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Subjects:
Engineering, Mechanical
Cold thermal energy storage (CTES) based on phase change materials (PCMs) has shown great promise in numerous energy-related applications. Due to its high energy storage density, CTES is able to balance the existing energy supply and demand imbalance. Given the rapidly growing demand for cold energy, the storage of hot and cold energy is emerging as a particularly attractive option.
- cold thermal energy storage (CTES)
- phase change materials (PCMs)
- heat transfer
1. Introduction
Fossil fuels are an important source of energy worldwide, accounting for 85.5% of total energy consumption. However, if humanity continues to use fossil fuels at this consumption level, a range of environmental problems will arise including shifting weather patterns and global temperature increases [1]. Therefore, with the development of energy-saving technologies, the use of new energy and renewable energy has become the focus of global attention. The energy supplied during the day and night varies greatly, which is detrimental to the reliability of the energy transfer system [2]. The most pressing issue is how to utilise the excess electricity generated at night during the day so that no stress builds up. Refrigeration equipment is one of the remedies to this issue and has developed into a crucial component of reducing the effect of the current crisis on the world’s electricity supply due to its distinctive influence on load shifting. Energy storage technology not only speeds up the uptake of new and renewable energy sources but also works well to correct the disparity between supply and demand in the market [3].
To find a solution to this problem, the intelligent use of thermal energy storage (TES) is essential. The technology behind TES has been extensively studied [4,5] and can be classified into three distinct thermal categories: chemical heat, latent heat, and sensible heat. Sensitive heat is the most common form of heat. Besides sensible heat and chemical heat, another way to efficiently utilise energy is to focus on the latent heat generated by phase change materials (PCMs) [6,7]. The phase transition process caused by changes in ambient temperature employs the latent heat method to store and release thermal energy. Constant temperature can be maintained by isothermal absorption and release of heat; this enables efficient use of energy in space and time [8,9,10,11]. PCMs are widely used in a wide range of energy conversion fields such as waste heat recovery, Li-ion batteries, building insulation, and solar energy storage [12,13,14,15,16,17,18,19,20,21,22,23].
The most familiar types of PCMs are eutectic and organic and inorganic. The term “eutectic PCM” refers to mixtures of two or more substances (organic and inorganic PCMs) that, when mixed in a particular proportion, have a lower melting point than any of the constituent parts. They experience a rapid and distinct phase transition while maintaining a steady temperature and releasing or absorbing thermal energy. Moreover, eutectic PCMs can be made so as to generate favourable properties such as a lower melting point or a specific phase change temperature. Salt hydrates, such as calcium chloride and sodium sulphate, are typical examples of eutectic PCMs [24]. Organic PCMs are single-component materials made from organic chemicals or hydrocarbons. Compared to eutectic PCMs, they may display a wider melting temperature range, but they have distinct phase change temperatures. Examples of organic PCMs frequently utilised for cold thermal energy storage include paraffin waxes and fatty acids [25]. Furthermore, solid inorganic phase change materials exhibit elevated latent heat values and high melting temperatures, resulting in effective energy storage and release during phase changes [26]. In this regard, PCMs have been successfully used in different sectors of CTES.
Veerakumar and Sreekumar (2015) [27] provided a detailed assessment of current breakthroughs and previous research projects using PCMs for cold thermal energy storage. These commercially available PCMs are classified and listed according to their melting point and latent heat of fusion. These PCMs have the potential to be utilised as materials for storing cold energy. Furthermore, methods to improve the thermo-physical properties of PCMs, including encapsulation, increased heat transfer, incorporation of nanostructures, and shape stability, were analysed and discussed in this paper. Corrosion of building materials was also found to affect the stability of the structure.
Nie et al. (2020) [28] revised TES for cold energy storage, focusing on a variety of cryogenic liquid–solid PCMs. A basic overview of the PCM classification system was presented. Recent technologies used to improve PCM performance, in particular their low thermal conductivity, liquid PCM leakage, and limiting their use in TES refrigeration applications with high degrees of sub-cooling, were intensively discussed in this study. Several strategies for improving thermal performance were compared, such as using composite PCMs and massive networks. The application of modelling and experimental research in the field of refrigeration was also highlighted. A number of applications for cold energy storage currently in use have been outlined such as air conditioning and free cooling.
Selvnes et al. (2021) [29] provided a comprehensive overview of recent advances and research surveys on CTES using PCMs in refrigeration systems. They focused on the latest developments in the field. The study included the classification of many types of PCMs used in a wide set of applications ranging from air conditioning (AC) (20 °C) to food freezing (below −60 °C). Besides providing a list of PCMs currently on the market that operate between 10 °C and −65 °C, the authors provided an indication of the thermo-physical characteristics of PCMs that may affect the behaviour and related approaches to characterise PCMs.
Radouane (2022) [30] focused on discussing the fabrication methods of PCMs including encapsulation, hybrid confinement, and polymerization in addition to addressing the enhancement of the thermal conductivity of composite PCMs. The author illustrated the successful utilisation of PCMs in different sectors of energy storage, energy conversion, and thermal management.
2. Conceptual Challenges of Using PCMs in Different Applications of CTES
Because of their capacity to store and release energy during phase transitions, PCMs have demonstrated significant potential in a variety of CTES applications. However, their use encounters conceptual difficulties that must be overcome in order to maximise usefulness and efficiency. Some of the most significant conceptual issues are as follows [31,32,33,34,35]:
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It is critical to select the appropriate PCM for a given application. PCMs are classified as organic, inorganic, or eutectic mixes, with varying melting and freezing points and latent heat capacities. Choosing a PCM that meets the temperature demands of the application while preserving stability and reliability might be difficult;
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To minimise corrosion problems in low-temperature applications, the construction materials of the container used to hold diverse eutectic PCMs for thermal energy storage must be considered;
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To minimise leakage and assure compatibility with current equipment, PCMs are often enclosed within containers. Finding acceptable encapsulating materials that are PCM-compatible, thermally conductive, and chemically stable might be difficult. Furthermore, the encapsulation technique should not interfere with heat transfer during phase change;
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The rate of heat transmission during the charging (melting) and discharging (solidification) processes determines the effectiveness of PCM-based thermal storage devices. PCMs have a lower and poor thermal conductivity than traditional materials such as metals. This might result in poorer heat transfer rates and might require the use of higher thermal conductivity structures or composites, complicating the process of design and production even more. In this regard, high heat transfer rates can be difficult to achieve, particularly for large-scale applications, because they may necessitate sophisticated heat exchanger designs and adequate interaction with current systems;
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During phase transitions, some PCMs experience considerable volume changes, which can cause mechanical stress and distortion of the incarceration structures. Controlling these volume variations in order to prevent system damage across several phase change cycles necessitates careful design and research;
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During their lifetime, PCMs are predicted to go through several phase change cycles. A crucial problem is guaranteeing the PCM’s stability and strength across many cycles without a substantial drop in effectiveness.
Overcoming these conceptual issues would necessitate collaborative efforts from material scientists, engineers, thermodynamics experts, and system designers. Solving these obstacles will result in enhanced effectiveness and broader adoption of phase change materials in cold thermal energy storage applications.
This entry is adapted from the peer-reviewed paper 10.3390/jcs7080338
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