Numerical Investigation of Thermal and Hydraulic Performance for Grid-type Earth-to-Air Heat Exchangers: A Comparative Study of U-Type and Z-Type Configurations

Abstract

Heating and cooling needs in greenhouses constitute a significant portion of total energy use, and therefore the adoption of passive and renewable approaches is important. Earth-to-air heat exchangers (EAHEs) exploit the relatively constant subsurface temperature to precondition ventilation air and may provide a sustainable option for greenhouse applications. Although grid-type configurations are particularly relevant to systems that require relatively large airflow rates, most previous studies have discussed isolated performance indicators under different geometrical and operating conditions, which makes fair comparison difficult. This study presents a numerical comparison of two grid-type EAHE configurations, Z-type and U-type, under identical geometric and operating conditions using four dimensionless performance parameters: thermal effectiveness ε, total pressure-loss coefficient K, airflow division uniformity coefficient Ω, and overall performance factor η* (defined as the ratio of heat-transfer rate to pumping power). The results show that the U-type layout improves flow distribution and reduces hydraulic losses. Ω increased from 0.79 to 0.87 for the U-type, while it decreased from 0.73 to 0.64 for the Z-type, resulting in a 38% improvement at high flow rates. Total pressure losses decreased by more than 28%, resulting in a corresponding reduction in fan-power demand. The thermal performance remained relatively similar, with ε differences ranging from 1% to 2%. However, η* was consistently higher for the U-type arrangement, exceeding the Z-type by more than 40% at the highest Reynolds number. Overall, the results suggest that the U-type EAHE provides the more favorable thermal–hydraulic performance for passive, energy-efficient greenhouse ventilation under the investigated conditions.