Solar Desalination Driven by Organic Rankine Cycles (Orc) and Supercritical CO2 Power Cycles: An Update

  Citation: Delgado-Torres, A.M.; García-Rodríguez, L. Solar Desalination Driven by Organic Rankine Cycles (Orc) and Supercritical CO2Power Cycles: An Update. Processes 2022,10, 153. https://doi.org/10.3390/pr10010153 Academic Editor: Chiing-Chang Chen Received: 31 October 2021 Accepted: 11 January 2022 Published: 13 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). processes Review Solar Desalination Driven by Organic Rankine Cycles (Orc) and Supercritical CO2Power Cycles: An Update Agustín M. Delgado-Torres 1,* and Lourdes García-Rodríguez 2 1Departamento de Ingeniería Industrial, Escuela Superior de Ingeniería y Tecnología (ESIT), Universidad de La Laguna (ULL), 38200 La Laguna, Spain 2Departamento de Ingeniería Energética, Escuela Técnica Superior de Ingeniería (ETSI), Universidad de Sevilla, 41092 Sevilla, Spain; [email protected] *Correspondence: [email protected]; Tel.: +34-922-84-60-45 Abstract: In the field of desalination powered by renewable energies, the use of solar power cycles exhibits some favorable characteristics, such as the possibility of implementing thermal energy storage systems or a multi-generation scheme (e.g., electricity, water, cooling, hydrogen). This article presents a review of the latest design proposals in which two power cycles of great potential are considered: the organic Rankine cycle and the supercritical CO 2 power cycle, the latter of growing interest in recent years. The designs found in the literature are grouped into three main types of systems. In the case of solar ORC-based systems, the option of reverse osmosis as a desalination technology is considered in medium-temperature solar systems with storage but also with low-temperature using solar ponds. In the first case, it is also common to incorporate single-effect absorption systems for cooling production. The use of thermal desalination processes is also found in many proposals based on solar ORC. In this case, the usual configuration implies the cycle’s cooling by the own desalination process. This option is also common in systems based on the supercritical CO 2 power cycle where MED technology is usually selected. Designs proposals are reviewed and assessed to point out design recommendations. Keywords: solar ORC desalination; solar supercritical CO 2 ; solar desalination; solar reverse osmosis 1. Introduction The need to change the global energy model and the increase in drinking water demand in important areas of the planet make desalination powered by renewable energies (REs) part of the solution to the future challenge of the energy–water nexus. According to International Energy Agency (IEA) data, almost 81% of the world’s energy supply had its origin in fossil fuels (coal, natural gas and oil) in 2019 [ 1 ]. If we focus on electrical energy, the percentage was 63% in the same year. These values explain the still-growing trend of global greenhouse gas emission levels. In addition to this, there is the problem of access to water. According to World Bank data, internal renewable freshwater resources per capita have fallen by 36% and 25% between 2002 and 2017 in areas such as Sub-Saharan Africa or the MENA (Middle East and North Africa) region, respectively [ 2 ]. These reductions are largely due to population increase, which is the expected trend in the coming decades. RE-powered desalination offers advantages and benefits compared to the fossil-fuelsdriven option, such as the coincidence of renewable resource availability—mainly solar— and water scarcity. In addition, the environmental impacts associated with the energy consumption of the processes are reduced [ 3 ]. In this sense, the Life Cycle Analysis (LCA) of desalination technologies systematically indicates that the environmental impacts associated with the energy consumption of the process are mainly attributable to the operation and maintenance phase in comparison to construction and decommissioning phases [ 4 ]. Therefore, the use of renewables as the energy input to these processes results in a significant reduction of the environmental impacts of freshwater production. On average, Processes 2022,10, 153. https://doi.org/10.3390/pr10010153 https://www.mdpi.com/journal/processes Processes 2022,10, 153 2 of 23 the global warming potential reduction is around 90% for conventional thermal desalination technologies—multi-stage flash (MSF) and multi-effect distillation (MED)—and 80% for reverse osmosis (RO) [4]. The above advantages justify the trend in scientific activity concerning RE-powered desalination shown in the bibliometric analysis of the scientific literature published on desalination between 2000 and 2020, as presented by Zapata-Sierra et al. [ 5 ]. Among other results, the clustering of publications is highlighted; the “Renewable energy desalination” cluster is the second most important of the eight clusters, defined with a weight of 26.1%, just one percentage point below the first ‘’Reverse osmosis” cluster). Within this frame, this work deals with solar-powered desalination by means of solar thermo-mechanic conversion through a power cycle, thus allowing the use of electricity and/or heat to drive desalination processes. The main aim of the paper is the review and assessment of recent design proposals existing in the literature to give design recommendations on such solar desalination systems. The latest general review of RE-desalination concludes that the techno-economic optimization of some aspects is still necessary to offer stable solutions in the long term [ 6 ]. Regarding the solar thermal-powered option, the need for its application to both small and industrial scales and the importance of thermal energy storage are emphasized. The need to expand the implementation of off-grid systems is also indicated in general. In the field of solar thermal-powered desalination, it is possible to combine this resource with all existing desalination technologies (see Figure 1). Only the methods associated with the use of power cycles for the thermo-mechanical conversion of solar thermal energy are shown since this is the route explored in this article. Therefore, the use of the electricity generated and/or the heat rejected by a solar power cycle are the options reviewed instead of the direct coupling of thermal desalination processes to a solar thermal collector field. Concerning direct coupling, we refer to Buenaventura Pouyfacon and García-Rodríguez [ 7 ]. Processes 2022, 10, x FOR PEER REVIEW 2 of 23 results in a significant reduction of the environmental impacts of freshwater production. On average, the global warming potential reduction is around 90% for conventional thermal desalination technologies—multi-stage flash (MSF) and multi-effect distillation (MED)—and 80% for reverse osmosis (RO) [4]. The above advantages justify the trend in scientific activity concerning RE-powered desalination shown in the bibliometric analysis of the scientific literature published on desalination between 2000 and 2020, as presented by Zapata-Sierra et al. [5]. Among other results, the clustering of publications is highlighted; the “Renewable energy desalination” cluster is the second most important of the eight clusters, defined with a weight of 26.1%, just one percentage point below the first ‘’Reverse osmosis’’ cluster). Within this frame, this work deals with solar-powered desalination by means of solar thermo-mechanic conversion through a power cycle, thus allowing the use of electricity and/or heat to drive desalination processes. The main aim of the paper is the review and assessment of recent design proposals existing in the literature to give design recommendations on such solar desalination systems. The latest general review of RE-desalination concludes that the techno-economic optimization of some aspects is still necessary to offer stable solutions in the long term [6]. Regarding the solar thermal-powered option, the need for