*4.1. Continuous Operation*

Figure 8 shows the simulation results of the GHE, which was operated on three consecutive summer days, with respect to the geological and climate conditions corresponding to Adelaide (South Australia). The amount of energy released by the GHE was calculated using Equation (21). It is observed that the lowest value of the outlet fluid temperature could be attained so the highest amount of energy can be released. As an example, Figure 9 provides the profile of the fluid temperature generated by operation of the horizontal, vertical, and the horizontal to the vertical GHE. This figure demonstrated that the outlet fluid temperature increased with the increase in the operation period. The accumulation of heat in the surrounding soil during the operation of the GHE led to a reduction in the heat transfer rate. As is shown in the Figure 9, the profile of the outlet fluid temperature of the vertical GHE was relatively stable when compared with those generated by the horizontal and the series operation modes. This phenomenon occurred because the ambient temperature, which fluctuated diurnally, did not have a significant effect on the performance of the vertical GHE. From Figure 8, it followed that the lowest energy was released when operating the horizontal GHE only. The horizontal GHE released 1002 kW less energy than that released in 3 days by the vertical GHE. The relatively stable temperature of the ground at a deeper layer might enhance the heat transfer capacity of the vertical GHE and led to better thermal performance. The amount of energy released could be increased by using the combined operation mode of the GHE, including the split flow and series operation modes. It can be seen that the series operation mode, from horizontal to vertical, could release slightly higher energy, namely 22 MJ more than the opposite operation mode. While the split flow mode, with a ratio of fluid flow at 50% in the horizontal GHE and 50% in the vertical GHE, released less energy than the series operation mode. The average heat transfer rate was calculated by dividing the amount of energy released with the operation period, namely 72 h. The single arrangement of the horizontal or the vertical GHE generated an average heat transfer rate of 6.3 kW and 10.2 kW, respectively. The combined arrangements produced a relatively higher heat transfer rate namely: 15.7 kW, 15.6 kW, and 15.4 kW for operation modes from the horizontal to the vertical, from the vertical to the horizontal GHE, and by splitting the fluid flow, respectively. Therefore, the horizontal or vertical GHE may be operated when the loading load was relatively low. At peak loads or when heating/cooling demands were relatively high, the combined GHE's operation could have a significant advantage, see Figure 8.

**Figure 8.** Energy released in 3 days and average heat transfer rate of the GHE under continuous operation condition in Adelaide (where the inlet fluid temperature = 50 ◦C, fluid mass flow rate = 0.6 kg/s, length of horizontal GHE = 200 m, length of vertical GHE = 200 m).

**Figure 9.** Profile of fluid temperature of the horizontal, vertical, and horizontal to the vertical mode in Adelaide.
