Small-Scale Phase Change Materials in Low-Temperature Applications: A Review

Abstract

Significant efforts have explored the field of Phase Change Materials (PCMs) for various applications. Research and real-world applications explore length scales that range from infrastructure to micro systems. A commonality of these efforts is the desire to utilize the phase change capability of the PCM to provide a steady temperature heat sink for thermal storage. Smaller scale efforts and materials are presented in this present review. A general challenge to the use of these PCMs regardless of application is the low thermal conductivity present as a baseline material property. Efforts to improve thermal conductivity have included the addition of underlying metal foam structures, heat pipes, or metallic fins inserted into the base PCM. Other efforts have investigated alterations to the base materials themselves by employing additives such as graphite to supplement thermal performance. Other additives are used to obtain form stability in the PCM as it melts. While the field of PCM research has been well established, the use of new materials and approaches that employ the use of natural materials continues to move research forward. This review captures significant efforts and presents a thoughtful comparison of common themes across centimeter and smaller-scale PCM use.

1. Introduction

For decades, there has been ongoing interest and research in the field of Phase Change Materials (PCMs) for the purpose of Thermal Energy Storage (TES). The applications have spanned scales and operating temperature ranges. These include investigations and implementations of solar energy-supporting TES in Megawatt scale applications [1,2] at temperatures of hundreds of degrees Celsius [3]. At the opposing end of scale and temperature, TES has been considered at the microchip level to promote hot-spot dissipation or in low temperature thermal storage for power generation.

The number of publications related to phase change materials has significantly increased in the past 15 years. Based on Scopus publication data since 2008, publications have increased more than 10-fold, eclipsing 2000 publications in 2022 alone. Multiple excellent review articles have been written across the decades of work represented by this unique field. In this effort, we focus on a review of PCM use primarily in lower-temperature applications for thermal storage. The intent of this review is to offer detailed and thoughtful insights into some specific efforts. We seek commonalities and key takeaways across the genre of work and present them as part of the governing PCM themes.

1.1. Thermal Energy Storage via PCM Use

Before investigating specific phase change materials, their use in Thermal Energy Storage (TES) systems is clarified. Thermal energy storage involves the storage of thermal energy in materials based on their thermal heat capacity, which can be released when needed. The choice of material for thermal storage is largely dependent on thermal heat capacity. In the past, materials such as water were commonly used for thermal storage due to their high thermal mass [4,5,6]. However, these materials only have a constant thermal mass when they are in a single phase. This necessitates larger amounts of material for higher energy storage and makes the system inefficient for small-scale applications. On the larger scale, however, significant improvements in working fluid characteristics have been achieved via conductivity enhancements such as multi-wall carbon nanotubes (MWCNTs) placed directly in fluid flows for heat exchanger use [7].

Phase change materials offer solutions to some of these issues by capturing heat not only through sensible heat transfer, but also through latent heat transfer. This enables them to store more heat given the same amount of material and makes them more efficient for thermal energy storage in various applications. Figure 1 shows a general schematic for different heat storage options, including the latent heat capture afforded through PCM use.

TES that employs phase change materials for thermal energy storage can be found across vastly different applications or storage media. Nonetheless, many of them share the commonality of energy storage via solid–liquid transitions. At higher temperatures, materials such as salts offer the base phase change media. At lower temperatures, energy storage is accomplished using materials such as waxes or other types of oils. In both, it is the heat of fusion or solidification that provides the constant temperature thermal reservoir that is advantageous for energy storage regardless of application. Additionally, common among these approaches and materials is a generally low thermal conductivity that hampers the speed of energy absorption or release. This has been a critical focus for this field, as many of the reviewed works attest.

Given lower thermal source temperatures that may not exceed 50 °C, a range of different phase change materials and approaches has been investigated. These include paraffin or eicosane waxes that melt at the lower temperatures required for TES. In addition, some recent efforts that are also reviewed have ventured into environmentally derived material use [8,9,10,11]. Researchers have achieved notable success in increasing the basic thermal conductivity of these storage materials through a variety of material enhancements or structural design improvements.

While the focus of this review is on materials or general approaches to the PCM challenge, several unique TES arrangements for application are also presented. Of particular interest to this review are designs with applications on the order of centimeters, and thermal power level on the order of Watts. Typically, these efforts utilize phase change materials well suited to low temperature phase change, with a variety of enhancements to improve operation. Larger, infrastructure-scale implementations are briefly noted, primarily due to their use of low temperature thermal energy as the heat source of interest.

1.2. Energy Challenges Drive Progress

There are many reasons for the work in this field. Thermal management and energy use have become synonymous with many engineering challenges that face the 21st century. The advent of the global COVID-19 pandemic, alongside energy and economic impacts of the war in Ukraine, have presented the “first truly global energy crisis” according to Dr. Fatih Birol, Executive Director of the International Energy Agency [12]. Components of the crisis include both access to and affordability of energy. Further, environmental costs of energy use are driving continuing technology change and commitments to cleaner sources [12,13,14]. Within this context, the use of TES presents a unique opportunity to capture, hold, and release thermal energy as part of a larger overall process. Through capture, what would be waste energy may be stored without the use of expensive battery technologies. This energy may be released as needed to continue the operation of the larger-scale systems the TES supports, thus improving the overall efficiency of the processes as a result.

1.3. General Commonalities across Approaches

Across the reviewed efforts, several commonalities are observed. At the material level, there has been growing consensus on the effects of liquid PCM and its impact on thermal conductivities and energy absorption rates [15,16,17]. Even on the small scale, the ability for a PCM to achieve internal convection is an important aspect of thermal absorption.

In addition to the internal fluid dynamics of melted PCM, the specific design for application is often stressed. Because of the different materials, melt points, or even mechanical properties of the PCMs under consideration, design specific to applications or thermal loads is cited as an important design attribute [18,19]. Both experimental and simulative approaches are used to guide these design efforts. The simulation approach has been used to develop different metrics or measures specific to design for given parameters [18,20,21,22,23,24]. A dimensionless mass parameter was identified that guided the use of PCM as a TES for Organic Rankine Cycle Applications, as one example [20]. Other simulative works focus primarily on PCM heat transfer enhancement and geometric optimization [24]. Additional approaches that offer insight into designs for different thermal loads or material considerations are reviewed in greater detail in the sections that follow.

As a final observation on commonality, many in the field have worked to couple thermal absorption capability alongside so-called “form stability”. While the phase change of solid to liquid represents a significant energy sink, it also presents a material challenge that requires the containment of liquified PCM. Form stability efforts employ additives or various internal structures to hold liquified PCM to shape even as it undergoes phase change. These and other interesting efforts are presented in this review.

2. Background and Overview

Recovering and capturing waste heat or other low temperature heat sources is a prominent area of phase change material research efforts. As an example, PCM as thermal energy storage for a low temperature organic Rankine cycle (ORC) was presented by Daniarta et al. [20] One of the most significant challenges to PCM utilization in general is a low thermal conductivity associated with many of the phase change storage materials. This limits both rates of storage and the retrieval of energy. This provides a general thrust in research efforts, even across the different applications where Thermal Energy Storage (TES) is found. This review focuses on the thoughtful presentation of the methods employed across the field, while observing some of the different and unique applications in which they are found.

