3.1 Waste Utilization

The requisite characteristics of a PCM for utilization in a structural context are elucidated in Chapter 1. In the event that these properties are not satisfied, a number of issues may arise within the structure. Such issues may manifest as insufficient thermal and chemical stability, elevated supercooling temperatures, reduced thermal conductivity, leakage problems, and low viscosity. The utilization of various waste materials serves to prevent such problems.

A review of the literature reveals that a considerable number of waste materials derived from biological, agricultural, industrial and construction sources are employed in the manufacture of thermal energy storage building materials. The principal reasons for the preference of these wastes are their low environmental impact, accessibility, cost advantages and biodegradability. However, it is also observed that wastes that are not biodegradable (such as tire rubber) or that may contain harmful components (such as electrical arc furnace dust) are also employed in the production of PCM (Figure 2).

In selecting waste materials for use in PCM, it is essential to consider their porosity and water absorbency, as these properties can prevent potential leaks that may occur during the phase change of the material. In addition to enhancing thermal performance, some wastes are also employed to improve acoustic performance, augment structural strength and regulate viscosity. For instance, the high porosity and absorbency characteristics of waste materials permit their effective utilization in form stabilization applications. Furthermore, waste cooking oils have been identified as a viable thermal energy storage agent in direct applications.

The utilization of waste materials in the construction industry plays a pivotal role in advancing environmental sustainability. Furthermore, the incorporation of waste into the circular economy and the production of energy-efficient materials derived from it also influences the embodied and operational energy consumption of buildings. It has been demonstrated that the incorporation of appropriately selected waste materials can enhance not only the energy performance but also the overall strength, fluidity, water absorbency and acoustic properties of materials.

In this context, waste materials and PCM are employed in the production of building materials designed to enhance energy efficiency and thermal comfort in buildings. These building materials are typically in the form of composites, panels, concrete, and mortar (Table 1).

3.2 Laboratory Analysis and Experiments

The tests and analyses employed during the production of building materials are typically conducted within a laboratory setting. The objective of these studies is to evaluate the chemical, thermal and physical characteristics of materials, as well as to ascertain their structural strength. Given that PCM-waste-based building materials are predominantly composite, it is of paramount importance that the components of these materials function in unison. The principal analytical techniques employed in the various studies, together with their intended applications, are set out below.

X-ray diffraction (XRD) is a technique used to analyze the crystal structure of a material. It generates pattern graphs that illustrate the chemical interactions and crystal structure of the components that constitute the material's structure. This analysis is employed for the identification of crystalline phases within the microstructure of the building material [4].

Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) are further analytical techniques that can be employed in this context. SEM provides microscopic images that can be used to determine the pore structure, homogeneity and physical compatibility of the components. The EDS method is employed to ascertain the chemical distribution of elements on the surface of the material, with this analysis conducted on the basis of the aforementioned images [39].

Fourier transform infrared spectroscopy (FTIR) is a technique used to analyze the molecular structure of materials. This technique is employed to examine the chemical constitution of composite materials and the compatibility of chemical bonds between their constituent components. This method is particularly well-suited to the analysis of the chemical structure of PCM-based materials. This method is typically employed to assess the chemical stability of a material subjected to prolonged thermal cycling [43].

DSC (differential scanning calorimetry) is employed to ascertain the thermal properties and thermal strength of materials. This analysis provides insight into the phase change temperatures of materials and their thermal behavior, including melting and freezing. The analysis is typically repeated after more than 100 thermal cycles to ensure the reliability of the results [41].

Thermogravimetric analysis (TGA) is a technique used to determine the thermal stability of materials. The technique is employed to quantify the loss of material and assess thermal stability as a function of temperature. This analysis examines the mass loss and degradation behavior of the material over a given temperature range, thereby providing information on its thermal stability [38].

Infrared thermography (IRT) is a technique that employs thermal imaging to analyze the infrared radiation emitted by objects. This technique is employed to ascertain the heat storage performance of materials. The distribution of temperatures across the sample surfaces is illustrated through the use of infrared thermal imaging. This method enables the analysis of heat transfer properties through the visual presentation of temperature differences on the surface of the material [46].

