High-Temperature Molten Salt Heat Exchanger Technology: Research Advances, Challenges, and Future Perspectives

Abstract

Molten salt heat exchangers are pivotal components in advanced energy systems, where their high-temperature stability and efficient heat transfer performance are critical for system reliability. This paper provides a comprehensive review of recent advancements in molten salt heat exchanger technology, focusing on their application in nuclear energy, concentrated solar power, and thermal energy storage systems. Key design considerations, including thermophysical properties of molten salts and operational conditions, are analyzed to highlight performance optimization strategies. The review traces the evolution from traditional shell-and-tube heat exchangers to compact designs like printed circuit heat exchangers, emphasizing improvements in heat transfer efficiency and power density. Challenges such as material corrosion, manufacturing complexities, and flow dynamics are critically examined. Furthermore, future research directions are proposed, including the development of high-performance materials, advanced manufacturing techniques, and optimized geometries. This review aims to consolidate dispersed research findings, address technological bottlenecks, and provide a roadmap for the continued development of molten salt heat exchangers in high-temperature energy systems.

1. Introduction

The series of climate issues caused by the greenhouse effect is increasingly driving the global energy transition [1]. Currently, the global energy landscape is characterized by the rapid rise in low-carbon energy sources such as solar and nuclear power [2,3], while traditional energy sources like coal, oil, and natural gas are gradually being replaced, though they remain significant in the energy mix [4,5]. The energy transition faces critical challenges. First, replacing fossil fuels with renewable energy requires mitigating the intermittency and volatility of renewables to ensure grid stability [6]. Second, improving the efficiency of thermal power plants remains imperative for carbon emission reduction, as they still provide over 67% of global electricity generation [7]. However, these plants are constrained by the performance limits of traditional working fluids [8].

High-temperature molten salts (MS) could address these challenges. For example, MS-based thermal energy storage (TES) enables renewable dispatchability in concentrated solar power (CSP) plants because of their superior temperature stability and heat transfer properties [9]. Furthermore, MS has high heat capacity, high working temperature limit, and low vapor pressure [10,11,12], which could allow efficient energy conversion at high temperature for the molten salt reactors (MSR) and CSP plants [13,14]. However, due to the strong corrosiveness, MS cannot be used directly via heat exchangers (HXs), where MS transfers heat to secondary fluids (e.g., water or supercritical CO₂) for power cycles. Thus, MS-HXs not only affect the overall energy conversion and utilization efficiency but also prevent the corrosion of power generation equipment by MS. Therefore, they are pivotal in the sectors of TES, MSR, and CSP.

Research on MS-HXs has grown significantly due to their critical role in energy systems. Currently, shell-and-tube type heat exchangers (STHXs) dominate industrial applications, with most studies enhancing heat transfer efficiency through surface optimizations. However, to address the low power density and effectiveness, recent efforts have shifted toward compact designs. Printed Circuit Heat Exchangers (PCHEs), a compact design, have gained attention due to their high power density, excellent thermal efficiency, and ability to operate under high-pressure and temperature conditions [15,16]. While extensive research has been conducted through experimental, numerical, and structural optimization methods, the field lacks comprehensive reviews that critically analyze these dispersed findings, reveal existing problems, pinpoint technological bottlenecks, and propose a feasibility-validated development roadmap. This study aims to fill these key gaps. First, the application fields of MS-HXs are introduced. Then, MS and its properties are concluded. Next, the research status of MS-HXs is stated. At last, the challenges and future perspectives of MS-HXs are pointed out.

2. Application Field of MS-HXs

The application scenarios of MS-HXs can be primarily categorized into three areas: MSR, CSP, and TES. These application areas are receiving significant attention worldwide and play a crucial role in accelerating the development and utilization of low-carbon energy, as well as in environmental and climate protection efforts [17].

2.1. Molten Salt Heat Exchangers in Molten Salt Reactors

The MSR technology originated from a series of studies at the Oak Ridge National Laboratory [18,19,20,21,22], which validated MS’s dual roles as coolant and fuel solvent [23,24]. After the Generation IV International Forum prioritized MSRs as a key development direction in 2001 [25,26,27], China’s TMSR-LF1 (Thorium-based Molten Salt Reactor-Liquid Fuel) reactor (2024) demonstrated thorium-uranium breeding at 2 MWt/650 °C [28].

The MSR power generation has three loops: the primary loop, the intermediate loop, and the power loop. Figure 1 illustrates the components of the MSR system. Molten salt is served as the fuel solvent in the primary and intermediate loops and coolant, while water is served as the working fluid in the power loop. There are two MS-HXs in the intermediate loop: one is the intermediate heat exchanger, and the other is the evaporator heat exchanger. Heat from the core is first transferred to the intermediate loop through the intermediate heat exchanger, and then transferred to the power loop through the evaporator heat exchanger. The intermediate loop design of the MSR reduces the temperature difference between the molten salt and the working fluid, lowering thermal stress on the devices, optimizing the operating temperature of the working fluid, and preventing radioactive elements from damaging the power system [29].

In recent years, CO₂ serving as the working fluid for the power loop has been investigated gradually because the supercritical CO₂ Brayton cycle has high efficiency, compact, and flexible features. The compression work in the supercritical CO₂ Brayton cycle is relatively low, thereby enabling high thermal efficiency (46.3–49.4%) in the high-temperature operating environment of the MSR [30,31,32,33]. Additionally, the compact turbine machinery not only reduces manufacturing costs but also facilitates modular design, enhancing system flexibility [34,35,36]. The supercritical carbon dioxide Brayton cycle can improve the utilization efficiency of low-carbon energy.

The stability of long-term operation is an important indicator for evaluating nuclear power systems. Long-term stability data for molten salt reactors (MSRs) are primarily derived from records of the Molten Salt Reactor Experiment (MSRE) conducted at Oak Ridge National Laboratory [37]. From reaching first criticality in 1965 until the end of operation in 1969, the MSRE accumulated over 15,000 h of operation, including more than 9000 h at full power. This extensive operational record robustly demonstrates the feasibility and reliability of the molten salt reactor concept. Throughout long-term operation, the chemical properties of the fuel salt remained stable; no uranium precipitation or significant accumulation of oxides—which could potentially impair reactor performance—was observed. Regarding materials, the Hastelloy-N alloy vessel exhibited a chromium depletion of only 47 ppm over three years (corresponding to a metal loss of <0.2 mil), and retained mechanical integrity after irradiation in accordance with design expectations. From a safety perspective, a dual-containment system (including a main circuit with zero leakage, confirmed by welded construction, and a reactor cell leakage rate of less than 1% per day) together with a freeze-valve emergency salt drainage system (activated 10–15 min after loss of power) ensured robust accident controllability.

2.2. MS-HXs in Concentrated Solar Power Systems

The CSP and associated thermal technologies decouple the direct conversion process from light energy to electrical energy, overcoming the limitations of photovoltaic power plants [38,39], and are emerging as pivotal solutions for dispatchable renewable energy provision.

Currently, scalable CSP systems mainly include tower and trough systems (Figure 2), both of which can use molten salt as the heat transfer medium [40,41]. Key components of a CSP system include heliostats, receiver, thermal energy storage tanks, and a power generation system, shown in Figure 3.

The molten salt, which has absorbed solar thermal energy, flows from the receiver into the high-temperature thermal energy storage tank. When power generation is required, the high-temperature molten salt is pumped into the steam generator, where it undergoes countercurrent heat exchange with the working fluid of the power generation system. Water is evaporated into high-temperature steam, which then drives the turbine to generate power. The low-temperature molten salt after heat exchange is pumped into the low-temperature thermal energy storage tank. Like the MSR system, CSP systems with extremely high operating temperatures also have the potential for collaboration with the supercritical CO₂ Brayton cycle.

In summary, the MS-HXs in a CSP system should have good heat transfer efficiency while also being capable of withstanding high temperatures, high thermal loads, and thermal stresses caused by temperature differences. High-performance MS-HXs are the essential hardware to ensure that CSP systems achieve high energy conversion efficiency.

2.3. MS-HXs in Thermal Energy Storage Systems

As renewable energy (especially solar and wind) accounts for a larger share of power generation [43,44], its inherent volatility and intermittency increasingly strain grid infrastructure. Curtailment of photovoltaic and wind power remains a persistent issue [45,46], which energy storage can mitigate this. The molten salt thermal energy storage system (MS-TES), with advantages like high energy density, low cost, and long cycle life, is a promising solution [47].

