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This article presents the heat transfer augmentation in a solar air heater (SAH) using arc-shaped ribs with multiple gaps. Multiple gaps in arc-shaped ribs are used to enhance the heat transfer by interrupting the growth of the boundary layer. Thermal performance analysis of a SAH has been carried out experimentally for the roughness parameters: number of gaps (Ng) ranging from 2 to 5 and flow parameters; Reynolds number (Re) ranging from 3000 to 18000 at a constant heat flux of 1000 W/m2. The data obtained from the experimental investigation are validated and compared with the smooth duct under similar conditions. For roughened SAH duct, the heat transfer and pressure drop are found to be 274% and 169% higher than that for smooth SAH duct at Ng = 3. Thermo-hydraulic performance is determined for considered parameters and maximum value is observed to be 2.75. [ABSTRACT FROM AUTHOR]

Solar air heater (SAH) is a popular and economical device which collects the solar energy and is employed for space heating, drying of agricultural products, food items, and leather, seasoning of timber, etc. Attachment of artificial roughness on the absorber surface is an appropriate method to augment the heat transfer from the heated surface to the air flowing through SAH duct. Use of artificial roughness has been an area of great interest for researchers as far as the cooling of turbine blades, and combustion chambers are concerned. This technique is effective in enhancing the heat transfer in a micro-channel heating/cooling system. Present paper holistically furnishes concise information about various kinds of V-shaped roughness geometries used in SAHs for improving its performance covering experimental, analytical, numerical and computational fluid dynamics (CFD) approaches. In this paper, 124 research articles have been referred, which provide a detailed comprehensive and comparative study revealing the effect of various geometrical parameters and different V-shaped roughness patterns on the performance of SAH. This article also brings in the information about the correlation developed by researchers for heat transfer and friction factor. This comprehensive review will be quite useful for technical persons and researchers working in the area of SAH. [ABSTRACT FROM AUTHOR]

Large-scale adaptation of solar air heating in industries and agro-processing will lead to clean energy processing as well as reducing the production cost for these industries. The solar air heater uses the principle of the greenhouse effect to heat air through the collected heat in the absorber. Among the various techniques employed by the researchers to augment heat transfer, the addition of artificial roughness elements/fins/corrugations on the heated surface is the promising one for heat transfer augmentation. In the present work, the effect of broken inclined ribs with rectangular cross-section on heat transfer and friction characteristics of the equilateral triangular passage duct has been analyzed using computational fluid dynamics. The effect of roughness parameters, viz., relative gap position and relative gap width, is being investigated for the Reynolds number (Re) ranging from 4000 to 18,000. The values of relative gap position (d/W) and relative gap width (g/e) are varied from 0.16 to 0.67 and 0.5 to 2, respectively, while a constant heat flux is supplied on the absorber side, other surfaces being insulated. The Nusselt number increased up to 2.16 times by using broken ribs than that of the smooth duct at d/W = 0.25 and g/e = 1. [ABSTRACT FROM AUTHOR]

Highlights • The Earth air tunnel heat exchanger system is capable to provide sufficient heating and cooling in buildings with significant energy savings. • The knowledge of thermo-physical properties of soil is crucial for the design of an EATHE system. • In multiple pipe system, the distance between the pipes should be reduced gradually along the length of pipe. • Too deeper excavation of soil does not result in any appreciable change in soil temperature. • EATHE should be used during daytime in summer and during night time in winter season. Abstract The principal drivers that led to the high growth rate in the renewable energy market in developing countries over the past twenty years include not only the dwindling fossil reserves, exorbitant increase in the price of crude oil, widening energy deficit, but also the extravagant lifestyle of profligate consumers of energy. Nearly one-third of total primary energy is consumed for the purpose of space cooling and heating. Renewable and passive systems can reduce the building cooling and heating energy demand. Earth air tunnel heat exchanger system is one of the promising technique used for space cooling and heating nowadays, thus reduce fossil fuel based energy demand as well as greenhouse gas emission, but its use has been restrained owing to its poor performance, land area requirement and financial viability. Various experimental, analytical and simulation studies have been conducted in last few decades to investigate the various parameters that may enhance its performance. In this article, the effect of thermo-physical parameters of soil, thermal and flow parameters of air, geometrical and material properties of pipe, different modes of operation on the performance of earth air tunnel heat exchanger systems have been critically reviewed. Following thorough review of the articles, it is concluded that if EATHE system is developed with proper design guidelines, then it may facilitate sufficient space heating and cooling with significant energy savings. [ABSTRACT FROM AUTHOR]