its application to both small and industrial scales and the importance of thermal energy storage are emphasized. The need to expand the implementation of off-grid systems is also indicated in general. In the field of solar thermal-powered desalination, it is possible to combine this resource with all existing desalination technologies (see Figure 1). Only the methods associated with the use of power cycles for the thermo-mechanical conversion of solar thermal energy are shown since this is the route explored in this article. Therefore, the use of the electricity generated and/or the heat rejected by a solar power cycle are the options reviewed instead of the direct coupling of thermal desalination processes to a solar thermal collector field. Concerning direct coupling, we refer to Buenaventura Pouyfacon and García-Rodríguez [7]. Figure 1. Possible indirect solar desalination options when a power conversion unit (power cycle) is considered as the solely final energy supply unit. MSF: multi-stage flash; MED: multi-effect distillation; MD: membrane distillation; HDH: humidification dehumidification; MVC: mechanical vapor compression; RO: reverse osmosis; ED/EDR: electrodialysis/electrodialysis reversal; CDI: capacitive deionization; EDI: electrodeionization. Adapted from [6]. The thermal processes shown in Figure 1 require a heat supply in the range of 50–120 °C [6]. Temperatures from 50 to 60 °C are only suitable for humidification dehumidification (HDH), appropriate only for small-scale use due to its relatively low energy efficiency [8]. Moreover, membrane distillation (MD) systems require 75–95 °C to achieve reasonable specific thermal consumption, up to 237 and 170 kJ/kg, respectively [7]. However, there are only small capacity units, thus limiting their application at an industrial scale. Figure 1. Possible indirect solar desalination options when a power conversion unit (power cycle) is considered as the solely final energy supply unit. MSF: multi-stage flash; MED: multi-effect distillation; MD: membrane distillation; HDH: humidification dehumidification; MVC: mechanical vapor compression; RO: reverse osmosis; ED/EDR: electrodialysis/electrodialysis reversal; CDI: capacitive deionization; EDI: electrodeionization. Adapted from [6]. The thermal processes shown in Figure 1require a heat supply in the range of 50–120 ◦C [ 6 ]. Temperatures from 50 to 60 ◦ C are only suitable for humidification dehumidification (HDH), appropriate only for small-scale use due to its relatively low energy efficiency [ 8 ]. Moreover, membrane distillation (MD) systems require 75–95 ◦ C to achieve reasonable specific thermal consumption, up to 237 and 170 kJ/kg, respectively [ 7 ]. However, there are only small capacity units, thus limiting their application at an industrial scale. Multi-effect distillation (MED) has been superior to multi-stage flash (MSF) distillation since the end of the 20th century due to its lower main and auxiliary energy consumption Processes 2022,10, 153 3 of 23 along with lower thermal consumption at the same top temperature. For the sake of comparison, the MED process achieves 230 kJ/kg of thermal energy consumption with a top temperature of 67 ◦ C, whereas the MSF would require at least 105 ◦ C to allow the said main energy consumption. In addition, the auxiliary energy required by the MSF process is much higher than that of MED, since the former requires brine recirculation. The industrial standard of conventional MED technology consists of a MED unit coupled to a thermal vapor compression process (MED-TVC). The MED unit is driven by the outlet flow of the TVC, which is fed by a steam extraction of the steam turbine of a Rankine cycle (motive steam) and steam generated within one of the last effects of the MED plant. Regarding desalination processes that consume shaft power or electricity, only mechanical vapor compression (MVC) and reverse osmosis (RO) are suitable for seawater desalination applications. This is due to the significant increase in energy consumption in electrodialysis (ED), electrodialysis reversal (EDR), capacitive deionization (CDI), and electrodeionization (EDI) with salt concentration. Therefore, they do not compete with RO for salinities within the range of seawater, about 0.035 kg of sea salts per kg of saline solution. In addition to conventional phase-change technologies, a MED unit can be coupled to a double-effect absorption heat pump (DEAHP). This technology has been developed at the Spanish research center Plataforma Solar de Almería-CIEMAT [ 9 – 11 ]. For a given flow and thermodynamic conditions of the external steam source, MED-DEAHP technology exhibits lower main and auxiliary energy consumptions than MED-TVC, thus resulting in higher thermodynamic efficiency of the integrated power and water production. Therefore, MED-DEAHP is superior to MED-TVC, although the former is not commercially available. Another advanced MED concept is a high-temperature MED process by using nanofiltration as seawater pretreatment. This was proposed and developed by Hassan [ 12 – 14 ]. Not only the concept but also the experimental assessment have been reported in the literature [ 15 ]. A representative industrial plant in which nanofiltration and distillation processes are integrated is the Sharjah plant, based on MSF distillation [16]. Analyses of integrating desalination processes within conventional solar power plants based on Rankine cycles with water as working fluid have been frequently reported in the literature, considering mostly RO, MED, and MED-TVC processes. Some exemplary papers are [ 17 , 18 ], among others. Regarding the integration of distillation processes, the main issue is the availability of operating models properly validated to simulate the distillation subsystem at part-load operation. Concerning MED-TVC plants, a performance model [ 19 ] has been developed to calculate the efficiency and water production of the MED-TVC distillation process as a whole within a suitable range of the external steam source. Some selected previous papers are Hanafi et al. [ 20 ], who described a thorough model of a thermocompressor for the said application, and Ameri et al. [ 21 ] and Mazini et al. [ 22 ], who presented useful models of MED-TVC plants. Additionally, the parametric analysis needed for plant design can be found in works by Kouhikamali et al. [ 23 ] and Esfahani et al. [ 24 ]. Finally, experimental data useful for validating models are provided by Temset et al. [ 25 ] and Al-Mutaz and Wazeer [ 26 ]. On the other hand, both MED and nanofiltration-MED processes can be modeled based on the aforementioned literature. Additionally, operation out of nominal conditions of a MED unit has been experimentally assessed by FernandezIzquierdo et al. [27]. Recent proposals of solar desalination system designs that fit the scheme of Figure 1 are presented throughout this paper. Minor innovations have been found concerning coupling conventional solar Rankine cycles and desalination. Indeed, innovative proposals rely on the organic Rankine cycle (ORC) and the supercritical CO 2 (sCO 2 ) cycle as power conversion units. Medium-temperature technology of solar thermal collectors consists of systems with top operating temperatures within the range of 150–400 ◦ C, thus requiring one-axis sun tracking to concentrate the solar irradiance on a linear focus in which the absorber tube is placed. Solar parabolic trough collectors (PTCs) and linear Fresnel concentrators correspond to medium-temperature collectors. Since conventional Rankine cycles operated with Processes 2022,10, 153 4 of 23 water exhibit quite limited efficiency at relatively low temperatures, significant research activity has been focused on organic Rankine cycles (ORCs), suitable even for the lowest temperatures within the said range. Indeed, ORCs operated with low-temperature solar collectors—i.e., stationary collectors—have also been developed. The first solar ORC desalination systems date back to the late 1970s and early 