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Review

Fatty Acids as Phase Change Materials for Building Applications: Drawbacks and Future Developments

by
Paola Herrera
,
Hector De la Hoz Siegler
and
Matthew Clarke
*
Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada
*
Author to whom correspondence should be addressed.
Energies 2024, 17(19), 4880; https://doi.org/10.3390/en17194880 (registering DOI)
Submission received: 12 August 2024 / Revised: 15 September 2024 / Accepted: 25 September 2024 / Published: 28 September 2024
(This article belongs to the Special Issue Energy from Agricultural and Forestry Biomass Waste)
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Abstract

:
The worldwide population growth and its increasing affluence have led to an increase in global building energy consumption. Therefore, developing sustainable energy storage materials to mitigate this problem has become a high priority for many researchers. Organic phase change materials (PCMs), such as fatty acids, have been extensively studied for thermal energy storage in building applications due to their excellent performance in absorbing and releasing energy within the environment temperature ranges. However, issues related to their thermal conductivity, stability, and flammability could limit the potential and require addressing. In this review, organic PCMs, with a special focus on fatty acids, are discussed. This review covers recent studies related to PCM synthesis from bio-sources, methods for PCM incorporation in building materials, methods for enhancing organic PCM thermal properties, flammability challenges, and life cycle assessment. Finally, future opportunities are summarized.

1. Introduction

Recent industrial and economic growth has significantly increased global energy consumption. The building sector, in particular, contributes significantly to this increase, accounting for 20% to 60% of total energy consumption worldwide [1]. Most of this energy is derived from fossil fuels, which produce substantial amounts of the greenhouse gases responsible for global warming [2,3,4]. This situation has sparked research interest in strategies to mitigate climate change and reduce global emissions associated with buildings, with a focus on the integration of renewable energy systems and the development of biomaterials for construction.
Phase change materials (PCMs) are latent heat storage substances that provide efficient and high energy storage capacity. This makes them a promising material for enhancing energy conservation, especially when used in conjunction with renewable energy sources. The use of PCMs in diverse applications has successfully been demonstrated, highlighting their versatility in thermal energy management in solar thermal power plants [5,6,7], building applications [8,9], thermal comfort in vehicles [10,11], greenhouses for temperature control [12,13,14,15], battery systems [16,17,18,19], and refrigerated transportation [20,21,22].
PCMs can be classified as inorganic, organic, or eutectic. Inorganic PCMs offer advantages such as high thermal conductivity and low cost but are characterized by a high degree of supercooling and corrosiveness. Organic PCMs are non-toxic, non-corrosive, and highly stable but face challenges such as flammability and low thermal conductivity [23,24,25]. Eutectic PCMs are combinations of two or more organic or inorganic substances, resulting in a specific transition temperature, known as the “eutectic point” [26].
Fatty acids are biobased PCMs, which can be derived from natural resources, such as biomass and vegetable oils [27]. These PCMs show favorable thermal properties compared to paraffins and are low-cost, non-toxic, and non-corrosive [28]. However, they are traditionally obtained from animal tallow or edible crops, raising environmental and food security concerns.
The aforementioned drawbacks are crucial challenges in the adoption of thermal energy storage materials for the construction sector. Some authors have reported various approaches, such as incorporating additives into bulk PCMs or using encapsulation techniques with supportive materials [29,30,31,32]. Despite these efforts, achieving good compatibility and balance between all components remains challenging, as it is essential to ensure a homogeneous mixture and prevent leakage or separation of the components over time.
Table 1 summarizes significant review articles published in recent years emphasizing the use of PCMs in construction materials. However, according to the examination of recent works, it is noticeable that, to date (even before 2018), no review paper has specifically focused on the use of fatty acids as PCMs in building applications. It is important to highlight that there is a need for a comprehensive review that addresses the methods to obtain fatty acids from different bio-sources, their common thermophysical properties, their incorporation into construction materials, and potential gaps to improve their principal limitations.
Accordingly, the objective of this review paper is to explore recent studies on the application of fatty acids-based PCMs for incorporation into common building materials.
The methodology followed in this literature review included a search of the published papers and research that includes the keywords: phase change materials, fatty acids, PCMs in building applications, extraction of fatty acids, thermal properties, and organic PCMs. The primary sources used were obtained from ScienceDirect, Google Scholar, and Scopus databases. The papers included in this review were published from 2012 to 2024, and a summary of the most relevant papers was created to categorize them based on the type of research.
This review begins with an introduction to building energy consumption and a brief overview of PCMs. Then, an outline of fatty acids as PCMs is presented, along with recent advancements focused on the production from novel non-food sources. Methods available for the incorporation of fatty acids into building materials are subsequently presented, followed by an overview of the properties of fatty acids and potential drawbacks in the building sector, including thermal characteristics, flammability, and life cycle assessment. Finally, we highlight potential areas of interest for future research, providing an outlook on development opportunities.

2. Energy Consumption

Global primary energy demand has risen by 10.1% [41] over the last decade due to population growth and industrialization. Currently, fossil fuels supply around 80% of the world’s energy consumption. However, the use of fossil fuels generates greenhouse gases, such as carbon dioxide, with detrimental environmental effects [2]. These environmental impacts, coupled with diminishing fossil fuel reserves, underscore the urgent need to develop greener energy systems, including thermal energy storage systems.
Thermal energy storage is an important strategy for increasing energy savings and efficiency from renewable sources, as it helps to balance energy demand and consumption, preventing overloads in networks and power plants [42]. Storage is accomplished through various systems, such as latent heat storage, where heat is transferred into a substance that uses it throughout its phase change transition without a noticeable temperature change. These substances are referred to as phase change materials (PCMs).

