Design And Operation Optimization Of Mid-deep Casing-type Buried Tube Heat Exchangers

In the context of energy conservation and emission reduction and energy structure transformation,geothermal energy utilization,as an important part of renewable energy,has great development and utilization potential.The deep buried pipe heat exchanger has the characteristics of large heat extraction capacity,high ground temperature,small footprint,no pollution to the environment and so on,which is gradually applied and popularized in domestic and abroad.However,there is still no reliable technology and means for determining the depth and borehole number of deep buried borehole heat exchangers.In this thesis,the economy of the two kinds of borehole design schemes,one is drilling more shallow boreholes and another one is driling less deep boreholes,are compared.Besides,the economy of the circulating pump operaiotn modes is also analyzed.The borehole design scheme is recommended according to the comprehensive cost of the initial investment and operation cost.The main research is as follows:(1)The heat transfer model of deep borehole coaxial heat exchanger is established to simulate the heat transfer process between ground and buried pipe,and calculate and analyze the temperature variation of circulating water and soil.(2)Based on the established model,the design calculation method of borehole depth is given,and the influencial factors of borehole depth are calculated.The borehole depth increases with the increase of the minimum inlet water temperature and heat extraction capacity,and decreases with the increase of ground heat flow,thermal conductivity and volume specific heat capacity.Borehole depth of buried pipe decreases with the increase of circulating water flow rate.However,power consumption of circulating water pump increases while circulating water flow rate increase,so the water flow rate should be controlled in a certain range.(3)By comparing and analyzing the influence of borehole depth,borehole number and water flow rate on the initial investment,it is found that the total drilling cost can be significantly reduced by adopting the less deep drillings scheme with same water flow rate.(4)The energy consumption and the influence of some parameters in operation modes with constant water flow rate and with variable flow rate within 30 years are explored.Constant temperature difference and variable flow was used to match the change of heat load and the data were compared with constant flow cycle.Parameters that affect system power consumption,including heat extraction capacity,circulating water flow rate,heat pump COP,as well as the power consumption of water pump and heat pump are analysed.The results show that the water flow variable operation mode is feasible for the scheme of less deep boreholes,which can effectively reduce system power consumption.