Study On Heat Transfer Characteristics In Ground Source Heat Pump System Horizontal Headers

Shallow geothermal energy is a clean,environmentally friendly,widely distributed,and abundant storage renewable energy,which is of great significance for achieving the goal of Carbon Peaking and Carbon Neutrality.The use of shallow geothermal energy is mainly based on heat pumps,which are used to provide building heating or cooling to achieve temperature regulation in the room.Due to the advantages of long service life,low maintenance cost,and strong environmental adaptability,ground source heat pump systems have been widely used in practical engineering.At present,the research on the ground source heat pump system is mainly concentrated on the vertically buried pipe heat exchanger,while the research on the horizontal header is less,and the energy loss of this part cannot be ignored.Therefore,this paper studies the heat transfer characteristics of horizontal headers through experiment and simulation.Taking summer operating conditions as an example.First of all,based on the similarity theory,this study built a sandbox test bench,and studied the heat transfer characteristics of the horizontal header and the influence of the horizontal header on the vertical U-tube heat transfer under the comprehensive effect of multiple factors through the orthogonal test,and obtained the influence degree of each factor on the horizontal header and the respective optimal control methods of the water supply section and return section of the horizontal header.Then,based on the actual engineering size,a simulation model is built to study the temperature change of the underground rock and soil layer,the heat exchange law of the horizontal header,the overall heat exchange change of the vertical U-tube,and the underground pipe system under different burial depths when the surface temperature changes,and the corresponding optimal burial method of the horizontal header is proposed for different climatic regions.The main findings are as follows:(1)The four factors’ influence order of buried depth,land surface temperature,buried pipe material,and pipe flow on the horizontal header is as follows: header buried depth >land surface temperature > buried pipe material > pipe flow and the buried depth has the greatest influence on the horizontal header heat transfer.(2)The increase of heat gain in the horizontal header water supply section is helpful to improve the heat exchange of the U-tube.However,the increase in heat gain in the backwater section will cause greater energy loss.The burying method of upper supply and lower return can not only improve the heat exchange performance of the U-tube,but also reduce the energy loss during the transportation process.(3)The greater the depth of the rock-soil layer,the smaller the impact of the surface temperature fluctuation,and the longer the thermal response time.When the water supply section and return section of the horizontal header is buried in parallel at the same depth,the buried depth should be increased as much as possible to reduce the impact of surface climate factors,and to improve the overall heat exchange of the underground pipeline system.(4)The optimal burying method of upper supply and lower return can be adopted in different climatic regions.In extreme climate areas,the length of the horizontal header should be shortened and the buried depth of the horizontal header should be increased as much as possible,or the horizontal header should be insulated and the pipes with small thermal conductivity should be used.