2.1. Building Energy Usage

According to the Energy Institute, primary energy consumption reached a record of 620 EJ in 2023, marking a growth rate of 0.6% above the average of the previous decade [43]. Globally, the transportation, industrial, and residential sectors are the largest consumers of energy, accounting for approximately 33%, 29%, and 27%, respectively [44].
Specifically, the building sector (residential and commercial) accounts for 20% to around 60% of the global energy consumption (See Figure 1). Notably, North American and European countries are significant contributors to this energy demand. According to the government of Canada, from 1999 to 2010, the commercial building sector grew by 22%, representing around 12% of the total energy consumption of the country [45].
The U.S. Energy Information Administration (EIA) released a report in 2022 showing the distribution of commercial building energy consumption in 2018 (See Figure 2). Roughly 41% of the energy consumed is associated with space heating and cooling. This demand is influenced by several factors, including climate conditions, geographical location, and energy efficiency [46].
Given that nearly half of building energy is consumed for space heating and cooling in North America, these sectors offer great potential for energy savings in building applications. Consequently, research efforts increasingly focus on enhancing energy efficiency and reducing energy consumption through green and renewable alternatives, such as thermal energy storage systems (TESs).
These systems use materials to store large amounts of energy. PCMs are key components, as they can store and release energy at nearly constant temperatures during phase transitions. Several studies have shown that incorporating PCMs into buildings can significantly reduce energy consumption with increasing comfort [47,48].

2.2. PCMs for Building Applications

The residential and commercial building sectors have been identified as having substantial potential for energy savings through the implementation of TESs based on PCMs.
Figure 3 illustrates the working cycle of PCMs through the phase transitions. The energy storage mechanism of PCMs involves the absorption of the latent heat in an endothermic process during melting and the release of this energy to the surroundings during crystallization [23,49].
Several researchers have investigated the use of PCMs incorporated into walls for building applications. In 2018, Stritih et al. [50] studied a fatty acid mixture of capric and myristic acids as core material in a prefabricated concrete panel. The results confirmed that incorporating PCMs into walls decreased building energy use, demonstrating a promising strategy for achieving net zero energy buildings in the near future.
Although PCMs generally indicate good efficiency in building applications, their optimal wall thermal performance is influenced by weather conditions. Oselen et al. [51] studied the incorporation of a PCM wallboard to the walls of a retrofitted building in Ottawa, Canada, where winter temperatures are about −14 °C. The results indicated a reduction of heat loss by 96%, with the PCM layer thickness not affecting the system’s effectiveness.
These findings, along with a numerical analysis by Rasool et al. in 2023 [41] and Calene et al. in 2024 [52], confirm that using PCMs is an effective technique for achieving energy efficiency criteria in net zero by 2025, as well as leading to a reduction of greenhouse gas emissions in the building sector.
PCMs can be classified mainly into inorganic, organic, and eutectic. Each of these types of PCMs has different advantages and drawbacks, emphasizing the need to assess the characteristics to maximize the thermal efficiency in building applications. A summary of the advantages and drawbacks of the types of PCMs is presented in Table 2.
Inorganic PCMs exhibit a wide range of melting temperatures, relatively high latent heat storage capacity, and high thermal conductivity. However, a significant drawback of these types of PCMs is their supercooling tendency. This refers to a delay in the solidification process due to slow nucleation rates, resulting in unpredictable behavior and challenges in thermal energy storage performance [54,55,56]. Recently, Lian et al. [57] prepared a modified inorganic PCM composed of disodium hydrogen phosphate dodecahydrate. The supercooling behavior was evaluated after the incorporation of sodium thiosulfate pentahydrate as a nucleating agent. The novel PCM developed evidenced a melting temperature of around 35 °C and a latent heat of fusion of 184.39 J/g. The incorporation of the nucleating agent decreased the supercooling rate to around 1.1 C, being a promising material to be applied for passive building thermal management. However, additional drawbacks, such as corrosiveness behavior, toxicity, low thermal stability, and potential leakage, can negatively affect heat transfer. In contrast, organic PCMs present a promising avenue for thermal energy storage applications.

3. Organic PCMs

Organic PCMs are carbon-based materials obtained from either fossil or natural, sustainable sources. Different authors have reported the study of several matrices based on organic compounds, including biomass composites, polyamide nanosheets/aerogels, polyvinyl alcohol, fiber membranes, and polyethylene glycol to be used as PCMs in the fields of energy storage and conversion, for instance, photothermal conversion, solar thermal systems, electromagnetic radiation, textile industry, and electronic devices [58,59,60,61,62].
These types of PCMs have gained significant attention, specifically in building applications, due to their advantages over inorganic materials. These advantages include the absence of supercooling, high thermal stability, high latent heat of fusion, non-corrosive properties, and good compatibility with construction materials [23,63].

3.1. Types of Organic PCMs

Organic PCMs can be categorized into two main types: paraffins and non-paraffins. Paraffin wax is the most commonly used PCM in commercial energy storage. Non-paraffin organic PCMs encompass a broader variety of materials, such as fatty acids, fatty esters, and alcohols [26]. Fatty acids, in particular, offer a wide range of phase change temperatures, making them extensively studied in various applications [64].

3.1.1. Paraffins

Paraffins are used in TESs because of their favorable properties, such as high heat of fusion, no supercooling behavior, and chemical inertness [49]. Paraffins are alkanes, are classified based on the number of carbons in their structure, and have melting temperatures ranging from 0 to 100 °C [65].
In 2021, Yasiri et al. [66] reviewed studies on paraffin as a PCM to improve building energy and thermal performance. Their review indicated that paraffin-based PCMs incorporated into building materials enhance thermal behavior by saving energy and improving thermal comfort.
Despite their effectiveness in building applications, some environmental concerns exist as paraffins are largely derived from non-renewable fossil sources. The production and disposal of paraffin can contribute to greenhouse gas emissions. Additionally, the toxicity of some paraffin formulations may present health risks during manufacturing, handling, and in the event of leakage or exposure [67]. These factors underscore the need for alternative PCMs that prioritize sustainability, safety, and reduced environmental impacts.

