A phase change material (PCM) is a substance that absorbs and releases thermal energy over a period of time. PCMs work by undergoing the processes of melting and solidifying to store and dispense heat. Thermal engineers use these materials in a variety of applications, including thermal insulation and thermal management.
These substances typically have a very high latent heat capacity. During these phase changes, latent heat is either absorbed or released — PCMs excel at harnessing this phenomenon. Engineers use PCM solutions to control heat transfer and achieve wide-ranging thermal objectives.
PCMs need to manage the release and absorption of latent heat effectively and sustainably. Hydrated salts and paraffin wax are two of the most common PCMs. In high-temperature settings, metal alloys are popular.
Thermal Energy Storage
Thermal energy storage refers to technology that captures heat in a medium. The system then dispenses the stored latent heat energy at a later time. For example, batteries store and dispense electrical power. Thermal energy storage systems do the same with heat.
Phase change materials are popular components in thermal energy storage systems. There are sensible heat storage (SHS) and latent heat storage (LHS) strategies. LHS holds an edge over SHS in that it may store and release far more heat energy across a given temperature difference.
Effective PCMs reliably change their phase with a given energy input. They release this energy when the phase change reverses.
Thermal engineers are on an ongoing quest for the best designs and PCMs for their systems. One essential factor engineers must consider is the number of thermal cycles required for the system. Some heat storage systems must cycle thousands of times, and this impacts their design requirements.
Engineers also incorporate other tools into these systems, including forced convection tools. For example, fins may dissipate heat, while venting may reduce the unit’s size and weight.
PCMs and Thermal Engineering
Effective PCM heat sinks can store thermal energy while remaining at a relatively stable temperature themselves. Engineers often place PCMs in hermetically sealed capsules or other enclosures, which maximize their effectiveness.
Absorbing heat via PCM is a straightforward concept. However, effective system designs demand engineering expertise. It is vital to select the correct PCM for each application. Then, manufacturers must adhere to specifications during volume production.
Types of Phase Change Materials
An excellent PCM will score high in multiple categories. Among them are the heat of fusion, thermal conductivity, and specific heat. A good PCM will demonstrate consistent freezing behavior and stand up to repeated cycling.
Melt temperature, weight, corrosion resistance, and cost are other considerations. To identify an ideal PCM, engineers must weigh the relative importance of these different parameters.
Categories of PCMs include organic, inorganic, and eutectic materials. Each has its advantages.
- Organic PCMs tend to be less prone to phase separation and are less corrosive.
- Inorganic materials often deliver higher storage capacities and better thermal conductivity.
- Eutectics combine two or more PCMs.
Engineers customize formulations to operate at specific melting points. Engineers may use micro-encapsulation to enhance thermal and mechanical performance. Encapsulated PCMs reduce overheating in everything from buildings to sportswear.
Common PCMs include hydrated salts, paraffin wax, non-paraffin organics, and metallics.
Hydrated salts
Salt hydrates are water-based PCMs. Their melt temperatures are higher than the melting temperature for water, which is 0°C. Some salt hydrates have melting temperatures as high as 117°C.
Hydrated salts are attractive as PCMs for various reasons, including:
- Hydrated salts have a high heat of fusion.
- Volume changes between the solid and liquid phases are modest.
- They are typically less expensive than other PCM options.
- They are not flammable.
However, there are disadvantages. Some downsides to salt hydrates include:
- Hydrated salts are corrosive.
- They are not always reliable for thousands of cycles.
Hydrated salts are popular in large thermal storage applications such as solar heating. They are also common in consumer products such as pre-filled ice packs.
Paraffin wax
Paraffins are one of the most commonly used PCMs, typically in either encapsulated or shape-stable designs. Paraffin wax melt temperatures range from 6°C to 108°C, although they usually fall between 46°C and 68°C. Benefits of paraffin wax include:
- Paraffin wax offers a high heat of fusion.
- It has a high latent heat capacity.
- It is chemically inert.
- It is non-corrosive.
- Thermal cycling is dependable.
Paraffin wax is popular in energy storage systems and electronics thermal management.
One downside to paraffin wax is that it is flammable and not well-suited to high-temperature applications.
Non-paraffin organics
Non-paraffin organics also excel as phase change materials. Examples include fatty acids, polyethylene glycol (PEG), polyalcohol, and polyethylene. However, non-paraffin organic PCMs are not suitable for high-temperature applications. They have a low flash point, making them highly flammable.
Metallics
Metal alloy PCMs have a vast range of melting temperatures from 150°C to 800°C. Metallics are effective in transient high-power settings to quickly move heat away from critical devices. They are popular in advanced electrical generation, for example, in concentrated solar power (CSP) plants.