Encapsulated Phase Change Material Slurries as Working Fluid in Novel Photovoltaic Thermal Liquid Systems: A Comprehensive Review

Abstract

Today, energy generation from renewable energy sources is of great interest. Photovoltaic (PV) systems, in this regard, have much to offer, but they suffer from low efficiency, which further deteriorates due to overheating under insolation. So, they need removal of heat from their bodies for better efficiency, which resulted in the introduction of PV-Thermal (PVT) systems, which feature heat transfer fluids (HTF) to draw the heat and deliver it to other systems that make use of it. Nevertheless, the best HTF has yet to be developed. Water-based fluids with additives or nanoparticles seemed like a good choice until HTFs that featured the use of encapsulated phase change materials (ePCM) were proposed. The findings of early studies and subsequent research revealed that the use of ePCM slurries (ePCM-Ss) as the working fluid in PVT systems increased the thermal efficiency, electrical efficiency, and overall efficiency without a notable increase in pumping power. However, preparation of ePCM-Ss is much more complex in many aspects compared to conventional HTFs, as it involves numerous parameters, including but not limited to the use of various shell and core materials, the variety of production methods, the homogeneity of the resulting capsules, the use of additives, the core to shell ratio, and the mass fraction of ePCM in the slurry. All these require an extensive and exhaustive study with quite a lot of background knowledge and interdisciplinary collaboration, as the proper selection of PCM materials and synthesis methods, as well as the correct concentration in the best CF, involve several aspects and expertise in a number of other fields. These parameters also significantly diversify and differentiate ePCM-S by affecting its suspension stability, rheological properties, and thermal properties. In recent years, PCMs have become an attractive research field for researchers due to their advantages. Although there are quite a number of studies addressing ePCM-S, none provide a holistic approach, and they just deal with a certain aspect of this broad topic. This study, therefore, aims to constitute a fundamental guide to refer to from the very beginning to the final implementation of the ePCM-Ss as the working fluid in PVT systems by addressing all steps, aspects, and almost all effective parameters in terms of advantages, disadvantages, challenges, and opportunities.

1 Introduction

Today, due to the rapid depletion of fossil-based energy sources (Al-Waeli et al. 2017b; Kara 2020; Nguyen et al. 2021; Öner et al. 2016) and the fact that the existing reserves are insufficient to meet (Deymi-Dashtebayaz et al. 2022) the rapidly increasing demand for energy (Chen et al. 2014a, b) for a long time, as well as the negative environmental effects of fossil fuels (Abdullah et al. 2018; Chen and Fang 2011); research aimed at the development of new technologies regarding the use of renewable energy resources and on improving the efficiency of existing applications have been gaining importance (El Chaar et al. 2011; Ghodbane et al. 2022). This need came into focus further after recent developments, such as the COVID-19 pandemic and its consequent impacts on the global energy system (Hoang et al. 2021b) and the growing international calls to cut down on GHG emissions, on which renewable energy sources have much to offer (Jäger-Waldau et al. 2020). A wide variety of research has been done on all renewable energy sources, and great progress has been made (Fu et al. 2021). Undoubtedly, solar energy is the most prominent (Adun et al. 2022) in terms of both total potential and accessibility (Allouche 2016; Awad et al. 2022), as well as in terms of diversity of applications apart from energy generation, such as drying, dehumidification, desalination, distillation etc. (Hoang et al. 2021a).

Solar energy has the potential to meet a significant portion of the world's energy needs (Al-Waeli et al. 2019a); and it is projected to play a crucial role in meeting the world’s electric supply by the year 2030 (Fu et al. 2021), especially as incorporated into smart energy systems in the smart cities of the near future (Hoang et al. 2021c). Besides, solar systems.

• Offer a quiet working regime (Messenger and Ventre 2003)

• Do not produce any unwanted wastes, pollutants, or emissions (Chandel and Agarwal 2017a)

• Offers high performance and reliability (Al-Waeli et al. 2019a)

• Have a satisfactory economic life of 20–30 years (Ibrahim et al. 2011)

• Have low operating and maintenance costs (Agyekum et al. 2021b; Al-Waeli et al. 2017b; Öner et al. 2016).

Nevertheless, solar energy, which exists with great potential in vast geographies worldwide (Awad et al. 2022), has some considerable handicaps that need to be overcome, such as the need for innovative absorber designs (Abdullah et al. 2018) due to non-uniform cooling requirements, a long payback time (Al-Waeli et al. 2019a), high initial investment and installation costs (Cartmell et al. 2004), limitation on the installation area due to lacking a proper shape and structure to integrate into existing buildings or other structures, need for large areas for discrete installations (Al-Waeli et al. 2019a; Öner et al. 2016).

Basically, there are two types of solar systems: solar cells (photovoltaic systems, PV) and solar panels (solar water heaters or collectors) (Fudholi et al. 2013; Said et al. 2022; Zondag 2008). PV systems convert solar radiation into electricity and are more industrial in terms of their application areas and utilization. However, they are under strong influence of a number of environmental and system parameters, which are mainly referred to as “sustainability parameters.” Not all incident solar irradiation can be converted into electricity, and the greater part accumulates as heat (Brahim and Jemni 2017) in the panel body, which in turn causes the electrical efficiency of the system to decline (Agyekum et al. 2021a; Chen et al. 2017). The photovoltaic thermal (PVT) combined systems have been put forth as capable of solving this problem, i.e., the accumulation of heat in the panel body and low efficiency brought on thereby (Chandel and Agarwal 2017a) by the use of heat transfer fluids (HTFs) to maintain cell temperature at or close to optimum operating temperature.

As such, a typical PVT system is made up of solar cells combined with a solar thermal system that uses various HTFs. PVTs have a very wide application area (Daghigh et al. 2011) and have diversified according to fluid type, cover, and absorber design (Gelis et al. 2022), as shown in Fig. 1. With the simultaneous cooling of the PV system by the thermal system, the increase in the electrical resistance caused by increased temperature can be prevented to a great extent, and the maximum efficiency value of the PV cells at the optimum operating temperature can be maintained.

Fig. 1

Classification of PVT Systems based on (Jia et al. 2019)

A typical PVT-air system, shown in Fig. 2, offers an electrical efficiency of 8% and around 40% thermal efficiency (Solanki et al. 2009) and they are low-cost and easy to integrate (Tonui and Tripanagnostopoulos 2007). However, air has a low heat capacity and has limited use as a warm/hot medium.

Fig. 2

A typical PVT air system (Choi and Choi 2022)

On the other hand, while PVT-water systems were shown to have provided an approximately 15% increase in the electrical efficiency (Gelis et al. 2022) compared to conventional PV, the quest for developing new and high-performance HTFs has been gaining momentum (Sardarabadi et al. 2017). The main idea has always been reclaiming the waste heat and preventing efficiency drops in exchange for pumping as little power as possible, as well as spending less on the HTF and relevant system components. In this regard, the new HTF should have a high specific heat capacity, be inexpensive and readily available, and require no great pumping power. As a result of numerous studies conducted so far, water-based special purpose fluids have stood out as reasonably good alternatives.

Thermal energy storage, as shown in Fig. 3, can be by chemical or thermal means, which have several alternative applications that can be used to implement active or passive cooling (Browne et al. 2015b) or an effective combination thereof (Agyekum et al. 2021a) in PVT systems.

Fig. 3

Thermal Energy Storage Systems (Sarbu and Sebarchievici 2018; Sharma et al. 2009)

As such, the heat storage density of an HTF can be increased by incorporating phase change materials (PCM), thereby achieving a better heat transfer rate as well as lower flow rates for a certain heat transfer rate (Alva et al. 2017; Salunkhe and Shembekar 2012). On the other hand, PCM suspended in HTF, which is called PCM slurry (PCM-S), was reported to store or carry a greater amount of thermal energy compared to PCM particles that are not in direct contact with HTF (Salunkhe and Shembekar 2012). Although PCM suspended in HTF increases the viscosity of the fluid, hence the required pumping power, such an effect remains within acceptable limits (Alva et al. 2017; Al-Waeli et al. 2019b; Cao et al. 2019; Trivedi and Parameshwaran 2020). Nonetheless, PCM-Ss have a few issues that must be addressed. PCM particles can solidify in the channels of the heat exchangers and cause clogging. Also, the stability of the PCM slurry is not very good above the melting temperature of the PCM, and the small PCM droplets may coalesce with each other over time, eventually becoming completely dissociated layers with the CF. In order to overcome such issues, PCM was dispersed as small droplets and encapsulated. No such incorporation problems occur when slurry preparation is carried out using ePCM instead of PCM, thanks to the shell material preventing contact between PCM and HTF (Alva et al. 2017). ePCM slurries (ePCM-Ss) offer all the benefits of a CF-PCM mixture, except for marginal effects induced by the shell material.