Additionally, it can store industrial waste heat and other forms of energy (Figure 4) [48]. Industrial waste heat and other thermal energy sources can be directly absorbed by molten salt. In contrast, photovoltaic or wind power must first be converted into thermal energy through a molten salt electric heater. The heated molten salt then flows into the high-temperature storage tank, completing the energy storage process. During power demand, the high-temperature molten salt is used to evaporate water into steam via a preheating heat exchanger. The steam subsequently passes through a superheater heat exchanger, where molten salt further heats it to control the dryness fraction, ensuring the steam meets the requirements for efficient Rankine cycle operation. When the molten salt temperature exceeds 500 °C, carbon dioxide can serve as an efficient working fluid, with its high density and low viscosity characteristics further enhancing the thermal-to-electric conversion efficiency.

2.4. Safety Evaluation and Analysis of Molten Salt Energy Systems

The energy production scenarios in which high-temperature molten salt technology is applied are highly safe and suitable for large-scale promotion. Take the molten salt reactor as an example, compared with the third generation nuclear power technology represented by light water pressurized water reactor, using molten salt as a heat transfer medium to replace water makes the system operating temperature more than double and realizes higher thermal efficiency. The operating pressure of the system is much lower than that of pressurized water reactors, which greatly reduces the risk of accidents such as high-pressure explosions and also realizes a more flexible fuel cycle and lower risk of radioactive leakage. Compared with liquid metal fast reactors, which are also a Generation IV nuclear power technology, molten salt as a heat transfer medium has higher chemical safety, a wider safety temperature range, better radiation control, more convenient waste management, and fuel cycle programs than liquid metal.

In the field of CSP solar power generation, molten salt replaces the previous generation of synthetic oil as the work material, with superior high temperature resistance to fully utilize the solar energy, not easy to decompose to reduce the fire hazard, larger heat capacity to make the system greatly enhance the potential of energy storage, and lower cost, less impact on the environment. Although the current molten salt application technology is still facing the corrosion of molten salt on metal equipment and low-temperature solidification blockage and other issues, the synergistic cooperation of multiple fields of research to make these problems have a more optimistic solution. And these problems have been solved in the long-term operation of the molten salt energy system.

3. Selection of Molten Salts

The selection of molten salt working fluids is application-dependent. Industrially, they are categorized by mixture formulations, which can be tailored to regulate ionic interactions, phase transitions, and thermalchemical properties [49,50,51]. Customized compositions address specific operational demands (e.g., high-temperature stability, corrosion inhibition, or cost-effectiveness). A systematic development protocol includes the following: (1) Objective specification: Defining functional requirements (thermal storage/heat transfer/nuclear cooling) and performance benchmarks (thermal conductivity, chemical inertness, and cost metrics). (2) Computational Design: Selecting components via eutectic phase diagrams and molecular dynamics simulations [52], prioritizing low toxicity and resource abundance. (3) Experimental Validation: Lab-scale synthesis and characterization (melting point, specific heat, thermal degradation thresholds), followed by corrosion screening and loop-scale dynamic testing. (4) Industrial Scaling: Formulation refinement based on long-term stability assessments [53], balancing performance and scalability. Subsequent sections evaluate prevalent molten salt system and their thermophysical constraints in heat exchanger design.

3.1. MS Used in Molten Salt Reactors

Molten salt photos refer to Figure 5.

The concept of using molten salt as a nuclear fuel for liquid fuel reactors was first publicly proposed by Weinberg and others at ORNL [19]. This design highlighted the use of fluoride molten salts as the reactor core liquid fuel. Researchers chose fluoride salts due to their low neutron absorption cross-section, which helps maintain a good neutron economy [56]. This ensures efficient nuclear reactions with minimal neutron loss. Additionally, fluoride salts can dissolve certain nuclear fuels, such as UF₄, allowing the fuel to be directly incorporated into the salt. This flexibility makes liquid fuel reactors particularly advantageous for online fuel processing and dynamic adjustment of fuel composition.

Grimes and others [57] suggested that FLiNaK salt (LiF-NaF-KF, 46.5-11.5-42 mol%) could be a promising coolant for molten salt reactors. Its low melting point, high heat capacity, and high-temperature stability make it suitable for heat transfer applications. However, after extensive experimental testing, researchers ultimately selected FLiBe (LiF-BeF₂, 66-34 mol%) as the preferred molten salt. FLiBe not only exhibits superior neutron moderation properties but also demonstrates excellent compatibility with radioactive environments. It can act as both a coolant and a fuel solvent, dissolving and carrying nuclear fuel within the reactor core. Furthermore, FLiBe’s low hygroscopicity and water solubility make it ideal for online nuclear fuel processing [58]. This molten salt was further utilized in the TMSR-LF1 project, part of the thorium-based molten salt reactor system research led by the Chinese Academy of Sciences. The choice of FLiBe reflects its dual role in maintaining neutron economy and enabling efficient fuel management, making it a critical component in modern molten salt reactor designs.

Although FLiBe molten salt has advantages over FLiNaK salt in terms of its applicability as a liquid fuel and core coolant in molten salt reactors, FLiNaK salt is also used as an ideal secondary coolant in these systems. FLiNaK offers several benefits, including lower toxicity, simpler processing requirements, and a more favorable specific heat capacity. As a secondary coolant, FLiNaK replaces FLiBe in the secondary loop to extract heat from the fuel solvent, thereby reducing potential environmental and equipment hazards.

Based on recent research on molten salt reactor working fluids and operating conditions [22,59,60,61], this study has compiled the working fluids and conditions for molten salt heat exchangers, as shown in

Table 1. Molten salts and operating conditions used in the primary and secondary loops of molten salt reactors.

	High-Temperature Working Fluid	Low-Temperature Working Fluid
Primary Loop Intermediate Heat Exchanger	FLiBe	FLiNaK or FLiBe
Conditions at the Working Fluid Inlet	T: 600 °C~700 °C P: 1~2 bar	T: 500 °C P: 1.5 bar
Secondary Loop Molten Salt-Water Evaporator	FLiNaK, FLiBe	Water or Steam
Conditions at the Working Fluid Inlet	T: 550 °C~600 °C P: 1.5 bar	T: 150 °C P: 1.5 Mpa

.

There has been continuous exploration of the properties of molten salts, and the experimental results for the thermal properties of the same molten salt often vary. This discrepancy is related to differences in experimental equipment, operational methods, molten salt purity, fitting methods, etc. Therefore, it is highly meaningful to organize, analyze, compare, and evaluate the credibility of the results from these independent studies. Romatoski et al. [22], to further promote the use of fluoride salts in fluoride salt-cooled high-temperature reactors, a large body of experimental research findings on FLiBe and FLiNaK provides helpful references for the properties of these two working fluids. In their study, they provided some suggested values, and detailed data is shown in

Table 2. Thermal Properties of FLiNaK Molten Salt and FLiBe Molten Salt.

Thermophysical Property	FLiBe 1	FLiNaK 2
Density (kg/m3)	ρ = 2413 − 0.488 × T	ρ = 2579 – 0.624 × T
Specific heat (J/kg-K)	2386 ± 3%	1884
Thermal conductivity (W/m-K)	1.1	K = 0.36 + 0.00056 × T
Viscosity (cP)	µ = 0.116 e3755/T	μ = 0.04 e4170/T

¹ (66 LiF-34 BeF₂) mol%. ² (46.5 LiF-11.5 NaF-42 KF) mol%.

.

A comparison of the thermophysical values of molten salts reveals a strong correlation with their type, specifically the proportion of molten salt formulation. In addition to these properties, which are critical for heat transfer media, different molten salts also vary in terms of corrosion capacity, toxicity, cost, and so forth. This underscores the necessity of judicious selection of molten salt based on the specific parameters of the application scenario. This underscores the necessity of determining the most suitable molten salt for a given application scenario. The aforementioned factors contribute to an increase in the complexity of the design and manufacture of molten salt heat exchangers.

3.2. MS Used in CSP Systems and TES Systems

The MS-HXs in CSP systems and TES systems generally use similar molten salts, as both often operate as an integrated system. However, differences may arise when TES utilizes low-grade heat sources (e.g., industrial waste heat) instead of solar energy, leading to lower operating temperatures.

