| Shallow geothermal energy（SGE）is a kind of clean and renewable energy,which is mainly developed and utilized for building heating and cooling by ground source heat pump（GSHP）technology.With its wide distribution,large reserves,high efficiency,energy saving and non-polluting characteristics,SGE can be used as an important energy form to promote clean heating in winter and realize the revitalization of villages in northern China,and also contribute to the realization of the goals of"30 carbon peak"and"60 carbon neutralization".As the first batch of ecological civilization demonstration areas,Chengde is located in the connecting belt between North China and Northeast China,with both cold and severe cold climates.It is of great demonstration significance to carry out research on the development and utilization of SGE in this area,promote clean energy to replace coal-fired heating and protect the ecological environment.Although Chengde area is with the abundant SGE resources,the development and utilization of SGE and related research are still in infancy.In order to maximize the development and utilization of SGE,it is necessary to scientifically and reasonably plan the suitability zoning of different development modes,more deeply study the heat transfer characteristics of underground heat exchange system and the dynamic evolution law of temperature field,explore and optimize the operation characteristics of GSHP system,and evaluate the stability,efficiency and energy saving and environmental protection of the system.Therefore,this thesis adopts the methods of theoretical analysis,in-situ test,numerical simulation and demonstration project research to carry out the research on the suitability evaluation of borehole heat exchanger（BHE）coupled GSHP and groundwater source heat pump（GWSHP）for the development of SGE,the efficiency of underground heat exchanger and the operation characteristics of GSHP system.The main research contents and conclusions are as follows:（1）Based on the theory of variable weights,the research on the suitability evaluation of the SGE development model in Chengde area was carried out.Collect relevant geological and hydrogeological conditions survey data as data sources,the suitability evaluation index system of BHE coupled GSHP and GWSHP is constructed.The weights are were determined by the analytic hierarchy process and fuzzy c-means clustering,the suitability of two development models in Chengde area is evaluated and partitioned by Arc GIS.The suitability evaluation zoning maps of constant weight and variable weight are obtained.The suitability evaluation was divided into five grades:most suitable,suitable,relatively suitable,low suitable and unsuitable.Based on the variable weight theory,the BHE coupled GSHP system in Chengde area has the most suitable area of5848.1 km2,the relatively suitable area of 9794.5 km2,the suitable area of 16775.9 km2,the low suitable area of 6809.3 km2,and the unsuitable area of 241.3 km2.The relatively suitable areas,suitable areas and most suitable areas accounted for 54.4%of the Chengde area,of which the most suitable areas were few and scattered.The most suitable area,suitable area and relatively suitable area of groundwater source heat pump system accounted for 82.1%in Chengde area.Comparing and analyzing the different partitions of constant weight and variable weight,it is believed that the dispersion and accuracy are more in line with the actual situation in the suitability evaluation of SGE based on variable weight theory.（2）The heat transfer characteristics of double U-shaped and a new type of high-efficiency coaxial casing heat exchanger（HCCHE）are studied by the method of in-situ test and indoor thermal physical property test.For three heat exchangers with 200 m double U-shaped,200 m and 300 m new high-efficiency coaxial casing,thermostatic thermal response experiment carried out to simulate 8 tests under the conditions of heat extraction in winter and heat removal in summer.The results show that the heat transfer performance of HCCHE is better than that of double U-tube,and its heat transfer per linear meter is 1.61 times that of double U-tube heat exchanger.However,the fluid inlet/outlet temperature is smaller than that of double U-tube.For the HCCHE,the heat rejection（heat extraction）only increases by 34.16%（27.93%）with the drilling depth increases by 100 meters.While the investment is not proportional to the output,and the energy consumption of the system is also increased.The thermal influence radius of HCCHE is more than 10 m.Different underground lithology has great influence on heat transfer performance of heat exchanger,the heat transfer rate of boreholes dominated by gneiss is 11.8%higher than that of boreholes dominated by Andesite strata.（3）Based on the in-situ test,the heat transfer mechanism of the double U-shaped heat exchanger under complex space-time conditions and the factors affecting the heat transfer performance were studied by using the numerical simulation method.The space-time dynamic evolution law and operation characteristics of the heat exchange temperature field between the underground heat exchanger and the surrounding rock and soil were revealed.The results show that the upper part of the double U-shaped heat exchanger is prone to cold/heat accumulation at the initial stage of heat exchange.Under long-term operation,the cold/heat accumulation gradually extends with the depth,which affects the entire borehole and reduces the heat exchange efficiency.The strategy of intermittent operation should be adopted in practical engineering.The cross arrangement of double U pipes in the borehole is conducive to sufficient heat exchange with the surrounding rock and soil and avoids abnormal heat exchange in the same working face.At the same time,the change of inlet temperature is inversely proportional to the heat exchange of the heat exchanger.In winter operation,the inlet temperature changes by2℃,and the heat exchange decreases by 24.35%.The change of initial ground temperature is directly proportional to the heat exchange.For every 3℃increase in initial ground temperature,the heat exchange increases by 57.7%.Increasing the inlet flow properly can improve the heat exchange,but the heat exchange of excessive flow in long-term operation is less than that of small flow.In the actual project,the flow should be adjusted appropriately.（4）On the basis of the above research,the experimental research of BHE coupled GSHP system project in severe cold area and GWSHP system reconstruction project was carried out.The operation characteristics of the two systems were analyzed.The efficiency,energy conservation,environmental protection and economy of the system operation were evaluated.The results show that the average COP and EER of the BHE coupled GSHP system are 3.22 and 2.22 respectively.Based on the calculation of 60 days of operation,it can save 453.6 yuan of electricity charge,reduce 734.8 kg of CO2 emission,22.11 kg of SO2 emission and 11.01 kg of NOx emission than conventional heating.After the renovation,compared with pumping and irrigating different wells,the single-well circulating groundwater source heat pump system increases the heat exchange efficiency of a single hole by more than 2 times,solves the difficult problem of recharging,and does not pollute and consume groundwater.The single-well circulating groundwater source heat pump system can save up to 54.1%for heating cost,and also has a high energy-saving emission reduction characteristic. |
