## 3 Active Heating Embankment and the DE-GSHP System
### Design of the DE-GSHP System for Embankment Heating  
Figure 4 shows the structure of the proposed DE-GSHP. Except for the ground heat exchangers, all the components were installed in a stainless steel container above the ground surface. The condenser and evaporator were both helically coiled; thus, this geometry promoted heat exchange. In addition, there existed an adiabatic part between the evaporator and condenser to prevent thermal interference. It should be indicated that the condenser height \(H_{\text{cond}}\) and evaporator height \(h_{\text{evap}}\) could be adjusted by the spiral pipe pitches, \(b_{\text{cond}}\) and \(b_{\text{evap}}\), respectively. Figure 3: Artificial heating embankment. (**a**) Helically coiled evaporator and helically coiled condenser, and (**b**) helically coiled evaporator and multi-U-tube condenser. 1. Compressor, 2. throttle valve, 3. helically coiled condenser, 4. helically coiled evaporator, 5. embankment, 6. ground base, and 7. multi-U-tube condenser. In practice, such mesoscale DE-GSHP systems can be widely used along both sides of an embankment, where the space to install ground heat exchangers is usually limited. Figure 5 presents a view plan of the DE-GSHP, in which the evaporator was installed vertically in case of undesirable thermal interference from the evaporator to the upper frozen layer of the embankment. Further, the condenser was installed at a variable inclined angle to reduce the freezing depth and damage induced by the frost heaving in the embankments, to ensure that embankment deformation did not hinder railway operations. In remote areas lacking a power grid, a small-scale photovoltaic system can be used for the \"island\" operations of DE-GSHP systems. A DE-GSHP system has the advantage of operating in a flexible management mode, allowing for adjusting the operating time and heat-supplying temperature output according to the heating requirements of the embankment. Consequently, the appropriate operational management, as well as an intermittent mode, is required for the sustainable and long-term operation of DE-GSHP systems. Figure 4: Detailed structure of the DE-GHPA. 1. Condenser, 2. evaporator, 3. compressor, 4. throttle valve, 5. filter, 6. bracket, 7. bolt, 8. copper block, 9. power source, 10. automatic controller, 11. helically coiled tubes, 12. base tube, 13. retainers, 14. temperature sensor, and 15. signal line. Figure 5: Artificial heating embankment. Generally, a hermetic piston-type compressor was selected for the DE-GSHP system due to its simple structure and robustness to leakages and vibrations. A passive capillary tube was applied in a simple small structure, and was thus highly favorable for the proposed mesoscale heating system. The entire system was controlled by a computer and operated automatically. The controller included integrated functions, including programmable control and regulation, constant temperature control, alarm control, and time programs. Furthermore, the system adopted the temperature control method of return difference. The simple structure, reliable control, and low cost served as advantages of the system. During practical applications, temperature sensors were deployed at the evaporator, condenser, and surrounding soil. When the target temperature was higher than a particular value, the DE-GSHP stopped, while it re-started automatically when the temperature was close to the preset limit value. Subsequently, by repeating the above process, the DE-GSHP achieved heating at a constant temperature, and this temperature could be adjusted along with the additional protection of the surrounding filling materials. Therefore, it could not only exhibit anti-freezing functions, but was also effective in energy saving. The need for high capital investment is a critical factor that limits the application of DE-GSHPs. More specifically, the average price of a conventional system is 1500 RMB.