The promise of phase change material for these low temperature applications is through the heat of fusion versus other methods. The challenge faced in real-world application is to achieve encapsulation of the phase change material as it undergoes fusion from solid to liquid. This must be undertaken while also increasing thermal conductivity to improve the rate of energy transfer. Efforts must also seek a balance that maintains a suitable working volume of PCM to store adequate energy despite additives or other material modifications. These often limit overall PCM volume, and hence the total storage capacity independent of any thermal conductivity enhancement.

2.1. Low Temperature Applications of PCM

Low temperature applications, as referred to in this review, refer to temperature ranges associated with low-grade residual or waste heat. Additionally included are ambient thermal sources such as solar energy. Section 2.2 looks at infrastructure applications within the typical operating temperature range of the materials considered.

TES using PCMs in low temperature applications offers several advantages. First, it allows for efficient and effective energy storage in a compact space, which is particularly important in areas with limited space for equipment installation. Second, it provides a stable and predictable thermal sink that ensures consistent temperature control and reduced energy waste. Third, it can reduce peak energy demand during periods of high energy consumption, reducing the need for expensive peak power generation and lowering energy costs. Even the shift towards renewable energy sources for power generation, which may be intermittent or variable, makes energy storage using PCMs an attractive solution for a stable thermal source of energy. The specific material properties that make a low temperature use of PCM in TES attractive are noted in Section 2.3.

Overall, low temperature applications of TES using PCM play a critical role in promoting sustainable energy practices, reducing energy waste, and ensuring a safe and comfortable environment for people and processes. Section 6 notes some of the specific, smaller scale application of these low temperature PCM uses.

2.2. Infrastructure and Other Uses of Low Temperature PCM

The focus of this review is on smaller-scale, lower temperature PCM and TES. However, it is observed that one common application of TES in a lower temperature application occurs on the large scale via infrastructure use. TES and the associated low temperature melt point PCMs in infrastructure have been investigated since the 1980s [25]. Efforts have focused on the reduction of electrical or other energy use within the buildings via energy loss mitigation, or the reduction in peak thermal loads associated with building interiors as external temperatures vary [13,14,25,26]. Melting points for the PCMs generally ranged between 16 and 26 °C, with heat of fusion values between 140 and 231 J/g [25]. This proves to be common among many materials reviewed for smaller scales in this paper, as well. The application of these low temperature TES solutions can be found in ceilings, walls, or flooring. Actual building materials include wallboards impregnated with PCM, concrete materials with PCM, or underfloor applications [25].

In infrastructure application, significant savings have been noted. As early as 1991, up to 15% of annual energy costs could be saved when applied to a residence in Wisconsin, USA [25,27]. More recent simulation analysis designed thermal management and PCM strategies integrated into office building architecture in a Saharan desert climate [13]. As with early demonstrations in Wisconsin, the projected savings for HVAC systems were between 12% and 15.6%, with PCM thickness bringing the variability.

Of recent note, these pursuits have been important to reduction in building carbon footprint [14] and have advanced into research specific to different climatic conditions via form-stable PCMs [26]. Form stability is an important consideration in the fusion process, where solid–liquid transformation causes PCM loss unless housed in a larger encasement. Increasing attention is paid to the strength and properties of building materials such as concrete or mortars and plasters in which they are combined. Stand-alone, form stable PCMs are noted more specifically in some of the micro and nano enhancements made to PCMs in the following sections.

Along with the expansive investigations of PCM use in infrastructure, other uses are included as part of this background but are beyond the scope of this review. Many excellent review articles are available in the fields of thermal storage applications regarding solar or other power plant generating applications, for example [1,28,29,30]. Other applications include, but are not limited to, the use of TES for solar water heating [31], advanced battery thermal management [32,33], shell and tube thermal storage [34], photovoltaic efficiency improvements [35], and solar greenhouse drying to preserve food [36]. The literature also reveals a convergence of specific material enhancements via Machine Learning (ML). This is being applied as specific to associated heat transfer fluids [37], as well as integrated with the thermal management of infrastructure systems where PCM may be incorporated [38].

2.3. PCM Properties and Low Temperature Applications

Phase Change Materials (PCMs) are important for Thermal Energy Storage (TES) applications, especially in low temperature situations, due to their unique properties. Some important properties of PCMs that make them suitable for TES applications are reviewed in this section.

One important aspect for PCM application in TES is the high latent heat that leverages the overall thermal absorption. High latent heat means that PCM can store large amounts of thermal energy via changing phase from solid to liquid in low temperature applications. Similarly, this allows the release of that energy via solidification back to a solid. Note that the constant temperature associated with melt or solidification has the added advantage of a constant temperature heat sink as part of a larger overall process. In fact, these PCMs have a narrow range between their melting and solidification temperatures. This ensures energy absorption or release at a constant temperature and, hence, predictable rates.

Thermal conductivity is another important aspect of PCM use in energy storage. However, unlike latent heat, many of the reviewed materials are based on waxes that have naturally low conductivity that impedes heat transfer rates and usability. As noted in Section 1.3, this has focused significant research on improving this base property. Coupled with the thermal conductivity and phase change property, the specific heat capacity of the material allows large amounts of thermal energy storage per unit mass, making them efficient for storing energy in a small space at a low temperature.

In addition to the properties of latent heat and thermal conductivity, the chemical stability of the PCM is important in any TES application. This allows a predictable set of properties that also include density to maintain the required volumes and overall heat capacity. Section 1.3 notes that this has driven some research to pursue form-stable material modifications that allow phase change without observable liquification of the PCM.

Section 6 includes a review of some of the small-scale TES applications where low temperature PCM use is observed in greater detail. These properties guide the build and the design of these systems.

2.4. Methods for the Use of PCM in Low Temperature TES Applications

There are several methods of Thermal Energy Storage using Phase Change Materials. When using encapsulation, PCMs are encapsulated in containers made of metal or plastic [39,40,41]. The containers can be arranged in a variety of shapes and sizes to fit the available space. The encapsulation provides physical and chemical stability to the PCM and allows for easy handling and transportation. Note that encapsulation also includes the micro-scale encapsulation of PCM within a spherical (or similar) shell. Several of these efforts are reviewed in subsequent sections.

In PCM slurries, TES employs PCM within a liquid that serves as the heat transfer medium for a larger-scale heat transfer or scavenging system [42]. In this manner, PCM is literally mixed with fluids and allowed to progress through the heat exchangers of the system. The properties of the PCM (Section 2.3) yield increased thermal capacity for the working fluid in general.

Applications have also utilized a packed bed of PCM particles that border a heat exchanger or thermal source for TES [43]. This is different than direct contact, where PCM is directly in contact with the material to be heated or cooled [44]. For example, PCM can be used to cool water in air conditioning systems, or to heat a room by releasing stored thermal energy. This method is simple and cost-effective, but requires careful design to avoid leakage and contamination.

3. Specific Small-Scale PCM Enhancement Approaches

As with larger scale or higher temperature PCM applications for TES, the grand challenge to the use of these materials is a low thermal conductivity that limits the thermal energy absorption rate [2,8,45]. In 2016, Jose et al. observed several significant areas of thermal conductivity research efforts, which included microencapsulation, the use of extended metallic surfaces (or matrices), carbon-based additives, or other nano-powders [45]. Some of these approaches are not specific to the small scale [46]; however, they form an accurate guide for work in the field. In addition to these general areas of investigation, efforts to create ‘form-stable’ liquified PCM have been of note. Many notable efforts are reviewed in this section, along the general guidelines regarding underlying structures for conductivity enhancements or the use of additives to the base PCM.

