Inorganic phase change materials in thermal energy storage: A review on perspectives and technological advances in building applications
Graphical abstract
Introduction
In recent years, owing to the environmental and economic constraints of conventional energy sources and their inadequacy to fulfill the current energy demands, renewable energy sources have gained attention around the globe. For instance, according to a report published in Reuter’s, the consumption of coal in China rose for a consecutive second year in 2018, but for the first time, there is 60% reduction of coal’s share in total energy consumption as it is being replaced by renewable energy sources [1]. Additionally, there is an urgent need for a smarter treatment of energy. Thermal energy storage (TES) is one of the most promising aspects of rational use of energy at a cost point where it can be utilized even at present in a variety of facilities. Therefore, in recent years a number of studies have been conducted through various critical aspects of TES to make it viable and sustainable for building applications. Amongst various types of technologies developed for energy storage, there are different options available today for cost-effective and reasonably stable thermal management solutions of buildings with increased power consumption and larger footprints. It is a fact that the reduction of building heat losses or more precisely the total energy consumption of buildings plays a significant role in minimizing the environmental burdens [2].
Two possible ways might be suitable at the building integration level: a conventional approach of sufficiently dense material that forms a TES mostly based on sensible heat storage (SHS) and an unconventional approach based on lightweight material with the different physical form of storing heat energy such as latent heat storage (LHS) [3], [4]. The former is typically focused on cement-based composites that might be suitable for encasement of various waste products, while the latter is exclusively based on latent TES using phase change materials (PCMs). LHS has drawn great attention among researchers due to its efficient thermal energy storage capacity, and now it is becoming a hot issue over the last few decades. PCM thermally operates during its internal phase change process, where the accumulation of the latent heat increases in relation to latent heat capacity for a certain boundary condition. The working principle of this physical method is based on three steps: thermal energy-absorbing, storing and releasing. The incompleteness of the charging and discharging process leads to a decrease in the overall energy potential of PCM integration into building systems. Therefore, a major benefit of integrating PCM is that it maintains its internal temperature during its phase transition [5]. PCMs are capable of storing a massive amount of thermal energy (TE) by a phenomenon termed as a change of phase from one to another (commonly used in building construction is based on the phase transformation from solid-liquid state and vice versa), at a specific narrow temperature range, and give away higher heat of phase transition (i.e., LHE) [6]. Fig. 1 represents the working diagram of PCMs to change their phase by absorbing and releasing a specific amount of heat with pertinent change in the thermal cycle.
The key attribute of PCM is the latent heat of fusion which is the thermal energy that is absorbed by a material during its phase transition process and calculated from the enthalpy difference between phases. PCM typically can absorb orders of magnitude more heat than any single-phase material but in a specific temperature range. The selection of PCM involves several factors that should be taken into account for building applications (Table 1) [7], [8]. Specifically, an ideal phase change material has the following general requirements [9]: (a) high specific heat, thermal conductivity, heat of fusion and density; (b) long-term reliability in repeated cycles; (c) steady freezing behavior; (d) environment-friendliness; (e) small volume change during phase transition. However, it is worth mentioning that no PCM can meet all these desirable requirements [10]. Moreover, PCM applications in various sectors such as air conditioning, refrigeration, construction, food, textiles, waste heat recovery and astronautics [11], [12], [13] mainly depend upon their phase change temperature [14], [15].
Nowadays, various methodologies can be applied to directly implement PCMs in building materials, composites, and structures: direct incorporation, immersion, encapsulation, vacuum impregnation, shape-stabilization, and form-stable composite. However, the group of pure PCMs suffers from two basic problems: low thermal conductivity (small rate of heat dissipation) and leakage (the capsulation method). These facts limit it for widespread applications in many sectors. According to the literature, the suitable PCM for building applications should have a phase temperature in the range of 18 °C to 40 °C (Fig. 2) [16].
PCMs are generally used in building in two basic ways: (a) as passive thermal storage incorporated into building elements, (b) as independent storage units coupled with heating ventilation and air conditioning (HVAC) systems. Since the PCMs are applied in different fields, Kenisarin [17] reviewed PCMs utilization in domestic buildings, military and industrial applications; while Khan et al. [18] conducted a review on PCMs employed in refrigeration and solar absorption systems. Though numerous types of PCMs are commercially available on the market, however, a few of them could be efficiently used in buildings. PCMs are mainly differentiated based on their melting temperatures which, together with PCM location and its quantity, are essential for designing its building incorporations. Overall system performances are dependent on PCMs integrated forms, heat transfer enhancement solutions, system operation modes, and optimal geometrical and operation parameters [19]. That is why for well-designed PCM systems, this temperature range must be as narrow as possible [20]. According to its material character, PCMs can be characterized into three types such as inorganic (iPCM), organic (oPCM), and eutectic (ePCM) [21], [22]. Each type of PCM has special advantages and disadvantages in this concern as presented in Table 2.
