Study On The Performance Of Earth-to-air Heat Exchangers And Its Integration Mode With Buildings In A Dynamic Outdoor Thermal Environment

In the process of creating indoor thermal environment,making full use of renewable energy can effectively reduce building energy consumption and alleviate environmental pollution.As one of the most common renewable energy,geothermal energy can be directly or indirectly applied to the indoor thermal environment regulation.Earth-to-air heat exchanger(EAHE),as a direct utilization technology of shallow geothermal energy,has attracted more and more attention worldwide in recent years.In the hot summer and cold winter regions,the outdoor climate varies greatly throughout the year,and the shallow soil temperature may also fluctuate significantly.Such dynamic thermal environment has a great influence on the heat transfer effect of EAHE.However,at present,researches on the heat transfer process and performance of EAHE under dynamic outdoor thermal environment are insufficient.In addition,the regulation effect of EAHE on indoor thermal environment is not completely depends on the performance of EAHE,but also affected by the coupling effect of EAHE and building and the EAHE ventilation model.At present,there is still lacks of theoretical models and quantitative methods to guide the combination of EAHE and buildings,which affects the promotion of this technology to a certain extent.Therefore,this paper mainly focuses on the performance of EAHE and its integration mode with buildings in a dynamic outdoor thermal environment.The main contents and results are as follows:A full-scale EAHE experimental platform was built in Chongqing,a typical hot summer and cold winter region.The experimental tests were carried out for five consecutive days in summer and winter,respectively.The performance of EAHE and its main influence factors in summer and winter were quantitatively studied.Experimental results show that EAHE can efficiently use shallow geothermal energy to pre-cool/heat the outdoor air in both summer and winter.Under the conditions of summer and winter,for EAHE with the same diameter and velocity,increasing the buried depth was beneficial to improve its performance,for EAHE with the same buried depth and velocity,decreasing the diameter was also beneficial to improve its performance;the air temperature in EAHE shows an exponential decrease/increase trend with the increase of pipe length;and the air temperature drop/rise bwteen the inlet and outlet of EAHE is linearly increasing/decreasing with the inlet air temperature.In addition,the drop/rise in the EAHE air temperature decreases with the increase of velocity.To further improve the performance of EAHE,this paper proposes an EAHE having flat rectangular cross section.In dynamic outdoor thermal environment,a heat transfer theoretical model of flat rectangular EAHE was established,and the fluctuation characteristics of EAHE outlet air temperature were quantified.Then,the performance difference between circular and flat rectangular EAHE was studied by numerical simulation and theoretical analysis.The results show that the disturbance range of the flat rectangular EAHE on the soil temperature is smaller than that of circular EAHE,and the buried pipe inner wall temperature is more stable in both winter and summer.Under the same cross-sectional area and pipe length,compared with the circular EAHE,the flat rectangular EAHE can obtain smaller outlet air temperature fluctuation amplitude and larger phase shfit.To quantify the regulation effects of EAHE on the indoor thermal environment under the constant air volume mechanical ventilation operation mode(EAHE with mechanical ventilation,EAHEMV),a full-scale experimental platform was built in this paper,and 24 hours of continuous experimental tests were carried out on typical days in summer and winter.Experimental results show that EAHE can significantly improve the indoor thermal environment under EAHEMV mode.Compared with the building without EAHE,EAHE can reduce the average indoor air temperature by 5.9°C in summer and increase by 4.29°C in winter,and the corresponding temperature fluctuation range is reduced by about 8.9°C.At the same time,the average relative humidity of indoor air can be increased by 17.85%and decreased by 21.2%,respectively.In addition,the average temperature of the inner surface of the building envelope can be reduced by 6°C in summe and increased by 5.8°C in winter,and the corresponding temperature fluctuation range decreased by 9.5°C and 3.8°C,respectively.In addition,compared with the building without EAHE,the average cooling/heating load decreases of the building with EAHE are 498.58W and 363.9W,and the corresponding average cooling/heating load per unit area decreases by about 55.4W/m~2and 40.43W/m~2,respectively.To utilize the buoyancy formed by the indoor heat source and EAHE to coordinately regulate the indoor thermal environment,this paper proposes a building ventilation mode of buoyancy-driven EAHE(EAHE with buoyancy-driven ventilation,EAHEBV).This ventilation mode relies on indoor buoyancy to drive the outdoor air flow in EAHE and building,so as to provide fresh air and cooling/heating energy for buildings at the same time.It is a pure passive ventilation mode.Therefore,a theoretical model for this ventilation mode was established in this paper,and a calculation formula for quantifying the fluctuation of ventilation flow rate and indoor air temperature was obtained,and the three-dimensional numerical simulation was used to simulate the ventilation mode.The theoretical model and numerical simulation results showed that in EAHEBV mode,the EAHE outlet air temperature,indoor air temperature and ventilation flow rate all presented periodic fluctuations,and the changes of the three were not synchronized,and the phase shfit of the ventilation flow rate obviously lagged behind the indoor air temperature.Then,the theoretical model is used to compare and analyze the operating effects of EAHEBV,EAHEMV and BV(Buoyancy-driven ventilation)modes.The results show that the improvement effects of EAHEBV mode on indoor air temperature is slightly weaker than that of EAHEMV mode in annual cycle.However,because EAHEBV mode is a purely passive operation mode,the energy-saving efficiency is obviously better than that of EAHEMV mode.In addition,EAHEBV mode has a much better improvement effects on indoor temperature than BV mode,especially in winter and summer.To efficiently combine EAHE with building and avoid the failure of using EAHE to achieve the specific indoor thermal environment control target,or excessive use of EAHE,it is essential to rationally configure the parameters of EAHE and building.To this end,this paper proposes a reverse matching method of EAHE based on indoor thermal environment regulation demand.Based on the dynamic balance relationship between EAHE cooling/heating capability and indoor cooling/heating demand,the parameter combination of EAHE and building is determined in reverse based on the indoor thermal environment regulation target.Then,three-dimensional numerical simulation is used to verify the reverse matching method.The results show that the parameter combination of EAHE and building determined by this method can achieve the expected control target,which proves the reliability of this method.At the same time,the study also shows that improving the thermal insulation of the building envelope and the ventilation flow rate of single pipe of EAHE can help to reduce the requirements for the configuration of EAHE parameters,and reducing the expected phase shfit of the indoor air temperature was an important measure to make the reverse matching successfully implemented and cost-effective.In addition,the main parameters of EAHE are complementary to each other to achieve the regulation objectives.