These analytical techniques are fundamental to ascertain the compatibility of the components employed in the manufacture of building materials and their thermal, chemical and physical properties. The repetition of such analyzes, particularly in the context of PCM-waste-based materials, is beneficial for the evaluation of the long-term performance and durability of the materials (Figure 3).

It can be observed that experimental studies are conducted within a laboratory setting in the majority of cases. Nevertheless, experimental setups designed to assess the performance of the materials under actual temperature conditions were incorporated into only half of the studies examined. The two distinct methodological approaches, namely theoretical and practical, yield results encompassing a multitude of parameters, including thermal, physical, chemical, acoustic, mechanical, and environmental properties of the materials under investigation (Table 2).

4.1 Energy Efficiency

The utilization of building materials that are capable of storing energy and facilitating its subsequent utilization represents an effective strategy for the reduction of non-renewable energy resource consumption. A review of the numerical data reveals that a single gram of the building materials produced within the scope of the studies examined can store a range of energy between 10 and 193 J during a phase change. The number of thermal cycles tested ranged from 100 to 1200. It is possible that chemical interactions between PCM and waste materials may have an adverse effect on the thermal performance of the material, even if the interaction is weak. It is therefore crucial to ensure that the selected waste materials do not induce chemical incompatibility or instability during the thermal cycles of PCM [4]. In related studies, it has been demonstrated that the energy storage capacity of building materials exhibits minimal reduction during thermal cycling periods. Based on a phase change rate of 300 times per year [47], these findings indicate that waste-based PCM may have a functional lifespan of approximately 4 years.

The phase change temperatures of PCM are situated in proximity to indoor comfort ranges, thereby performing an efficacious function in ensuring thermal comfort. However, it is also necessary to consider the climatic characteristics when selecting or adjusting the phase change temperature. A review of the literature reveals that all of the PCM types under examination are organic in nature. It has been documented that the thermal conductivity of alcohol derivatives is less than that of fatty acid derivatives, whereas paraffin exhibits a thermal conductivity value that falls between these two categories.

The low thermal conductivity of PCM impedes effective heat transfer, thereby limiting the material's potential for heat storage [48]. The addition of certain waste materials has been shown to significantly enhance the thermal conductivity of the resulting material. For instance, the thermal conductivity of a material can be enhanced by incorporating apricot kernel shell or tire rubber. As illustrated in Table 3, the thermal conductivity of the material can be increased by up to 2.61 and 2.3 times, respectively.

The utilization of waste materials enables the storage of energy without the loss of material that would otherwise occur during a change of state. The composite building material produced by impregnation method using bio-based waste materials, including cuttlebone and pomelo peel, demonstrated a 70% increase in thermal energy storage capacity compared to the reference case, where only PCM obtained from oil was utilized [4]. Similarly, it was demonstrated that the composite material obtained from coconut peat by vacuum impregnation has the potential to be employed as a wall panel that provides passive cooling in buildings, due to its energy storage capacity of over 100 J/g [37]. The heat storage capacity of another composite material produced by vacuum impregnation of alcohol into rubber was found to be 181 J/g, and a 2.3-fold increase in the thermal conductivity of the material was observed as a result of the waste material used [43].

The contribution of waste to the reduction of energy consumption is as significant in the production of building materials as it is in the operational phase of the building. Kumar et al. [44] asserted that the recycled expanded glass panels they produced in their study exhibited reduced embodied energy and carbon compared to the gypsum board panels currently available on the market. Furthermore, the energy consumption of active and passive houses during their respective use phases was calculated through the utilization of the EnergyPlus program for building energy simulation. In the study in which the life cycle energy and carbon emissions were calculated together, it was determined that the panel produced provided a 4.5% reduction in total energy and a 12% reduction in total carbon emissions in comparison to the reference. The aforementioned studies demonstrate that waste has yielded significant outcomes in the domain of thermal energy storage.

