| Excessive exploitation of traditional fossil energy has not only caused serious energy shortage,but also caused great damage to the earth’s environment and threatened the survival and development of human beings.Such an urgent situation makes energy conservation and emission reduction become the consensus of the society.The energy consumption of construction industry is an important part of the total energy consumption of the society,and effective reducing building energy consumption through technical improvements is an important part of energy conservation and emission reduction.On the basis of improving traditional energy technology,it is of great significance to rationally develop and employ green renewable energy,reduce energy consumption of building HVAC system and ensure good indoor environmental conditions.The earth-air heat exchanger（EAHE）which realizes heat storage and release by soil sensible heat is a common green energy technology used in buildings.More soil resources are needed for the application of this technology.Thus,how to effectively improve the performance of EAHE system is the key to the further application and promotion of this technology and to play a greater energy-saving benefit.In this paper,the thermal resistance analysis method is used to improve the thermal performance of the traditional EAHE from the perspective of thermal resistance in series and parallel.Compared with the sensible heat storage process of soil,the latent heat storage process of phase change material（PCM）has the advantages of high energy density and stable working temperature.Therefore,the thermal performance of such systems may be improved by reasonably applying PCM to EAHE.Based on this idea,the concept of PCM assisted EAHE（PCM-EAHE）and its two different structure schemes are proposed in this paper,namely the hollow cylindrical PCM assisted EAHE（HCPCM-EAHE）and cylindrical PCM assisted EAHE（CPCM-EAHE）.Meanwhile,in order to study the cooling/heating performance of this new heat exchanger,a three-dimensional transient numerical model of PCM-EAHE was established based on equivalent heat capacity method in this paper.Before the PCM-EAHE model was used to carry out relevant researches,a phase change heat storage material with phase change temperature of 28.16℃,supercooling less than 0.3℃,stable structure and good thermal conductivity was prepared in this paper by using the binary PCM of capric acid:palmitic acid=97:3（wt.%）as the phase change heat storage core material and 10%expanded graphite（EG）as the base material.Then,based on this material,the experimental device of PCM-EAHE system was built respectively under HPCM-EAHE and CPCM-EAHE schemes.The experimental test results of the device were used to verify the proposed PCM-EAHE numerical model.Based on the validated numerical model,the cooling performance of HCPCM-EAHE,CPCM-EAHE and a traditional EAHE（Trad-EAHE）with same specification under the meteorological conditions in the summer of a typical meteorological year in Chongqing was compared and studied in this paper.The results show that the cooling performance of two different PCM-EAHE system has been significantly improved.Compared with the Trad-EAHE with the same specification,the total daily cooling capacity of the HCPCM-EAHE and CPCM-EAHE in continuous high-temperature days in Chongqing was improved by 17.34%and 19.40%,respectively.According to their cooling output at outdoor peak temperature,the HCPCM-EAHE increased by 13.64%～18.40%,and the CPCM-EAHE increased by28.55%～39.74%.From the perspective of the heat transfer analysis,the enhancement of the thermal conductivity of the PCM around the air pipe is the main reason for the improvement of the cooling capacity of HCPCM-EAHE.The PCM unit has sufficient discharged can explain the significant improvement of the daytime cooling performance of CPCM-EAHE.In other words,the CPCM-EAHE system can use the cold in low temperature air at night with the help of PCM unit.Therefore,the utilization of the latent heat in PCM unit in CPCM-EAHE is much higher than HCPCM-EAHE.Furthermore,the effects of various factors on the cooling performance of CPCM-EAHE system are also discussed.Through the orthogonal experiment method based on numerical simulation,it is found that the influence of factors related to PCM unit on the cooling performance of PCM-EAHE is ranked as7)>（9）>8>>L,and7),(（9） have the most significant effects.Meanwhile,the influence of the phase change temperature on the utilization rate of the phase change heat storage capacity of PCM in CPCM-EAHE cannot be ignored.In addition,by comparing the cooling performance of CPCM-EAHE and Trad-EAHE at different depths,it is found that there is no significant relationship between the buried depth and the effect of PCM unit in CPCM-EAHE.Considering these research results,this paper uses the response surface analysis method and numerical simulation experiments to fit a multiple quadratic regression response surface model between the main factors D、、（9）、7、andand the indicators?8（6）and?（67）that characterize the CPCM-EAHE summer cooling performance in Chongqing.This model can reliably predict the cooling performance of CPCM-EAHE under the meteorological conditions of Chongqing in summer with values of D、、（9）、7、andwithin a certain range.The parameter analysis based on the response surface model shows that the cooling effect of CPCM-EAHE on air per unit mass increases with the increase of、（9） and7,and decreases with the increase of D and.Finally,the suitable operation mode of CPCM-EAHE is discussed as well.And through the comparative analysis on the cooling performance of the CPCM-EAHE operated in four modes of continuous operation,intermittent operation,intermittent operation combined with night ventilation under the summer meteorological conditions in Chongqing,it is found that the operation mode of combining intermittent operation and night ventilation can improve the cooling performance of CPCM-EAHE in the daytime,and increasing the wind velocity of night ventilation can further improve its cooling performance in the daytime. |
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