Study On Heat Transfer Characteristics Of Shallow Borehole Casing-shaped Buried Tube Heat Exchanger

As a representative renewable energy,geothermal energy has become one of the ways for the country and all sectors of society to achieve carbon neutrality because of its unique advantages such as abundant reserves,wide distribution,clean and environmental protection,and high utilization coefficient.As a key technical means,ground source heat pump technology has been widely used in geothermal energy exploitation.As the core component of geothermal exploitation,the heat transfer performance of ground heat exchanger directly affects the service life and operation efficiency of shallow ground source heat pump system.At present,the shallow geothermal energy development and utilization technology,based on U-shaped buried pipe ground source heat pump system,has problems such as large number of boreholes and low amount of heat transfer.Therefore,this paper uses theoretical analysis,numerical simulation,field test and other methods to study the heat transfer characteristics of shallow borehole casing-shaped buried tube heat exchanger(SBCBTHE).The main contents are as follows:(1)Based on the infinite long line heat source theory and slope method,the analytical process of constant power geotechnical thermal response test of SBCBTHE was analyzed.Through the self-built geotechnical thermal response test device and multi-point temperature measurement cable device,the heat transfer per unit well depth of SBCBTHE and U-shaped buried tube heat exchanger were compared and analyzed,and the initial average temperature of rock and soil,the comprehensive thermal conductivity of rock and soil,and the comprehensive volume specific heat capacity of rock and soil were obtained.(2)The effects of different inlet water temperatures,circulating water flow rates and burial depths on the heat transfer of these two types of buried pipes were analyzed by numerical models of double U-shaped buried tube heat exchanger(DUBTHE)and SBCBTHE.The single well heat transfer and heat exchange per unit well depth of these two types of buried pipes were compared under the optimal parameters.The influence degree of three factors,namely buried pipe depth,summer inlet water temperature and circulating water flow rate,were analyzed with the single well heat transfer rate as the evaluation index.(3)The resistance along the pipe of three kinds of outer pipe diameters under different inner pipe diameters was calculated,and three optimal matching forms of SBCBTHE were obtained.The effects of different inlet water temperature,thermal conductivity of inner pipe and thermal conductivity of backfill material on energy efficiency coefficient,thermal short circuit phenomenon and surrounding soil temperature field of SBCBTHE in summer and winter were analyzed respectively,and the influence rules of different factors on these three aspects were obtained.The research results enrich the key technologies of heating and cooling buildings by shallow geothermal energy,and provide reference and data support for optimizing the structure of SBCBTHE.