1980s, all of them using reverse osmosis as the desalination technology. Due to the high specific energy consumption of the process at that time, these first experiences were developed for brackish water desalination [ 28 – 30 ]. A review of state-of-the-art solar thermal reverse osmosis desalination systems up to 2009 can be found in [ 31 ]. Its conclusions indicate, among other issues, that the water–electricity cogeneration option has not been sufficiently explored. Design recommendations based on the work developed up to 2012 is reported in [ 32 ], and the update of the state of the art can be found in [ 33 ]. The existing systems and proposals are classified in the latter according to the ORC configuration into single stage (referring to simple cycles), two-stage (referring to double cascade cycles), and low-temperature systems. More recently, and covering a wider scope, the evaluation of solar thermal-powered desalination technologies has been conducted by [ 7 ] to identify market opportunities for these technologies. Among the options of growing interest is reverse osmosis powered by solar ORCs, which presents the possibility of incorporating thermal energy storage systems (TES) instead of batteries. In low demand scenarios (less than 100 m 3 /d), conventional distillation technologies (MED and MSF) and RO powered by solar ORCs are discarded against the photovoltaic (PV)–RO option, while in intermediate demand scenarios (100 to 25,000 m 3 /d), the solar ORCs with PTCs or Fresnel is one of the technologies with market opportunities. The following sections present the most recent proposals found in the scientific literature on solar desalination systems based on ORC as the main energy conversion units. As will be seen, the approaches based on polyor multi-generation are dominant. Lastly, sCO 2 Brayton cycles have been recently developed in order to achieve much higher energy efficiency, with top temperatures technically achievable with the current technology of solar tower power plants. Heliostats concentrate the solar radiation within a focal point on the top of a tower where the solar receiver is placed. Specifically, energy efficiency around 45% corresponds to the top temperature of 550 ◦ C with a Brayton cycle with regeneration and recompression. In the near future, 52% could be reliable with 700 ◦ C of top temperature [34]. 2. Solar ORC (Organic Rankine Cycle)-Driven Desalination Systems: An Update 2.1. Integration of Reverse Osmosis (RO) as Desalination Technology Reverse osmosis desalination requires electricity to power the main pump to pressurize the saline water feed above the osmotic pressure of the concentrate output flow. Pressurized feedwater flow circulates in parallel with the RO membrane surface, whereas part of the solvent passes through the membrane along with a minor portion of salts, thus generating the permeate flow (product). The remaining feed stream with increased salinity becomes the concentrate flow. Since this concentrate exits at a pressure slightly below the feed pressure, energy recovery devices are needed in seawater desalination plants. Recent proposals for solar RO desalination systems with ORC units as the prime mover generally correspond to the configurations described in the following subsections, dealing with mediumand low-temperature solar systems, respectively. 2.1.1. Medium-Temperature Solar Systems The scheme shown in Figure 2describes the latest innovative proposals of mediumtemperature solar ORC systems. As can be seen, a solar thermal plant with thermal energy storage (TES) drives a single-effect absorption refrigeration system by the heat rejected at the condenser. This scheme would therefore respond to a polyor multi-generational system if not the entire electrical energy output of the ORC is consumed by the RO unit. Additionally, part of this thermal energy rejected preheats the feed water of the RO system. This exploitation has two advantages: (1) the use of saline water as a cooling medium Processes 2022,10, 153 5 of 23 makes an additional cooling flow unnecessary; (2) the permeability of the RO membranes rises with feed water temperature, that is to say, the productivity of the plant is improved although this occurs at the expense of an increase in the salinity of the product. Processes 2022, 10, x FOR PEER REVIEW 5 of 23 storage (TES) drives a single-effect absorption refrigeration system by the heat rejected at the condenser. This scheme would therefore respond to a polyor multi-generational system if not the entire electrical energy output of the ORC is consumed by the RO unit. Additionally, part of this thermal energy rejected preheats the feed water of the RO system. This exploitation has two advantages: (1) the use of saline water as a cooling medium makes an additional cooling flow unnecessary; (2) the permeability of the RO membranes rises with feed water temperature, that is to say, the productivity of the plant is improved although this occurs at the expense of an increase in the salinity of the product. Figure 2. General scheme of a solar thermal-driven RO desalination and cooling system. A solar poly-generation system with a recuperative ORC unit with n-octane as the working fluid for electricity production is presented in [35]. The solar system considered consists of a parabolic trough collector field operated with Therminol-66 as the heat transfer fluid (HTF) and a two-tank thermal energy storage (TES) system with commercial molten salts. Hitec XL is considered in order to take advantage of its relatively low freezing point (120 °C) and cost. A fraction of the heat transferred in the ORC’s recuperator is exploited for the production of domestic hot water. Moreover, the heat rejection of the ORC is used in the generator of a single-effect absorption refrigeration cycle. The ORC’s electricity output is consumed by an electrolyzer for hydrogen production and by the seawater RO unit, where a Pelton turbine is considered as the energy recovery system. The water needed in the electrolyzer is preheated by the thermal oil flow at the outlet of the ORC’s solar heat exchanger. The proposed system is analyzed to produce between 200 and 500 kW electric and 450 ppm freshwater from 35,000 ppm feed water. The nominal capacity of the seawater reverse osmosis plant (SWRO) is 40 kg/s, and the absorption refrigeration system would have a cooling capacity of 500–800 kW. Both energy and exergy efficiency of the whole system is calculated, and techno-economic optimization is also performed using evolutionary algorithms. Another configuration of parabolic trough solar collectors with TES and Therminol VP-1 as HTF and storage medium is proposed by [36]. A conventional Rankine cycle is thermally driven by this solar energy system. The steam turbine’s output flow, after being partially expanded, acts as the primary flow of a steam ejector. The discharge mixed flow is the heat source of the ORC unit. After heating the ORC system, this stream is split into two flows. One of them completes the Rankine cycle and the second flow is used for a preliminary preheating of the RO feedwater and subsequently for cooling production. The cooling of the ORC is also carried out with the feedwater of the desalination unit to obtain Figure 2. General scheme of a solar thermal-driven RO desalination and cooling system. A solar poly-generation system with a recuperative ORC unit with n-octane as the working fluid for electricity production is presented in [ 35 ]. The solar system considered consists of a parabolic trough collector field operated with Therminol-66 as the heat transfer fluid (HTF) and a two-tank thermal energy storage (TES) system with commercial molten salts. Hitec XL is considered in order to take advantage of its relatively low freezing point (120 ◦ C) and cost. A fraction of the heat transferred in the ORC’s recuperator is exploited for the production of domestic hot water. Moreover, the heat rejection of the ORC is used in the generator of a single-effect