*4.5. Variations of the Fluid Mass Flow Rate*

Figure 16 shows the effect of the fluid mass flow rate on the amount of energy released by the GHE in 3 summer days in Adelaide. The performance of the GHE under two different fluid mass flow rates namely, 0.6 kg/s and 1 kg/s, was compared. The results of the simulation indicate that the amount of energy released by the GHE increased as the fluid flow rate increased. This tendency occurred because the mass flow rate affected the convective heat transfer coefficient of the fluid inside the GHE. The higher the mass flow rate, the higher the coefficient convective heat transfer was attained. As a result, it increased the heat transfer capacity and the amount of energy released by the GHE. As an example, Figure 17 shows the profile of the fluid temperature generated from the horizontal, vertical, and horizontal to vertical modes at two different mass flow rates namely, 0.6 kg/s and 1 kg/s. This figure shows that the outlet fluid temperature increased with the increase of fluid mass flow rate. This tendency occurred because at a higher mass flow rate, the time period during which the fluid makes contact with the pipe was shorter when compared to a relatively low flow rate. In addition, at a higher mass flow rate, the GHE released more heat into the surrounding soil and led to a quick increase in soil temperature, resulting in increased outlet fluid temperature. Varying the mass flow rate from 0.6 to 1 kg/s increased the amount of energy released by 2.4% for the horizontal mode and 3.3% for the vertical mode. The rate increases were 4.9%, 5.1%, and 5.6% for the horizontal to vertical, vertical to horizontal, and split flow mode, respectively.

**Figure 16.** Energy released in 3 days by the GHE with different mass flow rate in Adelaide (where the inlet fluid temperature = 50 ◦C, length of horizontal GHE = 200 m, and length of vertical GHE = 200 m).

**Figure 17.** Profile of fluid temperature of the horizontal, vertical, and the horizontal to vertical modes with variations in the fluid mass flow rates, in Adelaide.
