Phase change materials (PCMs) have emerged as a critical component in thermal energy storage and conversion systems due to their ability to absorb, store, and release large amounts of latent heat during phase transitions. Among these materials, organic PCMs such as paraffin waxes are widely studied for their high energy storage density, chemical stability, and tunable phase transition temperatures. Their integration with thermoelectric devices presents a promising pathway for improving energy harvesting efficiency by leveraging waste heat recovery and thermal management. This article examines the role of PCMs in thermal energy conversion, focusing on cyclic stability and synergistic coupling with thermoelectric systems.

Paraffin-based PCMs exhibit melting temperatures ranging from 20°C to 80°C, making them suitable for low-to-medium temperature energy harvesting applications. Their latent heat capacity typically falls between 150 kJ/kg and 250 kJ/kg, depending on the carbon chain length and purity. The primary advantage of paraffin lies in its negligible supercooling, reversible phase transition behavior, and compatibility with encapsulation materials. However, challenges such as low thermal conductivity (0.2–0.4 W/m·K) and volume expansion during melting necessitate the use of thermal conductivity enhancers like graphite, metal foams, or carbon nanotubes. Studies have demonstrated that dispersing 10–20 wt% of expanded graphite in paraffin can increase thermal conductivity by an order of magnitude while maintaining high energy storage efficiency.

Cyclic stability is a critical parameter for PCMs in energy harvesting applications, where repeated melting and solidification cycles must not degrade performance. Paraffin waxes generally exhibit excellent thermal reliability over thousands of cycles, with studies reporting less than 5% reduction in latent heat after 1,000 cycles. However, phase segregation and degradation can occur in composite PCMs if the supporting matrix is not chemically stable. Encapsulation techniques, including microencapsulation with polymer shells or macroencapsulation in metallic containers, have proven effective in preventing leakage and maintaining structural integrity. For instance, paraffin encapsulated in polyurethane shells retains over 95% of its initial storage capacity after 5,000 cycles, demonstrating long-term durability.

The integration of PCMs with thermoelectric generators (TEGs) enhances energy conversion efficiency by stabilizing temperature gradients across the device. Thermoelectric materials such as bismuth telluride (Bi2Te3) or lead telluride (PbTe) rely on the Seebeck effect to convert heat into electricity, but their performance is highly sensitive to transient thermal fluctuations. By embedding PCMs on the hot side of a TEG, the system can buffer temperature variations, ensuring a steady heat flux. Experimental results show that a paraffin-based PCM layer can improve TEG output power by 15–30% under intermittent heat sources by prolonging the temperature differential. The optimal thickness of the PCM layer depends on the heat input rate and melting characteristics, with 5–10 mm being typical for low-power applications.

Hybrid systems combining PCMs and thermoelectrics have been explored for waste heat recovery in automotive, industrial, and solar applications. In solar thermoelectric generators (STEGs), PCMs store excess thermal energy during peak sunlight hours and release it during low-insolation periods, enabling continuous power generation. A study using erythritol as a high-temperature PCM (melting point ~118°C) coupled with Bi2Te3 TEGs demonstrated a 22% increase in daily energy output compared to systems without PCM. For automotive exhaust heat recovery, paraffin composites with melting points around 50–60°C have been integrated into TEG modules, achieving up to 12% improvement in net power generation under real driving cycles.

Material compatibility and interfacial thermal resistance remain key challenges in PCM-thermoelectric integration. The large volume change of PCMs during phase transitions can induce mechanical stress on thermoelectric modules, potentially leading to delamination or microcracks. Solutions include the use of compliant interfacial layers such as thermally conductive elastomers or metallic foams that accommodate expansion while maintaining thermal contact. Additionally, the mismatch in thermal conductivity between PCMs and thermoelectrics can create bottlenecks in heat transfer. Graded composite structures, where the PCM is blended with progressively higher fractions of conductive fillers near the TEG interface, have shown promise in reducing thermal resistance without compromising energy storage density.

Future advancements in PCM-enhanced thermoelectric systems will likely focus on multifunctional composites that combine energy storage, thermal transport, and mechanical resilience. Nanostructured PCMs, such as paraffin infiltrated into graphene aerogels, offer simultaneous improvements in thermal conductivity (up to 5 W/m·K) and shape stability. Phase change slurries, where microencapsulated PCM particles are suspended in heat transfer fluids, enable dynamic thermal management in large-scale systems. For high-temperature applications, metallic PCMs like aluminum-silicon alloys are being investigated for integration with oxide thermoelectrics, though their higher cost and weight pose challenges.

The environmental impact and lifecycle analysis of PCM-thermoelectric systems must also be considered. Paraffin waxes are derived from petroleum, prompting research into bio-based alternatives such as fatty acids or sugar alcohols. These materials exhibit comparable thermal properties with the added benefit of renewability, though their long-term stability requires further validation. Recycling strategies for end-of-life PCM composites, particularly those containing conductive additives, are essential to ensure sustainability.

In summary, the strategic incorporation of PCMs into thermoelectric energy harvesting systems addresses critical limitations related to intermittent heat sources and thermal fluctuations. Through optimized material selection, encapsulation techniques, and system design, these hybrid solutions can significantly enhance the efficiency and reliability of waste heat recovery technologies. Continued research into advanced composites, interfacial engineering, and scalable manufacturing will further unlock the potential of PCM-thermoelectric integration for sustainable energy applications.