absorption refrigeration cycle. The ORC’s electricity output is consumed by an electrolyzer for hydrogen production and by the seawater RO unit, where a Pelton turbine is considered as the energy recovery system. The water needed in the electrolyzer is preheated by the thermal oil flow at the outlet of the ORC’s solar heat exchanger. The proposed system is analyzed to produce between 200 and 500 kW electric and 450 ppm freshwater from 35,000 ppm feed water. The nominal capacity of the seawater reverse osmosis plant (SWRO) is 40 kg/s, and the absorption refrigeration system would have a cooling capacity of 500–800 kW. Both energy and exergy efficiency of the whole system is calculated, and techno-economic optimization is also performed using evolutionary algorithms. Another configuration of parabolic trough solar collectors with TES and Therminol VP-1 as HTF and storage medium is proposed by [ 36 ]. A conventional Rankine cycle is thermally driven by this solar energy system. The steam turbine’s output flow, after being partially expanded, acts as the primary flow of a steam ejector. The discharge mixed flow is the heat source of the ORC unit. After heating the ORC system, this stream is split into two flows. One of them completes the Rankine cycle and the second flow is used for a preliminary preheating of the RO feedwater and subsequently for cooling production. The cooling of the ORC is also carried out with the feedwater of the desalination unit to obtain additional heating. Therefore, in this proposal, the ORC is not directly heated by the solar thermal plant. 2.1.2. Low-Temperature Solar Systems The other option commonly addressed in recent studies is the combination of solar ponds (SPs) and a low-temperature ORC unit (see Figure 3) to power a RO desalination Processes 2022,10, 153 6 of 23 system. The concept is not new; two of such systems were implemented in the USA— Los Baños (California) and El Paso (Texas) [ 37 ]. A solar pond is integrated in the same device for both solar–thermal energy conversion and long-term heat storage. With adequate procedures of creation and maintenance, three zones with different salinities remain throughout the lifetime of the SP. Thanks to the solar heating, thermal energy reaches the lowest layer (lower convective zone), consisting of high salinity solution. On top, a layer with an appropriate salinity gradient is created, so-called the non-convective zone (NCZ). Due to the corresponding density profile, its salinity gradient avoids convection in order to maximize the thermal insulation of the LCZ. The upper layer (upper convective zone, UCZ) provides insulation from the atmospheric phenomena. As Figure 3depicts, solar heat is extracted from the lower convective zone (LCZ) of the SP, whereas the heat rejected by the ORC is re-injected into the upper convective zone (UCZ). Processes 2022, 10, x FOR PEER REVIEW 6 of 23 additional heating. Therefore, in this proposal, the ORC is not directly heated by the solar thermal plant. 2.1.2. Low-Temperature Solar Systems The other option commonly addressed in recent studies is the combination of solar ponds (SPs) and a low-temperature ORC unit (see Figure 3) to power a RO desalination system. The concept is not new; two of such systems were implemented in the USA—Los Baños (California) and El Paso (Texas) [37]. A solar pond is integrated in the same device for both solar–thermal energy conversion and long-term heat storage. With adequate procedures of creation and maintenance, three zones with different salinities remain throughout the lifetime of the SP. Thanks to the solar heating, thermal energy reaches the lowest layer (lower convective zone), consisting of high salinity solution. On top, a layer with an appropriate salinity gradient is created, so-called the non-convective zone (NCZ). Due to the corresponding density profile, its salinity gradient avoids convection in order to maximize the thermal insulation of the LCZ. The upper layer (upper convective zone, UCZ) provides insulation from the atmospheric phenomena. As Figure 3 depicts, solar heat is extracted from the lower convective zone (LCZ) of the SP, whereas the heat rejected by the ORC is re-injected into the upper convective zone (UCZ). Figure 3. General scheme of solar-pond-driven RO desalination proposals. The investigation of a solar RO desalination system using an SP that provides thermal energy to a recuperative ORC unit is presented by [38]. For this purpose, an HTF is used to extract energy from the lower convective zone. This HTF is circulated through the ORC evaporator. As for the desalination system, the feed water is first preheated in the ORC condenser and then circulated into the upper convective zone of the solar pond before entering the desalination unit. The brine stream from the RO unit is partially evaporated and a portion is subsequently injected into the lower convective zone in order to replenish salts losses due to their diffusion to the upper layers. The seawater RO unit is electrically driven by the ORC. A RO desalination system based on a solar ORC powered by an SP is also presented in [39]. This configuration is proposed together with the analogous system, in which the ORC is replaced by a Kalina cycle. In the solar ORC configuration, an extracted fluid stream forms the LCZ and is circulated to the ORC evaporator, which incorporates recuperation. Part of the electrical power produced is used in the desalination unit while the rest is injected into the grid. It the use of the heat from the ORC condenser is also contemplated for a thermoelectric generation (TEG) system. A flow extracted from the upper convective zone is used as a cold sink for the TEG and re-injected into the upper convective zone after being heated. Seven different working fluids are considered for the ORC, resulting in an overall system exergy efficiency of up to 46.4% when using R227ea. Figure 3. General scheme of solar-pond-driven RO desalination proposals. The investigation of a solar RO desalination system using an SP that provides thermal energy to a recuperative ORC unit is presented by [ 38 ]. For this purpose, an HTF is used to extract energy from the lower convective zone. This HTF is circulated through the ORC evaporator. As for the desalination system, the feed water is first preheated in the ORC condenser and then circulated into the upper convective zone of the solar pond before entering the desalination unit. The brine stream from the RO unit is partially evaporated and a portion is subsequently injected into the lower convective zone in order to replenish salts losses due to their diffusion to the upper layers. The seawater RO unit is electrically driven by the ORC. A RO desalination system based on a solar ORC powered by an SP is also presented in [ 39 ]. This configuration is proposed together with the analogous system, in which the ORC is replaced by a Kalina cycle. In the solar ORC configuration, an extracted fluid stream forms the LCZ and is circulated to the ORC evaporator, which incorporates recuperation. Part of the electrical power produced is used in the desalination unit while the rest is injected into the grid. It the use of the heat from the ORC condenser is also contemplated for a thermoelectric generation (TEG) system. A flow extracted from the upper convective zone is used as a cold sink for the TEG and re-injected into the upper convective zone after being heated. Seven different working fluids are considered for the ORC, resulting in an overall system exergy efficiency of up to 46.4% when using R227ea. The double cascade concept is considered in the three designs investigated by [ 40 ], one of them without ORC, where an SP acts as the heat source of the topping cycle. The thermodynamic and thermoeconomic analysis of three configurations is performed. In two of them, a single ORC is thermally driven with energy extracted from the LCZ of the SP. The electricity output of this ORC is consumed by the RO unit. The heat rejection of the