A Hybrid MCDM Approach to Optimize Molten Salt Selection for Off-Grid CSP Systems

Abstract

Transitioning to sustainable energy systems demands the creation of innovative methods that deliver dependable and effective renewable energy technologies. CSP systems that integrate parabolic trough designs with thermal energy storage (TES) systems provide essential solutions to overcome energy intermittency challenges. Molten salts serve dual functions as heat transfer fluids (HTFs) and thermal energy storage (TES) media, making them critical to CSP system performance improvements. The study introduces a hybrid MCDM framework that combines the CRITIC method for objective weighting with the SWARA approach for expert-adjusted weighting and utilizes an enhanced Lexicographic Goal Programming to evaluate molten salt options for off-grid parabolic trough systems. The evaluation process considered melting point alongside thermal stability while also assessing cost-effectiveness, recyclability, and safety requirements. The use of Pareto front analysis helped identify non-dominated salts, which then underwent a tiered optimization process emphasizing safety, performance, and sustainability features. Results confirm that the ternary nitrate composition Ca(NO₃)₂:NaNO₃:KNO₃ offers the best overall performance across all tested policy scenarios, driven by its superior thermophysical properties. Solar Salt (NaNO₃-KNO₃) consistently ranks as a robust second choice, excelling in economic and sustainability metrics. The proposed approach provides a flexible, policy-sensitive framework for material selection tailored to enhance the efficiency and sustainability of off-grid CSP systems and support the renewable energy objectives.

1. Introduction

As the global world increases its interest in transitioning to sustainable energy systems, renewable sources have become the basis for the strategies aimed at mitigating gas emissions and providing stable and reliable electricity [1]. Among these sources, concentrated solar power (CSP) plays a pivotal role in this transformation by utilizing solar radiation to generate high-temperature thermal energy [2,3]. Unlike photovoltaic systems, CSP systems offer the advantage of continuous generation of power by integrating thermal energy storage (TES) solutions. TES can store excess heat generated during the day to be used later, thus ensuring a consistent energy supply even when sun radiation is not available [4,5], especially in regions blessed with abundant direct normal irradiance (DNI), such as the United Arab Emirates (UAE).

Another crucial factor for the efficient functionality of CSP systems is the heat transfer fluids (HTFs). These fluids play a crucial role in transferring and storing thermal energy, which is essential for maintaining the operational efficiency of solar thermal electricity (STE) installations [6]. To select the best material for the heat transfer and storage media, a wide range of materials has been studied, but molten salts have emerged as the most effective candidate, particularly in comparison to solid salts and other fluid alternatives [7,8]. Its excellent thermal stability and other thermophysical characteristics give the molten salt the ability to operate concurrently as both heat transfer fluids and a highly effective thermal energy storage medium [9]. This dual functionality helps in simplifying system design and enhances overall plant efficiency, making molten salt-based CSP a particularly attractive option for sustainable energy deployment [10].

The United Arab Emirates has recognized the critical role of solar energy, particularly CSP, in its sustainability initiatives, leveraging its abundant solar resources and ambitious renewable energy goals [11,12]. Optimizing the selection of molten salts is crucial for CSP technologies, especially in off-grid regions, to ensure thermal performance, environmental sustainability, and economic feasibility [13].

Several studies have been conducted on the application of molten salts in CSP systems, but some limitations exist when searching for studies that integrate sustainability factors and thermal characteristics when analyzing the performance of the parabolic trough configurations in off-grid settings. Also, the existing literature primarily focuses on the use of molten salts in central receiver systems, where traditional solar salts are commonly used as TES media in conjunction with distinct thermal transfer mediums, including synthetic oils. In contrast, the application of molten salts as a dual-function medium, that is, to serve as heat transfer fluids and thermal energy storage media simultaneously, has not been extensively investigated for trough-based CSP systems, especially for the systems that are designed for small-scale or off-grid applications. This difference is vital since parabolic trough systems generally operate at lower temperature levels relative to tower configurations, thereby imposing restrictions on melting point, heat conductivity, viscosity, and other thermal factors of molten salts [14] Due to off-grid plants distinct operational needs and financial constraints compared to large-scale or grid connected plants, off-grid CSP plants must prioritize simplicity of design, efficacy, and reliability. This makes the dual use of a single type of molten salt as the heat transfer fluid and thermal storage medium more appealing, but it requires the selection of molten salt compositions optimized for these particular conditions. The dual application of molten salts in parabolic trough systems remains an emerging field and has not yet attained the same level of maturity as in other concentrated solar power technologies.

In countries like the UAE, where the government is dedicated to integrating renewable energy and maintaining environmental responsibility, additional factors should be considered, such as the environmental impact, availability, cost, and recyclability of molten salts [15]. Also, these factors should hold the same importance as their thermal performance. Through the literature, limited studies have considered the life cycle analysis or investigated the potential of using recycled molten salts, which could substantially improve the economic feasibility and environmental aspects of CSP projects in remote or off-grid areas.

Although several multi-criteria decision-making techniques (MCDM) have been used in prior material selection studies, they may be limited in their analytical sophistication, as they tend to rely on subjective weighting systems or oversimplified ranking methods. A more reliable and transparent approach is offered in this research by using a hybrid MCDM framework that integrates objective weighting techniques (CRITIC—criteria importance through intercriteria correlation) with expert-based adjustments (SWARA—step-wise weight assessment ratio analysis) and then employs Lexicographic goal programming for the final selection, enabling a balanced consideration of performance and sustainability. Such comprehensive frameworks are seldom applied to the selection of molten salts, particularly for trough-based off-grid CSP systems.

This study aims to address this gap by identifying and evaluating molten salts—both commercial and recycled—that are most suitable for use as a dual medium for heat transferring and storing systems in off-grid parabolic trough CSP systems in the UAE. A hybrid MCDM approach will be used to balance technical performance with sustainability, using Pareto front analysis, CRITIC, SWARA-based expert judgment, and Lexicographic Goal Programming for final decision-making. The outcome will provide practical guidance for selecting the optimal molten salt that supports both energy resilience and the UAE’s long-term sustainability vision.

2. Literature Review

This section reviews existing literature on CSP systems, with particular emphasis on thermal energy storage and heat transfer media, molten salt technologies, sustainability aspects, and decision-making frameworks relevant to material selection.

2.1. Global Overview of CSP Technologies

CSP technologies have been considered as a core basis in the global transition towards renewable energy sources because of their reliable and effective approach in capturing solar radiation and converting it into electrical power [16]. These technologies are classified into four primary categories: parabolic trough collectors (PTCs), solar power towers (SPTs), parabolic dish systems (PDSs), and linear fresnel reflectors (LFRs). Each type has its own unique design principles, performance characteristics, and applications, as presented in

Table 1. CSP technologies summary.

CSP Technology	Description	Key Characteristics
Parabolic Trough	Use parabolic mirrors to focus solar radiations on a central receiver tube.	Most mature technology operates at temperatures below 500 °C, widely deployed globally [3,17]
Linear Fresnel	Employ flat mirrors to focus sunlight onto a fixed receiver.	Lower costs, simpler construction, suitable for centralized power generation [18]
Solar Power Tower	Utilizes heliostats to concentrate sunlight onto a central receiver.	High operating temperatures, flexible HTF options, integrates TES systems effectively [10,19]
Parabolic Dish	Dish-shaped mirrors concentrate sunlight onto a receiver for direct power generation.	Modular design, suitable for distributed generation, high energy transfer efficiency [18,20]

.

Among these configurations, PTCs are the most widely deployed technology, followed by SPTs [3,19]. Beyond their technical variety, several studies highlight the CSP’s contribution to climate change mitigation, especially its key role in reducing CO₂ emissions [21]. This environmental advantage provides substantial economic opportunities for technology providers in countries like Germany, which is considered the leading supplier in CSP technology [22]. Despite these advantages, CSP faces several challenges and barriers, including high capital and operational costs and growing competition from other cheaper systems such as the photovoltaic energy system [23,24]. To overcome these challenges, many advancements have been studied and implemented to enhance efficiency, reduce cost, and optimise the CSP systems. This guarantee will reduce the levelized cost of electricity (LCOE) for CSP systems, making them more competitive with other forms of power generation [19,25]. These advancements include utilizing new collector and receiver concepts that will increase thermal and optical performance [3,26]. Improvements on CPS components are not only conducted in design and concept but also in developing innovative materials such as nickel-based alloys [27], high-performance materials [18,28], phase change materials (PCMs), and molten chloride salts [3]. These advancements in CSP technologies have assured the competitive advantages of all CSP configurations over other solar systems in meeting the demand of grid-connected and off-grid areas, which led to energy security and sustainable developments [23].

Besides these improvements, integrating the TES with the CSP technologies, especially with the PTC, is considered the vital element that enhances the viability of these technologies, since it overcomes the significant challenge of the solar energy system, which is the inherent intermittency [29].

The subsequent section explores the role of TES in CSP systems, discussing the primary storage mechanisms, materials involved, and their significance—particularly in supporting off-grid and long-duration energy solutions.

2.2. Thermal Energy Storage (TES) in CSP

The use of thermal energy storage in CSPs is considered the best improvement in the capacity of CSP plants by increasing efficiency and generating electricity for longer operation hours. The process of storing thermal energy in the TES is carried out in three primary steps: charging, storage, and discharge [30]. These steps are common in all TES methods which are classified into three main categories: sensible heat storage (SHS), latent heat storage (LHS), and thermochemical storage (TCES). Each type offers unique advantages and challenges, and the selection of methodology is subjected to the particular application and its associated requirements.

Table 2. Comparison of TES methods in CSP systems.

TES Method	Storage Type	Key Materials	Temperature Range	Maturity	Advantages	Limitations
Sensible Heat Storage	Solid or liquid media	Molten salts, concrete [31], refractory bricks [32], liquid metals [33]	100–800 °C [31,32,34]	Commercially mature [31,34]	High reliability, low cost, easy implementation [35,36]	Lower energy density [31,34]
Latent Heat Storage	Phase change materials	Paraffin wax, fatty acids, molten salts, magnesium chloride, lithium fluoride [36,37]	100–420 °C [38,39]	Emerging [36,37]	High energy density, compact system [38,40]	Low thermal conductivity, higher cost [36,37]
Thermochemical Storage	Chemical reactions	Metal hydrides [41], metal oxides [41,42], carbonates, hydroxides [41]	200–1000 °C [42]	Early development [41]	Highest energy density, high-temperature [43,44]	Complex systems, material degradation. [45,46]

expresses these types along with their specifications, advantages, and limitations.

These methods are adopted and employed in several CSP projects around the world. The UAE is considered one of the best prime examples of the real-world application that has made significant investments in this technology. One of its major projects is the CSP on Demand (CSPonD) system, that created as a collaboration between Masdar and MIT. This project utilized a TES tank to save 400 kWh for duration of 16 h after the peak hours [47,48]. Another major project, which is Shams1, has a capacity of 100 MW; this project employs the PTC technology to handle the challenges in the desert and off-grid areas. Several research projects are being carried out to select the best parameters of the suggested TES [49]. Besides these projects, several universal projects are constructed.

Table 3. Several existing and planned CSP-TES projects.

Project/Initiative	Location	Capacity (MW)	TES Capacity/Duration	Technology	Status	Key Features
CSPonD [47,48]	Abu Dhabi, UAE	0.1	400 kWh/16 h	Volumetric receiver, integral TES	Prototype	Molten salt (Solar Salt), no pumping to tower top.
Shams 1 [49]	UAE	100	N/A	Parabolic trough	Operational	High live steam temperature (540 °C), designed for harsh conditions.
Proposed Plant in Abu Dhabi [50]	Abu Dhabi, UAE	100	N/A	Parabolic trough	Proposed	Expected to generate 333.15 GWh annually, 14.35% mean efficiency.
Noor Ouarzazate Solar Complex [51]	Morocco	580 (total)	Molten salt TES	Parabolic trough and solar tower	Operational	Largest CSP plant globally.
Solana Generating Station. [52]	Arizona, USA	280	6 h	Parabolic trough	Operational	Molten salt TES.
Gemasolar Thermosolar Plant [53]	Spain	19.9	Molten salt TES	Solar tower	Operational	Operated continuously for 24 h.
Cerro Dominador Solar Thermal Plant [54]	Chile	Not specified	17.5 h	Solar tower	Operational	Long heat storage duration.
Uzbekistan CSP Plant [55]	Uzbekistan	100	5 h	CSP	Planned	Masdar investment.
US DOE Funding Initiatives [56]	USA	N/A	N/A	CSP and TES technologies	Ongoing	Supports research and pilot projects.
NREL Sand-based TES Demonstration [57]	USA	N/A	Aims for 100 h	Sand-based TES	Demonstration	Long duration storage.
Westinghouse TES in Alaska [58]	Alaska, USA	N/A	Long duration	Thermal energy storage	Developing	Supports wind power integration.

summarises some of the best solar projects that uses the CSP technologies integrated with TES.