Molten salts are preferred in CSP due to their wide temperature range, high energy storage density, and low cost. The first large-scale CSP system, Solar One, used water/steam as the working fluid but faced challenges like intermittent operation and low efficiency. This led to the adoption of Solar Salt (60% NaNO₃ and 40% KNO₃) in the Solar Two project, which proved molten salt’s dual role as a heat transfer fluid and energy storage medium [62].

To further optimize performance, the Themis tower solar system in France tested Hitec (40% NaNO₂, 7% NaNO₃, 53% KNO₃) as a potential Solar Salt replacement [63]. Hitec’s melting point (142–145 °C) is significantly lower than Solar Salt’s 220 °C, reducing freezing risks. However, its maximum operating temperature is theoretically 538 °C, though practical systems often operate below 500 °C to avoid NaNO₂ oxidation. An even lower-melting-point alternative, Hitec XL (48% Ca(NO₃)₂, 7% NaNO₃, 45% KNO₃), was developed with a melting point of 120–130 °C, but its thermal stability degrades above 505 °C due to calcium nitrate decomposition. These trade-offs highlight the challenge of balancing low-temperature fluidity and high-temperature stability in molten salt design.

Due to the widespread use of Solar Salt and Hitec, extensive research on their thermal properties has been conducted over the past decades. However, like other molten salts, experimental results for these materials show significant variations between different studies. To address this issue, it is important to systematically organize and evaluate data from these independent investigations. For example, González et al. [64], together with other researchers [65,66,67,68], have comprehensively summarized the thermal properties of the three most commonly used molten salt mixtures in CSP systems. Relevant data is shown in

Table 3. Thermal properties of the three most commonly used molten salts in CSP systems and molten salt thermal energy storage systems.

Thermophysical Property	Solar Salt 1	Hitec 2	Hitec XL 3
Density (kg/m3) @300 °C	1889	1860	1992
Specific heat (J/kg-K) @300 °C	1495	1560	1447
Thermal conductivity (W/m-K)	0.5	0.48	0.43
Viscosity (cP) @300 °C	3.26	3.16	6.37

¹ (60 NaNO₃-40 KNO₃) weight%. ² (7 NaNO₃-53 KNO₃-40 NaNO₂) weight%. ³ (7 NaNO₃-45 KNO₃-48 Ca(NO₃)₂) weight%.

below.

Many researchers have investigated the potential application of chloride molten salts in CSP systems [69,70]. These salts offer advantages, including higher operating temperatures (typically above 700 °C) and lower material costs compared to conventional nitrate salts. Wang et al. [71] compared the heat transfer effectiveness of chloride salts with that of carbonates through numerical simulation studies and found that the overall performance of chloride salts is about 70% higher than that of carbonates in the application scenario of solar thermal power generation. However, their commercial adoption in CSP systems has been hindered primarily by severe corrosivity toward stainless steel and nickel-based alloys at high temperatures. Current research efforts are focused on developing effective corrosion mitigation strategies, including material coatings and composition optimization.

By combining the operational data of actual molten salt energy systems, this paper provides statistics on the characteristic temperatures of a variety of molten salts when used as heat transfer agents (

Table 4. The temperature range related to molten salt.

Heat Transfer Medium Temperature	FliBe	FliNaK	Solar Salt	Hitec	Hitec XL
The upper limit working temperature (°C)	663~700	700	565	538	425
The lower limit temperature (°C)	ca. 635	ca. 600	ca. 290	ca. 200	ca. 200
Boiling point temperature (°C)	1430	1570	—	873	—
Freezing point temperature (°C)	459	462	220	142	120

). These temperatures help the reader understand more intuitively the relationship between the operating temperature ranges of the molten salt agents and their melting and boiling points, and it is important to note that these data are estimates used as a reference, and there may be slight deviations.

4. Design and Optimization of MS-HXs

MS-HXs function as critical thermal management components in energy systems, enabling 15–20% thermodynamic efficiency improvements in power cycles and effective decarbonization through waste heat valorization. In renewable energy integration, advanced MS-HX designs couple thermal storage with power generation in CSP plants and stabilize grid output by providing 6–12 h of dispatchable power. Optimized waste heat recovery systems demonstrate 25–30% energy savings in heavy industries [72], successfully reinjecting low-grade heat (<300 °C) into production processes.

Currently, the design, research, and performance optimization of MS-HXs are characterized by multi-disciplinary collaboration, parallel optimization paths pursuing the same objectives, and rapid technological iteration in the industry. Both the quantity of related studies and published results show consistent annual growth, as systematically summarized in Figure 6.

Innovations in geometric structures and flow configurations (such as compact microchannel designs) directly address scaling challenges in emerging technologies like carbon capture and green hydrogen production. Industry-related research has evolved from designing heat exchangers for specific application scenarios to optimizing energy utilization efficiency, manufacturing technologies, and cost reductions in more mature technological directions. At present, there is an increasing focus on the growing demand for modularity in molten salt heat exchangers, alongside improved requirements for service life, maintenance convenience, and other operational aspects. From a materials science perspective, corrosion-resistant and high-temperature-resistant heat exchanger materials (such as nickel-based alloys or ceramic composites) significantly extend the equipment life in extreme environments (e.g., nuclear reactors) [27].

4.1. Shell-and-Tube MS-HXs

Shell-and-tube HXs represent the most prevalent MS-HX design across various application scenarios (Figure 7). As surface-type heat exchangers, they enable indirect heat transfer between molten salt and secondary fluids (e.g., water/steam, thermal oil, or compressed air) through isolated flow paths comprising a pressure vessel shell and tube bundles. This configuration dominates nuclear and CSP applications due to its unmatched reliability.

Advantages of Shell-and-Tube MS-HXs:

• High-temperature/pressure capability;
• Exceptional thermal cycle resistance;
• Streamlined fabrication using standardized components;
• Maintenance accessibility through removable tube bundles.

Beginning in 1962, the ORNL conducted extensive design work on shell-and-tube MS-HXs to support MS reactor development. They documented the operational conditions, manufacturing methods (such as welding techniques), and technical details of the tube bundle layout configurations used in these engineering projects. To address these requirements, they developed standard heat exchanger variants, such as MS evaporators (ΔT = 150–300 °C) and superheaters with a 50 °C approach temperature. Romanos and Kinyon [74] optimized the pressure distribution to design a high-temperature shell-and-tube steam generator compatible with molten salt/liquid metals HTFs, adapting the conventional shell-and-tube HX principles. Faugeras [75] proposed a novel STHX for low-capacity steam generation in light water reactors (LWRs), in which molten salt and water flow co-currently in separate tubes, and heat transfer occurs via an inert gas-filled shell side. Although patented, neither design reached commercialization due to technical and economic barriers.

In 2011, the Chinese Academy of Sciences adopted a tube-and-shell gas-cooled molten salt heat exchanger for the 2 MWt Thorium Molten Salt Reactor (TMSR) pilot project [76,77]. This heat exchanger employs high-density bundling and triangular tube arrangement with segmented baffles to enhance shell-side flow, and therefore, the footprint is 30~40% smaller compared to traditional STHXs. However, there are still some heat transfer efficiency limitations, such as dead zones and flow short-circuiting in the shell side, significant heat transfer coefficient disparity, and small mean temperature difference.

In CSP (concentrated solar power) systems, Spain’s 19.9 MW Gemasolar plant (2011) [78] demonstrated 24/7 baseload operation using 15 h molten salt storage—a milestone enabled by high-efficiency (>99%) MS-HXs. This breakthrough triggered global CSP deployments, with over 15 countries (including China, the US, and the UAE) launching projects. The rapid growth of renewable energy has significantly accelerated molten salt heat exchanger research.

Kong et al. [79,80] constructed a molten salt test loop (Figure 8) to investigate shell-and-tube molten salt-to-gas HXs. The results show that gas-side resistance is larger than molten salt-side resistance, dictating the overall performance of the heat exchanger. To address this, they developed an enhanced design featuring helical baffles (25° inclination) for shell-side and externally finned tubes (8 fins/inch) for gas-side. 500 h tests demonstrated molten salt’s superiority as a shell-side fluid, achieving 25% higher HTC than heat transfer oils.

Khedher et al. [81,82,83] studied ways to increase the melting and solidification rates of energy storage materials by optimizing the design of the heat exchanger. This improves the overall energy efficiency of the energy storage system.