In short, ePCM-Ss are HTFs that are multiphase in at least one region of the cycle and are prepared to take advantage of the latent heat of phase change materials without any problems, thus making them more efficient HTFs than single-phase fluids (Cao et al. 2019). It is generally formed by dispersing micro- or nano-sized capsules, consisting of a polymeric shell with a paraffin PCM in the core, in CFs such as water or glycerol, resulting in a nanofluid. Over the last decade, there have been approximately 150 thousand studies on renewable energy, about ten percent of which involved the use of nanofluids in renewable energy systems (Sharma et al. 2022b). ePCM-S nanofluids were also developed to improve the heat transfer rate of the CF and have found several applications, such as heating, cooling, air conditioning, heat exchangers, and ventilation (Barreneche et al. 2014; Said et al. 2022). Their utilization in PVT systems, however, is relatively new, and little research has been conducted to date (Jia et al. 2020).

2 PCM Slurries as Heat Transfer Fluid in Thermal Systems

2.1 Advantages of Using PCM Materials

PCMs help store energy as latent and sensible heat and defer thermal equilibrium due to the fact that a certain amount of energy (fusion energy or latent heat of fusion) is transferred at a constant temperature (Sharma et al. 2009), offer higher energy storage density (Yu et al. 2021; Zhang et al. 2010) and help maintain a relatively stable operating temperature (Chen et al. 2015).

As seen in Fig. 4, the amount of heat stored within the unit mass/volume of a medium is notably higher in the case of latent heat storage (the case that a phase change takes place within that ΔT temperature range), compared to sensible heat storage. Such an increase in the amount of energy stored per unit volume can be as high as 5–14 folds (Sharma et al. 2009; Yu et al. 2018). Further, some properties, such as the thermal conductivity and storage capacity, could be improved with the addition of nanoparticles to the PCM (Tarish and Alwan 2017).

Fig. 4

Comparison of total heat storage between sensible and latent storage at a given ΔT interval (Agyekum et al. 2021b; Browne et al. 2015a, b; Reyna 2018)

Moving from solid towards gas, the amount of phase-change latent heat increases. In this respect, solid–liquid PCMs have latent heat higher than solid–solid PCMs (Alehosseini and Jafari 2019) but lower than solid–gas and liquid–gas PCMs. Despite having higher latent heats, nevertheless, large volume change and pressure change greatly limit the applicability of solid–gas and liquid–gas PCMs (Su et al. 2015), which makes the solid–liquid PCMs more favorable.

While latent heat storage systems (LHSS), which make use of PCM, involve different types of applications, there is a common algorithm to follow in order to develop a proper and fit-for-purpose LHSS (Reyna 2018), which can be seen in Fig. 5.

Fig. 5

The algorithm for the development of LHSS (Abhat 1983; Reyna 2018; Rousse et al. 2009; Sharma et al. 2009)

In a thermal system, such as a PVT system, the thermal energy may not be converted into useful work instantly and often needs to be stored and/or transferred. If the energy is stored in the form of sensible heat, a medium with a high specific heat capacity has to be used, and it must be non-corrosive and non-toxic, and isolation/insulation is required during storage. However, owing to their ability to exchange heat at constant temperature and store energy in the form of latent heat, PCM materials offer higher energy storage density (Farid et al. 2004), and they provide important advantages, particularly in systems where an optimum working temperature must be maintained (Konuklu 2008; Sharma et al. 2015).

PCMs are available with a wide range of fusion energy at melting temperatures ranging from  − 5 to 150 °C (Kenisarin and Mahkamov 2007). When compared to sensible heat storage, the use of PCMs can increase the amount of energy stored per unit volume by 5–14 folds (Sharma et al. 2009). In order for PCMs to serve their purpose, they have to possess some thermodynamic, kinetic, physical, and chemical properties (Biçer 2009), e.g.:

• They should have the highest possible latent heat of fusion, as this determines the maximum amount of heat that can be stored per unit volume.

• As the solid-liquid phase change takes place at a constant temperature, the phase change temperature must be within the optimum operating temperature range of the system to be used.

• The volumetric change between the phases should be very small, and the vapor pressure should be low as it will be contained in a closed system.

• The thermal conductivity and density should be high, and the phase changes should be stable over time.

• In terms of kinetics, the rate of nucleation and the rate of crystal growth must be large.

• They must be non-toxic, have long economic life, are not harmful to health, are inexpensive and easily accessible (Akçay 2006; Garg et al. 1985; Hale et al. 1971; Pasupathy et al. 2008).

2.2 Incorporation of PCM

In taking advantage of PCM, one has to choose from different viable methods: direct incorporation, immersion in porous materials and shape stabilization, macro-encapsulation, and micro-encapsulation (Serale 2018; Zhou et al. 2012).

However, the intended use of PCM is determinative on the selection of incorporation methods (Fig. 6) that can function effectively (Yu et al. 2021). The fact that PCMs are not always suitable for use in circulation circuits makes the use of CF inevitable. As such, a new approach to the utilization of PCM was proposed in the last 10–15 years. A PCM-S is typically a solution that has PCM dispersed within a CF.

Fig. 6

Methods for incorporating PCM into CF (Serale 2018; Zhou et al. 2012)

Despite numerous alternatives, water is preferred as the CF for several advantages, such as having high thermal conductivity and considerable specific heat capacity (Cao et al. 2018), being compatible with PCMs, being easy to use, cheap, and safe (Jurkowska and Szczygieł 2016).

PCM-Ss, as outlined in the literature, can be prepared in different types, which can be seen in Fig. 7.

Fig. 7

Different types of PCM slurries based on (Delgado et al. 2012b; Qiu et al. 2019; Serale 2018)

Ice slurries are typical PCM-Ss that are naturally encountered on earth, and consist of ice particles stratified or floating in water. In ice slurries, the CF and PCM are of the same substance and become a pure substance in the liquid phase of the PCM content. The PCM-emulsions, on the other hand, are a mixture of a PCM and a CF, homogenized through the incorporation of an emulsifying agent and remain a slurry even in the liquid phase of the PCM. Encapsulation, where the PCM is wrapped in a shell material (Chandel and Agarwal 2017b) to avoid contact with other PCM particles or undergo other stabilization issues, and shape-stabilization, where solidified polymers are used as supporting materials (Melone et al. 2012) that absorb liquid PCM (Serale 2018) to prevent leakage (Qiu et al. 2019; Wu et al. 2020), are relatively new methods developed to prevent some drawbacks of micro-emulsions and offer great versatility in the application of PCMs in HTFs that circulate in more complex circuits. In encapsulation approach, encapsulation efficiency, i.e., the ratio of PCM to shell material, and mechanical strength is central whereas chemical compatibility and thermal stability is crucial in shape stabilization (Umair et al. 2019). Shape-stabilized PCM-Ss.

The relationship between the effective viscosity of the fluid and the thermal dilatation of the PCM with micro- or nano-capsules is one of the parameters to be investigated (Dutil et al. 2011). And despite the fact that the motion of the solid and liquid boundary layer and the mixing of the two phases are not well known (Prakash et al. 1985), numerical studies have shown that PCM can exhibit a single-phase feature in microcapsules and can simplify the solution of this problem with a reasonable error. As a result, regardless of the phase changes, the PCM-Ss act as a single phase fluid and have constant hydrodynamic properties at the macro level. This also allows the PCM-Ss to be pumped and be used as a HTF in thermal circuits, offering some advantages over similar solutions, including but not limited to latent heat exploitation, higher thermal diffusivity, reduced mass flow rate needs, and a high heat transfer rate.

Nevertheless, the PCM-Ss have some issues that need to be overcome in order to realize their full potential and benefit from their advantages. During a phase change heat transfer, the fact that the heat transfer cannot be determined precisely due to the non-linearity of the process, the volumetric change, and not knowing the exact heat transfer mechanism are the main obstacles to a full evaluation of the performance of a thermal system with phase change (Regin et al. 2008).

Encapsulation of PCMs provides performance improvement by both increasing the heat transfer surface area (surface area to volume ratio) (Farid et al. 2004) and reducing the PCM reactivity, as well as eliminating some other problems caused by volume change and low thermal conductivity (Jegadheeswaran and Pohekar 2009).

2.3 Encapsulation of PCMs

Encapsulation can roughly be defined as coating a core material (solid, liquid, gas, or even multiphase particles) with a shell (a film layer, generally of polymeric materials) (Chen et al. 2014b). The process can be named differently according to the size of the resulting capsules: macro-encapsulation (or simply encapsulation), micro-encapsulation, and nano-encapsulation. The first encapsulation process is believed to be studied by the National Cash Register Company within the framework of their project to produce carbonless copy paper (Benita 2005).

The shell of the capsules prevents the core material from interacting with the environment, which increases the stability of the material and prevents undesired exposure to the core or its interaction with the environment. The encapsulation process has long been executed and implemented primarily in drug-related or medicinal applications in pharmaceutical, chemical, and biological engineering. Over the following years, however, the technology has become widespread and found application in a variety of fields, including but not limited to thermal, mechanical, and structural engineering.

