   3.2 Experimental results analysis
According to the existing operating experience, in a stable heating season, the heat pump unit usually runs at the first boot 3-4 hours to reach a steady state [4] . Therefore, combined with the actual heating needs and heat pump operating conditions, the use of intermittent operation technology to make up for the slow underground heat exchangers, heat exchanger temperature recovery and improve overall efficiency. In this experiment, a cycle of 24 hours was used, and man-made control of on-off time was 1: 1 and 2: 5 [3,4] . The following experimental data are all measured results.
3.2.1 underground heat exchanger wall temperature change analysis
24-hour intermittent operation test from March 16, 2005 to March 19. It can be seen from Fig. 2 that the condition of ground temperature recovery is very favorable. After 5 hours of operation and shutdown, the temperature of the pipe wall is restored to about 0.2ºC , The unit continued to run for 4 hours to meet the user's heating needs, and then stop running, the wall temperature after 10 hours to restore the initial temperature. For the next cycle boot to provide better heat transfer conditions.
Figure 2 intermittent pipe wall temperature test results
Fig.2 The testing results of tube surface temperature
3.2.2 heat pump import and export of water temperature changes
The heat pump used in this experiment normal operation, the outlet temperature can not be lower than 4 º C, when below this temperature the heat pump will stop running. Therefore, we must ensure that the load to meet user needs, based on the import and export as stable as possible to ensure that the heat pump in the ideal operating conditions.
As can be seen from Figures 3 and 4, under intermittent operating conditions, the water temperature at the inlet and outlet of the heat pump is stabilized at a higher temperature, which is conducive to the operation of the unit under ideal operating conditions.
3.2.3 heat pump heat extraction from the soil heat exchanger
Due to the heat absorption of the buried heat exchanger, the temperature field of the soil changes. Just when the unit is running, the temperature of the water in the pipe is quite different from the temperature of the soil, the amount of heat is rapidly increased, and as the endothermic process progresses, the temperature of the soil is decreased. Therefore, for a period of time, the heat exchange of the underground pipe heat exchanger Reached a steady state.
Figure 5 shows the intermittent operation and continuous operation of the heat exchanger heat exchange curve. It can be seen from the figure that the heat exchange capacity of intermittent operation is obviously larger than that of the continuous operation under the same experimental conditions. Continuous operation of up to 3-4 hours, the heat exchange capacity of about 27.9W / m, with the increase of running time, the soil temperature decreases, the heat exchange between soil and heat exchanger will be affected, when running continuously for 24 hours , Heat transfer down to 26.5W / m. Intermittent ground temperature was restored (Figure 2), more conducive to the soil and heat exchanger full heat transfer; heat transfer can be guaranteed at 27.9W / m, compared with continuous operation heat transfer increased by 5 ?.
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 underground heat exchanger per meter depth of heat exchange
Fig 5 The heat flux a meter of ground source heat-transfer
4 Conclusion
(1) The controllable intermittent process can change the trend of the temperature change around the underground heat exchangers. Under winter operating conditions, it is possible to improve the equilibrium stable temperature and make the unit operate under the ideal operating conditions.
(2) Through reasonable human intermittent control, to achieve a good trend of ground temperature changes, to maximize the heat exchange capacity of underground heat exchangers.
(3) In the same conditions, intermittent operation and continuous operation compared to achieve full heat transfer, minimize the number of drilling, reduce initial investment.
(4) The technology of intermittent operation of ground-source heat pump proposed in this paper is of great significance and practical value for making full use of ground in air-conditioning system. It is necessary to further study the energy-saving characteristics of ground-source heat pump unit after applying intermittent operation technology.
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