3.2 Experimental Results Analysis
Based on previous operational experience, during a stable heating season, a heat pump unit typically requires 3-4 hours of continuous operation to reach a steady state [4]. Considering actual heating demands and the operating conditions of the heat pump, intermittent operation technology was applied to compensate for the slow temperature recovery of underground heat exchangers, thereby improving overall system efficiency. In this experiment, a 24-hour cycle was implemented, with manual control of on-off time ratios set at 1:1 and 2:5 [3,4]. All the experimental data presented here are measured results obtained from the field tests.3.2.1 Analysis of Underground Heat Exchanger Wall Temperature Changes
The 24-hour intermittent operation test was conducted from March 16 to March 19, 2005. As shown in Figure 2, the ground temperature recovery condition was very favorable. After 5 hours of operation followed by a shutdown, the pipe wall temperature recovered to approximately 0.2°C. The unit then ran for another 4 hours to meet user heating needs, after which it was shut down again. After 10 hours of downtime, the wall temperature returned to its initial level, providing better heat transfer conditions for the next startup cycle.Figure 2: Intermittent Pipe Wall Temperature Test Results
Fig. 2: Testing Results of Tube Surface Temperature
3.2.2 Changes in Heat Pump Inlet and Outlet Water Temperatures
During normal operation, the heat pump’s outlet temperature must not fall below 4°C; otherwise, the unit will stop running. Therefore, it is essential to ensure that the load meets user requirements while maintaining stable inlet and outlet water temperatures to keep the heat pump operating under ideal conditions. As illustrated in Figures 3 and 4, under intermittent operation, the inlet and outlet water temperatures of the heat pump stabilized at higher levels, which is beneficial for efficient unit performance.3.2.3 Heat Extraction from the Ground Heat Exchanger by the Heat Pump
Due to the heat absorption process of the buried heat exchanger, the soil's temperature field undergoes changes. When the unit is operating, the water temperature inside the pipes differs significantly from the surrounding soil temperature, leading to rapid heat extraction. As the endothermic process continues, the soil temperature gradually decreases. Over time, the heat exchange between the underground pipes and the soil reaches a steady state. Figure 5 compares the heat exchange curves of intermittent and continuous operations. It shows that under the same experimental conditions, the heat exchange capacity of the intermittent operation is significantly higher than that of continuous operation. During continuous operation, the heat exchange capacity reaches about 27.9 W/m within 3-4 hours. However, as the operation time increases, the soil temperature drops, affecting the heat transfer. After 24 hours of continuous operation, the heat transfer rate decreases to 26.5 W/m. In contrast, intermittent operation allows the ground temperature to recover (as shown in Figure 2), enabling more complete heat transfer. The heat transfer rate can be maintained at 27.9 W/m, representing an increase of about 5% compared to continuous operation.Figure 3: Heat Pump Inlet Water Temperature
Fig 3: In-Temperature of Heat Pump
Figure 4: Heat Pump Outlet Water Temperature
Fig 4: Out-Temperature of Heat Pump
Figure 5: Heat Exchange per Meter Depth of Underground Heat Exchanger
Fig 5: Heat Flux per Meter of Ground Source Heat Transfer
4 Conclusion
(1) The controllable intermittent operation can alter the temperature trend around the underground heat exchangers. Under winter operating conditions, it helps improve the equilibrium temperature and enables the unit to operate under ideal conditions. (2) Through reasonable human-controlled intermittent operation, the ground temperature change trend can be optimized, maximizing the heat exchange capacity of the underground heat exchangers. (3) Compared to continuous operation, intermittent operation under the same conditions achieves full heat transfer, reduces the number of drilling operations, and lowers initial investment costs. (4) The intermittent operation technology proposed in this paper holds great significance and practical value for fully utilizing the ground in air-conditioning systems. Further research is needed to explore the energy-saving characteristics of ground-source heat pump units after applying intermittent operation technology.Spare Part Of Fracturing Pump,Concrete Pump Spare Parts,Pump Spare Parts,Spare Part Main Pump
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