Encapsulation improves the thermal and mechanical properties of PCMs (Alva et al. 2017), increases the heat transfer surface area (Chandel and Agarwal 2017b, 586), thereby increasing the surface-to-volume ratio, and hence enhances the thermal capacity and efficiency of the CF significantly (Aslan 2014). In addition, the capsules allow the materials to be used as solid particles in their liquid state and compensate for the volume change in the shell during the phase change.

In recent years, a number of studies on the preparation and properties of suspensions prepared by mixing microcapsules having a PCM shell with certain CFs (also known as ePCM Slurry, ePCM-S) have been conducted and significant results have already been recorded (Chen and Fang 2011, 4625).

2.3.1 Material Selection

The shell and core should not interact chemically (Karellas et al. 2018). Therefore, the material selection should be made taking into consideration the intended use and based on the material of the other. This process must be carried out meticulously, and special attention should be paid to the properties of materials in order to ensure that both the shell and core materials can withstand the operating conditions (Yeşilyurt et al. 2019). In addition, it must be taken into account that a number of products harmful to the environment and human health are produced as a result of encapsulation methods (Bayés-García et al. 2010, 1235).

While the shell material functions as a container and is mostly regarded as important in terms of the mechanical strength of the capsule, it also has an important effect on the thermal performance as the heat transfer between the core PCM and the CF takes place through the shell. Therefore, it is also important that the shell material have good thermal properties, which will also bring on longer thermal life cycle (Salunkhe and Shembekar 2012).

The main general properties sought in the shell material and the importance of such properties can be summarized as shown in Table 1.

Table 1 Properties sought in a good shell material (based on Salunkhe and Shembekar (2012))

Among several organic and inorganic materials that have properties suitable for use as shell materials, polymers are the most commonly used. Polystyrene, polymethylmethacrylate, Arabic gum, gelatin, amino plastics, arabic gelatin-gum, urea formaldehyde resin, melamine formaldehyde resin, gelatin formaldehyde resin, and the like are also selected as shell materials (Su et al. 2017).

The core material is also very important and should be carefully selected by considering a large number of parameters among organic, inorganic, or eutectic solid–liquid phase change materials. The breakdown of solid–liquid PCMs and their respective melting enthalpies and melting temperatures can be seen in Figs. 8 and 9, respectively.

Fig. 8

Classification of PCMs (Bruno et al. 2015; Fong 2020; Karellas et al. 2018; Sharma et al. 2015; Su et al. 2015; Yeşilyurt et al. 2019)

Fig. 9

Melting temperatures and enthalpies of common solid–liquid PCM (Biçer 2009; Bruno et al. 2015; Ghasemi et al. 2022; Konuklu 2008; Mehling and Cabeza 2007; Yeşilyurt et al. 2019; Zondag et al. 2016)

Organic materials have good chemical and thermal stability (Bruno et al. 2015). Paraffins, which are suitable and practical organic materials, are often preferred for the production of ePCM in the solid–liquid phase change. Especially for PV system cooling, paraffins are more applicable because of their thermal stability with regards to cycling (Atkin and Farid 2015). Inorganic PCMs, which can be classified into metals, salts, and salt hydrates, offer high thermal conductivity and high energy storage density, but they are not preferred as core materials as they undergo high subcooling and phase separation, corrosion, and decomposition (Faraj 2021) during the phase change from liquid to solid (Chen and Fang 2011) and they lack thermal stability (Alehosseini and Jafari 2019). As a combination of organic and inorganic materials, eutectics enable the use of the superior properties of both material types.

When the core material of a capsule is considered; in thermodynamic terms; it is desirable that the latent heat of melting required for the unit volume to melt be as high as possible since it also makes the amount of heat that can be stored in the unit volume maximum (Yu et al. 2018). In addition, since the solid–liquid phase change takes place at a constant temperature, this value should be equal to or very close to the optimum operating temperature of the PV module used in the system. Another property to seek is that the volumetric change between phases be very small and the vapor pressure be low, as the whole process takes place in a closed system, i.e., the capsule. It is also preferred that the thermal conductivity and density are high and the phase changes are stable and congruent over time. From a kinetic point of view, the nucleation rate and crystal growth rate should be large. It should not be toxic, and its economic life should be long (Akçay 2006; Al-Mamoori 2017; Garg et al. 1985; Hale et al. 1971; Pasupathy et al. 2008). Just like the shell materials, some basic characteristics that the core materials should have can be listed as given in Table 2 (Yeşilyurt et al. 2019).

Table 2 Thermal, kinetic, and chemical properties sought in a core PCM (Alehosseini and Jafari 2019; Ali 2017; Aslan 2014; Biçer 2009; Browne et al. 2016; Chandel and Agarwal 2017b; Sarı 2011)

In parallel to the increasing interest in the utilization of LHSS, the proper selection of PCM for a certain application is ensured by the use of a database including several properties of these materials and software, taking into consideration different constraints (Barreneche et al. 2015b). Nevertheless, when a manual selection of PCM is to be done, the following table, which briefly summarizes the main advantages and disadvantages of organic, inorganic, and eutectic PCM, may help.

2.3.2 Encapsulation Methods

Encapsulation of materials can be accomplished as a combination of core and shell materials with different morphological structures by using different encapsulation methods. Types include single-core, multi-core, matrix, and multi-layered (Jurkowska and Szczygieł 2016).

Although there are quite a few encapsulation methods (Nadaroğlu et al. 2022), each has pros and cons that determine whether they are successful or unsuccessful in providing the desired properties for any given application. In general, encapsulation methods are classified into two groups according to how the capsulation mechanism takes place: chemical and physical. Figure 10 shows the classification of physical and chemical methods.

Fig. 10

Classification of Encapsulation Methods (Chandel and Agarwal 2017b; Chen and Fang 2011; Jamekhorshid et al. 2014; Liu et al. 2015; Salunkhe and Shembekar 2012; Su et al. 2015)

PCMs are encapsulated in a shell to prevent leakage of the PCMs as well as increase the thermal conductivity.

Figure 11 shows different types of capsules as mononuclear, polynuclear, and matrix in terms of core/shell composition, as single-layer or multilayer in terms of coating lamination of shell structures, and as regular and irregular in terms of the shape of the shell (Ghasemi et al. 2022; Jamekhorshid et al. 2014; Jurkowska and Szczygieł 2016). According to some researchers, however, the matrix composition of core and shell materials does not constitute a capsule but rather a sphere (Karthikeyan and Ramachandran 2014). While the morphology of the capsules, in different terms, may vary depending on several factors, such as the core material, shell material, encapsulation process, repetition coating process, as well as other process parameters such as stirring rate, surfactant type, type of emulsion, temperature of the reaction medium, etc., the most common morphological structure of encapsulated PCM is the mononuclear type (Ghosh 2010; Jamekhorshid et al. 2014; Mishra 2015). The structure of a typical mononuclear core/shell ePCM and the phase change within the capsule shell by absorbing / releasing heat is shown in Fig. 12.

Fig. 11

Morphology of different types of capsules (Jurkowska and Szczygieł 2016)

Fig. 12

The phase change of the core material during heat exchange (Ghasemi et al. 2022)

In order to obtain the capsule by coating the core material with the polymer shell material, the first step is to prepare an emulsion so that these two immiscible materials are dispersed into each other in such a way that the desired capsule size is assured.

Depending on the method of encapsulation, in addition to shell and core materials, an emulsifier, an initiator, a cross-linking agent, a nucleating agent, and a surfactant may also be used. Furthermore, other auxiliary materials such as NaOH, hydrochloric acid, triethanolamine, and acetic acid solutions may be needed as PH stabilizers in methods that include a polymerization process (Alva et al. 2017).

2.4 Issues with PCM/ePCM Slurries

Encapsulation serves as a means to overcome some common problems of PCM, such as sub-cooling, a low nucleation rate, and a low crystal growth rate, which is mainly dependent on droplet size after nucleation. With the opportunity to produce smaller-sized capsules in parallel with the developments in the technology, both in terms of devices and methods, such problems PCMs have can be eliminated partially. Studies have shown that the capsule size directly affects the crystallization temperature and that the subcooling temperature varies inversely proportional to the capsule size in the range of 5–100 μm (Safari et al. 2017). With respect to solving this problem, Cao and Yang (2014) developed a new technique to suppress subcooling in ePCMs by optimizing the composition of the capsule shell.

By virtue of the composite structure that comes out in the form of a shell and core after encapsulation and due to the fact that the phase change during heat exchange takes place within the shell, both problems such as precipitation and separation that may occur in the case of direct mixtures of PCMs and CF as well as the risk of leaking of the core can be prevented by taking advantage of different properties of shell materials. The capsule structure, shell, and core material to be preferred in heat transfer applications depend on the amount of heat to be drawn from the system per unit time, the stable operating temperature of the system, and some other system parameters. However, one of the most important points is that the phase change temperature of the core material to be selected should be equal to or close to the optimum operating temperature of the system. As important is that the shell material should not be damaged, broken, or torn during pumping (Yeşilyurt et al. 2019).

