Introduction

With rising global energy demand and environmental concerns, renewable energy sources, particularly solar thermal energy, have gained significant attention. Solar thermal energy systems are highly effective for sustainable power generation but face limitations due to the intermittent nature of sunlight. Heat storage systems are essential in overcoming this limitation, and phase change materials (PCMs) are being explored to store and release thermal energy efficiently (Wang et al., 2022). In hybrid heat storage systems, which integrate different energy storage technologies, PCMs offer a promising solution for improving thermal performance, reducing energy losses, and ensuring continuous energy supply even when sunlight is unavailable. This article evaluates the role and performance of PCMs in hybrid heat storage systems for solar applications, examining their benefits, challenges, and future potential.

2. Understanding Phase Change Materials (PCMs)

PCMs absorb and release significant latent heat during phase changes, usually between solid and liquid states. This latent heat enables PCMs to store substantial energy in a compact volume, making them highly effective for thermal energy storage applications (Nie et al., 2020). PCMs enhance storage capacity, thermal stability, and energy efficiency when integrated into hybrid heat storage systems.

2.1 Advantages of PCMs in Hybrid Heat Storage Systems

• High Energy Density: PCMs offer high energy storage density, allowing more energy storage in a smaller space, ideal for compact systems.
• Isothermal Operation: PCMs absorb or release heat at a constant temperature, which aids in maintaining steady output temperatures in heat storage systems.
• Improved Efficiency in Hybrid Systems: By combining PCMs with other storage technologies (e.g., sensible heat storage materials), hybrid systems achieve enhanced thermal stability and reliability.

2.2 Role of Hybrid Heat Storage Systems in Solar Thermal Energy Applications

Hybrid heat storage systems integrate multiple energy storage mechanisms, combining sensible heat storage (SHS) and latent heat storage (LHS) with PCMs (Suresh & Saini,  2020). This combination leverages the strengths of each mechanism, allowing efficient storage and release of thermal energy while minimizing the shortcomings of each storage method.

In solar thermal applications, hybrid systems that incorporate PCMs provide several advantages:

• Extended Heat Storage Duration: By storing thermal energy from sunlight during the day, PCMs enable the release of stored energy during non-solar periods, reducing dependency on sunlight availability.
• Enhanced Thermal Stability: The combination of SHS and LHS with PCMs allows hybrid systems to handle fluctuations in solar energy, ensuring a stable and continuous heat output.
• Cost and Space Efficiency: Hybrid systems maximize energy storage capacity without significantly increasing costs or requiring larger spaces, making them suitable for residential and industrial applications.

2.3 Evaluating Thermal Performance of PCMs in Hybrid Heat Storage

The effectiveness of PCMs in hybrid heat storage systems depends on their thermal performance, which is influenced by factors like melting temperature, thermal conductivity, and storage capacity (Liu et al., 2022). Optimizing these parameters is essential to maximizing PCM efficiency in solar applications.

• Melting Temperature: Selecting a PCM with an appropriate melting temperature for the desired operating range ensures optimal energy storage and release. PCMs with melting points around 50–100°C are commonly used for solar applications.
• Thermal Conductivity: PCMs with high thermal conductivity are preferred to enhance the heat transfer rate. Advanced techniques, such as adding conductive nanoparticles, can improve PCM thermal conductivity and performance in hybrid systems.
• Energy Storage Capacity: PCMs with higher energy density can store more thermal energy, allowing hybrid systems to operate efficiently with smaller storage volumes. By increasing the energy density of the PCM, the overall thermal capacity of the hybrid system is significantly enhanced.

2.4 Challenges and Future Prospects of PCM-Based Hybrid Systems

While PCMs offer numerous advantages, several challenges must be addressed to maximize their efficiency in hybrid heat storage systems. Key issues include thermal cycling stability (Liu et al., 2020), phase separation, and high material costs. Overcoming these challenges through innovative research and advanced materials will enable the broader adoption of PCM-based hybrid systems in solar thermal energy applications.

• Thermal Cycling Stability: Repeated heating and cooling can lead to the degradation of PCMs, reducing their effectiveness over time. Research into PCMs with high thermal stability and long cycle life is ongoing to enhance their durability.
• Phase Separation and Leakage: Some PCMs may experience phase separation or leakage, affecting performance and reducing storage efficiency. Encapsulation techniques are being developed to prevent leakage and ensure long-term reliability.
• Cost of Advanced PCMs: The high cost of some PCMs, especially those encapsulated or enhanced with additives, is a barrier to widespread adoption. Developing cost-effective and high-performance PCMs will make hybrid heat storage systems more accessible.

3. Conclusion

Integrating PCMs in hybrid heat storage systems holds great potential for enhancing the efficiency and reliability of solar thermal energy applications. By improving energy density, thermal stability, and overall efficiency, PCM-based hybrid systems enable the practical storage and release of solar energy, addressing one of the primary limitations of solar power. While challenges remain, advancements in PCM technology, encapsulation techniques, and cost-effective solutions will drive the future development of PCM-based hybrid systems. As solar thermal energy continues to grow as a sustainable power source, PCM-enhanced hybrid systems will play a pivotal role in optimizing energy storage and supporting the transition to renewable energy.

4. References

1. Wang, X., Li, W., Luo, Z., Wang, K., & Shah, S. P. (2022). A critical review on phase change materials (PCM) for sustainable and energy efficient building: Design, characteristic, performance and application. Energy and buildings, 260, 111923.
2. Nie, B., Palacios, A., Zou, B., Liu, J., Zhang, T., & Li, Y. (2020). Review on phase change materials for cold thermal energy storage applications. Renewable and sustainable energy reviews, 134, 110340. 
3. Suresh, C., & Saini, R. P. (2020). Thermal performance of sensible and latent heat thermal energy storage systems. International Journal of Energy Research, 44(6), 4743-4758.
4. Liu, Y., Zheng, R., & Li, J. (2022). High latent heat phase change materials (PCMs) with low melting temperature for thermal management and storage of electronic devices and power batteries: Critical review. Renewable and Sustainable Energy Reviews, 168, 112783.
5. Liu, J., Duan, Q., Ma, M., Zhao, C., Sun, J., & Wang, Q. (2020). Aging mechanisms and thermal stability of aged commercial 18650 lithium ion battery induced by slight overcharging cycling. Journal of power sources, 445, 227263.