Experimental And Numerical Investigations On The Thermal Performance Of Horizontal Spiral-coil Ground Heat Exchanger

With the increasing consumption of traditional energy sources,environmental pollution problems have been becoming more and more serious.The ground source heat pump is an air conditioning system that uses shallow geothermal energy as a heat source for heat pump.It has been widely promoted and applied due to its environmental protection,energy saving and high efficiency.The ground heat exchanger(GHE)is the key equipment to exchange heat with soil in the ground source heat pump system.The buried pipe form and heat transfer performance are important issues in the field of ground source heat pump.Compared with the vertical buried pipe,the horizontal GHE is favored because of its simple construction and low cost,but the traditional horizontal GHE has a large area and low heat exchange efficiency.To a certain extent,it limits its application and promotion.Therefore,the horizontal spiral ground heat exchanger(HSGHE)is proposed beacuase of its high-efficiency.This type of heat exchanger has the advantages of good heat transfer performance,convenient construction and low cost.In this paper,the numerical simulation and experimental tests are carried out on the thermal performance of HSGHE.The influence of different factors on the thermal performance is discussed.In numerical simulation,the numerical model of HSGHE was established by using COMSOL Multiphysics software.The influences of coil diameter,operation mode,soil type,groundwater types and long-term operating conditions on its thermal performance were discussed.The results show that increasing the diameter of the buried coil can increase the total heat exchange,but it will lead to the decrease of heat exchange per unit length and the increase of soil heat impact range.In the day and night intermittent operation mode with a cycle of 24h,the longer the system opening time,the greater the daily average heat transfer rate of the HSGHE and the larger the thermal influence range of the buried pipe horizontally.The soil type has great influence on the thermal performance.From the perspective of heat transfer performance,the stangstone is the most favorable to the heat exchanger,followed by sand,and the least is clay.However,from the perspective of reducing the thermal interference area,clay is the most favorable to the heat exchanger among sand,stone and clay.The increase of groundwater seepage velocity is beneficial to increase the heat exchange of the GHE,and the groundwater seepage in the vertical direction on the heat transfer performance of the GHE is more obvious.Under the operating conditions of the whole year,the intermittent operation mode is more conducive to improving the heat transfer performance of the HSGHE and there will be no hot and cold accumulation after the end of the year.In the experimental research,the model of the HSGHE was built,and the experimental study on the heat transfer performance and soil temperature distribution of the HSGHE under different conditions was carried out.The results show that the increase of the inlet water temperature in summer and the decrease of the inlet water temperature in winter will increase the average heat exchange of the HSGHE.But it will also aggravate the change of soil excess temperature,so it is necessary to determine the inlet water temperature.Reducing the center distance of the coil can increase the total heat exchange of the GHE,but the heat exchange of per unit length will decrease and the thermal interference to the soil around the buried pipe will increase.The increase of surface wind speed enhances the convective heat transfer between the ground surface and the air,and improves the heat transfer performance of the GHE.However,considering the stability of the heat transfer and the depth of the frozen soil,the depth of the GHE should not be shallow.In addition,the heat transfer performance of the HSGHE is better than that of the horizontal slinky GHE.The HSGHE has a thermal influence range of about 0.4 m in the horizontal direction and a thermal influence range of more than 0.2 m in the depth direction..The purpose of this paper is to investigate the influence of different factors on the heat transfer characteristics of HSGHE,in order to provide theoretical guidance and technical support for the popularization and application of HSGHE in practical engineering.