3.1.2. Fatty Acids

Fatty acids are characterized by the presence of a carboxylic acid group attached to an aliphatic chain with 2 to 80 carbon atoms [68]. These molecules can be classified as saturated or unsaturated, depending on the presence or absence of double bonds in the hydrocarbon chain [69]. Major sources of fatty acids include oleaginous crops and animal fats, such as tallow and fish [70].
The use of fatty acids as PCMs has received significant attention due to their high heat storage capacity, good chemical and thermal stability, congruent melting, non-toxicity, absence of supercooling, broad range of temperature transition, non-corrosiveness, and availability from renewable bio-sources [71,72].
Figure 4 shows the trend in research focused on capric, palmitic, stearic, lauric, and myristic acids from 1989 to 2024. The graph reveals a notable increase in studies involving these fatty acids over time, highlighting the growing interest in their use as PCMs due to their favorable properties and sustainable sourcing.
Table 3 summarizes the thermal properties of fatty acids evaluated in building applications, demonstrating their suitable phase transition temperature range and high latent heat of fusion.

3.2. Sustainable Sources and Processes for Obtaining Fatty Acids

Commercial fatty acids are primarily derived from animal fat or oleaginous crops, such as palm, soy, canola, and coconut, which are associated with detrimental environmental effects and concerns regarding land-use change and food supply [64,83,84]. Hence, recent research has focused on novel bio-sources, such as oleaginous microalgae and yeast, for obtaining pure fatty acids as an alternative to mitigate the global energy crisis linked to fossil fuels, pesticides, and fertilizers [85].
Microalgae biomass has been used as a source of bio-oil due to its high potential for lipid accumulation and rapid growth rate. Free fatty acids can be extracted through hydrolysis reactions, such as acid, chemical, and enzymatic [86,87]. As shown by Yaakob et al. [88], nutrient supply, primarily nitrogen and phosphorus, is crucial for enhancing lipid production and obtaining high-quality fatty acids.
Xue et al. [89] investigated the use of oleaginous yeasts, such as Rhodosporidium toruloides, Yarrowia lipolytica, Rhodotorulamucilaginosa, and Aureobasidium melanogenum, to obtain free fatty acids. These microbes can accumulate over 50% of their cell dry weight as lipids, thanks to their continuous acetyl-CoA production, which serves as a fatty acid precursor.
Chemical hydrolysis, a traditional method for extracting fatty acids, employs solvents under elevated temperatures and pressures. This method involves acids or alkaline agents, such as hydrochloric and sulfuric acids or sodium and calcium hydroxides [90]. Unfortunately, the required reaction conditions can lead to undesirable side reactions, such as oxidation and thermal decomposition, leading to lower fatty acid yields [91].
Enzymatic hydrolysis offers a milder and more efficient alternative using enzymes, such as lipases, to catalyze the hydrolysis reaction and thus requires more moderate pressures and temperatures [86,92]. For instance, lipases from Candida antarctica, Castor bean, Candida rugosa, and Candida cylindracea have been used to obtain free fatty acids from vegetable oils, used frying oil, fish oil, and rice bran oil. Several factors have been shown to influence the reaction’s yield, including reaction time, enzyme loading, and fatty acid composition [91,93,94]. Figure 5 represents the process of enzymatic hydrolysis of oils, including the reaction, followed by a filtration and evaporation process to recover the enzymes and solvent used. Finally, centrifugation leads to the separation of glycerol and fatty acids.
After hydrolysis, the recovery and purification of free fatty acids are key steps to ensure final product quality. Crystallization, steam distillation, adsorption, and molecular distillation are some of the methods available for separating fatty acids [95]. Fractional crystallization has emerged as a promising technique for obtaining high-purity saturated fatty acids with lower energy requirements. This method involves producing solid crystals in a liquid matrix through nucleation and crystal growth, followed by separation using techniques like centrifugation and vacuum filtration [96].
Maeda et al. [97] evaluated unsaturated fatty acid separation using this technique, examining the purification of a binary system of oleic and linoleic acids. They used a high-pressure system with a glass cell and free piston, achieving a composition of around 0.90 mol fraction of oleic acid at 90 MPa. This method shows promise for separating binary unsaturated fatty acids.

3.3. Fatty Acid Eutectic Mixtures

Eutectic PCMs are mixtures of two or more substances having a specific transition temperature, known as the “eutectic point,” which is lower than the melting points of the pure compounds. The eutectic point for a mixture can be studied through the development of a phase diagram based on the thermal properties of the pure constituents [98]. Figure 6a shows a typical phase diagram for a binary eutectic mixture, where point “E” represents the eutectic point. The area labeled as “L” represents the liquid phase that appears above the eutectic temperature; “A + L” and “B + L” represent regions where a liquid phase coexists with a solid phase (A or B); “A + B” represents the solid phase that is formed below the eutectic point. Figure 6b shows the comparison between experimental and simulated phase diagrams for a palmitic acid–stearic acid system, developed by Mailhé et al. [99]. The results evidenced a good correlation between numerical and experimental data.
It is important to note that not all mixtures form a eutectic point. A eutectic system occurs when the components of a mixture in a liquid phase inhibit the crystallization phase of one another, thus lowering the phase transition temperature and leading to the simultaneous formation of different solid phases [101]. This differs from peritectic systems, where a solid and liquid phase interact to form a new solid, potentially leading to incongruent melting and metastable phases, causing phase separation [102]. This incongruent behavior negatively affects the stability and efficiency of PCMs in thermal energy storage applications; thus, eutectic systems are preferable thanks to their sharp and well-defined phase change transition.
The design and synthesis of eutectic mixtures based on fatty acids have been explored by various authors [73,100,103,104,105,106]. These mixtures have shown potential as thermal energy storage materials for both low and high-temperature systems, offering superior properties compared to pure compounds. Table 4 summarizes recent relevant studies on the development of fatty acids-based eutectic PCMs and their implementation in building materials.
Up to now, most studies on fatty acids-based PCMs have focused on the characterization and improvement of melting point and heat storage capacity. However, due to the high number of possible eutectic combinations, available data is limited. One study that provides a more complete characterization is due to Duquesne et al. [114], who characterized eutectic mixtures of capric-myristic and capric-palmitic acids and reported data on their thermal diffusivity, energy densities, and volume expansion. In their work, Duquesne et al. highlighted the importance of these less-studied properties on indoor thermal regulation in buildings as they affect energy storage capacity, heat transfer rate, and physical stability of the PCMs. Moreover, the findings revealed that these properties are dependent on the composition of the fatty acid mixture. Future analyses should continue to explore these properties for a more comprehensive evaluation.

