Numerical investigation on the energy efficiency analysis of PCM enhanced PHC energy pile under building cooling

Energy pile is a novel technology to extract geothermal energy for building air conditioning. However, as the system operates, energy piles are facing the problem of rapid decline in energy efficiency and rapid increase in ground temperature. Phase change material backfill (PCMB), characterized by their substantial latent heat of fusion, elevated energy storage capacity, and minimal temperature fluctuation, are emerging as a novel and promising candidate for the thermal backfill medium in energy pile. Existing studies on PCMB in energy piles have predominantly focused on thermal performance, with limited exploration of their energy efficiency implications and an oversight of the coupled thermal interactions between heat pumps and energy pile systems. In this work, the coupled heat transfer models of energy pile and heat pump were developed and validated using data from in-situ tests to evaluate energy efficiency of prestressed high-strength concrete (PHC) energy pile under different PCM thermal conductivities, PCM transformation temperatures and PCM latent heats. This model established the relationship between the energy pile inlet temperature and the buildings load based on the correlation between energy efficiency and the heat pump inlet temperature (i.e., the energy pile outlet temperature) and iterated over each time step of the model to calculate the variation of energy efficiency. The results demonstrated that (i) PCMB significantly enhanced the thermal performance of PHC energy piles. Compared to traditional backfill (TB), the PCMB exhibited a 12.8 % increase in the energy efficiency ratio (EER), along with reductions of 11.41 % in heat pump power, 10.31 % in power consumption, and 9.10 % in maximum backfill temperature; (ii) EER increased with PCM thermal conductivity and latent heat. As the thermal conductivity rose from 0.64 W/m K to 2.44 W/m K, the EER increased by 78.09 %. As the latent heat rose from 79 kJ/kg to 229 kJ/kg, the EER increased by 6.46 %; (iii) the EER decreased with PCM transformation temperature. As the transformation temperature rose from 24 °C to 30 °C, the EER decreased by 5.80 %; (iv) meanwhile, PHC energy piles are more suited to PCMB than traditional grouted energy piles because the backfill in the center of the pile did not bear additional pressure. Hence, PCM had great potential in reducing power consumption of PHC energy piles. This work will guide the optimal design of energy pile PCMBs.