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Thermally conductive composites as polymer heat exchangers for water and energy recovery: From materials to products
1. Introduction
The demand for clean water and energy has been drastically increasing due to rapid population growth and urbanization [1], [2]. Therefore, water and energy recovery and savings have become increasingly important. However, energy generation via power plants heavily relies on carbon-emitting fossil fuels, contributing to climate change and environmental pollution [3]. Adopting renewable energy and improving energy efficiency are two strategies to reduce the reliance on fossil fuels [4], [5]. One third of the supply energy is consumed in industrial processes [6], and most is wasted as low-grade heat (< 250 ℃ [7]) during energy conversion processes [8]. Such energy lost comes from the latent heat in humid flue gas that contains water and acid/alkaline vapours. Reclaiming waste heat and other resources, such as water vapor and acidic/alkaline vapor, from humid flue gas can enhance energy efficiency, recycle valuable water and chemicals, and mitigate thermal release to the atmosphere [9]. Heat exchangers offer a promising solution for harnessing low-grade energy and reclaiming other resources from humid flue gas.
Heat exchangers achieve cooling or heating via heat transfer in various engineering systems [10]. Conventional heat exchangers are made of metals, such as stainless steel, copper, aluminium, and alloys [11], because metals meet the requirement where heat exchangers are required to transfer heat at high rates with minimum thermal expansion over working temperature ranges [12]. Metals have good thermal and mechanical properties, such as effective heat transfer [13], wide working temperature ranges [10], and high mechanical strength [14]. However, they also have limitations, such as high production costs [10], heavy weight [11], fouling, and are susceptible to corrosion [15], [16]. For example, insoluble particulates can easily deposit on the inner surface of metal heat exchangers, blocking the heat flow path [16]. Also, the degradation of metal heat exchangers can occur via corrosion due to their exposure to acidic/alkaline vapor [17]. Therefore, alternative thermally conductive materials with anti-fouling and anti-corrosion properties are desirable to develop heat exchangers.
Polymers have attracted great attention in past few decades [18], [19], [20] because of their low costs, easy manufacturing, anti-fouling, corrosion resistance, and low energy consumption in production [10], [11], [20], [21]. These advantages have led to their applications in various fields, such as membrane distillation [22], waste heat recovery [23], and solar water heat systems [24]. Importantly, polymers are economically competitive due to their lower energy consumption to produce a unit mass of materials and substantial weight and space saving for heat exchangers [25]. When designing heat exchangers with polymers, thermal and mechanical properties of the polymers are two major parameters. However, most polymers have a poor thermal conductivity (<1 W/(m⋅K) [26]), significantly lower than that of the generally accepted minimum of 10 W/(m⋅K) for conventional heat exchangers [27]. Developing thermally conductive polymer composites by adding thermally conductive fillers into a polymer matrix is an effective way to overcome this barrier of low thermal conductivity.
Adding suitable highly thermally conductive fillers into a polymer matrix can improve the overall thermal conductivity of polymer composites by forming thermal conduction pathways or networks [21], [28]. Thermally enhanced polymer composites can have special chemical and physical properties, making them promising for heat exchangers in unique applications [10]. Common fillers used in polymer composites can be particles, fibers, flakes, and laminas [11], in terms of filler geometry. Based on filler materials, they can be classified into metals, carbon, ceramics, or hybrid mixtures of these above [10]. Apart from the intrinsic thermal conductivity of the fillers, other factors, such as filler loadings, dimensions, and alignments also affect the effectiveness of the fillers on heat transfer in the composites [10], [21]. The filler-enhanced thermal conductivity of the composites is often lower than the theoretical value [21]. This is because adding fillers into polymer matrix can result in higher thermal resistance within new interfaces, imperfections, impurities, and defects, which will cause the increase of phonon scattering [29], [30].
Some review papers on thermally conductive polymer composites have been published. These review papers focus on strategies to improve thermal conductivity of polymer composites [31], [32], [33], [34] (e.g., using different filler types [35], and constructing connected filler structures [36]) and the basic applications where thermal management is required (e.g., thermal interfacial materials [37], and battery safety management [38]). However, considering the superiorities of polymers, few review papers on polymer composites for heat exchanging processes have been published, especially for water and energy recovery.
Despite the notable advancements in polymer heat exchangers, significant knowledge gaps remain. Existing literature primarily focuses on improving thermal conductivity of polymer composites, yet the impacts of filler properties (e.g., geometry, loading, and dispersion) on thermal conduction pathways have not been systematically assessed. Moreover, although advanced fabrication techniques have been proposed, their scalability and industrial feasibility are not yet fully explored. In addition, the long-term performance of polymer heat exchangers under harsh operational conditions, including resistance to fouling, chemical degradation, and mechanical stress, has not been adequately understood. This review fills these gaps by providing a detailed evaluation of strategies to enhance polymer composite performance, the feasibility of advanced manufacturing methods, and the potential applications of polymer heat exchangers for water and energy recovery.
This review aims to provide new insightful evaluation of utilizing polymers and polymer composites as heat exchangers to recover water and energy, including four key aspects: (1) strategies to improve the thermal conductivity of polymer composites, (2) fabrication techniques to produce thermally conductive polymer composites, (3) design of polymer heat exchangers, and (4) potential applications of polymer heat exchangers for water and energy recovery. Importantly, we provide our perspectives on the selections of polymers and fillers, and advantages and challenges of each fabrication technique. This review highlights the factors controlling the properties of the polymer composites since the properties of the polymer composites determine their application scenarios and the efficiency in water and energy recovery.
2. Polymers and polymer composites
2.1. Characteristics of polymers
Polymers are large organic molecules or macromolecules with a series of repeated units linked together [39]. Large molecules consequently have large molecular mass relative to small molecules, which brings them unique mechanical properties [11]. Polymers typically possess light weight, anti-fouling, corrosion resistance, easy processability and handling, and comparably low production costs [40], [41], [42]. These features have spurred recent investigations into polymers for heat transfer. However, the relatively low thermal conductivity of polymers (0.1–0.5 W/(m⋅K) [43]) limits their practical applications in heat exchanging processes. Thermal and mechanical properties are two key considerations for heat exchangers. The thermal properties include thermal conductivity (λ), specific heat capacity (Cp), melting point (Tm), service temperature range, and coefficient of thermal expansion (CTE). The mechanical properties to be considered are tensile strength, tensile modulus, and density (ρ).
Natural polymers (e.g., cellulose and starch) and artificial polymers are everywhere in our daily life. Polymers suitable for polymer heat exchangers include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polypropylene (PP), polyethylene (PE), polycarbonate (PC), and polyphenylene sulphide (PPS). Their basic thermal and mechanical characteristics are summarised in Table 1.
Table 1. Polymers suitable for polymer heat exchangers: basic thermal and mechanical characteristics and advantages.
| Polymers | ρ (g/cm3) | λ (W/(m∙K)) | Cp (J/(g∙K) | Tm (˚C) | Service temperature range (˚C) | CTE (10-6/˚C) | Tensile strength, yield (MPa) | Tensile modulus (GPa) | Advantages | Refs. |
|---|---|---|---|---|---|---|---|---|---|---|
| PVDF | 1.77 | 0.185 | 1.06 | 164 | −51.3 ∼ 143 | 140 | 41.4 | 1.56 |
|
[44], [45] |
| PTFE | 2.07 | 0.263 | 1.01 | 325 | −212 ∼ 260 | 108 | 20.5 | 0.564 |
|
[46], [47] |
| PP | 0.93 | 0.249 | 1.92 | 160 | −30.0 ∼ 111 | 114 | 31.6 | 1.67 |
|
[48], [49] |
| PE | 0.954 | 0.396 | 2.25 | 131 | −137 ∼ 96.7 | 143 | 26.2 | 0.976 |
|
[50], [51] |
| PC | 1.20 | 0.217 | 1.66 | 300 | −40 ∼ 124 | 66.1 | 62.4 | 2.39 |
|
[52], [53] |
| PPS | 1.60 | 0.264 | 1.38 | 281 | −17.5 ∼ 226 | 52.1 | 58.7 | 6.46 |
|
[54], [55] |
PVDF and PTFE are both fluorinated polymers, characterized by the presence of carbon–carbon (C–C) and carbon–fluorine (C-F) bonds in their molecular structures. These strong bonds are responsible for the exceptional resistance of fluoropolymers to corrosion by various chemicals. PVDF is a partially fluorinated polymer, whereas PTFE is a fully fluorinated polymer. The slight difference in their chemical structures leads to variations in their chemical and thermal stabilities. PTFE is the most chemically resistant and thermally stable polymer among the fluoropolymers, and it is impervious to nearly all known solvents [56]. This makes PTFE an excellent candidate for applications where extreme chemical resistance is required. In contrast, PVDF sacrifices some chemical and thermal resistance (i.e., less chemical inertness and lower melting point compared to PTFE) for improved mechanical properties at room temperature. While PVDF is still chemically and thermally stable compared to many other materials, it cannot withstand the same range of conditions as PTFE. The different temperature limits of these two fluoropolymers play a significant role in determining their suitable working temperatures in heat-related applications. PTFE can tolerate much higher temperatures than PVDF, making it the preferred option for applications where extreme thermal stability is crucial.
PP and PE are both known for their durability and lightweight characteristics. They find use in various applications, but each has distinct properties and advantages. PP is widely employed in a range of fields, including packaging, stationery, textiles, and more. Its non-toxic and non-staining properties make it suitable for applications where product purity and cleanliness are essential. Additionally, PP exhibits resistance to most organic solvents, but is susceptible to strong oxidants at room temperature. On the other hand, PE is known for its anti-corrosive properties and tolerance to gentle oxidants and reducing agents. Compared to PP, PE is more inert and stable, largely due to its lower static charge, which results in less attraction of dust and dirt to its surface. Consequently, PE is commonly used in laboratory apparatus, bottles, machine parts, and similar applications where cleanliness and resistance to corrosion are critical.
PC exhibits remarkable resistance to cracking or breaking even when subjected to substantial plastic deformations, allowing for room-temperature manufacturing and formation. In addition, PC is resistant to both inorganic and organic acid, such as sulfuric acid and hydrochloric acid. However, PC shows poor resistance to alkalis and solvents so that degradation of PC will occur at high pH and in some solvents. PC is popular for electronic components and other applications that benefit from its overall safety and reliability due to its combination of durability and resistance to cracking, and its performance in acidic environments.
PPS is an engineering polymer known for its exceptional thermal stability and is resistant to most solvents below 200 ℃. It retains its integrity and performance in high-temperature conditions, maintaining high strength, rigidity, and minimal degradation characteristics. PPS is also recognized for its excellent resistance to acid corrosion, further extending its applications in chemically harsh environments. Its anti-fouling properties are highly beneficial as well, as they prevent the accumulation of unwanted substances on its surface, making it easier to clean and maintain in various applications.
2.2. Polymer composites with enhanced thermal properties
Researchers have taken innovative steps by developing new polymer composites to address the limitations of polymers in various applications. These composites involve the addition of thermally conductive fillers, such as carbon materials and other advanced fibers. The incorporation of fillers can significantly improve both the thermal and mechanical properties of the resulting composites [10], [11], [21]. An additional advantage of these polymer composites is that they can retain the intrinsic chemical properties of the base polymers while it is unlikely that physical properties will decline [21], [57]. This unique combination of properties makes polymer composites highly promising for applications in heat exchangers and other contexts. Moreover, the flexibility of tailoring these composites allows them to meet specific and unique application requirements effectively.
