Performance evaluation of flat rectangular earth-to-air heat exchangers in harmonically fluctuating thermal environments

Abstract

An earth-to-air heat exchanger (EAHE) is an excellent passive technology that uses shallow geothermal energy directly to generate cooled or heated air for thermal regulation of buildings. Previous studies have focused on circular EAHEs, paying insufficient attention to the effect of cross-sectional shape. In this paper, we propose an EAHE having a flat rectangular cross section, develop a mathematical model to calculate its thermal performance in a harmonic thermal environment, and validate the theoretical results using a three-dimensional numerical model. Our results indicate that compared with a circular EAHE shape of the same cross-sectional area, the outlet air temperature fluctuation of the flat rectangular EAHE is 24.7% lesser and its phase shift is 22.67 days longer, and the soil's thermal disturbance range is shorter, resulting in a temperature wave in the soil around the buried pipe that decays faster and creates a more stable pipe wall temperature. The cooling/heating capacity of the flat rectangular EAHE is greater than that of the circular EAHE. The pressure drop and fan power consumption of the flat rectangular EAHE is greater than that of circular EAHE, and thus the coefficient of performance (COP) value of the flat rectangular EAHE is less than that of the circular EAHE.