PCMs: What They Are and How They Store Heat

PCM materials are materials capable of storing significant amounts of latent heat by exploiting the phase change phenomenon, which occurs at a constant temperature. Because of their unique ability to store heat while changing state, these materials are called Phase Change Materials (hence the acronym PCM by which they are known), or phase change materials.

In this article, we will explore the characteristics of the phase change of these materials and their classification into three main categories based on the temperature range in which this transition occurs.

But first, to better understand their nature and use, let's briefly review the main differences between PCM materials and traditional heat storage systems.

Differences Between PCM Materials and Traditional Storage Systems

While traditional heat storage systems involve materials that undergo temperature changes, in phase change thermal storage, latent heat is absorbed/released during the phase transformations of the material used. This allows for the storage and release of energy in reduced volumes, keeping temperature variations and the resulting losses to a minimum.

We also discussed this in the article "Complete Guide to Thermal Batteries: How They Work and What Advantages They Offer", where we introduced the distinction between sensible heat storage systems—those traditionally used—and latent heat storage systems using PCM materials.

The former store heat using materials such as water, sand, rock, and concrete and, for the same amount of energy stored, have a lower energy density with the frequent disadvantage of occupying large spaces. Moreover, these materials undergo greater thermal dispersion because as they release heat, they experience a continuous drop in temperature, which corresponds to a degradation of the useful energy (exergy) supplied to the user.

Latent heat storage systems using PCM materials, despite higher costs, solve these foundational issues: they possess a significantly higher energy density in smaller storage volumes.

How PCM Materials are Classified

Phase Changes of PCM Materials

In the previous article "PCM Materials: An Innovative Solution for Heat Recovery", we began explaining how these materials work, listing the objective benefits their application provides in terms of thermal storage/recovery. Now, let's delve into the technical characteristics of PCM materials, which, while increasingly used in the alternative energy sector, are still little known in their specifics.

As we've seen, a latent heat thermal energy storage system (LHTES) primarily uses the heat absorbed or released by a PCM material during its phase change. PCM materials are classified based on three main phase changes:

Solid-to-solid transitions
Liquid-to-vapor transitions and vice versa
Solid-to-liquid transitions and vice versa

Solid-solid systems show less phase transition and consequently lower values of latent heat. The transition from liquid to vapor has the highest latent heat values but involves a significant volume change between the two phases. The phase transition of a PCM material from solid to liquid, and vice versa, is the most widely used. Indeed, it ensures a high degree of latent heat without volume changes during the phase transition. Due to the suitable temperature range and relative ease of integration this system offers, it is especially widespread in thermal storage applications in equipment and in the building sector.

Main PCMs groups based on temperature range

Regarding classification, PCM materials can be further divided into three main categories based on the temperature range in which the phase change occurs. Specifically, they are distinguished into:

Low-temperature PCMs, where the phase change occurs below 15°C. Typical applications of these materials include cold storage for air conditioning and refrigeration.
Medium-temperature PCMs, where the phase change occurs between 15°C and 90°C. These are widely used for hot water storage, passive use in buildings, and electronic cooling.
High-temperature PCMs, where the phase change occurs at temperatures above 90°C. These are primarily used in industrial settings, solar power plants for energy production, and aerospace applications.

On this note, let's offer a small preview: i-TES exclusively uses organic PCM materials in the 1-80°C range, and we will explain some of the reasons for this choice in the next article, where we will delve into the types of PCM materials available on the market.

Critical issues of PCM materials and solutions

In concluding the first part of our exploration into PCM materials, it is crucial to emphasize the importance of working with solid and certified expertise in the TES (Thermal Energy Storage) field. While PCM systems are innovative for efficiently and sustainably storing heat, they are not without challenges. However, these can be easily addressed if managed correctly. For instance, some organic PCMs have low thermal conductivity; others may be flammable or corrosive. To prevent these issues, it is sufficient to use appropriate materials both for containment and to enhance thermal exchange.

Learn more about the i-TES PCM thermal battery to understand how these solutions are implemented to address the challenges associated with PCM materials. This battery exemplifies the practical application of such materials while mitigating common problems like thermal conductivity and safety risks through innovative design and material choice.

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