2.4.1 Sub-Cooling

Subcooling, a phenomenon that can be described as the ability of a liquid to cool down to a temperature lower than the fusion temperature without crystallization, is one of the main features of importance for PCMs. It represents the difference between crystallization and fusion temperatures. Since the use of latent heat of fusion, which is the main purpose of using PCMs, can be delayed or hindered because of subcooling, the degree of subcooling of PCMs should be as low as possible. Therefore, it is necessary to reduce this degree in a PCM that exhibits a high degree of overcooling. The issue can be solved by adding a nucleating agent to the solution (Chen et al. 2014b).

2.4.2 Stability

The most important factor to be considered in the synthesis and use of ePCM should be stability. Although the nature and type of physical properties required in an ePCM may vary based on the application areas, ePCMs for all applications should be able to maintain the properties they possess or have acquired under the operating conditions in order to perform the expected function in the same way. For example, ePCM slurries to be used as heat transfer fluid must remain stable both under mechanical and thermal loads and for long periods of time.

Stability is an indicator that can be evaluated based on whether any changes occur in the properties of the ePCM or ePCM slurry, such as particle size or shape, thermophysical properties, and viscosity of the slurry, under operating conditions or over time (Chen et al. 2014b).

Stability can be regarded in three aspects: physical stability (mechanical stability), structural stability, and thermal stability (Qiu et al. 2017).

2.4.2.1 Physical Stability

The physical stability of ePCM slurries, or of emulsions, in general, is very important for heat transfer and thermal energy storage (Qiu et al. 2019). The physical stability of an emulsion is directly related to certain emulsion parameters, such as the surfactants’ mass concentration, surfactants’ type, pH value, and density difference. While some of these parameters may be adjusted or regulated, some must be considered in the material selection stage.

For example, the suspension pH value can be adjusted by adding citric acid and triethanolamine, which are traditional pH value regulators (Qiu et al. 2018). But as for the density difference between the particles and CF, which is one of the dominant parameters influencing the physical stability, considerations should be made in advance.

As seen in Fig. 13, the physical stability issues, i.e., physical instability, roughly refer to five major physical stability issues: stratification (creaming or sedimentation), flocculation, coalescence, Ostwald ripening, or phase inversion (García-Pérez et al. 2019; Jurkowska and Szczygieł 2016; Kuroiwa et al. 2015; Qiu et al. 2017).

Fig. 13

Instabilities likely to be encountered in emulsions (Jurkowska and Szczygieł 2016; Kuroiwa et al. 2015; Qiu et al. 2017; Tadros 2004)

An ePCM-S with stability problems being used in a thermal cycle will surely underperform or require stirring to restore physical stability or to ensure homogeneous dispersion during operation in order to maintain normal performance (Alvarado et al. 2007; Wang et al. 2007; Yamagishi et al. 1999).

Creaming or Sedimentation These two phenomena take place when the densities of the difference between ePCM and the CF, i.e., the densities of the dispersed material and the continuous phase, are different. Therefore, the smaller the density difference, the more stable the suspension (Qiu et al. 2018). When the density of the ePCM is greater than that of the CF, e.g., as in an oil-in-water emulsion, creaming may occur, whereas sedimentation is likely to occur when the density of the CF is lower than that of the ePCM, as in an water-in-oil emulsion (Qiu et al. 2019).

Flocculation The flocculation phenomenon is described as the agglomeration of particles (Delgado et al. 2012b) within the solution due to affinity (Qiu et al. 2019). With smaller diameters, especially smaller than 10 µm, capsules are more robust, but in this case the solution may be prone to flocculation, which can be prevented by adding an anionic surfactant (Ali 2017). Flocculation is a dispersion problem and indicates a nonhomogeneous slurry (Yamagishi et al. 1999). When the volume concentration of the capsules gets higher, they come closer to each other; hence, the interaction between capsules also increases, resulting in the formation of larger agglomerates. In reverse, these agglomerates shear down to smaller pieces under shear forces. Flocculation increases the viscosity and shear stress (Cao et al. 2019).

Coalescence This type of instability involves the merging of two or more dispersed droplets into a new bigger droplet during contact.

Ostwald Ripening The Ostwald ripening is defined as the inhomogeneity that takes place over time in solid or liquid phases due to the solubility difference of the dispersed phase (Ghasemi et al. 2022), showing up as small crystals or solution particles dissolving, followed by redeposition into larger crystals or solid particles (Qiu et al. 2018).

Phase Inversion As the name suggests, it takes place when the continuous phase and dispersed phase convert to one another (Delgado et al. 2012b; Qiu et al. 2019).

Except for creaming, suppression of other instabilities can mostly be achieved by the shell protection provided by encapsulation. By taking advantage of different properties of the shell material, problems such as sedimentation and decomposition, which may occur in the mixture, can be eliminated while also preventing the penetration of the core and mixing with the CF. To prevent creaming, the use of a certain surfactant or a mixture thereof provides good emulsion stability, as seen in Fig. 14.

Fig. 14

a Creaming problem on ePCM slurries and b prevention with the use of surfactants (Al-Shannaq et al. 2015; Jurkowska and Szczygieł 2016)

PCM-Ss provide high thermal inertia conservation and higher thermal diffusivity while also reducing subcooling and phase segregation. Still, some problems such as leakage (Barreneche et al. 2015a) and precipitation are encountered, which limits their use in more advanced applications. At this point, the incorporation method comes in handy for the formation of PCM slurries, where a separation between the PCM and CF is also required (Serale 2018). One way to solve this problem is to obtain some kind of composite material by wrapping PCMs with different shell materials (Chandel and Agarwal 2017b: 586). Therefore, PCMs are widely used in microcapsules to prevent leakage and maintain the effectiveness of thermo-physical properties when applied in slurry active systems (Barreneche et al. 2014).

2.4.2.2 Structural Stability

Another issue with the ePCM-S is its structural stability, which relates to the potential rupture or breakage of capsules by a mechanical shear force induced by the pumping stress (Qiu et al. 2017). In this regard, the selection of a circulation pump is also a factor to consider for the structural stability of capsules, and centrifugal pumps are best for the purpose since they can pump a slurry for a long period of time without imposing any damage to the capsules (Cao et al. 2019; Gschwander et al. 2005). Structural stability is mainly dependent on the shell material, thickness, and capsule diameter. Capsules with smaller diameters up to 5 μm are strong enough to withstand 5000 pumping cycles without rupture (Yamagishi et al. 1996). Alvarado et al. (2007) confirmed this effect with several experiments on different ePCM particles. However, while capsules with smaller sizes can withstand the flow pressure, the smaller the capsules within the CF, the greater the viscosity of the slurry will get (Ali 2017).

Cracking of the capsule shell or its rupture can also be caused by volumetric expansion of the core PCM during phase change. Therefore, in order to ensure the structural stability of the capsule, PCM selection can be made paying due attention to the volume change between phases (e.g., select an inorganic PCM that features low volume change as outlined in Table 3). Yet, as it is nearly impossible to achieve all desired properties with any of the PCM candidates, this may not always be the best solution. Kim and Cho (2002), for instance, mixed volatile cyclohexane with the PCM and thus encapsulated the mixture with a polymer shell; then, during the phase change, cyclohexane evaporated and allowed the shell to remain unaffected, leaving room for the volume expansion of the PCM. On the other hand, any modification of parameters that leads to a smaller and more uniform capsule size and uniform shell thickness can be considered favorable for the structural stability. In experiments with n-eicosan and stearic acid microcapsules having shell wall thicknesses of 15% and 30% of total capsule size, it was reported that thin-walled microcapsules cannot withstand thermal cycles above the melting point (Roy and Sengupta 1991). Thick-shell capsules, on the other hand, were found to be less damaged during pumping, but thicker shells hampered heat transfer from the shell to the core. Therefore, an optimal choice should be made between these two parameters (Cao et al. 2019).

Table 3 Advantages and disadvantages of PCM Types (Islam et al. 2016; Karthikeyan and Ramachandran 2014)

The strength of particles can be evaluated by using an atomic force microscope (Ghasemi et al. 2022) or by experimentation. To conclude, the type of pump used in circulation, pump speed, capsule diameter, volume-to-weight ratio, low volume expansion of the core or larger room for expansion in the shell, shell material, and shell thickness are among the main effective parameters (Qiu et al. 2017).