He et al. [84] established an MWth-scale thermal loop to test MS-HXs, systematically quantifying turbulent heat transfer characteristics of Hitec molten salt in tube-side flow (Re = 5000–50,000). Their data revealed <10% deviation in Nusselt number correlations between Hitec salt and water, validating water as a cost-effective surrogate for molten salt heat transfer studies. Extending to shell-side applications, He et al. [85] developed a molten salt-specific heat transfer correlation and demonstrated that bow-shaped baffles (Figure 9b)could enhance shell-side HTCs by 35–40% at low Reynolds numbers (Re < 2000).

In addition to conventional baffle designs for shell-side heat transfer enhancement, Zhang et al. [86] later introduced rod baffles (arranged in a lattice pattern) were introduced to reduce the 40~60% shell-side pressure drop, eliminate flow-induced vibration (FIV) risks, and reduce dead zones (Figure 9c). By modifying the Colburn-j factor for undisturbed flow conditions, they derived a new heat transfer correlation specifically for molten salt flow with rod baffles, achieving ±12% prediction accuracy against experimental data.

Some similar work exists, while Xie et al. [87] designed a variety of perforated shell-range baffles and used numerical simulation methods for scheme optimization, combined with genetic algorithms to optimize the opening diameters of the perforations, which achieved the goal of reducing the heat exchanger’s flow dead zones and lowering the flow resistance. And Zhang et al. [88] applied similar perforated baffles to molten salt electric heaters and optimized the best scheme, which led to an improvement in the overall performance of the electric heaters by 15.29% to 17.18% compared to the pre-optimization period.

Figure 9. Enhanced heat transfer structure of shell-and-tube heat exchanger: (a) structure diagram of shell-and-tube molten salt heat exchanger [85]; (b) Baffle plate in shell-and-tube molten salt heat exchanger [85]; (c) Baffle rod in shell-and-tube molten salt heat exchanger [86]; (d) transverse tube bundle in heat exchanger [89].

Figure 9. Enhanced heat transfer structure of shell-and-tube heat exchanger: (a) structure diagram of shell-and-tube molten salt heat exchanger [85]; (b) Baffle plate in shell-and-tube molten salt heat exchanger [85]; (c) Baffle rod in shell-and-tube molten salt heat exchanger [86]; (d) transverse tube bundle in heat exchanger [89].

Beyond shell-side flow disruptors, researchers have investigated inlet condition effects on STHX performance. Bi et al. [89] engineered a transverse molten salt flow (0.5–2 m/s) across tube bundles in a Solar Salt steam generator. They found that when MS inlet temperature elevation and water flow rate augmentation are both enhanced, the Nusselt number (Nu) is increased by 15–30% at Re = 5000–20,000. System-level analysis also revealed that 70% of thermal resistance stems from shell-side flow maldistribution. Therefore, they prioritized shell-side optimization for CSP plants requiring rapid load-following and fluctuating inputs. In follow-up work, Bi et al. [90] investigated heat transfer in a concentric tube-in-tube configuration (inner Φ20 mm/annular gap 5 mm) under varying thermal conditions. They found that Nu is increased by 10% when the molten salt temperature is increased from 400 °C to 500 °C. They also suggested that the design of molten salt heat exchangers should prioritize controlling the molten salt temperature rather than the pressure, to limit thermal boundary layer growth. Bi et al. [91] then systematically mapped heat transfer coefficients for subcooled boiling, saturated boiling, and superheated vapor regions. This work addressed prior gaps by using Solar Salt and characterizing two-phase water flow. The data directly informs CSP steam generator design for high-pressure operation and load-following control.

Zeng et al. [92] studied the stress distribution characteristics of the heat exchanger components of a molten salt-water vapor generator applied in a solar thermal power station, and concluded that the heat transfer efficiency and stress deformation degree of the heat exchanger perform better when the water vapor is located on the tube side and the molten salt is located on the shell side. Liu et al. [93] used the dryness and pressure of water vapor in the molten salt-water heat exchanger as control variables to analyze the effect of the inflow conditions of the cold-side and hot-side work materials of the heat exchanger on the heat transfer performance, which provided a reference for optimizing the energy production efficiency.

Ding et al. [94] set up a molten salt loop to study the convective heat transfer characteristics of ternary nitrates outside a tube bundle, comparing it with smooth round tubes. Their experiment demonstrated the enhanced heat transfer performance of the tube bundle structure, promoting its application in STHXs, as supported by prior research on shell-side disturbance structures. In a subsequent study [95], they monitored the thermal performance of a steam generator by varying the molten salt inlet temperature, flow rate, and water inlet temperature. They found that as the molten salt flow velocity increased, the overall energy transfer efficiency initially rose but then declined, indicating an optimal flow rate and temperature. This trend was attributed to the expansion of the boiling region at higher water inlet temperatures, whereas excessively high molten salt temperatures or flow rates reduced efficiency due to heat loss or degraded heat transfer. Their conclusions highlighted the need to balance parameters like flow rate and temperature in designing molten salt steam generators to maximize both steam production and energy efficiency.

In parallel, improvements to shell-and-tube MS-HXs have been experimentally verified through the tube-side optimizations. For instance, Ding et al. [96] developed a U-tube bundle heat exchanger, which improved molten salt heat transfer efficiency by optimizing flow distribution, and identified the optimal inlet temperature and velocity. In a subsequent study [97], the same team designed a coiled-tube steam generator (Figure 10), achieving over 90% thermal efficiency under optimized molten salt temperature and water flow conditions. They further demonstrated that forced convection of molten salt increases the overall heat transfer coefficient but also raises heat loss. Jiang et al. [98] designed a molten salt–coal flue gas heat exchanger for use under molten salt thermal energy storage conditions, and focused on analyzing the thermal lag effect of the heat exchanger from the perspective of dynamic response.

Chen et al. [99] designed an STHX with periodic transverse grooves on the tube’s side (Figure 11a). Through experiments spanning Reynolds numbers of 300–60,000 and Prandtl numbers of 11–27, they demonstrated that the grooved tubes not only significantly enhanced heat transfer performance but also reduced the critical Reynolds number for laminar-to-turbulent transition. They further proposed heat transfer correlations for different flow states, providing key data for designing MS-HXs, especially those operating near the transition regime. In a follow-up study, Chen et al. [100] addressed molten salt reactor cooling needs by developing a serpentine-coil molten salt-to-air heat exchanger (Figure 11b). Their analysis revealed that the air-side thermal resistance contributed over 80% of the total thermal resistance, highlighting the importance of optimizing air-side flow (e.g., through finned surfaces) to improve overall efficiency.

Chen et al. [101] redesigned a STHX and studied fluoride salt heat transfer in a hairpin tube bundle (Figure 11c). They found that at high Prandtl numbers, molten salt behavior deviated from standard correlations due to boundary layer disruption from the dense packing and U-bends, enhancing heat transfer performance.

Chen et al. [102] designed and fabricated a fluoride salt-air heat exchanger with multiple rows of tube bundles arranged in a staggered manner for the FLiNaK molten salt applied in the cooling circuit of TMSR-LF1, and combined numerical simulations with experimental results to investigate the heat transfer process of the molten salt in this heat exchanger. After that, Li et al. [77] designed a new S-type tube bundle air-cooled fluoride salt heat exchanger with an integrated airflow deflector, which can improve the heat transfer efficiency while ensuring that the molten salt can be removed by gravity when the system is shut down, and it has excellent anti-freezing and plugging performance.

Since flow and heat transfer in smooth circular tubes exhibit highly predictable behavior, researchers at the Indira Gandhi Centre for Atomic Research (IGCAR) [103] simplified shell-and-tube MS-HXs in nuclear reactors using a porous-medium model. By approximating the tube bundles as a porous domain, they characterized flow resistance and heat transfer through porosity and permeability parameters. This approach enabled efficient transient event simulations, providing a robust numerical framework for safety analysis in complex engineering systems.

Hamdeh et al. [104] investigated the heat transfer characteristics of two-phase molten salt-water flow in parallel-flow STHXs through numerical simulations. Using a validated one-dimensional steady-state model, they systematically analyzed the heat exchange between shell-side molten salt and tube-side saturated water. Their results demonstrated that molten salt flow rate critically influenced the thermodynamic behavior in the two-phase zone. When the flow rate exceeded a threshold value, the system achieved complete water evaporation. The analysis revealed that the two-phase heat transfer coefficient decayed exponentially along the tube side. Although higher molten salt flow rates initially enhanced heat transfer, the coefficient subsequently dropped by over 80% post-evaporation. By integrating experimental correlations, they developed a phase-change heat transfer prediction model, highlighting the importance of flow rate control in managing the transition from two-phase flow to superheated steam.