Farid et al. [11] made a comparison for heat storage properties amongst LHS and SHS by using rock and water with iPCMs and oPCMS (Table 3). It is quite apparent from the comparison (as drawn in Table 3) that LHS has overall outperformed SHS by storage capacity. In accordance with the last studies, where PCMs have been investigated extensively, two promising candidates for building application include paraffins and salt hydrates [23]. iPCMs represent an important category of PCMs, which have almost double the heat storage capacity per unit volume (368 kJ/kg) in contrast to oPCMs (152 kJ/kg) as reported in Table 3. Additionally, they encompass a reasonably high thermal conductivity for nonmetals, and show minor volume changes between solid and liquid phases in comparison with oPCMs [11]. iPCMs incorporates mostly higher operating temperatures (18.5–116 °C) [24], higher thermal conductivity and lower operating cost as compared to oPCMs [25], [26], [27]. Conversely, iPCMs offer corrosive behavior to metals resulting in higher cost, reduced service life of the system and uncertainty in long-term reliability [23], [24], [38]. iPCMs as salt hydrates are drawing great attention for low-temperature domestic applications because of their wide availability, sharp melting point (Fig. 3a) [26], [28], [29], [30], economic advantages (Fig. 3b) and higher energy storage density [31], [32], [33], [34]. Due to these advantages, many studies have been carried out on iPCM to make it more viable to be used in thermal energy management systems [35], [36], [37], [38]. However, in the view of the construction industry, this valuable research data is still available in a scattered form due to a lack of efforts to properly summarize the available research data. Consequently, the current paper is focused on the actual literature review on iPCMs in the context of heat/energy storage in the building sector, summarizing its performance, applications, and challenges.
The methodology of this review (Fig. 4) has been initiated by identifying the scope of the review for building applications along with the problem statement with the close connection of a comprehensive review of the existing knowledge and recent literature reviewed mostly over the last decade (up to 2021). More than a thousand papers are initially selected for the review, which are systematically scrutinized to more than 250 papers. For the present review, relevant databases including but not limited to Science Direct, Web of Science, Scopus, and Google Scholar, are considered to acquire relevant literature from the past few decades. The paper is specifically focused on the research, development, and application of inorganic phase change materials. The main keywords were inorganic PCM, salt hydrates, thermal energy storage, building industry, composites, concrete structure, corrosion, building material, and latent heat. iPCMs performance factors such as thermo-physical and related chemical aspects and options for building integration were investigated for this work. After reviewing the literature, gaps and inconsistencies are figured out. Afterward, key technological advances in the application field are highlighted by reviewing recent papers between the years 2016 and 2021. Finally, potential research areas to be studied in future works are also derived.
Section snippets
Inorganic phase change materials
The family of iPCMs generally includes the salts, salt hydrates and metallics. Primarily, inorganic salt refers to salt and/or salt hydrates in PCMs and are generally expressed as AxBy.n(H2O), where n indicates the number of water molecules and AxBy denotes chloride, oxide, nitrite, sulfite, acetate, phosphate, or metal carbonate. The types of bonds that are mostly present in hydrated salts are ion–dipole bonds that contain H-H bonds or anion and a polar molecule. The H2O molecules are loosely
Challenges in thermophysical aspects of iPCMs
Generally, iPCMs possess various desirable properties like high latent heat storage, sharp melting point, high thermal conductivity, and inflammability. However, they are also associated with some disadvantages: corrosiveness, supercooling, high phase transitional volume changes and phase segregation resulting from poor nucleating ability and incongruent melting, respectively. A representative comparison of thermo-physical properties of PCM materials is presented in Fig. 5 [48].
In general,
iPCM based building materials and technological challenges
In recent times, iPCMs have been widely used in multiple sectors, e.g. smart electronics, construction industry for effective energy management, concentrated solar hot water systems for domestic use, food and textile industry [145], [146] etc. The PCMs application in building materials has exhibited tremendous performance, contributing to energy saving, electricity dependence, and human comfort [146], [147], [148], [149], [150]. On the other hand, PCMs used in the construction sector must
Performance evaluation of building applications of iPCM
As CO2 emission is a major problem in the modern world, PCMs blended in construction materials or directly using PCMs as passive building systems have considerable energy storage ability. In particular, four temperature groups can be closely be associated to building applications relative to the range of temperature [238], [239], i.e., (i) 18 °C to 28 °C for space heating and cooling of buildings; (ii) 29 °C to 80 °C for heating and cooling of water; (iii) 20 °C to 150 °C for solar energy
Summary and conclusions
In this review work, inorganic phase change materials (iPCMs) have been discussed with their properties and key performance indicators for building integration. The selection of these iPCMs mainly depends on thermophysical properties, mechanical properties soundness during phase transition and compatibility. Moreover, the current study reviews existing information on iPCMs focusing its utilization in thermal energy storage (TES) systems of the building sector based upon their properties,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by project GA 20-00630S “Climate responsive components integrated in energy and environmentally efficient building envelopes” supported by the Czech Science Foundation in Czechia and project VEGA 1/0680/20 supported by the Scientific Grant Agency of the Ministry of Education, Science, Research and Sport of the Slovak Republic. The authors also acknowledge the National Scholarship Program (NŠP) of the Slovak Republic for the Support of Mobility of Students, PhD
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