Addressing energy issues associated with high demand and utilization of energy demands efficient energy-saving strategies. Renewable energy sources are promising alternatives for promoting efficient energy usage and minimizing inherent environmental problems associated with non-renewable sources. Unfortunately, they are highly intermittent and unstable and thus cannot be connected to the grid directly. This problem can be addressed by using effective energy storage technologies such as thermal energy storage. In particular, phase change materials (PCM) are commonly used in various energy storage devices due to their remarkable isothermal, high energy density storage, and high latent heat properties. However, PCMs have low thermal conductivity limiting their heat storage efficiency and capacity.
Numerous studies have been conducted to improve the heat transfer efficiency of PCMs using different methods. For instance, composite PCMs formed by depositing PCMs into dispersed pores of metal foams exhibit improved heat transfer and storage properties. Additionally, thermal-enhanced composite PCMs have broad heat transfer and storage applications. This has resulted in extensive studies of heat transfer mechanisms and effective thermal conductivity (ETC) of metal/foam paraffin composite PCMs. Besides, various theoretical models have been developed to predict the ETC of the composites with high accuracy. However, most of the existing studies concentrate on porosity, with few focusing on the effects of internal voids and morphology of the copper ribs on internal heat transfer and thermal conductivity.
Herein, researchers from the Institute of Engineering Thermophysics, Dr. Xiao Yang, Dr. Xinghua Zheng, Dr. Zheng Yang, Dr. Ye Bai, and led by Professor Haisheng Chen, investigated the effects of morphology, internal voids and pore sizes of copper ribs on the thermal conductivity and heat transfer performance of copper foam/paraffin composite PCMs. A Hot Disk thermal conductivity meter was used to measure the ETC of the composites. The results were compared with the theoretically calculated ETC, and the differences were established through industrial CT scanning technology. Furthermore, the authors compared and analyzed the effects of pore size and different copper morphology structures on the heat transfer performance of five different copper structures using lumped parameter methods to calculate the thermal homogeneity and heat storage rate of the composite PCMs. The work is currently published in the journal, International Journal of Heat and Mass Transfer.
Results showed that the ETC of the composite decreased with an increase in the porosity. The thermal conductivity was significantly affected by the internal voids of copper ribs, decreasing the difference between theoretical and experimental data from 23.1% to 0.43%. The characteristic internal length of the PCM composites was identified as a key factor affecting the heat storage rate, which decreased with an increase in the porosity. Moreover, the characteristic length decreased with an increase in the pores per inch at the same porosity. The triangular prism structure exhibited improved thermal homogeneity and heat dissipation rate than other structures.
In summary, the effects of the pores per inch, internal voids, and porosity of the copper ribs on the thermal conductivity of composite PCMs as well as the effects of the copper morphology, pore sizes and porosity on the thermal homogeneity and heat storage rate of the composites were analyzed. The study was the first to use the industrial CT scan technique to resolve the discrepancies between the calculated and experimental data. The internal heat storage rate and internal thermal homogeneity were highly related to the various factors: porosity, copper morphology, pore sizes and internal voids. Therefore, to effectively apply copper foam/paraffin PCM composites to different thermal energy storage systems, these factors must be selected based on the application conditions to optimize the design and improve heat storage. In a statement to Advances in Engineering, lead author Professor Haisheng Chen said their study would facilitate the design of high-performance thermal energy storage systems.
Yang, X., Zheng, X., Yang, Z., Bai, Y., & Chen, H. (2020). Effects of morphology and internal voids of copper ribs on heat transfer performance in copper foam/paraffin composite phase change materials. International Journal of Heat and Mass Transfer, 152, 119526.