Page 60 - IJAMD-1-3

International Journal of AI for
Materials and Design
Phase change materials and digital twin technology in thermal energy

T pc T 2
Q = ∫ C solid + ∆+ ∫ C liquid (II)
h
T 1 T pc
where T pc is the temperature of the phase change, Δh is
the latent heat of fusion, and C solid and C liquid are the specific
heat capacities of the PCM in the solid and liquid phases,
respectively.
A PCM is utilized in LHS systems, with the temperature
at which the phase transition occurs referred to as the phase- Figure 3. Sensible and latent heat behavior of phase change materials with
change temperature. 45,47 In addition to offering high energy respect to temperature
density, LHS systems also face challenges related to low Abbreviation: PCM: Phase change material.
thermal conductivity and material degradation over time.
PCMs are commonly employed in LHS systems because of 3.1. Classification of PCMs
their ability to absorb and release large amounts of energy
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as they change phase, which alters their physical state. In PCMs can be primarily classified into three types based
the LHS process, heat is stored as the PCM undergoes a on their charging and discharging processes within the
phase transition at a nearly constant temperature, with the solid-liquid transition category: organic, inorganic,
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amount of heat being proportional to the material’s latent and eutectic PCMs. Each of these categories contains
heat of fusion. Due to their high energy capacity and ability additional sub-categories. Organic PCMs include both
to maintain a stable temperature throughout the phase paraffin and non-paraffin materials. Non-paraffin
transition, PCMs are widely considered ideal materials for materials are further subdivided into lipids, sugar esters,
LHS systems. 49 and carbohydrates. Inorganic PCMs, which undergo
phase transitions without requiring super-cooling, retain
2.3. Thermochemical energy storage their heat of fusion throughout cycling. Depending
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Thermochemical energy storage systems absorb and on their composition, inorganic PCMs can be metals,
release energy during reversible reactions, which molten salts, or salt-hydrates. Eutectic PCMs, in turn,
involve the breaking and reforming of chemical bonds. can be synthesized by combining organic and inorganic
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Chemical reactions that release heat form the basis components. Figure 4 provides a concise overview of
of thermochemical energy storage systems. The heat the different PCM classes.
involved in the calcium hydroxide cycle is given by Organic PCMs are primarily composed of carbon-
Equation III as follows: hydrogen bonds. These materials are non-corrosive,
CaO + H O ⇌ Ca(OH) + Heat (III) self-nucleating, and typically have consistent melting
2 2
points. While organic PCMs offer numerous advantages,
3. PCMs they also have several limitations that can reduce their
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PCMs are substances that can regulate temperatures effectiveness. They do not experience phase separation
within a narrow range by releasing latent heat during or degradation, meaning they can undergo continuous
phase transitions. As the surrounding temperature melting and freezing cycles. Key advantages of organic
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increases to the point where the PCM melts, an PCMs include their chemical stability, lack of super-
endothermic process begins, breaking the chemical cooling, non-corrosiveness, and recyclability. However,
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bonds and transforming the PCM from a solid to a liquid they are combustible and generally have low conductivity.
while absorbing energy. This process is referred to as the One widely used organic heat exchange material is paraffin
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PCM charging phenomenon. The next step involves the wax, which is composed primarily of alkanes. The formula
temperature decreasing until the PCM solidifies, at which of paraffin is C H 2n+2 , where “n” indicates the number of
n
point the bonds regenerate, and the PCM returns to its carbon atoms in the alkane chain.
solid state by releasing heat in an exothermic process. In contrast, inorganic PCMs are non-combustible,
Therefore, PCMs are defined as thermal reservoirs. have a higher LHS capacity, and exhibit more distinct
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When charged and discharged, the volume of PCMs phase transitions. However, their main drawbacks
changes by a negligible amount – approximately 10% include inherent corrosiveness, phase segregation, and
of their original volume. Figure 3 provides a visual super-cooling. Among organic PCMs, salt hydrates are
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representation of this process. particularly promising due to their higher LHS capacity
Volume 1 Issue 3 (2024) 54 doi: 10.36922/ijamd.4696