Polymer composites are materials composed of two or more dissimilar components. The primary component, which is continuous and makes up most of the material, is referred to as the matrix. The other components are known as fillers or enhancing phases. Typically, fillers play a crucial role in improving the properties of the matrix. The properties of polymer composites can be significantly different from those of individual constituents, and these properties are highly dependent on how the fillers are incorporated into the matrix. Fillers come in various shapes, and they can be classified based on their shapes into particles, fibers, flakes, and laminas [11]. Fig. 1 describes some possible distributions of fillers with different shapes in polymer matrices.
Fig. 1. Possible distributions of fillers with different shapes in polymer matrix.
Particle fillers in polymer composites include metals and ceramics (e.g., silver, aluminium, and aluminium oxide) [10], [11]. These particles are often employed to enhance the thermal conductivity of polymer composites due to their high intrinsic thermal conductivities. In contrast, fiber fillers, such as glass, carbon, and aramid fibers, are preferred for constructing polymer composites over particle fillers. This is because polymer composites with fiber fillers offer more adjustability in their properties, allowing for control over factors like the type of fiber, matrix material, or processing methods. Adding fillers into a polymer matrix is an effective means of increasing the thermal conductivity of polymers. However, the upper limit of thermal conductivity improvement is not as high as that achieved with electrically conductive polymer composites. This is because that thermal conductivity is a bulk property, and the maximum improvement in thermal conductivity is typically capped at around 20 times higher than that of the intrinsic polymers [58].
3. Thermally conductive polymer composites
The introduction of thermally conductive fillers to polymers is a straightforward and facile strategy to enhance the thermal conductivity of polymer composites [59]. The key to this strategy is creating thermally conductive pathways or networks that allow phonons (heat carriers) to move smoothly along the network with minimal scattering. However, there are important considerations in this approach. Increasing the filler volume fraction is essential for establishing effective thermal conduction networks but can lead to challenges related to the processability of the composite and potential deterioration of its mechanical properties [60], [61]. Balancing thermal conductivity improvements and other material characteristics becomes a crucial aspect of this strategy. Furthermore, achieving a homogeneous dispersion of fillers within the polymer matrix remains a significant challenge. Uneven filler dispersion can lead to inconsistencies in thermal conductivity and other properties of the composite, emphasizing the importance of effective fabrication techniques and thorough material processing methods. In this section, we discuss several factors that influence effective thermal conductivity in terms of fillers and fabrication techniques. We also explore the possible thermal conduction mechanisms in polymer composites to gain a comprehensive understanding of how heat is transferred within polymer composites.
3.1. Thermal conduction mechanisms in polymer composites
Different states of matter exhibit varying thermal conduction mechanisms. In solids, heat transfer primarily relies on two conduction mechanisms: free electron conduction in conductive solids and phonon conduction in non-conductive solids [33]. Phonons, often referred to as the normal mode energy quanta of lattice vibrations, serve as the main carriers of energy in crystalline regions [62], [63]. In most polymers, phonon transport is dominant due to the absence of free electrons in saturated systems. In addition, the low crystallinity of polymers and phonon scattering resulting from disordered molecular chains significantly restrict the thermal conduction of polymers, leading to their low thermal conductivity, typically lower than 0.5 W/(m⋅K) [64], [65].
To enhance the thermal conduction of polymers, a common approach is to develop polymer composites by incorporating thermally conductive fillers into the polymer matrix. This method is more convenient and effective compared to increasing the crystallinity of polymers by adjusting their molecular chains [33]. The addition of fillers not only boosts the thermal conductivity of polymers but also introduces greater complexity into the thermal conduction mechanisms of polymer composites compared to the intrinsic polymers. There are three main theories to explain thermal conduction in polymer composites: the thermal conduction path theory, thermal percolation theory, and thermoelastic coefficient theory [66], [67], [68]. Next, we will discuss these three thermal conduction theories.
3.1.1. Thermal conduction path theory
The thermal conduction path theory is a widely recognized explanation for thermal conduction mechanisms in polymer composites. According to this theory, when thermally conductive fillers are dispersed in the polymer matrix, thermally conductive paths form along the heat flux direction [21], [33]. The construction of these thermally conductive paths depends on filler loading levels. At lower filler loading levels, there is a lower chance of filler-filler connections, resulting in isolated fillers within the polymer matrix, referred to as the ‘sea-island’ system (Fig. 2a) [21]. Since no thermally conductive paths are formed, the thermal conductivity of the polymer composite is dominated by the intrinsic properties of the polymer.
Fig. 2. Thermal conductivity mechanism of composite materials. (a) Isolated state at low filler content. (b) Thermal conduction paths at high filler content. (c) Thermal percolation theory. (d) Thermoelastic coefficient theory.
As the filler loading level increases, the fillers begin to contact each other, leading to the formation of thermally conductive paths or networks (Fig. 2b). Heat flux can then easily transfer along these paths or networks, resulting in improved thermal conductivity. This theory suggests that the effective way for thermal conductivity enhancement is to reduce the thermal resistance between the polymer matrix and the thermally conductive fillers. The intrinsic properties of polymers limit the phonon transport from the fillers to the polymer matrix, meaning that the polymer matrix acts as a resistance to heat transfer.
3.1.2. Thermal percolation theory
The thermal percolation theory suggests that the thermal conductivity of polymer composites can be significantly improved at the percolation threshold, which is also known as the critical range of thermally conductive filler content [33] (Fig. 2c). This theory proposes that it is possible to achieve high thermal conductivity of polymer composites by adding a relatively few fillers to the polymer matrix. However, the validity of this theory remains controversial [69]. In many general polymer composites, a significant change in thermal conductivity is rarely observed, and the percolation threshold is not obvious.
The main reason for this lack of observable change is that the difference in thermal conductivity between most thermally conductive fillers and polymers is relatively small. This limited difference results in only a modest change in thermal conductivity of polymer composites at or near the percolation threshold. Additionally, the presence of defects, boundaries, and interfacial resistance can lead to phonon scattering, hindering phonon transport within the material. Consequently, substantial increases in thermal conductivity are rarely observed, regardless of the filler loading. For high thermally conductive fillers, the percolation threshold of polymer composites may be determined more easily [69].
3.1.3. Thermoelastic coefficient theory
The thermoelastic coefficient theory is based on the concept that the phonon propagation process in polymer composites is like the propagation of elastic coefficients and moduli [70], [71]. This theory suggests that the thermal conductivity of polymer composites is not dependent on the formation of thermal pathways or networks but rather on the overall properties of the composite [58]. In polymer composites, thermally conductive fillers and the polymer matrix represent two distinct phases with different thermal properties. On a macroscopic scale, the improved thermal conductivity of polymer composites results from the synergy of these properties. Specifically, the higher thermal conductivity of polymer composites arises from a larger thermoelastic coefficient and increased phonon transport efficiency, combining the properties of thermally conductive fillers and the intrinsic polymer. As shown in Fig. 2d, the thermal enhancement of polymer composites results from adding thermally conductive fillers to the polymer matrix [72]. The thermal conductivity gradually increases with increasing filler loading, without a sharp increase at a particular point. However, due to the presence of defects and interfacial resistance, phonon scatterings still exist and hinder thermal conduction within the material.
Although these three theories offer valuable insights into explaining the thermal conductivity enhancement of polymer composites, they do not perfectly correspond to real-world observations. One common limitation across these three theories is the impact of interfacial resistance at the boundaries between filler-filler and filler-polymer matrix. Interfacial resistance plays a significant role in impeding the thermal conductivity of polymer composites.
3.2. Filler types
There are three primary classes of fillers commonly used in polymer composites: metals, ceramics, and carbon materials [11], [21]. The selection of filler materials depends on the specific needs of the application, with careful consideration given to the balance between thermal and electrical properties, as well as other factors that may influence the overall performance of the polymer composite. For example, metal/carbon filled polymer composites are popular for heat exchangers, whereas ceramic filled polymer composites are usually designed for electrical insulation, such as printed circuit board.
3.2.1. Metal based fillers
Metal materials are popular fillers for incorporation into polymers because they can impart both thermal and electrical conductivity. Commonly used metallic fillers are powdered forms of aluminium, silver, copper, and nickel [73]. Table 2 provides a summary of the key characteristics of these metallic fillers, including their thermal conductivity, CTE, and density.
Table 2. Physical features of metallic fillers.
Copper is often selected when high thermal conductivity is a primary requirement. However, its relatively high CTE value poses the risk of inducing thermal stress, potentially leading to warping, especially when the CTE of copper significantly differs from that of neighbouring components at various temperatures [14]. Aluminium, while not having as high thermal conductivity as copper, is attractive for certain applications due to its low density, making it a suitable candidate when lightweight properties are essential [14]. Therefore, the selection of metallic fillers involves a trade-off among thermal conductivity, CTE, and density, and it should be determined based on the specific objectives and demands of the composite materials in practical applications.
Metallic fillers have various dimensionalities, which include three-dimensional isotropic shapes (e.g., spheres and cubes), two-dimensional forms (e.g., flakes and platelets), and one-dimensional shapes (e.g., fibers and rods) [82]. These different dimensionalities, as well as how these fillers are spatially distributed within a composite material, have a significant impact on the thermal and electrical properties of the metal-based polymer composites [83].
Well-dispersed metallic fillers within composites help create uniform paths or networks for thermal transport, which enhances the thermal conductivity of the material [10]. Additionally, the inclusion of metallic fillers can improve the mechanical properties of the composite. When polymers are filled with metal powders, properties such as modulus and stress at the point of failure tend to increase.
However, the addition of metallic fillers into polymers can introduce some issues, including increased material density and the potential for oxidation and corrosion of the metal particles [10]. To ensure the overall performance and longevity of metal-based polymer composites, it is often necessary to implement treatments or protective measures to prevent corrosion and surface oxidation of the metal fillers.
Luyt et al. [84] filled low-density PE (LDPE) and linear low-density PE (LLDPE) with copper powder and investigated the mechanical and thermal properties of the resulting composites. Their experimental findings revealed that the thermal conductivity of LDPE and LLDPE, when filled with copper, was more than twice as high as that of the neat materials. However, the composites exhibited poor mechanical performance, with the stress at break of LLDPE significantly decreasing from 27.1 to 11.0 MPa as the copper content increased. The researchers explained that this reduction in tensile properties was due to poor adhesion and the formation of filler clusters, which negatively impacted the mechanical strength.
Similarly, Krupa et al. [85] obtained a similar result when studying high-density PE (HDPE) filled with nickel. They observed that as the nickel content increased, the thermal conductivity of the HDPE/nickel composites also increased. At lower loading levels, the stress at break decreased noticeably with the addition of nickel. However, with the further addition of nickel powder, the stress at break rebounded, nearly reaching the initial level of neat HDPE. The researchers attributed this rebound in the stress at break to the reinforcing effect of the fillers.
