| As one of the main renewable energy using technologies, the ground source heat pump system (GSHP) has been attracted more and more attentions taking advantages of energy saving and environmental protection, and correspondingly, applications of GSHP systems were also increased rapidly in recent years. Because the performance of GSHP system is mainly determined by the Ground Heat Exchanger (GHE), widely investigations have been conducted on the structure、heat transfer mechanism and simulation model of the GHE. However, due to the complexity for heat transfer procedure of the GHE and the special coupling relationship between the GHE and the dynamical building cooling/heating load, knowledge on the thermal characteristics of GHE is far from sufficient, which lead to the applications of GSHP system with significant blindness and limitations. Several typical problems, including too low operational efficiency、too expensive initial investment of the GHE and seriously insufficient heat supplying during heating period etc., are still exist. Thus, in this paper, experimental and numerical studies, which concerns on the heat transfer procedure and the dynamical response to the building cooling/heating load of the GHE, have been conducted to improve the thermal performance and the operation efficiency of the GSHP system. The main researches and corresponding findings of this study are listed as follows:1) For the first time, this study focused on a GSHP system used in an actual project located at loess plateau region with cold climate and long term on-site measurements have been conducted. In comparison with studies conducted in laboratory rooms, this study is more real ity to reveal the superimposed heat transfer effect of GHE groups. In this study, temperature of the rock-soil surrounding the GHE is monitored by using the 1-wire bus type temperature sensors. This innovative measuring method effectively avoid impact from traditional intraductal monitoring system to the fluid flow and heat transfer of the GHE, and thus greatly improve the reality and accuracy of the measuring results. In current rock-soil heat response testing standard, the initial temperature of the rock-soil body is approximated by using the average value of the inlet and outlet fluid temperature in the GHE. In Ms study, measurement results for the thermal properties and initial temperature field of the rock-soil body reveals that this method may lead to large deviation, and specialized testing well is suggested to be used to improve the measuring accuracy.2) Operation parameters for both the GHE and the building air-conditioning system of the GSHP system are continuously monitored for two years, by using a self-developed automatic real-time GSHP monitoring system. The testing results indicated that recovery of the rock-soil temperature field were mainly occurred at a short period after running stopping of the GSHP system at the end of the cooling or heating period. Taking advantages of the rock-soil body self-recovery capacity, the rock-soil body temperature field imbalance phenomena caused by long-term operation of the GSHP system is effectively alleviated by using the intermittent operation control strategy, and thus greatly improved the operation efficiency of the GSHP system. Vertically temperature gradient distribution of the rock-soil body is obviously, which indicates that the geothermal energy is directly work on the recovery of the rock-soil temperature field. Furthermore, the difference of thermal properties indifferent rock-soil layers and the groundwater seepage effect greatly influence the heat transfer of the GHE and lead to non-linear water temperature distribution in the GHE along with the flow direction. Because thermal conductivity ofthe shallow rock-soil layer in the loess plateau region is relatively small, the solar radiation and air convection heat transfer from the earth’s surface to the rock-soil body is greatly influenced, which lead to severe thermal interference to the heat transfer of GHE. The research also firstly revealed that heat transfer at lower region of the buried pipes is the main cooling or heating source of the GHE. Since the strong self-adjust function of rock-soil body to the thermal balance, the heat transfer of GHE just affects thermal imbalance of rock-soil body for short period and slightly influences the long period thermal balance of rock-soil body. Therefore, the "thermal balance" calculation method of rock-soil body used currently is required to be further improved.3) According to the testing results, the heat transfer procedure between the U-tube GHE and the surrounding rock-soil is quantitatively analyzed. The "thermal interference" phenomena of GHE during cooling and heating period are investigated separately and two thermal disturbance evaluation methods for the GSHP system is contrasted. It was found that the "thermal interference" between water supplying and returning pipes and thermal disturbance between water returning pipe and its surrounding shallow rock-soil layer greatly deteriorated the thermal capacity of GHE. Special attentions are required to be paid at design stage on thermal loss of water returning pipe during heating or cooling period. Reducing the thermal conductivity of backfilled materials is beneficial for alleviating the thermal disturbance phenomena and reducing the invalid thermal loss of GHE. However, too low thermal conductivity of backfilled materials also restricts the valid heat transfer between GHE and its surrounding rock-soil body, and aggravates the "thermal interference" phenomena. Therefore, reasonable selection of the backfilled materials is one of the key points to improve the thermal capacity of GHE. The testing results also reveal that heat transfer of the water returning pipe has no positive effects to the variation of the fluid temperature in GHE. Locations of the minimum and maximum fluid temperature in the GHE should be considered when designing the depth of the heat source well. Using the coefficient of thermal efficiency to evaluate the thermal capacity of GHE is much more reliable than using the loss coefficient of heat exchange capacity.4) A new three-dimensional simulation model for the GHE, integrating the GHE heat transfer model and the porous medium seepage model, is developed based on the testing results, which focused on several problems exist in previous simulation models. Impacts from the operation parameters to the heat transfer of GHE are considered in the newly developed simulation model, including the building dynamical heating/cooling load variations、the non- uniform temperature distributions of the rock-soil body、the difference of thermal properties in different rock-soil layers and its influence to the temperature recovery of rock-soil body、 the groundwater seepage effect、the thermal disturbance among ground heat source wells、the "thermal interference" phenomena in the u-tube and the superimposed effect of multiple heat sources. The simulation results are compared with the measurement data in different operation conditions of the GSHP system. The comparison results indicate that new models greatly improved the simulation accuracy of the heat transfer characters of GHE, which is beneficial for the design and operation regulation of the GSHP system.5) By using the newly developed three-dimensional simulation model, influences from the difference of thermal properties indifferent rock-soil layers、the thermal disturbance among ground heat source wells、the rock-soil temperature variation during the recovery period、the groundwater seepage effect and the thermal interference between water supplying and returning pipes of the u-tube to the thermal performance of vertically buried ground heat exchangers are further investigated. Effective method is suggested to improve the heat transfer capacity of the GHE and reduce the invalid heat loss during heat transfer procedure. Based on that, an innovative configuration of the GHE named "One returning pipe with multiple supplying pipes" is developed. The thermal performance of GHE with new configuration is numerically investigated. The simulation results reveal that with new configuration the heat transfer between GHE and its surrounding rock-soil is highly enhanced, and thermal disturbance between water supplying and returning pipes is obviously reduced. The thermal capacity of GHE with new configuration is much better than that of traditional GHE with single U-tube or double U-tube. |
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