2.4.2.3 Thermal Stability

The thermal stability is directly related to the thermal stability of the PCM and the shell material when encapsulated (Gong et al. 2009). Paraffin wax is the most readily available PCM type in the market with high thermal stability (Abdelrazik et al. 2020). The thermal stability of any PCM of choice, on the other hand, can be evaluated and assessed. Thermal Gravimetric Analysis (TGA), which is basically a measurement of the weight change (gains and losses) as a function of temperature or time, provides information about the material’s thermal stability and compositional analysis through the determination of loss on drying and phase transition temperatures (Ahuja and Scypinski 2001; Alkan et al. 2009; Allouche 2016). However, in general, a simultaneous SEM analysis and/or Differential Scanning Calorimetry (DSC) analysis are carried out to visualize and identify the origin of the PCM degradation (Allouche 2016; Barreneche et al. 2015a). Fei et al. (2015) and Fu et al. (2017) have experimentally examined the effect of encapsulation on thermal stability and confirmed a significant improvement. The protection provided by the shell material (Karthikeyan et al. 2014) was reported to differ depending on not only the material itself but also the structure of the shell. Huang et al. (2019) reported that the thermal stability of the network polyurethane shell was remarkably enhanced compared to linear polyurea shell. In this regard, the modification of shell composition has been a practice.

Thermal stability of PCM microcapsules is crucial for practical applications (Al Shannaq and Farid 2015). Therefore, numerous studies have been aimed at the examination and improvement of thermal stability. Salunkhe and Shembekar (2012) reviewed the effects of capsule size, shell thickness, shell material, and encapsulation geometry on the performance of thermal energy storage systems and reported that heat storage capacity and thermal stability strongly depend on the core-to-coating mass ratio. PCM stabilization can also be achieved through the development of 3D-structured supporting matrices that can also increase latent energy storage capacity of composites, and nanomaterials can be used to fabricate organic PCM composites with increased thermal stability (Alehosseini and Jafari 2019).

Another factor that improves thermal stability is the availability of expansion space in the microcapsule, which allows PCM to expand freely without exerting stresses on the shell when the temperature rises. For some encapsulation methods, process parameters, such as stirring rate and emulsifying content were also reported to be effective on thermal stability (Al Shannaq and Farid 2015).

3 Hydrodynamic and Thermal Characteristics of ePCM Slurries

3.1 Density and Viscosity

The ePCM is a very fine granular powder (Fig. 15a). It can be added to a CF, such as water, which has been determined to be the most ideal CF and is the most used liquid for ePCM slurries due to its easy availability, cheapness, high thermal conductivity, and high specific heat capacity (Cao et al. 2019), at different weight or volume ratios. The resulting mixture is called ePCM slurry (ePCM-S) (Fig. 15b).

Fig. 15

ePCM (Qiu et al. 2017) and ePCM slurry (Qiu et al. 2016)

The fluid and flow properties of the ePCM-S are governed mainly by the densities of the core PCM and the shell material, the diameter of the capsule, and the concentration of capsules in the CF. These parameters affect the density and viscosity of the slurry. The higher the viscosity of the fluid, the greater the power required to circulate the fluid through the system. Therefore, the fluid’s viscosity is preferred to be as low as possible. The density of the ePCM slurry is then calculated as follows:

$${\rho }_{slurry}={\rho }_{carrier fluid}*\left(1-\phi \right)+{\rho }_{capsule}*\phi$$

(1)

where $\phi$ is the volume concentration of ePCM in the CF.

As for the viscosity of the slurry, assuming that the suspension is hydrodynamically homogeneous, a theoretical formula proposed by Einstein to predict the viscosity of dilute suspensions of hard shell spheres can be applied:

$${\mu }_{slurry}={\mu }_{carrier fluid}(1+k\phi )$$

(2)

where k takes the value 2.5 for rigid spherical particles based on rigidity and Brownian motion (Vand 1945). However, when ePCM particles move relative to each other under the shearing motion of fluid, they collide and roll over each other, the duration of which is directly proportional to the concentration of ePCM particles in the CF.

Considering the fact that the viscosity of the slurry is greater when the ePCM particles are in contact with each other than when they aren’t, the equation to reflect the true viscosity can be expressed as follows:

$${\mu }_{slurry}={\mu }_{carried fluid}{\left(1-\phi -{q\phi }^{2}\right)}^{-k}$$

(3)

with k = 2.5 and q = 1.16, the viscosity values obtained from the above equation gives very consistent results with the experimental data obtained with Ostwald viscometers.

The relative viscosity of the ePCM slurry is almost negligible for concentrations below 5%. In order to test the accuracy of Vand’s formulation, Wang et al. (2007) conducted experiments and reported an acceptable agreement (Fig. 16).

Fig. 16

Change of relative viscosity with capsule concentration (Wang et al. 2007)

Vand (1945) correlation was also used by Goel et al. (1994) and Qiu et al. (2019) to calculate solution viscosities and was reported to be valid. Furthermore, Qiu et al. (2019) reported that the constant q with a value of 3.7 fits best for commercial ePCM, testing it in their study addressing the use of ePCM-S in solar systems.

$$\frac{{\mu }_{\mathrm{sl}}}{{\mu }_{fl}}={\left(1-\phi -\mathrm{3,7}{\phi }^{2}\right)}^{-\mathrm{2,5}}$$

(4)

Thomas (1965) made a comparison of the values given by the Vand formulation with experimental results and reported that the degree of agreement is 97.5%, which gradually decreases to 60% for a concentration of 40% and to an unacceptable degree of 8.7% when the concentration reaches 60%. As a result, a better-fit version of the formulation was proposed by Thomas (1965) in order to maintain high agreement rates even for greater concentrations:

$${\mu }_{eff} = {\mu }_{fl}+ 2.5\phi + 10.05{\phi }^{2} + 0.00273\mathrm{exp}\left(16.6\phi \right)$$

(5)

But the Vand formulation still remains valid as ePCM slurries reported in the literature is generally below 25–30%.

3.2 Specific Heat Capacity and Thermal Conductivity

Thermal properties, such as the specific heat capacity (Cp) and thermal conductivity (k), of ePCM-Ss, as is the case for viscosity or other hydrodynamic and physical properties, depend on the thermal properties of the CF and ePCM. Thanks to the precious studies conducted so far, correlations and formulations have been provided in the literature that allow us to predict/design, or even calculate such properties.

As a function of temperature, the specific heat capacity of an ePCM changes with the temperature, but this change is substantial depending on whether the temperature is in the phase change temperature range of the PCM or not. As can be seen in Fig. 17, specific heat capacity of the capsule (C_p,capsule) is considered equal to the specific heat capacity of the core PCM (C_p,PCM) when the temperature is outside the phase change temperature range, and when within the phase change temperature range, it can be calculated as given as follows (Kuravi 2009):

Fig. 17

Change of ePCM's specific heat capacity with temperature (assuming equal Cp values of PCM in solid and liquid state) (Kuravi 2009)

$${C}_{p,capsule}={C}_{p,PCM}+\left\{\frac{\pi }{2}\left(\frac{{h}_{s,fluid}}{{T}_{m,e}-{T}_{m,o}}-{C}_{p,PCM}\right)sin\pi \left[\frac{T-{T}_{m,o}}{{T}_{m,e}-{T}_{m,o}}\right]\right\}$$

(6)

Further, irrespective of the phase change but considering the composite structure of the capsule as core and shell, the C_p of the capsule can be calculated as follows (Goel et al. 1994; Qiu et al. 2019):

$${C}_{p,capsule}=\frac{\left(R\times {C}_{\mathrm{p},\mathrm{PCM}}+(1-\mathrm{R})\times {C}_{p,shell}\right){\rho }_{\mathrm{PCM}}{\rho }_{shell}}{\left(\mathrm{R}\times {\rho }_{shell}+(1-\mathrm{R})\times {\rho }_{\mathrm{PCM}}\right){\rho }_{capsule}}$$

(7)

or as (Guo et al. 2017):

$${C}_{p,capsule}=R*{C}_{p,PCM}+\left(1-R\right)*{C}_{p,shell}$$

(8)

where R refers to the encapsulation ratio, i.e., how much by weight of the capsule consists of core PCM and how much of it consists of the shell material.

Based on the above, the bulk specific heat capacity of the slurry (C_p,slurry) can be calculated as follows (Guo et al. 2017; Languri et al. 2013):

$${C}_{p,slurry}=\xi *{\mathrm{C}}_{\mathrm{p},\mathrm{capsule}}+\left(1-\xi \right)*{C}_{p,fluid}$$

(9)

where ξ is the mass fraction of the capsule in the fluid.