3.2.2. Carbon based fillers
The introduction of carbon-based fillers into polymer composites offers significant enhancements in thermal and electrical conductivity, as well as mechanical properties [86]. Various carbon-based fillers are available, such as carbon black, carbon fibers, graphite, graphene, carbon nanotubes, and carbon nanosheets [10]. Table 3 provides a summary of the thermal conductivities of some typical carbon based fillers.
Table 3. Thermal conductivity of carbon-based materials.
| Carbon fillers | λ (W/(m∙K)) | CTE (10-6/˚C) | Density (g/cm3) | Refs. |
|---|---|---|---|---|
| Carbon black | 6–174 | 1.0 | 1.7–1.9 | [90], [91] |
| Carbon fibre (Pitch) | 530–1100 (axis) | 1.5 | 1.9–2.25 | [90], [92] |
| Carbon nanotubes | 2000–6000 (axis) | 10 | 1.3–2.1 | [93], [94] |
| Graphite | 100–400 (in-plane) | 27 | 2–2.25 | [90], [95] |
| Graphene | 5000–6000 | −8 | 1.36–2.23 | [96], [97] |
Carbon materials exhibit higher thermal conductivity due to the strong carbon–carbon covalent bonds [87]. Graphene, in particular, displays the highest thermal conductivity and is excellent for improving heat transport [87]. Carbon nanotubes are also renowned for their high thermal conductivity, with values of around 3000 W/(m⋅K) for multiwall carbon nanotubes [88] and approximately 3500 W/(m⋅K) for single-wall carbon nanotubes [89] at room temperature. Carbon fibers exhibit good thermal properties along their long axis. The carbon structures possess low CTE and have the additional benefit of being resistant to corrosion and chemical decomposition. These stable properties render carbon fillers inert and compatible with thermal polymers.
However, there are challenges in effectively enhancing the thermal conductivity of the polymer matrix using carbon-based fillers. For example, the thermal conductivity of carbon fibers is directional, with much lower thermal conductivity in the transverse direction compared to the axis direction. Carbon nanotubes can be challenging to disperse evenly within a polymer matrix, which can influence the overall thermal enhancement. Achieving good dispersion is essential to maximize the benefits of carbon-based fillers in polymer composites.
3.2.3. Ceramic based fillers
Ceramic based fillers are another important category of fillers used to enhance the thermal properties of polymer matrices while maintaining electrical resistance. These fillers are known for their low CTE. Common ceramic-based fillers include aluminium nitride (AlN), boron nitride (BN), silicon carbide (SiC), and beryllium oxide (BeO) [10]. The key physical properties of typical ceramic-based fillers are summarized in Table 4.
Table 4. Physical features of ceramic-based fillers.
AlN and BN stand out for their high thermal conductivity due to their graphene-like hexagonal planar structure and the combination of ionic and covalent bonds among their atoms [98], [99], [100]. While ceramic-based fillers may not match the thermal conductivity of metallic fillers, their advantages lie in their low CTE and low density. These characteristics make them suitable for applications where not only high thermal conductivity is required but also lower shrinkage and dimensional stability are essential. The low CTE of ceramic-based fillers also allows them to operate effectively at high service temperatures.
However, ceramic-based fillers are often more expensive to prepare and make compatible with thermoplastics when compared to metallic and carbon materials. This cost aspect is a consideration when selecting an appropriate filler for a given application.
3.2.4. Hybrid fillers
Researchers have encountered challenges in achieving the theoretical thermal conductivity of polymer composites by adding single fillers. These challenges arise from phonon scattering caused by defects, interfaces, and other factors [21], [105], [106]. As a result, hybrid thermally conductive fillers are often employed to develop polymer composites. Combining different fillers in a hybrid approach has a synergistic effect, leading to enhanced thermal and mechanical properties compared to composites with a single type of filler [107].
Fillers of different shapes and/or sizes have been used to facilitate the construction of thermal networks. This strategy is mainly through the construction of bridges to form thermal conductive networks and increase the bulk density. Pornea et al. [108] constructed a continuous 3-dimensional (3D) structure by Al2O3, hexagonal BN (h-BN), and BN nanotubes (BNNT) and synthesized a poly(dimethylsiloxane) (PDMS)-based composite. The thermal conductivity of the PDMS composites was 7.50 W/(m⋅K). In this system, the thermal conductivity was further improved by incorporating BNNT onto spherical Al2O3 and planar h-BN to construct bridges for connecting phonon pathways in various directions.
A higher thermal conductivity from the synergistic effect implies that the overall filler loading of these hybrid fillers can be reduced to compensate for the loss of processability and mechanical properties of the polymer composites. Choi et al. [109] investigated the influence of different sizes and proportions of Al2O3 and AlN at a near-maximum pack loading on the thermal conductivity of epoxy composites. The results imply that the optimization of the proportion of different fillers can improve the mechanical properties of the composites even though high filler loadings often lead to low mechanical properties.
The improvements by hybrid thermally conductive fillers in polymer composites can be attributed to various factors, including enhanced filler dispersion in the polymer matrix, the creation of better thermal conduction paths, and reduced interfacial resistance within the polymer matrix [110], [111], [112].
Table 5 provides a summary of common hybrid thermally conductive fillers used in these applications.
Table 5. Common hybrid fillers and their properties.
3.3. Filler features and influences
The addition of highly thermally conductive fillers to a polymer matrix is expected to significantly enhance the thermal conductivity of the polymer composites. However, in practice, the achieved thermal conductivity is often far behind the value determined by theoretical calculation. This disparity arises because various factors come into play that affect the ability of the fillers to enhance heat transfer within the polymer matrix [21]. These factors can act as hindrances to the smooth transport of heat flux, ultimately impacting the final thermal conductivity of the polymer composites. Important factors include the filler loading, their dimensions, and other relevant parameters, all of which have a substantial influence on the thermal conductivity of the composites.
3.3.1. Filler loading
Filler loading plays a crucial role in determining the thermal conductivity of polymer composites (Fig. 3). Generally, increasing filler loading can enhance the thermal conductivity of polymer composites (i.e., monotonous increase, Fig. 3a and b) because more connected heat conduction pathways or networks are gradually formed as the increase of the filler loading [119], [120]. At low filler loading levels (e.g., <30 vol% [21]), achieving high thermal conductivity is challenging due to insufficient contact between fillers, leading to poor thermal conduction paths or networks. Increasing filler loading can facilitate enhanced filler contact and build more effective thermal conduction paths or networks, thereby improving the thermal conductivity of polymer composites (Fig. 3a and b). For example, Bashir et al. [121] achieved the maximum through-plane thermal conductivity of 4.63 W/(m⋅K) when 57 wt% of BN and 3 wt% of rGO were added into thermoplastic polyurethan. However, a high filler loading may lead to some drawbacks, such as reduced mechanical stability, processing difficulties, and increased costs [10]. Huang et al. [122] found that the tensile strength of PP significantly decreased up to 34 % at 20 wt% of graphene nanosheets compared to pristine PP. Moreover, there is always a limitation for theoretical maximum addition volume of thermally conductive fillers, such as 0.637 for spherical rigid fillers with equal radii [123]. Therefore, reducing filler loadings while maintaining high thermal conductivity in polymer composites is another approach. For instance, Guo et al. [124] fabricated horizontally aligned graphene/silver (f-GE/Ag) heterostructures within PVDF composites by stress-induced assembly. The resultant f-GE/Ag structure displayed a high thermal conductivity of 25.22 W/(m⋅K) at 15 wt% filler loading.
Fig. 3. Examples of thermal conductivity increase with the rise in filler loading: monotonous increase (a) Thermoplastic polyurethane (TPU)-based composites [121], and (b) PDMS-based composites [119]; non-monotonous changes (c) PVDF-based composites [125], and (d) poly(lactic-co-glycolic acid)-based composites [126].
However, the thermal conductivity initially increases and then decreases as the filler loading level raises [125], [126] in some cases (Fig. 3c and d). One possible explanation is that the filler agglomerations disrupt the thermal conduction pathways or networks. Simultaneously, agglomerated fillers increase the effective size of fillers, negatively impacting the thermal conductivity and physical properties of the composite. Guiney et al. [126] investigated the influence of BN loadings on the thermal conductivity of 3D printed BN/poly(lactic-co-glycolic acid) composites. They found that after an initial increase of the thermal conductivity of the composites with the increase of the BN loadings, the thermal conductivity began to drop beyond 40 wt% of the BN loadings. Similarly, Zhang et al. [125] found that the thermal conductivity of electrospun modified-BN (mBN)/PVDF composite films reached the maximum of 7.29 W/(m⋅K) with 30 wt% of mBN, and the thermal conductivity decreased with further increasing the mBN loading to 40 wt%.
3.3.2. Filler shape, size, and aspect ratio
The geometry and size of thermally conductive fillers can influence the thermal conductivity of the polymer composites. Different filler shapes lead to various types of contact and contact areas between the fillers. The contact areas play a crucial role in affecting the thermal conductivity of polymer composites because they directly influence the thermal resistance between fillers [127], thereby impacting the thermal conductivity of the polymer composites [128], [129], [130]. Fig. 4 shows the schematic representation of different types of contact between fillers. In general, face contact has the highest contact area, followed by line contact, while point contact has the lowest contact area. Compared to spherical fillers, linear (CNTs, CFs, nanowires, etc) and lamellar (BNNS, expanded graphite, graphene, etc) fillers can form larger contact areas between the fillers and the polymer matrix. The larger contact areas provide phonon transport with a broader path, thus improving the thermal conductivity of the composites by reducing the thermal resistance between the interfaces [131], [132]. Fu et al. [133] proved that lamellar fillers (face/line contact) were more favourable than the spherical fillers (point contact) for the enhancement of the thermal conductivity of epoxy, while the sharp-corner shaped fillers (mostly point contact) was the most difficult to form thermal conduction pathways. Moreover, the orientation of the fillers in the polymer composites should be also considered due to the anisotropic characteristics for linear fillers, and isotropic properties for spherical fillers.
Fig. 4. Schematic representation of different types of contact between fillers [32].
The size of fillers is important to the thermal conductivity of polymer composites. Large size fillers contribute to improving the thermal conductivity because of the reduced interfacial thermal resistance between the filler-polymer interfaces [134] and increased contact areas between the filler-filler interfaces [135]. Zhou et al. [136] showed that large BN spheres had a better enhancement on the thermal conductivity of silicone rubber composites than small BN spheres. However, small size fillers can be more effective in enhancing thermal conductivity in some cases [137], [138]. Shtein et al. [139] presented that the large size graphene nanoplatelets (25 μm) into the epoxy resin did not have the thermal conductivity as high as the small size graphene nanoplates (15 μm). Similarly, Ren et al. [140] demonstrated that the PDMS composites filled with small size BN spheres (89.0 μm) had a higher thermal conductivity than the composites filled with large size BN spheres (160.0 μm). In general, at low loading levels, compared to large size fillers, small size fillers are more efficient in forming thermal conduction paths or networks due to their greater number, leading to a significant improvement in thermal conductivity. At an optimal loading level, thermal conduction paths or networks are already well-established. Beyond this point, larger fillers become beneficial as they reduce interfacial resistance, which further enhances thermal conductivity [141], [142]. However, the accumulation/aggregation of small size fillers is an issue in polymer composites at high filler loadings, which results in the increase of interface defects [143]. Moreover, the reduced spacing between the fillers will destroy the continuity of the polymer matrix, thus cracks or fractures happen easily [144]. Therefore, it is more desirable to select large size fillers to improve the thermal conductivity of the polymer composites considering their much lower interfacial resistance than that of small size fillers.