As the integration of CSP technologies with TES systems advances, as demonstrated in the previous projects, a sustainable, efficient, and economically competitive solution for energy storage for small-scale, off-grid operations is presented [59]. This solution can also be accomplished by integrating the TES systems with other renewable energy sources, such as solar power [60,61], or with photovoltaic systems, or with heat pumps [62]. Such integrations will help in storing the excess solar energy for use during peak demand periods, which means that it increases the capacity factor of the CSP and reduces reliance on fossil fuels; this will surely support off-grid operations.

Despite these advantages in using the TES in off-grid areas, challenges such as storage capacity limitations [63], complex infrastructure [3,23], and selecting the best material to be used in the TES systems still need more research and investigations [60]. Most of the ongoing research is centred on creating innovative high-temperature heat transfer fluids and storage materials [64], designing novel thermal energy storage systems [65], advancing thermochemical storage technologies [66], and applying artificial intelligence for optimization [67].

This research will focus on the progress of selecting the best molten salt to overcome the aforementioned challenges. Therefore, the next section will provide a comprehensive overview of the molten salts used in the CSP technologies, examining their properties, roles in energy storage and heat transfer, applications, and limitations. This will set the stage for their evaluation as dual-purpose materials in trough systems.

2.3. Molten Salts

2.3.1. Molten Salts Overview

Molten salts are a class of ionic compounds that are liquid at high temperatures, characterized by their efficient electricity and heat conduction [68,69]. Molten salts comprise four categories: nitrates, chlorides, fluorides, and carbonates. Several studies have been conducted on these categories and have examined their suitability in different applications. For example, nitrate salts showed their excellent efficiency in storing energy at high temperature, which makes it a very suitable option for being a stable energy supply source [69]. Chloride salts, on the other hand, are favourable in the CSP plants due to their high thermal stability and relatively low cost, but with a probability of corrosion of the system’s material [70]. The third category, the fluoride salts, has a high thermal stability characteristic that makes it the best option used in nuclear applications, besides its ability to dissolve a wide range of inorganic compounds [68]. Carbonate salts are considered the best energy conversion technologies, but with a corrosion drawback [71].

Table 4. Applications of molten salts across industries.

Industry/Application	Key Uses	Challenges and Innovations	References
Energy Storage (CSP, Wind, Nuclear)	Thermal energy storage in CSP plants, wind power integration, and nuclear energy storage. High thermal stability (up to 550 °C) enables efficient energy dispatch.	Challenges: corrosion at high temperatures, limited thermal stability. Innovations: nano fluids (nanoparticle-doped salts) improve heat capacity and reduce corrosion.	[72,73,74,75,76,77]
Industrial Process Heat	High-temperature heat supply (400 °C+) for food processing, chemical synthesis, and metal production. Lowers LCOH (5–13 cEURO/kWhth).	Challenges: high initial costs, material degradation. Innovations: advanced parabolic trough designs with binary molten salts.	[78,79,80]
Metallurgy and Metal Extraction	Electrolysis for Al, Ti, and rare earth metals; recovery from waste (e.g., scrap, nuclear waste). Reduces reliance on primary sources.	Challenges: electrolyte/electrode degradation. Innovations: novel electrolytic cells for complex waste processing.	[81,82,83]
Nuclear Applications (MSRs, Waste Mgmt.)	Coolant/fuel solvent in MSRs, actinide burning for waste reduction, high-temp operation (>700 °C). Passive safety features.	Challenges: corrosion under radiation, material compatibility. Innovations: LiF-based salts for tritium breeding; SMR integration.	[84,85,86,87]
Environmental Protection	CO2 capture (conversion to carbon/CO), catalytic SO2 oxidation, hazardous waste treatment.	Challenges: scalability, reaction efficiency. Innovations: molten carbonate-based carbon capture.	[88,89,90]
Electrochemistry and Advanced Materials	High-temperature batteries (3.1 V+), fuel cells, nanotubes/nanoparticle production.	Challenges: electrode–electrolyte stability. Innovations: ceramic separators for molten salt batteries.	[91,92,93]

summarizes the applications of the molten salts and highlights their challenges and suggested innovations.

2.3.2. Thermophysical Properties of Molten Salts

In this research, we are interested in CSP technologies, especially the parabolic trough system. Several studies have been conducted to examine their thermophysical properties that influence their functionality in solar power systems as an HTF or a TES medium. The first critical property is the melting point. Molten salts with lower melting points are preferable as the risk of freezing is reduced, which is considered one of the significant concerns in CSP operations. Researchers have developed ternary and quaternary salt mixtures to achieve melting temperatures below 100 °C to be a suitable choice to be used in parabolic trough systems [94]. Thermal stability is another vital property, as it sets the maximum operating temperature and determines the durability of the salt under elevated temperatures. The best type of molten salts in thermal stability is the nitrate salts, which can still be stable up to 500 °C [95,96]. Specific heat capacity and thermal conductivity are other crucial properties that should be taken into consideration to ensure efficient heat transfer and storage. Salts with higher values indicate better efficiency in storing and transferring thermal energy [97,98]. Density and viscosity are the last important properties, especially when using the molten salt as HTF, as they affect its flow characteristics; also, lower viscosity reduces pumping energy requirements [99]. Overall, the selection of molten salts for CSP systems is a balance of these properties to optimize performance and cost-effectiveness [8,100].

Besides the molten salts, researchers investigate the possibility of using different materials for enhancing the efficiency of the thermal energy systems, such as synthetic oils, liquid metal, and phase change materials (SCMs), and different solid salts. The following section provides an evaluation of the molten salts versus these materials based on several factors. This comparative evaluation aims to interpret the status and the role of molten salts relative to other media, especially when considering their utilisation in the parabolic CSP systems in off-grid areas.

2.3.3. Evaluation of Molten Salts Against Other Alternatives of Thermal Media

As mentioned earlier, the properties of molten salts make them a suitable alternative to serve as heat transfer and energy storage media. But in this section, we will evaluate some of the other alternatives that have their unique specifications, advantages, and limitations that may make them preferable for specific applications under specified operating conditions. Table 5 will summarise this comparison. This evaluation focuses on system efficiency, sustainability and environmental impact, cost, and innovation in technology.

Regarding the efficiency of molten salts, it is found that molten salts deliver better performance than thermal oils in parabolic trough power plants by achieving higher operational efficiency while reducing the levelized cost of energy (LCOE) for extensive storage systems. The ternary salt mixture functions effectively between 170 °C and 500 °C and remains cost-effective, while thermal oils require supplementary heat exchangers, leading to higher operational costs and diminished system efficiency [101]. Another research demonstrates that molten salts offer better thermal performance when applied in concentrated solar power (CSP) systems, they demonstrated thermal efficiencies reaching 63.61% and exergy efficiencies between 63.55% and 73.36% [102], which significantly surpass the typical performance of liquid air/nitrogen storage systems that operate below 50% efficiency according to Yang [103]. Molten salts maintain long-term stability while phase change materials (PCMs) suffer from degradation through repeated cycles according to Ong et al. [104] and solid fillers need bigger storage spaces because of their lower energy density, as shown by Delise et al. [105]

In the context of sustainability and environmental benefits evaluation, molten salts are much better than thermal oils in avoiding leakage problems. Also, they are better than LAES/LNES systems, which require significant energy for liquefaction [84,103]. Proper management results in nitrate and nitrite salts from mineral sources having low environmental impact [105]. Material compatibility problems with particular rock types in storage systems remain unsolved according to Forsberg [84], even though new salt formulations promise to resolve these problems [104].

To evaluate the economic benefits of the molten salts, a study conducted by Xue et al. [106] demonstrates that using molten salts as TES in waste heat recovery industrial applications produces financial returns in five years, which means that the molten salts have a long-term economic benefit even though they have high initial costs. According to Aljudaya et al. [107] concentrated solar power systems with substantial storage facilities achieve lower levelized cost of electricity values. Dersch et al. found that when using the thermal oils as HTF, the system will require higher operational costs because of maintenance and heat exchange demands [101], but both LAES and LNES systems demand substantial capital investment because of their intricate liquefaction processes [103].

Molten salts provide substantial benefits compared to alternative thermal solutions with respect to functional effectiveness and environmental sustainability while maintaining cost-effectiveness in the long run. Molten salts face initial cost and material compatibility challenges, but technological progress and research continue to resolve these problems, thereby establishing molten salts as a viable and credible alternative for thermal energy systems in CSP and other applications.

Table 5. Comparison of molten salts with other storage media.

Storage Media	Advantages	Disadvantages	Citation
Molten Salts	High energy density, long thermal stability, low environmental impact, and scalable	High initial cost, material compatibility challenges, complex system design	[101,104,105]
Thermal Oils	Lower cost, established technology, compatible with existing infrastructure	Lower thermal stability, higher operational costs, environmental risks	[84,101]
Liquid Air/Nitrogen	High energy density, low environmental impact, suitable for urban applications	Low efficiency, high energy required for liquefaction, limited scalability	[103]
Solid Fillers	Low cost, simplicity of design, compatibility with various heat transfer fluids	Lower energy density, larger storage volume required, limited thermal conductivity	[84,105]
Phase Change Materials	High energy density, compact storage, suitable for low-temperature applications	Degradation over cycles, lower thermal conductivity, higher cost	[104,105]

Table 5. Comparison of molten salts with other storage media.

Storage Media	Advantages	Disadvantages	Citation
Molten Salts	High energy density, long thermal stability, low environmental impact, and scalable	High initial cost, material compatibility challenges, complex system design	[101,104,105]
Thermal Oils	Lower cost, established technology, compatible with existing infrastructure	Lower thermal stability, higher operational costs, environmental risks	[84,101]
Liquid Air/Nitrogen	High energy density, low environmental impact, suitable for urban applications	Low efficiency, high energy required for liquefaction, limited scalability	[103]
Solid Fillers	Low cost, simplicity of design, compatibility with various heat transfer fluids	Lower energy density, larger storage volume required, limited thermal conductivity	[84,105]
Phase Change Materials	High energy density, compact storage, suitable for low-temperature applications	Degradation over cycles, lower thermal conductivity, higher cost	[104,105]

2.3.4. Molten Salt Role in CSP Technologies

As stated earlier, the dual functionality of molten salts as HTFs and TES media in CSP technologies, combined with their ability to operate at different temperature ranges and store thermal energy, makes them suitable for high efficiency and off-grid solar power applications [108,109]. Building on this fundamental rule, researchers investigate several molten salts as an alternative to HTF media, or as TES, or a combination of both [21]. The most investigated salt is Solar Salt, as it is the most common commercial salt used in CSP technologies. Researchers continue investigating ternary mixtures, chloride-based salts, carbonates, sulphates, and hybrid combinations to enhance thermal stability, viscosity, corrosion resistance, and environmental impact to support high-efficiency and cost-sensitive CSP deployment needs as documented by several studies [70,96,108,109,110,111,112,113].

Table 6. Summary of molten salt types and compositions for CSP applications.