topping cycle is used as heat absorbed by the bottoming cycle, which, in one of the configurations, Processes 2022,10, 153 7 of 23 is a Kalina cycle, and in the other, a simple ORC. In both cases, the cooling of the bottom cycle’s condenser is carried out with water from the UCZ of the SP. A thermodynamic and thermoeconomic analysis is performed, assuming a SP surface area of 10,000 m 2 , a temperature of 90 ◦ C in the LCZ, and a RO unit conversion of 30% for seawater salinity of 42,485 g/kg. 2.2. Integration of Thermal Processes as Desalination Technology When a thermal desalination process is considered to be integrated with a solar ORC system, recent proposals have mostly matched the layout shown in Figure 4. As can be seen, the integration implies the use of the heat rejected by the ORC to drive the thermal desalination process, which, in most of the proposals, involves a MED or MED-TVC unit. There are also some proposals with HDH or an MSF unit. Those desalination processes are described below. Processes 2022, 10, x FOR PEER REVIEW 7 of 23 The double cascade concept is considered in the three designs investigated by [40], one of them without ORC, where an SP acts as the heat source of the topping cycle. The thermodynamic and thermoeconomic analysis of three configurations is performed. In two of them, a single ORC is thermally driven with energy extracted from the LCZ of the SP. The electricity output of this ORC is consumed by the RO unit. The heat rejection of the topping cycle is used as heat absorbed by the bottoming cycle, which, in one of the configurations, is a Kalina cycle, and in the other, a simple ORC. In both cases, the cooling of the bottom cycle’s condenser is carried out with water from the UCZ of the SP. A thermodynamic and thermoeconomic analysis is performed, assuming a SP surface area of 10,000 m2, a temperature of 90 °C in the LCZ, and a RO unit conversion of 30% for seawater salinity of 42,485 g/kg. 2.2. Integration of Thermal Processes as Desalination Technology When a thermal desalination process is considered to be integrated with a solar ORC system, recent proposals have mostly matched the layout shown in Figure 4. As can be seen, the integration implies the use of the heat rejected by the ORC to drive the thermal desalination process, which, in most of the proposals, involves a MED or MED-TVC unit. There are also some proposals with HDH or an MSF unit. Those desalination processes are described below. Figure 4. Integration of thermal desalination processes in solar-ORC-based poly-generation systems. The industrial standard of the MED process comprises a MED unit coupled to a thermocompressor, as follows: • MED unit. A set of several chambers (so-called effects) are kept under vacuum conditions with decreasing pressures, corresponding to the saturation pressure at decreasing temperatures from 50 to 67 °C, up to 45–35 °C. Normally there is a single heat exchanger consisting of a horizontal tube bundle and the evaporator. An external flow, providing the external heat source of the MED process, circulates within the evaporator tubes of the first effect. Seawater feed with slight preheating is sprayed on the surface of the tube bundle, thus resulting in a thin film that is partially evaporated. The remaining brine is discharged by the bottom of the effect, whereas the steam generated is sent to circulate inside the tubes of the evaporator of the next effect. The steam generated in the last effect is condensed in the end condenser. A seawater flow circulates through the end condenser; part of this flow (seawater cooling) is discharged back to the sea and the rest of the flow is the preheated seawater that enters all effects in parallel. Industrial plants normally have 8–12 effects in MED units and 4–8 effects in MED-TVC, although designs with 14 effects are feasible, with an Figure 4. Integration of thermal desalination processes in solar-ORC-based poly-generation systems. The industrial standard of the MED process comprises a MED unit coupled to a thermocompressor, as follows: • MED unit. A set of several chambers (so-called effects) are kept under vacuum conditions with decreasing pressures, corresponding to the saturation pressure at decreasing temperatures from 50 to 67 ◦ C, up to 45–35 ◦ C. Normally there is a single heat exchanger consisting of a horizontal tube bundle and the evaporator. An external flow, providing the external heat source of the MED process, circulates within the evaporator tubes of the first effect. Seawater feed with slight preheating is sprayed on the surface of the tube bundle, thus resulting in a thin film that is partially evaporated. The remaining brine is discharged by the bottom of the effect, whereas the steam generated is sent to circulate inside the tubes of the evaporator of the next effect. The steam generated in the last effect is condensed in the end condenser. A seawater flow circulates through the end condenser; part of this flow (seawater cooling) is discharged back to the sea and the rest of the flow is the preheated seawater that enters all effects in parallel. Industrial plants normally have 8–12 effects in MED units and 4–8 effects in MED-TVC, although designs with 14 effects are feasible, with an average of around 2.5 ◦ C of temperature gradient between adjacent effects. With 14 effects, the thermal energy consumption would be about 230 kJ/kg in a MED unit and 166 kJ/kg in MED-TVC. • Termocompressor. A high-pressure steam flow (motive steam) is mixed with a lowpressure steam flow, thus resulting in a steam flow with intermediate pressure and temperature. This outlet steam flow drives the MED process: the low-pressure flow is the steam generated at the last effect or one of the last effects, whereas the motive Processes 2022,10, 153 8 of 23 steam is generated by the available heat source. In conventional MED-TVC plants, the motive steam is a turbine extraction. Steam at about 140–225 ◦ C is required in MED-TVC plants. Coupling the thermocompressor to the MED plant at the last effect makes the end condenser unneccesary. This results in avoiding the seawater cooling flow that requires significant electricity consumption, along with increased capital cost attributable to seawater intake infrastructure and pumps. Moreover, MSF plants rely on steam generation due to the quick reduction of the pressure (flash) of the saline solution heated by an external heat source. An MSF plant consists of many flash chambers, the so-called stages, connected in series with a tube bundle (condenser) at the top. Steam generated in the process preheats the seawater feed as it circulates within the condenser tubes. After being preheated, seawater enters an external heat exchanger (brine heater) driven by the external heat source. Industrial plants include brine recycling since only a small portion of steam is generated. Then, brine, instead of seawater, suffers successive flash processes within the flash chambers, coupled in series. Condensers of several stages at the tail are cooled by an external seawater cooling flow. MSF plants exhibit very high auxiliary energy consumption, attributable to the cooling flow, the vacuum system, and the large flow of brine circulating through the flash stages. Finally, the HDH process is based on increasing the relative humidity of the air by means of spraying water within an air flow at the humidifier. At least one of those flows, water and/or air, should be previously heated by an external heat source. Finally, at the dehumidifier, part of the steam within the hot moist air is condensed on the outer surface of colling tubes, thanks to the circulation of a cooling flow. Water and air circuits can be either open or closed. Moreover, their circulation can be forced or natural. A solar poly-generation scheme is presented in [ 41 ]. The ORC unit with o-Xylene as the working fluid is powered by a solar thermal plant with PTCs (Sky Trough model) and a sensible TES. In both systems, Therminol VP-1 is used as the thermal