The aspect ratio, defined as the ratio of diameter to thickness, or length to diameter, is a useful indicator to discuss the influence of the fillers on the thermal conductivity of polymer composites considering the complexity of the geometry and size of the fillers [145], [146]. High aspect ratio fillers (CNTs, CFs, BNNS, etc) can facilitate the formation of thermal conduction pathways or networks at low loading levels due to more easy phonon transports along the linear and lamellar fillers [135], thus resulting in a lower critical volume fraction [147]. They can also easily overlap with each other to form high aspect ratio aggregate structures, which helps to form thermal conduction pathways or networks with reduced thermal resistance [148]. Considering the advantages of the high aspect ratio fillers in thermal conduction, many studies are dedicated to thermally conductive fillers with high aspect ratio, particularly BNNS.
For examples, Jiang et al. [149] prepared BNNS by ball milling and synthesized BN spheres (BNSs) by spray drying. They added the BNSs into polyvinyl alcohol (PVA) matrix to fabricate the composites. The highest in-plane thermal conductivity of the composites was up to 10.6 W/(m⋅K) (∼3700 % enhancement). Similarly, Zhou et al. [150] prepared BNNS by liquid-phase exfoliation and modified it by polydopamine (PDA). The developed PVA/BNNS@PDA composites showed the highest in-plane thermal conductivity of 16.6 W/(m⋅K). The theoretical in-plane thermal conductivity of two-dimensional (2D) BN is up to 400 W/(m⋅K) [151]. Cai et al. [102] prepared a high-quality monolayer BNNS with an outstanding thermal conductivity of 751 W/(m⋅K) by chemical vapor deposition. Although thermally conductive fillers with a high aspect ratio is beneficial to the thermal conduction of the polymer composites, the high aspect ratio should be reasonably selected. The excessively high aspect ratio of the fillers indicates the quite large size of the fillers in a certain dimension, which increases the possibility of defects in the composites, thus decreasing the overall performance of the composites.
3.3.3. Filler orientation
The spatial orientation of thermally conductive fillers in a polymer matrix also affects the thermal conductivity of the polymer composites [152], [153], [154]. One-dimensional (1D) and 2D materials with high aspect ratios show a higher thermal conductivity along the longer dimensions rather than the transverse directions [155], [156]. For example, BNNS exhibits a high in-plane thermal conductivity of 600 W/(m⋅K) [157] but a relatively low through-plane thermal conductivity of 30 W/(m⋅K) [158]. Achieving a high thermal conductivity with a relatively low filler loading is possible by aligning these fillers in their longer dimensions [159], [160]. During the processing of these composites, the anisotropic fillers tend to align along the flow direction, resulting in an enhancement of the thermal conductivity and mechanical properties of the composites in parallel to the flow direction [67]. The alignment of the anisotropic fillers can be adjusted by various methods, such as applying external fields (e.g., electric or magnetic fields [161], [162]), electrospinning [163], filtration [164], and hot pressing [165]. However, we discuss the fabrication techniques which cover the details of the alignment of the fillers in Section 3.4. Here, we brief some examples to illustrate how to adjust the orientation of the fillers (Fig. 5).
Fig. 5. Examples to adjust the orientation of the fillers in polymer composites. (a) Electric field was applied to prepare directional assembly of the fillers [167]. (b) Vacuum filtration assisted the preparation of the composite film [171]. (c) Electrospinning process controlled the alignment of modified BNNS along the fiber [173]. (d) Hot-pressing was used to fabricate orderly aligned BN/PC composites [165].
An electric/magnetic field can be applied to control the directional alignment of electrically/magnetically responsive fillers in the composites. The electrically/magnetically responsive fillers are placed between two electrodes (Fig. 5a) where the fillers can align along the electric/magnetic field direction via electrophoresis [166]. Wu et al. [167] achieved the alignment of BN and Al2O3 in PDMS along the electric field. The aligned fillers formed chain-like structures which assisted the thermal conduction. The result showed that the thermal conductivity of the PDMS composites with aligned fillers (0.228 W/(m⋅K)) was higher than that of the PDMS composites with random fillers (0.215 W/(m⋅K)). Guo et al. [168] developed vertically aligned NdFeB@Ag arrays and a styrene-ethylene-propylene-styrene (SEPS) crosslinked network within paraffin (PA). The composite achieved a maximum thermal conductivity of 2.59 W/(m⋅K), representing a 12.3-fold increase compared to pure PA.
Vacuum-assisted filtration can achieve directional orientation of fillers in polymers, especially for 2D materials. The effect of gravity can help the fillers align along the parallel direction in the polymers [169], thus forming thermal conduction pathways along the plane direction. Therefore, the in-plane thermal conductivity of the composites can be improved significantly [170]. Fu et al. [171] prepared cellulose nanofibers (CNFs)/BNNT composites by vacuum-assisted filtration (Fig. 5b). They achieved the highest in-plane thermal conductivity of 20.9 W/(m⋅K) for the composites, which was approximately 14.3 times higher than that of conventional polymers.
Electrospinning is a simple and efficient method to produce polymer fibers from polymer solutions or melt under a high voltage electric field. The electrospun fibers exhibit highly aligned orientations, facilitating the arrangement of the fillers within the fibers in a highly oriented manner [172]. Zhao et al. [173] controlled the alignment of modified BNNS along the fiber via electrospinning while the silver nanowire (AgNW) was sprayed to overlap with the modified BNNS (Fig. 5c). The aligned fillers in polyimide (PI) enhanced the in-plane thermal conductivity of the composites up to 8.38 W/(m⋅K) due to the formation of thermal conduction pathways along the hybrid network.
Hot-pressing can achieve orderly orientation of fillers along the in-plane direction by applying external forces at high temperatures. It can also produce layer-by-layer assembled filler structures with minimal defects during fabrication [155]. Su et al. [165] prepared highly ordered BN plates by hot-pressing and prepared BN plates/PC composites (Fig. 5d). The maximum in-plane thermal conductivity of the composites was 3.09 W/(m⋅K) since the orderly arrangement of the BN plates provided pathways for phonon transport.
3.3.4. Others
Some other factors also affect the effective thermal conductivity of polymer composites, such as filler dispersions and interfaces within the composites. Various strategies, such as surface functionalization (e.g., chemical grafting [174] and silane coupling [175]) and surface treatment (e.g., plasma treatment [176]) have been used to improve the surface properties of fillers to enhance their dispersibility within the polymer matrix. When fillers are uniformly dispersed in a polymer matrix (i.e., the fillers are isolated with each other) with polymer chains surrounding them, it can hinder the effective heat transfer [177]. On the other hand, when the thermally conductive fillers are segregated within the polymer matrix in close distances, heat conduction paths can form within these segregated areas, increasing the final thermal conductivity of the composite [178]. Many preparation techniques, such as template- and equipment-assisted methods have been designed to facilitate the distribution of fillers in thermally conductive networks, achieving high thermal conductivities for the composite, even at low filler contents [179], [180].
The interface between the polymer and the fillers is a critical factor affecting heat transfer. The structure of the interface changes when the dispersed phase (thermally conductive fillers) differs from the polymer matrix [181]. The interface usually consists of three main parts: the original contact area between the polymer and the fillers, a solid–liquid layer formed by diffusion between the polymer and the fillers, and surface coating introduced through the surface modification of the fillers [36], [182]. This complexity makes the chemical composition and structure of interfaces in polymer composites intricate. Interfaces exhibit three primary properties: transmission, blocking, scattering and absorption [32] (Fig. 6). The transmission effect (Fig. 6a) acts as a bridge between the polymer and the fillers, ensuring the continuity of both components and facilitating the effective transfer of properties between them [183]. The blocking effect (Fig. 6b) involves mechanically preventing crack propagation and reducing stress concentration. Scattering and absorption effects (Fig. 6c) refer to the ability of the interface to scatter and absorb various waves, including light waves, acoustic waves, thermoelastic waves, and more [184], [185]. These effects collectively impact the thermal conductivity of polymer composites.
Fig. 6. Illustrations of three main interface properties: (a) transmission, (b) blocking, and (c) scattering and absorption. Take spherical fillers uniformly dispersed in polymer matrix as an example.
3.4. Fabrication techniques
Fabrication techniques also significantly influence the thermal conductivity of polymer composites by controlling the dispersion and distribution of thermally conductive fillers in the polymer matrix and introducing or avoiding potential defects [10]. Table 6 summarises some strategies to address the challenge of insufficient thermal conductivity in the polymer composites. The most popular approach is to blend single or hybrid fillers with polymers, which utilizes filler synergy or is with the help of various filler features (i.e., filler dimensions, aspect ratios, etc.). Vacuum-assisted filtration can produce polymer composite films with a highly hierarchical structure for the fillers by adjusting the filler arrangement in polymer composite solutions under vacuum filtration [186]. However, the filtration process requires a long time, limiting its practical applications. The template method is a new approach to develop 3D networks within polymer composites for highly thermal conduction pathways. The complete and uninterrupted thermal conduction pathways or networks ensure the heat transfer in the polymer composites, although complex template preparation and polymer filling are still challenging. Besides, equipment-assisted assembly achieves thermal conduction networks by adjusting the filler orientation via external fields, such as electric and magnetic fields. Electrospinning is a straightforward technology to fabricate highly ordered composites [187]. Next, these methods will be discussed as follows.
Table 6. Summary of different fabrication techniques for thermally conductive polymer composites.
| Fabrication methods | Features | Strategies | Advantages | Challenges | Solutions | |
|---|---|---|---|---|---|---|
| Blending |
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| Vacuum-assisted filtration |
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| Template assembly |
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| Equipment-assisted assemble | Hot-pressing |
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| Electrospinning |
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| External fields |
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3.4.1. Blending
Solution blending and melt blending are two simple methods to prepare thermally conductive polymer composites by mixing single or hybrid fillers with polymer solutions [188] or directly mixing fillers with polymers at high temperatures. However, improvement of the obtained thermal conductivity through this way is far below expectations due to the inefficient construction of heat conduction networks [189].
Solution blending (Fig. 7a) requires a relatively high loading of the fillers to form thermal conduction pathways or networks [190], [191]. For example, Xu et al. [192] prepared BN nanosheet-filled polyamide 6/polyethylene terephthalate composites by solution blending and achieved a thermal conductivity of 3.28 W/(m⋅K) at 55 wt% filler loading. Cao et al. [193] modified CNTs with (3-aminopropyl) trimethoxysilane (APTMS) and subsequently prepared APTMS-modified CNTs / PVDF composites by solution blending. The resultant composite demonstrated a thermal conductivity of 2.15 W/(m⋅K) when the APTMS-modified CNT concentration was 40 vol%.
Fig. 7. Schematic of (a) solution blending and (b) melt blending [197].