Salt Type	Composition/Example	Melting Point (°C)	Max Temp (°C)	Application	Advantages	Challenges	Citations
Nitrate-based	NaNO3-KNO3 (Solar Salt)	240	600	HTF/TES	Commercial, stable, good heat capacity	Decomposes >600 °C	[108,109,114]
Ternary Nitrate	NaNO3-KNO3-Ca(NO3)2	<150	550	HTF/TES	Lower melting point, improved stability	Nitrate instability, Ca compatibility	[96]
HITEC	NaNO3-KNO3-NaNO2	142	450	HTF	Low melting, pumpable	Nitrite toxicity, lower thermal stability	[110]
Chloride-based	NaCl-KCl-MgCl2	~385	800+	HTF/TES	High temp stability, low cost	Severe corrosion	[70,109,115]
Chloride-based	NaCl-KCl-ZnCl2	~250	700+	HTF	Thermal stability, good properties	Corrosion, volatility, toxicity	[100,116]
Carbonate-based	Li2CO3-Na2CO3-K2CO3	400–450	800	TES	High heat capacity, good thermal stability	CO2 reactivity, cost	[108,109]
Sulfate-based	Na2SO4-K2SO4	500–600	800	TES	Stable, non-toxic	High melting point	[96,108,117]
Mixed Anion	NaNO3-KNO3-Ca(NO3)2	<150	500+	TES	Eutectic behavior, improved performance	Complex synthesis, nitrate limits
Hybrid Chloride/Carbonate	NaCl-KCl-Na2CO3	~300	800	TES	Wide temperature range, better stability	Compatibility, complex design	[113]
Alkali Chlorides	NaCl, KCl	~770	>800	HTF	Good for extreme temps	Very high melting point, corrosion	[70]
Ternary Chlorides	MgCl2/KCl/NaCl	<400	~800	HTF	Corrosion can be controlled, scalable	Hygroscopic, corrosive	[70,111]

summarizes the most relevant molten salt types and compositions, including their melting and maximum operating temperatures, roles in CSP systems (As HTF or TES), advantages, limitations, and corresponding literature references.

The comparative evaluation shows that no single molten salt composition proves best for all scenarios. The appropriate selection of a salt for CSP applications requires a balance between thermal properties and corrosion behaviour alongside environmental impact and system cost. Nitrate-based salts remain the top choice for commercial systems, especially parabolic trough systems, but chloride-based and hybrid alternatives now show increasing adoption for high-temperature uses and standalone systems. Further research in corrosion-resistant materials and salt recyclability, alongside performance optimization research, will enable maximum utilization of molten salts, particularly for off-grid thermal systems in the UAE. The next section explores a comprehensive assessment of the molten salt sustainability, including the previously mentioned challenges.

2.3.5. Sustainability of Molten Salts in CSP Applications

Considering the sustainability of the molten salts in CSP systems is very critical, especially when deploying them in off-grid parabolic trough applications, particularly in the UAE, where environmental and economic priorities drive renewable energy development. Following the previous section’s discussion, this section evaluates the sustainability of molten salts across its four major dimensions as summarised in Table 7: environmental, economic, social, and technical dimensions, with a special focus on recyclability, life cycle assessment (LCA), durability of molten salt materials, and functional productivity. Beginning with the recyclability of molten salts that is the basis of sustainability, studies show that nitrate salts are chemically stable, allowing the reuse of these salts for long periods with the same thermophysical properties [118,119]. Also, several studies focused on the techniques of restoring the thermophysical properties of molten salts to be reused. For example, electrochemical purification that is performed at Idaho National Laboratory removes impurities like iron from FLiBe using inert electrodes, while chemical precipitation with K₂S in LiCl-KCl achieves 90% recovery of the virgin properties [89,120]. Moreover, another recycling method results in high efficiency when recycling tin-doped indium oxide into alloys using NaCl-KCl eutectic [77]. In CSP, recycled molten salts maintain 95% discharge efficiency, reducing raw material demand for virgin salt production and waste [118]. Thermal processing also facilitates contaminant separation during biomass upcycling. However, high recycling costs (USD 60/kWh for chloride TES) and corrosion risk affect the economic feasibility, especially for the UAE’s off-grid systems, where cost-effectiveness is critical [70,118]. So, selecting recycling methods that are scalable and have low-cost recycling is critical to aligning with the UAE’s Net Zero by 2050 goals [121].

To study the environmental impacts of using molten salts, LCA studies show that using molten salts in CSP emits greenhouse gases that are significantly lower than traditional fossil fuel plants. Also, the use of dry cooling reduces the water use in dry regions like the UAE [36]. However, the process of virgin salt production involves energy-intensive mining, which emits CO₂ and causes ecosystem disruption [122,123].

To mitigate these environmental impacts, recycling and local sourcing can be adopted, and this will align with the UAE’s Net Zero by 2050 initiative [121].

Moving to the material durability and corrosion aspect, studies show that some salts like chloride-based salts are highly corrosive at elevated temperatures, which impose durable alloys like Hastelloy-N, which will result in increasing costs and environmental impacts [70]. Also, studies show that using advanced coating techniques and proton irradiation may increase material durability, thus enhancing sustainability by reducing changeovers [124].

To evaluate the economic and social aspects, studies indicate that using molten salts in CSP systems will extend the operational hours, which will result in reducing the levelized cost of electricity (LCOE) in high-irradiance regions like the UAE [125]. Another study explores a CSP plant with 100 MW capacity and indicates that using the molten salts in the TES systems will stimulate the local economies by employing over 1 million person-days [126]. Also, some studies explore the ethical sourcing of Li and K salts and show that this is critical to avoid social impacts from mining in dry regions [122].

Regarding the last critical aspect, which is operational efficiency, previous studies indicate that the molten salts achieve thermal efficiencies up to 63.6%, which is higher than efficiencies of other alternatives like liquid air storage [102]. Also, using advanced nanofluid salts will improve the heat transfer, thus reducing storage volume, especially for off-grid applications [124].

As a result, molten salts offer sustainability advantages, but risks like corrosion and recyclability still exist. These factors will inform the hybrid MCDM framework proposed in Section 2.4, optimizing salt selection for off-grid UAE applications.

Table 7. Sustainability criteria for molten Salts in CSP.

Criterion	Strengths	Challenges	Relevance to UAE Off-Grid
Recyclability	95% discharge efficiency post-recycling; reduces raw material use [118]	High purification costs; corrosion risks [34]	Cost-effective recycling critical for remote systems
Environmental Impact	Low emissions; dry cooling saves water [36]	Mining-related CO2 emissions [122]	Aligns with Net Zero by 2050 goals
Material Durability	Advanced coatings extend lifespan [124]	Costly alloys increase environmental footprint [70]	Low-maintenance materials needed for off-grid
Economic/Social Impact	Low LCOE; 1 M+ person-days employment [126]	Ethical sourcing challenges [122]	Supports remote community development
Operational Efficiency	Up to 63.6% thermal efficiency [102]	Impurity-related energy losses [124]	High efficiency reduces system size for off-grid

Table 7. Sustainability criteria for molten Salts in CSP.

Criterion	Strengths	Challenges	Relevance to UAE Off-Grid
Recyclability	95% discharge efficiency post-recycling; reduces raw material use [118]	High purification costs; corrosion risks [34]	Cost-effective recycling critical for remote systems
Environmental Impact	Low emissions; dry cooling saves water [36]	Mining-related CO2 emissions [122]	Aligns with Net Zero by 2050 goals
Material Durability	Advanced coatings extend lifespan [124]	Costly alloys increase environmental footprint [70]	Low-maintenance materials needed for off-grid
Economic/Social Impact	Low LCOE; 1 M+ person-days employment [126]	Ethical sourcing challenges [122]	Supports remote community development
Operational Efficiency	Up to 63.6% thermal efficiency [102]	Impurity-related energy losses [124]	High efficiency reduces system size for off-grid

2.4. Decision-Making Frameworks for CSP Material Selection

Selecting the best molten salts for the CSP applications, especially the off-grid parabolic trough systems in arid areas, requires considering multiple criteria such as thermophysical properties, sustainability, and cost, with significant weight for each criterion. Using MCDM frameworks will enable a systematic evaluation of suggested molten salts across multiple criteria. This section reviews the MCDM techniques used for CSP material selection over published studies, identifies limitations, and then introduces a hybrid MCDM framework that integrates CRITIC, SWARA, and LGP.

The traditional MCDM methods, such as the analytic hierarchy process (AHP) and technique for order preference by similarity to ideal solution (TOPSIS), are commonly used in literature for selecting HTFs and TES media in CSP. For example, Cavallaro used the AHP method in his study, and the result was selecting nitrate salts, which were prioritized based on thermal stability and cost; however, the AHP method relies on subjective pairwise comparisons, which risk bias [127]. Other researchers used TOPSIS to rank the candidate materials used in CSP by selective criteria like density and corrosion, but the method faces challenges with conflicting criteria, such as cost and sustainability [128]. The WSM (weighted sum method) and WPM (weighted product method) are other conditional MCDM techniques that are simpler than other methods but less effective for integrating sustainability aspects like recyclability [100]. These methods show limitations when considering the intercriteria correlations, thus limiting the effective use of these methods in off-grid applications that need balanced optimization [129].

Advanced MCDM methods overcome these challenges. For example, the criteria importance through intercriteria correlation (CRITIC) method objectively gives the weights for each criterion based on contrast intensity and correlations, which will reduce the subjectivity [130]. On the other hand, the SWARA (stepwise weight assessment ratio analysis) method considers expert judgement for flexible weight adjustments [131]. Lexicographic Goal programming is used to find the optimal molten salt that suits all conflicting objectives through minimizing the deviations from the target and managing conflicting criteria such as efficiency and environmental impact [132]. Despite the improvements of the advanced methods, analysts seldom merge these methods with molten salt selection for off-grid CSP systems.

Table 8. Evaluation of MCDM techniques for CSP material and technology selection.

In-Text Citation	MCDM Technique(s) Used	Weighting Approach	Sustainability Focus	Application Context	Key Limitation(s) Related to Gaps
[133]	AHP, PROMETHEE II, VIKOR	Subjective (AHP)	Partial (Environ.)	Grid-Connected (Implied)	Uses subjective AHP weighting; environmental criteria included but not full sustainability spectrum; Grid-focused technology selection.
[134]	Fuzzy SWARA, GIS	Hybrid (SWARA)	Partial (Environ./Tech.)	General Site Selection	Uses hybrid weighting (SWARA); focus is site suitability, not specific off-grid HTF/TES selection; limited sustainability scope.
[135]	Spherical Fuzzy AHP (SF-AHP), Spherical Fuzzy MACONT (SF-MACONT)	Subjective (AHP base)	Comprehensive	General ESS	Explicitly uses sustainability criteria; uses fuzzy AHP (subjective weighting base); focus on general ESS ranking, not specific off-grid CSP application.
[136]	QFD-AHP-PSI	Subjective (AHP)	Partial (Soc./Environ./Tech.)	Off-Grid (Implied Arid)	Considers arid region context (potentially relevant to off-grid); uses AHP (subjective); limited sustainability scope defined by QFD/criteria.
[137]	Fuzzy AHP, Fuzzy TOPSIS	Subjective (AHP base)	Partial (Soc./Environ./Econ.)	Grid-Connected (Implied)	Uses traditional MCDM (fuzzy); considers developing country context but not explicitly off-grid; uses AHP base (subjective).
[127]	Fuzzy TOPSIS	Equal (implied)/unspecified	Partial (Environ./Econ.)	Grid-Connected (Implied)	Uses traditional method (Fuzzy TOPSIS); weighting potentially subjective/unclear; lacks full sustainability and off-grid focus. (Cited re: weight limitations).
[138]	Graph Theory and Matrix Approach (GTMA)	Objective (implied by method)	Partial (Tech./Econ.)	General Material Sel.	Non-standard MCDM; focus on thermochemical materials, not salts/off-grid/full sustainability.
[139]	SWARA-ARAS	Hybrid (SWARA)	Comprehensive	General EST	Explicitly uses sustainability index; hybrid weighting (SWARA); focus on general EST ranking, not specific off-grid CSP application.
[140]	AHP-VIKOR	Subjective (AHP)	Partial (Environ./Safety/Econ.)	Grid-Connected (Implied)	Uses subjective AHP weighting; includes multiple criteria but not framed as comprehensive sustainability; grid-focused HTF prioritization.
[141]	MCDA (unspecified)	Subjective (implied)	Partial (Environ./Econ.)	Grid-Connected (Implied)	Compare system configurations based partly on environmental/economic criteria; weighting likely subjective; not explicitly off-grid.
[142]	MCDM (unspecified), Techno-economic Analysis	Unclear	Partial (Econ./Tech.)	Grid-Connected (Implied)	Uses MCDM for TES duration ranking, but method/weighting unclear; limited sustainability/off-grid scope.
[143]	AHP	Subjective (AHP)	Partial (Safety/Tech.)	General Component Sel.	Uses subjective AHP weighting; focuses on technical/safety criteria for component selection, not full sustainability or specific application context (off-grid).
[144]	TOPSIS, Grey Relational Analysis (GRA)	Unclear (likely objective/equal)	Partial (Environ./Safety/Econ.)	General HTF Sel.	Uses traditional MCDM; weighting approach unclear; includes relevant criteria but not framed as comprehensive sustainability; not off-grid specific.
[145]	Improved Pythagorean Fuzzy TOPSIS	Hybrid (entropy-based)	Partial (Implied Econ/Tech)	General System Opt.	Proposes improved MCDM method with objective entropy weighting but applies it to optimizing hybrid systems, not specific to off-grid salt selection.
[146]	AHP (+LCA)	Subjective (AHP)	Partial (Environ. via LCA)	General Component Sel.	Uses subjective AHP; considers environmental sustainability via LCA; not focused on off-grid context or full sustainability spectrum.
[147]	MCDM (Multiple unspecified)	Unclear	Partial (Implied Tech/Econ)	General Material Sel. (TES)	Uses MCDM for TES material selection (PCM), but specific methods, weighting, sustainability focus, and context (off-grid) unclear from snippet.
[148]	MAIRCA, SPOTIS, COMET, CRITIC	Objective (CRITIC)	Yes (Explicit)	General Tech. Ranking	Uses objective weighting (CRITIC) and sustainability focus; but applied to general electricity tech ranking, not specific CSP/salt/off-grid application.
[149]	Multi-choice Goal Programming	Implicit (GP priorities)	Partial (Environ./Soc./Econ.)	Grid-Connected (Implied Planning)	Uses Goal Programming (handles conflicting goals); considers some sustainability aspects; weighting implicit in goal setting; not explicitly objective like CRITIC; not off-grid specific.

