Study On The Layered Heat Transfer Model Of Ground Heat Exchangers In Bedrock Region

As a kind of renewable energy,geothermal energy has been widely concerned and studied.With the popularization and application of heat pump technology,there are many problems in putting into use GSHP.Take the bedrock area of chongqing for example:(1)Chongqing area is dominated by lithosphere,the drilling cost is high;(2)Unbalance of heat and cold load throughout the year,which reduces the performance of long-term operation system.(3)The cooling season lasts for a long time and the heat pump system runs according to the designed load,as a result,the energy consumption of the system is growing.Therefore,in view of the above problems,the paper takes the source heat pump system of an office building in the bedrock area of Chongqing as the experimental platform,and studies the scheme of improving the heat exchange of the buried pipe heat exchanger,reducing the initial investment and reducing the energy consumption of the system.First of all,a layered heat transfer model was established based on the GSHP project of an office building in chongqing,and the reliability of the model was verified by TRT thermal response experiment.A composite pipe with a layer of high thermal conductivity material injected into the wall thickness of traditional pipes was proposed.The thermal resistance of composite pipes and PE pipes with different thickness of high thermal conductivity layers was analyzed theoretically.FLUENT software was used to conduct seasonal intermittent simulation for different pipes and to compare the changes of various parameters.The results show that the thickness of the high thermal conductivity layer in the composite pipe increases,and the thermal resistance in the borehole decreases.Compared with the PE pipe,the maximum thermal resistance can be reduced by 22.42%.In the cooling season and heating season,the heat transfer performance of the composite pipe can be improved by 18.21% and 20.12%,respectively,compared with the PE pipe.The heat transfer performance improvement is mainly reflected in the inlet pipe,while the heat transfer performance in the upper part of the return pipe is relatively lower than that of the PE pipe.On this basis,according to the coefficient of energy efficiency in the section pipe buried in different depths to determine the buried depth of compound pipes,the study found that part of the pipe(in the rest back to central and upper with PE pipe pipe with composite pipe)in heat is small compared with the pipe unit well depth,and with the increase of the running time,the heat transfer difference decreases,and considering the heat transfer performance and the pipe cost,some better products.Then,the effects of different inlet flow rates,inlet water temperature and thermal conductivity of backfilling materials on the heat transfer performance of different pipes were studied.Due to the existence of buried pipes in the form of pipe groups,the heat transfer characteristics of different pipes at different positions in the pipe groups were also studied in this paper.The results show that after running for 20 days,thermal interference appears in the pipe group,ranging from small to large: corner Wells,side Wells and middle Wells,among which the thermal interference of some composite pipes is more obvious.After running for 90 days,the heat transfer performance of some composite pipes in each position of the pipe group is still higher than that of PE pipes at the same position.Finally,the GSHP system model of the office building was established with the software of TRNSYS,and the load side and the source side flow rate of the unit were optimized according to the hourly load of the building.The COP and EER of the unit within 10 years after the flow optimization met the requirements of the code,and the energy saving rate of the system was up to 10.83%.Based on the flow optimization model,the system performance and economy of different pipe heat pump systems were compared and analyzed for 30 years,so as to provide references for practical engineering applications.The results show that the soil temperature and system energy consumption of some composite pipes are slightly higher when the distance between buried pipes is the same,but the initial investment and operation cost of the system can be reduced by 201,600 yuan.With the same buried pipe area,the performance difference between some composite pipes and PE pipe systems is relatively small,but the initial investment of adopting some composite pipe systems can be reduced by 215,100 yuan.