First, investigations involving the use of structures inserted into PCMs are reviewed, typically on the mm–cm scale. Second, examinations of micro or nano additives are presented. These specifically investigate the challenge of both thermal conductivity and heat transfer characteristics as well as form stability. Where possible, reported values of both thermal conductivity, and heats of fusion are noted for comparison across different approaches. Section 5 provides a summary of some of these reported values.

3.1. Conductive Foam Structures: Experimental Evaluations

Weiss et al. utilized eicosane wax as the phase change media for thermal storage [47]. The aim of this work was to serve as a thermal battery for harvested waste heat energy, as part of a larger meso-scale system. To improve the thermal conductivity performance of the wax, multiple structures were examined for integration, sharing the commonality of a low-cost approach. These included a porous copper foam with 90 PPI and a manually fabricated copper matrix. The overall dimensions were on the order of 35 mm by 35 mm by 15 mm height. Liquified wax was allowed to enclose these structures, and then cool to form the combined storage material. Results showed an improvement from the base wax thermal conductivity of 0.5 W/mK to 3.7 W/mK for the copper matrix and 3.8 W/mK for the copper foam. A maximum thermal power absorption was recorded of 900 W/m². Both numerically and experimentally, the results indicated diminishing returns for absorption rates as average thermal conductivities increased beyond 4–6 W/mK. This was specifically true for the working temperatures of about 75 °C [47]. Figure 2 shows the copper foams that formed the basis for the PCM effort.

Similar efforts have been conducted by other groups [2,48,49]. Pore size was directly considered by Jin et al. in a visualized study of paraffin wax that had been incorporated within copper foam [48]. The melt of the wax storage media was monitored via infrared camera. The pore sizes were 15, 30 and 50 PPI, and testing indicated a diminishing return with increased copper content. Both 30 and 50 PPI foams were similar in rates of fusion for the wax, and were notably faster than the 15 PPI sample. This matched similar observations in other efforts [47].

Other efforts by Mancin et al. [17] examined in greater detail the solid–liquid phase change process of three different paraffins with melt points of 53, 57 and 59 °C. Further, three heat fluxes were utilized in the studies, from 6.25 to 18.75 W/m². With this combination, three copper foams with densities of 5, 10 and 40 PPI were used as the underlying copper framework for the PCM. The dimensions of the copper foams were 40 × 40 × 40 mm, and care was taken to assure the complete filling of the copper with the various waxes under consideration. The detailed time of melt temperature vs. foam PPI was studied, and results confirmed those of Thapa et al. [47]; diminishing returns of thermal conductivity exceeding 4–6 W/mK. Little difference in melt time performance was recorded across the pore density selections. The authors note that the thermal conductivity is improved by the copper interconnects that conduct heat, but is ultimately limited by the low thermal conductivity of the base PCM.

To improve the generally observed limitations of thermal conductivity and thermal absorption performance, other efforts have focused on the creation of unique knitting aryl network polymers (KAPs) atop a copper foam [2]. This process was inspired by the natural environment, considering the growth of corals. Paraffin wax was used as the storage media, incorporated within the structure. A maximum thermal conductivity of 55.37 W/mK was attained through testing a sample based on 30 PPI copper foam and an encapsulation percentage of 62%. This result was significantly higher than other foam-based tests, and shows the general promise of the technique. The encapsulation rate suggested that the overall storage capability was less than other approaches, albeit with a much higher thermal conductivity. A latent heat of 105.6 J/g was recorded, as well. This was the result of a high copper surface area that formed the base of the structure. Thermal cycling was conducted more than 300 times without any latent heat density reduction, indicating a very stable storage system. The authors noted that it compared very favorably to other work in the field [2].

Unique efforts have been recently undertaken by Galvagnini et al. that utilize a polypropylene syntactic foam with hollow microspheres of glass (HGM) [50]. The PCM selected for use was an encapsulated paraffin from Microtek Labs Inc (Dayton, OH, USA), with the paraffin core held in a melamine-formaldehyde capsule that could maintain stability up to 250 °C. The spheres encapsulated a PCM with a melt point of 57 °C and 210 J/g. The capsule diameter was about 34 µm. This was used to form a new TES-capable matrix for low temperature application. Multiple mixtures were investigated, as shown in Figure 3. Sample H0-P0 represented the ‘neat’ structure of PolyPropylene (PP) with zero void content. This was contrasted with increasing additions of different constituents and finally all materials combined (H20-P20) in a 57:20:20:3% PP:HGM:PCM:compatibilizer mixture. [50].

Of note was the increasing void content with the inclusion of PCM in these cases, likely developed during mixing stages, and the partial evaporation of the PCM leaking from its encapsulation. The included figure shows some of the fractured capsules, particularly in H0-P30 (Figure 3b) [50]. The authors noted that the surrounding matrix, however, served as a secondary encapsulator for any leaked PCM. Thermal properties were investigated via DSC analysis. The expected enthalpy of fusion based on nominal PCM content was compared to the different mixtures prepared. The best heats of fusion were, as expected, attained with higher concentrations of PCM included at 62 J/g for a PP-PCM only mix (Figure 3b) [50]. The expected values differed little from the measured values, showing a successful encapsulation and generation of the material for use. Thermal conductivities ranged from about 0.275 W/mK to 0.175 W/mK, dependent on the mix, but were lower for the incorporation of the hollow glass microspheres. Mechanical properties of the matrices showed the benefit of adding HGM, while increasing PCM percentages tended to reduce general strength properties. Through this effort, it was noted that tailoring a material to a specific application was possible through the consideration of structural strength needs and TES needs.

Yu et al. examined the problem of liquid-phase leakage from PCM directly through two methods [51]. First, microencapsulation was considered followed by a porous infiltration investigation. Microsphere and 3D porous aerogels were found to be successful in maintaining the form-stability of the PCM without leakage during melt. Three-dimensional porous aerogel support was also noted to further offer an increased volume of available PCM due to the porous nature of the material itself. The PCM considered for this effort was paraffin. A cross-linked graphene aerogel proved to further reduce volume shrinkage during filling, while maintaining a high level of latent heat of fusion. The microencapsulation of PCM without leakage was accomplished through a solvent-vapor method. When in distilled water, pure PCM adopts a microsphere shape due to hydrophobicity. The shell material was dissolved into the solution, and then absorbed into the microspheres through the gradual evaporation of the solvent. This yielded the desired microspheres enclosing PCM. Different mixtures were studied, including a Graphene Oxide additive for form stability increase and conductivity improvement. The heats of fusion were generally comparable across techniques; however, DSC analysis showed the benefit of aerogel supported composites with heats of fusion of 178.9 J/g. Figure 4 shows the graphene supported PCM composite from this work [51].

