3.2 Experimental Results Analysis
Based on existing 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 operational conditions of the heat pump, intermittent operation technology was employed to compensate for the slow recovery of underground heat exchangers and improve overall system efficiency. In this experiment, a 24-hour cycle was used, with manually controlled on-off ratios of 1:1 and 2:5 [3,4]. All the experimental data presented here are based on actual measurements.3.2.1 Analysis of Underground Heat Exchanger Wall Temperature Changes
The 24-hour intermittent operation test took place 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 1 hour of shutdown, the pipe wall temperature recovered to approximately 0.2°C. The unit then ran for an additional 4 hours to meet user heating requirements, after which it shut down again. Following a 10-hour shutdown period, 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: The 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 should not fall below 4°C; otherwise, the unit will stop running. Therefore, it is essential to ensure that the load meets user needs and that the inlet and outlet water temperatures remain as stable as possible to maintain the heat pump in ideal operating conditions. As shown in Figures 3 and 4, under intermittent operation, both the inlet and outlet water temperatures of the heat pump stabilized at higher levels, which is beneficial for the unit's efficient operation.3.2.3 Heat Extraction from the Ground Heat Exchanger by the Heat Pump
Due to the heat absorption by the buried heat exchanger, the soil temperature field changes over time. When the unit is operating, the water temperature in the pipes differs significantly from the soil temperature, leading to rapid heat extraction. However, 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 shows the heat exchange curves of the heat exchanger under both intermittent and continuous operation. As seen in the figure, the heat exchange capacity under intermittent operation is significantly higher than that under continuous operation under the same experimental conditions. During continuous operation, the heat exchange capacity reaches about 27.9 W/m within 3–4 hours, but as the operation time increases, the soil temperature drops, reducing the heat exchange efficiency. After 24 hours of continuous operation, the heat transfer rate decreases to 26.5 W/m. In contrast, the intermittent operation allows the ground temperature to recover (as shown in Figure 2), enabling more complete heat transfer between the soil and the heat exchanger. This ensures a consistent heat transfer rate of 27.9 W/m, representing a 5% improvement compared to continuous operation.Figure 3: Heat Pump Inlet Water Temperature
Fig 3: The In-Temperature of Heat Pump
Figure 4: Heat Pump Outlet Water Temperature
Fig 4: The Out-Temperature of Heat Pump
Figure 5: Heat Exchange per Meter Depth of Underground Heat Exchanger
Fig 5: The 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 allows the unit to operate under ideal conditions. (2) With reasonable manual control of the intermittent periods, a favorable trend in ground temperature changes can be achieved, maximizing the heat exchange capacity of the underground heat exchangers. (3) Compared to continuous operation, intermittent operation under the same conditions enables full heat transfer, reduces the number of drilling operations, and lowers initial investment costs. (4) The intermittent operation technology proposed in this paper has significant practical value for maximizing the use of ground heat in air-conditioning systems. Further research is needed to explore the energy-saving characteristics of ground-source heat pump units when using intermittent operation technology.Quintuplex Pump,Quintuplex Plunger Water Injection Pump,Quintuplex Plunger Pump,High Pressure Quintuplex Plunger Pump
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