‘Derating Factor’ new concept for evaluating thermal performance of earth air tunnel heat exchanger: A transient CFD analysis Vikas Bansal a,⇑ , Rohit Misra b , Ghanshyam Das Agarwal b , Jyotirmay Mathur a a Mechanical Engineering Department, Govt. Women Engineering College Ajmer, India b Mechanical Engineering Department, Malaviya National Institute of Technology, Jaipur, India highlights " Transient 3-D Computational Fluid Dynamics analysis of Earth Tunnel Air Heat Exchanger. " Introduction of new concept ’Derating factor’ for evaluating thermal performance of Earth Tunnel Air Heat Exchanger. " Effect of period of continuous operation on thermal performance of Earth Tunnel Air Heat Exchanger. " Effect of thermal conductivity of soil on thermal performance of Earth Tunnel Air Heat Exchanger. article info Article history: Received 16 April 2012 Received in revised form 30 June 2012 Accepted 20 July 2012 Available online 30 August 2012 Keywords: Earth air tunnel heat exchanger Derating factor Computational fluid dynamics FLUENT abstract A new term ‘Derating Factor’ is devised for evaluating deterioration in thermal performance of Earth Air Tunnel Heat Exchanger (EATHE) under transient operating conditions in predominantly hot and dry cli- mate of Ajmer (India) using experimental and computational fluid dynamics modeling with FLUENT soft- ware. Maximum air temperature drop obtained using steady state approach for EATHE of pipe length 100 m, pipe diameter 0.2 m and at air velocity of 5 m s 1 is 18.4 °C, 18.7 °C and 18.4 °C for soil thermal conductivity of 0.52, 2.0 and 4.0 W m 1 K 1 respectively. However, the maximum air temperature drop obtained using transient approach during 24 h of operation vary between 18.3 °C and 14.0 °C, 18.3 °C and 17.2 °C and 18.6 °C and 18.0 °C for soil thermal conductivity of 0.52, 2.0 and 4.0 W m 1 K 1 respec- tively. The derating factor is found to be a function of thermal conductivity of soil, duration of continuous operation of EATHE and length of pipe. The analyzed cases have shown the range of derating to be as min- imal as 0.2% to as high as 68%, which if ignored while designing may lead to poor performance of earth air heat exchangers. Maximum value of derating factor is observed after continuous operation of EATHE for 24 h. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Earth Air Tunnel Heat Exchanger (EATHE) has been used as a passive heating/cooling method for reducing consumption of pri- mary energy resources in space heating/cooling. EATHE uses the ground as heat source/ sink for space heating/cooling. A lot of research work on ground heat exchangers has been carried out. Khatry et al. [1] presented an analysis of the periodic variation of ground temperature with depth which takes into ac- count the periodicity of solar radiation and atmospheric tempera- ture. Explicit expression for the temperature as a function of time and depth was derived. Jacovides et al. [2] studied several statisti- cal characteristics of the soil temperature in Athens/Greece through Fourier analysis of a 74 year record (1917–1990) of soil temperatures at the surface and at various depths and for both bare and short-grass-covered areas. Bau [3] presented analytical solutions for heat losses from a buried pipe. Puri [4] performed parametric study of a single pipe carrying warm fluid buried in medium wet sand for pipe diameter, initial soil moisture concen- tration and temperature, and fluid-tube interface temperature 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.07.027 Abbreviations: EATHE, Earth Air Tunnel Heat Exchanger; PVC, poly vinyl chloride; RPM, revolution per minute; CFD, Computational Fluid Dynamics; DBT, dry bulb temperature of air (°C); RTD, resistance temperature detectors; GSHP, ground source heat pump; GCHP, ground coupled heat pump; GHEs, ground heat exchangers. ⇑ Corresponding author. Address: Govt. Women Engineering College Ajmer, Mechanical Engineering Department, Nasirabad Road, Makhupura, Ajmer 305 001, India. Tel.: +91 9982378283. E-mail addresses: bansal_vikas1@yahoo.com (V. Bansal), rohiteca@rediffmail. com (R. Misra), gdagrawal2@gmail.com (G.D. Agarwal), jyotirmay.mathur@gmail. com (J. Mathur). Applied Energy 102 (2013) 418–426 Contents lists available at SciVerse ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy