Phase Change Materials (PCMs): Classification, Properties and Application

Last Updated on 27/05/2022

What are Phase Change Materials (PCMs)?
In the last decade one of emerging technologies is microencapsulated phase change materials (PCMs), which are being developed to provide significantly enhanced thermal management for fibers, foams and textiles with applications to apparel and technical textiles.

Phase change is a process of going from one physical state to another. The three fundamental phases of matter, solid, liquid and gas, are known but others are considered to exist, including crystalline, colloid, glassy, amorphous and plasma phases. Substances that undergo the process of phase change are known as phase change materials (PCMs). PCMs possess the ability to change their physical state within certain temperature ranges.

Phase change materials (PCMs) are substances which absorb or release large amounts of so-called ‘latent’ heat when they go through a change in their physical state, i.e. from solid to liquid and vice versa. In a heating or a cooling process, this phase change takes place as soon as the material reaches its specific phase change temperature. During the latent heat absorption or latent heat release, the temperature of the PCM remains constant. The PCM’s property of absorbing and releasing large amounts of heat in a controlled way can be utilized to improve the thermal performance of various end-use products to which the PCMs are applied. The latent heat absorbed by the PCM can be stored therein. Therefore, PCMs are considered to be highly efficient thermal storage means.

Phase change materials (PCMs) are theoretically able to change state at nearly a constant temperature and therefore to store a large quantity of energy to regulate temperature fluctuations. PCMs can exist in at least two different phases (an amorphous and one or more crystalline phases), and they can be switched repeatedly between these phases.

Phase change materials (PCMs) materials have high heats of fusion so they can absorb a lot of energy before melting or solidifying. A PCM temperature remains constant during the phase change, which is useful for keeping the subject at a uniform temperature. It is one of the application of technical textile.

PCM have been studied for use in direct thermal energy storage, solar energy applications and more recently, in response to growing international concern to climate change, building temperature regulation.

PCM can undergo solid-solid, liquid-gas and solid-liquid transformations. Solid-solid PCMs generally have high transition temperatures which are beyond the scope of practical use. Liquid-gas PCMs are not considered usable for most applications due to the large volume change that occurs at phase change. Solid-liquid PCMs have seen the most use in research and applications as they have high latent heat capacities and good thermal conductivity, making them practical for use. There are however several difficulties that can arise when using certain solid-liquid PCMs such as, subcooling, phase segregation and corrosiveness.

Energy and environment are the two major issues facing human beings nowadays. Industrial developments and population boom in the past few centuries have resulted in an enormous increase in energy demand with an annual increasing rate at about 2.3%. The increasing demand for energy-saving and environment-friendly technology is driving the growth of the global phase change material (PCM) market.

Phase Change Materials in building construction
Fig: Phase Change Materials in building construction

Building and construction currently forms the largest application market due to the globally increasing demand for cooling buildings, which in turn has arisen due to the shift from heavy thermal mass design to lightweight architecture. Phase Change Materials (PCMs) provides a high heat storage density and has the capability of storing a large amount of heat during the phase change process with a small variation of PCM volume and temperature.

PCM Materials and Their Properties / Characteristics:
Different kinds of materials were used as PCM. In principal materials should fulfill different criteria in order to be suitable to serve as a PCM.

  1. Suitable melting temperature
  2. High melting enthalpy per volume unit [kJ/m³]
  3. High specific heat [kJ/(kg.K)]
  4. Low volume change due to the phase change
  5. High thermal conductivity
  6. Cycling stability
  7. Not flammable, not poisonous
  8. Not corrosive

As one of the goals of latent energy storage is to achieve a high storage density in a relatively small volume, PCMs should have a high melting enthalpy [kJ/kg] and a high density [kg/m³], i.e. a high volumetric melting enthalpy [kJ/m³].

Classification of Phase Change Materials:
Based on phase change state, PCMs fall into three groups: solid– solid PCMs, solid–liquid PCMs and liquid–gas PCMs. Among them the solid–liquid PCMs are most suitable for thermal energy storage. The solid–liquid PCMs comprise organic PCMs, inorganic PCMs and eutectics.

Classification of Phase Change Materials
Fig: Classification of Phase Change Materials

Comparison of different kinds of PCMs:

Classification Advantages Disadvantages
Organic PCMs
  1. Availability in a large temperature range
  2. High heat of fusion
  3. No supercooling
  4. Chemically stable and recyclable
  5. Good compatibility with other materials
  1. Low thermal conductivity (around 0.2 W/m K)
  2. Relative large volume change
  3. Flammability
Inorganic PCMs
  1. High heat of fusion
  2. High thermal conductivity (around 0.5 W/m K)
  3. Low volume change
  4. Availability in low cost
  1. Super cooling
  2. Corrosion
Eutectics
  1. Sharp melting Temperature
  2. High volumetric thermal storage density
  1. Lack of currently available test data of thermo-physical properties

Application of Phase Change Materials (MCMs):
The application of PCMs to a garment provides an active thermal insulation effect acting in addition to the passive thermal insulation effect of the garment system. The active thermal insulation of the PCM controls the heat flux through the garment layers and adjusts the heat flux to the thermal circumstances. The active thermal insulation effect of the PCM results in a substantial improvement of the garment’s thermophysiological wearing comfort. The intensity and duration of the PCMs’ active thermal insulation effect depend mainly on the heat-storage capacity of the PCM microcapsules and their applied quantity. In order to ensure a suitable and durable effect of the PCMs, it is necessary to apply proper PCMs in sufficient quantity into the appropriate fibrous substrates of proper design. The PCM quantity applied to the active wear garment should be matched with the level of activity and the duration of garment use.

Furthermore, the garment construction needs to be designed such that it assists the desired thermoregulating effect. Thinner textiles with higher densities readily support the cooling process. In contrast, the use of thicker and less dense textile structures leads to a delayed and therefore more efficient heat release of PCMs. Further requirements on the textile substrate in a garment application include sufficient breathability, high flexibility, and mechanical stability.

Others applications of phase change materials are given below:

  1. Cooling Packs
  2. Thermasorb Capsules
  3. Body Cooler
  4. Bridge Warmer
  5. NASA Dive Suit
  6. Building Insulation

References:

  1. Intelligent Textiles and Clothing Edited by H. R. Mattila
  2. Advanced Textile Engineering Materials Edited by Shahid-ul-Islam and B.S. Butola
  3. Smart Textile Coatings and Laminates Edited by William C. Smith
  4. Phase Change Materials: Science and Applications By Simone Raoux and Matthias Wuttig

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