In general, energy conservation and consumption are particularly important in this application, where any HVAC or secondary system use of stored battery energy can dramatically impact the vehicle operational range. A work by Baumeister et al. [52] looked at the high loading and unloading power requirements that are present for the application of the PCM materials in these real-world situations. A unique demonstrator system was investigated that would be part of an HVAC installation in an electric vehicle. In this effort, a liquid coolant could be passed through a tube system embedded in the constructed PCM device for thermal capture. Aluminum foam was utilized as the metallic scaffolding that held the PCM, and further improved its overall thermal performance. The aluminum foams for the effort were produced through the infiltration of aluminum (melted) into structures via a high-pressure die-casting process [53]. The pore size was adjustable via this method, and even fluid ducts such as tubes could be integrated. Polymer tubes were embedded into the structure following the fabrication. Figure 5 shows the combined preform used for the aluminum foam fabrication, along with tubing inserted. Additional tubes were inserted into other foam test pieces as part of the work to increase the amount of flow through the setup.

The PCM selected for this effort was a paraffin based RT18HC from Rubitherm Technologies GmbH. The initial latent heat of fusion was 230 J/g, with a phase change point of 18 °C [53]. To maintain form stability in the PCM, a thickening agent (10% polybutadiene-based) was added to melted PCM prior to foam insertion. Aluminum foams were then filled with PCM via a vacuum process. The authors investigated both pressure losses from flow through the inserted tubes serving as the heat exchangers, as well as water temperatures flowing in and out of the PCM during testing [52]. The results indicated a successful removal of thermal energy from the incoming fluid flows. The initial inlet temperatures were 28 °C. Up to 10 °C gradients of the fluid flow were measured at the lower flow rates of 1 L/min. Increasing the flow rate (and thus decreasing the liquid dwell times in the PCM) reduced the measured gradients. Over time, the temperature gradient reduced as the PCM absorbed more and more energy from the flows, as expected. The authors targeted a 500 W consumption point for 5 min of operation. The recorded thermal absorption achieved this 500 W flux, depending on the temperature gradient that was maintained [52]. An effective foam design that balances PCM volume vs. overall thermal conductivity allows specific designs to be constructed for real world application.

In a modification to the use of the underlying foam structure, eicosane was also investigated as a PCM with internal capillary tubes employed for thermal conductivity enhancement [54]. Figure 6 shows the basic setup of the work.

Twenty-five tubes were constructed using 2 mm OD, 1.3 mm ID copper. These were filled with different amounts of 3M^TM HFE 7200 working fluid. This working fluid had a boiling point of 72 °C, which made it a suitable candidate for low temperature application and a good match for the melting point of the base PCM. When heated from the lower surface, the tubes generated an internal heat-pipe operation that aided in the heat transfer and thermal absorption of the PCM that was encased around them. A capillary fill percentage of 25% yielded the highest performance, with 4.1 kW/m² power absorption and a maximum average thermal conductivity of 8.2 W/mK [54].

3.2. Conductive Foam Structures: Numerical Studies

The use of numerical studies for PCM research is an important aspect of fundamental property optimization and understanding. In this section, works that elucidate the metallic structure PCM enhancements are considered and offer insight into many of the commonalities observed in experimental efforts. Other studies have gone beyond the study of additives, and include the use of rotation and magnetic field applications to enhance PCM thermal properties [55]. This shows the significant value and promise of numerical study in the field.

The numerical simulation of PCMs enhanced with a thermally conductive embedded matrix has been the focus of multiple efforts [18,19,49,56]. Wang et al. developed a detailed melt study of open foam metal at pore-scales using paraffin as the PCM [49]. The approach was based on the representative elementary volume (REV) method. Other materials, such as air and water, were also considered within the structure for comparison. Of note, the study indicated that pore size had little effect on the overall thermal conductivity enhancement, which was instead driven by the porosity and conductivity of the metal [49]. This numerical effort further highlighted the role of conduction as the primary means of heat transfer. Simulations confirmed the visualization of Jin et al. [48] of the propensity for liquefaction of the PCM closest to the heat source and foam metal frames under study. The center of the pores of the different forms were last to melt. The largest temperature gradients were noted in the liquid portions of the PCM. It is interesting to note that the liquid phase fraction within the structure grew swiftly at the onset of thermal application, but became increasingly limited as liquid fraction grew. Heat transfer occurred primarily via the melted liquid PCM, with weak natural convection contributions to the overall energy transfer. As with other studies and experiments previously noted, this implied a natural limit of overall thermal conductivity enhancement given the propensity of the energy to transfer via the melted PCM itself [17,47].

A study of the metallic structure and its impact on PCM performance was undertaken directly by Venkata et al. [18] Pore radius, overlap and overall porosity were considered in the model. Modeling focused on the resolution of melt within individual pore structures for the comparison study. Paraffin was the PCM of focus in this work, with the overall size of the system being 25 × 25 × 25 cm. Aluminum was selected as the metal for the underlying structure, and was used for comparison with experimental efforts in the field. Of note was the consistency of the slowing melt rate with time; however, the authors predicted a steady rate until up to 90% of the PCM was melted. The study indicated that pore overlap had a significant influence on energy absorption; however, different structures could be suitable for different desired thermal applications, and no single structure proved most efficient at melting the PCM for all thermal inputs considered across all time scales. The general trend suggested that lower porosity and lower pore overlap were desirable for a shorter duration absorption, with longer duration thermal absorptions benefiting from higher porosity and increased pore overlap [18]. This study, calling attention to the needed design attributes of a specific real-world application, is significant for the use of PCMs in lower temperature functions.

Numerical techniques were also applied by Nada et al. [19] These efforts considered copper foam structures and different phase change materials with different melt points for the purpose of electronic temperature management. Because of this focus, thermal loads of up to 96 kW/m² were simulated, higher than other waste thermal energy studies [17,47]. A volume averaging technique was undertaken and, as with Venkata et al., recommendations indicated that specific design variances across porosity, PCM, and even overall volume were needed depending on the specific application or thermal loads. Porosity was allowed to range from 0.6 to 0.5, and the underlying foam structure adopted ranges of thermal conductivity in this work from 26 to 400 W/mK, with thermal conductivities aligned well with general eicosanes ranging from 0.22 to 0.4 W/mK. PCM melt points were studied from 50 to 70 °C, and the overall PCM module height was varied to allow for the study of different total storage capabilities. Of note for the thermal management of electronic temperatures, the PCMs can either be under constant cycle loading or something akin to a steady state temperature load. Based on the model suggestions, a higher porosity foam combined with a PCM with a lower melt point but a high heat of fusion provided the best case for electronic temperature control [19]. In the case of steady state heat application, however, low porosity copper foams and PCM with a high heat of fusion and max thermal conductivity provides the best case results. Generally, reducing PCM melt points can achieve lower electronic temperature initially, although the longer-term temperature steady-state is not greatly affected.

Electronics cooling was the target of efforts via Alshaer et al. [56], who performed a numerical analysis of advanced PCMs enhanced using carbon nano tubes (CNTs) in carbon foam matrices of different porosity values. These were compared to published experimental efforts [57]. Different variations of combinations along with the base PCM were investigated via CFD analysis. In the modeling, adiabatic enclosures bounded a constant heat flux wall and isothermal cold wall to monitor performance of the PCM within the confines of the structure. Dimensions were 50 mm by 39 mm in the effort. Paraffin wax was selected as the PCM under study. The baseline was established using only the carbon foam within the modeled space. Subsequent studies included the paraffin and then paraffin/CNT combination. Several trends were of note. The base hot surface temperature was reduced by 10 °C (from 80 to 70 °C) through the addition of PCM, CNT, and base foam. This was distinct from the evaluation of foams alone or foam and PCM, which performed similarly across different foam porosity levels studied. The lowest operating temperatures of the heated side were attained with the lowest porosity foams, 55% in these studies. This, again, points to the value of increased thermal conductivity enhancement with the foam structures; however, it also includes the observed reduction in total storage capacity of the PCM. The use of CNTs further improves the operation of the PCM across all porosities studied, indicating the importance of the average thermal conductivity increase of the phase change material itself in this field [56].

3.3. Conductive Metal Fins Investigations

In addition to foam-based approaches to hold and improve PCM performance, several groups have investigated fins of various dimensions and scales inserted into PCM materials [15,21,58,59]). Fins were examined in both vertical and horizontal implementations. An example of PCM with horizontal fins is shown in Figure 7. Vertically positioned fins swapped the heated surface and fins from the vertical side to the lower, horizontal face.

Experimental efforts investigating horizontal fins were conducted by Kamkari et al. [15]. Heat was introduced through the vertical y-axis, and fins were mounted to the hot side, protruding into the PCM. Lauric acid was selected as the PCM, with a melt point of 43.5 °C and latent heat of fusion of 187.2 J/g. Fins with mm-scale geometries were milled from aluminum, and thermocouples monitored temperatures at different points across the experimental enclosures. The solid–liquid interface was monitored across fin geometries, including a baseline case of no fins. In this work, the natural convection of the liquid PCM played an important role in the process, similar to foam-enhanced observations where the melt of the liquid itself was a determining factor in overall PCM thermal performance [48]. The increasing quantity of liquid PCM becomes a rate limiting factor for thermal absorption [15]. Direct impacts of the added fins on base thermal properties such as heat of fusion were not noted; however, they would be minimal in this approach, where fins take up a smaller percentage of the overall enclosed volume and allow the wax to remain effectively unaltered. Based on the set of conditions considered, the total melt times improved by up to 37% for the max-finned (three) arrangement. At a max heated wall temperature of 70 °C, more than 62 W was absorbed using the three finned arrangement compared to 40 W for 1 fin and 30 W for PCM only. With an increase in liquid PCM, absorption rates decreased following the normalized trend of Nusselt number across all geometries [15].

This detailed analysis [15] helped to confirm work carried out by Saha et al. [16] which represented some of the earlier work on the topic, establishing a vertical pin/fin geometry with a heated lower surface. In that effort, the target application was electronic chip cooling and eicosane was used as the PCM with an initial heat of fusion of 241.0 J/g. Pins were 25 mm in height and 2 × 2 mm in footprint atop an overall footprint of 42 × 42 mm in an insulated enclosure. Peak performance was found using 36 pins, or an 8% by volume pin fraction. This effectively allowed convection regions in the molten section of the PCM and improved the thermal energy absorption of the system [12].

RT42 was used as the PCM in efforts by Ji et al. [60], with horizontal aluminum fins allowed to vary in length. A ratio was developed of the top fin length/lower fin length. RT42 was used as a PCM, with a melt point of 38 °C, thermal conductivity of 0.2 W/mK and heat of fusion of 165 J/g. A numerical setup was adopted to study the melt in detail. Thermal energy was again added from the vertical side of an enclosure where the horizontal fins were protruding. With ratios at 0.25 (smaller upper fin, larger lower fin), the melt time could be reduced by 25% and further reduced depending on the heated wall temperatures and overall fluxes. The authors note particularly that with length ratios less than 1, natural convection currents were intensified, which would yield improvement over fins of constant length when PCM liquifies, in good correlation to other studies [15]. There was sensitivity to ratios greater than 1, where the upper fin was longer than the lower. This hampered some of the positive effects of convection currents and reduced performance.

Other similar efforts identified the horizontal fin arrangement as directly preferable via numerical efforts [59]. To demonstrate, heat flux was applied to a lower boundary and then allowed to progress through the enclosure via fin arrangements. The total time for the enclosed system to reach a melting point was monitored. Depending on elements such as scale and fins, the fin length ratios and number for a particular arrangement could be identified for real-world applications. This matched well the general findings of other numerical efforts in the field, which promote specific designs for specific thermal loads, scale, and cycle times.

Jmal and Baccar [61] added to the fundamental understanding of the basic fin and phase change phenomena via the numerical study of an enclosed PCM with fins present and attached to external, cooling walls. In this study, the solidification front was of particular interest. This effort addressed the question of energy extraction from the PCM vs. heat addition. This can be of critical importance to systems where PCM may be used as a thermal battery, for example [47,54]. The basic dimensions were similar to others in this field, with air moving along the top and bottom surfaces of a 40 mm thick PCM enclosure. The side lengths were expanded compared to other studies at 1.2 m. This would serve to minimize some edge effects in the modeling approach, and the authors were able to study longer durations of solidification behaviors. Fins were attached to the cooling surfaces, protruding into the PCM. Two paraffin PCMs were studied: RT27 (179 J/g latent heat) and C18 (244 J/g latent heat). Both pinned and unpinned geometries were considered, with pins having a protrusion of 30 mm into the PCM from either side.

Unlike other studies with vertical PCM positioning, the efforts by Jmal and Baccar indicated a smaller contribution of internal convection in the solidification process [61]. It was noted that the construction of the system itself would tend to negate some of these effects, however, because of the trapped PCM between two constant temperature plates. Increasing the number of fins further decreased any convective effects, effectively trapping PCM into a conduction-only thermal exchange between fins. The results also indicated that even a small improvement in the overall thermal conductivity in PCM resulted in a significant performance improvement in thermal energy transfer. RT27 had a thermal conductivity of 0.24, and C18 had 0.15 W/mK. The switch to RT27 yielded 55% total solidification of the PCM vs. 40% of C18 at 90 min of cooling time [61].

Mahdi et al. elevated fin use to a more complicated arrangement that involved triplex-tube storage in a detailed numerical study [62]. This was noted as an improvement in the performance of nanoparticle use within PCMs to achieve improved overall thermal conductivity and energy absorption rates. As with work by Ji et al. [60], variable length fins proved advantageous to accelerate melting. Unlike the structures established previously, however, a concentric cylindrical structure was investigated. An inner core and outer annulus of working fluid surrounded the RT82 PCM material, with a thermal conductivity of 0.2 W/mK and latent heat of 175 J/g. The total exterior dimension was 200 mm in diameter, and the inner working fluid core was 50.8 mm in diameter. The PCM annulus between the fluids had an outer diameter of 150 mm where it met the outer fluid volume. Figure 8 shows the general setup of a coaxial system with water flow passages and PCM [62].