Bonilla et al. [105] developed and compared four dynamic heat transfer models for a dual-unit shell-and-tube MS-HX in solar TES systems. By comparing with experimental data from the molten-salt test platform, they verified the models’ accuracy and applicability under both steady-state and transient conditions. He et al. [106] focused on the heat transfer characteristics in the transition zone of an STHX without baffles. Through experimental data and numerical simulations, they analyzed the variations in Nusselt number and Reynolds number for Hitec molten salt on the shell side under transition flow conditions. Using the Wilson plot method, they established two new empirical correlations. Their simulations not only confirmed the applicability of the RNG k-ε turbulence model but also demonstrated that the temperature gradient distortion near the shell-side inlet and outlet areas resulted from flow dead zones.

Research combining multi-objective optimization algorithms with CFD for heat exchanger optimization is rapidly advancing. For instance, Ozden et al. [107] aimed to increase the shell-side heat transfer coefficient while reducing pressure drop. They employed a particle swarm optimization algorithm to optimize structural parameters such as the tube spacing, number of tube rows, and tube bundle rotation angle, significantly improving the overall heat transfer performance. Although this study did not focus on molten salt fluids, its optimization methods remain applicable to shell-and-tube MS-HXs, as the underlying fluid dynamics principles are similar. Furthermore, numerical simulations of shell-and-tube MS-HXs are increasingly exploring microscopic mechanisms, such as local turbulence and thermal boundary layer effects. Selected results from these studies are presented in Figure 12.

Nuerlan et al. [108] overcame the limitations of traditional system-level simulations in their study of a sodium-cooled fast reactor coupled with a molten salt thermal storage system. They employed a distributed-parameter model to discretize the heat exchanger into multiple control volumes and established a three-dimensional thermal-fluid–structure coupling model. This approach enabled accurate prediction of molten salt velocity field effects on tube bundle vibrations, offering new perspectives for heat exchanger structural optimization. Their multi-scale simulation methodology, spanning from system to component levels, represents an important transition in molten salt heat transfer research from macroscopic characteristics to microscopic mechanism analysis.

4.2. Compact MS-HXs

Since the rise in CSP systems and molten salt thermal storage, there has been an increasing demand for improved comprehensive heat transfer efficiency of MS-HXs in energy system operations. Traditional ST-MS-HXs face several limitations, including relatively low heat transfer efficiency, large physical footprint, and slow thermal response, all of which constrain overall system efficiency improvements in applications like molten salt thermal storage.

Compact heat exchangers (CHEs) offer significant advantages over traditional designs:

• Reduced material requirements: CHEs use less material in manufacturing, lowering production costs.
• Compact size and weight: Their smaller dimensions and reduced weight simplify system integration and transportation.
• Faster thermal response: With lower thermal inertia, CHEs provide quicker response times, enabling systems to adapt more flexibly to load variations.
• Modular design: The inherent modularity of CHEs facilitates easy system expansion and maintenance, enhancing overall adaptability and reliability.
• Higher power density: CHEs demonstrate significantly greater volume-specific power density compared to conventional STHXs.

The development of MS-HXs has evolved from large shell-and-tube configurations to more efficient compact designs, with related research concentrated in the past decade. In 2011, Lippy [72] proposed an intermediate compact heat exchanger design for high-temperature molten salt applications, bridging the gap between traditional STHXs and high-efficiency CHEs. The compromise solution aimed to enhance the volume power density of conventional MS-HXs while circumventing the maintenance challenges typical of CHEs. Lippy developed a modular CHE where molten salt and water flowed through adjacent rectangular channels for heat exchange. Through parametric simulation analysis, the channel geometry was optimized for both performance and maintainability, yielding a final design featuring high-aspect-ratio rectangular channels (0.5 mm height × 6 mm width). This CHE achieved equivalent thermal performance to ORNL’s STHX while occupying only 5% of the volume, and it reduced corrosion-resistant Hastelloy N alloy usage by 90%, significantly lowering material costs. Notably, comparative analysis revealed that the high-aspect-ratio rectangular channels provided superior material efficiency heat transfer performance compared to the conventional circular channel.

The printed circuit heat exchangers (PCHE) concept was first proposed in 1985, with the world’s first PCHE prototype developed shortly thereafter in Australia. This innovative design was subsequently commercialized by Heatric, a UK-based company [109]. PCHEs offer several technical advantages, including high pressure and temperature resistance, compact footprint and modular design, exceptional heat transfer efficiency, and superior sealing properties [110,111,112,113]. These characteristics make PCHEs ideally suited for molten salt applications in CSP, TES, and MSR systems.

In 2012, Sabharwall et al. [114] from the Idaho National Laboratory conducted a comprehensive feasibility analysis of high-temperature molten salt PCHEs for advanced molten salt reactors. Their technology development roadmap study identified that the PCHEs offer substantial cost advantages and superior heat transfer performance, making them particularly promising for high-temperature, high-pressure, and corrosive environments. However, they also identified several technical challenges specific to molten salt PCHEs, such as stringent requirements for diffusion bonding technology and precision manufacturing, structural integrity concerns due to thermal stress fatigue and high-temperature creep, flow maldistribution issues in the header and plate regions, and insufficient understanding of thermal-hydraulic dynamic response characteristics. These limitations were found to be common across PCHEs using different working fluids.

While advances in compact heat exchanger technologies such as PCHEs bring significant improvements in heat transfer performance and system responsiveness, they also exacerbate issues related to thermal stress. The high surface-area-to-volume ratio and reduced wall thickness, while beneficial for heat transfer, result in larger and more abrupt temperature gradients during operation, especially under thermal cycling conditions. These factors lead to higher localized thermal stresses, which in turn drive fatigue, creep, and potential failure mechanisms. Therefore, high performance must always be evaluated alongside mechanical durability. Future design and material optimization must focus on minimizing thermal stress concentrations through both structural and operational strategies, ensuring that the gains in compactness and heat transfer do not compromise long-term reliability.

Sabharwall et al. [115] extended their research through comprehensive experimental investigations evaluating molten salt corrosion resistance of candidate alloys and the mechanical integrity of diffusion-bonded joints. Their work confirmed that Hastelloy N and Alloy 242 exhibit suitable material properties for molten salt PCHE applications. Through detailed microscopic characterization, the researchers validated both the corrosion resistance and mechanical strength of the diffusion-bonded interfaces. Representative experimental results are shown in Figure 13. This study enhanced process control for diffusion bonding in PCHE fabrication and demonstrated the technical feasibility of the manufacturing approach. Aakre et al. [116] experimentally investigated the pressure drop and heat transfer performance of diffusion-bonded zigzag channel heat exchangers. In addition, a computed tomography (CT) system was employed to perform non-destructive inspection of the internal channels and header regions of the heat exchanger.

In 2016, Kim et al. [117] from Ohio State University investigated PCHEs for reactor systems using FLiNaK and supercritical CO₂. Through combined experimental and numerical analysis, they evaluated straight, zigzag, S-shaped, and offset strip fin channel configurations [118]. As illustrated in Figure 14, a comprehensive assessment of thermal efficiency, pressure drop, and cost factors identified the OSF design as the optimal channel geometry for molten salt applications.

Collaborative research between Ohio State University and the University of Wisconsin-Madison adopted conservative round-channel designs for their molten salt reactor system [119,120]. Certification documents revealed that this design selection criterion focused on system stability, standardization feasibility, and manufacturing reliability. the heat exchanger component design certification was focused on ensuring stability and ease of employing a conservative design. This conservative approach reflected the immaturity of molten salt PCHE manufacturing technology could not meet the stringent commercial requirements, possibly due to technical challenges in precision fabrication and high manufacturing costs.

In the same year, Lu et al. [121,122] from Sun Yat-Sen University pioneered the structure of molten salt PCHE channels. Their innovative sinusoidal channel design departed from conventional zigzag angle (Figure 15) optimization approaches. Numerical simulations showed that sinusoidal PCHEs reduced pressure fluctuations by 20–30%, decreased temperature fluctuations by 15–25%, and enhanced heat transfer through suppression of flow separation. He et al. [123] systematically evaluated various PCHE channel configurations for different working fluids (water and supercritical CO₂) [124,125,126,127], concluding that discontinuous finned channels offered optimal performance for molten salt applications. To address CSP system requirements, the team developed a compact 316 stainless steel heat exchanger using Hitec molten salt and synthetic oil (Figure 16a). The experimental results demonstrated that finned channels achieved two-fold and seven-fold higher heat transfer coefficients than zigzag and straight channels, respectively. This research significantly advanced molten salt PCHE miniaturization while demonstrating successful technology transfer from supercritical CO₂ to molten salt systems.