To overcome the limitations of conventional tube in plate type solar collector such as low collection and conversion efficiency, direct absorption solar collectors using nanofluid as heat transfer media has been proposed. Direct absorption of solar radiation and mixing of nanoparticles in the base liquid improves the optical and thermo physical properties of the base liquids, resulting to an increase in the efficiency of solar collectors. In the present experimental work, the effect of Al2O3-H2O nanofluid, as heat transfer fluid on the efficiency of a direct absorption solar collector was investigated. The ASHRAE standard was used to calculate the instantaneous efficiency. The results show that, in comparison with water as direct solar absorption medium, nanofluid increases the efficiency. For 0.001 vol% the increased efficiency was 12.8%. There is a continuous increase in the efficiency by increasing the volume fraction from 0.001% to 0.01% and beyond that increase in volume fraction values, the solar collector efficiency gets a reverse trend and becomes constant. [ABSTRACT FROM AUTHOR]

The conventional tube-in-plate type flat plate solar collectors have low efficiency and higher heat losses due to surface based solar energy absorption and indirect transfer of heat from hot absorber surface to working fluid flowing through tubes. A full scale direct absorption solar collector having gross area of 1.4 m 2 and working on volumetric absorption principle was developed to perform experimental study using thin film of Al 2 O 3 -H 2 O nanofluid. Use of nanofluid as working fluid improves the optical and thermo physical properties that result into an increase in the efficiency of the collector. Experimentation was carried using four different volume fractions of 20 nm Al 2 O 3 nanoparticles, 0.001%, 0.005%, 0.01% and 0.05%. ASHRAE standard 93-86 was followed for calculation of instantaneous efficiency of solar collector. Improvement in efficiency of solar collector has been recorded in all four cases of using nanofluids in place of water. Instantaneous efficiency enhancement of 22.1%, 39.6%, 24.6% and 18.75% has been observed for 0.001%, 0.005%, 0.01% and 0.05% volume fraction respectively. The experimental results also indicated that collector efficiency peaked at certain volume fraction, and decreased for lower and higher values of volume fraction. [ABSTRACT FROM AUTHOR]

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]

Thermal performance of ground-air heat exchanger (GAHE) depends on the rate of heat transfer between air and soil, which is governed by thermal properties of soil surrounding the GAHE pipe. Soil thermal properties around GAHE pipe can be improved either by increasing its moisture contents or by using some thermally enhanced backfilling materials. In the present study experimentally investigates the thermal performance of ground-air heat exchanger system using a sand-bentonite mixture (dry as well as wet) and compare their performance with the ground air heat exchanger system having native soil (dry as well as wet) as backfilling material. The study acknowledges the highest cooling capacity (125 W) for GAHE with wet sand-bentonite as backfilling material, and after 6 h of continuous operation, it is 38.4%, 18.4% and 11.1% higher than that obtained with dry native soil, dry sand-bentonite and wet native soil, respectively. The study also revealed that thermal performance deterioration factor (TPDF) increases with the duration of the operation. At airflow velocity of 5 m/s, after 6 h of continuous operation highest TPDF is noticed for ground-air heat exchanger with dry soil (0.22); whereas, lowest TPDF is observed for ground-air heat exchanger system with wet sand-bentonite (0.07). Schematic of laboratory experimental setup of GAHE system. Image 1 • Development of a laboratory scale experimental set up for GAHE system. • To evaluate the effect of sand-bentonite backfilling material (dry as well as wet) on performance of GAHE system. • Maintaining a certain level of moisture into the soil in the close vicinity of GAHE pipe. • Better thermal performance of GAHE due to BFM resulting reduction in required pipe length. [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]