4.2 Effective Water Use

The efficient utilization of water in buildings is of paramount importance in order to avert the looming threat of water scarcity, which is one of the most significant challenges of our time. In this study, the water efficiency of the wastes incorporated into the structure of PCM was evaluated through an assessment of their water retention properties. The majority of the waste materials employed in the studies were observed to have the capacity to trap water due to their porous structures. As a result of these characteristics, they are capable of providing impermeability to the material and preventing the necessity for water supplementation by ensuring the continuous utilization of water.

The use of shape stabilizers is frequently employed to avert leakage issues that may arise when PCM transitions to the liquid phase. The primary characteristics of the stabilizers employed include high porosity, recyclability, innocuousness, and economic viability [4]. It is feasible to employ a range of waste materials as shape stabilizers in the production of composite PCM. The aforementioned materials include hemp-shive, coconut peat, cuttlebone, and pomelo peel, which are biologically based wastes. Furthermore, fly ash and concrete powder, which are industrial by-products, can also be employed to impart sealing properties to the material.

Despite undergoing a drying process, coconut peat's hydrophilic structure enables it to resist evaporation, thereby allowing its incorporation into the PCM structure as a shape stabilizer [37]. Cuttlefish are renowned for their capacity to regulate the volume of water within their bones, enabling them to maintain a vertical position in aqueous environments. The capacity of this bone structure to retain water endows the PCM with a leak-proof quality, which can benefit up to 100 thermal cycles [4].

Conversely, the processing of waste materials entails the consumption of energy and water. Baldassarri et al. [40] posit that improvements can be made to the water, electricity, and natural gas consumption during the waste oil treatment process in order to facilitate the incorporation of waste oil into building materials and utilize it as thermal energy storage. Moreover, the utilization of waste materials in PCM production is of significant importance in ensuring the fluidity of materials processed by 3D printing. In a study where fly ash was incorporated into microencapsulated paraffin wax, it was observed that thermal stability and the desired viscosity were achieved over 100 thermal cycles in the production of miniature 3D-printed houses [41] (Table 4).

4.3 Raw Material Conservation

PCM optimizes the utilization of materials in the construction industry, thereby contributing to sustainability. The utilization of waste and recycled materials in the production of PCM serves to reduce the consumption of natural resources. The findings of studies conducted by Sarı et al. [43] and Baldassarri et al. [40] indicate that PCM produced from recycled materials exhibits enhanced energy storage capabilities while necessitating a reduced reliance on new materials. This constitutes a notable contribution to the conservation of natural resources. As materials with energy storage capacity, PCM reduces the necessity for insulation of buildings, thereby reducing the quantity of material utilized in construction projects. In a similar vein, Gencel et al. [38] posit that hemp-shive and PCM-based building materials can provide the same thermal performance with less material than conventional building materials. This contributes to environmental sustainability while reducing costs. The incorporation of PCM into building materials enhances their durability, thereby reducing the necessity for renovation. Kumar et al. [44] asserted that building materials incorporating PCM demonstrate prolonged performance, thereby reducing the necessity for the periodic replacement of building components. Consequently, the utilization of materials is also made more efficient.

In all of the studies reviewed, the utilization of waste material as a substitute for raw materials serves to reduce the necessity for raw materials and to guarantee the continuity of resources. Nevertheless, in certain studies, the characteristics of waste materials that enhance thermal performance also diminish the necessity for supplementary insulation. To illustrate, the utilization of carbonized waste rubber derived from tires to enhance thermal conductivity and serve as a PCM support represents a valuable contribution to the circular economy [43].

4.4 Environmental Impacts and User Health

A significant proportion of waste is generated by sectors such as agriculture, construction, and industry. In the construction sector alone, millions of tons of waste are generated worldwide every year as a consequence of the utilization of building materials [49]. The term “embodied energy” is used to describe the energy consumed during the production, use, and lifetime of building materials. This energy can be 20 to 50 times the annual operational energy consumption of a building [50]. It is therefore of great importance to consider the energy consumption and greenhouse gas emissions associated with the materials to be used in the construction of the building.

