In the present paper, a simple but accurate transient model to evaluate the thermal performance of earth to air heat exchangers (EAHEs) is developed. Governing equations for both pipe flow and soil surrounding the pipe are derived for the transient state and are solved using Laplacian transform. To validate the analytical model, the 3D numerical model of the EAHE is also simulated. Results of the analytical model and numerical simulations are compared with already reported experimental data. In heating period at velocity of 2 m/s, the discrepancy between the analytical model and reported experimental results is 0.87% and 1.31% for steel and PVC pipes, respectively. For velocity of 5 m/s this was as low as 0.4% for both pipe materials. For the cooling condition, a maximum discrepancy occurs for PVC pipe at 2 m/s equal to 9.55%. Analytical results were compared with numerical simulations for both constant and time-varying inlet temperatures, and great correlation was observed. Thermal diffusion into the soil was also investigated after 6 h and 120 h of continuous operation. Results revealed that for the fluid velocity of 5 m/s, after 6 and 120 h of continuous operation, heat diffused up to 10 cm and 50 cm distance of the pipe axis, respectively, which showed the thermal saturation of the soil with passing the time in continuous operation mode. Results show that thermal saturation of the soil should be taken into the model as ignoring it can cause an overestimation in system performance.
