Phase Change Materials (PCMs) are substances that absorb and release energy during a phase transition, typically from solid to liquid and vice versa. They store and release this energy as latent heat, without a temperature change. This property makes them effective for thermal energy storage, managing temperature fluctuations.
How Phase Change Materials Work
PCMs utilize latent heat, which is energy absorbed or released during a change of state without a corresponding temperature change. When a PCM melts, it absorbs a large amount of heat from its surroundings at a nearly constant temperature. This energy breaks molecular bonds, storing latent heat.
In contrast, sensible heat involves a direct relationship between heat addition or removal and a change in temperature, such as when water heats up. A PCM, however, can absorb a substantial amount of energy during its phase change, like ice melting, which requires 333.55 Joules per gram, compared to only 4.18 Joules per gram to raise the temperature of water by one degree Celsius. This allows PCMs to store significant thermal energy in a compact volume.
When the surrounding temperature drops, the PCM solidifies, releasing the stored latent heat back into the environment. This maintains a stable temperature as the material transitions back to solid. This isothermal energy exchange, like a “thermal battery,” is a defining characteristic of PCMs.
The melting process in some PCMs can be faster than solidification due to natural convection within the liquid phase, which aids in heat transfer. During solidification, this convection is reduced as the material becomes solid, potentially slowing the heat release. This mitigates temperature swings and provides thermal regulation.
Practical Applications of Phase Change Materials
PCMs offer diverse benefits across many applications due to their unique thermal storage capabilities. In buildings, PCMs are integrated into walls, floors, and ceilings to passively regulate indoor temperatures. They absorb excess heat when the environment warms, melting and preventing overheating. They then release stored heat when temperatures drop, solidifying and providing warmth. This dynamic process reduces reliance on traditional HVAC systems, leading to energy savings and enhanced occupant comfort.
PCMs are widely used for cooling electronic devices, absorbing heat generated by components like processors. This prevents performance degradation and shortened lifespan. Paraffin waxes are often chosen for this purpose due to their high heat of fusion and tailored melting points, effectively managing thermal fluctuations in compact electronics.
In temperature-controlled packaging, PCMs maintain stable conditions for sensitive goods like pharmaceuticals, vaccines, and food during transit. Unlike conventional ice or dry ice, PCMs can be formulated to melt and freeze at precise temperatures. This ensures products remain within their specified range, preserving quality and complying with cold chain logistics requirements.
PCMs enhance smart textiles by providing personal thermal regulation. Incorporated into fabrics, they absorb body heat during high activity or warm conditions, offering a cooling sensation. Conversely, they release stored heat as the wearer cools down, providing warmth and improving comfort in sportswear, outdoor gear, and specialized garments.
The automotive industry benefits from PCM technology, managing thermal conditions for vehicle cabins and electric vehicle batteries. PCMs can store “cold” during driving to maintain comfort when the engine is off in start-stop systems, reducing fuel consumption. They also regulate battery temperatures, important for extending lifespan and ensuring optimal performance by preventing overheating or excessive cooling.
Choosing the Right Phase Change Material
Selecting a PCM requires careful consideration of properties to match application needs. The most important factor is the desired operating temperature range. The PCM’s melting or freezing point must align with the temperature at which thermal regulation is needed. This ensures effective phase transition for energy absorption or release.
Latent heat capacity, or heat of fusion, quantifies the energy a PCM stores or releases per unit of mass or volume. Higher capacity means less material manages more thermal energy. High density also contributes to compact storage.
Thermal conductivity is important for efficient performance. A PCM needs high thermal conductivity in both solid and liquid states for rapid heat transfer. Many organic PCMs, while suitable, often have low thermal conductivity, limiting charge and discharge rates unless enhanced.
Beyond thermal properties, practical considerations include long-term stability across melting and solidification cycles, ensuring consistent performance. Safety aspects like flammability, toxicity, and corrosiveness are paramount, impacting suitability for different environments and containment. Volume change during phase transition is also considered to prevent encapsulation issues. Cost and availability also play a role, as no single material is universally ideal.