Numerical simulation of heat transfer through rock envelope of a plant factory in an underground mine tunnel

• Conventional plant factories are energy intensive and affected by surface climate. • Underground mine tunnels provide suitable environmental conditions for plant growth. • Rock envelope reduces HVAC loads of underground plant factories (UGPF). • No research exists on heat transfer through surrounding rock envelope of the UGPFs. • This study developed a transient heat transfer model to predict HVAC loads in UGPF. Conventional plant factories (indoor vertical farming) are energy intensive and affected by surface climate whereas underground mine tunnels have a stable environment, and heat transfer through the surrounding rock envelope is expected to reduce cooling and heating loads of the environment control systems of the plant factories. The aim of this study is to numerically analyze short and long term heat transfer through the surrounding rock envelope for an underground plant factory (UGPF) located in a mine tunnel using ANSYS Fluent software. The model is compared with analytical results with discrepancies of 2.5 and 2.7 % for light and dark periods, respectively, indicating the numerical model is acceptable for further utilization in thermal modeling of the UGPF. For the base case of 284 K virgin rock temperature, 2.75 W/m.K rock thermal conductivity, 303 and 297 K indoor air temperature in light and dark periods, the time weighted average overall heat transfer coefficient is high initially with 4.04 W/m 2 ·K, but reduces rapidly in the first 6 months to 0.75 W/m 2 ·K, then slows down to 0.48 W/m 2 ·K in 3 years when the system almost reaches the equilibrium status. The heat transfer rate and overall heat transfer coefficient reduction with time follow power relationships. The results indicate that long term heat loss to rock envelope instead of initial values should be used in determining the cooling capacity of the plant factory considering the worst-case scenario. The sensitivity analysis shows that the rock thermal conductivity is more influential on heat transfer rate than indoor air temperature and virgin rock temperature, the latter two have similar levels of influence.