Transient analysis based determination of derating factor for earth air tunnel heat exchanger in summer

Thermal performance of earth air tunnel heat exchanger (BATHE) under transient operating conditions has been evaluated for predominantly hot and dry climate of Ajmer (India) using experimental and computational fluid dynamics modeling. Effect of time duration of continuous operation and thermal conductivity of soil on thermal performance of EATHE under transient conditions has been analyzed. Results show that the transient thermal performance of EATHE is significantly dependent on thermal conductivity of the soil and duration of its continuous operation. Under steady state condition, a drop of 18.8 degrees C is obtained in air passing through BATHE having 0.1 m diameter and 60 m length, at 5 m s(-1) flow velocity, whereas, the transient analysis shows that for soil having thermal conductivity of 0.52 Wm(-1) K-1, the cooling of air reduces from 18.7 degrees C to 16.6 degrees C, after 24 h of continuous operation. Change in cooling effect after 24h of operation for soil thermal conductivity of 2.0 W m(-1) K-1 and 4.0 W m(-1) K-1, reduced from 18.8 degrees C to 18.5 degrees C and 18.8 degrees C to 18.7 degrees C respectively. The analyzed cases have shown the range of derating to be as minimal as 0% to as high as 64%, which if ignored while designing may lead to poor performance of earth air heat exchangers. (C) 2012 Elsevier B.V. All rights reserved.