With a similar approach, the thermal conductivity of the ePCM-S can be calculated using Maxwell’s relation as follows:

$$\frac{{k}_{slurry}}{{k}_{fluid}}=\frac{2+\frac{{k}_{capsule}}{{k}_{fluid}}+2\phi \left(\frac{{k}_{capsule}}{{k}_{fluil}}-1\right)}{2+\frac{{k}_{capsule}}{{k}_{fluid}}-\phi \left(\frac{{k}_{capsule}}{{k}_{fluid}}-1\right)}$$

(10)

Again, the thermal conductivity of the capsule can be obtained using (Guo et al. 2017; Qiu et al. 2016):

$$\frac{1}{{k}_{cap}{d}_{cap}}=\frac{1}{{k}_{\mathrm{c}}{d}_{\mathrm{c}}}+\frac{{d}_{cap}-{d}_{\mathrm{c}}}{{k}_{sh}{d}_{cap}{d}_{\mathrm{c}}}$$

(11)

Equation 2 can be re-arranged as follows (Languri et al. 2013):

$${k}_{slurry}={k}_{fluid}\left[2+\frac{{k}_{capsule}}{{k}_{fluid}}+\frac{2\phi \left(\frac{{k}_{capsule}}{{k}_{fluid}}-1\right)}{2}+\frac{{k}_{capsule}}{{k}_{fluid}}+\phi \left(\frac{{k}_{capsule}}{{k}_{fluid}}-1\right)\right]$$

(12)

Considering the fact that the thermal conductivity is also governed by the particle/fluid interaction, the effective thermal conductivity need to be reframed as:

$${k}_{eff}=f{*k}_{sl}$$

(13)

where f is a constant defined as:

$$f=1+B+\phi P{e}_{cap}^{m}\left\{\begin{array}{cc}B=0, m=1.5 & Pe<0.67 \\ B=1.8, m=0.18 & 0.67\le Pe\le 250\\ B=3.0, m=1/11& Pe>250\end{array}\right\}$$

(14)

considering that the Peclet number for spherical micro/nano particles is:

$$P{e}_{cap}=\frac{\gamma {d}_{capsule}^{2}}{{\alpha }_{fl}uid}$$

(15)

where α is the thermal diffusivity of CF, γ is the shear rate, and d is the capsule diameter (Languri et al. 2013).

Compared to water, although the specific heat capacity of the slurry is lower (as can be seen in Fig. 18a), within the melting temperature range, the ePCM slurry has a significantly higher effective specific heat capacity, boosted by the latent heat of fusion, increasing in direct proportion to the ePCM concentration in the slurry (Fig. 18).

Fig. 18

Specific heat capacity of ePCM-S and water (Zhang et al. 2011)

On the other hand, the increase in the viscosity of the slurry was almost negligible in comparison. The pumping power required to draw the same amount of heat is much lower in ePCM-S than in pure water (Wang et al. 2017).

3.3 Hydrodynamic Performance of ePCM-S

The main problems of ePCM slurries are the high flotation rate and the limited operating temperature range. In addition, agglomeration of microcapsules can cause various problems, such as increased viscosity, clogged channels, or even pump failure. While agglomeration can be reduced by determining a reasonable microcapsule concentration or using surfactants, using smaller capsules to prevent flotation and balancing the density of the capsules and carrier medium may offer solutions (Cao et al. 2019, pp 180).

Although there are partial increases in pressure drop in some cases, there are many theoretical and experimental studies showing that ePCM slurry increases convective heat transfer. Volumetric slurry concentrations below 25% can be assumed to be Newtonian and analyzed as such. Under ideal conditions, the Nusselt number of the ePCM slurry is 1.5–4 times higher than that of the single-phase flow. While Stefan number, volumetric concentration of microcapsules, dimensionless subcooling degree, dimensionless phase change temperature range, and the diameter of the microcapsules are found to be the dominant parameters that affect the heat transfer enhancement of the ePCM slurry, the shell of the microcapsule, the form of the PCM specific heat function, the ratio of specific heats, and thermal conductivities have little effect on heat transfer properties (Hu and Zhang 2002). ePCM-S has great potential as a thermal energy storage medium and HTF simultaneously (Yang et al. 2019). Since an increase in the capsule concentration will increase the heat capacity and energy storage density in the phase change temperature range, the required pumping power can be reduced by cycling at lower flow rates. However, in turn, the viscosity of ePCM-S may increase with the capsule concentration, increasing the pumping resistance (Alva et al. 2017; Cao et al. 2019). Therefore, the rheological properties of ePCM solutions should also be investigated. Many studies have evaluated ePCM solutions as Newtonian fluids, but many such systems are non-Newtonian and exhibit time-dependent behavior (Cao et al. 2019).

4 Thermal Performance of ePCM-S

ePCM-Ss have potential applications as an HTF for many systems, such as micro-channel heat exchangers, solar panels, and thermal power plants. Theoretical and experimental studies investigating the effects of different parameters have reported the Stefan number and the concentration of ePCM in the slurry as the most effective parameters on the heat transfer characteristics of ePCM-Ss in a laminar flow systems. As revealed by numerical studies, the Nusselt number of ePCM-S, in comparison to a single-phase fluid, is 1.5–4 times greater (Salunkhe and Shembekar 2012). In various studies, different parameters were experimentally investigated with regards to their effects on the heat transfer performance in both laminar and turbulent flow conditions (Alva et al. 2017) and are briefly summarized in Table 4.

Table 4 Heat transfer improvement of ePCM-S by the changes of different parameters and dimensionless numbers in turbulent and laminar flow cases (Qiu et al. 2017)

PVT systems have also been included in the research on the usage areas of PCM and ePCMs. Although there are studies on systems in which PCMs are used directly, which are often referred to as PVT/PCM in the literature, PVT-nanofluid systems using ePCM slurries as working fluid are the most prominent systems in terms of making use of the advantages offered by PCMs by eliminating their negative aspects.

As seen in Table 4, all parameters effect heat transfer in laminar and turbulent flow conditions in the same direction. And it is also noteworthy to mention that most of the parameters studied are positively related to the improvement of heat transfer. The increases in the inlet subcooling, phase change temperature, and Stefan number were found to be inversely related to heat transfer enhancement.

Since turbulent eddies and slurries containing PCM capsules as small as the laminar substrate thickness will exhibit single-phase flow characteristics; heat transfer coefficient increases with the effective heat transfer coefficient of the fluid.

Slurry mixes containing phase change material have the potential to significantly increase the heat transfer coefficient, whether under laminar or turbulent flow conditions. As a matter of fact, an experimental study conducted with an aqueous mixture containing paraffin particles at temperature differences between 10 and 20 °C showed that the heat transfer coefficient could be increase by three folds (Qiu et al. 2017).

It is well known that PCM solutions show physicochemical changes after many thermal cycles. However, the load effective in damaging the capsules and breaking the shells is also variable and a function of the operating temperature, so temperature is the primary consideration in the design and operation of PCM-S systems (Barreneche et al. 2014).

In short, it is a fact that the heat transfer coefficients of ePCM-S increase, and even this increase is effective in increasing the heat transfer capacity, however, the occurrence of a phase change significantly increases this improvement in heat transfer. It was determined that melamine increased thermal stability and that the homogeneity of composites was maintained after melamine was added (Acar 2014). Su et al. (2006) used melamine formaldehyde (MF) to prevent PCMs from being affected by the environment and environmental materials and to ensure long life in practice.

According to the DSC results of micro/nano-encapsulated phase-change materials synthesized by emulsion polymerization using n-heptadecane as the core and polystyrene as the shell, the melting and freezing temperatures were found to be 21.48 and 21.37 °C, and the latent heat of melting/freezing was 136.89 and 134.67 J/g, respectively. After five thousand thermal cycles, it decreased from 136.89 to 128.27 J/g due to damage to some capsules during pumping. Thermal gravimetric analysis (TGA) results showed good thermal stability of Micro-/nano-PCM in the process (Sarı et al. 2014).

5 Use of PCMs in Photovoltaic Systems

More and more research has been carried out on PVT systems, developed to save on installation space and costs, for they have a higher overall yield than both PV systems and solar collectors. While most researchers recommend new models/designs, some have been conducting studies to examine the performance of existing models and configurations under different climatic and environmental conditions. However, it is also very important to screen and examine existing studies in order to identify the basic ideas and principles to be considered in future studies and to know the limits of research in this field (Al-Waeli et al. 2016). One important research topic aimed at improving the performance of PVT systems addresses the use of various cooling means, including but not limited to air, water, nano-fluids, PCM, and their combinations (Al-Waeli et al. 2019d, pp 178).

The most common PVT system is PVT air systems, which are commercially and technologically advanced, but the electrical and thermal efficiencies that can be achieved with this system, at a maximum, are 8% and 39%, respectively. Another drawback of PVT air systems is that the application area of hot air is limited. On the other hand, PVT liquid systems are also common, a technology with greater electrical and thermal efficiencies of 9.5% and 50%, respectively, depending on the flow rate and temperature of the HTF, the shape, size, and geometry of the flow channel.

Yet, the presence of a characteristic technical problem, –increasing fluid temperature during operation– limits the possibility of improving these systems. Despite innumerable studies on PVT liquid systems, there still exist some technical problems that have not been completely resolved, such as the working fluid temperature being higher than the optimum operating temperature of the system, insufficient heat absorption, fluid freezing in the system at night in cold climate conditions, or fluid leakage out of the system.

Using refrigerants as the HTF in PVT systems has long been considered, and they were shown to be able to increase the solar energy utilization rate compared to PVT-water and PVT-air systems, and that they could reach 10% and 65% electrical and thermal efficiency, respectively. However, due to the fluid leaking from the system, the phase change, the uneven distribution of the fluid in the system, and the technical problems in providing pressure control in the operating conditions, it seems that such systems may only be used integrated with heat pumps and may become popular only in the future.

