*4.2. Intermittent Operation*

The GHE can also be operated in the intermittent condition to cope with cyclic load conditions. Figure 10 shows the amount of energy released by the GHE operating intermittently in three summer days, see Figures 5 and 6. The GHE ran for 8 h (during the working hours) and was off for 16 h daily. The results display that during 24 h of operation, the horizontal GHE released 41% less energy than the vertical GHE. The combined operation modes of the GHE could increase the amount of energy released, as the contact area, where heat was exchanged with the surrounding soil, increased. It is seen that the series operations could release 40.8% and 39.6% more energy than the vertical mode, for the horizontal to vertical and vice versa modes, respectively. In the split flow mode, the fluid is split to flow with a ratio of 50% in the horizontal GHE and 50% in the vertical GHE. The results demonstrate that the split flow mode released 2.8% less energy than the horizontal to vertical mode. Even though, the amount of energy released in the intermittent operation regime was less than that generated by the continuous operation due to the total operation period in the intermittent condition was less compared with the continuous condition. However, the average heat transfer rates increased. They were 60.1% for the horizontal mode, 68.5% for the vertical mode, 54% for the horizontal to vertical mode, 53.5% for the vertical to horizontal mode, and 52.6% for the split flow mode. In the intermittent operation condition, the deterioration of the ground temperature during the operation hours is possible to recover during the time when the system is switched off. As a result, it increases the heat transfer rate of the GHE and produces lower outlet fluid temperature over the next day's operation as reflected by an example of fluid temperature generated by the vertical to horizontal mode (see Figure 11).

**Figure 10.** Energy released in 3 days and average heat transfer rate of the GHE under intermittent operation in Adelaide (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 11.** Profile of the fluid temperature of the horizontal to the vertical mode in Adelaide.

#### *4.3. Split Flow Operation*

In this section, the effect of the fluid mass flow rate ratio in the split flow operation mode was investigated. The ratio of the fluid mass flow rate varied as follows: 30%:70%; 40%:60%; 50%:50%; 60%:40%; and 70%:30% for the horizontal and the vertical GHE, respectively. From the current numerical simulations, it was found that the ratio of fluid mass flow rate in the split flow mode did not significantly affect the amount of energy released by the GHE, as shown in Figure 12. As an example, Figure 13, presents the outlet fluid temperature of the GHE at three different flow rate ratios, namely: 30%:70%; 50%:50%; and 70%:30% for the horizontal and the vertical GHE, respectively. It is found that the difference in the outlet fluid temperature was relatively small. The highest fluid outlet temperature was yielded by the GHE with a flow rate ratio of 70%:30%. It can be seen that the GHE operated with a flow rate ratio of 70% horizontal and 30% vertical released the lowest amount of energy across three consecutive days of operation, namely, 3% less than the one with a flow rate ratio of 60% horizontal and 40% vertical. The amount of energy released gradually increased with the increase of the ratio of the fluid mass flow rate in the vertical GHE and declined at the ratio of 30% horizontal and 70% vertical

GHE namely, 0.7% less than that with a ratio of 40% horizontal and 60% vertical GHE. This tendency was due to a significant reduction in the fluid mass flow rate in the horizontal GHE (only 30% of the total fluid flow rate). At a lower mass flow rate, the thermal resistance of the horizontal GHE increased as a reduction in fluid mass flow rate was directly proportional to the decrease in the convective heat transfer coefficient between the working fluid and the inner pipe surface. As a result, it decreased the heat transfer rate of the horizontal GHE. In addition, with 70% of mass flow rate in the vertical GHE, the surrounding soil temperature deteriorated quickly due to heat accumulation, leading to degradation in the heat transfer capacity of the vertical GHE. From the Figure 12, it can be seen that the highest heat transfer rate was produced by the GHE with a ratio of flow rate of 40% horizontal and 60% vertical namely, 15,490 kW.

**Figure 12.** Energy released in 3 days by the GHE with different ratio of mass flow rate in Adelaide (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 13.** Profile of the outlet fluid temperature of the GHE (with a split flow operation mode) under different flow rate ratios (Adelaide case).
