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In the present experimental work, two identical real field experimental setups (with dry soil and wet soil) of ground-air heat exchanger (GAHE) have been developed in order to evaluate the influence of change in inlet air temperature, airflow velocity, diameter of pipe and soil moisture content on the thermal performance of GAHE system. Influence of these parameters on the required pipe length to produce a specified drop in air temperature has also been investigated. Results reveal that the drop in air temperature achieved from GAHE having dry soil is 11.2 °C at 60 m pipe length, whereas, for GAHE with wet soil having 20% moisture, the same amount of drop in air temperature is obtained at a pipe length of 28 m only. Moreover, upon increasing the airflow velocity from 2 m/s to 10 m/s, cooling capacity of GAHE with dry soil, wet soil with 5% moisture and wet soil with 20% moisture increases by 122%, 185% and 220%, respectively, after 6 h of operation, but the effectiveness of the GAHE system with dry soil, wet soil with 5% moisture and wet soil with 20% moisture decreases by 55.4%, 42.5% and 36.2%, respectively. • Development of an outfield experimental set up for GAHE system with dry and wet soil. • 53% length of GAHE pipe can be reduced by using wet soil with 20% moisture. • By increasing air flow velocity 2 m/s to 10 m/s, cooling capacity increases 220% in wet soil. • By increasing air flow velocity 2 m/s to 10 m/s, effectiveness decreases 36.2% in wet soil. • Required length of GAHE pipe highly depends on soil moisture, air velocity and pipe diameter. [ABSTRACT FROM AUTHOR]

• Earth air heat exchanger system is one of the promising passive techniques for building heating and cooling application. • Efficiency and utility of the EAHE system increase by coupling it with other active/passive systems. • EAHE system may be a good option for pre-heating and pre-cooling of air in industrial applications. • Large land area required to install EAHE is a major constraint for the adaptability of EAHE system in urban areas. • The required pipe length of EAHE can also be reduced by using soil of high thermal conductivity at the vicinity of the pipe. Earth air heat exchager (EAHE) is a promising passive technique that utilises the constant temperature of the underground soil for heating/cooling of air. The EAHE system possesses several advantages and can be used stand-alone as well as in hybrid mode for heating/cooling of air for building and commercial/ industrial applications. This article reviews the available literature pertaining to technology integration and applications of earth air heat exchanger system in the last four decades and also account for the recent significant developments in EAHE to indicate the emerging trends in the domain. The comprehensive review of the literature suggests that a standalone EAHE system can does not cater the requirements of thermal loads entirely and therefore has not grown much popular and still vying for commercial/ industrial applications, worldwide. The requirement of auxiliary devices so as to supplement the standalone EAHE system to achieve thermal comfort conditions entailing huge expenses conflicts the economics of the hybrid EAHE systems. The exorbitantly increasing costs of pipes, land and labour necessitate the paradigm-shift in the major thrust of the research associated with thermal comfort so as to incorporate the economics of system as a whole to enhance the acceptability of EAHE system. [ABSTRACT FROM AUTHOR]

Earth air tunnel heat exchanger (EATHE) is a promising passive technique to provide thermal comfort condition in buildings. EATHE system uses undisturbed temperature of the ground for heating/cooling of air. Despite the several advantages, this technique has not become much popular owing to its high capital cost (mainly pipe cost and trench excavation cost) and large land area requirements. The primary objective of this study is to present a comprehensive review of different EATHE pipe layouts, pipe properties and positioning of the pipe with their advantages and limitations. It is observed that the ring pipe layout is the most cost-effective pipe-layout for small size EATHE system because it saves excavation cost by using a trench of the existing foundation of the building. However, Grid pipe-layout is an ideal layout for a large size EATHE system. Multi-layer pipe layouts should be used to reduce the land area requirement significantly. Moreover, EATHE system can be installed beneath the building (under building foot-print) to eliminate extra land area requirement for the installation of EATHE system. This review article shows that the Initial capital cost and land area requirement for the EATHE system can be substantially reduced by using appropriate pipe layout. It can be concluded that if EATHE system is installed with proper design strategies, it will be a clean and cost-effective method for building heating/cooling with significant power savings. [ABSTRACT FROM AUTHOR]

• Evaluated economic feasibility of backfilling materials for GAHE system. • Developed a CFD simulation model for GAHE system with backfilling materials. • Use of backfilling materials to improve the overall economics of GAHE system. • Required length of GAHE pipe is determined with different BFMs. The ground-air heat exchanger (GAHE) system is an energy-efficient system for building cooling/ heating, but its initial cost is higher due to the requirement of longer pipes to achieve the desired heat exchange between air and soil. To reduce required pipe length, secondary soil (i.e. soil of high thermal conductivity) can be used as backfilling material in the vicinity of GAHE pipe. However, due to the use of secondary soil, the installation cost of the GAHE system may increase. Therefore, the key objective of this study is to explore the economic feasibility of using backfilling materials in the vicinity of GAHE pipe. For this, a three-dimensional CFD simulation model of GAHE system with backfilling material has been developed using FLUENT 15.0, and the required length of GAHE pipe to achieve a specified drop in air temperature with backfilling materials has been determined. Based on this, the initial installation cost of the GAHE system (i.e. pipe cost, trench excavation cost, backfilling material cost) has been estimated. The study revealed that the initial installation cost of a GAHE system could be reduced by 25.7%, 32%, and 38% using dry sand-bentonite, wet sand-bentonite, and wet native soil respectively, as backfilling materials in place of dry native soil. [ABSTRACT FROM AUTHOR]

• Development of a laboratory scale experimental set up with slinky-coil pipe layout. • Use of sand–bentonite mixture as backfilling material for GAHE system. • Required length of the trench is reduced by using slinky-coil pipe layout. • Substantial saving in terms of excavation cost and piping cost. The ground-air heat exchanger (GAHE) is the promising passive technique for building heating/cooling, but due to its high installation cost (mainly pipe cost and excavation cost), it could not become popular. The required length of pipe can be reduced by using thermally enhanced backfilling material (BFM), while, the length of the trench can be reduced by using slinky-coil pipe layout. In the present study, an experimental laboratory setup of GAHE system has been developed with a slinky-coil pipe layout and sand-bentonite as BFM. For this, a flexible PVC pipe of 75 mm diameter and 30 m length, has been arranged in slinky-coil pattern and installed horizontally in a soil box of 10.2 m length. For the study, variation in soil temperature (23 °C–27 °C), inlet air temperature (38 °C–46 °C), air flow velocity (5 m/s–20 m/s) and soil conditions (dry and wet), have been considered. Study reveals that a drop-in air temperature as high as 18.7 °C at 30 m pipe length (trench length of 10.2 m) can be achieved with soil at a temperature of 27 °C and inlet air temperature of 46 °C. The study also revealed that the pipe length could be reduced by 20% with wet sand-bentonite as compared to dry sand-bentonite. [ABSTRACT FROM AUTHOR]