The utilization of waste materials can yield both beneficial and detrimental consequences for the natural environment. The incorporation of waste into the system can facilitate resource continuity, reduce the necessity for raw materials, and mitigate the burden on landfills. The negative effects of waste include the potential presence of harmful compounds, lack of biodegradability, and the release of volatile organic compounds into the environment. These factors, which also impact indoor air quality, are addressed in section 4.6 on comfort parameters. This section will examine the behaviors that fall within the scope of environmental impact classes as defined in international standards.

Kumar et al. [44] found that using waste can reduce lifecycle energy by 4.5%. Therefore, energy-related carbon emissions decreased 12%. In another study employing vegetable oils derived from waste cooking oil as phase-change materials (PCMs), wall panels were subjected to energy simulation in 50 distinct countries and four different climate zones [40]. In the study in which the environmental impacts of PCM-containing panels designed as single and double layers were tested, it was observed that the utilization of waste oils had a mitigating effect on the negative impacts, particularly in regard to poisoning, non-cancer health threats, eutrophication, clean water and land use. The study highlighted the potential for glycerin derived from waste oils to serve as a substitute for paraffin, a material currently prevalent in the market. It is hypothesized that the elevated levels of toxicity and non-cancer health risks associated with oil derivatives in the market are attributable to the nitrogen and phosphate fertilizers employed in the cultivation of the seeds from which the oil is derived. Furthermore, other activities, such as soil cultivation and irrigation, can result in eutrophication and significantly impact the availability of clean water and land use (Figure 4). In this regard, the selection of waste materials can be regarded as an environmentally benign decision.

Moreover, the utilization of waste as building materials within the context of circular economy principles confers environmental benefits, including a reduction in resource consumption and the circumvention of raw material processing. The recycling of waste materials and their utilization as building materials serves to reduce energy consumption and contribute to the conservation of natural resources and the advancement of environmental sustainability. Conversely, the disposal of waste materials in the natural environment can result in environmental contamination. Given their low economic value, coconut shells are discarded in considerable quantities on an annual basis, which can result in adverse environmental consequences. However, its cellulosic structure, biodegradability, ease of accessibility, and low cost make it a potentially viable bio-based PCM supporter [37].

No health analysis was conducted in the selected studies. However, Barreneche et al. [42] conducted a leaching test to find out if industrial waste used as wall material contained any hazardous substances.

4.5 Economic Perspective

One of the principal criteria for the utilization of PCM in construction projects is its economic viability. In the event that solutions designed to enhance the energy performance of buildings have a payback period that exceeds the lifespan of the building in question, the aforementioned solutions will effectively become devoid of any meaningful purpose. It is therefore of great importance to develop cost-effective solutions. Inorganic PCM types are typically less expensive than organic and eutectic types. However, the application method of PCM can also have a significant impact on the total cost [28]. For instance, while encapsulation techniques can assist in preventing leakage, they are typically not the preferred option due to the high initial costs involved [42, 44].

Consequently, more cost-effective techniques have been devised that can prevent leakage without the utilization of capsules in PCM production. One such method is the form-stabilization technique, whereby the PCM is impregnated into a porous matrix. An alternative approach is the shape-fixing method, whereby the PCM is melted and mixed with a supporting matrix [51]. Nevertheless, these techniques may result in a reduction in the heat transfer performance of the composite materials produced. Accordingly, the incorporation of fillers with high thermal conductivity may prove an effective means of enhancing heat transfer. Due to their low cost, these fillers are typically selected from waste materials [43].

Waste materials can be employed not only as filler material but also, on occasion, as PCM itself. For example, Baldassarri et al. [40] produced cement-based composite panels using waste cooking oil and employed this waste oil as a PCM. Such applications result in reduced material costs and contribute to environmental sustainability. Furthermore, the utilization of recyclable waste materials in PCM production renders these materials a more economically viable option. In conclusion, in order to achieve cost-effectiveness in PCM production, the utilization of waste materials, non-encapsulated fixation methods, and low-cost filler materials represents a significant contribution to the overall cost-effectiveness of the process.