Building on previous work, Li et al. [127] numerically evaluated the thermal-hydraulic performance of an airfoil-shaped channel for MS-HXs (Figure 16b). Their simulations demonstrated that the airfoil-shaped channels consistently outperformed conventional straight and zigzag designs across the operational temperature range of CSP systems. This finding holds significant implications for enhancing the efficiency of MS-HXs. Li et al. compared the important heat transfer performance indexes of zigzag as well as airfoil channels in detail based on the numerical simulation results, and the performance comparison of the two channels can be seen in Figure 17.

In recent years, there have been many studies on winged molten salt PCHE. Yang et al. [129] tested the combined enhanced heat transfer effect of various winged fin shapes in molten salt channels through numerical simulation, and finally concluded that a winged molten salt channel with a full center had the best thermo-hydraulic performance. Ding et al. [130] screened out a suitable design of the wing-shaped channel for FNaBe molten salt by changing the geometrical features such as the thickness of the wing-shaped fins and the staggered pitch of the rows in the channel, while Arora et al. [131] comprehensively investigated the flow and pressure drop characteristics of the wing-shaped molten salt heat exchanger by combining experimental and numerical simulation methods, with an emphasis on discussing the transition process of molten salt from laminar to turbulent flow. In parallel with airfoil-shaped channel research, ongoing optimization efforts focus on zigzag molten salt channels. While airfoil designs offer superior thermal-hydraulic performance, zigzag configurations retain critical advantages of structural integrity, lower fabrication costs, and reliability in high-pressure environments for MSR. Japanese researchers [132] advanced this technology by evolving traditional zigzag channels into sinusoidal wave configurations. Their thermos-mechanical analysis revealed that sinusoidal wave-shaped heat exchangers could achieve comparable heat transfer efficiency to zigzag designs, lower pressure drops by 18–22%, and reduce fouling susceptibility due to smoother flow paths.

Zhou et al. [133] and Niu et al. [128], respectively, investigated and simulated the flow and heat transfer processes of molten salt in straight channels and Z-shaped channels with semicircular cross-sections. A comparison of their results reveals that there is significant potential for optimizing the flow performance of molten salt in Z-shaped channels. This conclusion is consistent with the findings of Jiragoontansiri et al. [134], whose comprehensive comparison of molten salt flow and heat transfer performance in airfoil-finned, zigzag, and slotted finned channels also indicated that the pressure drop in zigzag channels is excessively high.

Che et al. [135] developed a critical structural integrity assessment methodology for PCHEs used in MSRs. Through parametric optimization of zigzag channel geometries- particularly corner radius, they reduced stress concentration factors by 40–50%, enhanced failure resistance under thermal cycling conditions, and improved overall safety margins for high-temperature operation. Their analysis further identified the heat channel exit region as a high-risk zone for thermal-mechanical failure, providing essential design guidelines to mitigate crack initiation in molten salt PCHEs.

Ding et al. [136] conducted numerical simulations to study the heat transfer in molten salt PCHEs and found that the performance of the heat exchanger is largely influenced by the pressure drop in the molten salt channels and the heat transfer resistance in the low-temperature channels. By increasing the width of the molten salt channels, the pressure drop can be significantly reduced, while the overall heat transfer coefficient remains relatively stable. This trade-off ultimately leads to improved overall performance. Their research deviates from the conventional design approach of maintaining similar diameters for hot and cold channels. The proposed widening of molten salt channels is particularly promising, as it mitigates practical challenges such as blockages caused by freezing or the high viscosity of molten salts-key issues in molten salt CHEs. Notably, similar asymmetric channel designs have been investigated in evaporator studies, highlighting broader applicability of this concept [137].

Beyond airfoil-shaped and zigzag channel designs, researchers have explored innovative approaches to enhance the compactness of MS-HXs. Ding et al. [138] proposed a corrugated-plate heat exchanger featuring periodically staggered convex and concave structures for heat transfer between molten salt and sCO₂ (Figure 18). This design significantly improved heat transfer performance while maintaining low pressure drop. Tano et al. [139] leveraged additive manufacturing (AM) to fabricate a molten salt channel with staggered pin-fin arrays. Experimental results confirmed its high power density, low pressure drop, and modular scalability (Figure 19). In follow-up studies, they further optimized channel head structures and AM efficiency, reducing manufacturing costs [140].

The factors that affect the comprehensive heat transfer performance of the heat exchanger have other important factors in addition to the structure of the heat transfer channel, and a large number of excellent research results have been born in these aspects. Zhu et al. [141] investigated the flow inhomogeneity due to the structure in molten salt heat exchanger, proposed the concept of flow inhomogeneity matching, and proposed a new method to improve the heat exchanger performance by matching the flow inhomogeneity. Tian et al. [142], on the other hand, quantified the buoyancy parameter in the horizontal channel for the phenomenon of mixed convective heat transfer of molten salts in the horizontal tube due to gravity and temperature difference, and this study helps to improve the accuracy of heat exchanger design. Molten salt solidification clogging is a difficult problem that cannot be bypassed in heat exchanger design, Prantikos et al. [143] used machine learning principles to construct a method to invert and locate the specific clogging location in the heat exchanger channel by detecting temperature anomalies in the molten salt heat exchanger. This has impacted the notion that machine learning cannot be well applied to the field of heat exchanger design.

Currently, the compact design of MS-HXs has diversified into multiple research directions, demonstrating significant potential for industrial applications. The coexistence of competing technological approaches further drives rapid technological iteration in this field. Valuable insights can be drawn from existing PCHE research on supercritical CO₂ and other fluids. For example, studies extensively investigated the impact of airfoil fin geometric distribution on heat transfer performance in sCO₂, helium, and other working fluids [144], along with fin-shape optimizations [145] and twisted airfoil fin design (Figure 20a) [146]. These findings may offer transferable strategies for MS channel optimization.

The optimization must balance competing objectives, including maximizing heat transfer efficiency, maintaining acceptable pressure drop, preventing freezing-induced blockages, and ensuring long-term corrosion resistance. These factors must collectively serve as the multicriteria optimization framework for MS-HXs. In addition to thermal-hydraulic performance, the fabrication processes and production cost-effectiveness (e.g., etching technology, brazing, and diffusion bonding), the application-specific requirements (e.g., evaporators, superheaters), performance stability during operational transients, and the transient response characteristics under variable loads should also be incorporated as critical optimization considerations.

Research trends often outpace practical implementations. Over the past five years, academic publications on zigzag and airfoil-shaped channel PCHEs have shown steady growth. However, in real-world world applications-particularly for sCO₂ power system demonstrations and engineering projects, straight-channel PCHEs continue to dominate. This preference stems from their superior manufacturability, cost-effectiveness, and balanced performance in terms of pressure drop versus system efficiency. Additionally, straight-channel designs benefit from established engineering experience, offering proven long-term reliability and standardization advantages that complex geometries currently cannot match.

Optimal compact designs for MS-HXs are highly application-dependent. For MSR systems operating under extreme conditions (e.g., high neutron flux, thermal cycling), conservative channel geometries are prioritized to ensure structure integrity and operational stability. Conversely, in lower-temperature MS-TES applications, designers frequently adopt more aggressive channel configurations (e.g., enhanced turbulator, complex fin arrays) to maximize thermal exchange efficiency, accepting marginally high pressure drops for improved energy conversion performance.

5. MS-HX Fabrication: Status and Challenges

The MS-HX optimization research is increasingly focused on aspects such as compactness and modularity. However, translating innovative research into engineering applications requires careful consideration of manufacturability of the design and other technical challenges, including material corrosion resistance and cost-effective mass production.

In this section, we examines MS-HX manufacturing processes, materials compatibility issues, and viable strategies for implementing these advanced designed in practical applications.

