Operation Performance Research On Heat Source/Sink Well Of Groundwater Heat Pump With Pumping & Recharging In The Same Well (GWHPPRSW)

The heat source / sink wells (HSSW) of groundwater heat pump with pumping and recharging in the same well (GWHPPRSW) have two types now, one is pumping & recharging well (PRW), the other is standing column well (SCW). They both can finish the operations of pumping and recharging in only one well. For PRW, the well is divided into three parts by clapboards: low pressure (production) space in the low part of the well, seal section in the middle part and high pressure (injection) space in the top part. When the submersible pump is running, groundwater is sent to heat exchanger at the wellhead, where it releases heat, and then is sent back to the injection space through the same well. SCW can be regarded as a transfiguration of the coaxial heat exchanger of ground-coupled heat pump (GCHP). It cuts down the outside pipe of the coaxial pipe in competent bedrock, and lets fluid circulate directly in the borehole to exchange heat with rock of the borehole. During peak load periods, SCW can bleed some circulation water from the borehole to induce original groundwater flow.In this work, the operation performance of these two types of HSSW was studied in detail and systematically. The main works and results of this dissertation are listed as following:(1) The in-situ experiment of PRW was performed, and nearly two years'experimental data were obtained. Based on in-situ test, the phenomenon of thermal transfixion was delimited as the slow-response thermal transfixion and the fast-response thermal transfixion. When the fast-response thermal transfixion was existence in the system, the pumping temperature would vary along with the recharging temperature. The bleed tragedy was introduced to PRW, and the HSSW was called single-double wells mixed GWHP system. At the same time, the test results showed that for full-year-operation PRW the seasonal thermal energy storage (STES) played an important role in the heat source for PRW.(2) The PRW were sorted as non-backfill PRW and gravel-backfill PRW according to the technology of well construction and the instruction of the in-situ test results. The models of groundwater flow and heat transfer for non-backfill PRW, gravel-backfill PRW and SCW were established, respectively. The methods of numerical solution were provided. The mathematic models were validated more completely through the experimental data collected from Technical University of Denmark for non-backfill PRW and from Pennsylvania State University for SCW as well as conducted by us for gravel-backfill PRW.(3) The analytic drawdown equation for non-backfill PRW in a unitary homogenous confined aquifer was acquired through the principle of superposition based on the analytic drawdown equation for partially penetrating well presented by Hantush. Equations of steady drawdown, seepage velocity, quasi steady time and ideal well distance were gained through the analytic drawdown equation. The characteristic analyses results showed that the phenomena of thermal transfixion would happen when PRW kept running. Even though the existence of thermal transfixion the PRW could burden big load because of the large thermal effective radius. Compared with the non-backfill PRW, the gravel-backfill PRW could reduce the pressure of pumping and recharging and had more serious thermal transfixion, too. For SCW, the original groundwater exchange burdened much large part of load, which made SCW can undertake larger load than the coaxial heat exchanger. Meanwhile, the load ability of SCW was smaller than that of PRW for the smallness of groundwater velocity in aquifer and the thermal effective radius. In addition, the phenomena of thermal transfixion were studied quantitatively.(4) The parametric study was presented. For PRW, the results showed that the coefficient of permeability was critical for pumping and recharging and the ratio of permeability coefficient (horizontal / vertical) was the key factor for the variation of pumping temperature. The operation strategy of small flow rate and large temperature difference was feasible to PRW. SCW didn't have the trouble of recharging. The coefficient of permeability was also the key parameter to SCW. The load ability of SCW could be promoted greatly if SCW was in the aquifer with good permeability. The increase of borehole length was an effective method for SCW to enhance the ability of load. The bleed strategy could alleviate and postpone the change of pumping temperature. But there was unbalance between the rate of bleed and its impact. Therefore, the bleed strategy should be adopted as an emergency method to delay the sharp change of recharging pressure and pumping temperature.(5) The phenomena of seasonal thermal energy storage (STES) in the operation period were discussed on the basis of the analyses of perennial behavior of GWHPPRSW. The calculation results showed that when PRW ran perennially the phenomena of STES appeared apparently and the STES provided much large part of heat source or sink to PRW. When the accumulated load kept unbalance, for perennial-operation PRW the pumping temperature could increase or reduce much annually, which even made PRW not work. However, for SCW the phenomena of STES didn't occur and the thermal energy storage ratio was nearly zero. There was little effect to the later pumping temperature of SCW due to its previous operation. Thus, the problem of load unbalance didn't need to be considered at all for SCW.The works of this dissertation provided the theroical references to comprehensively understand the HSSW of GWHPPRSW and offered the evidences to distinguish the confusion of PRW with SCW.