## 4 Superiority and Challenges  
This study proposed a new geothermal extraction method of EGS-E and investigated the performance of its core heat extraction structure over a 20-year operating cycle using numerical simulation. In summary, the advantages of the proposed system are as follows. The volume of the geothermal reservoir is customizable. The volume of geothermal reservoirs of EGS is affected by hydraulic fracturing, making it difficult to form geothermal reservoirs in scale and difficult to accurately control the volume of geothermal reservoirs. The EGS-E uses a tunnel system to delineate the geothermal reservoir. Adjusting the structural parameters of the tunnel system, such as length, diameter, quantity, etc., to establish the different volumes of geothermal reservoirs more precisely to meet different requirements. The heat extraction rate can be adjusted. The circulation rate of the working fluid plays an important role in the rate of heat extraction. EGS cannot even guarantee a stable output flow rate, so the controllable circulation speed is more difficult to achieve, and thus it is impossible to realize the adjustment of the heat extraction rate. For EGS-E, the heat extraction rate can be adjusted in a controlled manner according to changes in external demand. The heat extraction rate is regulated by adjusting the driving device to control the circulation rate of 1\(\mu\) working fluid or by adjusting the 2\(\mu\) working fluid injection rate. Low energy is consumed to operate the system. For EGS, the working fluid must overcome huge resistance when flowing in hydraulic fracturing fractures and HDR with low permeability. In contrast, for EGS-E, 1\(\mu\) working fluid flows in the tunnels and the large fractures formed by gravel accumulation with minimal flow resistance, thus significantly reducing frictional energy consumption in operation. Good safety and environmental friendliness can be manifested. The high safety of EGS-E is reflected in the high level of its own safety and the low risk to the outside environment. In terms of its own safety, considering the convenience of tunnel construction, tunnel support methods such as wet shotcrete support, bolt support, etc., can be used to reinforce tunnels and shafts in the EGS-E structure. In terms of low risk to the outside environment, EGS-E does not induce earthquakes as it does not require hydraulic fracturing. The mining industry has triggered small micro-earthquake events during its development, but large-scale earthquakes have never been reported. The tunnel walls are sealed by a low permeability coating so that working fluids do not leak and therefore do not pollute the groundwater. As with other new technologies, EGS-E also faces many challenges. In terms of technical feasibility, how to implement it under the current technology is a problem, such as excavating tunnels in high temperatures and ultra-deep rock mass and maintaining their long-term stability. The Mponig gold mine reaches a depth of over 4350 m, which shows us that it is possible to build tunnel structures kilometers underground [42]. However, the rock temperature at this mine is only 60 \({}^{\circ}\)C, whereas HDR often exceeds 200 \({}^{\circ}\)C. Consequently, deep excavation remains a challenge, especially with the presence of ultra-high temperatures. Recently, the German company of Herren Knecht has developed a shaft boring machine that can drill tunnels with a diameter of 7-12 m in hard rock formations with the in-situ stress level of up to 120 MPa [43]. This brings us inspiration. Intelligent mining robots are rapidly iterating and evolving and can be expected to gradually replace humans for deep, high-temperature underground work as their accuracy and power increase. In addition, the efficient transfer of heat from the ground to the surface is also a problem that EGS-E needs to face. In 2019, Evor's project demonstrated the effectiveness of heat transfer based on the thermosiphon principle [44]. The depth of the borehole reached 2400 m, and the working fluid was water.