5.1. Corrosion and Materials Selection for MS-HXs

MS-HXs materials require exceptional high-temperature stability and corrosion resistance to withstand these aggressive environments. Corrosion mechanisms in MS environments can be classified into two primary categories: one is intrinsic reactions including oxidation, dissolution, and electrochemical corrosion processes. The other is environment drivers encompassing thermal gradients-induced corrosion and impurity-induced attacks, which are influenced by operational conditions and salt purity [147,148]. For example, in nitrate salts (e.g., Solar Salt), dissolution corrosion dominates, where temperatures above 600 °C accelerate like chromium (Cr) leaching from stainless steel, increasing corrosion rates by 200–300% compared to inert atmospheres [149]. This occurs via oxide layer destabilization and selective element dissolution.

Different molten salt systems (e.g., chlorides, fluorides, nitrates) exhibit distinct oxidative potentials, acidities, and solubilities, which govern their corrosion behavior toward metals.

Table 5. The Corrosion Resistance Performance of Key Alloys.

Key Alloys	Type of Molten Salt	550–600 °C (μm/Year)	650–700 °C (μm/Year)
Hastelloy N	FLiBe	<2	<10
316L	FLiBe	10–20	20–40+
Hastelloy N	Solar Salt	1–5	5–10
316L	Solar Salt	10–30	30–50+

contains the corrosion resistance performance of some important alloys. Patel et al. [150] systematically analyzed alloy corrosion mechanisms across fluoride, nitrate, chloride, and sulfate salts, while also evaluating corresponding corrosion assessment methodologies. These findings offer substantial value for MS-HX design. For example, corrosion rates (μm/year) measured via weight loss methods enable direct service life estimation at operational temperatures. Comparative corrosion data for different facilitate cost-effective material choices, balancing performance and maintenance expenses.

Quantitative studies on the corrosion rate of metal substrates of heat exchangers in different operating environments can provide engineers with a reference basis for rapid heat exchanger design work. Palacios et al. [151] studied the high temperature corrosion behavior of four metals (mild steel A1045, stainless steels 304H and 316L, and nickel alloy Inconel 600, Special Metals Corporation, New York, United States) in Solar Salt, and systematically studied and summarized: temperature and time, alloy type, salt composition and purity, environment and testing methods, and corrosion products and a series of parameters on the metal under the molten salt system. Figure 21b illustrates some of the phenomena in the experiment. They systematically investigated and summarized the effects of a series of parameters, such as temperature, time, alloy type, salt composition and purity, impurities, environment and testing methods, and corrosion products, on the corrosion rate of metals in molten salt systems, and quantified and discussed in detail the roles of these controlling factors and their interplay mechanisms by means of a large amount of experimental data and cases. The experimental data in the study indicate that the mass loss per unit area of metals in molten Solar Salt shows an approximately parabolic relationship with time, i.e., the square of mass loss is proportional to exposure time, implying that the corrosion process is diffusion-controlled and the corrosion rate decreases with increasing exposure duration. Meanwhile, the data also demonstrate that increasing the operating temperature significantly accelerates the corrosion rate, exhibiting a typical temperature dependence.

The corrosion resistance of metals varies significantly based on their elemental composition and alloy characteristics. Advanced characterization techniques such as SEM/EDX (for elemental mapping) and XRD (for phase identification) enable quantitative analysis of corrosion products, including their particle size distribution, chemical composition, and solubility behavior. A critical design consideration arises from the low solubility of these corrosion product in molten salts. Particulate deposits tend to accumulate in low-flow zones (e.g., channel bends or manifolds), increasing the risk of flow blockage. Therefore, channel geometry optimization must account for flow distribution uniformity to minimize dead zones and erosion-resistant designs to mitigate particle adhesion. From a materials selection perspective, designs should prioritize alloys demonstrating proven corrosion resistance, stable passive layer formation, and mechanical robustness.

Fluoride salts, valued for their high thermal stability and low melting points, are extensively utilized in MSR systems. However, their high chemical reactivity with metal surfaces leads to alloying element dissolution and oxide film degradation (Figure 21a). Nickel-based alloys (e.g., Hastelloy N, Haynes 230) exhibit superior corrosion resistance, making them the preferred choice for MS-HXs. In contrast, stainless steels (e.g., 316) suffer rapid degradation-in 700 °C FLiBe and 316SS loses Cr six times faster than Hastelloy N [153]. Notably, adding Be (<1 wt%) to molten salts reduces oxidative attack on alloys. In MSR environments, materials face additional degradation from neutron irradiation-induced effects and synergistic corrosion-irradiation effects, which accelerate the degradation of material’s corrosion resistance [154].

In molten salt CSP and TES systems, nitrate salt systems (e.g., Solar Salt and Hitec) are widely used. 316L stainless steel exhibits a relatively low corrosion rate in nitrate salts, as its Cr element forms a passivation layer that inhibits molten salt penetration [151]. However, its corrosion rate under dynamic conditions remains higher than under static conditions. Nickel-based alloys such as Hastelloy N, which contain high molybdenum (Mo), demonstrate even better corrosion resistance in nitrate environments. Therefore, for nitrate salt heat exchangers, corrosion-resistant alloys like 316L Hastelloy N are recommended [155]. In contrast, chloride salt systems offer cost and heat capacity advantages but are significantly more corrosive to metals [154]. Studies suggest that adding magnesium oxide (MgO) to chloride salts or using magnesium treatment can effectively reduce their corrosivity [117]. Beyond material selection, molten salt purification to minimize impurities is another key protection strategy for high-temperature components like heat exchangers.

Another salient factor influencing the corrosion of the heat exchanger by molten salt is the flowability of the molten salt within the molten salt heat exchanger. This phenomenon elucidates the rationale behind the initial research findings on enhanced heat exchange with molten salt, wherein the transverse corrugated scheme within the tube was eliminated due to the potential stagnation of molten salt within the grooves oriented perpendicular to the flow direction. This stagnation, in turn, has been shown to accelerate the corrosion of the heat exchanger within the molten salt stack. Besides this, geometric features such as sharp corners, sudden expansions, dead-end channels, and areas with poor fluid distribution can lead to local flow stagnation and sedimentation, which are hotspots for localized corrosion phenomena including pitting and crevice corrosion. Smoother channel transitions, rounded corners, and optimized channel arrangements help maintain uniform flow, minimizing stagnant zones and facilitating the removal of corrosive byproducts. Incorporating turbulence promoters or adopting helical and wavy channels can further enhance the self-cleaning effect of flowing salts, reducing the accumulation of corrosion products. Therefore, geometry-driven design not only improves heat transfer performance but also plays a crucial role in mitigating corrosion risks in molten salt heat exchangers. A comprehensive evaluation of the f-index of molten salt flow in the heat exchanger channel and the flow line smoothness, and the flow dead zone of molten salt should be the focus of molten salt heat exchanger design.

Molten salt corrosion significantly impacts the design of MS-HXs. However, research on molten salt corrosion specifically tailored to heat exchanger applications remains limited, lacking systematic synthesis and analysis. While numerous quantifiable metrics exist to evaluate metal corrosion resistance, the precise relationships between these factors and heat exchanger design have yet to be fully elucidated. To address this gap, future studies should prioritize a comprehensive analysis of molten salt corrosion for heat exchanger designs. By fitting experimental data on metal corrosion rates under varying molten salt working conditions, empirical formulas could be developed, providing a scientific foundation for the preliminary design and optimization of MS-HXs.

5.2. Fabrication Technology of MS-HXs

The fabrication of MS-HXs benefits from interdisciplinary advances in manufacturing technologies. Brazing, a mature technique, has been widely used to produce STHEs and PFHEs through years of optimization. In contrast, the diffusion bonding technology, which is essential for PCHEs, has shown significant progress, but challenges persist in high-temperature/pressure or corrosive environments [156,157,158,159,160,161]. With further maturity of this technology, it is expected to significantly reduce the R&D costs by streamlining production, while improving stress stability and long-term reliability under extreme operating conditions. High-precision PCHE channel etching faces inherent geometric deviations, as shown by Wang et al. [162]. Their work compared heat transfer performance between chamfered and regular airfoil-shaped channels. They verified several schemes by CFD numerical simulation and found that changes in the transverse spacing of the fins can cause significant changes in the thermal and hydraulic performance of the chamfered channel, while the longitudinal spacing of the fins has little effect on the Nu and f of the chamfered channel.