4.6 Comfort Parameters

In the context of sustainable architecture, social sustainability is concerned with enhancing the quality of human life. Consequently, it is closely associated with human necessities pertaining to health and well-being, as well as cultural and psychological aspects [52]. The provision of thermal, acoustic, visual comfort and indoor air quality enables the design of comfortable buildings. A deficiency in thermal comfort can impede physical and mental work, whereas poor indoor air quality can endanger human health by precipitating a range of illnesses.

The quality of the air within buildings is of great importance. The utilization of waste materials in the production of building materials has the potential to negatively impact indoor air quality. It is therefore essential to ascertain whether the waste in question contains any harmful substances, and if so, to what extent, and whether such substances are released into the indoor or outdoor environment. In accordance with the European Directive, the heavy metal content of waste is categorized as inert, harmless, or harmful. In light of the aforementioned considerations, the present study was designed to assess the ability of heavy metals to be filtered through the use of a leaching test. A leaching test is a chemical analysis method that is used to detect the presence of hazardous substances in waste. In the study in which electrical arc furnace dust (EAFD), an industrial waste, was employed in the production of PCM, it was asserted that this material is hazardous and should be deposited in landfills with caution [42]. In order for such hazardous waste materials to be employed in construction projects, a number of remediation techniques can be proposed. One such method is encapsulation. Nevertheless, this approach represents a provisional solution, given its high cost and lack of specificity in addressing the issue of heavy metal removal [42]. While fixation and solidification techniques are relatively inexpensive, the elevated pH value of the leachate can potentially give rise to certain issues. It is therefore imperative that professionals from a range of disciplines, including environmental and materials engineers, collaborate to develop remediation methods that are tailored to the specific characteristics of the waste in question. Despite the utilization of waste in all of the reviewed studies, no supplementary studies were identified.

The concept of thermal comfort is defined as the condition of mind that expresses satisfaction with the thermal environment. The test cabinets produced in the studies allow for the determination of the temperature values and time delays reached in comparison to the reference situation. In a study conducted by Gencel et al. [38], two 20 × 20 × 20 cm modules were designed using hemp shive and lauryl alcohol (LA). The modules were equipped with skylights and examined under real atmospheric conditions to analyze the temperature behavior of the wall panels produced. One of the modules serves as a reference cube, devoid of PCM but containing solely hemp shive and encased in an outer layer of 5 cm-thick expanded polystyrene (EPS). The second comprises a cabin with 45% PCM (LA) and hemp shive. The experiment {3} was conducted over a four-day period in August in Bartın, Turkey. The temperature data obtained from the temperature meters placed on the inner surface of the prepared cabin revealed a temperature difference of 6°C between the two cabinets [38].

The configuration of {6}, comprising miniature test chambers produced via 3D printing, was evaluated within a temperature range of 15°C–30°C, reflecting typical diurnal fluctuations. The temperature transitions were conducted over a 12-h period. Among the tested cabinets, the GPP-0% model does not contain macroencapsulated PCM, whereas the GPP-20% model contains 20% PCM. The temperature differential between the two chambers was 1.2°C [41].

In a separate study {7}, the temperature differential between two cabinets, one equipped with PCM and the other without, tested at a temperature differential of 18°C–28°C was found to be 3°C [42]. In the experimental setup {9} consisting of a wood panel and insulation material, tested in a simulated environment in Australia, it was found that extreme temperatures were reduced by 12.4% and carbon emissions from cooling could be reduced by 14% thanks to the concrete panel with PCM [44]. Finally, in {10}, which was constructed with material comprising MDF, EPS, and recycled concrete powder, a 5°C decrease in the highest daily temperatures and a 3.6°C increase in the lowest temperatures were observed in the PCM-containing cabinet in comparison to the reference cabinet without thermal storage [45]. The manufactured experimental cabinets are illustrated in Figure 5.

The acoustic comfort of the environment is a key consideration in the design of any acoustic space. In their 2016 study, Barreneche and colleagues observed that the industrial waste (EAFD) utilized in their investigation provided 4 dB of acoustic insulation in the PCM-containing cabin when compared to the reference cabin. The high density of heavy metal oxides present in the material has an effect on the improvement of the acoustic performance of the waste material.