summarizes the reviewed MCDM studies applied to CSP molten salt material selection and CSP technology selection, highlighting the approaches and challenges used.

The proposed hybrid MCDM framework integrates CRITIC for objective weighting, SWARA for expert-driven adjustments, and goal programming for optimisation. It evaluates molten salts using criteria from Section 2.3.2 and Section 2.3.5 aligned to the UAE’s off-grid needs. This approach addresses traditional MCDM’s mentioned gaps and selects the best molten salt that is adequate for heat transferring and energy storage, considering the sustainability criteria, and subjective and objective evaluation of the used criteria.

2.5. The Potential and Challenges of Off-Grid CSP Applications

The UAE is developing major CSP projects through its solar resources and strategic measures, while research for off-grid and small-scale CSP developments remains an essential but largely neglected area. The UAE’s energy vision for 2050 sets multiple objectives that endorse off-grid viability, by considering the energy demand of remote populations, powering industrial sites in remote regions, and improving electricity resilience through decentralized generation forms [150]. Small-scale CSP systems can play an essential role in the UAE’s energy transition, presenting a modular, cost-effective, and dispatchable alternative to grid-scale CSP systems.

Table 9. Key Aspects of small-scale CSP systems in the UAE.

Aspect	Description	Citation
Technological Innovation	Modular design, latent heat storage, and advanced materials like recycled aluminium alloys.	[151,152,153]
Cost-Effectiveness	Lower capital costs compared to grid-scale CSP; competitive LCOE with PV systems.	[151,152]
Policy Support	Financial incentives, tax exemptions, and alignment with the UAE’s sustainability goals.	[154,155,156]
Challenges	High initial costs, infrastructure requirements, and regulatory hurdles.	[152,153,154]
Market Dynamics	Growing demand for decentralized energy and increasing cost-competitiveness.	[151,152,155]

summarises the key aspects of small CSP systems in the UAE and beyond.

The global improvement and adoption of small-scale CSP systems face technical and regulatory challenges. Some studies have explored the required infrastructure for small-scale CSP systems; these systems require specialized thermal energy storage systems and advanced control systems that are complex and costly to implement [152,153]. Moreover, integrating small-scale CSP systems into existing power grids is difficult due to technical issues and regulatory requirements [155]. While the UAE has achieved remarkable progress in promoting renewable energy, the lack of technical capacity and comprehensive regulations for off-grid systems has created uncertainty for investors and developers, thereby impeding the development of off-grid systems [154,157].

This analysis reveals a notable gap: The studies have not yet been conducted precisely using detailed methodologies to evaluate and design the system’s essential components, such as molten salts, for off-grid parabolic trough CSP systems that meet the UAE’s and the entire world’s specific requirements for operation and sustainability.

The MCDM framework discussed in Section 2.4 addresses optimized molten salt selection to achieve the UAE’s Net Zero by 2050 goals.

2.6. Research Gaps and Study Justification

The literature reviewed highlights the concepts and types of the CSP system and advancements in its molten salt TES, but gaps for trough parabolic type, especially for off-grid areas in the UAE, still exist. This section highlights these gaps and clarifies the current study’s focus on selecting the best molten salt that acts as a heat transfer medium and storage medium at the same time using a hybrid MCDM framework.

First, studies on molten salts as dual HTF and TES media in CSP applications, especially in parabolic troughs, are limited, with most studies focusing on power towers using Solar Salt [108]. Parabolic troughs, which operate at 150–500 °C, require salts with low melting points and high stability, which makes it critical for off-grid simplicity [97]. Second, sustainability aspects, such as recyclability and LCA, are underexplored for CSP salts. While recycling is technically feasible, as mentioned earlier, standardized processes and cost analyses for the recycling process are lacking, especially in arid regions [143]. Third, traditional MCDM methods like AHP and TOPSIS fail to integrate sustainability criteria or address off-grid constraints, relying on subjective weighting [127]. Fourth, off-grid CSP applications receive less attention than grid-connected systems, despite the UAE’s need for energy resilience in remote areas [50].

This study addresses these gaps by evaluating the current commercial salts for dual HTF/TES roles in off-grid parabolic troughs, using a hybrid CRITIC-SWARA-LGP programming MCDM framework. Leveraging the UAE’s policy support (e.g., Federal Law No. 12 of 2018) and MISP’s research, the study contributes to sustainable CSP deployment, aligning with the UAE’s Energy Strategy 2050 [121].

3. Methodology

3.1. General Framework of the Hybrid MCDM

This research explores and evaluates the candidate suitable molten salt that can be used simultaneously for heat transferring and energy storing within off-grid parabolic trough systems while meeting operational and sustainability standards that meet the UAE policies. This can be carried out by the application of a hybrid MCDM framework that evaluates the candidate salts using predefined criteria with calculated weights, then adjusting and refining these weights based on the experts’ opinion, followed by multi-objective optimization, and initial Pareto front screening. A summary of the methodology used is represented in Figure 1. The selected techniques and their integration are detailed below.

The proposed hybrid MCDM framework integrates four techniques: Pareto front analysis, CRITIC method, the SWARA method, and Goal Programming. The Pareto front analysis is used as the starting step in this model as it screens all potential salts and evaluates them based on key technical and thermal criteria. These salts are then classified into dominated and non-dominated alternatives. The dominated salts are the salts for which another alternative performs better across all selected evaluation criteria, while the non-dominated ones are not. These non-dominated salts are considered the optimal Pareto solutions that proceed to further analysis using the rest of the MCDM techniques. Following the Pareto screening, the CRITIC method [130] is used to determine the weights for the comprehensive set of evaluation criteria. This method is based on considering contrast intensity (standard deviation) and the conflict between criteria (correlation). The criterion that has higher variability and lower correlation with other criteria will get a higher objective weight; this means that this criterion has a greater impact on making the decision. To ensure relevance to the UAE strategies, and to determine the weight based on quantitative and qualitative data, the SWARA method is applied to adjust the qualitative weights generated by the CRITIC method. A group of experts with different backgrounds (academics in material science, industry professionals, and policymakers) rank the criteria based on the context of off-grid applications in the UAE. Then the adjustment factors are calculated and applied to the objective weights to gain a new set of hybrid weights that are quantitatively and qualitatively assessed and balanced. After weighing the criteria, the final selection of the optimal molten salt alternative is made using Goal Programming (GP) [132]. The GP model is formulated to consider multiple, potentially conflicting objectives at the same time, such as maximizing thermal efficiency while minimizing cost and minimizing environmental impact. Through this GP model, the optimal molten salt will be the one that meets technical, economic, environmental, and safety goals, ensuring compatibility with off-grid parabolic trough CSP systems in the UAE, where simplicity, reliability, affordability, and strong sustainability alignment are critical.

3.2. Justification of the Model

Regarding the application context, the UAE’s strategic energy and environmental goals (e.g., Net Zero by 2050) are clearly considered in the criteria weighing (via SWARA) and goal setting (for Lexicographic Goal Programming). Furthermore, the adoption of this hybrid MCDM framework is justified by its ability to address the limitations of traditional material selection methods and the complexity of this research problem. This approach provides several advantages; the first one is the comprehensive evaluation of the problem by considering thermal, technical, economic, environmental, and safety criteria. Moreover, the model reduces subjectivity by considering the CRITIC method, which evaluates the alternatives based on data characteristics and gives objective weights [158]. Although the model uses objective weights, it does not neglect the effect of expert incorporation when performing the SWARA method to consider the off-grid trough application. To justify the model based on efficiency, the Pareto fronts’ initial screening analysis shortlisted the most promising alternatives to be the focus of more analysis, instead of evaluating all salts each time. Also, this model considered the balancing of multiple conflicting goals at the same time, which led to more realistic decisions.

This methodology ensures that the selection process is based on real data and context-aware at the same time, sustainable, reliable, and practically applicable.

4. Data Analysis and Discussion

Based on the literature review, several studies evaluate the process of selecting the molten salt as HTF and TES medium based on several criteria. The selected criteria in this model are derived based on the most needed thermophysical requirements for both HTF and TES, sustainability, economic, and safety considerations. In this model, two sets of criteria are used: one for the Pareto front analysis, and the other for the rest of the techniques, as shown in

Table 10. Comprehensive evaluation criteria and their role in the MCDM framework.

#	Criterion	Category	Role in Evaluation and Justification	Usage in Analysis
1	Melting Point (°C)	Technical (Minimize)	Lower values reduce the risk of freezing and simplify operation in off-grid CSP systems. Also, affects system cold-start behaviour and prevents solidification in moderate-temperature troughs.	Initial Screening and Full Evaluation
2	Maximum Operating Temp (°C)	Technical (Maximize)	Higher values enable longer operational range and storage potential without decomposition.	Initial Screening and Full Evaluation
3	Specific Heat Capacity (J/kg·K)	Technical (Maximize)	Improves storage capacity and reduces tank volume requirements.	Initial Screening and Full Evaluation
4	Cost-effectiveness ($/kg)	Economic (Minimize)	The initial material cost is a primary driver of project feasibility, especially in remote or cost-sensitive installations.	Initial Screening and Full Evaluation
5	Safety (toxicity, flammability)	Operational/Sustainability	Addresses hazards related to salt handling, storage, and containment.	Initial Screening and Full Evaluation
6	Corrosion Resistance	Durability (Maximize)	Ensures material compatibility, lowers maintenance, and extends component life.	Initial Screening and Full Evaluation
7	Operating Temperature Range (°C)	Technical (Maximize)	Indicates the usable window between melting and degradation; a wider range offers greater operational flexibility.	Full Evaluation Only
8	Thermal Conductivity (W/m·K)	Technical (Maximize)	Enhances heat transfer efficiency within solar collectors and storage systems, improving overall performance.	Full Evaluation Only
6	Viscosity (mPa·s)	Technical (Minimize)	Lower values reduce pumping power and system pressure drops.	Full Evaluation Only
7	Density (kg/m3)	Technical (Neutral)	Influences TES tank size and fluid dynamics in circulation loops.	Full Evaluation Only
11	Environmental Sustainability	Lifecycle/Policy	Considers emissions, resource depletion, and end-of-life recyclability.	Full Evaluation Only
12	Availability and Scalability	Logistical/Strategic	Reflects the commercial maturity and robustness of the supply chain, ensuring widespread deployment is feasible.	Full Evaluation Only

. For the Pareto front analysis, the criteria are chosen based on the trade-offs between the performance of molten salt and its feasibility.