The performance of the PCM was assessed using variable fins that extended from the inner and outer working fluid passages into the PCM. Lengths were varied to study the most effective means of thermal absorption by the PCM. Natural convection occurring within the PCM was of direct interest to the authors. It was found that increasing the length of fins where conduction was the primary means of heat transfer and reducing length where internal convection was more dominant (i.e., in melted sections), would significantly improve the performance [62]. Normalized Nusselt numbers could be increased by almost 3× by leveraging this technique in the upper, liquid sections of melted PCM. This proved more effective than the use of AL₂O₃ nanoparticles, as enhancements in the modeling and the reduced size fins in the upper half of the annulus enhanced convection into the PCM. There are several larger-scale implementations of tube bundles and finned-tube bundles for latent heat storage in specific devices and constructions. These are well reviewed by Delgado-Diaz et al. [63]

In efforts by Nallusamy et al. [58], paraffin was selected as the PCM (61 °C melt point) but held in spherical capsules on the order of cm-scale. Fins were attached to the spherical capsules and placed in an enclosed volume where water was used as the heated energy source. The scale of the effort makes comparison with other noted work difficult; however, the use of external attachments such as fins did improve the ability of the PCM to absorb and store thermal energy from the working fluid, which in this effort was water flow. On a similar scale, Castell et al. created a cylindrical container filled with sodium acetate trihydrate as PCM [64]. This was placed in a cylindrical, external water tank (45 cm diameter, 45 cm height). Fins were attached to the internal PCM chamber and increased thermal transfer with the surrounding water. The temperature transfer to the PCM was monitored and used to evaluate different fin geometries. The authors noted an important new design aspect in this external fin approach, which was the interference and interaction of the fins within the heated water flow itself.

The overall effectiveness of these fin approaches can be significant. In work by Lohrasbi et al. [23], a comparative study between the Latent Heat Thermal Energy Storage System (LHTESS) with optimized fin structure in PCM and a nano-enhanced Phase Change Material (NEPCM) was completed. Findings suggested that the addition of the fin structure had superior heat transfer to the nanoparticles dispersion.

A commonality across each of these techniques has been to improve the thermal performance of the PCM under consideration through the addition of a mechanical structure. The approaches reviewed have primarily examined low temperature thermal systems and held several common themes. First, the liquefaction of the PCM itself causes significant change in the conduction or convection characteristics of the PCM material. This has led several to conclude that the liquefaction of the PCM tends to limit the overall thermal conductivity enhancement capability regardless of the underlying mechanical structure or approach. In addition, systems designed to specifically improve the internal convection characteristics of a PCM undergoing phase change tended to outperform those that limited its ability to establish internal fluid movement. These findings also align well with observations that even a small thermal conductivity enhancement of the PCM itself can improve performance measurably [61].

Combined, this collective set of efforts reveals a general guide to PCM use for maximum heat transfer in and out of the material. When designed for specific sizes and applications, these can have significant value for real-world applications.

4. PCM Investigations with Micro/Nano Additives

Micro and nano additives to base PCMs have been targeted by many research efforts for multiple years. These additives range from naturally occurring ones [8,9,10] to micro particle additives such as copper or graphite [8,56,65]. While the addition of mechanical structures such as foams or fins has shown great promise, there may be further gains through the use of micro or nano-additives in the PCM, depending on use and application intentions.

The basic goals of these efforts remain similar to structural efforts that utilize metallic foams or matrices combined with PCM. The first goal is to increase overall thermal conductivity and, relatedly, reduce the thermal cycle times for energy storage and release. However, the utilization of micro materials can also bring additional effects such as phase-change form stability. In these applications, the PCM can undergo melt without leakage due to the combination of materials within the hybrid materials. Some of these unique materials and additives, as well as the results, are detailed in this section.

Form stability combined with strength considerations have been examined by several authors. For example, high density polyethylene was filled with micro-encapsulated RT42 paraffin wax in efforts by Karkri et al. [66] Paraffin served as the PCM in this effort, and was enclosed via microcapsules of melamine-formaldehyde resin. The shell was approximately 1.5 µm thick, and the overall diameter of the capsules was 15 µm. The combination of high strength polyethylene with the encased PCM provided both form stability as well as form rigidity and strength for applications where this could be a challenge for use. Different combinations of polyethylene and PCM capsules were considered in this effort, including 60/40 (HDPE/capsule), 50/50, and 40/60 by mass. Slurries formed the basic mixture of these materials, which were then dried and pressed to rectangular shapes for testing. Based on the shape of the samples, a guarded hot plate method for thermal characterization was employed to evaluate performance. The thermal conductivity of the mixtures was monitored and reduced from about 0.49 W/mK for simple polypropylene to 0.236 W/mK for 60% paraffin samples. Both the PCM as well as the encapsulating resin that formed the spheres represented significant insulators in this respect. No direct heats of fusion were reported; however, the maximum latent heat storage was found with 60% PCM microcapsules at 91.3 J/g [66]. Despite the lower thermal conductivity associated with this approach, the structural soundness and form-stability due to the encapsulation technique may have benefits for some applications.

Sari et al. evaluated encapsulated PCM in polystyrene shells [67]. Micro capsules of PCM were sourced for inclusion in the base material. However, C24 (n-tetracosane) was selected as a base PCM ingredient. The heat of fusion for C24 was significant at 276 J/g; however, the melting point of about 50 °C was too high for the anticipated low temperature applications. Hence, the team prepared a eutectic mixture with n-octadecane (C18). This formed a C24/C18 mix of 10/90% by weight. Following the fabrication of the encapsulated PCM, measures indicated an average diameter between 11 and 4 µm depending on the exact mixtures with polystyrene. The final heats of fusion were measured via DSC to be 242.6 J/g for the C24/C18 mixture alone. Incorporating the shells reduced the performance to a best case of 156.4 J/g in a 1:2 mixture of shells to PCM. Melt points were maintained at about 25 °C across different samples. The successful incorporation and performance of the encapsulated PCM positioned it well for inclusion in other materials or applications. The use of styrene would also tend to give it good strength capacity where mechanical stress or strain could impact installed PCMs.

Other investigations on PCM encapsulated in shells for strength and thermal enhancement were conducted by Cui et al. [68,69] This investigation began with the observation that cement composite with PCM is an effective way to gain PCM advantages within infrastructure projects; however, it also mandates the environmentally costly manufacture of concrete base material. Because of this, alkali-activated slag was used as a cement alternative and then combined with different amounts of graphite enhanced, microencapsulated PCM and carbon fibers. The base PCM utilized was paraffin, which was encapsulated in polyurea micro-capsules with a graphite additive [54]. The final combined materials with carbon fiber had sizes of 30 × 30 × 120 mm and served as the specimens. In addition to the thermal property investigation, strength comparisons to the alternative concretes were conducted with good results. The compressive strengths of the new materials were up to 30% greater than cement with only encapsulated PCM. This was largely attributed to the use of carbon fiber as a strength enhancement included at 1% by weight.

These investigations also examined thermal aspects. Thermal conductivities were measured, and the use of carbon fiber to enhance the thermal conductivity of the included PCM was effective [69]. The new materials were generally less conductive than their cement counterparts; however, the use of carbon fiber improved the results of overall conductivity up to 0.77 W/mK. This represented a 10% by weight content of PCM with 1% carbon fiber. The baseline concrete had a conductivity value of 1.1 W/mK [55]. While this represented reduced thermal conductivity, the thermal storage capacity was noted to increase based on different maximum temperature values during testing. Given the strength characteristics and the promise of thermal energy storage, this approach of a microencapsulated PCM had promise for use in building applications. This also highlights the need to consider strength and thermal properties in material fabrication depending on the intended applications.

