3 to 5 kWh / m³. Driven by sustainable water supply objectives, it is necessary to improve the functioning of these membranes with more efficient materials such as, coated ceramics with nano-particle (nano = 10^-9) catalytic, zeolitic, organic-inorganic nano-compounded, or bio-inspired ones that include biomimetic polymer-protein, block isoporous copolymer and aligned nanotubes. The bioinspired are the best functioning but still about 5 to 10 years away, research for commercial availability. However, nanocomposite membranes with significant improvements are already commercially available.

As for the desalination process, reverse osmosis (RO) processes are promising as they are expected to be reinforced in the future by the high efficiency development of aquaporin and graphene in membranes. Aquaporin proposed by Agre, Sasaki and Chrispeels (cited in ^{Shahzad et al., 2017}), is an element that controls the flow through biological membranes with protein channels. Reverse osmosis membrane covered with 75% aquaporin increases the permeability of commercially available membranes by one order of magnitude; however, they have not been massively applied due to the difficulty in producing large areas of protein. However, there is a potential to incorporate biological aquaporin into RO membranes in the future, so more research is required to optimize the formation of biological structures in terms of selection, robustness, material cost, scalability, and in the process of RO, specific energy consumption less than 2 kWh / m². The author also concludes that thermal hybrid desalination processes can reach between 20 and 25% efficiency, with 1 or 2 years of experience, which would be sustainable and save energy, protect the environment and be consistent to COP21 objectives.

^{Amya et al. (2017)} explore the state of the art in seawater desalination practices, with an emphasis on membrane-based technologies, while identifying future opportunities in improvements by conventional technology stages and the development of out of trends emerging and potential technologies, through advances in materials sciences, engineering processes and integration of systems. In this article, seawater RO is considered as the starting technology. The discussion goes beyond desalination processes in membranes that are based on energy production processes with salinity gradients. The future scenario of desalination and energy membranes with saline gradients is projected to include hyper-high permeability RO membranes, renewable energy for desalination and emerging processes, including closed RO circuits, distillation membranes, future osmosis, delayed osmosis pressure and reverse electrodialysis, according to several application and / or combined niches (hybrids), operating separately or in conjunction with RO.

Given the problem of variable salinity due to the collection of brackish water in pumping barriers with beach wells for protection against saline intrusion, it´s necessary to consider that changes in pressure and number of membranes in RO, as well as reducing the efficiency of feed pumps to absorb changes in salinity, are issues that are related with the cost that will ultimately affects the user.

On the other hand, ^{Raval and Maiti (2016)} emphasize that the electrical efficiency of the photovoltaic (PV) solar panel decreases with the increase in its temperature and therefore, the heat transfer from the panel is a very important point to be considered. Capitalizing the heat transferred for some useful purpose is of first importance since the solar panel (PV) has a conversion efficiency of only 5-17% and consequently, much of the solar radiation remains unused. This article pointed out the control of the temperature of the PV solar panel, mentioning the possibility of establishing a direct contact of the heat exchanger with the feed water flowing to the RO system from the upper end of the panel, thus recovering the energy together with a better operation of the PV panel. Also, the RO system at high temperature results in a better membrane flow. In addition, the modification of the membrane morphology by controlling the treatment with sodium hypochlorite increases the efficiency of the flow by increasing the ability to mix, dissolve or moisten the membrane; and as an evidence we get the declination of the contact angle that goes from 48.05 ° to 26.22 °. Thus, two techniques, the temperature control of the PV panel and the heat transfer and the adjustment of the membrane morphology towards the increase in capacity, helped to significantly improve the flow permeability in the RO system and in the electrical operation of the PV panel. As a result, the total energy consumed by RO has been reduced by 40%. This novel approach opens the channels to significantly reduce energy consumption of RO systems for brackish water.

Research on "hybrid" technologies spectral beam splitting concentrated photovoltaic/thermal (SBS CPVT), having started in the ‘80s, have been improving until the full use of solar irradiation of its entire spectrum using photovoltaic cells (PV) and thermal absorbers. Different SBS attempts were used to achieve better conversion efficiencies, including several types of filters (^{Xing et al., 2017}) and ^{Fenga, Zhengb, Wangc and Mab (2016)}, based on the optical refraction principle, calculated the dimensions of each Fresnel lens element. An optical simulation was performed to obtain the optimum efficiency of the concentrator, at the same time as ^{Widyolara et al. (2017)} designed, simulated and tested an innovative photovoltaic-thermal hybrid solar collector (PV/T). The PV/T system uses a non-reflective composite parabolic concentrator (CPC) to focus sunlight towards a receiver with a high concentration battery of the simple arsenic-gallium junction, solar cells that simultaneously generate electricity and high temperature thermal power. Finally, ^{Srivastava (2017)}, in his article presents the linear and parabolic thermal and electrical analyzes through a photovoltaic concentrator collection system (CPV) under different design and operation conditions. A composite parabolic collector (CPC) was incorporated as a secondary reflector to homogenize the flow. Below we present a global example of water supply and required power, as well as a comparative series of the energy required per unit of water obtained by desalination systems.