In recent years, the design of heat exchangers via additive manufacturing has enabled the development of many high-performance corrosion-resistant substrate materials, such as ceramics and novel alloys. Kelly et al. [157] were the first to successfully fabricate a ceramic heat exchanger with a triply periodic minimal surface (TPMS) structure using binder jetting additive manufacturing technology. This method enables the realization of complex geometries, providing greater design freedom for heat exchangers. Du et al. [158] also employed binder jetting additive manufacturing to produce ceramic heat exchangers for high-temperature and high-pressure applications, and validated their thermal performance through combined experimental testing and numerical simulation. Ren et al. [159] successfully used directed energy deposition (DED) technology to fabricate gradient nickel-based alloys (Hastelloy N and Haynes 282), which can simultaneously meet the stringent high-temperature operational requirements of both molten salt and supercritical CO₂ (sCO₂) heat exchangers. Both simulation and experimental investigations demonstrated the considerably high-temperature operational potential of these heat exchanger types.

Gradient porosity and multifunctional channel walls exemplify the application of advanced design methodologies, such as topology optimization, to meticulously calibrate the heat exchanger configuration at the micro and meso scales. These mechanisms facilitate the streamlined organization of fluid flow, the augmentation of heat transfer processes, and the optimization of material utilization efficiency. In the context of molten salt heat exchangers, which are subject to harsh operating conditions and stringent performance requirements, these techniques have the potential to overcome the limitations of traditional designs, thereby facilitating more efficient, compact, and reliable heat transfer. Algorithms for topology optimization have the capacity to automatically generate optimal structural solutions with these complex features based on set objective functions (e.g., maximize heat transfer, minimize pressure drop) and constraints.

Improved precision in additive manufacturing now supports complex heat exchanger designs. A compelling example is three-dimensional topology optimization (TO), which has been successfully applied to supercritical CO₂ heat exchanger (Figure 22b) design [163]. Although current molten salt applications focus on pseudo-3D airfoil-shaped channel optimization (Figure 22a) [164], higher-dimensional TO methods show promising potential. As these technologies advance, both the performance and production efficiency of heat exchangers will be significantly enhanced.

5.3. Phase Transformation and Operational Risks

Phase transformation risks, including salt solidification and thermal decomposition, are critical concerns for molten salt heat exchangers. These challenges significantly affect operational reliability and system longevity, and thus require analysis and effective mitigation strategies.

Molten salt energy systems are typically designed with stringent operational temperature requirements, primarily due to the low-temperature freezing characteristics of molten salts. Once the molten salt heat exchanger freezing blockage occurs, it will lead to an increase in flow resistance, a reduction in the effective heat transfer area, a sharp decline in the heat transfer coefficient, the system flow/heat cycle imbalance, the all-round deterioration of operational safety, and in serious cases, can also lead to system shutdown or damage to the equipment. In order to prevent the flow stop and subsequent solidification of the working fluid, engineers in the heat exchanger design will be pre-built in the heat exchanger electric heating wire and other heating mechanisms. Alternatively, the heat exchanger and associated systems can be preheated and insulated with another thermal fluid (e.g., thermal oil, hot air, low-melting-point molten salts, or even steam) through a separate jacket or coil [59].

In the context of novel high-temperature molten salts (e.g., chloride molten salts, fluoride molten salts), there is a heightened imperative to investigate the impact of impurities (particularly oxides and hydroxides) on their melting points and corrosion behavior. The development of efficient online purification techniques, such as electrochemical purification methods, as well as the study of the characteristics and migration behavior of the corrosion products of molten salts, can facilitate a fundamental reduction in the introduction and accumulation of impurities. The evacuation design in shell and tube heat exchangers is also effective in solving the problem of molten salt clogging in the case of unplanned shutdown. A large number of thermocouples are arranged on the molten salt heat exchange equipment, and the operating procedures will set multi-level temperature alarm thresholds, which can also detect the molten salt blockage in time.

In addition to the effective prevention and treatment of clogging in the molten salt heat exchanger, the improvement of the channel structure of the heat exchanger to reduce the probability of molten salt condensation clogging is also necessary. Zhang et al. [165] investigated the influence of molten salt flow rate, channel diameter, channel wall thickness, and other factors on the critical length of molten salt freezing in the molten salt freezing problem. The investigation was conducted by combining calculation and numerical simulation. In the design of the molten salt heat exchanger, increasing the molten salt flow rate appropriately and increasing the diameter of the molten salt channel of the heat exchanger can effectively reduce the risk of molten salt condensation blockage.

In the event of frozen blockage or partial blockage that has not yet fully developed in the molten salt, the channel diameter of the heat exchanger will undergo a decrease. Studies have demonstrated that this inhomogeneous structural change, in conjunction with the operating conditions, will result in vibration [166]. This, in turn, will accelerate the corrosion of the molten salt on the heat exchanger substrate and potentially inflict additional damage from vibration. Concurrently, localized blockage will exacerbate the inhomogeneity of molten salt flow within the heat exchanger, potentially leading to local temperature accumulation. This, in turn, will augment the thermal stresses on its metal substrate, thereby reducing the heat exchanger’s expected lifespan and increasing the likelihood of leakage. Consequently, it is imperative to meticulously monitor the flow of the mass within the heat exchanger to avert the potential for severe clogging of the heat exchanger by molten salts. Zeng et al. [122] took the wall temperature of the heat exchanger, the length of the channel and the cooling temperature of the molten salt when the molten salt flowed into the heat exchanger as the analytical objects, and established a molten salt heat exchanger freezing and blocking discrimination model, which can effectively predict the risk of freezing and blocking within the molten salt heat exchanger, and it can provide the basis for the set up of the safe operating conditions of the molten salt system such as the molten salt reactor.

The boiling points of molten salts are sufficiently high, and under current application scenarios, these temperatures are rarely reached. Moreover, since most molten salts exhibit decomposition behavior at elevated temperatures, engineers often impose an upper limit on the operational temperature to prevent thermal degradation. For example, in concentrated solar power (CSP) systems, the molten salt temperature is typically maintained below 565 °C to ensure chemical stability. The decomposition of molten salt at elevated temperatures has the potential to generate gaseous components, which can induce stress concentrations within the heat exchanger. This phenomenon can lead to complications such as leakage in the heat exchanger and the accelerated corrosion rate of the molten salt on the metal substrate, consequently reducing the heat exchanger’s operational lifespan. Therefore, in all types of molten salt energy systems, it is very difficult for the molten salt as a heat transfer medium to boil, and the upper limit of the operating temperature of the molten salt is usually several hundred degrees Celsius lower than its boiling point.

6. Conclusions

MS-HXs are indispensable for high-temperature energy systems, including MSRs, CSP, and TES, where they enhance efficiency and enable renewable energy integration. This review highlights significant advancements in MS-HXs design and optimization have been discussed, including performance improvements and the transition from traditional STHXs to more compact designs like Printed Circuit Heat Exchangers (PCHE). Additionally, the challenges surrounding material corrosion and manufacturing processes have been thoroughly reviewed. Using high-performance alloys like Hastelloy N and developing new corrosion-resistant materials are crucial for improving durability. Advanced manufacturing techniques like brazing, diffusion bonding, and additive manufacturing show promise but require further research to address precision, scalability, and cost control issues.

Compact heat exchangers, especially PCHEs, offer enhanced heat transfer performance and operational efficiency. While research is in early stages, optimization of PCHE designs and addressing corrosion challenges remain a focus. One of the significant challenges identified is the trade-off between heat transfer efficiency and pressure drop in compact heat exchanger designs. While innovations like PCHEs offer higher thermal efficiency, they come with fabrication complexities and cost concerns. Furthermore, the difficulty in accurately predicting molten salt flow dynamics, especially in high-temperature and two-phase flow regimes, complicates the design process for heat exchangers.

Therefore, the following investigations should be carried out based on the above analysis in the future:

• The improvement of molten salt properties includes enhancing the performance of molten salts in flow heat transfer, thermal energy storage, and other aspects; reducing the production and purification costs of molten salts; and modifying molten salts in various ways to mitigate their corrosion effects on metal components.
• Continuously focus on the development of new alloys, enhancing their corrosion resistance while increasing the machinability of metals in additive manufacturing and etching processes, and reducing alloy manufacturing costs.
• Keep a continuous focus on the application and development of multi-objective optimization methods and topology optimization in the research of heat exchanger channel structure optimization. Improve the operational precision of diffusion bonding, additive manufacturing, and etching technologies, and accelerate the iteration speed and practical application of these technologies.