Abstract: Thermal performance of Earth Air Tunnel Heat Exchanger (EATHE) under transient operating conditions has been evaluated for predominantly hot and dry climate of Ajmer (India) using experimental and Computational Fluid Dynamics modeling. Effects of time duration of continuous operation, thermal conductivity of soil pipe diameter and flow velocity on thermal performance of EATHE under transient conditions have 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. 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 EATHE. Study reveals that the effect of pipe diameter due to prolonged use of EATHE system on its thermal performance is least in case of soil with higher value of thermal conductivity. Results show that the increase in flow velocity leads to deterioration in thermal performance of EATHE system. Under steady state condition, drop of 18.8°C in air temperature is obtained, whereas, under transient conditions cooling of air reduces from 18.7°C to 16.6°C for soil thermal conductivity of 0.52Wm−1 K−1, after 24h of continuous operation. [Copyright &y& Elsevier]

Abstract: Thermal performance of earth air tunnel heat exchanger (EATHE) 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°C is obtained in air passing through EATHE having 0.1m diameter and 60m length, at 5ms−1 flow velocity, whereas, the transient analysis shows that for soil having thermal conductivity of 0.52Wm−1 K−1, the cooling of air reduces from 18.7°C to 16.6°C, after 24h of continuous operation. Change in cooling effect after 24h of operation for soil thermal conductivity of 2.0Wm−1 K−1 and 4.0Wm−1 K−1, reduced from 18.8°C to 18.5°C and 18.8°C to 18.7°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. [Copyright &y& Elsevier]

Abstract: Thermal performance of Earth Air Tunnel Heat Exchanger (EATHE) under transient operating conditions in winter season has been evaluated for predominantly hot and dry climate of Ajmer (India) using experimental and Computational Fluid Dynamics (CFD) 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 rise of 19.6°C is obtained in air passing through EATHE having 0.1m diameter and 60m length, at 5m/s flow velocity, whereas, the transient analysis shows that for soil having thermal conductivity of 0.52Wm−1 K−1, the heating of air reduces from 19.4°C to 17.2°C, after 24h of continuous operation. Heating effect after 24h of operation for soil thermal conductivity of 2.0Wm−1 K−1 and 4.0Wm−1 K−1, reduced from 19.6°C to 19.2°C and 19.6°C to 19.5°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. [Copyright &y& Elsevier]

Abstract: Earth–air–tunnel heat exchanger (EATHE) systems are insufficient to meet the thermal comfort requirements in summer conditions. Therefore, evaporating cooling has been integrated with EATHE to enhance the thermal comfort hours. Economics of integrated EATHE–evaporative cooling system to reduce/avoid the use of air-conditioner and electric heater is analyzed by evaluating simple Internal Rate of Return (IRR) on investment. For this analysis a transient and implicit model based on computational fluid dynamics has been used for evaluating the energy saving obtained by the use of EATHE system integrated with evaporative cooling. Four base cases of existing systems, i.e. air-conditioner and electric heater have been considered for carrying out the economic analysis. Moreover, three different types of blower (i.e. energy efficient blower, standard blower and inefficient blower) have been considered for evaluating the energy saving and financial viability of the proposed system. These results of IRR calculation shows that wherever, the existing air conditioner and heaters are efficient, their replacement with proposed EATHE system is not a technically and economically viable option. It is observed that the IRR value is greatly dependent on the electric tariff and the energy efficiency of the blower used in the proposed EATHE system. [Copyright &y& Elsevier]