4. Properties of Fatty Acid-Based PCMs

A variety of fatty acid-based PCMs have been explored for thermal energy storage applications, including lauric acid, stearic acid, and palmitic acid [115]. Despite their satisfactory performance, these PCMs have limitations, such as leakage during phase transitions, low thermal conductivity, and high flammability. These properties are important for PCMs used in the construction sector. Some strategies have been developed to overcome these limitations, including the design and production of shape-stabilized PCMs (composites) employing techniques such as impregnation and encapsulation, incorporating additives that act as supports or fillers [115,116,117].

4.1. Stability

The stability of PCMs for latent heat storage applications is defined as the ability of the material to ensure negligible changes in its chemical bonds and thermal properties, such as melting point and latent heat. This parameter determines the long-term performance of the material [105,118]. Various authors have reported that the stability of fatty acid-based PCMs is associated with sample degradation during temperature fluctuations. Physical and chemical phenomena, including chain scission, moisture absorption, ramification, polymorphism, and oxidation, can cause the loss of stability [119,120,121]. Strategies to improve the stability of fatty acids involve chemical modification, synthesis of eutectic mixtures, and incorporation of various materials [122].
Thermal cycle tests are crucial for evaluating long-term performance. Figure 7 shows the traditional experimental setup to evaluate the stability of PCMs for determined cycle numbers. The system consists of one heating and one cooling circulating bath. The samples are immersed in these baths until the phase transition is completed, and then the samples are swapped [120].
Majo et al. [119] analyzed the degradation of capric and myristic acids using 1000 thermal cycles, finding a decrease in the latent enthalpy of fusion for capric acid, which was associated with chemical degradation. Yang et al. [118] exposed fatty acids to 10,000 thermal cycles, reporting that although the chemical structure remains unchanged, the phase transition temperature and latent heat decreased. These studies recommend the use and study of accelerated thermal cycle tests for fatty acids before their use in latent heat storage applications and to consider additional encapsulation methods to enhance stability. Building performance simulation tools, like Energy Plus, can predict the thermal behavior of PCMs in different climates [24].
Cellat et al. [123] investigated the thermal behavior of binary mixtures of capric, myristic, lauric, and palmitic acids in building applications by direct addition to concrete mixtures. Altering the fatty acids loading in the composites provided the ability to adjust the melting temperature to achieve human comfort ranges (22 °C to 27 °C). Thermal cycle tests indicated suitable durability at temperatures up to 120 °C, suggesting these PCMs could be used for solar energy storage in prefabricated building materials.
Kumar et al. [124] analyzed the effectiveness of gypsum, a commonly used building material, loaded with a composite PCM composed of lauric acid, graphite, and zeolite as a supporting material. The results showed good chemical and thermal stability after 1000 thermal cycles, with the integration of the novel material in building roofs and south-facing walls providing excellent thermo-regulating performance compared to raw gypsum.
Comparatively, the thermal and chemical stability of fatty acids has been found to be superior to that of paraffins [105,125,126], making fatty acids more suitable for building energy conservation purposes [92,112,113].

4.2. Thermal Conductivity

Thermal conductivity is a measure of a material’s ability to transfer heat. High thermal conductivity enables faster charging and discharging rates. Unfortunately, low thermal conductivity is a major drawback for many promising PCMs, making thermal conductivity enhancement a key priority to foster the use of fatty acids as PCMs in building materials [127].
Fatty acids have inherently low thermal conductivity compared to inorganic PCMs. To enhance their thermal conductivity, various strategies have been proposed, as summarized in Figure 8. These include the addition of various heat-conducting components, such as metal nanoparticles, expanded graphite, carbon nanotubes, and carbon nanofibers [32], and the encapsulation of the PCMs in conducting shells.
Dan et al. [31] synthesized a shape-stabilized PCM using stearic acid combined with Co3O4 and expanded graphite, achieving a thermal conductivity of 2.53 Wm−1K−1, which is 7.67 times higher than pure stearic acid. A nine-fold improvement in thermal conductivity has, likewise, been shown in lauric acid when using a variety of graphite-supporting materials, including graphite sheets and graphitic porous carbon [31,129,130,131].
Innovative fillers, including functionalized carbon-based nanomaterials and copper nanoparticles, have shown a potential to enhance thermal conductivity. Al-Ahmed et al. [132] investigated the effect of graft carbon nanotubes on the stability and thermal degradation of capric, palmitic, and stearic acids, finding that the grafting of carbon nanotubes increased latent heat, thermal conductivity, and stability. In the same way, Rezaie et al. [133] incorporated copper nanoparticles into a eutectic mixture of palmitic and lauric acid, resulting in a composite with good thermal conductivity, stability, and tensile strength.
Building materials like perlite and diatomite have been used to produce composites incorporating fatty acids to evaluate their performance and thermal properties. Wen et al. [134] prepared a form-stable composite PCM with a eutectic mixture of capric and lauric acids (67.6:32.4 mole ratio), expanded graphite, and diatomite using vacuum impregnation. Similarly, Zhang et al. [135] incorporated a nano Al2O3-modified binary mixture of capric and palmitic acid into expanded perlite to obtain a novel building material. The melting points reported for the composites were about room temperature (24 °C) with high latent heat (up to 98.6 kJ/kg). These results have shown that the addition of carbon and metal-based fillers significantly increased the thermal conductivity of the composites, making them promising for building and solar energy conservation systems.

