Energy storage potential analysis of phase change material (PCM) energy storage units based on tunnel lining ground heat exchangers

Low geo-temperature geothermal energy in the surrounding rock can be extracted by tunnel lining ground heat exchangers (GHEs) and stored in phase change material (PCM) plates to realize the cold energy utilization. However, the research of the coupling heat transfer of tunnel lining GHEs and PCM plates has been rarely reported. In this study, a 3D coupling heat transfer model of tunnel lining GHEs and PCM plates was developed to investigate the cold energy storage performances of PCM plates under different PCM and tunnel parameters. The circulative iteration calculation is used to solve the coupling model. The results show that the cold energy storage of PCM plates utilizing tunnel lining GHEs for energy storage is feasible. The PCM melt fraction and solid phase PCM proportion within the PCM plates decrease and increase respectively with an increase in the PCM thermal conductivity and melting temperature and a reduction in the surrounding rock temperature. The cold energy storage efficiencies of PCM plates improve by 77.8% and 34.1% as the PCM thermal conductivity and melting temperature increase by 1 W/(m K) and 4 & DEG;C. Moreover, the cold energy storage efficiency of PCM plate enhances by 68.5% as the surrounding rock temperature reduces from 10 to 1 & DEG;C. A larger difference between the surrounding rock temperature and PCM melting temperature is efficient for the cold energy storage of PCM plates, and the cold energy storage time and temperature difference show a power function relationship. Finally, increasing the tunnel lining GHEs length is conducive to enhancing the cold energy storage efficiency of PCM plates, whereas this enhancement is limited. In actual applications, selecting a reasonable tunnel lining GHEs length is necessary.