## 2 Progress in Enhanced Geothermal System Based on Excavating
The function of the heat exchange tunnels is to gather the high-temperature fluid flowing out of the heat extraction zones and provide a place for the second-stage heat exchange of \"fluid-fluid.\" The maintenance tunnels connect the connection tunnels with the heat extraction and heat exchange tunnels to facilitate access for maintenance. The entrance gates of tunnels are provided with multiple holes for various types of pipes to pass through, facilitating connections to the outside. The piping system refers to all kinds of pipelines located in the shaft, connection tunnels, and heat exchange tunnels, mainly including 1# injection pipeline, 2# injection pipeline, collection pipeline, and heat exchange pipeline. The drive system mainly includes fluid pumps and other equipment, and its function is to promote the working fluid to circulate in the heat extraction zone and heat exchange tunnels. According to needs, the basic unit geothermal energy extraction can be arranged around the shaft and the connecting tunnel in any combination to form a larger-scale geothermal reservoir. ### Physical Components of EGS-E  
The system operation process reflects the heat exchange process. The extraction of deep geothermal energy is accomplished through a two-stage heat exchange: \"fluid-rock\" and \"fluid-fluid.\"  
For the first-stage heat exchange process, the 1# working fluid is transported through the 1# injection pipeline to heat extraction zones and heat exchange tunnels. When the 1# working fluid fills the above spaces, then the 1# working fluid injection is stopped. The  
Figure 4: Schematic illustration for cross-section structure with regards to a heat extracting tunnel. drive devices outside the gates are opened to drive the 1# working fluid to start circulating in the above spaces and harvest heat from the hot tunnel walls and gravel. After full heat exchange, the 1# working fluid converges into the heat exchange tunnels, thus completing the first-stage heat exchange. For the second-stage heat exchange process, the 2# working fluid is injected into the spiral heat exchange pipes through the 2# injection pipeline, and the 2# working fluid in the spiral heat exchange pipe exchanges heat with the 1# working fluid filled in the heat exchange tunnels and vaporizes. Due to the thermosiphon effect, the vaporized 2# working fluid floats to the ground through the collection pipe under the action of thermal buoyancy and enters the heat utilization equipment. After utilization is completed, the 2# working fluid is cooled and liquefied and flows into the spiral heat exchange pipe through the 2# injection pipe again under the action of gravity to perform self-circulation and reciprocate in this way to realize the second-stage heat exchange. In summary, during the geothermal energy extraction process of EGS-E, the heat is first transferred to the 1# working fluid through the hot tunnel walls and gravel. Secondly, the 1# working fluid acts as a carrier medium to collect heat to the heat exchange tunnels. Finally, the heat is transferred to the 2# working fluid in the spiral heat exchange pipe and then to the utilization equipment located on the ground. The flowchart of heat transfer in EGS-E is shown in Figure 5.