4.3. Flammability

Fatty acids are organic materials with a long hydrocarbon chain attached to a carboxylic group, making them combustible when exposed to heat or an ignition source. When a flammable compound comes into contact with a heat source and reaches its ignition temperature, decomposition takes place, releasing gaseous products that can serve as fuels. In the presence of oxygen and an ignition source, these flammable gases can ignite, leading to the combustion of the material [136]. The flammability of fatty acids requires special attention for PCMs used in building applications where fire risk is a concern. Mclaggan et al. [137] investigated the flammability of a polymeric macro-encapsulated PCM composed of fatty acids within mat insulation and assessed the material’s fire risk, indicating a high heat release rate (1040 kWm−2). This information helps in understanding fire risks and designing effective fire barriers of fatty acids in building applications, through the selection of a maximum quantity of PCM for an acceptable behavior.
Flame-retardant materials can be used to mitigate flammability risks and improve the safety of fatty acid PCMs. Flame retardants reduce or prevent the ignition and spread of flames [138] by minimizing the concentration of flammable gases and the diffusion of oxygen (See Figure 9). These materials can be classified into halogenated and non-halogenated types. Current regulations in the United States and Europe limit the use of halogen compounds due to the release of toxic fumes and associated health risks [139].
Various tests are used to evaluate the flammability of PCMs through the quantitative and qualitative assessment of relevant parameters, such as heat release rate, auto-ignition temperature, flash point, and limiting oxygen index (LOI) [141]. The industry standards are vertical and horizontal flammability tests, which assess burning velocity, flame extinguishing time, and dripping behavior. The LOI is a quantitative measure of the minimum concentration of oxygen needed to support the combustion process in a controlled environment. Cone calorimetry measures heat release during combustion, including the total heat of combustion, heat release rate, and peak heat release rate. These values indicate the flames’ intensity and provide information that can be used to analyze flammability behavior in real-world systems [29,142].
Several studies have enhanced the flame retardancy of fatty acids by using non-halogenated compounds, providing a comprehensive understanding of the synergistic behavior in the final composites via the tests described above [141]. Palacios et al. [143] added 50% weight of magnesium hydroxide to capric acid:myristic acid (73.5%:26.5%) and capric acid:palmitic acid (75.2%:24.8%) eutectic mixtures, showing improved fire performance and self-extinguishing behavior. However, the addition of more than 40% of flame retardant decreased the thermal storage capacity of the PCM. Palacios et al. [30] also investigated commercial endothermic aluminum hydroxide, magnesium hydroxide, and an aluminum polyphosphate–melamine flame retardant in fatty acids of capric, myristic, and palmitic acids, finding improved flame retardancy using the dripping test with an increase of 10% to 26% in ignition times. However, the flame retardant addition affected melting temperature and enthalpy of fusion in a composition-dependent way. The authors also reported that the flame retardant requires a well-dispersion method in the PCM volume to achieve a proper effect.
A novel flame-retardant PCM composed of lauric acid and resorcinol bis (diphenyl phosphate) incorporated into expanded perlite via impregnation showed good thermal properties and reduced flammability [144]. Similarly, a nano-enhanced PCM made from stearic acid, graphite powder, and magnesium hydroxide nanoparticles showed improved flame retardancy, thermal conductivity, and phase transition temperature [145]. The addition of nanosized particles provided a superior dispersion in comparison to micro-sized particles, leading to a more compact char during the combustion.
The mixing of flame retardants into organic PCMs is the most common technique to reduce their flammability; however, the good dispersion of the material, especially for macro- and microparticles, is a key challenge to avoid the migration of the compounds for long-term applications [138]. In contrast to physically blending the PCM and the flame retardant, chemical transformations offer a promising and innovative approach to enhance the flame retardancy of organic compounds through the integration of flame-retardant moieties. Initial reports have shown that the bonding of halogen materials to alcohols and polyethylene glycol results in materials that can self-extinguish in air, indicating suitable flame-retardant properties and stability, avoiding issues of phase separation [29,146,147].
From the research described, it is evident that the integration of flame retardants into fatty acids bulk PCMs is an effective method to enhance the flame resistance of these materials. However, their incorporation can show certain limitations, such as the homogeneous distribution of the compounds and phase separation over time.

4.4. Life Cycle Assessment

Life cycle assessment (LCA) evaluates the environmental impacts of materials throughout their life cycle, considering factors like source production, manufacturing processes, and final disposal or waste treatment. Building technologies aim to reduce CO2 emissions by using environmentally friendly materials and processes to mitigate the negative effects related to climate change [148].
Ji et al. [149] conducted an LCA of capric and stearic acids (9:1 mass ratio) using the EnergyPlus (version 24.1.0) software [150] to simulate PCM’s whole-life performance in buildings. They studied the total energy saving, consumption, and carbon reduction and reported that, at an environment temperature above 24 °C, the potential annual energy savings were as high as 57.3 kWh/m2, with an associated reduction in carbon emissions of 23.3 kg/m2.
Frahat et al. [151] investigated the carbon emissions and energy savings of a novel building material composed of wood fiber (from 25% to 52% wt) and a eutectic mixture of stearic and capric acids (17:83 mass ratio). It was reported that higher fatty acid content in the insulation material increased energy savings (up to 21 kWh/m2) and reduced carbon emissions (up to 26.58 kg/m2).
These studies demonstrate that fatty acid-based PCMs incorporated into building materials provided a promising energy-saving mechanism compared to conventional materials. LCAs help identify opportunities for reducing environmental footprints and promoting more sustainable PCMs.

