Performance evaluation of a novel nano-enhanced phase change material for thermal energy storage applications

Ground source heat pump systems (GSHP) are one of the sustainable energy resources that provide heating and cooling to buildings. A stable ground temperature has a key role in the performance of the GSHP, in which the overuse (extract/release) of the underground energy can destabilize the ground temperature, leading to the system's failure. A GSHP's performance can be improved by stabilizing the ground temperature, which can be achieved by adding thermal energy storage (TES) to the system. This study aims to present a potential solution to improve the thermal performance of the GSHPs by coupling them to a latent heat TES system. PCMs are widely used for latent heat TES application; however, their poor thermophysical properties are a drawback, so adding nanoparticles has been considered as one of the solutions to address this drawback and improve PCM's properties. This study investigated the thermal performance of a nano-enhanced phase change material (NE-PCM) as an underground TES by developing a finite element numerical model and validation of the experimental apparatus. To this end, PDA@hBN/MXene as a thermal enhancer has been used to develop the NE-PCM. Then, the NE-PCM was utilized in a storage tank with a diameter of 30 cm and a height of 60 cm. The experimental apparatus consists of 8 NE-PCM pipes and four borehole heat exchangers. The experimental temperature of the NE-PCM pipes was used to validate the numerical model for heating and cooling the system within 0.3 °C and 0.2 °C, respectively. Then, the numerical model was used to study the potential of the proposed TES in three scenarios, including no PCM, base PCM, and NE-PCM, with both parallel- and serial-connected heat exchangers. The total heat transfer with NE-PCM was increased by 38.4 % compared to no PCM case and 24.6 % compared to the base PCM. Lastly, the results of three different flow rates showed up to 11.5 % enhancement of the thermal storage efficiency by increasing the flow rates. The results show the potential of using the new NE-PCM in GSHP applications, adding the NE-PCM as the thermal storage medium can improve the performance of the hybrid TES-GSHP system.