Review of analytical models for heat transfer outside borehole of vertical ground heat exchangers

Based on the assumed shapes of heat source, divides the analytical models for heat transfer outside borehole into three types such as line heat source model, surface heat source model and solid heat source model. Compares and analyses the three types of models. The results show that for practical design they are almost the same. The comparison between the infinite and finite heat source models indicates that obvious difference between the two types of models can be observed only after a long time of heat transfer process , and the influence of the heat exchanger’s length can be neglected with a balance of thermal load. Without the seepage in the ground, if the thermal load is kept balanced, the infinite line heat source model should be applied in practical design because it is simple. If the seepage velocity reaches 10-6 m/s, its effect should be considered.