4.1. Candidate Molten Salts

In this model, a diverse range of molten salts, including traditional, advanced, and salts with potential for recycling, are considered as alternatives for evaluation within the proposed hybrid MCDM framework. These candidates have been reviewed for their general suitability for parabolic trough systems and their potential relevance to the operational and environmental requirements of off-grid applications in the UAE. The resulting alternatives are listed in

Table 11. Molten salt list.

Molten Salt Composition(s)	Category	Classification	Rationale for Selection
Solar Salt (NaNO3-KNO3)	Nitrate-Based	Traditional	Widely commercialized and extensively studied benchmark salt for CSP TES; well-understood properties and operational experience in parabolic troughs.
HITEC (NaNO3-KNO3-NaNO2)	Nitrate-Based	Traditional	Commercial salt with a lower melting point than Solar Salt, making it suitable as an HTF in parabolic troughs and potentially for dual use where a lower operational limit is beneficial.
Binary Nitrate Mixtures (e.g., KNO3-NaNO2, NaNO2-KNO3)	Nitrate-Based	Advanced	Explored in literature for achieving lower melting points, which is advantageous for reducing freezing risk and extending the operational window in parabolic trough systems.
Ternary Nitrate Mixtures (e.g., Ca(NO3)2:NaNO3:KNO3, NaNO3-KNO3-NaNO2)	Nitrate-Based	Advanced	Advanced compositions specifically developed to achieve significantly lower melting points and/or improve other thermophysical properties relevant to parabolic trough operation.
LiNO3-KNO3 (potentially with nanoparticles)	Nitrate-Based/Advanced	Advanced	Explores the potential of using Lithium-based nitrates and nanoparticle additives to enhance thermal properties, such as specific heat capacity or thermal conductivity.
MgCl2-LiCl	Chloride-Based	Advanced	Explored for potential use at higher temperatures than nitrates, offering a wider operational range if corrosion challenges can be managed.
KCl-MgCl2	Chloride-Based	Advanced	Similar to MgCl2-LiCl, another chloride mixture investigated for higher temperature applications in CSP.
NaCl-MgCl2	Chloride-Based	Advanced	A binary chloride salt considered for its properties, though potentially higher melting point than some other candidates.
KCl-ZnCl2	Chloride-Based	Advanced	Noted for a relatively lower melting point among some chloride salts, making it potentially suitable for parabolic troughs, but safety concerns (volatility, toxicity) require careful evaluation.
LiCl-KCl (59–41 mol%)	Chloride-Based	Advanced	Used in various high-temperature applications, including some energy systems; properties are relatively well-documented.
NaCl-AlCl3 (66–34 mol%)	Chloride-Based	Advanced	Characterized by a very low melting point, which is highly desirable for low-temperature operation or startup, but faces significant challenges with corrosivity and hygroscopy.
LiCl-RbCl (50–50 mol%)	Chloride-Based	Advanced	A binary chloride mixture with documented properties, considered an alternative within the chloride family.
Ternary Chloride Mixtures (general)	Chloride-Based/Advanced	Advanced	Various combinations are being explored in research to optimize properties like melting point, thermal stability, and corrosivity for high-temperature CSP applications.
LiF-CaF2 (50–50 mol%)	Fluoride-Based	Advanced	Known for very high thermal stability, typically considered for high-temperature CSP tower systems, but included for comparison if its melting point is within a feasible range or if its high stability offers advantages.
LiF-NaF (50–50 mol%)	Fluoride-Based	Advanced	A binary fluoride salt with high thermal stability.
FLiBe (LiF-BeF2)	Fluoride-Based	Advanced	Noted for high specific heat capacity, a desirable property for TES, but the toxicity of Beryllium requires rigorous safety protocols and environmental considerations.
FLiNaK (LiF-NaF-KF)	Fluoride-Based	Advanced	A eutectic fluoride mixture with high thermal stability, often considered for higher temperature applications.
KF-ZrF4 (58–42 mol%)	Fluoride-Based	Advanced	Fluoride mixtures involving Zirconium, explored for their thermophysical properties and potential in high-temperature systems.
NaF-ZrF4 (50–50 mol%)	Fluoride-Based	Advanced	Fluoride mixtures involving Zirconium, explored for their thermophysical properties and potential in high-temperature systems.
RbF-ZrF4 (50–50 mol%)	Fluoride-Based	Advanced	Fluoride mixtures involving Zirconium, explored for their thermophysical properties and potential in high-temperature systems.
LiF-ThF4 (75–25 mol%)	Fluoride-Based	Advanced	Primarily relevant for nuclear applications due to Thorium content, but included for comprehensive comparison of fluoride salt properties if data is available, acknowledging its specific application context.
Li2CO3-Na2CO3-K2CO3	Carbonate-Based	Advanced	Explored for high heat capacity and thermal stability, potentially suitable for higher temperature TES, but reactivity with CO2 needs to be considered.
Na2SO4-K2SO4	Sulfate-Based	Advanced	Noted for stability and non-toxicity, which are desirable sustainability traits, but typically have high melting points that might limit their direct use in parabolic troughs without specific modifications.
NaCl-KCl-Na2CO3	Hybrid/Mixed Anion	Advanced	Compositions combining different anions are explored to potentially achieve a balance of desirable properties from different salt classes (e.g., lower melting point from chlorides, high heat capacity from carbonates).

. To ensure a comprehensive evaluation, the properties of both virgin and recycled (potentially) candidate molten salts are assessed, with special focus on their implications for economic feasibility and environmental sustainability metrics.

4.2. Evaluated Properties of Candidate Molten Salts

Following the identification of candidate molten salts in Section 4.1, a comprehensive dataset was compiled for detailed evaluation within the hybrid MCDM framework. The preliminary screening phase utilized a selected group of essential criteria, which helped to select the top molten salt candidates. This step focused on streamlining the selection process by minimizing the alternatives that needed thorough multi-criteria evaluation.

The data is collected from the studies outlined in the literature and MSTDB (Molten Salt Thermal Properties Databases). In regard to the evaluation of the qualitative criteria used, a standardized scale from 1 to 5, where a score of 5 denotes outstanding performance. The complete rubric is provided in Supplementary Table S1. The evaluation scores for each salt were based on literature analysis, technical reports examination, and database reviews when precise numeric performance data could not be found. Justification by Criterion:

• Safety (Toxicity, Flammability, and Handling): The PubChem and ECHA databases offer individual compound data, including LD₅₀ and flash points, while mixture properties, which can vary, remain insufficiently reported. NFPA 704 flammability ratings provide partial insight. The adoption of a qualitative scoring system ranging from 1 to 5 occurred based on evaluations of toxicity levels, handling complexity, and fire hazards.
• Corrosion Resistance: Information on corrosion rates under CSP-specific conditions remains limited. Studies from authors such as Pillai et al. [159] and Svobodova et al. [75] provide crucial insights, but their findings remain restricted to particular materials and testing conditions. The qualitative scoring system evaluated compatibility trends with standard CSP alloys and assessed the aggressiveness of different salt chemistries, such as chlorides, compared to nitrates.
• Environmental Sustainability: Limited comprehensive LCA information exists for many new or non-commercial salt mixtures. The scores were deduced by analyzing how constituent materials affected extraction processes, together with their energy requirements and toxicity levels. Salts that contained rare elements or required high energy production received lower scores whereas nitrate-based salts with extensive deployment history earned better ratings.
• Availability and Scalability: USGS reports and industrial analyses provide details about separate elements but fail to consistently evaluate if specific blends can be produced on an industrial scale. The scoring system evaluated commercial maturity together with resource abundance and supply chain outlook.

Standard nitrate salts (Solar Salt and Ca(NO₃)₂:NaNO₃:KNO₃) are extensively acknowledged as the industrial standard for safe handling practices, attaining a safety rating of 4/5 owing to their intrinsic thermal stability and non-flammable nature within their specified operational temperature ranges [160,161]; however, their categorization as potent oxidizing agents necessitates compliance with specific handling and containment protocols, thereby precluding the attainment of a perfect score. Empirical evidence accumulated over numerous decades of industrial application consistently illustrates that these standard nitrate mixtures manifest markedly low and predictable corrosion rates when engaging with conventional stainless steel alloys at temperatures below 600 °C (4/5) [161], which serves as a principal determinant of their extensive commercial adoption and operational efficacy; the constituent salts are abundant industrial commodities characterized by well-documented and relatively minimal life-cycle environmental impacts (4/5) [162], with their extensive usage across a multitude of industrial sectors ensuring the presence of mature and highly efficient production methodologies, Solar Salt is commercially available on a global scale and is easily procurable in the large amounts needed for major utility operations (5/5), while the individual elements of the Ca(NO₃)₂ formulation are also widely obtainable, thus facilitating the scalability of the formulation (4/5), although it does necessitate the performance of on-site blending activities [163]. Nitrite-nitrate compositions (KNO₃-NaNO₂, NaNO₂-KNO₃, and NaNO₃-KNO₃-NaNO₂) are assigned a safety score of 3/5 because of the integration of sodium nitrite (NaNO₂) within these mixtures, which will enhance the chemical toxicity compared to pure nitrate mixtures, therefore requiring more handling practices and protocols as outlined in the industrial chemical safety manuals. These mixtures are composed of the same fundamental component of the traditional nitrate salts, so they share their ability of corrosion resistance, environmental impact, and their commercial availability (4/5 for each criterion) [164,165].

The use of chloride-based salts (MgCl₂-LiCl and KCl-MgCl₂) presents notable challenges in the literature in terms of safety and corrosion resistance. They receive a safety score of (2/5), since they release hazardous hydrogen chloride (HCl) gas when exposed to elevated thermal conditions [166]. Regarding corrosion resistance, these salts got a score of (1/5) as the molten chlorides show extreme corrosiveness towards standard metallic alloys, causing extensive damage that demands the use of costly nickel-based superalloys for containment. Regarding corrosion resistance, these salts got a score of (1/5) as the molten chlorides show extreme corrosiveness towards standard metallic alloys, causing extensive damage that demands the use of costly nickel-based superalloys for containment [166]. Environmentally, scores of (2/5) for MgCl₂–LiCl and (3/5) for KCl–MgCl₂ are given for the salts from the fact that these salts require purification processes to attain anhydrous states; the MgCl₂–LiCl salt is also constrained by the environmental impacts associated with lithium extraction [167]. In terms of availability, these salts score 3/5 and 4/5, respectively, as they depend on specialized supply chains that are less established than those for conventional nitrates [167,168].

Fluoride-based salts, such as LiF–CaF₂, achieved the most unfavorable safety score of (1/5) because of their tendency to interact with atmospheric moisture, resulting in the generation of hydrogen fluoride (HF), which is characterized by its extreme toxicity and corrosive properties [169,170,171]; hence, the corrosive nature is rated at (2/5), as it also requires the use of costly containment solutions. In terms of availability, these salts are considered limited as they are used in specific sectors, such as the field of nuclear energy [169,170]. This reason and its environmental consequences justify a moderate availability score of (2/5).

For the LiNO₃–KNO₃ (Nano) and MgBr₂-based mixtures, uncertainty is the major challenge due to the lack of empirical data on testing performance and the potential health effects of nanoparticles. As a result, they got moderate to low safety scores (3/5 for LiNO₃–Nano and 2/5 for MgBr₂). Regarding the corrosion resistance scores, LiNO₃–Nano performs well (4/5), showing notable behaviour compared to the conventional nitrate salts. In contrast, the MgBr₂ mixture is rated lower (2/5) due to its halide composition [172].