fluid. The heat rejected by the ORC drives the MED unit and a single-effect absorption refrigeration system. The system is studied from energetic and exergetic points of view. The use of a recuperative ORC unit driven by a solar thermal plant with PTC and TES, both with thermal oil as the thermal fluid, is proposed by [ 42 ], among other options. This configuration is compared with the analogous system comprised of a conventional Rankine cycle and molten salts as HTF and storage medium. In both cases, a MED plant is considered. A system based on a solar thermal plant with PTCs and TES with thermal oil as the HTF and storage medium to drive an ORC unit is analyzed in [ 43 , 44 ]. N-octane is the working fluid. The electricity produced by the ORC is consumed in an electrolyzer for hydrogen production. The heat rejected by the ORC is used in an HDH desalination unit and also to preheat the desalinated water output, which will be consumed by the electrolyzer. A solar poly-generation system to meet the cooling and water demands of a greenhouse is studied in [ 45 ]. The solar thermal plant with PTCs and TES provides thermal energy to an ORC unit and an absorption cooling system. Part of the electricity produced by the ORC—with toluene as the working fluid—is used in an electrolyzer, while the heat rejected is used to drive an MSF plant. The desalinated water is then used both in the electrolyzer and as a supply for the greenhouse. The oxygen and hydrogen produced are stored to produce backup thermal energy during periods of no solar radiation using a hydro-oxy combustor. A solar poly-generation system based on an ORC unit integrating a steam ejector placed at the turbine’s outlet is analyzed in [ 46 ]. The discharge mixed flow of the ejector is condensed with seawater, while the secondary flow of the ejector is previously used for cooling production. A MED plant fed with the vapor from an extraction at the ORC turbine is proposed as the desalination unit. Seawater is considered as the heat transfer fluid in the solar thermal plant. In addition to the above, there are other recent proposals in the literature that do not match the scheme of Figure 4but incorporate ORC units to supply the energy demanded Processes 2022,10, 153 9 of 23 by a thermal desalination process. Examples of this are the proposals where the waste heat recovered from the exhaust gas of a gas turbine is used to supply heat to the ORC [47,48]. A compressed air energy storage (CAES) system to consume electricity during the offpeak hours of electrical demand is proposed by [ 47 ]. An intercooled two-stage compression process is considered, and aftercooling prior to storage also takes place. The heat recovered in both processes is used in the production of domestic hot water. During peak demand hours, the stored compressed air is used to produce electricity by means of a gas turbine. Prior to its combustion chamber, the air is preheated in a solar dish system. The exhaust gases from the turbine are used to supply the heat demanded by the simple cycle ORC unit, with toluene as the working fluid. The condenser of this unit is cooled with a MED plant. A 10 m diameter solar dish and a 6-effect MED plant with a top brine temperature of 70 ◦ C and a feed of 40,000 ppm are analyzed. The results indicate an ORC output of 17.4 kW and 2.5 m 3 /day of freshwater production with an overall exergy efficiency of 41.7%. As a storage system, the calculated configuration has a round trip efficiency of 65.2%. Two multi-objective optimizations with evolutionary algorithms are also performed by [ 47 ], resulting in optimal exergy efficiencies of around 51%. A poly-generation system composed of a solar/fossil hybrid recuperative gas turbine cycle with an intercooled two-stage compression system is proposed in [ 48 ]. The air coming from the recuperator is solar-heated before entering the combustion chamber. The exhaust gases from the gas turbine are used to drive the ORC unit. Moreover, there is an absorption refrigeration system and a desalination unit, which consists of a single-stage flash process. The seawater feed is preheated in the ORC condenser and then within the condenser of the desalination unit. After being preheated, the seawater flow is separated into two streams. One of them is directly sent to the condenser of the refrigeration system. The second acts as the cooling flow of the two-stage compression before entering the refrigeration system, passing through the generator. The latter finally enters the desalination unit as feed water. At the outlet of the flash unit, the brine is air-cooled for low-grade heat production. For the thermodynamic analysis, concentrating solar collectors are considered, while R134a is the working fluid in the simple ORC. The proposed multi-generation configuration has an exergy efficiency of 27%. This configuration is compared with another in which the solar hybrid gas cycle configuration is maintained but the ORC, cooling, and desalination systems are replaced by a Kalina cycle. 2.3. Proposals with Integration of Hybrid RO/Thermal Desalination Recent hybrid desalination configurations with a solar-driven ORC unit can also be found in specific literature. A solar–wind hybrid system, in which a simple solar ORC operated with toluene is connected to a solar parabolic trough field without TES, is presented in [ 49 ]. In addition to the ORC, electricity production from a wind turbine is also considered. The total electricity output drives a seawater RO desalination plant and the surplus is sold to the grid. The rejected heat of the ORC is also used in a MED-TVC plant. Jaubert et al. [ 50 ] present the analysis of three RO-MED hybrid solar desalination systems powered by a PTC field without TES. Therminol VP-1 is selected as the heat transfer fluid. Part of the RO brine flow is used in the MED plant. As for the ORC, two possible working fluids are considered in this work. On one hand, there is isopentane, a fluid usually considered in ORC studies, and on the other hand, ethyl butyrate, which represents a case not studied so far and, according to the authors, shows promising results. In the three proposed configurations, the heat rejected by the ORC preheats the feed water of the reverse osmosis unit and the brine flow of the reverse osmosis unit before it enters the MED plant. As for the ORC architectures, the simple cycle, simple regenerative and simple regenerative, and solar reheating of the vapor are considered. Processes 2022,10, 153 16 of 23 where TSF is the average inlet/outlet heating temperature of the HTF, and a 1 and a 2 are the heat losses coefficients. Values of these coefficients are derived from the information available at [ 69 ]. Parameters selected for the evaluation of the solar field performance at design conditions are given in Table 3. Two extreme values are considered for the a 1 coefficient to quantify the influence of the ORC’s condensation temperature in scenarios of different solar-to-thermal energy conversion efficiency values. The two resulting curves are depicted in Figure 9. Table 3. Location and design conditions for the efficiency computation of the PTC solar field. Location Fuerteventura (Canary Islands, Spain) Latitude, longitude 28.317–14.071◦ Design conditions Ambient temperature, Tamb 23 ◦C Direct normal irradiance, Gb,N 850 W/m2 Peak optical performance, ηopt,0075 Cleanliness index, Fe0.97 Availability, avSF 0.95 Design point Solar noon, 21 June Orientation North-South (East-West tracking) Incidence angle, θ4.87◦ Optical efficiency, η0,SF 0.67 Heat losses coefficient, a1(low–high value) 0.02–0.4 W/m2×K Temperature dependence of the heat loss coefficient, a20.001 W/m2×K2 Processes 2022, 10, x FOR PEER REVIEW 16 of 23 For the comparison presented in this section, it is assumed that a solar field is composed of PTCs. In that case, the optical efficiency is computed by taking into account the shadow (ηshadow) and end losses (ηendloss), the cleanliness of the solar collectors (Fe), the availability