Melt blending (Fig. 7b) normally is followed by mixture extrusion from single or twin-screw extruders or injection moulds [194]. For example, Zhang et al. [195] applied a powerful shear flow field in an extruder to fabricate highly oriented BN/HDPE composites. The highest thermal conductivity of the composites reached 3.57 W/(m⋅K) when 40 wt% BN was added. Similarly, Li et al. [196] prepared oriented short CF/polyether ether ketone (PEEK) composites by adjusting the shear stress within the extruder. The through-plane thermal conductivity of the composite was 0.63 W/(m⋅K) with an improvement of approximately 174 %.
3.4.2. Vacuum-assisted filtration
Vacuum-assisted filtration is an efficacious approach for the fabrication of oriented fillers within polymer composites. This compounding technique operates with the support of hydrodynamic forces, engendered by water flow, thereby facilitating the deposition of fillers to assume an in-plane configuration at the base of the filtration device [143], [198]. For example, 2D materials, such as graphene and BN, exhibit remarkable properties, such as chemical stability, high surface area, and notably high in-plane thermal conductivity. Nevertheless, fully harnessing these favorable properties of 2D materials to construct macroscopic materials with exceptional thermal conductivity remains challenging. One viable solution is to assemble 2D materials into structures resembling paper or film, ensuring the parallel arrangement of fillers. This configuration creates a horizontally interconnected network that greatly enhances in-plane thermal conductivity. Zhao et al. [199] fabricated PI composites with ordered modified BN structure by vacuum-assisted filtration and other steps as illustrated in Fig. 8. The resultant composite demonstrated an in-plane thermal conductivity of 17.44 W/(m⋅K) when 18.79 vol% modified BN was loaded.
Fig. 8. Schematic of the vacuum-assisted filtration process [199].
3.4.3. Template method
Templating utilizes a 3D cross-linked filler framework to immerse into a polymer matrix to fabricate polymer composites [200]. Two strategies can achieve 3D frameworks, including using ice as the template or an existing porous framework. Ice templating is a classic and cost-effective method, which uses the orientated ice growth, via temperature difference, to align the filler along the ice crystal direction [201]. Water is the most common freezing medium, though sometimes organic solvents are also used. Four main growth directions can be controlled, including isotropic [202], unidirectional [203], bidirectional [204], and radial freezing [205].
The vertical growth of ice crystals is commonly applied when high through-plane thermal conductivity is required. Hu et al. [206] utilized bidirectional freezing assembly to create a vertical fluorinated BNNS/GO skeleton in PDMS composites, achieving a through-plane thermal conductivity of 3.28 W/(m⋅K) at 9.5 vol% filler loading. Similarly, Hu et al. [207] employed the same method to fabricate a vertically aligned dopamine-modified boron nitride/silicon carbide aerogel scaffold in PEG composites, resulting in a through-plane thermal conductivity of 3.94 W/(m⋅K), 12.31 times higher than that of the pure PEG. Zeng et al. achieved a notable in-plane thermal conductivity of 2.85 W/(m⋅K) at a relatively low loading of BN nanosheets, specifically 9.29 vol%, employing the ice-templated assembly method (Fig. 9) [208]. Other studies [209], [210], [211], [212] also demonstrate the significant enhancement in thermal conductivity of polymer composites by constructing 3D structures using the template method.
Fig. 9. Schematic illustration of the template assembly process [208].
In addition to ice, alternative materials have been employed as templates for the construction of 3D networks. Chen et al. [213] utilized salt as a template, amalgamated with BN and PVDF in acetone. Given the insolubility of salt in acetone, a contiguous network was formed by driving BN into the interstitial space among salt particles after acetone evaporation and salt removal (Fig. 10). through vacuum-assisted impregnation, epoxy was introduced into the interstices of the BN-PVDF scaffold, resulting in the formation of epoxy/BN-PVDF composites featuring a conductive BN-PVDF network. This composite showed a thermal conductivity of 1.23 W/(m⋅K) when 21 wt% BN was loaded. The inclusion of PVDF enhanced the viscosity and prevented the sedimentation of BN. After thermal treatment, the epoxy/ BN-PVDF composites can be further transformed into epoxy/BN-C composites, thereby achieving a heightened thermal conductivity of 1.47 W/(m⋅K) due to the mitigation of phonon scattering at the interfaces.
Fig. 10. Schematic diagram of the synthesis procedure for BN-PVDF scaffolds and epoxy/BN-PVDF composite [213].
3.4.4. Equipment-assisted assembly
Enhancing thermal conductivity at low filler fractions remains a formidable challenge. Although certain strategies have been discussed for achieving filler orientations, composite materials often necessitate the use of templating, which involves multiple treatment steps. Considering these issues, some other strategies are also used to enhance the efficiency of filler utilization. These strategies include magnetic field assistance, electrospinning, and one-step hot pressing. These approaches yield superior performance compared to mere blending, allowing to adjust performance direction to a certain extent in accordance with specific requirements in some cases [214], [215], [216].
The magnetic field assisted method is to use a magnetic field to orientate the packing with magnetic response functionalization in low viscous polymer matrix. Magnetic responsive materials, such as Fe3O4, are used to functionalize the fillers. For example, Lin et al. [217] combined epoxy with BN coated with Fe3O4 by adjusting the alignment of the fillers under a magnetic field during curing. The result showed that the enhanced thermal conductivity was 104 % higher than that of the unaligned composites. However, adjusting the filler orientation heavily relies on the effect of magnetization. The process also involves the introduction of other low thermal conductivity materials and additional treatment steps.
Electrospinning can achieve effective alignment of fillers and polymer fibers [218], [219], [220]. Electrospun fibers can be arranged in a particular way followed by hot pressing to retains the filler alignment for customizing the properties of final composites. For example, Chen et al. [172] fabricated PVDF/BNNS composites by electrospinning. The nanocomposite films exhibited a high in-plane thermal conductivity of 16.3 W/(m⋅K) at 33 wt% BNNS, and electrospinning has significant potential for large-scale production. However, it is influenced by various factors, such as solution concentration, stability, and processing humidity. The potential solutions to mitigate the limitation is the combination of electrospinning and in-situ polymerization, which can further improve the uniform dispersion and stability of the mixture. Vertically oriented cylindrical composites can also be established by rolling the composite fabric. Winding times and orientation control the thermal conductivity in a desirable direction.
Hot pressing is usually a successive process after vacuum-assisted filtration, electrospinning, or the template method. One obvious advantage of hot pressing is that the designed structure can be partially preserved with less steps compared to the template method. One-step hot-pressing is also suitable for a variety of substrates. Yi et al. [221] fabricated PVDF/CF composites by using one-step hot pressing and achieved a maximum through-plane thermal conductivity of 2.89 W/(m⋅K).
4. Polymer heat exchangers
4.1. Polymers vs. Metals in heat exchangers
Designing heat exchangers using polymers is a recent promising development [19], [224]. This shift from metals to polymers for heat exchangers is due to the unique properties of polymers, even though they have lower thermal conductivity and inferior mechanical properties compared with metals. Table 7 summarizes the basic thermal and mechanical properties of commonly used polymers and metals in heat exchangers at 25 °C. Polymers typically have lower maximum operating temperatures compared with metals. For instance, commonly used polymers, such as PVDF and PTFE, have upper service temperature limits of approximately 143 and 260 °C, respectively, whereas metals, such as stainless steel or copper, can operate at temperatures exceeding 1000 °C (Table 1). In addition, polymers have lower pressure-handling capabilities due to their reduced mechanical strength and modulus. As a result, polymer heat exchangers are generally suitable for low- to moderate-pressure applications [10]. However, metals exhibit significantly higher tensile strength and can withstand both higher pressures and temperatures. For example, stainless steel and titanium heat exchangers are commonly used in high-pressure environments (>10 MPa [225]) and at elevated temperatures.
Table 7. The basic thermal and mechanical properties of common polymers and metal materials at 25 °C.
| Materials | Density (g/cm3) | λ (W/(m∙K)) | Tensile Strength (MPa) | Melting Point (℃) |
|---|---|---|---|---|
| PVDF | 1.78 | 0.19 | 44 | 160 |
| PTFE | 2.17 | 0.27 | 12 | 330 |
| PP | 0.94 | 0.11 | 31 | 160 |
| HDPE | 0.96 | 0.51 | 45 | 136 |
| LDPE | 0.91 | 0.40 | 18 | 121 |
| PC | 1.2 | 0.2 | 62 | 300 |
| PPS | 1.43 | 0.3 | 69 | 280 |
| Aluminium 3003 | 2.7 | 169 | 179 | 629–652 |
| Cu (99.9 %) | 8.89 | 391 | 221 | 1083 |
| Stainless Steel 304 | 7.92 | 52 | 207 | 1393 |
| Titanium | 4.51 | 17 | 517 | 1691 |
| Cu-Ni 90/10 | 8.9 | 50 | 140 | 1100 |
The comparison of the thermal conductive properties between polymers and metals is a significant consideration. In general, polymers exhibit lower thermal conductivities, typically below 1 W/(mK) [226], [227], whereas metals have substantially higher thermal conductivities, in the range of 50–500 W/(m·K). This considerable difference often makes the preference toward metals in scenarios that require efficient heat transfer. However, the material selection for a heat exchanger depends on various factors, such as the nature of the fluids involved in the process. For example, when the heat exchanger encounters acid vapors or seawater, metals may not be a good option due to the corrosion risks.
Only some metals, such as titanium, and Cu-Ni alloy, are resistant to corrosion. Titanium, for instance, emerges as the first option because of its exceptional anti-corrosion properties, low density, and high tensile strength, although its thermal conductivity (17 W/(m·K)) may not be as high as other metals. Similarly, Cu-Ni alloy has good resistance to corrosion, but its high cost limits its wide application. To enhance the thermal performance of metals with lower thermal conductivities, engineers often use heat exchangers with thinner walls [228], [229]. This method improves heat conduction, compensating for the lower thermal conductivity of some metals. The option to use polymers or metals in heat exchanger designs depends on operational conditions, cost, and material properties that best meet the specific requirements in practical applications.
Table 8 shows some examples of polymer and metal heat exchangers and their heat transfer performances. Metal heat exchangers, such as aluminium and copper, exhibit significantly higher thermal conductivity than polymers. As a result, metal heat exchangers typically display higher overall heat transfer coefficients, such as 5800 W/m2·K observed for copper heat exchangers in water-nanofluid systems [235]. In contrast, shell and tube polymer heat exchangers, such as PVDF and PTFE, demonstrate lower overall heat transfer coefficients of 612 [230] and 391 W/m2·K [231], respectively, in water-water systems. These differences highlight the inherent thermal advantages of metals, which contribute to their widespread use in high-performance applications requiring maximum heat flux and compact designs. However, for thermally enhanced plate PPS heat exchangers, the maximum overall heat transfer efficient was up to 1850 W/m2·K in a water-water system [229], suggesting that thermally enhanced polymer composites can partially bridge the performance gap between polymers and metals. This also suggests that the incorporation of thermally conductive fillers can significantly enhance the heat transfer capabilities of polymer heat exchangers, making them more competitive in applications where chemical stability and corrosion resistance are crucial.