Form-stabilized efforts have been examined by Trigui et al., as well [65]. The authors note that, in addition to the well-documented poor thermal conductivity of PCMs themselves, the difficulty of retaining shape upon phase change adds additional complications to any overall design for use. In this effort, hexadecane was utilized as the organic PCM with a phase change temperature between 18 and 20 °C. The material also has a high latent heat of 224 J/g. Microparticle copper with particles sized less than 120 µm was utilized as the thermal conductivity enhancement. For shape stability, styrene-ethylene-butylene-styrene (SEBS) was combined with low-density polyethylene (LDPE) due to compatibility with hexadecane. Multiple mixtures of the base wax (hexadecane), LDPE and SEPS were created and varied by weight percent. SEBS ranged from 0 to 20%, HEX was maintained at 75%, LDPE ranged from 0 to 25% and Cu additive was varied from 0 to 15% by weight. DSC heating and cooling were utilized across the different mixtures. Of note were results indicating form stability increase and improved cycle performance (melt-solidify-melt) operations with 15% Cu particles, 5% LDPE, 75% wax, and 5% SEBS. This mixture also showed the lowest reduction in enthalpy of solidification, as well, at about 201.3 J/g, and was well suited to low temperature applications where periodic cycling could be expected [65].

Naturally Occurring Additives Use

While many efforts reflect the general combination of thermal conductivity enhancement and form stability, there have been interesting projects that look to naturally occurring additives to boost PCM performance. In an effort by Zhao et al. [8], paraffin was sourced as the PCM with a target application of low temperature thermal energy storage. Unique, naturally occurring halloysites were utilized as additives to the PCM. Halloysite, or clay nanotube, is formed through the rolling of alumosilicate sheets with an ideal formula of (Al₂Si₂O₅(OH)₄ × nH₂O) with silicon oxide that makes up the tube outer surface [70,71,72]. Halloysite clay nanotubes are an inexpensive (ca. USD 5 per kg), abundantly available natural material which is environmentally friendly. Contrary to many other synthetic nanomaterials, it is biocompatible and hydrophilic (surface electric zeta-potential in water is −30 mV), and may be well dispersed in water, alcohol and many polar polymers (epoxy, PDMS, PLA, polyamides, PVA, PMMA, and most biopolymers) [72]. In 2020, its world usage was ca 50,000 tons—for high quality porcelain, dye formulations, polymer composites, cosmetics and traditional Chinese medicine. The overall dimensions are typically on the order of 1 µm in length and 50–70 nm in diameter.

In addition to the halloysite additive, graphite nanocomposites and carbon nanotubes were added with different percentages by weight across multiple mixtures and tests. SEM images of various mixtures are shown in Figure 9. Pure paraffin PCM was utilized as a baseline, followed by a 50/50 mixture of wax and halloysite [8]. These percentages were then varied to allow for up to 10% graphite, or a maximum of 10% carbon nanotube, both with 45/45% mixtures of PCM and halloysite. The addition of halloysite allowed a form-stable melted composite through these efforts and was very effective in 45% mixtures. Further DSC analysis showed the expected reduction from a pure wax latent heat of 171 J/g to 87 J/g for the 50/50 wax-halloysite mixture. A mass loss up to the wax melt point of 53 through 70 °C was monitored, and losses of less than 0.55% were evident from the 45/45/10 wax–halloysite–graphite mixtures. In addition to the demonstrated form stability, the thermal conductivity of this mixture was 1.4 W/mK vs. the neat wax conductivity value of 0.25 W/mK, as measured via DSC [8].

Pinecone biochar (PB) was the basis for a cost effective form-stable PCM investigated by Wan et al. [10] This biochar was utilized as the support material for palmitic acid (PA) with a melt point of 75 °C. Of note was the effort to incorporate an inexpensive and environmentally friendly material as part of the work to achieve form-stability. The Pinecone biochar (PB) was fabricated based on abundance, as well as the ease of obtaining burned residue (char) from local practices. Pinecone was first washed in distilled water, and then baked for 24 h at 105 °C. This dried sample was crushed to powder and pyrolyzed in a nitrogen environment. The PCM had a melt point of 62–64 °C and was dissolved in ethanol. The PB was added in different concentrations and stirred to achieve good mixing under vacuum. Mixtures ranged from mass ratios of 3:7 to 6.5:3.5, PA:PB. The results indicated that form stability was particularly strong for mixture ratios of 6:4. Melt points for the mixture via DSC analysis were found to be 59.13 °C. The divergence from pure PA behavior was attributed to the impurity of having the PB additive affecting the crystalline region of the PA. The heats of fusion ranged from 219.6 J/g (pure PA) to 84.7 J/g for the 6:4 ratio material [10]. The resulting thermal conductivity was 0.39 W/mK. The authors also examined the thermal performance with up to 50 melt-freeze cycles with good stability results. Of note was that neither the PA nor PB represented materials with significant thermal conductivity; however, in combination, when the PCM displaced air pockets within the biochar, the overall average was increased. This work represented a significant use of environmentally friendly materials to achieve form-stability, with the heats of fusion and thermal conductivity well aligned with other efforts reviewed.

Jujube branches were selected as a biomass component for work by Lv et al. [11] This naturally occurring material was first obtained in char form, carbon activated and then used as an additive for Polyethylene Glycol (PEG) as the studied PCM. Important to the study was the identification of an overall process that represented improved energy efficiency in manufacture with reduced environmental impact. Investigations included several preparations of base activated jujube char, combined with different volumes of PEG ranging from 50 to 80%. Experiments indicated that 80% PEG was preferred, and thermal property evaluations were conducted via DSC. Several outcomes were noted. First, the melt point of neat PEG (17.3 °C) shifted slightly higher depending on the specific Jujube base considered, similar to findings by Wan et al. [10]. The heats of fusion shifted with the increase of additive to the PEG, dropping from 145.39 J/g (neat PEG) to minimums of 90.97 J/g depending on the exact material composition. The best performing composite materials brought together large surface areas and staggered pore structures that allowed significant PCM uptake. Surface areas of 1082.2 m²/g in the supporting jujube charcoal performed with the highest heat of fusion of 114.9 J/g [11]. Significant form-stability was found using these materials, which demonstrated a very capable encapsulation ability of the support biochar.

5. Thermal Properties Summary

There have been many varied approaches that have addressed the combined challenge of thermal conductivity, heats of fusion, and form stability in these reviewed efforts.

Table 1. Summary of reported average values from different PCM studies reviewed.

PCM	Conductivity Enhancement	Form Stable Additive	Avg. Thermal Conductivity (W/mK)	Heat of Fusion (J/g)	Authors
Average Baseline Waxes in Use	-	-	0.2–0.5	140–230
Icosane	Cu foams	-	3.8	-	[47]
Paraffin	Cu foam, knitted arly network	-	55.4	105.6	[2]
Paraffin	-	Encapsulated in syntactic foam	0.18–0.28	62	[50]
Paraffin	Graphene composite	Encapsulated in graphene aerogel foam	-	178.9	[51]
Paraffin	-	Encapsulated in polyethylene	0.24	-	[66]
C24/C18	-	Encapsulated in polystyrene	-	156.4	[67]
Paraffin	Graphite particles	Encapsulated in polyurea, carbon-fiber additive	0.77	-	[68,69]
Palmitic Acid	-	Pine biochar	0.39	84.74	[10]
Icosane	Graphite particles	Halloysite	1.4	87	[8,9]
PEG	-	Jujube branch char	-	114.9	[11]
Hexadecane	Copper particles	SEBS + LDPE	-	201.3	[65]

brings together some of the reported results that focus on these key outcomes from the different works. Note that reported values of heats of fusion and thermal conductivity reflect the modified, experimental results and not the baseline PCM values.