of the solar field (avSF), and the influence of the incidence angle ( θ ) on its peak optical performance (ηopt,0) via the incidence angle modifier (K): 𝜂, =𝜂,∙𝐾∙𝐹 ∙𝜂 ∙𝜂∙𝑎𝑣 (3) On the other hand, thermal losses are modeled as: 𝑄󰇗, =󰇟𝑎∙󰇛𝑇  −𝑇)+𝑎∙󰇛𝑇  −𝑇)󰇠∙ 𝐴  (4) where 𝑇  is the average inlet/outlet heating temperature of the HTF, and a1 and a2 are the heat losses coefficients. Values of these coefficients are derived from the information available at [70]. Parameters selected for the evaluation of the solar field performance at design conditions are given in Table 3. Two extreme values are considered for the a1 coefficient to quantify the influence of the ORC’s condensation temperature in scenarios of different solar-to-thermal energy conversion efficiency values. The two resulting curves are depicted in Figure 9. Table 3. Location and design conditions for the efficiency computation of the PTC solar field. Location Fuerteventura (Canary Islands, Spain) Latitude, longitude 28.317°–14.071° Design conditions Ambient temperature, Tamb 23 °C Direct normal irradiance, Gb,N 850 W/m2 Peak optical performance, ηopt,0 075 Cleanliness index, Fe 0.97 Availability, avSF 0.95 Design point Solar noon, 21 June Orientation North-South (East-West tracking) Incidence angle, θ 4.87° Optical efficiency, η 0,SF 0.67 Heat losses coefficient, a1 (low–high value) 0.02–0.4 W/m2×K Temperature dependence of the heat loss coefficient, a2 0.001 W/m2×K2 Figure 9. Solar field’s efficiency according to the parameters fixed in Table 3. Figure 9. Solar field’s efficiency according to the parameters fixed in Table 3. Figure 10 shows the CSP − ORC + (MED + RO) configuration for which a condensation temperature of 70 ◦ C is studied. To assess only the desalination application, the remaining net power output ( . Wnet ) not consumed as auxiliary by the MED plant (Specific Electricity Consumption, ( SECMED ) is dedicated to an RO unit ( SECRO ). Under these assumptions, the volumetric flow rate of freshwater ( . Vp) is computed with the following expressions: . Vp,MED =. Qin·(1−ηORC) STECMED (5) . Vp,RO = . Wnet SECRO − . Vp,MED·SECMED SECRO (6) Processes 2022,10, 153 17 of 23 This configuration is compared with the one shown on the left side, for which a condensing temperature of 40 ◦ C is assumed and whose net power produced is fully consumed by an RO unit. In this case, . Vp,RO can also be computed with the expressions above, making . Vp,MED = 0. Table 4shows the numerical results of the comparison, assuming the same total desalination capacity in both cases. Processes 2022, 10, x FOR PEER REVIEW 17 of 23 Figure 10 shows the CSP − ORC + (MED + RO) configuration for which a condensation temperature of 70 °C is studied. To assess only the desalination application, the remaining net power output (𝑊󰇗) not consumed as auxiliary by the MED plant (Specific Electricity Consumption, 𝑆𝐸𝐶) is dedicated to an RO unit (𝑆𝐸𝐶). Under these assumptions, the volumetric flow rate of freshwater (𝑉󰇗) is computed with the following expressions: 𝑉󰇗, =𝑄 󰇗 ∙󰇛1−𝜂) 𝑆𝑇𝐸𝐶 (5) 𝑉󰇗, =𝑊󰇗 𝑆𝐸𝐶 − 𝑉󰇗, ∙𝑆𝐸𝐶 𝑆𝐸𝐶  (6) This configuration is compared with the one shown on the left side, for which a condensing temperature of 40 °C is assumed and whose net power produced is fully consumed by an RO unit. In this case, 𝑉󰇗, can also be computed with the expressions above, making 𝑉󰇗, =0. Table 4 shows the numerical results of the comparison, assuming the same total desalination capacity in both cases. Figure 10. Configurations considered to be compared: CSP − ORC + RO (left) and CSP − ORC + (RO + MED) (right). As can be seen, increasing the condensing temperature to 70 °C to include the MED unit requires an ORC unit with less net power due to the utilization of the heat rejected by the cycle for freshwater production output (118 kW instead of 125 kW per 1000 m3/day of capacity, 6% less). However, the negative effect of the above is clearly reflected in the solar field area and storage volume requirements. In the first case, 11% more area would be required for solar multiple (SM) = 1 due to the higher thermal energy input of the ORC unit, while in the second case, the increase would be 24%. The configurations to be compared when the heat rejected by the cycle is consumed in the generator of an absorption cooling system are shown in Figure 11. To make the comparison consistent, a mechanical vapor compression (VC) refrigeration system is analyzed when the condenser’s heat is not used for this purpose. For the CSP − ORC + (RO + ARS) configuration, the auxiliary electrical consumption of the absorption refrigeration unit is considered negligible, in which case, both the cooling capacity (𝑄󰇗,) and the desalinated water flow rate (𝑉󰇗,) are easily computed as: G M edium tem perature ORC Condenser Turbine Pump Recuperator Evaporator - superheater Preheater 1 2 3 y x 4 Brine Freshw ater Electricity Multi-Effect Distillation unit Feedw ater Brine Freshw ater RO unit G M edium te m p e ra tu re ORC Condenser Turbine Pump Recuperator Evaporator - superheater Preheater 1 2 3 4 y x Electricity Brine Freshw ater RO unit Feedw ater Figure 10. Configurations considered to be compared: CSP − ORC + RO ( left ) and CSP − ORC + (RO + MED) (right). Table 4. Comparison of the CSP − ORC configuration for desalination (same desalination capacity). SECRO = 3 kWh/m3;SECMED = 2 kWh/m3; PR = 10. Direct TES with thermal oil Therminol VP-1. CSP −ORC + RO CSP −ORC + (MED + RO) ORC’s condensation temperature, Tcond 40 ◦C 70 ◦C ORC’s evaporation temperature, Tevap 270 ◦C 270 ◦C ORC’s thermal efficiency, ηORC [%] 24.98 21.37 HTF’s outlet temperature, THEX,o 226.5 234.5 Average solar field temperature [◦C] 263 267 Solar field’s efficiency, ηSF (low a1/high a1) 0.608 0.501 0.606 0.500 Storage medium volume/stored energy [m3/MWh] 25.9 29.1 Storage medium volume/equivalent electric stored energy [m3/MWhe]103.7 136.1 ORC thermal input/total desalination capacity [kW/(1000 m3/day)] 500 554 Net ORC power output/total desalination capacity [kW/(1000 m3/day)] 125 118 Solar field’s area (SM = 1)/total desalination capacity [m 2 /(1000 m 3 /day)] (low a 1 /high a 1 ) 968 1175 1075 1303 Storage medium volume (1 h)/total desalination capacity [m3/(1000 m3/day)] 13.0 16.1 Processes 2022,10, 153 18 of 23 As can be seen, increasing the condensing temperature to 70 ◦ C to include the MED unit requires an ORC unit with less net power due to the utilization of the heat rejected by the cycle for freshwater production output (118 kW instead of 125 kW per 1000 m 3 /day of capacity, 6% less). However, the negative effect of the above is clearly reflected in the solar field area and storage volume requirements. In the first case, 11% more area would be required for solar multiple (SM) = 1 due to the higher thermal energy input of the ORC unit, while in the second case, the increase would be 24%. The configurations to be compared when the heat rejected by the cycle is consumed in the generator of an absorption cooling system are shown in Figure 11. To make the comparison consistent, a mechanical vapor compression (VC) refrigeration system is analyzed when the condenser’s heat is not used for this purpose. For the CSP − ORC + (RO + ARS) configuration, the auxiliary electrical consumption of the absorption refrigeration unit is considered negligible, in which case, both the cooling capacity ( . Qc,AR ) and the desalinated water flow rate ( . Vp,RO) are easily computed as: . Qc,AR =COPAR· . Qin·(1−ηORC)(7) . Vp,RO =. Qin·ηORC SECRO (8) Processes 2022, 10, x FOR PEER REVIEW 18 of 23 𝑄󰇗, =𝐶𝑂𝑃  ∙𝑄󰇗 ∙󰇛1−𝜂) (7) 𝑉󰇗, =𝑄 󰇗 ∙𝜂 𝑆𝐸𝐶 (8) For the CSP − ORC + (RO + VC) configuration, the 𝑉󰇗, can be obtained from: 𝑉󰇗, =  ∙󰇣𝜂 ∙𝑄󰇗 −󰇗, 󰇤 (9) where 𝑄󰇗, is the cooling capacity of the vapor compression refrigeration system. Figure 11. Configurations considered to be compared: CSP − ORC + (RO + VC) (left) and CSP − ORC + (RO + ARS) (right). Table 4. Comparison of the CSP − ORC configuration for desalination (same desalination capacity). SECRO = 3 kWh/m3 ; SECMED = 2 kWh/m3 ; PR = 10. Direct TES with thermal oil Therminol VP-1. CSP − ORC + RO CSP − ORC + (MED + RO) ORC’s condensation temperature, Tcond 40 °C 70 °C ORC’s evaporation temperature, Tevap 270 °C 270 °C ORC’s thermal efficiency, η ORC [%] 24.98 21.37 HTF’s outlet temperature, THEX,o 226.5 234.5 Average solar field temperature [°C] 263 267 Solar field’s efficiency, η SF (low a1/high a1) 0.608 0.501 0.606 0.500 Storage medium volume/stored energy [m3/MWh] 25.9 29.1 Storage medium volume/equivalent electric stored energy [m3/MWhe] 103.7 136.1 ORC thermal input/total desalination capacity [kW/(1000 m3/day)] 500 554 Net ORC power output/total desalination capacity [kW/(1000 m3/day)] 125 118 Solar field’s area (SM = 1)/total desalination capacity [m2/(1000 m3/day)] (low a1/high a1) 968 1175 1075 1303 G M edium te m p e ra tu re ORC Condenser Turbine Pump Recuperator Evaporator - superheater Preheater 1 2 3 y x 4 Electricity Brine Freshw ater C o o lin g Heating Feedw ater Absorption Refrigeration System RO unit G Medium tem perature ORC Condenser Turbine Pump Recuperator Evaporator - superheater Preheater 1 2 3 4 y x Electricity Brine Freshw ater RO unit Feedw ater Vapor Compression Refrigeration Cooling Heating Figure 11. Configurations considered to be compared: CSP − ORC + (RO + VC) ( left ) and CSP − ORC + (RO + ARS) (right). For the CSP −ORC + (RO + VC) configuration, the . Vp,RO can be obtained from: . Vp,RO =1 SECRO ·"ηORC· . Qin − . Qc,VC COPVC #(9) where . Qc,VC is the cooling capacity of the vapor compression refrigeration system. The numerical results of the comparison for the same cooling and desalination capacity are shown in Table 5, where a condensing temperature of 90 ◦ C is assumed for the operation of the absorption refrigeration unit. Processes 2022,10, 153 19 of 23 Table 5. Comparison of the CSP − ORC configuration for desalination and refrigeration (same desalination and cooling capacity). SEC RO = 3 kWh/m 3 ; direct TES with thermal oil Therminol VP-1. CSP −ORC + (RO + VC) CSP −ORC + (RO + AR) ORC’s condensation temperature, Tcond 40 ◦C 90 ◦C ORC’s evaporation temperature, Tevap 270 ◦C 270 ◦C ORC’s thermal efficiency, ηORC 24.98% 19.12% HTF’s outlet temperature, THEX,o 226.5 ◦C 239.6 ◦C COP 4 0.7 Average solar field temperature [◦C] 263 270 Solar field’s efficiency, ηSF 0.608 0.501 0.604 0.493 Solar field’s area (SM = 1), ASF [m2]4553 5526 3440 4215 Storage medium volume/stored energy [m3/MWh] 25.9 31.5 Storage medium volume/equivalent electric stored energy [m3/MWhe]103.7 164.9 ORC thermal input/total desalination capacity [kW/(1000 m3/day)] 871 654 Net ORC power output/total desalination capacity [kW/(1000 m3/day)] 218 125 Solar field’s area (SM = 1)/total desalination capacity [m2/(1000 m3/day)] 1685 2045 1273 1560 Storage medium volume (1 h)/total desalination capacity [m3/(1000 m3/day)] 22.6 20.6 Unlike the use for MED desalination, the use of the condenser’s heat for cooling combined with RO desalination has advantages over a configuration in which the cooling production is performed by a compression system to avoid increasing the condensation temperature of the cycle. Moreover, this situation is observed for a relatively high COP value of the compression chiller. Even in that case, the better performance of the ORC does not lead to a smaller size of the ORC unit, which finally implies a larger storage volume and a larger required solar field area. 5. Conclusions and Recommendations The following trends have been detected after the update of the design proposals recently found in the specific literature on solar desalination systems based on ORCs. In the medium-temperature range, it is common to find proposals in which the heat rejected by the cycle is used, in general, to drive thermal desalination processes (mainly MED) or absorption cooling processes. In the low-temperature range, there are numerous proposals for the use of solar ponds as solar collection and energy storage systems. Regarding the coupling of desalination systems with solar supercritical CO 2 power cycles, the most common option is the use of the heat rejected by the cycle by means of MED units. In addition, the critical analysis of the reviewed configurations leads to the following conclusions and design recommendations: • In remote locations, solar desalination systems based on solar-pond-driven ORC/RO should be studied in comparison to solar PV/RO plants to supply only the freshwater demand. • Concerning the selection of thermal desalination versus reverse osmosis, design proposals should consider the extra costs attributable to cooling flow, namely, capital expenses of seawater intake and pumping systems, along with pumping electricity consumption (see Figure 5). Processes 2022,10, 153 20 of 23 • A comparative analysis of solar ORC/RO combined with cooling technologies has been performed. The use of the heat rejected by the cycle to drive an absorption refrigeration system is superior since results show that a bigger solar field and storage system would be needed to supply the same cooling capacity with a mechanical vapor compression refrigeration system (see Table 5). • A comparative analysis has also been carried out to compare solar ORC/MED and solar ORC/RO desalination. Auxiliary electricity consumption of 0.75 kWh/m 3 is assumed in seawater RO desalination, attributable to the control system and feed pumping, whereas 2 kWh/m 3 has been considered in MED plants due to the additional requirements of a vacuum system and cooling flow pumping (see Figure 5). Results (see Table 4) show that thermal desalination is not recommended to exploit the heat rejection of the power cycle since the condensing temperature must be increased to couple the MED unit. • On the contrary, heat rejection of sCO 2 cycles in CSP plants may be economically exploited by innovative MED desalination plants with the so-called cascade design depending on both local regulations of discharge temperature and the corresponding auxiliary pumping of cooling flow. Water production is limited to the waste heat available, so in the case of higher water demands, RO desalination is the only option recommended. Either seawater feed or concentrate flows of the RO plant can be used as the cooling flow of the power cycle, up to the maximum limit of temperature discharge. Based on the results of the present literature update and assessment, research on the improvement of solar ORC configurations for poly-generation, with freshwater production through desalination, is considered a future work of interest, in addition to solar sCO 2 coupled to novel MED plants with enhanced energy efficiency. Author Contributions: Conceptualization, A.M.D.-T. and L.G.-R.; writing—original draft preparation, A.M.D.-T. and L.G.-R.; writing—review and editing, A.M.D.-T. and L.G.-R.; funding acquisition, L.G.-R. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the European Regional Development Fund, Interreg Atlantic Area, and the EERES4WATER Project (Second Call, Priority 2, EAPA_1058/2018). The University of Seville is also gratefully acknowledged for supporting this research through its Internal Research Programme (Plan Propio de Investigación) under contract no. 2019/00000359. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Acknowledgments: L. García-Rodríguez wishes to thank the European Regional Development Fund, Interreg Atlantic Area, for its financial assistance within the framework of the EERES4WATER Project (Second Call, Priority 2, EAPA_1058/2018). Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. References 1. International Energy Agency, IEA. Data and Statistics Page. Available online: https://www.iea.org/data-and-statistics/databrowser?country=WORLD&fuel=Energysupply&indicator=TESbySource (accessed on 31 October 2021). 2. The World Bank. DataBank. World Development Indicators. Available online: https://databank.worldbank.org/source/worlddevelopment-indicators (accessed on 31 October 2021). 3. 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