*4.4. Climate Condition*

The performance of the GHE installed in a temperate climate corresponding to Adelaide was compared with that installed in a subtropical climate, such as near Brisbane. Figure 14 summarises the performance of the GHE, which was installed in two different regions, with different climate conditions. It was found that during 72 h (3 days: 10–12 January) of the continuous operation, the GHE installed in Adelaide's temperate climate could release 34.3% and 26.9% more energy compared with that installed in Brisbane, for the horizontal and vertical mode, respectively. While for the combined modes including horizontal to vertical, vertical to horizontal, and split flow were 31.7%, 31.8%, and 31% higher, respectively. It is observed that operating the vertical GHE only, in the temperate climate, Adelaide, could release an amount of energy that was almost the same as that released by the combined operation mode, in the subtropical climate. This tendency occurred because the climate condition influenced the initial soil temperature in both regions. The initial soil temperature in Adelaide was lower than that in Brisbane, as seen in Figure 5. In addition, the difference in soil types in both locations contributed to the heat transfer capacity of the GHE. As a result, the GHE installed in a temperate climate, Adelaide, produced a lower outlet fluid temperature as demonstrated in Figure 15.

**Figure 14.** Energy released in 3 days by the GHE under different climate conditions (where the inlet fluid temperature = 50 ◦C, fluid mass flow rate = 0.6 kg/s, length of horizontal GHE = 200 m, and length of vertical GHE = 200 m).

**Figure 15.** Profile of fluid temperatures of the horizontal, vertical, and the horizontal to vertical modes under different climate conditions.
