Types of PCM Materials: What They Are and Their Characteristics

What are the types of PCM materials available on the market, how are they made, and what are their performance characteristics? These are the questions we will answer today, continuing our in-depth look at PCM materials started in the previous article "PCM Materials - What They Are and How They Store Heat."
The type of PCM materials we are referring to is the solid-liquid based type, the most common, which allows the transition from a solid to a liquid state and vice versa. This type of PCM material is further divided into three classes and respective subclasses, illustrated in the image below:

1. Organic PCM Materials
   1. Paraffin waxes
   2. Non-paraffin materials
2. Inorganic PCM Materials
   1. Hydrated salts
   2. Metal alloys
3. Eutectic PCM Materials
   1. Organic-organic materials
   2. Inorganic-inorganic materials
   3. Inorganic-organic materials

If we recall the classification based on the melting temperature of PCM materials discussed in the article "PCM Materials: What They Are and How They Store Heat," we can easily associate these three groups of PCM with specific corresponding materials. For example, ice and eutectics fall into the low-temperature PCM category as their phase change occurs below 15°C. Hydrated salts, organic PCM, and polymers undergo phase changes within a temperature range of 15°C to 90°C, populating the medium-temperature PCM category. Finally, molten salts, metal alloys, and paraffins have phase changes at very high temperatures, typically above 90°C.

Let's now look in detail at the specific characteristics of the three existing families of PCM materials.

Organic PCM Materials

Organic PCM materials exhibit a melting process that maintains an almost constant temperature over a large number of melting/solidification cycles. This ensures that the latent heat of fusion remains constant for an extended period. Additionally, they do not present subcooling issues, are non-corrosive, and are available on the market at reasonable prices.
Let's examine them one by one in detail. Organic PCM materials are divided into:

• Paraffin waxes
• Non-paraffin materials (fatty acids, alcohols, and glycols)

Paraffin waxes or paraffins are saturated hydrocarbons. The crystallization of the CH3 chain releases a considerable amount of latent heat. The longer the hydrocarbon chain (thus the greater the number of carbon atoms), the higher the phase change temperature and the latent heat of fusion. Paraffins are among the most widely used PCM materials for various positive reasons:

• Stability
• Safety
• Affordability
• Non-corrosiveness
• Low vapor pressure
• They cover a wide range of temperatures

However, they have low thermal conductivity (about 0.2 W/mK), are moderately flammable (with an index between 15 and 50), and exhibit significant volume change during liquefaction.

Non-paraffin organic materials are a broader category of PCM and thus present a greater variety of properties. These can be divided into two subgroups:

• Fatty acids, composed of CmHnO2 where m and n are atomic numbers
• Other non-paraffin organic PCM such as esters, alcohols, and glycols

Unlike paraffins, which have similar properties, each material in this category possesses specific characteristics. But generally, they are characterized by:

• High latent heats
• Low thermal conductivity (like paraffins)
• Low flash points
• Various levels of toxicity
• Instability at high temperatures
• Higher cost compared to paraffins

Inorganic PCM Materials

The second category introduced above is inorganic PCM materials, which include:

• Hydrated salts
• Inorganic compounds
• Metal alloys based on bismuth, lead, tin, or indium

Hydrated salts are the most important and studied category. They can be considered alloys between an inorganic salt and water that form a typical crystalline solid (A) indicated by a general formula AnH2O. The liquefaction and solidification of hydrated salt are actually dehydration and hydration of the salt. Hydrated crystals decompose into anhydrous salt and water, or into a less hydrated salt and water. Disadvantages associated with using PCM as phase change materials include incongruent melting and subcooling. Incongruent melting occurs when not enough water is released during crystallization to completely dissolve the solid phase present. Due to higher density, the less hydrated salts deposit at the bottom, making the phase change increasingly irreversible over time. Subcooling is caused by the low nucleation rate of hydrated salt crystals, resulting in latent heat release at a temperature lower than the melting point. To mitigate subcooling and incongruent melting problems, several solutions can be adopted, such as introducing nucleating agents to promote crystal formation, mechanical agitation, PCM encapsulation, using excess water, confining some crystals in a cold and limited region to act as nuclei, or modifying the system's chemical composition.
Some metals and low-melting metal alloys have been examined for use as heat storage materials because they have advantageous characteristics:

• High thermal conductivity
• Low vapor pressure
• Limited volumetric variation coefficient during phase change
• High latent heat per unit volume (due to high densities), generating interest for specific applications such as cooling laser systems

However, the high cost and latent heat of fusion that tends to degrade over time currently limit their spread.

Eutectic PCM Materials

Let's now examine the third category introduced at the beginning of the article: eutectic PCM materials, which are composed of a mixture of substances with a low melting point, lower than that of the individual substances that constitute it. In a eutectic material, all elements melt and solidify congruently and simultaneously, without segregation. What makes these substances particularly advantageous compared to other types of PCM is:

• Their ability to regulate the melting point through the weighted combination of phase components.
• High thermal conductivity values

However, the latent heat of solidification and specific heat values are much more limited compared to hydrated salts and paraffins.

Summarizing PCM Materials

In conclusion, to summarize, the characteristics of PCM materials can be compared and usefully represented according to the image below. The graph provides an intuitive visual summary of the performance characteristics of the various PCM materials presented.
The organic PCM materials of biological origin chosen by i-TES perfectly align with i-TES's mission of being both environmentally friendly and part of a circular and sustainable economy to ensure maximum energy efficiency objectives. In fact, these bio-PCM possess a fairly good thermal storage capacity, even though it is lower than that of hydrated salts, completely eliminating some phenomena such as subcooling and corrosion, making them easy to manage and maintain.