Table 8. Examples of polymer and metal heat exchangers and their heat transfer performances.
| Materials | λ (W/(m∙K)) | Configurations | Working Conditions | Overall Heat Transfer Coefficients (W/m2∙K) | Refs. | |
|---|---|---|---|---|---|---|
| Temperature (℃) | System | |||||
| PVDF | 0.2 | Shell and tube (hollow fiber thickness: 150 µm) | 80 | Water-water | 612 | [230] |
| PTFE | 0.2 | Shell and tube (hollow fiber thickness: 200 µm) | 55 | Water-water | 391 | [231] |
| PP | 0.18 | Shell and tube (hollow fiber thickness: 60–80 µm) | 75 | Air-water | 339 | [232] |
| HDPE | 0.5 | Plate (length: 290 mm, width: 155 mm, thickness: 150 µm) | 50 | Air-water | 35–120 | [233] |
| PPS (PPS/graphite composite) | 2.4 | Plate (length: 158 mm, width: 90 mm, thickness: 2.5 mm) | 80 | Water-water | 1280–1850 | [229] |
| Aluminium | 202.4 | Finned tube (fin thickness: 0.1 mm, outer diameter of tube: 5.1 mm) | 60 | Air-air | ∼110 | [234] |
| Copper | 387.6 | Finned tube (fin thickness: 0.1 mm, outer diameter of tube: 5.1 mm) | 60 | Air-air | ∼100 | [234] |
| Copper | N/A | Plate (length 375 mm, width 120 mm, thickness: 4 mm) | 50–70 | Water-nanofluid | ∼5800 | [235] |
Although material properties are fundamental to heat exchanger performance, other factors such as heat exchanger configurations, fluid properties, and operating conditions also play a significant role in overall heat transfer performance. Design options, such as finned tube structures and hollow fiber configurations, directly influence heat transfer efficiency. Similarly, fluid type and flow characteristics affect convective heat transfer [236], [237], with liquid–liquid systems generally exhibiting higher overall heat transfer coefficients than air-based systems. Additionally, polymer heat exchangers operate in moderate temperature ranges due to material limitations, whereas metals can withstand significantly higher temperatures and pressures. Therefore, selecting a heat exchanger material requires a comprehensive approach that should balance thermal performance with chemical resistance, mechanical durability, and long-term maintenance requirements.
Table 9 presents a comparative ranking of metal and polymer heat exchangers based on key design criteria, including thermal conductivity, weight, cost, corrosion resistance, mechanical strength, manufacturing complexity, chemical stability, and flexibility in design. Metals for heat exchangers, such as aluminium and copper, exhibit superior thermal conductivity and mechanical strength, making them suitable for high-performance applications. Polymer heat exchangers provide significant advantages in weight reduction, corrosion resistance, and cost-effectiveness. These factors play a crucial role in selecting the appropriate material for specific operating conditions.
Table 9. Comparative ranking of metal and polymer heat exchangers based on key design criteria.
| Criteria | Metal Heat Exchangers | Polymer Heat Exchangers |
|---|---|---|
| Thermal conductivity | 5 (High) | 1–2 (Low to Moderate with fillers) |
| Heat transfer | 5 (Good heat transfer efficiency) | 1–3 (Proper configurations help heat transfer) |
| Weight | 2 (Heavy) | 5 (Lightweight) |
| Cost | 2–3 (Expensive, especially for high-performance alloys) | 4–5 (Generally lower cost) |
| Corrosion resistance | 2–3 (Depends on material; stainless steel and titanium are better) | 5 (Excellent, especially for PTFE, PVDF, etc.) |
| Mechanical strength | 5 (High, excellent pressure and temperature resistance) | 3–4 (Lower, but composites can reinforce it) |
| Manufacturing complexity | 3–4 (Complex, especially for intricate geometries) | 4–5 (Easier for extrusion and moulding) |
| Chemical stability | 3–4 (Good, but can degrade in extreme chemical environments) | 5 (Excellent resistance to harsh chemicals) |
| Flexibility in design | 2–3 (Limited to metal forming techniques) | 5 (Can be extruded, moulded, or 3D-printed) |
| Overall performance | 4 (Best for high-performance, extreme conditions) | 3–4 (Best for corrosion resistance, low-cost applications) |
Although the inherent properties of polymers limit the operational range of polymer heat exchangers, ongoing advancements in polymer composites, such as reinforced structures and hybrid materials, are gradually expanding their applications. In certain cases, polymer heat exchangers demonstrate competitive performance with metal heat exchangers, particularly in environments where high corrosion resistance, low weight, and cost-effectiveness are required. For instance, thermally enhanced polymer composites, optimized geometries, and innovative manufacturing techniques (e.g. 3D printing) have significantly improved the heat transfer efficiency of polymer heat exchangers. Future research may focus on further optimizing polymer materials and refining heat exchanger configurations (e.g., increasing wall thickness, integrating thermally conductive fillers, or employing reinforced structures) to overcome existing limitations while preserving their inherent advantages in cost, chemical stability, and design flexibility.
4.2. Advantages of polymer heat exchangers
The disparities between polymers and metal materials in terms of thermal and mechanical properties are presented in Table 7. The utilization of polymers in heat exchanger production poses challenges during the design and manufacturing phases attributable to their relatively diminished thermal conductivity, substantial thermal expansion characteristics, and diminished mechanical strength. Nevertheless, polymers offer certain advantages that serve to offset these limitations. These benefits encompass their considerable resistance to chemical corrosion within conditions characterized by moderate pressures and temperatures. Additionally, polymers exhibit cost-efficiency and provide space-saving attributes [238]. Importantly, polymer heat exchangers can achieve comparable heat transfer performance with metal heat exchangers under certain conditions [239]. The following mathematical relationships can prove that it is rational to use polymer heat exchangers rather than metal heat exchangers for some special applications [238].
The heat transfer coefficient (U) in a cylindrical tube can be formulated as follows [240]:(1)where di and do are the diameter of interior and exterior tubes, respectively; hi and ho are the heat transfer coefficient of interior and exterior films, respectively; k tube is the thermal conductivity of the tube; fi and fo are the fouling coefficient of interior and exterior tubes, respectively.
To simplify, the formula-based on heat transfer through-plane wall is used as Eq. (2).(2)where kwall is the thermal conductivity of the tube wall, and t is the thickness of the tube wall.
In the analytical calculations, we take Ni-Cr-Mo alloy and PVDF as examples to specify the difference. We assume that only the tube materials used are different, while other parameters are kept the same, and set the t is 1 mm, hi = ho = 4000 W/m2⋅K, and fi = fo = 4000 W/m2⋅K. The calculation results show that the heat transfer coefficients of the alloy and PVDF are 890 W/m2⋅K and 145 W/m2⋅K, respectively. Therefore, the heat transfer coefficients ratio is 890/145 = 6.
Consequently, the PVDF exchange surface should be six times larger than the of the alloy to achieve the same heat transfer coefficient. However, if we consider PVDF and Ni–Cr–Mo tubes of the same diameter and same thickness, the weight ratio of the heat exchange unit surface is expressed by the ratio of density. The densities of the alloy and PVDF are 8.9 g/cm3 and 1.78 g/cm3, respectively. Therefore, the weight ratio of the alloy and PVDF is around 5.
Moreover, if we introduce the price of materials in the form of tubes, then the price of materials for the essential part of the exchanger can be obtained. Here, we set US $4500 per unit area for the alloy, and US $1500 per unit area for PVDF [241]. This means that if all other conditions remain equal, a tube bundle of Ni–Cr–Mo alloy will cost 2.5 times as much as a PVDF bundle. The price difference is significant enough to recommend the development of PVDF heat exchangers.
This quantitative assessment shows the benefits of adopting polymers over metals in heat exchangers. However, industrial-scale polymer heat exchangers are still not common, representing a niche market. Their widespread adoption in the process industries has yet to be fully realized. This may be due to a lack of awareness regarding the advantages offered by polymer heat exchangers, coupled with a dearth of reliable design methodologies and insufficient investigations under real operational conditions. The scope of industrial applications for these heat exchangers appears to be constrained only by the creative ingenuity of process designers, particularly given the increasing use of polymers in various plant components, including pumps and valves.
4.3. Design of polymer heat exchangers
Traditional metal heat exchangers operate over a wide range of temperatures up to 1000 ℃. Fig. 11 shows the typical heat exchanger configurations across different temperature ranges. These configurations include plate and frame heat exchangers, welded plate heat exchangers, plate-fin/tube heat exchangers, and printed circuit heat exchangers. Each design has specific thermal and mechanical requirements. However, due to the temperature limitations of polymers, polymer heat exchangers are primarily utilized in low-temperature applications. Therefore, in the following sections, the discussion will focus on common polymer heat exchanger designs that are suitable for lower operating temperatures.
Fig. 11. Types of heat exchangers and their typical temperature limits [225].
Polymers are not the immediate choice for heat transfer applications due to their inherently low thermal conductivity. Nevertheless, they can be competitive with metal materials in heat exchangers when considering overall system performance, due to several distinct advantages, such as low-temperature processing, malleability, ease of shaping and joining, anti-fouling properties, and corrosion resistance [10]. Achieving equivalent heat transfer performance to traditional metal heat exchangers necessitates a thorough and tailored design approach. This design should consider the selection of thermally conductive polymer composites and comprehend how thermal conductivity, processability, and mechanical strength interact with one another. Moreover, optimizing the heat transfer performance of polymer heat exchangers is achievable through the concurrent selection of suitable polymer composites and careful design. Recognizing the merits that support the deployment of polymers as a viable alternative to traditional metal heat exchangers, we examine three primary categories of polymer compact heat exchangers available in the industry: plate heat exchangers, heat exchanger coils, and shell-and-tube heat exchangers. In addition, we highlight the emerging 3D-printed polymer heat exchangers, which offer unique design flexibility and customization opportunities, further expanding the potential applications of polymer-based thermal systems.
Plate heat exchangers. Plate heat exchangers has widespread applications in various industrial contexts due to their inherent advantages, including compactness, design versatility, and cost-effectiveness [242]. These heat exchangers are composed of multiple plates stacked and interconnected (Fig. 12). Each plate features a chevron pattern to increase the overall surface area and promote turbulent flow, thereby enhancing heat transfer efficiency [10]. Given their straightforward design, it is feasible to construct plate heat exchangers using standard polymer resins. Polymers offer excellent processability, and the uncomplicated structure of the plate elements allows for extrusion or moulding with ease [243]. Polymers have been established in high volume manufacturing processes, thus the barrier to entry into the market is simple [244], [245]. However, it is essential to undertake significant redesign efforts when using polymer plates since they exhibit lower mechanical strength compared to their metal counterparts. Despite the associated challenges, there are instances showcasing the potential of using polymers in the production of plate heat exchangers. Christmann et al. [246] created a falling film evaporator using PEEK and examined the mechanical stability of the polymer film. Their experimental findings indicated that a 25 μm thick film, in combination with an appropriately designed spacer geometry, provided sufficient mechanical stability for the heat exchange unit without compromising the overall heat transfer coefficient.
Fig. 12. Assembled heat exchanger. (a) Real view including resistance thermometer; and (b) exploded view [229].
Heat exchanger coils. Heat exchanger coils typically comprise a coiled tube or piping arrangement, serving as a conduit through which one of the fluids circulates (Fig. 13). The other fluid, often at a dissimilar temperature, flows either within the coil or around it. The partition between these two fluids is typically formed by the wall of the coil, constructed from thermally conductive materials, commonly metals. However, polymer heat exchangers are emerging as a viable alternative for specific applications where corrosion resistance, weight reduction, or cost-effectiveness is crucial. Polymer coil heat exchangers offer significant advantages in handling highly corrosive fluids or in environments requiring lightweight structures, such as desalination, wastewater treatment, or heat, ventilation, and air conditioning (HVAC) systems. These features make them competitive when the polymer materials well match the operational conditions. For instance, a commercial fluoropolymer heat exchanger coil from the Junkosha company is designed for harsh environment applications [247].