5. Techniques to Incorporate PCMs into Building Materials

To improve thermal storage capacity, PCMs can be integrated into construction materials through different methods. In 1993, Hawes et al. [152] reported the three most promising methods for the incorporation of PCMs into building materials, such as gypsum wallboard and concrete block:direct incorporation, encapsulation, and immersion. Shape stabilization is also a suitable method for long-term applications [128].
Studies have evaluated the compatibility of fatty acids with building materials. Rui et al. [153] studied a ternary fatty acid mixture of lauric, myristic, and palmitic acids for use as PCMs in cement concretes via direct impregnation treatment. Results showed that while fatty acids aggregate reduced temperature fluctuations, they also decreased the thermal conductivity and compressive strength of concrete. Recently, Alireza et al. [154] evaluated a vacuum impregnation method to incorporate lauric and myristic acid mixtures into diatomite, resulting in high compressive strength, significant thermal energy storage, and reduced energy consumption, suggesting its usability as a building material.
These studies demonstrate that the amount of PCMs incorporated and the technique used significantly influence the thermal and other physical properties of the final building composite. Therefore, these parameters are key when developing novel PCMs for building applications.

5.1. Direct Incorporation

Direct incorporation is the simplest technique and the most economical method. This technique involves mixing the PCMs directly with the building material during manufacturing. However, weak bonding between the compounds affects the mechanical properties of the building material and may cause leakage during the melting process [155].

5.2. Immersion

In this technique, manufactured building materials are immersed into the melted PCMs. Absorption of the PCMs in the porous structure of the material occurs due to capillary forces [156,157]. Figure 10 illustrates the laboratory experimental process to incorporate the melted PCMs into solid particles.
Li et al. [131] prepared a diatomite-based composite PCM via vacuum impregnation with a lauric-stearic acid mixture and expanded graphite to enhance thermal conductivity. The resulting material demonstrated suitable thermal properties and stability, indicating its promise for indoor thermal comfort systems. However, leakage, incompatibility, and corrosion are problems associated with long-term use of materials produced using this method [157]. An additional encapsulation process may be necessary to seal the material and address leakage [155].

5.3. Encapsulation

Encapsulation involves introducing the PCMs (core) into a supportive material that acts as a shell, as illustrated in Figure 11. This technique prevents leakage, increases heat transfer rates, and reduces compatibility issues related. Encapsulation can be done at macro-, micro-, and nanolevels based on the capsule size and properties of the materials being encapsulated [116].
Macro-encapsulation, the most commonly used technique, involves using containers larger than 1 mm, such as tubes, spheres, and panels [155]. Although the construction structure is impacted less, this encapsulation method yields materials with low thermal conductivity. On the other hand, micro-encapsulation packs the PCMs in polymeric particles with diameters ranging from 1 µm to 1000 µm, and nanoencapsulation consists of the use of capsule diameters smaller than 1 µm [155,157]. For instance, Konuklu et al. [160] developed a thermal energy storage material using palmitic acid as the core and poly(allyl methacrylate (AMA)) as the shell. The synthesized microcapsules were used in mortar-based composites, exhibiting good thermal performance and stability after 60 min of heating, making it a promising alternative for thermal management in buildings.

5.4. Shape Stabilization

PCMs can be dispersed into a supporting material, such as high-density polyethylene, to form a stable composite. The materials obtained using this method feature large apparent specific heat, suitable thermal conductivity, and shape retention throughout the phase change process, with long-term thermal stability [161].
Zhang et al. [130] synthesized a shape-stabilized PCM by incorporating stearic acid into a 3D ultrathin-wall graphitic hierarchical porous carbon. Results indicated improved shape stability after 600 thermal cycles and increased thermal conductivity. However, the materials also exhibited a lower melting temperature compared to the corresponding pure fatty acid. Similar results were obtained by Li et al. [31] for a shape-stabilized PCM synthesized using stearic acid and a porous zeolitic imidazolate framework, showing remarkable thermal properties, including a 7.7 times higher thermal conductivity and effective heat transfer rates.
Shape stabilization allows the incorporation of a higher amount of PCM compared to micro-encapsulation. However, studies on the use of fatty acids as PCMs in this context are limited.

6. Perspectives on Future Research Directions

Based on the previous discussion, several areas for future study have been identified. Future research should focus on developing novel strategies to enhance current limitations and address existing challenges and knowledge gaps.
The extraction of fatty acids from bio-sources presents several challenges, including the energy efficiency and environmental impacts of the extraction methods and the productivity associated with land-use change and cultivation practices. Progress in these areas is needed to ensure that high-quality fatty acids for various applications, including building materials, are sustainably sourced. For instance, in the future, studies can be carried out, focusing on the use of relevant and promising microorganisms, such as oleaginous yeasts, to produce high-quality free fatty acids and on their application in building materials.
As discussed in Section 4, the thermal properties of fatty acids are important parameters that affect their suitability for building energy storage and thermal comfort applications. Several authors have proposed alternatives to improve the thermal conductivity, flammability, and thermal/chemical stability of fatty acid-based PCMs. However, few studies have explored the use of additives to enhance these properties without negatively impacting others. For example, nanoparticles could be used to create composites that maintain heat storage capacity while improving thermal conductivity. Additionally, further research is needed to evaluate the mechanical and physicochemical properties of fatty acid-based PCMs incorporated into building materials, such as viscosity, density, and diffusivity.
Regarding the flammability of fatty acids, the homogeneous distribution of flame-retardant additives within the PCM is a key challenge, often leading to localized areas with lower flame retardancy. Over time, some composites may exhibit phase separation and components migration. Thus, it is important to investigate the effects of various flame-retardant materials on a range of fatty acid-based PCMs and to evaluate methods to prevent phase separation, such as chemical transformation.
Despite significant advancements in the synthesis and evaluation of fatty acid-based eutectic mixtures, gaps remain in developing mixtures involving fatty acids and other chemical families, such as alkanols, esters, and phenols. Their properties, stability, and cross-interactions have not been thoroughly investigated, leading to uncertainties about their performance in real-world building applications.
Further research into the composition, enhancing additives, long-term performance, and flame-retardancy enhancement of fatty acid-based PCMs will contribute to the development of efficient and durable TESs for buildings.