4.3. Pareto Front Analysis

The study evaluated molten salt candidates that possessed complete data for all six selected criteria—four quantitative and two qualitative—outlined in the previous section. This balanced methodology confirms that no salt is prematurely disregarded based on a singular weakness, provided it offers compensating strengths in other criteria. A thorough pairwise comparison was performed on the entire set of ten candidate molten salts.

Table 12. Summary of data used for Pareto front screening.

Molten Salt Composition(s)	Melting Point (°C)	Max Operating Temp (°C)	Specific Heat Capacity (J/kg·K)	Cost-Effectiveness (Initial $/kg)	Safety (1–5)	Corrosion Resist. (1–5)
Solar Salt (NaNO3-KNO3)	220 [95,173]	565 [95,173]	1500 [95,173]	0.50 [118]	4	4
KNO3-NaNO2 (Binary Nitrate)	142 [120,173,174]	450 [120,173,174]	1550 [120,173,174]	0.45 [118]	3	4
MgCl2-LiCl (Chloride)	420 [166]	800+ [166]	1050 [166]	0.60 [118]	2	1
LiNO3-KNO3 (with Nanoparticles)	142 [175]	550 [175]	1600 [175]	0.55 [118]	3	4
MgBr2-based Ternary Mixture	200 [176]	580 [176]	1380 [176]	0.70 [176]	2	2
Ca(NO3)2:NaNO3:KNO3 (32:24:44 wt%)	80 [160,177]	600 [160,177]	1520 [160,177]	0.63 [118]	4	4
NaNO2-KNO3 (Binary Nitrite-Nitrate)	135 [120,173,174]	480 [120,173,174]	1480 [120,173,174]	0.48 [118]	3	4
LiF-CaF2 (Fluoride)	769 [169,178]	1100 [179,180]	1750 [180]	0.75 [118]	1	2
KCl-MgCl2 (Chloride Mixture)	426 [166]	750 [166]	1300 [166]	0.58 [118]	2	1
NaNO3-KNO3-NaNO2 (General Ternary Nitrate)	130 [164]	520 [164]	1500 [164]	0.50 [118]	3	4

contains the summarized data that was utilized for the Pareto front screening process. The full, comprehensive dataset for all ten initial candidates across all twelve criteria, including specific literature citations for each data point, is provided in Supplementary Table S2.

Dominance relationships were established by evaluating each candidate through pairwise comparisons against the remaining candidates.

Table 13. Dominance classification of candidate molten salts.

Salt Name	Status	Justification for Status
Solar Salt (NaNO-KNO3)	Non-Dominated	Offers a balanced profile with no single salt being superior across all six criteria.
KNO3-NaNO2 (Binary Nitrate)	Non-Dominated	Superior cost and melting point prevent dominance by other salts.
MgCl2-LiCl (Chloride)	Non-Dominated	Superior max operating temperature prevents dominance.
LiNO3-KNO3 (with Nanoparticles)	Non-Dominated	Superior specific heat and melting point prevent dominance.
MgBr2-based Ternary Mixture	Dominated	Dominated by Ca(NO3)2:NaNO3:KNO3 across all six screening criteria.
Ca(NO3)2:NaNO3:KNO3 (32:24:44 wt%)	Non-Dominated	Top performer on many criteria, not dominated by any other salt.
NaNO2-KNO3 (Binary Nitrite-Nitrate)	Non-Dominated	Superior cost and melting point prevent dominance.
LiF-CaF2 (Fluoride)	Non-Dominated	Superior max operating temperature and specific heat prevent dominance.
KCl-MgCl2 (Chloride Mixture)	Non-Dominated	Superior max operating temperature prevents dominance.
NaNO3-KNO3-NaNO2 (General Ternary Nitrate)	Non-Dominated	Superior melting point and viscosity prevent dominance.

displays the screening results, which show the classification status of each salt as either dominated or non-dominated. One candidate, MgBr₂-based Ternary Mixture, was identified as dominant. This is the Ca(NO₃)₂:NaNO₃:KNO₃ salt performs strictly better across all six Pareto criteria: the Ca(NO₃)₂ mixture has a lower melting point (80 vs. 200 °C), higher maximum operating temperature (600 vs. 580 °C), higher specific heat capacity (1520 vs. 1380 J/kg·K), lower initial cost (USD 0.63 vs. USD 0.70/kg), a superior safety score (4 vs. 2), and a superior corrosion resistance score (4 vs. 2). As the Ca(NO₃)₂:NaNO₃:KNO₃ salt is strictly better in every evaluated dimension, the MgBr₂-based Ternary Mixture is dominated and is excluded from subsequent analysis.

Nine candidates passed the non-domination test and advanced to further evaluation stages, which include CRITIC analysis followed by SWARA and LGP methods. Although the full Pareto front screening was conducted across the four criteria, the visualization presented in Figure 2 illustrates a two-dimensional projection based on two of the most thermally critical properties: melting point and thermal stability. The visualization functions as an illustrative example but final dominance classification depended on evaluating all six selected criteria together. In this figure, the non-dominated salts connect at the red dashed Pareto boundary, while MgBr₂-based Ternary Mixture appears as a separate grey point below the frontier.

After applying Pareto front Analysis to find non-dominated molten salt candidates, another extensive assessment with the CRITIC method is performed. The technique objectively distributes weights across the complete evaluation criteria set by evaluating each criterion’s variability and its conflict level with other indicators.

4.4. Objective Criteria Weighting Using the CRITIC Method

The CRITIC method was applied to determine objective weights for evaluating molten salts dual HTF/TES roles for off-grid CSP systems. A total of 12 criteria were used in this analysis, encompassing thermophysical, safety, economic, environmental, and logistical dimensions. These include melting point (°C), thermal stability (°C), operating temperature range (°C), specific heat capacity (J/kg·K), thermal conductivity (W/m·K), viscosity (mPa·s), density (kg/m³), corrosion resistance (Score 1–5), safety (Score 1–5), environmental sustainability (Score 1–5), cost-effectiveness (USD/kg), and availability and scalability (Score 1–5). The thermophysical data were compiled from MSTDB [181], corrosion studies [159], industry reports [57,58,182], and qualitative sources such as PubChem, ECHA, and NACE. A purely quantitative approach would lead to excluding numerous viable candidates because of missing data or necessitate thorough lab testing and site-specific analysis, which exceeds this study’s boundaries. Quantitative information, such as alloy corrosion rates or specific production process LCA emissions, exists but lacks comprehensive availability across all relevant salts and their operating conditions. The study implements a structured qualitative scoring method for chosen criteria to bridge existing research gaps and ensure a complete system-oriented assessment. This study includes important safety and sustainability measures as part of its quantitative MCDM assessment.

This approach quantifies criterion importance through two metrics:

Contrast Intensity: Represented by the standard deviation (${\sigma }_j$) of normalized data for criterion $j$, reflecting its ability to discriminate between alternatives.

Conflict: Calculated as ${\sum }_{k=1}^n(1-r_{jk})$ where $r_{jk}$ is the Pearson correlation coefficient between criteria $j$ and $k$, identifying redundant criteria.

The final weights ($w_j$) were derived as follows:

$$w_j=\frac{{\sigma }_j\times \sum _{k=1}^n(1-r_{jk})}{\sum _{j=1}^m{\sigma }_j\times \sum _{k=1}^n(1-r_{jk})}$$

(1)

All criteria were normalized on a 0–1 scale using min-max normalization, ensuring comparability across diverse units and scales. The calculated values are presented in

Table 14. CRITIC weights based on 12 criteria across 10 molten salts.

Criterion	σ	Conflict	Information (Cj)	CRITIC Weight (w1)
Safety (toxicity, flammability)	0.316	7.92	2.51	0.103
Maximum Operating Temp (°C)	0.301	8.24	2.48	0.102
Corrosion Resistance	0.444	6.75	3.00	0.123
Cost-effectiveness (USD/kg)	0.324	6.64	2.15	0.088
Operating Temperature Range (°C)	0.280	7.42	2.08	0.085
Specific Heat Capacity (J/kg·K)	0.269	6.81	1.83	0.075
Melting Point (°C)	0.296	5.51	1.63	0.067
Environmental Sustainability	0.385	6.13	2.36	0.097
Thermal Conductivity (W/m·K)	0.299	6.09	1.82	0.075
Availability and Scalability	0.293	5.48	1.60	0.066
Viscosity (mPa·s)	0.302	5.67	1.71	0.070
Density (kg/m3)	0.310	4.02	1.25	0.051

.

Corrosion resistance received the highest weight at 0.123, while safety and maximum operating temp received weights of 0.103 and 0.102, respectively. This study’s findings demonstrate substantial differences among the salt candidates while showing no dependency on other evaluation criteria, which confirms their fundamental function in CSP systems’ safe and reliable performance. Density (0.051) and viscosity (0.070) received less significant weights due to their low variability and strong correlation with other thermophysical properties, even though they remained relevant. The cost-effectiveness property (0.088) received similar importance as the operating temperature range (0.085), which emphasizes economic viability as a crucial factor for CSP deployment, especially in areas without grid access. The study successfully includes essential dimensions through its incorporation of structured qualitative evaluations. The scoring system utilizes CRITIC-based objective weights, together with expert input from the SWARA method, to produce a clear and complete evaluation of molten salt alternatives for CSP implementation in the UAE.

4.5. SWARA Analysis

The SWARA method is used to evaluate expert perspectives on technical, economic, and sustainability criteria for off-grid parabolic trough CSP systems implementation in the UAE. The method proves effective when strategic factors, including environmental sustainability and safety, achieve importance beyond numerical variance measurements. This method starts with listing all evaluation criteria in descending order based on their perceived importance. To establish the ranking for the twelve evaluation criteria, a group of twelve experts was formed and selected based on their extensive and relevant experience in this field. This panel of experts was structured to include wide range of critical perspectives, including academics with expertise in material sciences and renewable energy systems, senior engineers with strong background knowledge in the industrial CSP systems, and policy makers with specialisation in sustainable energy policies in the MENA region. The methodology of gathering their evaluation followed a structured approach that began with a detailed questionnaire shared with the experts. This was followed by a series of in-depth interviews to discuss and clarify their reasoning and responses in selecting and ranking the criteria based on their importance. This approach ensured that the final weights used in the SWARA method accurately reflect the informed judgment of the expert panel.

A coefficient of relative importance (s_i) was given to each successive criterion beginning from the second to show how much less important it was compared to its predecessor. The coefficients are then used to create correction factors (k_i), unnormalized scores (q_i), and then the final normalized weights (w_i). Starting with safety as the highest-priority criterion, expert-derived values were computed using the standard SWARA approach and shown in

Table 15. SWARA calculations.

Rank	Criterion	si	ki = 1 + si	qi	SWARA Weight (wi)
1	Safety (toxicity, flammability)	0.00	1.00	1.0000	0.0954
2	Maximum Operating Temp (°C)	0.01	1.01	0.9901	0.0945
3	Corrosion Resistance	0.05	1.05	0.9430	0.0900
4	Cost-effectiveness (USD/kg)	0.01	1.01	0.9336	0.0891
5	Melting Point (°C)	0.01	1.01	0.9244	0.0882
6	Specific Heat Capacity (J/kg·K)	0.02	1.02	0.9063	0.0865
7	Thermal Conductivity (W/m·K)	0.04	1.04	0.8714	0.0831
8	Operating Temperature Range (°C)	0.05	1.05	0.8300	0.0792
9	Environmental Sustainability	0.01	1.01	0.8217	0.0784
10	Availability and Scalability	0.04	1.04	0.7805	0.0745
11	Viscosity (mPa·s)	0.03	1.03	0.7578	0.0723
12	Density (kg/m3)	0.05	1.05	0.7217	0.0689

.

SWARA objective weighting shows safety with a score of 0.0954 at the top of the criteria ranking, highlighting its critical role in ensuring reliable and safe operation in remote regions. Also, having a safe system means less control is needed. Maximum operating temp (0.0945) and corrosion resistance (0.0900) follow closely, which assures the long-term material degradation and accommodates the environment of the UAE. The environmental sustainability criterion did not achieve the highest weight but still has a strong weight. The ranking position does not indicate reduced importance but shows that deployment of thermal systems in remote locations necessitates addressing immediate operational issues before environmental sustainability.