The use of nanofluids as HTF has also become widespread and has proliferated since their thermal conductivity is higher than that of CFs. Currently, nanofluids are widely used in different applications. Their use in heat exchangers and PVT systems is also increasing. However, nanofluids exhibit different characteristics in terms of performance and applicability. There is a lot of research aimed at finding the best HTF for use in PVT systems (Al-Waeli et al. 2019b, c) including novel hybrid nanofluids that comprise more than one nanomaterial dispersed in the CF (Sharma et al. 2022a).

As another novel approach, studies on the use of PCM in the thermal management of PV systems, and hence the number of articles submitted to the literature, have been increasing regularly and gradually since 2002, 24 years after 1978, when the first study in this field was made. Figure 19 shows the total number of studies on using PCM in PV systems published each year as stacked bars showing the particular field in which the studies are conducted. The interest in the thermal regulation of PV modules and in the use of PCM for their thermal management began many years before these two fields were combined. Over the last decade, studies on the thermal control and management of PV, CPV, and PVT systems by using PCM have increased and diversified rapidly (Browne et al. 2015b).

Fig. 19

Distribution of studies on the use of PCM for thermal stability of PV and PVT systems by year (Browne et al. 2015b)

In addition to studies on the utilization of PCM in PV and PVT systems becoming notable in 2000s, they also diversified over the years to cover micro- and nano-encapsulation of PCM and utilization of ePCM slurries, topping out at some two thousand papers by the year 2021, as seen in Fig. 20.

Fig. 20

Distribution of studies on the use of PCM, micro- or nano-encapsulated PCM and PCM Slurries combined (Ghasemi et al. 2022)

In order to make a system thermally more efficient using PCM, the main requirement is to maximize the heat transfer between the PCM and the environment (Rady 2009). However, it is known that many PCMs, especially organic PCMs, exhibit low thermal conductivity. Therefore, the main problem on which studies in this field are focused is, for sure, increasing the thermal conductivity of PCMs. In the literature, different techniques such as the use of fins, the incorporation of metal matrices with high thermal conductivity into the PCM, the dispersion of micro or nanoparticles with high thermal conductivity particles within the PCM, and micro- or nanoencapsulation were reported to have positive effects on increasing the thermal conductivity of the PCM (Velraj et al. 1999).

Al-Waeli et al. (2019c) examined the thermo-physical properties of three different types of nano-fluids comprising nano-SiC as an additive and setyl-trichlorite ammonium bromide as a surfactant and tried to find the best CF for use in PVT applications with a mixture of water, 35% ethylene glycol, and 35% propylene glycol. They reported that glycol solutions are more stable than water, although the thermal conductivity of the three nano-fluids is close to each other in the studied temperature range.

Song et al. (2007) showed that the addition of silver nanoparticles provided a strengthening of the shell structure of microencapsulated bromo-hexadecane microcapsules. In addition, the thermal and structural stability of PCM microcapsules combined with silver nanoparticles were found to be significantly higher than conventional PCM microcapsules.

Zhang et al. (2004) found in their studies on phase change properties and thermal stability with ePCM obtained from urea-melamine–formaldehyde shell and n-octadecane core materials that the best performance was obtained in the ratio of 0.2:0.8:3 mol. It has been reported that thermal stability can be increased up to 163 °C. Stability could be increased to 200 °C by adding cyclohexane. Salaün et al. (2009) studied the effect of the formaldehyde / melamine (F/M) ratio on the mechanical properties of paraffin encapsulated with amino resin and found that a low F/M ratio causes a significant reduction in the structural strength of the microcapsule, although it offers a smooth capsule surface. They reported that ePCM exhibited better mechanical properties at higher F/M ratios.

In addition to the thermal conductivity enhancement methods such as adding metal spheres/screens (Ettouney et al. 2004) or graphite (Sarı 2004) directly into the PCM, the studies on creating a conductive layer on the shell structures of the encapsulated PCMs by various methods have also yielded positive results (Bellemare 2009).

Al-Waeli et al. (2019d, pp 178) developed a mathematical model for a new nano-fluid/nano-PCM-based PVT system and experimentally tested it. The proposed mathematical model has been reported to be satisfactorily compatible with the results of the experiment. The study revealed that the electrical and thermal efficiencies for the mathematical and experimental methods were 13.7%, 13.2% and 72%, 71.3%, respectively. The maximum temperatures recorded in glass, PV cells, wax, and nano-fluid were 41.2, 39.92, 38.8 and 36.5 degrees, respectively.

With a solar panel on the top of the storage tank in which cylindrical containers with PCM were placed, only a marginal improvement effect has been observed compared to the conventional solar panel, and even a loss of performance has been reported in some cases (Talmatsky and Kribus 2008). Ibáñez et al. (2006), who performed a similar study, both experimentally and numerically, with sodium acetate trihydrate PCM encapsulated within aluminum bottles, reported that the fraction of solar energy in the total energy need increased from 4 to 8% due to the inclusion of encapsulated PCM in the system. However, it was also noted that the addition of PCM has a critical value for performance improvement, and after the critical threshold is exceeded, it will not increase the performance of the system; on the contrary, it will decrease it.

Another PCM application that has attracted great attention recently is taking advantage of their latent heat by mixing them into CFs as encapsulated within micro- or nanoscale capsules, which offers improvements in circulation circuits as well as better electrical and thermal power obtained from PV/PVT systems. This approach is also effective in overcoming the technological constraints and handicaps associated with solar collectors (Baronetto et al. 2014).

PVT systems using ePCM-S as the working fluid are almost a new technology that can be considered still in the early research phase.

Within the scope of the research on the sustainability of heating and cooling needs in environmentally friendly Energy-Plus houses, where the energy produced from renewable energy sources throughout the year is greater than the annual consumption, ePCM-Ss were proposed and used as the working fluid in PVT systems and were shown to be capable of improving the electrical and thermal efficiency of the system better than conventional PVT-liquid systems (Al-Waeli et al. 2017a). The ePCM-S-based PVT systems are generally connected to a second heat-pump system through a common heat exchanger.

The experimental system by Qiu et al. (2016), shown in Fig. 21, consists of a PVT module (4), an ePCM-S-refrigerant heat exchanger (evaporator) (3), a compressor (2), a refrigerant-water heat exchanger (condenser) (7), a water tank (1), an inverter (9) and other necessary accessories such as pumps (5), valves (6), an electrical resistance (8) and controller (10). During operation, the PVT module absorbs the incident solar irradiation and converts a portion of it into electricity and the rest into heat. ePCM-S circulating through the serpentine pipe attached to the back surface of the PV cells draws heat from the cell body, causing the PCM particles in the ePCM-S to melt. ePCM-S then flows into the evaporator of the heat pump, where heat transfer takes place between the ePCM-S and the working fluid (R134a) in the heat pump cycle, causing the PCM particles in the ePCM-S to solidify and the refrigerant (R134a) of the heat pump cycle to evaporate. The temperature of the refrigerant increased approximately from 15 to 70 °C and the heat was released into the water passing through the condenser of the heat pump cycle, thus supplying hot water for use in the building. The condensed refrigerant then passes through an expansion valve to complete the cycle and return to the evaporator. An electric pre-heater was used to adjust the temperature of the ePCM-S at the entrance of the PVT module in order to create a stable working regime.

Fig. 21

PVT system and cycles using PCM slurries (Qiu et al. 2016)

5.1 ePCM-S Based PVT Systems

In general, all the thermodynamic properties of fluids used in heat extraction from thermal systems are important. The specific heat capacity and viscosity of the fluid are determinative in terms of the pumping power required to circulate the fluid in the system. For air and water, the flow rate of the fluid that corresponds to the total heat required to be drawn from the system increases depending on the specific heat capacities. The greater flow rates require more power (fan power or pump power) to circulate the fluid. As can be seen in Fig. 22, the pumping power required to draw a certain amount of heat with pure water and encapsulated PCM slurries, it can be easily seen that it is much higher for pure water.

Fig. 22

Required pumping power versus heat drawn (Delgado et al. 2012a; Jurkowska and Szczygieł 2016)

ePCM-S helps increase the amount of energy stored per unit volume by 5–14 times (Sharma et al. 2009) as they also store some of the energy they gain during the heat transfer as latent heat. As a result, the same amount of heat can be drawn with less fluid or at less flow rates, i.e., with up to 5 folds less pumping power. For example, in order to draw 1000 Watts of heat, with pure water we need almost five folds of pumping power. Also, the relationship between the pump power and the heat drawn is parabolic for pure water, while it is almost linear for aqueous mixtures containing encapsulated PCM. Also, the relationship between pump power and heat absorbed is parabolic for pure water, whereas it is almost linear for ePCM-S.