7. Conclusions

This paper provides a state-of-the-art review of recent studies evaluating fatty acid-based PCMs in various building materials and applications. The research summarized here demonstrates that fatty acid-based PCMs can be viable alternatives for energy conservation in the building sector.
From the papers analyzed in this work, it is important to highlight that there is substantial potential for evaluating and understanding additional aspects associated with the production of fatty acids to be used as PCMs and also with the improvement of their thermo-physical properties and their performance in building applications. The main recommendations for future research can be summarized as follows:
-
Obtaining pure fatty acids from more sustainable bio-sources remains a critical area of study, alongside LCAs aimed at assessing the sustainability of these novel technologies.
-
Incorporating novel additives has been shown to enhance the properties and performance of these composites. However, a deeper understanding of the synergistic effects on all relevant properties is still needed.
-
Reducing the flammability of fatty acid-based PCMs is especially important to mitigate fire risks in building construction materials. Additional research is crucial to fully harness the potential of fatty acid-based PCMs, advancing sustainable building practices and supporting global efforts in energy management. Future studies should focus on understanding the mechanisms of flame-retardancy compounds on pure and complex matrices of fatty acids, as well as on the evaluation of different incorporation techniques and their effect on the long-term performance of the novel PCMs.

Funding

Funding was provided by the Natural Sciences and Engineering Research Council of Canada (Project number: 10025417).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Percentage of energy consumption in the buildings sector globally. Modified from [1].
Figure 1. Percentage of energy consumption in the buildings sector globally. Modified from [1].
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Figure 2. Distribution of energy used in the Unitd States in 2018. Modified from [46].
Figure 2. Distribution of energy used in the Unitd States in 2018. Modified from [46].
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Figure 3. Principle of phase change cycles of PCMs. The pink shell is an encapsulating material, which prevents leakage during melting. Modified from [49].
Figure 3. Principle of phase change cycles of PCMs. The pink shell is an encapsulating material, which prevents leakage during melting. Modified from [49].
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Figure 4. Time trend of research developed in the last years involving the main fatty acids used for thermal energy storage applications.
Figure 4. Time trend of research developed in the last years involving the main fatty acids used for thermal energy storage applications.
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Figure 5. Enzymatic hydrolysis process to obtain fatty acids and glycerol. Modified from [91].
Figure 5. Enzymatic hydrolysis process to obtain fatty acids and glycerol. Modified from [91].
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Figure 6. Phase diagram of binary eutectic systems. (a) Typical diagram for a binary system. (b) Diagram developed for a Palmitic–Stearic acid system taken from [99,100].
Figure 6. Phase diagram of binary eutectic systems. (a) Typical diagram for a binary system. (b) Diagram developed for a Palmitic–Stearic acid system taken from [99,100].
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Figure 7. Thermal cycling test system used to evaluate the stability of PCMs.
Figure 7. Thermal cycling test system used to evaluate the stability of PCMs.
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Figure 8. Thermal conductivity enhancement methods for PCMs. Modified from [128].
Figure 8. Thermal conductivity enhancement methods for PCMs. Modified from [128].
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Figure 9. Flame-retardant mechanism of a novel flame retardant (POCODA) synthesized from polylactic acid (PLA) and ammonium polyphosphate (APP). Modified from [140].
Figure 9. Flame-retardant mechanism of a novel flame retardant (POCODA) synthesized from polylactic acid (PLA) and ammonium polyphosphate (APP). Modified from [140].
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Figure 10. Vacuum impregnation method to incorporate fatty acid-based PCMs into solid particles. Modified from [158].
Figure 10. Vacuum impregnation method to incorporate fatty acid-based PCMs into solid particles. Modified from [158].
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Figure 11. Working principle and structure of PCM encapsulation. Modified from [159].
Figure 11. Working principle and structure of PCM encapsulation. Modified from [159].
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Table 1. Summary of Recent Review Papers Focused on PCMs for Building Applications.
Table 1. Summary of Recent Review Papers Focused on PCMs for Building Applications.
YearType of PCMBuilding MaterialHighlightsReference
2024Inorganic, Organic, EutecticBrick wallsInvestigate how the type of PCM, its location, and quantity integrated into brick walls influence energy efficiency enhancement[23]
2024PCMs derived from wasteBuilding envelopesFocus on minimizing greenhouse gas emissions and analyzing the use of machine learning to predict the material’s behavior[33]
2024Nano-PCMs (nanoparticles integrated into PCMs)Building envelopes (walls, floors, ceilings)Provide information on the role of nano-PCMs in boosting energy efficiency and identifies the advantages regarding the thermal properties for the application on residential and industrial scales[34]
2023Bio-PCM (plant-based oils)Building-integrated PCMExamine the thermophysical properties of biobased PCMs directly using plant-based oils, building integration techniques, and lifetime impacts[35]
2023Inorganic, Organic, EutecticBuilding envelopesReview passive cooling benefits from PCM integrations in buildings in tropical climates. The study detailed information to simulate access to buildings enhanced with PCMs. [24]
2023Inorganic, Organic, EutecticBuilding materialsExplore the problems associated with the selection of PCMs and techniques to encapsulate them for heating and cooling applications[36]
2022Inorganic: Hydrated saltsBuilding materials, building envelopes, and air-conditioning systemsAnalysis of methods to enhance thermal properties of hydrated salts, as well as encapsulation techniques and their application in thermal energy storage systems[37]