For consistent and relevant results, the final weight for each criterion is recalculated using a weighted CRITIC-SWARA with 60% weight for SWARA weights and 40% weight for CRITIC methods. This will ensure the alignment with the UAE region while maintaining the quantitative analysis.

Table 16. Hybrid weights (SWARA 60%, CRITIC 40%).

Criterion	SWARA Weight	CRITIC Weight	Hybrid Weight
Safety	0.0954	0.103	0.0984
Maximum Operating Temp (°C)	0.0945	0.102	0.0975
Corrosion Resistance	0.0900	0.123	0.1032
Cost-effectiveness (USD/kg)	0.0891	0.088	0.0887
Melting Point (°C)	0.0882	0.067	0.0797
Specific Heat Capacity (J/kg·K)	0.0865	0.075	0.0819
Thermal Conductivity (W/m·K)	0.0831	0.075	0.0799
Operating Temperature Range (°C)	0.0792	0.085	0.0815
Environmental Sustainability	0.0784	0.097	0.0858
Availability and Scalability	0.0745	0.066	0.0711
Viscosity (mPa·s)	0.0723	0.070	0.0714
Density (kg/m3)	0.0689	0.051	0.0617

displays the combined weights that will be utilized in the next Goal Programming model.

Corrosion resistance is the leading factor that shapes isolated energy installations through its requirement for risk mitigation and maintaining operational reliability. Safety and maximum operating temp show how crucial it is for materials to withstand both extreme temperatures and aggressive conditions. The mid-tier criteria of cost-effectiveness, environmental sustainability, and specific heat capacity serve as vital components for achieving effective energy storage while maintaining system affordability. Operating temperature range, thermal conductivity, and melting point significantly support the UAE’s decarbonization objectives, but their impact is controlled by their relationship with additional variables. Availability and scalability, along with viscosity and density, present minimal constraints when selecting initial materials, allowing more flexibility in design.

To determine the optimal molten salt, this study employs a Goal Programming (GP) model informed by hybrid weights derived from the CRITIC and SWARA methods. These hybrid weights serve as the weighting factors in the GP objective function, directly influencing the multi-objective optimization process that underpins the final material selection. In the general GP approach, the first step is to define the decision variables. In this model, $x_i$ is a binary variable indicating whether molten salt $\left(i\right)$ is selected ${(x}_i=1)$or not ${(x}_i=0).$ The second step is to set a target for each criterion $\left(j\right)$, where $j=1$, …,12. The target value $\left(T_j\right)$ is the ideal performance, which achieves a score of 1 in beneficial criteria such as thermal stability and safety because higher values represent better outcomes in the normalized matrix. The third step is to define the deviations for each criterion $j$ and salt $i$. $d_{ij}^{+}$ is the positive deviation (overachievement) from the target and $d_{ij}^{-}$ negative deviation (underachievement) from the target. Now the objective function can be formulated as minimizing the weighted sum of negative deviations (since we want to minimize underachievement) as follows:

$$minZ={\sum }_{i=1}^9{\sum }_{j=1}^{12}{w}_j{d}_{ij}^{-}{x}_i$$

(2)

The constraints of this model include the following:

$$\mathrm{Only}\mathrm{one}\mathrm{salt}\mathrm{can}\mathrm{be}\mathrm{selected}:{\sum }_{i=1}^9{x}_i=1$$

(3)

$$\mathrm{Goal}\mathrm{constraint}\mathrm{for}\mathrm{each}\mathrm{criterion}:T_j=s_{ij}+d_{ij}^{-}-d_{ij}^{+}x$$

(4)

$$\mathrm{Non}\text{-}\mathrm{negativity}:d_{ij}^{-},d_{ij}^{+}\ge 0$$

(5)

$$\mathrm{Binary}\mathrm{constraint}:x_i\in \{0,1\}$$

(6)

where $s_{ij}$ is the normalized score of salt $i$for the criterion.

The traditional linear GP model may reduce the significance of essential safety and durability criteria when multiple objectives are linearly minimized together. That is, it may select a molten salt with a major weakness, like an unsafe and corrosive salt. An Enhanced Lexicographic Goal Programming (LGP) method has been developed to overcome this limitation. This approach introduces a tiered prioritization mechanism, ensuring that the most important technical and safety constraints are satisfied before optimizing lower-priority constraints like cost and other criteria with low hybrid weight. This enhancement addresses major drawbacks identified in the literature: the risk of selecting unsafe or corrosive salts, the lack of resilience testing against minor policy or operational changes, and oversimplification of thermophysical property trade-offs through linear normalization.

The enhanced LGP model has the following three-tier hierarchy of objectives:

• Tier 1: Critical criteria for off-grid UAE conditions: This tier includes safety and corrosion resistance criteria, which are essential for long-term deployment in off-grid UAE conditions. These criteria are non-negotiable and must meet minimum thresholds $(\ge 0.75)$ to qualify salt for further evaluation in the next tier. The normalized matrix score of 0.75 stands for a qualitative rating of about 4 out of 5 on the original scale based on linear normalization, where 5 maps to 1.00 and 3 maps to 0.50. A rating of 4 means “good” or “above average” performance, which confirms that chosen salts show strong safety profiles, including low toxicity and minimal handling risks, together with excellent corrosion resistance and minimal system component degradation.
• Tier 2: Core thermophysical performance: This tier includes all criteria that are critical to technical performance, such as melting point, maximum operating temperature, operating temperature range, specific heat capacity, and thermal conductivity. These determine how effectively the salt stores and transfers heat within the expected operational range in the UAE $(~300-400 °\mathrm{C}).$
• Tier 3: Sustainability and economic feasibility: This tier includes viscosity, density, environmental sustainability, cost-effectiveness, and availability and scalability. These criteria are critical to the overall lifecycle performance, cost, and deployment possibility.

In this enhanced model, we will overcome the linearity limitation by evaluating each salt using non-linear normalizing scores $s_{ij}^{\prime }$, this will capture realistic engineering trade-offs. These scores will be generated for the melting point, viscosity, and thermal stability criteria. The score of other criteria will be linear because the threshold values have less effect. The updated scores are calculated as follows:

• For the melting point criterion, the Sigmoid transformation is used to emphasize low values:

  $$s_{ij}^{\prime }={\displaystyle \frac{1}{1+e^{0.05(s_{ij}-0.9)}}}$$

  (7)

The purpose of the sigmoid function is to select low melting points close to 150 °C to avoid freezing risks in CSP systems but penalize melting points above 240 °C, which demand additional energy to keep fluids liquid. In this equation, $s_{ij}$ is the original normalized score (0–1) where 1 represents the lowest (best) melting point (e.g., $~80 °\mathrm{C} \mathrm{for} \mathrm{Ca}{\left({\mathrm{NO}}_3\right)}_2:{\mathrm{Na}\mathrm{NO}}_3:{\mathrm{KNO}}_3)$ and 0 represents the highest (worst) (e.g., $~ 769 °\mathrm{C} \mathrm{for} \mathrm{LiF}-{\mathrm{CaF}}_2$). The 0.9 value is the reference point for the sigmoid function, which is the normalized score near a melting point of $~150 °\mathrm{C}$, which is ideal for parabolic trough CSP systems since it minimizes the risk of freezing in UAE’s ambient conditions ($5-50 °\mathrm{C}$) and reduces heating energy compared to higher values like $240 °\mathrm{C}$ as found in the solar salts. The 0.9 value is near the top scores, aligning with salts that perform well like Ca(NO₃)₂:NaNO₃:KNO₃ with a score of 1.00 and NaNO₃-KNO₃-NaNO₂ with a score of 0.927. The 0.05 value is chosen as the steep parameter of the function, which determines the rate at which scores change around the reference point of 0.9. (e.g., Solar Salt at 220 °C scores 0.501, Ca(NO₃)₂:NaNO₃:KNO₃ at 80 °C scores 0.499). Specifically, these values have been carefully selected to establish a “soft threshold” to ensure a sufficiently sharp distinction between the desirable and undesirable values of the melting points without being sensitive to the small variation in these values, which will make a realistic trade-off.

• For viscosity: Exponential decay to favor low-resistance flow

  $$s_{ij}^{\prime }{=e}^{-2\left({1-s}_{ij}\right)}$$

  (8)

This function is selected to ensure the selection of salts with low viscosity (e.g., $<3 \mathrm{m}\mathrm{P}\mathrm{a}·\mathrm{s}$) to reduce energy needed for pumping in CSP systems, emphasizing scores near 1.0 while penalizing higher viscosity values. The 2 value represents the decay rate constant, which controls how quickly the score ${(s}_{ij})$ decreases as viscosity increases. This value was chosen to show the non-linear impact of viscosity of the power of pumping the molten salts, since the viscosity rapidly increases when the frictional losses in the piping of the system increase. This will align with the operational energy costs of the real-world applications. This approach is employed to emphasize performance gains from low viscosities $(<3 \mathrm{m}\mathrm{P}\mathrm{a}·\mathrm{s}$) while penalizing higher energy requirements (e.g.,$0.310 \mathrm{f}\mathrm{o}\mathrm{r} ~3.8 \mathrm{m}\mathrm{P}\mathrm{a}·\mathrm{s} \mathrm{v}\mathrm{s}. 1000 \mathrm{f}\mathrm{o}\mathrm{r} ~2.1 \mathrm{m}\mathrm{P}\mathrm{a}·\mathrm{s}$).

• Thermal Stability: A piecewise linear function (amplification = 1.2, capped at 1.0)

  $$s_{ij}^{\prime }=\min (1.2·s_{ij},1)$$

  (9)

This will amplify the scores up to 0.833 (e.g., Solar Salt at 565 °C) but caps at 1.0 for higher values.

For all criteria, negative deviation values are computed as follows:

$$d_{ij}^{-}=\mathrm{m}\mathrm{a}\mathrm{x}(\mathrm{0,1}-s_{ij}^{\prime })$$

(10)

The LGP objectives are defined as follows:

$$\mathrm{Tier} 1 \mathrm{objective}: Z_1=w_s·d_{is}^{-}+w_{CR}·d_{iCR}^{-}$$

(11)

where $s$ annotation is for the safety criterion and $CR$ is for the corrosion resistance criterion

$$\mathrm{Tier} 2 \mathrm{objective}: Z_2={\sum }_{j\in \mathit{Tier}2}{w}_j·d_{ij}^{-}$$

(12)

$$\mathrm{Tier} 3 \mathrm{objective}: Z_3={\sum }_{j\in \mathit{Tier}3}{w}_j·d_{ij}^{-}$$

(13)

Only salts that achieve $Z_1=0$ (satisfy safety and corrosion resistance thresholds) are evaluated in subsequent tiers. This structure ensures that no amount of improvement in economic or performance metrics can compensate for critical failures in system safety or durability. Following the hybrid weighting and non-linear normalization strategy, the model is applied to the five molten salt candidates that met the Tier 1 threshold conditions (safety ≥ 0.75 and corrosion resistance ≥ 0.75) as shown in

Table 17. Tier 1 candidates.

Salt	${\mathit{s}}_{\mathit{i}\mathit{S}}^{\prime }$	${\mathit{d}}_{\mathit{i}\mathit{s}}^{-}$	${\mathit{s}}_{\mathit{i}\mathit{C}\mathit{R}}^{\prime }$	${\mathit{d}}_{\mathit{i}\mathit{C}\mathit{R}}^{-}$	${\mathit{Z}}_1$
Solar Salt (NaNO-KNO3)	1.000	0.000	1.000	0.000	0.0000
KNO3-NaNO2 (Binary Nitrate)	0.667	0.333	1.000	0.000	0.0328
MgCl2-LiCl (Chloride)	0.333	0.667	0.000	1.000	0.1688
LiNO3-KNO3 (with Nanoparticles)	0.667	0.333	1.000	0.000	0.0328
Ca(NO3)2:NaNO3:KNO3 (32:24:44 wt%)	1.000	0.000	1.000	0.000	0.0000
NaNO2-KNO3 (Binary Nitrite-Nitrate)	0.667	0.333	1.000	0.000	0.0328
LiF-CaF2 (Fluoride)	0.000	1.000	0.333	0.667	0.1672
KCl-MgCl2 (Chloride Mixture)	0.333	0.667	0.000	1.000	0.1688
NaNO3-KNO3-NaNO2 (General Ternary Nitrate)	0.667	0.333	1.000	0.000	0.0328

:

The table shows that only Solar Salt and Ca(NO₃)₂:NaNO₃:KNO₃ achieve perfect scores (i.e., $Z_1=0$) and are passed to Tier 2. To calculate the $Z_2$ performed in Tier 2, the updated score for each criterion is listed in

Table 18. Updated score $s_{\mathit{i}\mathit{j}}^{\prime }$ for each criterion.