As noted in Section 2.3, thermal conductivity is a key element of research efforts and PCM performance in TES use. The values presented in Table 1 show that the average thermal conductivity of PCMs can be significantly improved through various enhancements. In addition, critical form stability is achievable. For instance, the thermal conductivity of eicosane is enhanced by adding Cu foams, resulting in a reported value of 3.8 W/mK. Similarly, paraffin exhibits higher thermal conductivity when combined with Cu foam or a knitted arly network. Encapsulating paraffin in Syntactic Foam or Polyethylene also improves thermal conductivity. The use of graphite particles and aerogel foam also enhances thermal conductivity for some PCMs, such as paraffin.

In addition to the success of the thermal conductivity enhancement and form stability, Table 1 reviews the overall heats of fusion of different approaches. Combined, the key properties of PCM guide both the design for TES use and different application approaches. As a summary of the reviewed research, Table 1 highlight the importance of selecting the appropriate PCM and adding conductivity enhancement or form stable additives to achieve optimal thermal performance in TES systems.

6. PCM Applications at a Smaller Scale

It is interesting to note the different materials as well as the different approaches that have been employed to address common challenges to PCM use. Among these, thermal conductivity and form-stability have risen to be the top considerations across many of the efforts summarized. Multiple applications have already been mentioned as part of the material review. It is interesting to review additional applications where these low temperature PCMs find use in smaller (non-infrastructure) scale applications.

PCMs have found many smaller-scale general use cases with applications that include electronic thermal management [73], photovoltaic cooling [74], heat sink use [75], active thermal protection in hot environments [76], and even applications in MEMS resonators [77]. Further, there has been interest in improving the duration of thermoelectric generator operations through the use of these materials, creating a so-called “thermal battery” [47,54,78].

As work with electronics attests, PCM materials are crucial for managing heat in electronic systems that use conductors with electric resistance. The resistance of conductors is caused by the collision of electrons with the conductor lattice, leading to heat generation that can cause components to melt. Heat sinks made of metals are often used to solve this problem; however, they have fixed shapes and thermal masses. This limits their ability to reject heat quickly and consistently. However, phase change materials can improve thermal management systems by providing different cooling characteristics, tuned or designed for specific use cases. According to research by Wu et al. [73], phase change material board (PCMB) technology is suitable for many electronic devices that work intermittently for short periods, such as smartphones, digital cameras, and personal digital assistants. By contrast, a more traditional ribbed sink or radiator is useful for devices that work continuously over long periods [73].

In photovoltaic use, an experimental study showed that using PCM in the PV module to enhance the cooling performance increased the generated power up to 26.2% [74]. The use of PCM in these cases allowed the modules to maintain a lower average temperature during operation, significantly enhancing outputs.

By contrast, PCM use has been explored to protect devices on the smaller scale when involved in deep hole exploration [76]. Deep hole exploration exposes sensitive electronics and devices to higher temperatures and pressures than surface applications. Instruments that can be used on the surface with little or no thermal management are useless for deep hole exploration because they can produce erroneous results when used above their designed operational temperature. As a result, smaller and more compact heat transfer units or sinks are required. Because of the size constraints and thermal absorption capabilities within a narrow temperature range, PCM becomes a unique thermal sink [76].

It is useful to compile a table of the various applications and methods used throughout the course of this review.

Table 2. Small-scale PCM Applications Summary.

General Application(s)	Configuration or PCM Type	Comments	Authors
Low-grade thermal energy storage and capture	Conductive Foam enhanced PCM Heat pipe enhanced	Improved thermal conductivity via metallic inserts or heat pipes, requires external volume for containment of liquified PCM. Reduces overall volume for thermal storage with increasing foam or heat pipe inclusions.	[47,54]
HVAC efficiency improvement	Graphene-based foam enhanced	Shows form stability within the structure of the foam. Overall thermal storage reduced based on volume percentages of foam design.	[51]
Electronics cooling and related thermal temperature management in different environments for electronics	Nano-additive PCM	Improved thermal conductivity.	[19]
	Encapsulated, enhanced PCM	Contained PCM with thermal conductivity enhancement, best suited for intermittent electronic cooling needs with periodic temperature increases.	[73]
	Sheep fat PCM with CuO nanoparticles	Unique use of more sustainable PCM with added thermal conductivity enhancement improves photovoltaic performance, not directly form stable.	[74]
	Heat-sink enclosed PCMs	Different PCMs offer performance improvement for electronics cooling as heat sinks. Conductivity enhancement not employed, PCM enclosed to control liquification.	[75]
Heat exchanger enhancement	Encapsulated, packed	Direct contact with thermal surface improves heat transfer and operation of the exchanger.	[43]

brings together the general application areas for the PCMs under consideration, as well as general comments about the use and application. Of note is the large contribution of PCM for thermal management in electronics-related application. Sustainable materials such as sheep fat are even being considered [74]. Thermal energy scavenging is also of importance, and may even be paired with electronics applications to power devices such as thermoelectric generators [47]. As noted previously, every PCM implementation requires its own specific design for use that includes space, thermal energy absorption requirements, and packaging. This makes generalities about common positives or negatives to the different approaches difficult.

7. Summary and Conclusions

There have been many significant efforts to explore and utilize Phase Change Materials (PCMs) on the smaller-scale at lower operating temperatures across the past multiple decades. Many of these employ the use of PCMs that have melting points in the temperature range of expected waste thermal energy or ambient heat sources. Paraffin waxes, for example, are one common PCM explored in this field.

A general challenge to the use of these PCMs, regardless of application, is the low thermal conductivity that is present as a baseline material property.

Several commonalities and challenges emerge through the review of this field:

• The use of metallic foams or other metal-based structure to improve average thermal conductivity;
• Base wax alterations to employ nano-scale additives, such as graphite for increased thermal conductivity;
• The form stability of melted PCM through the addition of material additives. Natural additives such as halloysite nanotubes have been employed as one example of this technique.

Efforts to alter thermal conductivity or maintain form shape when melted must be weighed against the cost of PCM volume reductions. For example, through the increased use of nano-particle additives or metallic foam structures, the overall storage capability of the wax is reduced per volume. Achieving specific designs for specific applications are thus noted by multiple investigations in the field as critical.

While the field of PCM research has been well established, the use of new materials and approaches that employ the use of natural materials continues to provide evolution. Further, as energy efficiency and thermal management become increasingly important, this remains an important and active area of research even after several decades have passed since some of the earliest work in the field. Looking ahead, the use of PCMs in TES applications is expected to grow, driven by this increasing demand for sustainable and efficient energy solutions. Future research in this field will focus on developing novel PCMs with improved thermal properties, even guided by advancing ML. New methods will explore further enhancements to the thermal conductivity of PCMs and optimizing the design of TES systems. This will ensure maximum efficiency and reliability while increasing the more sustainable approaches of some recent advances.