With ePCM-S made of Eicosane and a mineral oil, up to 80% heat transfer enhancements could be obtained with concentrations as low as %1. However, concentrations above 5% decreased heat transfer due to particle clumping (Chen et al. 2014b). Comparing the pumping power required to draw a certain amount of heat with pure water and encapsulated PCM slurries, it can be easily seen in Fig. 22 that it is much higher for pure water. Viscosity and flow rate are the two determinant parameters on the required pumping power, i.e., the higher the viscosity of the fluid or the greater the flow rate, the greater the power required to circulate the fluid in the system gets, so it is always preferable that the viscosity of the fluid be low or the flow rate be smaller.

Examining Figs. 22 and 23 together, the pressure drops that occur in the system against mean flow velocities for different concentrations of ePCM-Ss in comparison with that of pure water suggest that the pressure drops that occur in the system are at an acceptable level in compensation for the low pumping power provided.

Fig. 23

Pressure Drop versus mean flow rate for a PVT-water system and a PVT-ePCM-S system compared (Yamagishi et al. 1999)

Erdoğan (2017) examined the thermal conductivity of ePCM-S comprising different weight concentrations of ePCM and reported that ePCM addition at any percentage resulted in a greater thermal conductivity. As can be seen in Fig. 24, 0.5% ePCM content induced an almost 15% increase in the thermal conductivity of the fluid. Nevertheless, since it also increases the viscosity of the fluid and hence the pumping power required to circulate the fluid in the system, the optimum percentage for the ePCM content was reported to be around 2–5% by weight, it may still vary depending on the shell material and particle size though.

Fig. 24

Variation of thermal conductivity of ePCM slurry with temperature (Erdoğan 2017)

Liu et al. (2017) reported that they achieved an improvement in both the thermal and electrical efficiency of their PVT system by employing ePCM-S as the working fluid at approximately the same pumping power. They compared the thermal and electrical efficiencies of water and ePCM-S at different mass flow rates (Fig. 25a and b) and measured the pumping power required at those flow rates, concluding in the net efficiency of the two PVT-Liquid systems (Fig. 25c and d). A significant heat storage capacity improvement was reported to have been achieved at v = 0.015 m/s and $\dot{m}$=0:67 kg/s, proving the superiority of ePCM-S over water as a HTF. Nevertheless, both initial setup costs and operating costs of ePCM-S-based PVT are higher when compared to conventional PVT-liquid systems (Ali 2017).

Fig. 25

a Thermal Efficiency b Electrical Efficiency c Pumping Power d Net Efficiency as compared to a normal PVT-water system (Liu et al. 2017)

A number of ePCMs have been tested in Pakistan in an outdoor C-PVT system. Lauric acid was the best performing PCM, reducing the PV temperature by 22 °C and the second best was palmitic acid with 19.5 °C. According to the results of the experiments, the optimal PCM selection should be made depending on the application, and it should be noted that a PCM that is optimal for one application may not be suitable for another (Browne et al. 2015b).

In their outdoor experiments in Selangor, Malaysia, Al-Waeli et al. (2017a) used paraffin FDM with a SiC nano-fluid circulation circuit to control the heat capacitance of the system, both to maintain electrical efficiency and increase overall efficiency. At the highest insolation period (12:30–01:30) at a fluid flow rate of 0.17 kg/s, the cell temperature decreased by 30 °C. Thus, the proposed PVT-nano-PCM nanofluid system increased the open circuit voltage from 11–13 to 20–21 V, the output power from 61.1 to 120.7 W, and the electrical efficiency from 7.1 to 13.7%, whereas the thermal efficiency of the system was recorded to be 72%.

6 Discussion

It is no longer a matter of debate whether PV cooling is necessary, but it is still a hot topic as to which cooling method is the most effective. There have been innumerable studies all around the world. The topic is so broad and deep that it is not even possible for a researcher or a group of researchers to address all cooling techniques in all aspects. Therefore some researchers have had to make comprehensive reviews of previous studies in order to structure or plan their own research, each addressing different methods and aspects of PV cooling (Agyekum et al. 2021a; Ali 2020; Bahaidarah et al. 2013, 2016; Baloch et al. 2015; Chandel and Agarwal 2017a; Feng et al. 2021; Liu et al. 2017; Mojumder et al. 2016; Nižetić et al. 2017, 2018; Ren et al. 2018; Shahsavar et al. 2020; Shen et al. 2021; Shukla et al. 2017; Zhe et al. 2019). Being the most effective cooling method depends on several sub-factors, such as cost-effectiveness, applicability, cost of fluid/cooling media, cost recovery factor, enhancement factor, etc. Therefore, any cooling may or may not be appropriate for application in certain cases. For example, Chandel and Agarwal (2017a) reported that a PV-PCM system would be effective only in regions that receive high insolation throughout the year and where inter-seasonal climatic variation is less. Yet, this might still be subject to change.

Once PV power generation was only a lab-scale application and was deemed to be an option for only extraterrestrial power generation for satellites, but now that it has become a viable option to resist, mitigate, and even replace fossil fuel-based power generation (Misha et al. 2019). Similarly, just two decades ago, ePCM-Ss were only lab-scale. Over the years, technology has developed and enabled these special fluids to be viable and yet efficient options for heat transfer processes (Alvarado et al. 2007; Liu et al. 2021; Yuan et al. 2022). In a recent study, Trivedi and Parameshwaran (2020) proved that ePCM-Ss exhibited Newtonian fluid properties and were viable for thermal energy storage. Ghaziani et al. (2012) used porous media to further improve the heat transfer by preventing ePCM particles from moving far from the surfaces and hence not taking part in heat exchange.

The overall efficiency of PVT systems, therefore, depends on not only the improvement of the electrical efficiency of the PV module but also on the thermal efficiency of the integrated thermal system. It is therefore worthy of consideration, with regards to the efficiency of PV cooling systems/mechanisms, whether or not the heat drawn from the PV panel could be converted into useful heat (Misha et al. 2019). Despite numerous studies carried out on PVT systems over the past three to four decades, there have been only a few efficient PVT systems on the market (Sathe and Dhoble 2017).

7 Conclusions

Studies on renewable energy are important within the framework of sustainable development and clean energy strategies. PVT systems are becoming increasingly common among solar energy applications, which have the highest potential and the widest application area among renewable energy sources. Efforts to improve both electrical and thermal efficiency of PVT systems continue, with different approaches on different system parameters. Although studies aimed at drawing more heat from the system with less pumping power by using ePCM-Ss have increased in number and diversity in recent years, much more dynamic simulations and experimental studies are needed in this field in order to more precisely establish real climatic conditions and operating parameters.

ePCM-S systems are much more complex in many ways compared to conventional PVT-liquid systems. Numerous parameters such as the use of various shell and core materials, the variety of production methods, and the homogeneity of the resulting capsules, the use of additives, the core to shell ratios, and the mass fraction of ePCMs in the slurry, make it difficult to determine the properties of these fluids accurately and precisely. These parameters also significantly diversify and differentiate ePCM-S by affecting the suspension stability, rheological properties and thermal properties of ePCM-S. As a result, it becomes very difficult to compare the data and findings obtained from different studies. In recent years, PCMs have become an attractive research field for researchers due to their advantages.

The use of ePCM-Ss in PVT systems requires an extensive and exhaustive study with quite a lot of background knowledge and interdisciplinary collaboration, as the proper selection of PCM materials and synthesis methods as well as the correct concentration in the best CF involve several aspects and expertise in a number of other fields.

The findings of early studies and subsequent research revealed that the use of ePCM-S as the working fluid in PVT systems increased the thermal efficiency, electrical efficiency, and overall efficiency at almost the same pumping power. In some specific cases, the yields were reported to be better.

However, efforts should be intensified to achieve improvements in the size and composition of shell and core materials or to develop new materials. And operational issues still need to be fully addressed for the full implementation of ePCM-S to be successful. In the present study, the need and value of encapsulation, phase change materials, and their synthesis and characterization methods, as well as their advantages and suppression of their disadvantages, were addressed in a comprehensive way with special reference to their use as a HTF in PVT systems. This study, therefore, aims to constitute a fundamental guide to refer to from the very beginning to the final implementation of the ePCM-S as the working fluid in the PVT system by addressing almost all effective parameters in terms of advantages, disadvantages, challenges, and opportunities.

Future research on the implementation of encapsulated PCM in dilute solutions of water or of other liquids as working fluids in thermal systems, either as heat transfer fluid or as thermal storage medium, is recommended to be aimed at designing and developing binary, ternary, or even quaternary ePCM slurries in order to enable transfer of heat as latent heat in a wider temperature range. The narrow window of phase change temperature range functions as a constraint on the flow rate, forcing it to be low so as to allow the PCM to melt within short exchange circuits, but this, on the other way round, may bring on problems in other parts of the cycle. In longer exchange circuits, on the other hand, the phase change would take place at a shorter portion of the exchange circuit, resulting in sensible heat storage in the rest of the circuit. Furthermore, future research should be aimed at the incorporation of single- or multi-walled carbon nanotubes, as well as other nanoparticles and even graphene, into the shell or core of the capsules in order to further enhance their properties.