Coupled heat and moisture transfer model to evaluate earth-to-air heat exchangers exposed to harmonically fluctuating thermal environments

Abstract

The heat and moisture transfer process of an earth-to-air heat exchangers (EAHE) determines its ability to cool/heat an indoor environment. In humid climates, the moisture content of the EAHE inlet air is relatively high and exhibits a different fluctuation behavior than the air dry bulb temperature. In this study, a mathematical model was accordingly developed to evaluate the coupled heat and moisture transfer in an EAHE. Two prerequisites were considered by the model: the inlet was exposed to both harmonically fluctuating outdoor air temperature and moisture content, and the surface of the EAHE was directly formed by the surrounding material and always saturated due to water infiltration from this material. Based on the harmonic analytical method and a numerical approach, solutions for the amplitude dampening and phase-shifting of air temperature and moisture content were obtained. The model was validated using the results of well-regulated experiments, which indicated that phase-shifting and amplitude dampening of both air temperature and moisture content fluctuations occur in an EAHE. The coupled heat and moisture transfer between the humid air and the wet EAHE walls was found to intensify the phase shift but considerably attenuates the air enthalpy fluctuation amplitude. Then, application examples were analyzed corresponding to typical humid and dry cities-Chongqing and Beijing, respectively. The results showed the normalized fluctuation amplitude of the outlet air enthalpy for the Chongqing and Beijing environments decreased by 56.7 % and 61.1 %, respectively, indicating that moisture transfer considerably influences the air enthalpy fluctuation at the EAHE outlet.