Numerical Investigation Of Heat Transfer Performance Of Single Well Geothermal Systems

The world’s energy consumption is growing and fossil energy is dominant.However fossil energy is not renewable and easy to cause environmental pollution.Therefore,it is of great significance to develop clean new energy.As a clean renewable energy,geothermal energy has the advantages of high reserves,safety and stability.It can be used for heating and power generation.However,the traditional technology is mainly for obtaining shallow geothermal energy,which has certain defects.Single Well Geothermal System(SWGS),aiming to obtain medium and deep geothermal energy,avoids underground water waste and pollution through closed circulation,and achieves heat extraction without water extraction.In recent years,related research has been carried out by scholars from various countries to improve geothermal energy development.In this paper,a heat and mass transfer model of a SWGS with water as the working fluid is established.Ansys Fluent and other software is used to calculate the model numerically to obtain the change law of the SWGS’s heat transfer performance.The evaluation parameters mainly include heat extraction rate and outlet temperature.This paper also pay attention to the rock temperature field.Through the control variable method,the change law of the heat transfer performance of the system with flow parameters and directions,rock physical quantities SWGS geometric parameters,and pipe properties are summarized and sensitivity analysis is performed.It is found that reducing inlet temperature,increasing inlet velocity,increasing the axial size of SWGSs,improving the thermal insulation performance of inner pipe,and enhancing the thermal conductivity and temperature field of the rock can increase heat extraction rate.However,reducing inlet temperature and increasing inlet velocity will lower outlet temperature and enhance the performance degradation.Increasing radial dimension will reduce the temperature difference between the inlet and outlet.Therefore,the heat extraction rate cannot be improved blindly.Increasing the axial dimension increases the heat extraction rate,but increases the cost of drilling.The properties of outer pipe have little effect on the heat transfer performance,and the inner pipe needs to ensure enough thermal insulation performance.Aiming at the sustainability of the heat transfer process of SWGSs,this paper analyzes the attenuation law of the system heat transfer performance during a single heating season and the more heating seasons.It is founded that the heat transfer performance continuously decreases in a single heating season.It is close to a linear change finally,and the change trend is very small.Under more heating seasons,the natural recovery capacity of rock heat storage in non-heating seasons is too low to provide enough energy,and the heat transfer performance will deteriorate more severely after more heating seasons.The effect of SWGSs on the rock temperature field during heating seasons is kept within a certain radial range.