Abstract: Earth–pipe–air heat exchanger (EPAHE) systems can be used to reduce the cooling load of buildings in summer. A transient and implicit model based on computational fluid dynamics was developed to predict the thermal performance and cooling capacity of earth–air–pipe heat exchanger systems. The model was developed inside the FLUENT simulation program. The model developed is validated against experimental investigations on an experimental set-up in Ajmer (Western India). Good agreement between simulated results and experimental data is obtained. Effects of the operating parameters (i.e. the pipe material, air velocity) on the thermal performance of earth–air–pipe heat exchanger systems are studied. The 23.42m long EPAHE system discussed in this paper gives cooling in the range of 8.0–12.7°C for the flow velocities 2–5m/s. Investigations on steel and PVC pipes have shown that the performance of the EPAHE system is not significantly affected by the material of the buried pipe (pipe). Velocity of air through the pipe is found to greatly affect the performance of EPAHE system. The COP of the EPAHE system discussed in this paper varies from 1.9 to 2.9 for increase in velocity from 2.0 to 5.0m/s. [Copyright &y& Elsevier]

Abstract: Earth–pipe–air heat exchanger (EPAHE) systems can be used to reduce the heating load of buildings in winter. A transient and implicit model based on computational fluid dynamics is developed to predict the thermal performance and heating capacity of earth–air–pipe heat exchanger systems. The model is developed inside the FLUENT simulation program. The model developed is validated against experimental investigations on an experimental set-up in Ajmer (Western India). Good agreement between simulated results and experimental data is obtained. Effects of the operating parameters (i.e. the pipe material, air velocity) on the thermal performance of earth–air–pipe heat exchanger systems are studied. The 23.42m long EPAHE system discussed in this paper gives heating in the range of 4.1–4.8°C for the flow velocities 2–5m/s. Investigations on steel and PVC pipes have shown that performance of the EPAHE system is not significantly affected by the material of the buried pipe. Velocity of air through the pipe is found to greatly affect the performance of EPAHE system. [Copyright &y& Elsevier]

• 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]

Earth air tunnel heat exchanger (EATHE) systems are inadequate to meet the thermal comfort requirements in semi-arid regions as they require large pipe lengths. The effectiveness and performance of such system can be increased by increasing soil moisture content, making as wet soil configuration. In the present work, the thermal performance of the EATHE system with dry and wet soil has been evaluated during the peak summer season. Further, the thermal behavior of these two EATHE systems has been compared on the basis of temperature of air flowing through buried pipes at different sections along the length. For the same a theoretical model is developed and validated with experimental results and found in good agreement with an error ranging from 2.44% to 10.85%. Moreover, the exergetic analysis has been carried outto investigate the work potential and scope of maximum utilization of energy. Results reveal that thepipe length can be reduced by 12–14 m with wet EATHE system as compared to dry EATHE system for the same cooling performance. The second law analysis shows that the maximum exergetic efficiency is 52.25% and 53.18% for dry and wet soil EATHE systems respectively. [ABSTRACT FROM AUTHOR]

An EATHE system performs well for short periods but its thermal performance gets deteriorated during long continuous operation due to sub-soil thermal saturation in the vicinity of buried pipe. Effect of soil thermal saturation can be reduced to some extent in longer EATHE pipe but it is uneconomical. In the present study sub-soil moisture content is increased to enhance soil thermal properties and its effect on EATHE thermal performance and pipe length requirement for certain temperature drop has been investigated experimentally for summer cooling in hot and arid climate. In the study, two identical experimental set-ups have been developed at Ajmer city (India). A water impregnation system has been introduced to maintain different soil moisture contents in the close proximity of EATHE pipe. Knee point is obtained at a length of 29 m, 28 m, 27 m and 26 m from pipe inlet section with 5%, 10%, 15% and 20% moisture in sub-soil respectively, as compared to 41 m length in dry soil, after 10 h of continuous operation. The average heat transfer rate and COP increased by 24.1% and 24.0% respectively for 20% moisture content at 30 m EATHE pipe length as compared to dry system. [ABSTRACT FROM AUTHOR]