Fig. 13. Schematic illustrations of heat exchanger coils [248].
Shell-and-tube heat exchangers. Shell-and-tube heat exchangers represent another prevalent design, comprising multiple tubes (Fig. 14). This type of heat exchangers consists of fundamental components, including the tube, shell, front and rear end headers, along with baffles or fins [249]. Heat exchange transpires between the fluid flowing inside the tubes and the fluid surrounding the exterior of the tubes. Several critical parameters, such as tube wall thickness, tube wall fraction coefficient, and fluid flow rates, are important to the design of these heat exchangers. Surface topology modifications are often employed to enhance heat transfer efficiency by augmenting the contact area and generating a more turbulent flow. The use of polymers in fabricating shell-and-tube heat exchangers is advantageous due to the ability to extrude tubes to desired dimensions and the versatility in manufacturing the shell through various methods. Furthermore, the shell-and-tube heat exchangers based on polymers have enhanced mechanical strength compared with plate heat exchangers [224]. Enhancements in mechanical strength can be achieved by adjusting the wall thickness and diameter of the tubes to withstand specific pressures for shell-and-tube heat exchangers.
Fig. 14. A bundle of the hollow PTFE fibers (a), the cross-section (b), and appearance (c) images of the hollow fiber PTFE membrane heat exchanger [231].
3D-printed polymer heat exchangers. 3D printing, also known as additive manufacturing, is an emerging technology that allows to produce polymer heat exchangers with complex structures [250]. This method can create complex designs, such as customized flow channels and surface patterns, that are difficult to achieve using conventional manufacturing techniques [251]. These features can improve the surface-area-to-volume ratio and heat transfer performance by promoting turbulence in fluid flow, which can help offset the low thermal conductivity of polymers [252]. The layer-by-layer manufacturing process in 3D printing reduces material waste and enables rapid prototyping, making it suitable for cost-effective manufacturing [253].
Recent studies have highlighted the potential of 3D printing for fabricating polymer heat exchangers with advanced geometries and improved performance. 3D-printed polymer heat exchangers have been developed with complex lattice and channel designs to enhance heat transfer efficiency while maintaining lightweight structures [254], [255]. For example, Luo et al. [256] reported that 3D-printed polydimethylsiloxane heat exchangers with microchannels had uniform heat dissipation and improved heat transfer characteristics. In addition, polymer composites with thermally conductive fillers have been explored for further performance enhancement [257]. Li et al. [258] used graphite and pitch-based carbon fibers as fillers to enhance polyethylene terephthalate glycol by 3D printing. The 3D-printed polymer heat exchangers showed excellent thermal conductivity and stability, and the potential for efficient low-temperature heat transfer.
Also, 3D printing of polymer heat exchangers faces some challenges, such as concerns in mechanical strength, filler dispersion, and scalability for large-scale production [253], [257], [259]. Despite these issues, the advancements of 3D printing technology have enabled customization of heat exchanger designs to suit specific applications, such as HVAC systems and compact energy recovery devices [255], [260]. Further research is needed to address these challenges and expand the application of 3D-printed polymer heat exchangers in more areas.
5. Applications of polymer heat exchangers in water and energy savings
Polymer heat exchangers present opportunities across various domains, particularly in sectors requiring high corrosion resistance, such as the chemical industry and desalination [224]. Table 10 provides some examples of polymer heat exchangers for water and energy saving applications. For instance, polymeric hollow fiber and film heat exchangers have been successfully applied in water desalination processes, demonstrating high thermal efficiency and effective water recovery. Additionally, polymer heat exchangers have been implemented in waste heat recovery systems, where materials such as nylon and polypropylene have achieved heat exchanger effectiveness values comparable to stainless steel alternatives [261].
Table 10. Examples of polymer heat exchangers in water and energy savings.
| Application areas | Heat Exchanger Types | Research Methods | Specifications | Remarks | Refs. |
|---|---|---|---|---|---|
| Water desalination/distillation | Hollow fiber heat exchanger | Numerical simulation and experimental |
|
|
[309] |
| Film heat exchanger | Experimental |
|
|
[310] | |
| Hollow fiber heat exchanger | Experimental |
|
|
[311] | |
| Hollow fiber heat exchanger | Experimental |
|
|
[312] | |
| Plate heat exchanger & tube heat exchanger | Experimental |
|
|
[313] | |
| Waste heat recovery | Fin-tube heat exchanger | Experimental |
|
|
[314] |
| Heat exchanger coils | Numerical simulation |
|
|
[261] | |
| Webbed-tube heat exchanger | Numerical simulation |
|
|
[315] | |
| Hollow fiber heat exchanger | Experimental |
|
|
[262] | |
| Compact heat exchanger | Theoretical and experimental |
|
|
[316] | |
| Evaporative cooling | Film heat exchanger | Experimental |
|
|
[263] |
| Hollow fiber heat exchanger | Experimental |
|
|
[317] | |
| Film heat exchanger | Experimental |
|
|
[318] | |
| Hollow fiber heat exchanger | Experimental |
|
|
[319] | |
| Film heat exchanger | Experimental |
|
|
[320] |
A notable example is the use of polymer compact heat exchangers in air–water systems, where they provide high overall heat transfer coefficients and maintain competitive pressure drops compared to traditional finned-tube radiators [262]. Similarly, polymer film heat exchangers have shown excellent energy efficiency in evaporative cooling applications, achieving high coefficients of performance while maintaining low-pressure drops [263]. These examples highlight the advantages of polymer heat exchangers, such as corrosion resistance, and adaptability to various heat transfer processes, including water desalination, waste heat recovery, and evaporative cooling. Despite their lower thermal conductivity compared to metals, innovative polymer composites, structural optimizations, and advanced coatings have enabled these heat exchangers to achieve competitive heat transfer performance in certain applications.
To further explore their potential, the following sections will explore several key areas where polymer heat exchangers are making significant impacts, such as water desalination, waste heat recovery systems, evaporative cooling systems, and solar thermal systems. Through these applications, this section demonstrates the versatilities and benefits of polymer heat exchangers across different fields.
5.1. Water desalination/distillation applications
The scarcity of freshwater on earth has driven the development of seawater and brackish water desalination [264], [265], [266], [267]. Metal heat exchangers are often used in desalination. Metal materials, such as high steel alloy, copper-nickel alloys, are popular to produce conventional heat exchangers. However, fouling and corrosion easily happen for these metal materials in corrosive environments, and the production cost of metallic products is often high [268]. Using polymers for heat exchanging in desalination has attracted growing interest in recent years.
El-Dessouky and Ettouney [25] employed compact plastic heat exchangers constructed from PTFE in a single-effect desalination system. They explored thin-walled polymer tubes and plates with small tube diameters ranging from 0.05 to 0.1 mm. The study revealed the necessity for spacers to prevent the structure from collapsing and the need for very fine filtering should this unit be constructed. The research delved into the heat transfer areas concerning the boiling temperature of hot brine in a PTFE heat exchanger and compared them with those made from metals like titanium, high alloy steel, and Cu–Ni alloys. The heat transfer areas of the PTFE preheaters and evaporators were 2 to 4 times larger than those of metal heat exchangers across various boiling temperatures of the brine. Notably, the polymer heat exchanger exhibited the advantage of being the most cost-effective option.
Christmann et al. [269] produced a falling film plate evaporator for use in multi-effect distillation (MED) plants. This evaporator was constructed from high-performance PEEK. The mean overall heat transfer coefficients for evaporation ranged from 3182 to 3765 W/m2⋅K. These values were comparable to the reported values for metallic falling film heat exchangers. Additionally, the heat transfer simulation demonstrated a strong agreement between the theoretical predictions and the experimental data, highlighting the effectiveness of the PEEK-based falling film plate evaporator in the MED plants.
Song et al. [268] fabricated hollow fiber polymer heat exchangers using PP in laboratory and commercial scales. These heat exchangers were used to remove salt from a hot brine system as well as a steam system. The experiment results showed that the overall heat transfer coefficient was up to 2000 W/m2⋅K, which is closed to the limit imposed by the PP wall thickness of 2660 W/m2⋅K. Compared with metal heat exchangers, these polymer heat exchangers had larger surface area per unit volume but significantly reduced weight. Song et al. [230] produced hollow fiber PVDF heat exchangers to investigate the heat transfer performance. The experiment results indicated that the maximum overall heat transfer coefficient was 1168 W/m2⋅K in condensation and evaporation systems. They also emphasized that the phase change remarkably improved the heat transfer performance.
Polymer heat exchangers (e.g., PTFE, PEEK, PP, and PVDF-based in these cases) have shown promising results in improving heat transfer efficiency in desalination processes. These results demonstrate the promise of polymer heat exchangers as durable and cost-effective solutions for seawater and brackish water desalination, enhancing the sustainability and efficiency of water treatment processes.
5.2. Waste heat recovery systems
Waste heat recovery focuses on capturing and utilizing the waste heat generated as a byproduct of industrial processes [270], combustion engines [271], electrical power generation [272], or other operations [273], [274]. Such waste heat is often released into the atmosphere or discharged into cooling systems without being used, representing a significant potential energy source [275]. By implementing waste heat recovery, industries can capture this otherwise wasted heat energy and convert it into useful forms, such as steam, hot water, or electricity [276], [277], [278]. These recovered energy sources can then be used internally within the facility or exported for other industrial processes, heating applications, or electricity generation [279], [280], [281]. Waste heat recovery systems have various units, such as heat exchangers, economizers, cogeneration (combined heat and power) systems, and organic Rankine cycle systems [282], [283], [284].
Conventional waste heat recovery units are made of metallic materials. However, they are vulnerable to corrosive environments at high temperatures thus corrosion and fouling easily happen. In new design of heat exchangers, polymers and polymer composites are popular because of their anti-corrosion and anti-fouling properties. They are used to produce polymer heat exchangers for waste heat recovery, especially for low-graded waste heat. Here we discuss some examples on polymers and polymer composites applied in waste heat recovery systems, including economizers and air preheaters.
Economizers (e.g., finned tube heat exchangers) are used to capture low to medium waste heat for heating liquids. The unit consists of tubes that is normally covered by metallic fins to maximise the surface area of heat absorption and the heat transfer rate [282]. Robust polymers (e.g., PTFE) can be used as economisers and withstand the acidic condensate deposition on the surface of the heat exchangers [285]. Iliopoulos et al. [286] investigated polyurethane and epoxy as a protection layer to stainless steel tubes for economizers in a harsh corrosive environment at room temperature and 60 ℃. The results exhibited better corrosion protection efficiency values of 40 % and 39 %, respectively. Their study supports the application of polymers in the economizer fabrication.