2020Paraffin and EutecticGlazing unitsSummary of experimental and numerical research focused on PCMs incorporated into glazing units, as well as challenges and future works[38]
2019Macro-encapsulated PCMsBuilding envelopesReview of the available PCMs suitable for macro-encapsulation and their influence in building envelopes[39]
2018Inorganic and OrganicMortar based-materialsSummary of details regarding various PCM-mortar combinations, their benefits, drawbacks, and application in buildings[40]
Table 2. Benefits, Drawbacks, Average Melting Points, and Latent Heat of Fusion of Diverse Types of PCMs. Taken and modified from [23,53].
Table 2. Benefits, Drawbacks, Average Melting Points, and Latent Heat of Fusion of Diverse Types of PCMs. Taken and modified from [23,53].
PCMBenefitsDrawbacksMelting PointsLatent Heat of Fusion
Organic
-
Broad range of phase change temperatures
-
High latent heat of fusion
-
Compatible with construction materials
-
No/low supercooling behavior
-
Physically and chemically stable
-
Thermal stability
-
Low thermal conductivity
-
High flammability
-
High volumetric change
-
Costly
−12 to 187 °C130 to 260 kJ/kg
Inorganic
-
High thermal storage capacity
-
Good thermal conductivity
-
Low volumetric expansion
-
Available at low cost
-
Non-flammable
-
Low vapor pressure
-
Segregation and supercooling
-
Corrosive
-
Incongruent melting.
-
Considerable volumetric change
11 to 120 °C25 to 200 kJ/kg
Eutectics
-
Sharp melting temperatures
-
High storage capacity
-
No phase segregation
-
Limited thermophysical properties data
-
High cost
4 to 93 °C100 to 230 kJ/kg
Table 3. Thermal Properties of Most Common Pure Fatty Acids Used as PCMs in Building Applications.
Table 3. Thermal Properties of Most Common Pure Fatty Acids Used as PCMs in Building Applications.
Fatty AcidMelting Temperature (°C)Melting Latent Heat (J/g)Freezing Temperature (°C)Freezing Latent Heat (J/g)Reference
Capric acid32.14156.4032.53154.24[73]
30.92163.3727.69167.95[74]
30.48169.17--[75]
Palmitic acid59.40218.5358.23216.46[73]
59.66209.3558.90212.48[76]
61.71206.6859.48204.25[77]
Stearic acid68.86252.7268.91254.12[78]
72.09200.964.65194.42[79]
Lauric acid43.93178.1140.63178.98[80]
42.8172.1941.2170.26[81]
Myristic acid54.28191.2751.69194.36[80]
53.6199.451.8199.0[82]
Table 4. Recent Developments of Organic-Organic Eutectic PCMs in Building Applications.
Table 4. Recent Developments of Organic-Organic Eutectic PCMs in Building Applications.
YearEutectic MixtureHighlightsReference
2024Capric acid—Stearic acid
-
Expanded perlite was used as supporting material with 50% weight of the PCM.
-
The melting temperature and latent heat were reported as follows: 24.66 °C and 77.90 J/g.
-
The novel eutectic PCM was utilized to obtain a based cementitious mortar. The results showed an improvement in thermal properties and a good compatibility of the materials.
-
Further works focused on the mechanical properties require attention.
[107]
2024Stearic acid—Palmitic acid
-
Assessment of the impact of aging on binders containing an organic eutectic PCM
-
Incorporating the PCM improves the resistance, stability, fatigue performance, and cracking behavior of asphalt.
-
A decrease of 5% in the melting enthalpy after 60 h was observed
[108]
2023Lauric acid—1 hexadecanol
-
Addition of powdered seashell
-
Effect of particle size and mixing ratio of thermal enhancer in final properties
-
Comparison of thermal conductivity with PCM containing TiO2
-
Evaluation of melting temperature, enthalpy, and thermal stability
[109]
2023Palmitic acid—Lauryl alcohol
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Transition temperature and latent heat were 20.25 °C and 155.59 J/g, respectively.
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Thermophysical study included corrosion test, thermogravimetric test, thermal conductivity, and thermal stability.
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Evaluation of stainless steel and aluminum to be used with the PCM developed
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Implementation of the PCM into vapor-absorption refrigeration chamber
[110]
2022Capric acid—Myristic acid
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Nanostructures were incorporated into the eutectic PCM (silver-antimony tin oxide and silver-graphene nanoplatelets).
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A comparison of the thermal properties of pure and modified PCMs was carried out, focusing on transition temperatures and stability.
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Improvement of thermal conductivity (18.2%) was reported.
[111]
2021Capric acid—Stearic acid
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Silica fume was impregnated into the eutectic PCM to prevent the leakage of the material.
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The developed PCM was integrated with cement mortar in three weight fractions: 10%, 15%, and 20%.
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The melting point, latent heat capacity, stability, mechanical properties, and thermoregulation were evaluated, reporting suitable properties for the regulation of indoor temperatures in buildings.
[112]
2018Methyl palmitate—Lauric acid
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The stable eutectic mixture was modified with nanoscale additives (nano-graphene platelets).
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Evaluation of specific heat, thermal diffusivity, thermal conductivity, and latent enthalpy
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The study provides useful data for developing simulation tools.
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Promising energy storage material for wallboards, ceilings, floors, and building structures. Compatibility requires further investigations.
[113]
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Herrera, P.; De la Hoz Siegler, H.; Clarke, M. Fatty Acids as Phase Change Materials for Building Applications: Drawbacks and Future Developments. Energies 2024, 17, 4880. https://doi.org/10.3390/en17194880

AMA Style

Herrera P, De la Hoz Siegler H, Clarke M. Fatty Acids as Phase Change Materials for Building Applications: Drawbacks and Future Developments. Energies. 2024; 17(19):4880. https://doi.org/10.3390/en17194880

Chicago/Turabian Style

Herrera, Paola, Hector De la Hoz Siegler, and Matthew Clarke. 2024. "Fatty Acids as Phase Change Materials for Building Applications: Drawbacks and Future Developments" Energies 17, no. 19: 4880. https://doi.org/10.3390/en17194880

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