Criterion	${\mathit{w}}_{\mathit{j}}$	${\mathit{s}}_{\mathit{i}\mathit{j}}^{\prime }$ Solar Salt	${\mathit{s}}_{\mathit{i}\mathit{j}}^{\prime }$ Ca(NO3)2:NaNO3:KNO3
Melting Point	0.0797	0.501	0.499
Thermal Stability	0.0975	0.212	0.277
Operating Temperature Range	0.0815	0.175	1.000
Specific Heat Capacity	0.0819	0.643	0.671
Thermal Conductivity	0.0799	0.152	0.391
Viscosity	0.0714	0.468	0.381
Density	0.0617	0.583	0.450
Environmental Sustainability	0.0858	1.000	1.000
Cost-effectiveness	0.0887	0.833	0.767
Availability and Scalability	0.0711	1.000	0.750

.

The resulted $Z_2$of the Solar Salt is 0.2808, and for the Ca(NO₃)₂:NaNO₃:KNO₃ is 0.1860. This means that Ca(NO₃)₂:NaNO₃:KNO₃ performs significantly better in the thermophysical trade-offs (Tier 2), largely due to its vastly superior operating temperature range and better thermal conductivity, which result in a lower total deviation score. Moving to Tier 3, $Z_3$ of the Solar Salt is 0.0785, and for the Ca(NO₃)₂:NaNO₃:KNO₃ salt is 0.1166.

While Solar Salt is the most robust choice for Tier 3 (economic and sustainability), Ca(NO₃)₂:NaNO₃:KNO₃ is the clear winner for Tier 2 (thermophysical performance).

Following this approach (threshold-based exclusion) maintains system integrity, it may prematurely exclude promising salts with high recyclability and thermal performance, such as ${\mathrm{N}\mathrm{a}\mathrm{N}\mathrm{O}}_2-{\mathrm{K}\mathrm{N}\mathrm{O}}_3 \mathrm{o}\mathrm{r} {\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{l}}_2-\mathrm{L}\mathrm{i}\mathrm{C}\mathrm{l}$. Also, it may ignore potential for risk mitigation strategies, like using protective alloys or redundancy. Thus, a more comprehensive framework is needed.

This comprehensive approach deals with the 3 Tiers as the main categories to be arranged to give the best rank for the molten salt alternatives. The general approach is to calculate a weighting score for each Tier, considering all salts among all criteria in each Tier. Then a weighted score is calculated based on the following equation:

$$Z_{Total}=w_{T1}·Z_1+w_{T2}·Z_2+w_{T3}·Z_3$$

(14)

where, $w_{T1}, w_{T2}, w_{T3}$ are the Tier weights. These values will be assigned based on the priorities set for off-grid CSP systems in the UAE. The values of Z scores for each molten salt alternative are shown in

Table 19. Z scores for each molten salt alternative.

Molten Salt Alternatives	Z1 (Tier 1)	Z2 (Tier 2)	Z3 (Tier 3)
Solar Salt (NaNO-KNO3)	0.0000	0.2808	0.0785
KNO3-NaNO2 (Binary Nitrate)	0.0328	0.2678	0.0845
MgCl2-LiCl (Chloride)	0.1688	0.2037	0.2396
LiNO3-KNO3 (with Nanoparticles)	0.0328	0.2464	0.1654
Ca(NO3)2:NaNO3:KNO3 (32:24:44 wt%)	0.0000	0.1860	0.1166
NaNO2-KNO3 (Binary Nitrite-Nitrate)	0.0328	0.2842	0.0910
LiF-CaF2 (Fluoride)	0.1672	0.1205	0.3227
KCl-MgCl2 (Chloride Mixture)	0.1688	0.2222	0.1558
NaNO3-KNO3-NaNO2 (General Ternary Nitrate)	0.0328	0.2588	0.0874

.

5. Results

To map the various policy priorities in molten salt selection for off-grid CSP systems in the UAE, five multi-tier weighting scenarios were utilised within the enhanced Lexicographic Goal Programming (LGP) model. The assessment of each scenario relies on unique weight distributions across the three evaluation tiers, as shown in

Table 20. The weight distribution for each Tier in each scenario.

Tier–Scenario	Z1 (Tier 1)	Z2 (Tier 2)	Z3 (Tier 3)
Scenario A–Equal Weight Policy	33.3%	33.3%	33.3%
Scenario B–Safety-Oriented Policy	50%	30%	20%
Scenario C–Performance-Focused Policy	20%	60%	20%
Scenario D–Sustainability Priority	20%	30%	50%
Scenario E–Balanced with Technical Focus	30%	50%	20%

.

The results of Z scores for each alternative in each scenario are summarized in

Table 21. Result of Z scores for each alternative in each scenario.

Salt	Scenario A (Rank)	Scenario B (Rank)	Scenario C (Rank)	Scenario D (Rank)	Scenario E (Rank)
Ca(NO3)2:NaNO3:KNO3	0.1008 (1)	0.0791 (1)	0.1349 (1)	0.1141 (1)	0.1163 (1)
Solar Salt (NaNO3-KNO3)	0.1196 (2)	0.0999 (2)	0.1842 (3)	0.1235 (2)	0.1561 (3)
KNO3-NaNO2	0.1282 (3)	0.1136 (3)	0.1841 (2)	0.1292 (3)	0.1606 (4)
NaNO3-KNO3-NaNO2	0.1294 (4)	0.1138 (4)	0.1783 (4)	0.1309 (4)	0.1565 (2)
LiNO3-KNO3 (with Nanoparticles)	0.1482 (5)	0.1232 (5)	0.1844 (5)	0.1632 (6)	0.1660 (5)
NaNO2-KNO3	0.1360 (6)	0.1197 (6)	0.1939 (6)	0.1354 (5)	0.1700 (6)
LiF-CaF2 (Fluoride)	0.2035 (7)	0.1820 (7)	0.1706 (7)	0.2558 (9)	0.1748 (7)
KCl-MgCl2 (Chloride Mixture)	0.1823 (8)	0.1823 (8)	0.2003 (8)	0.1782 (7)	0.1931 (8)
MgCl2-LiCl (Chloride)	0.2040 (9)	0.1925 (9)	0.2012 (9)	0.1950 (8)	0.2026 (9)

.

The Ca(NO₃)₂:NaNO₃:KNO₃ mixture is the top-performing candidate across most policy scenarios as presented in Table 21, followed by the Solar salt as a second candidate.

6. Discussion

The results of the LGP show that choosing the best molten salt depends on policy-driven priorities as indicated by the tier weighting scenarios. The study reveals that Ca(NO₃)₂:NaNO₃:KNO₃ ranked first in four of five scenarios, including A, B, C, and E, with Solar Salt consistently achieving the second rank. These two nitrate-based mixtures demonstrate strong performance across a range of priority criteria. An analysis of the scenarios provides deeper insight into the trade-off between thermophysical and sustainable metrics.

Scenario A (Equal Weight): Ca(NO₃)₂:NaNO₃:KNO₃ gets the top position because it demonstrates consistent performance across all evaluation categories when each tier has equal influence. Scenario B (Safety-Oriented): The highest importance is assigned to Tier 1. Ca(NO₃)₂:NaNO₃:KNO₃ again ranks first. This salt demonstrates superior performance in essential safety and durability dimensions over competing materials, which matches its established traits and widespread utilization. Scenario C (Performance-Focused): Ca(NO₃)₂:NaNO₃:KNO₃ achieves the best ranking when Tier 2 (Thermal Performance) is given maximum importance. Its superior thermal properties play a major role in its performance outcomes when performance metrics are the primary consideration.

Scenario D (Sustainability Priority): Ca(NO₃)₂:NaNO₃:KNO₃ remains in first place. Even when its weakest tier (Tier 3) is prioritized. This result is significant, as it shows that its outstanding Tier 2 advantage is large enough to overcome the better cost and sustainability scores of Solar Salt. Scenario E (Balanced with Technical Focus Ca(NO₃)₂:NaNO₃:KNO₃ takes the top position when thermal performance receives increased importance because its thermal properties significantly contribute to its overall appeal.

Consistent Lower Ranks: The composite materials MgCl₂-LiCl (Chloride) and KCl-MgCl₂ (Chloride Mixture) always achieve the lowest rankings of 9 and 10 across all evaluated scenarios. Though these materials show possible benefits such as high thermal stability for fluorides, their performance across more important criteria like safety, environmental sustainability, and corrosion causes them to deviate more from policy objectives, which results in lower overall preference. Their lower rankings stem from the significant impact of toxicity associated with fluorides and the severe corrosion caused by chlorides when evaluating safety and durability-focused scenarios.

Mid-Tier Performance: The nitrate-based mixtures, such as General Ternary Nitrate and Binary Nitrate typically typically secure positions in the middle of the rankings, showing variation depending on specific circumstances. This indicates they offer a balance of properties but do not consistently outperform the top two candidates across all priority structures.

The LGP analysis performed under various policy scenarios reveals crucial trade-off information for selecting molten salts to use in off-grid CSP systems throughout the UAE. While Solar Salt demonstrates robust performance, particularly in economic and sustainability metrics (Tier 3), the Ca(NO₃)₂:NaNO₃:KNO₃ mixture emerges as the unequivocally superior choice across all tested scenarios, driven by its dominant performance in the crucial thermophysical criteria (Tier 2). The repeated poor performance of fluoride and chloride salts demonstrates how essential it is to solve safety and corrosion problems to make these substances practical for this specific use. The findings demonstrate the importance of selecting molten salts based on specific policy priorities while offering data-driven guidance to decision-makers in the UAE renewable energy sector.

7. Conclusions

The study introduces a systematic method for selecting molten salts that perform dual roles as HTF and TES in off-grid parabolic trough CSP systems while addressing both the UAE’s arid climate and its sustainability objectives. The combined multiple criteria decision-making framework that includes Pareto front analysis, CRITIC, SWARA, and Goal Programming presents a reliable and clear methodology that extends beyond the UAE case study. The Ca(NO₃)₂:NaNO₃:KNO₃ mixture consistently emerges as the top-performing candidate across all policy scenarios, driven by its superior thermophysical properties. Solar Salt is a robust second choice, offering reliable safety and excelling in economic and sustainability metrics, but the ultimate choice must match the priorities of the use case. The UAE needs to weigh the benefits of higher thermal performance (offered by Ca(NO₃)₂:NaNO₃:KNO₃) against better cost-effectiveness and availability (offered by Solar Salt) to achieve its sustainable development objectives. Future research should expand the use of this framework by performing detailed life cycle evaluations of molten salts and examining CSP system performance during different operational scenarios.

A limitation of this study is the use of a fixed 60– 40% weighting ratio to combine the qualitative and quantitative weights. This ratio was chosen to highlight the strategic policy insights obtained from the expert panel (particularly from stakeholders with strategic, technical, and policy expertise) while maintaining the analysis of objective data. The justification of this weighing is threefold: (1) Limited long-term operational data for many molten salts, specifically within the UAE’s unique arid climate. This makes the expert experience more valuable. (2) As Al-Aomar stated in his research that the off-grid CSP systems are reliability-critical, and this requires more engineering judgment for the key decisions that may not be fully captured by statistical data alone [183]. (3) The process of aligning the sustainability and policy goals needs a qualitative expert assessment that extends beyond statistical correlation.

However, a comprehensive sensitivity analysis, which would test various weighting combinations (e.g., 50/50, 70/30), was beyond the scope of the current work but is crucial and strongly recommended for future research to confirm the robustness of the findings.