Air preheaters are mainly used for recovering heat from exhaust air of low to medium temperatures. Such applications can include gas turbine exhausts and heat recovery from furnaces, ovens, and steam boilers [282]. Kim et al. [287] evaluated the heat transfer performance of PTFE and nylon rotary air preheaters. The results showed that the effectiveness of these two polymer rotary air preheaters was 10 % higher as that of the rotary air preheaters made of stainless steel and aluminium. Their study experimentally confirmed that polymer rotary air preheaters can exhibit heat transfer performances as good as metals.
Considering the corrosion resistance properties, polymer heat exchangers can also be applied for wastewater and flue gas heat recovery. Lyu et al. [288] created a patented cross flow tube polymer heat exchanger to recover waste heat from wastewater. Based on their experimental and theoretical analysis, the patented polymer heat exchanger showed stable global heat transfer coefficients ranging from 100 to 110 W/m2⋅K and achieved 67–92 % performance of metal heat exchangers in the same configuration. The soft heat exchanger can be enhanced by 30 % when it is under oscillation. Yi et al. [231] investigated a hollow fiber PTFE membrane heat exchanger to recover heat in a counter-current water-water system. The highest overall heat transfer coefficient was up to 391 W/m2⋅K. The neglectable change in mechanical strength indicated the superiority of PTFE membrane heat exchangers in corrosion environments.
Building ventilation systems are another application of polymer heat exchangers for heat recovery. In a modern building, more than 50 % of total thermal losses come from the ventilation systems [289], [290]. Therefore, it is significant to improve the efficiency in building ventilation systems. Fernández-Seara et al. [290] designed an air-to-air polymer plate heat exchanger for heat recovery in residential buildings. The polymer heat exchanger achieved a heat transfer rate and a thermal efficiency of 672 W and 80 %, respectively. With the increase of the air flow rate from 50 to 175 m3/h, the heat transfer rate increased by ∼65 %. Al-Zubaydi et al. [291] investigated the influence of plate geometry on the thermal performance of plate polymer heat exchangers. The experiment results indicated that the dimpled surface heat exchangers performed better than the plate one. The cooling capacity was 50–60 % higher and the highest coefficient of performance (COP) was 6.6 at an air operating temperature of 32.6 ℃. Chen et al. [292] applied a shell-and-tube hollow fibre polymer heat exchanger for heat transfer in water-to-water applications, and reported overall heat transfer coefficients ranging from 258 to 1675 W/m2∙K. The polymer heat exchanger offered 2–8 times higher conductance per unit volume than conventional metal heat exchangers.
Polymer-based economizers, air preheaters, and heat exchangers for heat recovery from exhausted gas or liquid streams have demonstrated comparable thermal performance to metal heat exchangers. Polymer heat exchangers have also shown promising applications in building ventilation systems, significantly enhancing thermal efficiency. These examples demonstrate the suitability of polymer heat exchangers for a wide range of waste heat recovery applications, providing a sustainable and efficient alternative to conventional metal heat exchangers.
5.3. Evaporative cooling systems
Evaporative cooling technologies have been developed over the past decades in industrial processes like cooling, HVAC, microclimate cooling, and etc [293], [294]. Evaporative cooling relies on the evaporation of water to cool the air, thereby regulating the temperature of the surroundings by absorbing heat when water evaporates [295]. The advantages of evaporative cooling include significant energy and cost savings, reduced CO2 emissions, improved life-cycle cost efficiency, etc [296], [297].
Evaporative cooling technologies can be categorized into direct evaporative cooling (DEC) and indirect evaporative cooling (IEC). DEC offers enhanced heat exchange efficiency using specific medium (e.g., air or water) or systems (e.g., cooling towers) [295], [298]. However, the energy efficiency of DEC potentially reduces due to the indirect process of heat relocation, where heat must be transported by water from the hot surface before being exposed and rejected to the ambient air [299]. IEC can achieve effective cooling without direct exposure of the cooled air to the evaporating water, potentially offering a cleaner process. However, the performance of IEC is significantly affected by the quality of water sources [300]. Therefore, membrane technologies are emerging for enhanced cooling. The selection of the materials is important for the efficiency and cost of the membrane evaporative cooling processes. Polymers are suitable in evaporative cooling because they can have high water-absorbing capacities, appropriate thermodynamic properties, corrosion resistance, and antifouling properties [295].
Zhao et al. [301] fabricated composite hollow fiber membranes using polyacrylonitrile (PAN) as the substrate and PDMS as the coating. They utilized dry-jet wet spinning for produce PAN fibers, followed by dip-coating in a PDMS solution. These membranes demonstrated significant water vapor transport properties, reducing the water vapor concentration from 18–22 g/m3 to 13.5–18.3 g/m3 using a low vacuum force of 0.78 bar absolute pressure. Compared to conventional air conditioning processes, the membrane dehumidifier achieved an energy saving of up to 26.2 %.
Chen et al. [302] utilized a hollow fiber PVDF module and configured it in a spindle shape to maximize contact between air and water, thereby enhancing heat and mass transfer in an evaporative cooling system. The performance of the system was experimentally evaluated under various conditions. The results demonstrated significant improvements in cooling capacity, wet bulb effectiveness, and dew point effectiveness compared to traditional designs. The spindle shape of the hollow fiber bundles mitigates the shielding effect between adjacent fibers, leading to enhanced cooling performance.
Cui et al. [303] introduced a semi-direct evaporative cooler using a hollow fiber polymer membrane to cool and humidify process air, while addressing the water droplet carryover issue. The hollow fiber polymer membrane with a large surface area and excellent selective permeability, enhanced heat and mass transfer. The evaporative cooler effectively cooled and humidified air, with performance dependent on various parameters, such as the inlet air velocity, temperature, and humidity. The study highlighted that polymers in evaporative cooling systems offer substantial energy savings and improved air quality.
Recent studies on polymer membranes, such as PAN-PDMS composite hollow fibers and PVDF-based membrane modules, have demonstrated improved cooling capacity, energy savings, and enhanced heat and mass transfer. These developments highlight the role of polymer membranes in enhancing evaporative cooling technologies, enabling more sustainable and cost-efficient thermal regulation.
5.4. Other applications
Polymer heat exchangers can also be used in some other fields, such as electronic device cooling [304] and liquid desiccant cooling. However, this reviews mainly focuses on the energy and water recovery from waste heat or sustainable sources. Utilizing solar energy is a sustainable approach because solar energy is abundant, renewable, and environmentally friendly, making it an ideal source for energy conversion and water recovery applications [305]. Researchers have been investigating the use of polymers to fabricate units capable of harnessing solar energy.
For example, Wang et al. [306] fabricated a photovoltaic (PV)-membrane distillation-evaporative crystallizer (PME) system, utilizing a 1-mm-thick stainless steel mesh, hydrophobic PTFE membrane, and stainless steel sheets and pipes for the multi-stage membrane distillation component. The PME system used the waste heat from the solar cell to produce clean water, simultaneously cooling the solar cell and increasing electricity production. The device demonstrated a clean water production rate of 2.35–2.45 kg/m2/h from seawater and a significant reduction in solar cell temperature, thus addressing the global water-energy nexus effectively.
Tang et al. [307] developed a polyacrylamide (PAAm) conical aerogel embedded with multi-walled CNTs for solar steam generation and passive cooling. The aerogel was prepared by dissolving acrylamide and cross-linking agents in deionized water, followed by polymerization and freeze-drying. The CNTs were treated with acid and dispersed in water before being coated onto the PAAm aerogel. The resultant CNT-PAAm-conical aerogel exhibited excellent solar absorption, significantly enhancing water evaporation and providing passive cooling. This innovation aimed at achieving sustainable and efficient solar thermal applications.
Agyekum et al. [308] explored the use of a cotton wick mesh for passive cooling of photovoltaic panels. The experimental setup included a polyvinyl chloride pipe system for water distribution, relying on capillary action to cool the panels. This method proved effective in high-temperature regions, enhancing the efficiency of the PV systems. The study emphasized the importance of water flow rate and proposed improvements to prevent clogging of the water distribution system, contributing to sustainable energy solutions.
To conclude, in waste heat recovery, polymer heat exchangers, including economizers, air preheaters, and ventilation systems, demonstrate competitive thermal performance while resisting harsh environments. Evaporative cooling technologies benefit from polymer membranes that enhance heat and mass transfer, leading to improved energy efficiency. Additionally, polymer materials are utilized in solar-driven systems for clean water production, passive cooling, and photovoltaic efficiency enhancement. These advancements emphasize the potential of polymer heat exchangers in addressing energy and water challenges, contributing to more efficient and sustainable thermal management solutions.
6. Conclusions and perspectives
Polymer heat exchangers present a potential alternative to conventional metallic heat exchangers, particularly in applications where low cost, easy manufacturing, anti-fouling, and corrosion resistance are expected. Polymers, such as PTFE and PEEK, demonstrate good performance under high temperatures, making them suitable for various industrial purposes. Enhancing the thermal and mechanical properties of polymers is essential to extend their applications, with strategies such as constructing connected thermal conduction pathways within the polymer matrix and using advanced fabrication techniques, such as electrospinning and vacuum-assisted filtration. Although challenges remain, such as achieving higher thermal conductivity and maintaining mechanical strength at high filler loadings, ongoing advancements and smart engineering designs continue to fill the performance gap between polymer and metal heat exchangers. Consequently, polymer heat exchangers are increasingly considered for applications in water and energy recovery, offering a promising solution to address clean water scarcity and energy shortages.
In the future, the following aspects on polymer heat exchangers should be considered:
- •
Enhancing thermal conductivity remains a primary focus. This can be achieved by developing novel polymers and fillers with enhanced thermal conductivity but without compromising mechanical properties of the material. Additionally, exploring new filler geometries and orientations can create more efficient thermal pathways or networks.
- •
Reducing filler loadings is also important. Hybrid fillers can synergistically improve thermal conductivity at lower loadings. Optimizing filler dispersion to achieve uniform distribution and minimize agglomeration is also essential.
- •
Improving the mechanical properties of polymer composites is necessary to balance the trade-off between thermal conductivity and mechanical strength. Enhancing the interfacial bonding between polymer matrix and fillers can improve the overall performance of the composites.
- •
Scaling up fabrication is essential for the commercialisation of polymer heat exchangers. Developing cost-effective and scalable fabrication methods for large-scale production of polymer heat exchangers is critically important for practical applications. Advanced manufacturing techniques, such as 3D printing can be utilized to create polymer heat exchangers with complex design and optimized performance.
- •
The long-term stability and durability of polymer heat exchangers under harsh operational conditions need further investigation. Extensive testing under real conditions is needed to evaluate the durability and stability of polymer heat exchangers over time. It is also necessary to improve the resistance of polymer composites to environmental factors, such as UV radiation, moisture, and chemical exposure.
Overall, polymer heat exchangers represent a promising technology with the potential to significantly contribute to sustainable energy and water management solutions. These heat exchangers offer unique advantages, making them suitable for a wide range of applications. However, continued research and development are essential to overcome the current limitations. By addressing the challenges related to thermal conductivity, mechanical properties, fabrication processes, and long-term stability, full potential of polymer heat exchangers can be realized. This will make the way for their broader adoption and commercialization, ultimately leading to more efficient and sustainable technologies for managing energy and water resources.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
G. Yi gratefully acknowledges the Deakin University Postgraduate Research Scholarship.
Data availability
Data will be made available on request.
February 11, 2025 at 01:12PM
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